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Ammonia from Gym Socks

a Simple Experiment in Biochemistry

by C. Thorsten

Introduction: One of the best ways to illustrate a principle of chemistry is to use an unforgettable classroom demonstration. We can either stage an impressive visual show, or-- to borrow a phrase from Stephen King-- we can "go for the gross-out". This is one such experiment. Your students will not soon forget it.  Though it's quite harmless if done properly, some of them may not be brave enough to participate. For the faint-hearted, this experiment is better kept as a lecture topic. (Handling someone's dirty gym socks also introduces the risk of infection with e.g. athlete's foot.)

Sweaty gym socks
Disposable gloves
Sodium bicarbonate
Plastic wrap / plastic bag
Spray bottle of warm water
Phenolphthalein paper

gym socks The sweaty gym socks should be obtained as soon after a vigorous workout as possible.
Some people sweat more profusely during exercise than others;  this is a desirable trait for our experiment.

Make sure you have an equal-size pair of clean, dry gym socks for the volunteer to wear when the experiment is done. 

This experiment works best if the socks are obtained immediately after vigorous exercise, and it's preferable if they are from a person who eats a fairly high-protein diet (i.e., someone who maintains a highly positive nitrogen balance).

Procedure: Put on the gloves. Place the sweaty gym socks on plastic wrap, a flattened-out plastic bag, or other waterproof surface. Sprinkle powdered sodium bicarbonate on the socks.
Almost immediately, the characteristic, penetrating odor of ammonia is produced. Although only a relatively small amount of ammonia forms, you shouldn't inhale it directly. You can test for it with a piece of filter paper moistened in phenolphthalein solution. Hold the paper directly above the ammonia source and it should begin to turn pink. Take care not to let the indicator paper touch the socks, or else some bicarbonate could contaminate the paper and give a false positive reaction.
If the reaction is slow, spray a very small amount of warm water mist on the socks. There need be only a small amount of moisture in the socks for the reaction to proceed.
Unless you're into "extreme science" or you have a will of iron, actually "smelling" the socks is ill-advised. If enough ammonia is produced, it can cause lung, sinus, and mucous membrane irritation. In either case, the moistened litmus paper or phenolphthalein paper should serve well. Lab instructors may consider doing this experiment with a gas-delivery tube setup, in which the "ammonia generator" is a flask containing the gym socks.  The flask is placed in a water bath;  the temperature is gradually increased to about 40C. Warming the bath will cause the "gym sock" reactions to speed up and also make the gas expand out of the flask. The flask should of course have a one-hole stopper with a tube that runs into a bottle of water-- this serves as the receiving bottle for the generated gas. A couple of drops of phenolphthalein in water in this receiving bottle would quickly indicate the generation of ammonia.

What happens: human sweat contains water, salt, small amounts of urea (NH2CONH2), and sebacious gland secretions (squalene, fats, cholesterol esters, waxes, etc.). Fats themselves are esters of glycerol and various carboxylic acids. Bacterial enzymes quickly break down these esters into butyric, caprylic, and caproic acids (among others). These compounds are largely responsible for the unpleasant, rancid smell of gym socks.
Bacteria living on the skin (and, consequently, in the gym socks) also produce the enzyme urease, which converts the urea to ammonia and carbon dioxide. When the pH is low (as in the presence of the carboxylic acids), the enzyme does not attack urea as readily. Adding sodium bicarbonate raises the pH to the point where the enzyme becomes quite active, and the conversion to NH3 and CO2 is favored. Note also that ammonia itself then promotes the enzyme's activity, resulting in a kind of positive feedback system.

Discussion: Countless reactions are catalyzed by enzymes at room temperature. Indeed, life as we know it would not be possible without enzymatic reactions-- many of them would otherwise require such high activation energies that they could not take place within a living organism. As mentioned before, this experiment revