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Why is it advised to avoid bubble formation during mixing?

Why is it advised to avoid bubble formation during mixing?


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I have been told not to vortex solution containing protein. The reason I was given is bubble formation. Here I am interested in the effect of bubble formation in general.


A "bubble" per se would do no harm unless you have cells (without walls like mammalian cells) in the suspension. If you agitate the protein mixture vigourously then it may lead to denaturation of proteins by extensive intermolecular collisions. The "froth" formation is an indication of denaturation as denatured proteins stabilize these foams [1, 2].


References:

  1. Zayas, Joseph F. "Foaming properties of proteins." Functionality of proteins in food. Springer Berlin Heidelberg, 1997. 260-309.

  2. Schmidt, Isabelle, et al. "Foaming properties of protein/pectin electrostatic complexes and foam structure at nanoscale." Journal of colloid and interface science 345.2 (2010): 316-324.


  • Problems with stadard curve fit
  • Inconsistent results and high coefficient of variation
  • Difficulty obtaining a signal
  • High background

Before performing quantitative ELISA, you need a standard curve that performs well. This ensures you can reliably determine the concentration of your samples.

Note: If using one of our ELISA kits, the measurement values can vary considerably from the examples shown on the datasheet or protocol booklet. This is usually to be expected, so long as the curve has a good fit, as measured by the regression coefficient (R 2 ). As long as the R 2 0.9, the standard curve can be used with confidence.

Common problems encountered when setting up a standard curve include:

Issues with standard solution

Standard solution has not been diluted correctly.

Confirm dilutions are made correctly.

Standard improperly reconstituted.

Briefly spin the vial before opening inspect for undissolved material after reconstituting.

Store and handle the standard as recommended.

Use calibrated pipettes and proper pipetting technique.

Curve fitting model is not working with the data

You may need a different curve fitting model.

You should always follow manufacturer's instructions in the first instance. However, if the curve-fit doesn't seem to work, try plotting using different models.


Diseases of Peaches and Plums

Some of the diseases that attack peaches in Mississippi are very aggressive, and missing one or two key sprays can result in the loss of most of a peach crop, especially if sprays are missed when weather conditions are favorable for disease development. Fungicides protect the plant or fruit from infection they do not eliminate the infection once it has occurred. While fungicide sprays are necessary to grow peaches in the Deep South, much of the real protection from these diseases will come from removing and destroying the inoculum (or “seed”-producing structures) of these diseases.

The following disease descriptions may seem extensive to the point of “too much,” but they will help you identify these disease infections so that you can prune and remove these structures from your trees, reducing the disease pressure. Similarly, descriptions of weather conditions necessary for a disease may seem unnecessary, but knowing the conditions that encourage the disease can help you decide how important it might be to get out and spray before or between rains.

When tree parts suspected of harboring disease are removed or pruned from the tree or surrounding soil, immediately place them in a plastic bag. Tightly close the bag and destroy it. If the limbs are too large to fit in the bag, place them well away from and downwind of the trees. Burn or otherwise remove them as soon as possible. Do not allow them to accumulate.

Brown Rot

Brown rot is a serious peach disease, but it is not very common on plums in Mississippi. The disease attacks many plant parts (blossoms, twigs, shoots, and fruit) from spring through harvest. Fungicides will help suppress the disease but control it only moderately when conditions favor the disease, especially in late season near harvest. Nonetheless, fungicides are almost a necessity in our climate.

The fungus that causes brown rot (Monilinia fruticola) overwinters in twig cankers, fruit mummies, and peduncles (stem-like structures that attach the flower/fruit to the branch). Removing these overwintering sites after harvest will reduce disease pressure the next season.

The brown rot fungus becomes active in early spring, about the time the flower buds develop into the “pink” stage. Warm, humid, wet weather favors rapid spread and disease development. The optimal temperature for disease development is 75°F, but slower disease development can occur as cool as 39°F and as warm as 86°F. Storms are a perfect time for spore movement because the free water (rain, dew, irrigation) on the trees provides the moisture for these seeds (spores) to germinate and infect the plant. The free water will need to be present for longer periods the further the temperature is from the 75°F optimum.

As the fungus grows, it produces spores, or seed-like structures. They are very small (like very small pollen) and easily carried by wind and rain. The fruiting areas that produce the spores are small, ash-gray tufts that emerge from the surface of the brown-colored infected tissue. Infections in mature fruits show these spores clearly (Figure 1).

Figure 1. Peach brown rot. The “fuzzies” on the left side of the peach are the spores produced by the fungal disease. Notice the light brown discolored area between the “fuzzies” and the bright color of the peach. This is the leading edge of infection where the fungus is invading and consuming the fruit spores will soon appear on the outside.

Twig cankers are dead (brownish), sunken areas. The canker may stay on one side of the twig or may girdle (encircle) it. A weak or dead twig or fruit spur will emerge from the canker. Some cankers may be small and difficult to find. Larger infected twigs or spurs may ooze sap, which looks like a bubble of dark brown viscous gum. This is called gummosis. The amount of gummosis varies from none to a fair amount and will only occur on larger twigs and branches.

Mummified fruit is a favored location for many diseases to overwinter. The “mummies” are fruit that have dried, leaving an unappetizing mock fruit. They might be hanging from the tree, lying on the ground, or, worse, partially buried in the soil near the tree (Figure 2). Infected fruit mummies that have been buried or partially buried in the soil may produce small, brown, cup-shaped mushrooms (apothecial stage of the fungus). The mushrooms produce a different kind of spore that infects the trees. Retrieving and destroying all mummies will be very beneficial.

Figure 2. The peaches here have been infected with brown rot and have fallen to the orchard floor. They will continue to produce spores and infect other peaches. They may mummify and, especially if partially buried, produce some small mushrooms that produce a second kind of spore that will infect spring blossoms.

The fungal spores commonly infect the flower, fruit, peduncle, and twigs. The peduncle is the stout stem that connects the flower/fruit to the tree branches. Early-season infection of the twig and blossoms creates the small cankers from which the fungus produces more spores. These early-season infections can substantially influence fruit infections later in the season.

In Mississippi, attacks on the flower by brown rot disease are not common. When they do occur, it seems to foretell a very challenging season for the grower, because the disease becomes prevalent. Symptoms of flower infection are called blossom blight. The blossoms will brown and probably collapse. The blight appears 3–6 days after infection, which will probably have occurred during a rain, irrigation, or long, heavy dew event.

Symptoms of shoot and twig infection will occur 3–4 weeks after infection. They may or may not follow from infected blossoms, from which the fungus travels down the peduncle into the twig or branch. As these infections progress, whole clusters of blossoms or leafy branches may wilt and die. This is because the canker cuts off the flow of water to these parts of the tree. Prune these out by cutting into healthy wood below the lesion as soon as possible. Remember to place the cut parts in a plastic bag, and seal and destroy it.

Brown rot may attack fruits at any time, but older fruit are more susceptible. Infection may occur directly through the skin of the fruit, through natural openings, and through wounds, especially those made by insects.

Direct your spraying and sanitation controls toward the sources of infection. Remove old, mummified fruit, peduncles, and infected twigs/branch parts from the tree and ground before spring. If harvest weather favors the disease, regular and thorough sprays will be necessary if you want to save your fruit from destruction by brown rot. Fungicides work preventively—they cannot eradicate an infection. This means you must be proactive and keep these protective sprays on the targets the fungus most likes to infect.

Peach Scab

Scab is a fungal disease caused by Cladosporium carpophilum. Although the primary damage caused by this disease is visual, it can provide entry wounds for brown rot. Heavy infections may also cause the peach to split.

The disease symptoms are velvety, olive-green spots on the fruit, leaves, or twigs. The spots are about one-sixteenth of an inch and enlarge to one-eighth of an inch. You will begin seeing these spots about 3 weeks after petals fall. When the spots are on the fruit, they will usually be on the stem-end side. When infections are numerous, they may merge and may cause the fruit to split. The fruit spots are confined to the skin they do not enter the flesh.

Like brown rot, peach scab overwinters in twig lesions. Infections of twigs occur on new growth and are difficult to see. They start as raised, oval to circular areas that are pretty much the same color as the surrounding tissue. As they age, they may turn brownish. By season’s end, the lesion edges may be somewhat purple and the lesions may have grown to one-fourth to one-half of an inch. The second season of infection is when these lesions will produce most of the spores. The spores are both air- and water-borne and require 24 hours of high relative humidity to germinate.

Peach Leaf Curl

Peach leaf curl disease is caused by the fungus Taphrina deformans. Peach leaf curl does not occur regularly on most peach and plum trees, but it can be a serious disease. Standard fungicide sprays used to control other diseases, such as brown rot, normally control this disease.

The disease is favored by moderate temperatures (48–81°F optimal temperature for development is 68°F) and wet weather during early bud development. The humidity needs to be above 98 percent.

Two stages of the fungus make this disease unique. One type of spore is produced from curled (infected) leaves in the spring. The fungus can infect either side of the leaf. Infected leaf symptoms include yellow to reddish areas that get thicker as the fungus grows. The infected and thickening portion of the growing leaf causes that part of the leaf to grow more slowly than the rest of the leaf, causing the leaf to curl. These thick areas produce spores that, when germinated, produce a different phase of the fungus that grows on and along with the shoot tips, keeping up with their growth.

Sanitation and cultural controls are not effective for this disease. Some peach cultivars have been bred for resistance to this disease, so resistant cultivars and fungicides are the primary management tools. Copper sprays during tree dormancy, as well as in-season applications, are important. Once established in a group of trees, even radical pruning to remove infections will have only modest success controlling the disease.

Shot Hole

Shot hole is a fungus disease (Wilsonomyces carpophilus) that gets its name from the leaf symptoms—smallish brown spots that fall out, leaving a “shot” pattern in the leaf. The disease is present in Mississippi.

This fungus starts to cause problems during wet winter months when buds and twigs infected the previous season produce spores. The fungus infects and kills dormant buds. Some buds may have a varnished appearance, which results when tree gum seals the infection from the rest of the plant.

Stem lesions range from about one-tenth to three-eighths of an inch in diameter. Leaf and fruit lesions start as small, purplish areas that expand and turn brown. All may have a velvety, brownish mass of fungus in the middle during moist and humid weather. When the weather turns warm, the leaf lesions will fall from the leaf, leaving the “shot hole” appearance. Fruit lesions will be on the upper (stem) side and will become rough-textured, almost corky.

To manage this disease, you must protect the dormant buds. A single application of fixed copper or Bordeaux mixture before fall/winter rains provides winter-long protection. Growing shoots and fruits also need protection. A spray application immediately after fruit set is most common. Usually Captan is used because copper fungicides used at this time of year can cause plant injury (phytotoxicity). No resistant cultivars are available.

Bacterial Spot

As the name indicates, this disease is caused by a bacteria (Xanthomonas arboricola pv. pruni). It can be very aggressive in the eastern United States because of generally higher humidity, wetter conditions, and longer dew periods than in the western states. Very susceptible cultivars cannot be grown here at all.

The bacteria depend upon free moisture (dew, rain, irrigation) to reproduce and for lesion growth. Rain driven by wind spreads the bacteria through the tree and among trees. Infections will be worse on the sides of the trees facing the winds that brought the infection. The optimal growth temperature is 75–84°F. The disease affects twigs, shoots, leaves, and fruits.

Leaf symptoms start as a water-soaked dark green spot that expands until it meets the veins inside the leaf. Because the leaf veins keep the lesion from spreading for a while, angular lesions (lesions with sharp corners) about one-sixteenth to one-eighth of an inch are a key that bacterial spot is the problem. If warm, wet weather continues, the lesions may enlarge and merge. As the lesions age, the insides will turn from a water-soaked dark green to a light purple color. As the weather dries, the lesions may turn brown and fall from the leaf. The lesions will be more common in areas of the tissue where water sits for any period of time, such as along the leaf midrib, on leaf tips, or along lower areas of the leaf margins. Leaves with numerous lesions may turn chlorotic (yellow) and fall from the tree.

This bacterial pathogen usually enters twigs through leaf scars, which are places where a leaf has fallen from the twig. Lesions that develop on the previous year’s growth are called “spring cankers” or “black tip.” They were infected by the bacteria moving through the leaf scars the previous autumn. Spring cankers appear as slightly raised blisters. They can expand to as long as an inch along the twig. Black tip is confined to the terminal bud area of the twig. The bud fails to open, and a dark canker can extend up to 1 inch down the twig from the bud.

Summer cankers form on newly growing shoots and are seen in late spring or very early summer. Favorable weather conditions may cause rapid bacterial growth, and the infection may kill the shoot.

Fruit symptoms first become apparent several weeks after petal fall. They appear as small, water-soaked, brownish lesions that might be mistaken for insect damage. As infection progresses, gum may ooze from the lesions during periods of high humidity. As the fruit and the infection age, the lesions may crack open and perhaps sink.

Bacterial infections can only be managed with proper sanitation, copper-based products, or antibiotic sprays and host plant resistance. There are cultivars with resistance to this disease. Common resistant cultivars include Redskin, Redhaven, Loring, Candor, Biscoe, Dixired, Sunhaven, Jefferson, Madison, Salem, Contender, Harrow Beauty, and Harrow Diamond. Bacterial spot is a very difficult disease to manage. If you are planting peaches or plums, please select a resistant cultivar.

Black-knot

Black-knot is caused by the fungus Apiosporina morbosa. The primary symptom in established infections occurs on wood and consists of outgrowths or knots on shoots, spurs, branches, and trunks. Old knots are hard, dark, almost black, raised areas. The raised areas are often invaded by insects whose damage may, in turn, be invaded by secondary pink or white fungi.

Infection starts in the spring when the tree enters the green tip stage, with most infection occurring between very early bloom and the end of petal fall. Spores released from 2-year-old infected tissue are moved by wind and splashing rain to new shoot growth. For the spores to be made, at least 6 hours of rain are needed at 70°F, which is close to the optimal growth temperature for the fungus.

Symptoms of new shoot infection are difficult to detect. Perhaps the most obvious symptoms are the branches growing at right angles. Less obvious are the small, olive-green knots that might be firm to somewhat corky. The knots later turn hard and will probably break off easily.

Black-knot can be a problem in Mississippi plum trees, usually when those trees are within about 600 feet of wild plums and cherries or when the trees have not received care for a substantial length of time. Fungicides apparently suppress the disease, but pruning out black-knot cankers anywhere on the tree is a necessity. Wild plums and cherries within 600 feet should be removed if possible. Prune infections in wood about 4 inches below the lowest symptom of infection. Midsummer pruning is the most effective since the outer swelling is the closest to the infection on the inside of the wood. Fungicides should be applied during the time of active shoot growth if the disease is a problem in your area.

Plum Pockets

The fungus Taphrina causes plum pockets disease, but, while present in Mississippi, it has not been a serious problem. It is included here because it occurs frequently enough for many people who raise plums to see it. Although the fungus infects leaves, shoots, and fruit, symptoms are most obvious on fruits. Symptoms become obvious on all plant parts 6–8 weeks after bud break.

Fruit become enlarged (up to 10 times their normal size), wrinkled, and distorted. The centers of the fruit are spongy or hollow and may or may not contain a pit. When the fruits dry, they turn brown to black and are called “bladder plums,” “mock plums,” or, most often, “plum pockets.”

Twisting and curling are the most common signs of leaf and fruit infections, but these symptoms may not be present.

If planting new trees, select resistant cultivars. The most effective fungicide practice is a single fungicide spray in late autumn or before spring budbreak. Bordeaux mix, chlorothalonil, and liquid lime sulfur are effective treatments.


Why is it advised to avoid bubble formation during mixing? - Biology

How do you make lots of foam very easily? It turns out to be suprisingly hard to come up with a recipe for success, with many complicating factors. If you are concerned with foam making, sit back, relax and enjoy the read. The 2020 update later down the page provides a state-of-the-art summary which says, yes, it's complicated but the practical rules aren't too hard. I've also added a section on different foam making methods, based on what I've learned in the past years.

It is trivially easy to make a foam - just mix air and liquid with some energy and bubbles will form. If these bubbles reach the surface with a liquid fraction &epsilon in the 0.1-0.2 range then they are a kugelschaum ("kugel" means "sphere" and "schaum" means foam). These foams are not really considered in these apps. When &epsilon <0.1 then we have a polyederschaum (polyhedral), the classic foam that is the central concern of Practical Foams. Although it is easy to create a foam, in most cases it is totally unstable. So the question of making foam is not so much about how to make them (which is trivial) but how to make them stable (which is not). In the AntiFoam section we will discuss the even more difficult question of how to make a stable foam unstable.

  1. Elasticity. The first reason surfactants help create foams is that the surface becomes elastic. This means that the bubbles can withstand being bumped, squeezed and deformed. A pure water surface has no such elasticity and the bubbles break quickly. It also means that those systems which produce more elasticity (see the Elasticity section) will, other things being equal, produce more stable foams. As discussed in the Rheology section, in general a wall which is both stiff and elastic provides a foam with a greater ability to resist a pushing force and therefore a higher yield stress. Smaller bubbles also give a higher yield stress
  2. Disjoining pressure. The second reason that surfactants help create foam is that the liquid in the foam walls is naturally sucked out of the walls into the edges. This is nothing to do with drainage (as explained in Drainage, the walls contain an irrelevant fraction of the liquid), it is just simple capillarity. The capillary pressure will keep pulling liquid out unless a counter pressure ("disjoining pressure") acts against it. This can be produced by charges on the surfactant either side of the wall and/or by steric interactions between surfactant chains. These effects are discussed in DLVO, but because the charge effect operates over large distances (50nm) compared to the small distances (5nm) of steric effects, in general ionic surfactants are much better at creating stable foams.
  3. Resistance to ripening. The Ostwald ripening effect means that small bubbles shrink and large ones grow. As the Ostwald section shows, this is partly controlled by the gas (CO2 falls apart quickly, air/N2 is slower and C2F6 much slower) but also by how good a barrier to gas diffusion the "wall" of surfactant at the surface provides.
  4. Resistance to drainage. The more water around the foam the less risk (in general) of it becoming damaged. So a foam that drains quickly is more likely to become damaged. As we will see, to resist drainage you need high viscosity and small bubbles, though the surfactant wall has some effect on the drainage process with stiffer walls giving (usually) slower drainage.
  5. Resistance to defects. If oil or a hydrophobic particle can penetrate the foam wall it can cause the wall (and therefore the foam) to break. Although there are plausible and simple theories (discussed in AntiFoams) of Entry, Bridging and Spreading coefficients they turn out to be of limited predictive value. Once again they are necessary but not sufficient. The key issue is the Entry Barrier. When this is high the foam is resistant to defects.

These principles are so easy, yet creating foams efficiently is surprisingly hard. Why? The key issue is timescales. If a surfactant is marvellously elastic and has a strong disjoing pressure and is a good gas barrier and has a high entry barrier it might (and usually does) fail to form a foam because it takes too long to reach the liquid/air interface and form its strong resistant domain so the foam has already collapsed. On the other hand, a surfactant that quickly reaches the surface to create an adequate elasticity and disjoing pressure will produce large volumes of foam - though the foam will collapse quickly, especially in the presence of oily impurities such as grease being washed from one's hands.

This leads us to the issue of Dynamic Surface Tensions. It would be wonderful to provide an app that fully described the complexities of DST and which therefore allowed you to produce a mixture with very rapid decrease of ST to give the fastest possible foaming behaviour. But my reading of the literature is that it is quicker to measure the DST behaviour using (most usually) a Maximum Bubble Pressure device (which creates bubbles over different timescales and therefore gives the surface tension at each of those timescales) than it is to attempt to describe the behaviour via theories. In particular, there are great debates about whether DST is limited by diffusion, by barrier entry and/or via the need to come out of a micelle before entering the interface. My reading of the excellent review by Eastoe 1 is that simple diffusion dominates and that the existence of micelles largely makes no difference because the timescale for a surfactant molecule to partition from the micelle is very fast even though the timescale for micelle formation/collapse is very slow. Of course one can find real cases of entry barriers and real cases of micelle-limited diffusion. But it is even more complicated. An extensive analysis from U. Sofia shows that there are 4 possible outcomes in systems containing micelles, two of which are indistinguishable (to the casual observer) from simple diffusion kinetics and two of which might be confused with barrier kinetics. Finally, distinguishing entry-barrier and micellar effects from the effects of small amounts of impurities in the surfactants is surprisingly difficult and for the practical formulator using commercial, unpurified surfactants there is little hope of understanding the subtleties of DST curves. The take-home message is "Don't formulate foams without measuring DST, but don't spend too much time theorising about why you get great results for some specific surfactant combination." I don't like writing such advice as I usually find that good models are the best way to avoid lots of lab experiments. However, the 2020 review paper, discussed below, contains a master-class on the relevant theory and concludes "The theory doesn't really help - just measure the DSTs" .

The harsh reality is that successful foaming agents tend to be mixtures, with all the complexities they induce. The ubiquitous SLES/CAPB (Sodium Laureth Sulfate/CocoAmidoPropyl Betaine) mixture happens to be made from two excellent fast foamers. The CAPB on its own produces a lot of stable foam, but is rather expensive. CAPB is especially good at creating a high entry barrier so is resistant to oils during the creation of foam. SLES on its own produces a lot of relatively unstable foam. A mix of the two provides a good balance of cost, foam and stability. However, adding a small % of lauric or myristic acid has a dramatic effect on foam stability. It increases elasticity but also slows down bubble growth (Ostwald ripening) dramatically, so the foam remains small. This has a big impact on the ability of water to drain from the foam - drainage speed goes as Diameter² - and the drier the foam the more easy (other things being equal) it is to break it apart. The long-chain acids on their own are useless as foaming agents (and as sodium salts are of modest foaming ability as common soap, easily wrecked by hard water). The combination of SLES/CAPB/Long-chainAcid is a potent mix for creating a foam with small bubbles and a long life-time. Indeed, a simple way to transform a hand-soap to a shaving foam is to add a few % of the long-chain acid.

But what about my surfactant system?

The rules for creating a good, stable foam (or, indeed, the rules for making sure that such a foam is not created) are simple and clear. So why is it so hard to create new foam formulations? The answer is that if you have the right set-up to measure all the basics: CMC, &Gammam, disjoining pressure v film thickness, interfacial elasticity and entry barrier then it's rather straightforward to make the best out of any set of surfactants and foam boosters you happen to want to use. The measurements can largely be automated so lots of formulation mixes can be screened quickly. One problem, as mentioned above, is timescales. Most measurements are made after comparatively long times so it needs extra time-dependent experiments to see if the appropriate parts of a surfactant blend will get to the surface fast enough to create a foam which then becomes stabilised as the slower components arrive to form a tougher surfactant layer. The other problem is that small additions of co-surfactants, foam boosters etc. can make a large difference, so it is necessary to carry out measurements on large numbers of samples. A robotic lab set up to do lots of high-throughput screening can do a lot of the hard work, but most of use don't have access to such a lab.

In the longer term, a theory that could predict the interfacial behaviour of mixtures of ingredients would make development of foam much more rational. But such a theory seems to be a long way off.

The view from 2020

I wrote this page in 2014-15 and had no reason to update it till 2020. To my surprise, what I wrote has stood the test of time. I've not changed any of the previous text, other than the DST sentence that refers the reader to here. But a masterly review 2 , backed by a serious amount of experiment and theory, allows us to be a bit more specific. Again it is the team at Sofia, led by Prof Tcholakova, who have clarified the situation with five key points.

  1. Although both non-ionics and ionics can produce excellent foaming, the non-ionics need to be above 95% of the full surface coverage of the interface (with a Gibbs Elasticity over 150 mN/m) before they will foam well - it's a sort of all or nothing. Ionics can start producing credible foam at 30% of their surface coverage (even with Gibbs Elasticity of just 50 mN/m), with a with a steady increase in production as you head to 100%. The reason is clear: steric stabilization of the foam interface works well, but only when there is near-full coverage the interface can break easily if there is even a 5% gap in coverage. Charge stabilized ionics are much more forgiving.
  2. The speed at which the surfactants generate the surface coverage is critical. Basically, if they get to the interface in a few 10s of ms, you'll easily get lots of good foam. This speed depends on concentration, CMC, surface mobility, salt concentratioin in no way that is readily extractable with 2020's theory/experiment (for some hints of the complexity, see DST-Choice, and read the master class on the theory within the paper, which concludes that it's not much help). This is sad in one way, but liberating in another. Just measure the dynamic surface tension at a 10ms timescale and tweak the formulation till you find a large reduction in surface tension. On a typical Maximum Bubble Pressure Tensiometer this 10ms timescale is measured at

Foaming techniques

I had generally paid little attention to the different foaming techniques, but the remark in the previous section about foams being self-limiting made me realise that I've come across quite a few different methods.

  1. Shaking cylinder. Put, say, 10ml of solution into a 130ml measuring cylinder and oscillate it, checking the volume of foam after a given number of shakes. If you get 90% trapped air then you are at 100ml, so finding whether you have 91, 92 . gets tricky in a 130ml cylinder. My impression is that this sort of foam is relatively coarse, but I might be wrong
  2. Ross-Miles. Put some test solution in the bottom of a tall cylinder. Now dropwise add more of the solution from the top. The drops smashing into the liquid below produce a foam. Measure the volume at the end of the addition, then, for stability, the volume after a few minutes. Amazingly, this is an industry standard test.
  3. Blender. Just get a big blender and put in enough liquid to cover the blades. Whizz away and measure the volume by pouring the contents into a measuring cylinder. The fact that this can be done suggests that the foam is rather coarse, because a fine foam would be hard to pour..
  4. Planetary mixer. Take you Kenwood Chef or equivalent with a wire whisk and watch what happens as the whisk turns on its axis while moving around on the other axis. A paper from the Sofia group shows a clear self-limiting effect once the foam gets thick enough to squash the surface waves which initially trapped the air, so this seems good for testing for the ability to create finer foams.
  5. Sparging column. Blow air through a frit at the bottom of a column containing your foaming solution. You get some idea of the foamability and stability from the stable height of the foam, and/or you can measure the weight of foam coming over the top in a given time. More details are available on the Foam Fractionation page.
  6. Micro-foam test. I once had to measure foamability using mg of surfactant and μl of solution. This was remarkably easy to do with a steady stream of air blowing through a very fine syringe needle into the solutions in micro-titre plates. It's a very good high throughput technique (which is why we developed it) to distinguish low, medium and high foamers and short, medium and long-life foam. It's crude but amazingly effective.
  7. Compressed air foam. Mix your surfactant solution with some high-pressure air, let it travel down a pipe, expanding as it goes, and burst out onto, say, an oil storage vessel in flames. I once wrote an app for a fire-fighting project that required the theory of such a foam and needed some measurements to parameterise the theory. Unfortunately the live experiments on a full-sized test rig failed because the rig burned down during one of the tests.
  8. Aerosol foams. This is a variant of the previous one, on a smaller scale. The propellant in a can (typically a hydrocarbon gas blend) is beautifully mixed into the surfactant mixture so creates a mass of fine bubbles when it suddenly expands. A typical example is a shaving foam which has to be fine in order to have the high viscosity and yield stress to stay on the face.
  9. Hand rubbing. I know that foaming has no significance in terms of washing - the craving for it is psychological, not physics. So I'd never bothered to see how much foam one could create with imaginative hand rubbing. It's quite a lot, but in my view not worth the effort.
  10. Shaving brush. I had never understood shaving brushes. They didn't produce an interesting amount of foam and just seemed a complicated way of spreading soap over my face. But then I'd never bothered to learn how to do it. If you whisk away onto a blob of wet soap on one's hand, nothing much seems to be happening. That's because all the foam is in the brush. Just squeeze the brush in any way, and out comes a mass of very fine, stable foam, perfect for placing on the face. I was very impressed.
  11. Foaming net. Take a few cm of a fine net and rub it hard between your hands with the wet soap. As with the shaving brush, nothing much happens if you don't know what you are trying to do - I had to go to YouTube to find out. If you pull the net between your fingers, a large amount of foam emerges. Repeat this a few times and you get an awesome amount of fine, stable foam. The fine net is clearly good at breaking up larger bubbles into smaller ones. Why anyone bothers to spend their time creating this mass of foam bubbles is not a question I am qualified to answer.
  12. Measurements of key parameters.
    • Obviously foam height, where appropriate, and the ratio of the total height to the amount of liquid in the bottom of the container, and how this changes over time.
    • A conductivity meter across a known gap, calibrated with the conductivity of the water used in the experiment, gives you a good idea of the volume fraction of air.
    • Put a large prism in contact with the foam and couple light into and out of it. A video shows a strong contrast between contact with water (white) and air (black) and it is then easy to use image analysis to measure the foam. Experiments have shown that the prism has a surprisingly small perturbation on the foam itself so the measurements are relevant. It's incredibly hard to get good image analysis from images of free foam because there's seldom reliable good contrast between walls and the rest.

Oil foams

It seems obvious that you can't make foams in oils. The surface tensions of oils are low and a surfactant can't make much difference and therefore the crucial elasticity stabilising effect cannot come into operation. This is generally true for simple hydrocarbon oils. To produce foams in these you need to use clever particulate tricks such as lyotropic phases of specific surfactants (such as mono-Myristylglycerate) or hydrophobised silicas (look up Binks in Google Scholar). But the real oil industry has massive problems with foams and the art/science of finding defoamers for each specific crude oil is a major challenge. Why do many crude oils foam?

The clearest scientific description of this comes from work by Callaghan and colleagues at BP 3 . They carefully extracted all the acidic components from a wide range of oils (these typically represented only 0.02% by weight) and found that the oil showed (a) no elasticity and (b) no foaming. If they added the extracts back to the no-foam oil then both elasticity and foaming returned. The acids were rather simple long(ish)-chain alkanoic acids such as dodecanoic. Although this paper did not record the surface tensions of the crude, other papers show typical values in the low 30mN/m but which can be reduced to the mid 20's by additions of simple surfactants or defoamers. This is not a huge decrease and, therefore, the elasticity effects cannot be large. However, in crude oils the pressures can be very high so the bubbling can be very violent when the crude reaches atmospheric pressure, so it doesn't need a very strong surfactancy effect to cause massive foaming.

Going back to the other type of foam stabilisation, crude oil is usually complicated by the presence of asphaltenes which can readily crystallise/cluster at the air/oil interface and provide foaming in that manner. And, as we will see, foam stability is greatly enhanced by high viscosity which many oils can readily supply. But nothing is simple: asphaltenes have been shown to be veryy modest surfactants that can produce foaming in toluene where they are (by definition) soluble.

Fire Fighting Foams

This is a huge subject. The only point raised here is that for oil/petrol fires the surfactant should not be good for emulsifying the oil with the water in the foam. The standard theory therefore states that the system needs a large "Spreading Coefficient" (see the Antifoam section) which in practice can only be achieved with fluorosurfactants. Such foams are astonishingly good at being jetted through huge flames to land nicely on the surface of the burning liquid (which, to the surprise of many, is "only" at its boiling point - not some super-high temperature) and put out the fire. For really robust foams adding a protein surfactant is a good idea - usually as part of a fast/slow mix of a normal fast surfactant to get the foam going and the slow protein which reaches the interface after a time and renders the whole thing remarkably solid. Alternatively some high MWt polymers can perform this function to create an AR-AFFF Alcohol Resistant-Aqueous Film Forming Foam which means one that works not only on non-polar fires but also on polar fires for which a conventional foam might be too compatible with the liquid.

However, with the move away from fluorosurfactants (seemingly inevitable, justifiable or not) my view is that it's necessary to focus on creating what I call LRLP foams, Low Radius and Low Permeability, created with standard surfactants. If you explore foam rheology, drainage, Ostwald ripening you will see that small-radius foams are stiffer and tougher. So you can gain foam lifetime via smaller bubbles. And with tricks like adding myristic acid, you can make a foam Low Permeability by making the interface stiffer. This helps reduce the rate at which warm vapours can move through the foam, reducing the risk of them re-igniting.

1 J. Eastoe, J.S. Dalton, Dynamic surface tension and adsorption mechanisms of surfactants at the air/water interface, Advances in Colloid and Interface Science, 85, 2000, 103-144

2 B. Petkova, S. Tcholakova, M. Chenkova, K. Golemanov, N. Denkov, D. Thorley, S. Stoyanov, Foamability of aqueous solutions: Role of surfactant type and concentration, Advances in Colloid and Interface Science 276 (2020) 102084

3 IC Callaghan, et al, Identification of Crude Oil Components Responsible for Foaming, SPE Journal, 25, 1985, 171-175

Surfactant Science: Principles in Practice

My free book expands on the content in Practical Surfactants but is linked to the apps so you go straight from the eBook page to the app of interest. It can be downloaded for iBooks, for the iPad, for Kindle and as PDF.


Hemodialysis

Dialysis Solution

Dialysis fluid can be considered a drug to be adjusted to the individual patient's needs. In modern machines, dialysate is made by mixing two concentrate components, which may be provided as liquid or dry (powder) concentrates. The bicarbonate component contains sodium bicarbonate and sodium chloride the acid component contains chloride salts of sodium, potassium (if needed), calcium, magnesium, acetate (or citrate), and glucose (optional). These two components are mixed simultaneously with purified water to make the dialysate. Dialysate proportioning pumps ensure proper mixing. The relative amounts of water, bicarbonate, and acid components define the final dialysate composition. Bicarbonate has replaced acetate as the dialysate buffer in most countries. Typical concentrations of dialysate components are given in Figure 89.4 . Dialysate containing citrate (0.8 mmol/l) has recently been introduced, which may allow a reduction in heparin dose. Dialysate composition can be further modified by changing the mixing fraction and by adding salt solutions potential advantages and disadvantages of dialysate modifications are shown in Figure 89.5 . Modern machines allow an alteration of the bicarbonate concentration by changing the mixing ratio of water to bicarbonate. A variable sodium option allows the adaptation of the dialysate sodium concentration to the patient's needs. Glucose is usually added to prevent intradialytic hypoglycemia, but glucose concentrations of 200 mg/dl (11 mmol/l) may result in hyperglycemia and hyperinsulinemia.

To avoid the need for large volumes of water, spent dialysate can be regenerated by sorbents. These systems may need as little as 6 liters of tap water for a regular dialysis treatment, which makes them particularly attractive for home HD or arid areas.


Why is it advised to avoid bubble formation during mixing? - Biology

last updated Monday, December 30, 2013

After a reaction is completed, the solution often times does not only contain the desired product, but also undesired byproducts of the reaction, unreacted starting material(s) and the catalyst (if it was used). These compounds have to be removed in the process of isolating the pure product. A standard method used for this task is an extraction or often also referred to as washing. Strictly speaking, the two operations are targeting different parts in the mixture: while the extraction removes the target compound from an impure matrix, the washing removes impurities from the target compound i.e., water by extraction with saturated sodium chloride solution. Washing is also used as a step in the recrystallization procedure to remove the impurity containing mother liquor adhering to the crystal surface.

Many liquid-liquid extractions are based on acid-base chemistry. The liquids involved have to be immiscible in order to form two layers upon contact. Since most of the extractions are performed using aqueous solutions (i.e., 5 % NaOH, 5 % HCl), the miscibility of the solvent with water is a crucial point as well as the compatibility of the reagent with the compounds and the solvent of the solution to be extracted. Solvents like dichloromethane (=methylene chloride in older literature), chloroform, diethyl ether, or ethyl ester will form two layers in contact with aqueous solutions if they are used in sufficient quantities. Ethanol, methanol, tetrahydrofuran (THF) and acetone are usually not suitable for extraction because they are completely miscible with most aqueous solutions. However, in some cases it is possible to accomplish a phase separation by the addition of large amounts of a salt (“salting out”). Commonly used solvents like ethyl acetate (8.1 %), diethyl ether (6.9 %), dichloromethane (1.3 %) and chloroform (0.8 %) dissolved up to 10 % in water. Water also dissolves in organic solvents: ethyl acetate (3 %), diethyl ether (1.4 %), dichloromethane (0.25 %) and chloroform (0.056 %). Oxygen containing solvents are usually more soluble in water (and vice versa) because of their ability to act as hydrogen bond donor and hydrogen bond acceptor. The higher water solubility lowers the solubility of weakly polar or non-polar compounds in these solvents i.e., wet Jacobsen ligand in ethyl acetate. Other solvents such as alcohols increase the solubility of water in organic layers significantly because they are miscible with both phases and act as a mediator. This often leads to the formation of emulsions.

The most important point to keep in mind throughout the entire extraction process is which layer contains the product. For an organic compound, it is relatively safe to assume that it will dissolve better in the organic layer than in most aqueous solutions unless it has been converted to an ionic specie, which makes it more water-soluble. If a carboxylic acid (i.e., benzoic acid) was deprotonated using a base or an amine (i.e., lidocaine) was protonated using an acid, it would become more water-soluble because the resulting specie carries a charge. Chlorinated solvents (i.e., dichloromethane, chloroform) exhibit a higher density than water, while ethers, hydrocarbons and many esters possess a lower density than water (see solvent table), thus form the top layer (see solvent table). . One rule that should always be followed when performing a work-up process:

Never dispose of any layer away until you are absolutely sure (=100 %) that you will never need it again. The only time that you can really be sure about it is if you isolated the final product in a reasonable yield, and it has been identified as the correct compound by melting point, infrared spectrum, etc. Keep in mind that it is always easier to recover the product from a different layer in a beaker than from the waste container or the sink. In this context it would be wise to label all layers properly in order to be able to identify them correctly later if necessary.

In order to separate compounds from each other, they are often chemically modified to make them more ionic i.e., convert a carboxylic acid into a carboxylate by adding a base. Standard solutions that are used for extraction are: 5 % hydrochloric acid, 5 % sodium hydroxide solution, saturated sodium bicarbonate solution (

6 %) and water. All of these solutions help to modify the (organic) compound and make it more water-soluble and therefore remove it from the organic layer. More concentrated solutions are rarely used for extraction because of the increased evolution of heat during the extraction, and potential side reactions with the solvent.

What do I use when to extract?

a. Removal of a carboxylic acid or mineral acid

In order to remove an acidic compound from a mixture, a base like NaOH or NaHCO3 is used. The carboxylic (or mineral) acid and the base react to form a sodium salt, which is usually exhibits a higher solubility in aqueous solutions due to its negative charge and higher polarity (as indicated by a more negative log Kow value i.e., CH3COOH: -0.17, Na + CH3COO - : -3.72).

Which of the two reagents should be used depends on the other compounds present in the mixture. Sodium hydroxide is usually easier to handle because it does not evolve carbon dioxide as a byproduct. In addition, the concentration can be increased significantly if is needed. However, if compounds were present that are sensitive towards strong bases or nucleophiles (i.e., esters, ketones, aldehydes, etc.), sodium bicarbonate should be used. It does not react with these compounds because it is a weaker base and a weak nucleophile (due to its resonance stabilization). Note that the formation of carbon dioxide as a byproduct causes a pressure build-up in the separatory funnel, the centrifuge tube or the conical vial. Thus, additional precautions (i.e., frequent venting) have to be taken to prevent any accidents resulting from the pressure build up in the extraction vessel. The target compound can subsequently be recovered by adding a mineral acid to the basic extract i.e., benzoic acid in the Grignard experiment in Chem 30CL.

b. Removal of a phenol

Most phenols are weak acids (pKa=

10) and do not react with sodium bicarbonate, which is a weak base itself (pKa(H2CO3)=6.37, 10.3). However, they do react with a strong base like NaOH. This difference in acidity can be exploited to separate carboxylic acids and phenols from each other in an organic layer. While many phenols dissolve poorly in water (8.3 g/100 mL at 20 o C, log Kow=1.46), phenolates dissolve very well in aqueous solutions. After the extraction, the phenol can be recovered by adding a mineral acid to the basic extract.

c. Removal of an amine

Depending on the chain length, amines might or might not be soluble in water i.e., propylamine is miscible with water (log Kow=0.48), triethylamine displays a limited solubility at room temperature (17 g/100 mL, log Kow=1.44), while tributylamine hardly dissolves at all (0.37 g/100 mL, log Kow=4.60). Amines are basic and can be converted to ammonium salts using mineral acids i.e., hydrochloric acid. The resulting salts dissolve in water. However, the solubility of the ammonium salts decreases as the number and size of R-groups increases. Ammonium salts from primary amines are much more soluble in water than salts from tertiary amines due the increased ability to form hydrogen bonds [(H3NEt)Cl: 280 g/100 g H2O, (H2NEt2)Cl: 232 g/100 g H2O, (HNEt3)Cl: 137 g/100 g H2O (all at 25 o C)].

After separation of the organic and the aqueous layer, the amine can be recovered by addition of a strong base like NaOH or KOH to the acidic extract i.e., lidocaine synthesis. Note that amides are usually not basic enough to undergo the same protonation (pKa of conjugate acid:


d. Isolation of a neutral species

Most neutral compounds cannot be converted into salts without changing their chemical nature. Many of these neutral compounds tend to react in undesired ways i.e., esters undergo hydrolysis upon contact with strong bases or strong acids. One has to keep this in mind as well when other compounds are removed. For instance, epoxides hydrolyze to form diols catalyzed by acids and bases. Ketones and aldehydes undergo condensation reactions catalyzed by both, acids and bases. Esters also hydrolyze to form carboxylic acids (or their salts) and the corresponding alcohol. In order to separate these compounds from each other, chromatographic techniques are often used, where the compounds are separated based on their different polarities (see Chromatography chapter).


e. General Separation Scheme

Based on the discussion above the following overall separation scheme can be outlined. Which sequence is the most efficient highly depends on the target molecule. There is obviously no reason to go through the entire procedure if the compound sought after can be isolated in the first step already. Note that many of these steps are interchangeable in simple separation problems.



For instance, if the target compound was the base in the system, the extraction with HCl should be performed first. Whatever remains in the organic layer is not of interest anymore afterwards, unless one of the other compounds has to be isolated from this layer as well. If the target compound was an acid, the extraction with NaOH should be performed first. This strategy saves steps, resources and time, and most of all, greatly reduces waste.

Practical Aspects of an Extraction

An extraction can be carried out in macro-scale or in micro-scale. In macro-scale, usually a separatory funnel (on details how to use it see end of this chapter) is used. Micro-scale extractions can be performed in a conical vial or a centrifuge tube depending on the quantities. Below are several problems that have been frequently encountered by students in the lab:

a. Which layer is the aqueous layer?

Most solutions are relatively diluted (

5 %) and their density is not much different from that of water (i.e., 5 % HCl: 1.02 g/cm 3 , 5 % NaOH: 1.055 g/cm 3 ). Thus, the density of a solid i.e., sodium hydroxide (2.1 g/cm 3 in the solid) does not provide the information sought. The density is determined by the major component of a layer which is usually the solvent. About 5 % of a solute does not change the density of the solution much. However, this can change if very concentrated solutions are used (see table in the back of the reader)! Thus, diethyl ether and ethyl acetate, which are both less dense than the dilute solutions that are usually used for extraction, form the top layer, while dichloromethane and chloroform form the bottom layer (currently both of them are not used in Chem 30BL or Chem30CL due to safety concerns!).

b. Why are three layers observed sometimes?

It is not uncommon that a small amount of one layer ends up on top of the other. Mixing with a stirring rod or gentle shaking usually takes care of this problem. Small amounts (compared to the overall volume of the layer) should be discarded here.

c. Why do the layers not separate?

This would usually happen if the mixture was shaken too vigorously. Subsequently, an emulsion is formed instead of two distinct layers. In such an event, the mixture can be stirred slowly with a glass rod to bring the small droplets together a little faster, which ultimately leads to the formation of a new layer. In some cases, a careful draining of the existing lower layer can also be helpful because it pushed the bubbles together in the smaller part of the extraction vessel. In cases, where the phases have similar polarity or density, the addition of more solvent can assist the separation. Sometimes, the addition of a salt (or salt solution) can also lead to a better phase separation (“salting out”). In many cases, centrifugation or gravity filtration works as well. When it is known, through experience, that some mixtures may form emulsions, vigorous shaking should be avoided. Instead, gently rocking the separatory funnel back and forth for 2-3 minutes will accomplish sufficient degree of mixing while minimizing the formation of emulsions.

d. How do we know that we are done extracting?

Strictly speaking, hardly ever all of the solute will be extracted since there is finite distribution coefficient for the compound (see also Extraction II). As a general rule, multiple extractions with small quantities of solvent or solution are more efficient than one extraction using the same amount of solvent (see below). The amount of material left behind after two or three extractions is usually very small (less than 5 % in most cases) and does not justify the effort and resources (solvent and time to perform the extractions and to remove the solvent later on). Excessive washing will also lower the yield of the product, if the desired compound dissolves noticeably in the other phase.

e. Why does the extraction container (vial, centrifuge tube, separatory funnel) make funny noises?

This phenomenon will often be observed if sodium bicarbonate is used for the extraction in order to neutralize or remove acidic compounds. The reaction affords carbon dioxide (CO2), which is a gas at ambient temperature. Pressure builds up that pushes some of the gas and the liquid out. The container should be vented immediately before the pressure build-up can cause an explosion, an ejection of the stopper on the top or excessive spillage upon opening. A similar observation will be made if a low boiling solvent is used for extraction. The shaking of the mixture increases the surface area, and therefore the apparent vapor pressure of the solvent. In addition, many extraction processes are exothermic because they involve an acid-base reaction.

f. The centrifuge tube leaks

Often times the cap is either the wrong cap in the first place or it is not properly placed on the top. If NaHCO3 is used for extraction, the centrifuge tube has to be vented more frequently.

g. The separatory funnel leaks

Before using the separatory funnel, the user should check if the stopcock plug and the stopcock fit together well. In addition, the stopper on the top has to fit into the joint on the top to prevent leakage there (for more details at the end of this chapter).

h. Why is a centrifuge tube, a conical vial or a separatory funnel used for the extraction and not a beaker or test tube?

The conical shape of these pieces of equipment makes it easier to collect the solution on the bottom using a Pasteur pipette because of the smaller interface. The task of getting a clean phase separation will be more difficult if the liquids are spread out over a large, flat or curved surface.

i. Which layer should be removed, top or bottom layer?

The bottom layer is always removed first independently if this is the one of interest or not because it is much easier to do. If a centrifuge tube or conical vial was used, the bottom layer should be drawn using a Pasteur pipette. From this point of view, a solvent with higher density than water would be preferential, especially when very small quantities are used. This will allow to minimize the number of transfer steps required.


TechTalks Discussion

I am trying to make very thin crackers something like Italian Sfoglie, with different toppings to flavor. The dough is made with flour, potatoes flakes, and potato starch, with water and oil. I use ammonium bicarbonate as a chemical leaven.

When we prepare the dough it is beautiful and soft. The dough we make has yeast and we leave it to ferment for 16 to 18 hours.

After fermentation, we add the amonium bicarbonate and the dough gets very humid and we must add more flour to dry it, if the dough isn´t very dry it is very hard to laminate as thin as we need, but it is very hard to form a ball because it must be so dry that it falls apart in pieces that once we put on the laminator we must be able to form a dough and star laminating and folding for 3 times, then we reduce the thickness to 0 we must pass it for three-time until it reaches the correct thickness, but it is very frustrating and time-consuming because it breaks very easy and we have to start all over again. We try to give the dough time to rest and we keep it warm and in a humid container hoping it will be more pliable.

Is the kneading time and rph very important? what would the ideal time? and at what rph would you suggest please? My equipment is small, more like bakery-style than industry, I have a kneader, a laminator, and a rotative oven.

As a former product manager and now a consultant for tunnel oven belts, I would be interested to ask for your experience in changing the principal type of belt used in your tunnel oven and for what reason. Did you change from solid steel belt to mesh belt or (multiple spiral) CB5-belt to Z-belt (rolled baking oven belt)? Or was it vice versa? Or are there intention and thoughts to do something like this?

Ee baked for the first time biscuits. We start with a soft biscuit (two types with/without cocoa powder). We have a direct-fired oven, 40m long, with four-zone. We faced with two problems: -the line of unbaked dough in the middle of the biscuit

-The bigger challenge for us is little bumps and air bubbles on a soft biscuit. (We have docker holes on each biscuit.)


Dissolved Oxygen in Surface Water

Dissolved oxygen (DO) measurements calculate the amount of gaseous oxygen dissolved in surface water, which is important to all oxygen-breathing life in river ecosystems, including fish species preferred for human consumption (e.g. bluegill and bass), as well as decomposer species critical to the recycling of biogeochemical materials in the system.

The oxygen dissolved in lakes, rivers, and oceans is crucial for the organisms and creatures living in it. As the amount of dissolved oxygen drops below normal levels in water bodies, the water quality is harmed and creatures begin to die. In a process called eutrophication, a body of water can become hypoxic and will no longer be able to support living organisms, essentially becoming a “dead zone.”

Eutrophication occurs when excess nutrients cause algae populations to grow rapidly in an algal bloom. The algal bloom forms dense mats at the surface of the water blocking out two essential inputs of oxygen for water: gas exchange from the atmosphere and photosynthesis in the water due to the lack of light below the mats. As dissolved oxygen levels decline below the surface, oxygen-breathing organisms die-off in large amounts, creating an increase in organic matter. The excess organic matter causes an increase in the oxygen-breathing decomposer populations in the benthic zone, which further depletes the remaining dissolved oxygen levels during the metabolic decomposition activity. Once the oxygen levels become this low, mobile oxygen-breathing species (e.g. fish) will move away, leaving no aerobic life in the water and creating a dead zone.

The Azide-Winkler titration method uses titration to determine the concentration of an unknown in a sample. Specifically, sodium thiosulfate is used to titrate iodine, which can be stoichiometrically related to the amount of dissolved oxygen in a sample.

Principles

The Azide-Winkler method is used to measure DO on site, where surface water is collected. Manganese(II) sulfate and potassium hydroxide are added to the sample, and the dissolved oxygen in the sample oxidizes the manganese and forms a brown precipitate. Azide is added in the form of a purchased alkaline iodide-azide reagent to correct for the presence of nitrites, which are found in wastewater samples and can interfere with the Winkler oxidation procedure.

MnSO4 + 2 KOH Mn(OH)2 + K2SO4

4 Mn(OH)2 + O2 + 2 H2ل Mn(OH)3

Sulfuric acid is then added to acidify the solution, and the precipitate dissolves. Under these conditions, the iodide from the alkaline iodide-azide reagent in the solution is converted into iodine.

2 Mn(OH)3 + 3 H2SO4 Mn2(SO4)3 + 6 H2O

Mn2(SO4)3 + 2 KI 2 MnSO4 + K2SO + 2 I 2

Thiosulfate is then used to titrate the iodine in the presence of an added starch indicator.

4 Na2S2O3 + 2 I2 2 Na2S4O6 + 4 NaI

4 moles of S2O3 2- 1 mole of O2

At the endpoint of this titration, the blue solution will turn clear. The amount of dissolved oxygen in the sample is quantified in direct proportion to the amount of thiosulfate required to reach the endpoint.

X mL S2O3 X mg/L O:

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Procedure

1. Sample Dissolved Oxygen Measurement

  1. At the water collection site, use a calibrated pipette to add 2 mL manganous sulfate to a clear 300-mL BOD bottle filled with the sample water. Be careful not to introduce oxygen into the sample by inserting the pipette tip under the sample surface and carefully dispensing manganous sulfate. This will avoid creating bubbles until the sample is “fixed” and prevents change to the dissolved oxygen concentration.
  2. Using the same technique, add 2 mL alkaline iodide-azide reagent.
  3. Immediately insert the stopper, tilting the bottle slightly and quickly pushing the stopper in place so no air bubbles are trapped in the bottle.
  4. Carefully invert several times (without creating air bubbles) to mix. A floccule (floc) will form from a precipitated aggregation of material with a cloudy appearance (Figure 1).
  5. Wait until the floc in the solution has settled. Again, invert the bottle several times and wait until the floc has settled. The sample is now fixed to prevent change in dissolved oxygen content, and can be transported back to the lab and stored for up to 8 h, if needed, in a cool and dark condition.
  6. If storing, samples should be sealed using a small amount of deionized water squirted around the stopper, and the stopper should be wrapped in aluminum foil, secured with a rubber band.
  7. Pipette 2 mL of concentrated sulfuric acid into the sample by holding the pipette tip just above the sample surface. Invert carefully several times to dissolve the floc (Figure 2).
  8. In a glass flask, and using a calibrated pipette, titrate 200 mL of sample water with 0.025 N standardized sodium thiosulfate, swirling and mixing continuously until a pale straw color forms (Figure 3).
  9. Add 2 mL of starch indicator solution with a dropper and swirl to mix. Once the starch Indicator is added, the solution will turn blue (Figure 4).
  10. Continue the titration, adding one drop at a time until one drop dissipates the blue, causing the colorless endpoint. Be sure to add each drop of titrant carefully and to evenly mix each drop before adding the next. Holding the sample against a white piece of paper can help enhance visualization of the endpoint.
  11. The concentration of DO is equivalent to the volume (mL) of titrant used. Each milliliter of sodium thiosulfate added to the water sample equals 1 mg/L dissolved oxygen. 


Figure 1. A sample after the alkaline iodide-azide reagent has been added and mixed, showing floc formation at the top of the sample before settling.


Figure 2. A sample with dissolved floc after addition of sulfuric acid.


Figure 3. A sample after addition of sodium thiosulfate displaying a pale straw color.


Figure 4. A sample showing the blue color after the starch indicator is added and mixed.

Dissolved oxygen is crucial for river and lake ecosystems to support aerobic life. The Azide-Winkler titration method allows quantification of the amount of dissolved oxygen in surface water samples.

Gaseous oxygen dissolved in surface water is required for the survival of the organisms living in it decomposers critical to recycling of biogeochemical materials in the ecosystem, or fish species preferred for human consumption. As oxygen levels fall below normal in water systems, water quality is harmed and organisms begin to die.

The Azide-Winkler titration method is a standard test to determine the concentration of dissolved oxygen in a sample. Sodium thiosulfate is used to titrate iodine, which is stochiometrically related to the amount of dissolved oxygen in the sample.

This video will illustrate the principles behind dissolved oxygen quantification, the process of performing the Azide-Winker titration, and the interpretation of dissolved oxygen measures.

Eutrophication is the introduction of excess nutrients into an ecosystem. This causes algae populations to grow rapidly into dense mats, known as algal blooms. These mats can lead to hypoxia, or low oxygen levels, by blocking out gas exchange at the surface, and prevent photosynthesis by blocking sunlight. Oxygen breathing organisms begin to die, causing an increase in organic matter, which in turn causes an increase in oxygen dependent decomposers, depleting oxygen resources yet further. Finally, mobile oxygen-dependent organisms move away, leaving a dead zone with no aerobic life.

To test the level of dissolved oxygen in a water source, the Azide-Winkler method can be used to measure dissolved oxygen directly in the field, or samples can be fixed and taken to the laboratory for further analysis.

Manganese sulfate and potassium hydroxide are added to the sample, forming manganese hydroxide. This reduces the dissolved oxygen, forming a brown precipitate. Alkaline iodide-azide reagent is added to correct for the presence of nitrates found in wastewater samples that can interfere with the oxidation procedure.

Added sulfuric acid acidifies the solution and dissolves the precipitate. This new compound oxidizes the iodide from the alkaline iodine-azide reagent to iodine.

Next, a starch indicator is added that will turn blue in the presence of iodine. Thiosulfate, which turns iodine back into iodide, is used to titrate the iodine. When the titration is complete, the blue solution will turn colorless. The amount of dissolved oxygen in the sample is proportional to the amount of thiosulfate required to turn the solution from blue to colorless.

Now that we are familiar with the principles behind measuring dissolved oxygen in water samples, let's take a look at how this is carried out in the field and the laboratory.

The experiment will begin at the collection site. First, collect the sample water in a clear 300-mL BOD bottle. Next, measure and record the temperature of water from the water source. Carefully add 2 mL manganous sulfate to the sample by inserting the pipette tip under the water surface and slowly dispense to avoid creating bubbles.

Using the same technique, add 2 mL alkaline iodine-azide reagent, and immediately insert the stopper, tilting the bottle slightly so no air is trapped in the bottle.

Carefully invert several times to mix the solution, taking care not to create air bubbles. A precipitate will form, causing a cloudy appearance. Let the precipitate in the solution settle, and then mix thoroughly by inverting the bottle several times before letting it settle again. Samples should be sealed using a small amount of deionized water squirted around the stopper, then wrapped in aluminum foil and secured with a rubber band. The sample is now fixed, and can be transported back to the laboratory.

Once the samples have been fixed, they are transported to the lab for further analysis. First, holding the pipette tip just above the sample surface, add 2 mL of concentrated sulfuric acid into the sample. Invert several times to dissolve the precipitate. Using a glass flask and calibrated pipette, titrate 200 mL of the pre-treated sample water with 0.025 N standardized sodium thiosulfate, swirling and mixing continuously until a pale straw color forms.

Once the solution is straw colored, add 2, 1-mL droplets of starch indicator solution and swirl to mix. The solution will turn blue. Continue the titration, adding one drop of sodium thiosulfate at a time and mixing slowly using a stir bar until the blue dissipates and the solution becomes colorless. Hold the sample against a white piece of paper to enhance visualization. Record the volume of thiosulfate added.

The concentration of dissolved oxygen is proportional to the volume of sodium thiosulfate added to the sample. Each milliliter added is equivalent to 1 mg/L, or parts per million, dissolved oxygen.

The maximum amount of oxygen that can be dissolved in water varies by water temperature. Dissolved oxygen measurements in mg/L are converted to percent saturation using water temperature and a conversion chart. Saturation of 91 to 110% dissolved oxygen is considered excellent between 71 and 90% is good, 51-70% is fair, and below 50% is poor.

Dissolved oxygen levels of 6 mg/L are sufficient to support most aquatic species. Levels below 4 mg/L are stressful to the majority of aquatic animals, so biodiversity will be affected. Water containing less than 2 mg/L dissolved oxygen will not support aerobic aquatic life.

The ability to quantify the amount of dissolved oxygen in a water source also has alternative methods, and many relevant practical applications. Some of these are explored here.

Dissolved oxygen and temperature can also be measured using a handheld LabQuest monitor with dissolved oxygen and temperature probes. For dissolved oxygen, plug the probe into channel 1. Units should be in mg/L. Submerge the probe into the water sample, circulating the probe slowly through the sample to avoid consuming oxygen in a localized area. When the readings appear to stabilize, record the value.

Most fish require moderate to good levels of dissolved oxygen in their habitats to thrive and reproduce. For fish farms, which may occupy man-made or natural lakes or streams, being able to test dissolved oxygen levels can help farm managers to choose a good initial set-up site, or to keep track of the health of their pools or streams.

Monitoring dissolved oxygen can also be useful for habitat management and conservation. If a lake or river region contains protected or endangered flora or fauna, monitoring of dissolved oxygen levels can give an indication of the health of the ecosystem. If levels change rapidly, this could indicate danger for the protected species, and may indicate that a management intervention strategy should be implemented.

The United States Environmental Protection Agency, the EPA, suggests a number of measures to correct dissolved oxygen levels in ecosystems. These include correct and minimal use of fertilizers, proper wastewater treatment, not discharging sewage from boats, and preserving adjoining rivers, streams, and wetlands. Reducing nitrogen oxides by minimizing electricity and automobile use and choosing more efficient boat engines can also help to maintain appropriate dissolved oxygen levels in water resources.

You've just watched JoVE's introduction to measuring dissolved oxygen in surface waters. You should now understand the principles behind dissolved oxygen measurement, how to quantify dissolved oxygen in your own water samples, and how to interpret your findings and their implications for the environment. Thanks for watching!

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Results

A dissolved oxygen level of 6 mg/L is sufficient for most aquatic species. Dissolved oxygen levels below 4 mg/L are stressful to most aquatic animals. Dissolved oxygen levels below 2 mg/L will not support aerobic aquatic life (Figure 5).

The maximum amount of oxygen that can be dissolved in water varies by temperature (Table 1).

DO measurements in mg/L are converted to % saturation using water temperature and the conversion chart below (Figure 6).

DISSOLVED OXYGEN LEVELS (% SATURATION)
Excellent: 91 – 110
Good: 71 – 90
Fair: 51 – 70
Poor: < 50


Figure 5. DO measurements are converted to % saturation using the water’s temperature. The water’s temperature on the top horizontal axis and the measured DO value on the bottom horizontal axis. Use a ruler to draw a line between the two values and record where the line meets the middle diagonal axis for % saturation.


Figure 6. A dissolved oxygen level of 6 mg/L is sufficient for most aquatic species. Dissolved oxygen levels below 4 mg/L are stressful to most aquatic animals. Dissolved oxygen levels below 2 mg/L will not support fish and below 1 mg/L will not support most species.

Temp. (°C) DO (mg/L) Temp. (°C) DO (mg/L) Temp.(°C) DO (mg/L) Temp.(°C) DO (mg/L)
0 14.60 11 11.01 22 8.72 33 7.16
1 14.19 12 10.76 23 8.56 34 7.16
2 13.81 13 10.52 24 8.40 35 6.93
3 13.44 14 10.29 25 8.24 36 6.82
4 13.09 15 10.07 26 8.09 37 6.71
5 12.75 16 9.85 27 7.95 38 6.61
6 12.43 17 9.65 28 7.81 39 6.51
7 12.12 18 9.45 29 7.67 40 6.41
8 11.83 19 9.26 30 7.54 41 6.41
9 11.55 20 9.07 31 7.41 42 6.22
10 11.27 21 8.90 32 7.28 43 6.13

Table 1. Maximum amounts of oxygen that can be dissolved in water by temperature.

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Applications and Summary

Slow-moving rivers are particularly vulnerable to low DO levels, and in extreme cases, these DO levels can lead to hypoxic conditions, creating “dead zones” where aerobic life is no longer supported by a body of water (Figure 7). Once plants and animals die-off, the build-up of sediment that occurs can also raise the riverbed, allowing plants to colonize over the water and could lead to the loss of the river all together (Figure 8). Surface waters at higher altitudes are also more vulnerable to low DO levels, as atmospheric pressure decreases with increasing altitude, and less oxygen gas is suspended in the water.

Low DO levels support life forms considered unappealing or unfit for human use, including leeches and aquatic worms (Oligochaeta).


Figure 7. Map of dissolved oxygen concentrations across the Louisiana shelf showing the dead zone region.


Figure 8.  Photograph of the Caspian Sea showing severe eutrophication in the north end.

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Transcript

Dissolved oxygen is crucial for river and lake ecosystems to support aerobic life. The Azide-Winkler titration method allows quantification of the amount of dissolved oxygen in surface water samples.

Gaseous oxygen dissolved in surface water is required for the survival of the organisms living in it decomposers critical to recycling of biogeochemical materials in the ecosystem, or fish species preferred for human consumption. As oxygen levels fall below normal in water systems, water quality is harmed and organisms begin to die.

The Azide-Winkler titration method is a standard test to determine the concentration of dissolved oxygen in a sample. Sodium thiosulfate is used to titrate iodine, which is stochiometrically related to the amount of dissolved oxygen in the sample.

This video will illustrate the principles behind dissolved oxygen quantification, the process of performing the Azide-Winker titration, and the interpretation of dissolved oxygen measures.

Eutrophication is the introduction of excess nutrients into an ecosystem. This causes algae populations to grow rapidly into dense mats, known as algal blooms. These mats can lead to hypoxia, or low oxygen levels, by blocking out gas exchange at the surface, and prevent photosynthesis by blocking sunlight. Oxygen breathing organisms begin to die, causing an increase in organic matter, which in turn causes an increase in oxygen dependent decomposers, depleting oxygen resources yet further. Finally, mobile oxygen-dependent organisms move away, leaving a dead zone with no aerobic life.

To test the level of dissolved oxygen in a water source, the Azide-Winkler method can be used to measure dissolved oxygen directly in the field, or samples can be fixed and taken to the laboratory for further analysis.

Manganese sulfate and potassium hydroxide are added to the sample, forming manganese hydroxide. This reduces the dissolved oxygen, forming a brown precipitate. Alkaline iodide-azide reagent is added to correct for the presence of nitrates found in wastewater samples that can interfere with the oxidation procedure.

Added sulfuric acid acidifies the solution and dissolves the precipitate. This new compound oxidizes the iodide from the alkaline iodine-azide reagent to iodine.

Next, a starch indicator is added that will turn blue in the presence of iodine. Thiosulfate, which turns iodine back into iodide, is used to titrate the iodine. When the titration is complete, the blue solution will turn colorless. The amount of dissolved oxygen in the sample is proportional to the amount of thiosulfate required to turn the solution from blue to colorless.

Now that we are familiar with the principles behind measuring dissolved oxygen in water samples, let's take a look at how this is carried out in the field and the laboratory.

The experiment will begin at the collection site. First, collect the sample water in a clear 300-mL BOD bottle. Next, measure and record the temperature of water from the water source. Carefully add 2 mL manganous sulfate to the sample by inserting the pipette tip under the water surface and slowly dispense to avoid creating bubbles.

Using the same technique, add 2 mL alkaline iodine-azide reagent, and immediately insert the stopper, tilting the bottle slightly so no air is trapped in the bottle.

Carefully invert several times to mix the solution, taking care not to create air bubbles. A precipitate will form, causing a cloudy appearance. Let the precipitate in the solution settle, and then mix thoroughly by inverting the bottle several times before letting it settle again. Samples should be sealed using a small amount of deionized water squirted around the stopper, then wrapped in aluminum foil and secured with a rubber band. The sample is now fixed, and can be transported back to the laboratory.

Once the samples have been fixed, they are transported to the lab for further analysis. First, holding the pipette tip just above the sample surface, add 2 mL of concentrated sulfuric acid into the sample. Invert several times to dissolve the precipitate. Using a glass flask and calibrated pipette, titrate 200 mL of the pre-treated sample water with 0.025 N standardized sodium thiosulfate, swirling and mixing continuously until a pale straw color forms.

Once the solution is straw colored, add 2, 1-mL droplets of starch indicator solution and swirl to mix. The solution will turn blue. Continue the titration, adding one drop of sodium thiosulfate at a time and mixing slowly using a stir bar until the blue dissipates and the solution becomes colorless. Hold the sample against a white piece of paper to enhance visualization. Record the volume of thiosulfate added.

The concentration of dissolved oxygen is proportional to the volume of sodium thiosulfate added to the sample. Each milliliter added is equivalent to 1 mg/L, or parts per million, dissolved oxygen.

The maximum amount of oxygen that can be dissolved in water varies by water temperature. Dissolved oxygen measurements in mg/L are converted to percent saturation using water temperature and a conversion chart. Saturation of 91 to 110% dissolved oxygen is considered excellent between 71 and 90% is good, 51-70% is fair, and below 50% is poor.

Dissolved oxygen levels of 6 mg/L are sufficient to support most aquatic species. Levels below 4 mg/L are stressful to the majority of aquatic animals, so biodiversity will be affected. Water containing less than 2 mg/L dissolved oxygen will not support aerobic aquatic life.

The ability to quantify the amount of dissolved oxygen in a water source also has alternative methods, and many relevant practical applications. Some of these are explored here.

Dissolved oxygen and temperature can also be measured using a handheld LabQuest monitor with dissolved oxygen and temperature probes. For dissolved oxygen, plug the probe into channel 1. Units should be in mg/L. Submerge the probe into the water sample, circulating the probe slowly through the sample to avoid consuming oxygen in a localized area. When the readings appear to stabilize, record the value.

Most fish require moderate to good levels of dissolved oxygen in their habitats to thrive and reproduce. For fish farms, which may occupy man-made or natural lakes or streams, being able to test dissolved oxygen levels can help farm managers to choose a good initial set-up site, or to keep track of the health of their pools or streams.

Monitoring dissolved oxygen can also be useful for habitat management and conservation. If a lake or river region contains protected or endangered flora or fauna, monitoring of dissolved oxygen levels can give an indication of the health of the ecosystem. If levels change rapidly, this could indicate danger for the protected species, and may indicate that a management intervention strategy should be implemented.

The United States Environmental Protection Agency, the EPA, suggests a number of measures to correct dissolved oxygen levels in ecosystems. These include correct and minimal use of fertilizers, proper wastewater treatment, not discharging sewage from boats, and preserving adjoining rivers, streams, and wetlands. Reducing nitrogen oxides by minimizing electricity and automobile use and choosing more efficient boat engines can also help to maintain appropriate dissolved oxygen levels in water resources.

You've just watched JoVE's introduction to measuring dissolved oxygen in surface waters. You should now understand the principles behind dissolved oxygen measurement, how to quantify dissolved oxygen in your own water samples, and how to interpret your findings and their implications for the environment. Thanks for watching!


Weed has an active ingredient that affects a part of your brain known as the hippocampus and alters how your mind processes information.

Marijuana can affect how your brain forms memories. It can lead to cognitive impairment in adulthood, especially if you continually use it during your adolescence.

If you are really wondering if it affects the brain, then let’s dive into a study done in New Zealand.

It was conducted using IQ test scores over a period of time, ages 13 to 38. Those who were physically dependent on marijuana before the age of 18 had a decrease in IQ by the age of 38. This is because the younger the brain, the more negative impact it has on memory performance.


Side Effects

Most people tolerate Taxol well, especially in low doses. It does have side effects, however, which include:  

    (nerve damage) (a low platelet count)
  • Bone and muscle aches (Neulasta and Neupogen also frequently cause bone pain)
  • Hair loss
  • Nausea
  • Vomiting
  • Mild diarrhea (irritation of mucous membrane inside the mouth) (absence of menstruation)  

There are ways to prevent some of the problems these side effects can cause. Before you begin treatment with Taxol, your doctor will probably have you take supplements of an amino acid called L-glutamine to reduce your risk of neuropathy. It is also become increasingly common to use you injections of either Neupogen (filgrastim) or Neulasta (pegfilgrastim) to boost white blood cell counts. These must be given at least 24 hours after your chemotherapy infusion has been completed, but early enough to stimulate formation of white blood cells before they hit their lowest point (called the nadir).

Furthermore, to avoid risky interactions, you will be advised not to drink alcohol while you're being treated with Taxol, and to avoid medications that include aspirin.

Furthermore, to avoid risky interactions, you will be advised not to drink alcohol while you're being treated with Taxol, and to avoid medications that include aspirin.

Most side effects of chemotherapy resolve rapidly after treatment is completed, although some long-term side effects of chemotherapy may persist. In particular, peripheral neuropathy may sometimes be permanent, and fatigue may sometimes take years to fully improve.


Icing

Ice collects on and seriously hampers the function of not only wings and control surfaces and propellers, but also windscreens and canopies, radio antennas, pilot tubes and static vents, carburetors and air intakes. Turbine engines are especially vulnerable. Ice forming on the intake cowling constricts the air intake. Ice on the rotor and starter blades affects their performance and efficiency and may result in flame out. Chunks of ice breaking off may be sucked into the engine and cause structural damage. The first structures to accumulate ice are the surfaces with thin leading edges: antennas, propeller blades, horizontal stabilizers, rudder, and landing gear struts. Usually the pencil-thin outside air temperature gauge is the first place where ice forms on an airplane. The wings are normally the last structural component to collect ice. Sometimes, a thin coating of ice will form on the windshield, preceded in some instances by frosting. This can occur on take-off and landing and with sufficient rapidity to obscure the runway and other landmarks during a critical time in flight.

Icing of the propeller generally makes itself known by a slow loss of power and a gradual onset of engine roughness. The ice first forms on the spinner or propeller dome and then spreads to the blades themselves. Ice customarily accumulates unevenly on the blades, throwing them out of balance. The resulting vibration places undue stress on the blades and on the engine mounts, leading to their possible failure. If the propeller is building up ice, it is almost certain that the same thing is happening on the wings, tail surfaces and other projections. The weight of the accumulated ice is less serious than the disruption of the airflow around the wings and tail surfaces. The ice changes the airfoil cross section and destroys lift, increases drag and raises the stalling speed. At the same time, thrust is degraded because of ice on the propeller blades and the pilot finds himself having to use full power and a high angle of attack just to maintain altitude. With the high angle of attack, ice will start to form on the underside of the wing adding still more weight and drag. Landing approaches and landing itself can be particularly hazardous under icing conditions. Pilots should use more power and speed than usual when landing an ice-laden airplane.

If ice builds up on the pilot tube and static pressure ports, flight instruments may cease operating. The altimeter, airspeed and rate of climb would be affected. Gyroscopic instruments powered by a venturi would be affected by ice building up on the venturi throat. Ice on radio antennas can impede VOR reception and destroy all communications with the ground. Whip antennas may break off under the weight of the accumulating ice.

Primary Force Icing Effect on Force Resulting Effect on Aircraft
Lift Decreased Excessive loss of lift will cause aircraft to lose altitude
Weight Increased Excessive weight will cause aircraft to lose altitude
Thrust Decreased Excessive loss of thrust will cause aircraft to lose airspeed and lift
Drag Decreased Excessive drag will cause aircraft to lose airspeed and lift

TYPES OF ICING

The three main types of ice accretion, in order of their hazard to flying, are as follows:

Clear ice or glaze ice is a heavy coating of glassy ice which forms when flying in areas with high concentration of large supercooled water droplets, such as cumuliform clouds and freezing rain. It spreads, often unevenly, over wing and tail surfaces, propeller blades, antennas, etc. Clear ice forms when only a small part of the supercooled water droplet freezes on impact. The temperature of the aircraft skin rises to 0°C with the heat released during that initial freezing by impact of the part of the droplet. A large portion of the droplet is left to spread out, mingle with other droplets before slowly and finally freezing. A solid sheet of clear ice thus forms with no embedded air bubbles to weaken its structure. As more ice accumulates, the ice builds up into a single or double horn shape that projects ahead of the wing, tail surface, antenna, etc. on which it is collecting. This unique ice formation severely disrupts the airflow and is responsible for an increase in drag that may be as much as 300 to 500%.

The danger of clear ice is great owing to (1) the loss of lift, because of the altered wing camber and the disruption of the smooth flow of air over the wing and tail surfaces, (2) the increase in drag on account of the enlarged profile area of the wings. (3) the weight of the large mass of ice which may accumulate in a short time, and finally (4) the vibration caused by the unequal loading on the wings and on the blades of the propeller(s). When large blocks break off, the vibration may become severe enough to seriously impair the structure of the airplane. When mixed with snow or sleet, clear ice may have a whitish appearance. (This was once classified as rime-glazed but it is now considered to be a form of clear ice).

Rime ice is an opaque, or milky white, deposit of ice that forms when the airplane is flying through filmy/stratiform clouds. It is dependent on a low rate of catch of small supercooled water droplets. It accumulates on the leading edges of wings and on antennas, pilot heads, etc. For rime to form, the aircraft skin must be at a temperature below 0°C. The drop will then freeze completely and quickly without spreading from the point of impact. Thus, the droplets retain their spherical shape as they freeze, creating air packets between the frozen particles. This process creates an irregular shape of the ice.

The deposit has no great weight, but its danger lies in the aerodynamic alteration of the wing camber and in the choking of the orifices of the carburetor and instruments. Rime is usually brittle and can easily be dislodged by de-icing equipment. Occasionally, both rime and clear ice will form concurrently. This is called mixed icing and has the bad features of both types.

Mixed icing, as the name implies, has the properties of both clear and rime icing. Large and small supercooled droplets coexist. Appearance is whitish, irregular and rough. Favorable conditions include liquid and frozen particles found in the colder portion of the cumuliform cloud and wet snow flakes. The formation process for mixing icing includes that of clear and rime icing. Mixed ice can accumulate rapidly and is difficult to remove.

A white semi-crystalline frost which covers the surface of the airplane forms in clear air by the process of deposition. This has little or no effect on flying but may obscure vision by coating the windshield. It may also interfere with radio by coating the antenna with ice. It generally forms in clear air when a cold aircraft enters warmer and damper air during a steep descent. Aircraft parked outside on clear cold nights are likely to be coated with frost by morning. The upper surfaces of the aircraft cool by radiation to a temperature below that of the surrounding air.

Frost which forms on wings, tail and control surfaces must be removed before take-off. Frost alters the aerodynamic characteristics of the wing sufficiently to interfere with take-off by increasing stall speed and reducing lift. Frozen dew may also form on aircraft parked outside on a night when temperatures are just below freezing. Dew first condenses on the aircraft skin and then freezes as the surface of the aircraft cools. Frozen dew is usually clear and somewhat crystalline, whereas frost is white and feathery. Frozen dew, like frost, must be removed before take-off. In fact, any snow or moisture of any kind should be removed since these may freeze to the surface while the airplane is taxiing out for take-off. The heat loss due to the forward speed of the airplane may be sufficient to cause congelation.

INTENSITY OF ICING

Icing may be described as light, moderate, and severe (or heavy). In severe icing conditions, the rate of accretion is such that anti-icing and de-icing may fail to reduce or control the hazard. A change in heading and altitude is considered essential. In moderate icing, a diversion may be essential since the rate of accretion is such that there is potential for a hazardous situation. Light icing is usually not a problem unless the aircraft is exposed for a lengthy period. Clear ice is considered more serious than rime ice since the rate of catch must be high to precipitate the formation of clear ice. The seriousness of an icing situation is, of course, dependent on the type of aircraft and the type of de-icing or anti-icing equipment with which the aircraft is equipped or the lack of such equipment.


Icing Intensity Classification
Intensity Rate of Accumulation
Trace Perceptible, no significant accumulation
Light Significant accumulations for prolonged flight (over 1 hour)
Moderate Significant accumulations for shorter periods of flight
Severe Rapid, dangerous accumulations

Icing Intensity Effects
Icing
Itensity
Airframe
Ice Accumulation
Trace Usually not hazardous event if de-icing/anti-icing equipment is NOT used
Light Occasional use of de-icing/anti-icing equipment removes/prevents accumulation
Moderate Rate of accumulation is such that event short encounters become potentially hazardous
and use of de-icing/anti-icing equipment or flight diversion is necessary
Severe De-icing/anti-icing equipment fails to reduce or control the hazard. Immediate flight
diversion is necessary

Trace
• Stratus clouds
Light
• Stratus clouds and weak weather-producing system
• Widespread weak cumulus or stratocumulus clouds
Moderate
• Nimbostratus clouds and weather-producing system
• Stratocumulus and turbulent mixing
• Light freezing rain, freezing drizzle
• Extensive vertically-developed cumuliform clouds
Severe
• Nimbostratus clouds and strong weather-producing system
• Freezing rain
• Cumulonimbus

ICING IN CLOUDS AND PRECIPITATION

Cumulus. Severe icing is likely to occur in the upper half of heavy cumulus clouds approaching the mature cumulonimbus stage especially when the temperatures are between 0°C and -25°C. The horizontal extent of such cloud is, however, limited so that the aircraft is exposed for only a short time. The zone of probable icing in cumuliform clouds is smaller horizontally but greater vertically than in stratiform clouds. Further, icing is more variable in cumuliform clouds because many of the factors conducive to icing depend to a large degree on the stage of development of the particular cloud. Icing intensities may range from generally a trace in small supercooled cumulus to often light or moderate in cumulus congestus and cumulonimbus. Icing is generally restricted to the updraft regions in a mature cumulonimbus, and to a shallow layer near the freezing level in a dissipating thunderstorm. Icing in cumuliform clouds is usually clear to mixed.

Stratus. Icing is usually less severe in layer cloud than in cumulus type clouds but it can be serious if the cloud has a high water content. Since stratus cloud is widespread in the horizontal, exposure to the icing condition can be prolonged. Icing is more severe if cumulus clouds are embedded in the stratus layer. The likelihood of structural icing is greatest in the temperature range from 0°C to -10°C. The likelihood decreases, but is still possible between -10°C to -20°C. Research findings indicate icing is most intense near the top of stratiform clouds. The vertical extent of icing layers does not usually exceed 3,000 feet (a change in altitude of only a few thousand feet may take the aircraft out of icing conditions, even if it remains in clouds).

Freezing Rainis common ahead of warm fronts in winter. Serious icing occurs when the aircraft is flying near the top of the cold air mass beneath a deep layer of warm air. Rain drops are much larger than cloud droplets and therefore give a very high rate of catch. In freezing temperatures, they form clear ice.

Freezing Drizzle.Drizzle falls from stratus clouds with a high water content. As the droplets fall through the clear air , prompt action on the radio is important when icing starts. Information about the latest weather for altitudes above and below will help the pilot to make the decision on what action to take. The final alternative would be to turn back, or, if the accumulation of ice has already become serious, to make a precautionary landing immediately. In any event, the decision must be made rapidly since once ice has started to form, the condition may become critical in a matter of approximately six minutes.

ICING ASSOCIATED WITH FRONTAL SYSTEMS

It is rather difficult to represent frontal icing conditions by an idealized model, since the structure of the clouds in frontal regions and in regions of intense low-pressure systems is very complex. In general, frontal clouds have a higher icing probability than other clouds. It has been estimated that 85 percent of the observed aircraft icing occurs in the vicinity of frontal zones. Usually, the greatest horizontal extent of icing is associated with warm fronts, and the most intense icing with cold front.

Warm Frontal Icing:This may occur both above and below the frontal surface. Moderate or severe clear icing usually occurs where freezing rain or freezing drizzle falls through the cold air beneath the front. This condition is most often found when the temperature above the frontal inversion is warmer than 0°C and the temperature below is colder than 0°C. Icing above the warm frontal surface, in regions where the cloud temperatures are colder than 0°C, is usually confined to a layer less than 3,000 feet thick. Research studies indicate that there is a definite possibility of moderate icing, usually mixed or clear, within 100 to 200 miles ahead of the warm-front surface position. This was particularly noticeable for fast-moving, active, warm fronts. Light rime ice was noted in the altostratus up to 300 miles ahead of the warm-front surface position.

Cold Frontal Icing:Whereas warm-frontal icing is generally widespread, icing associated with cold fronts is usually spotty. Its horizontal extent is less, and the areas of moderate icing are localized. Clear icing is usually limited to supercooled cumuliform clouds within 100 miles to the rear of the cold-front surface position, and is usually most intense immediately above the frontal zone. Light icing is often encountered in the extensive layers of supercooled stratocumulus clouds which frequently exist behind cold fronts. Icing in the stratiform clouds of the widespread anafront type of cold front cloud-shield is more like icing associated with warm front.

Other Frontal Icing:Icing conditions associated with occluded and stationary fronts are similar to those of a warm or cold front, depending on which type the occlusion or stationary front most resembles. Moderate icing conditions are frequently associated with deep, cold, low-pressure areas in which the frontal systems are quite diffuse.

OROGRAPHIC INFLUENCE

High or steep terrain, particularly mountains, causes icing to be more intense than is usual under identical conditions over low, flat terrain.

Icing is greater over the ridges than over valleys and greater on the windward side than on the leeward side.

Moderate icing, usually clear, is commonly experienced in convective clouds over mountainous terrain. Windward, mountainous coasts in winter are especially subject to extensive aircraft-icing zones.

The lifting of the fresh maritime polar air by the mountains results in the formation of more-or-less continuous supercooled clouds. Also, the orographically-induced updrafts permit the air to support larger cloud droplets than otherwise, so that the icing is more intense.

SEASONAL EFFECTS

High Latitudes - Most frequent during Fall and Spring
Mid Latitudes - Most frequent during Fall, Winter and Spring
Low Latitudes - Most frequent during Winter

CATEGORIES OF AIRCRAFT ICING

Structural Icing - forms on aircraft structures

Induction System Icing - forms in air intake of engines and carburetors
• Can occur with or without structural icing conditions
• Air intake icing usually requires the aircraft surface to be 0°C or colder with visible moisture present
• Carburetor icing is most likely with the air temperatures between -7°C(20°F) and 21°C(70°F) with relative humidity above 80%

Instrument Icing - forms in pitot tube, other exterior instruments, and antennas
• Generally occurs with structural icing conditions

RULES TO FOLLOW

Pilots flying in light airplanes which are not fitted with an outside air temperature gauge will be well advised to have one installed as this instrument will warn of temperatures that are conducive to icing conditions. To avoid icing problems, here are a few rules to follow:

PROTECTION FROM ICING

Many modern airplanes that are designed for personal and corporate use, as well as the larger transport type airplanes, are fitted with various systems designed to prevent ice from forming (anti-icers) or to remove ice after it has formed (de-icers).

1. Fluids. There are fluids which are released through slinger rings or porous leading edge members to flow over the blades of the propellers and the surfaces of the wings. A fluid is an anti-icing device since it makes it difficult for ice to form.

2. Rubber Boots. Membranes of rubber are attached to the leading edges. They can be made to pulsate in such a way that ice is cracked and broken off after it has already formed. This is a de-icing device.

3. Heating Devices. Heating vulnerable areas is a method for preventing the buildup of ice. Hot air from the engine or special heaters is ducted to the leading edges of wings, empennages, etc. Electrically heated coils protect pilot tubes, propellers, etc.

ICING AVOIDANCE

Few single engine airplanes, or even light twin-engine types incorporate any means of ice prevention. A few tips for pilots flying airplanes in this category will therefore be in order.

When ice formation is observed in flight, there is only one certain method of avoiding its hazards and that is to get out of the ice-forming layer as quickly as possible. This may be done by climbing above the ice forming zone. This alternative would obviously require an airplane that has good performance and is fitted with radio and proper instruments for flying over the top. The next alternative would be to descend and fly contact below the ice forming zone. The advisability of this course would depend on the ceiling and visibility along the route at the lower level concerned.

Do not remain in icing conditions any longer than necessary. For that reason, during climbs or descents through a layer in which icing conditions exist, plan your ascent or descent to be in the layer for as short a time as possible. However, keep your speed as slow as possible consistent with safety. Speed of an airplane affects accretion of ice. The faster an airplane moves through an area of supercooled water drops, the more moisture it encounters and the faster will be the accumulation of ice.

If ice has started to build up on the airplane, do not make steep turns or climb too fast since stalling speed is affected by ice accumulation. Fuel consumption is greater due to increased drag and the additional power required. Land with more speed and power than usual. Do not land with power off. With the advent of the jet age, the problem of icing has taken on some surprising new aspects. At one time, the pilots of airplanes flying through high cirrus clouds did not worry about ice forming on the airplane as cirrus clouds are composed of ice crystals rather than water droplets. With the increased speeds of which jet airplanes are capable, the heat of friction is sufficient to turn the ice crystals in the cloud to liquid droplets which subsequently freeze to the airplane.


Watch the video: Η ΘΕΡΜΟΤΗΤΑ ΩΣ ΜΟΡΦΗ ΕΝΕΡΓΕΙΑΣ - Eureka Mεταγλωττισμένο (June 2022).


Comments:

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  2. Nikus

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  3. Samura

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  4. Aragrel

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  5. Rae

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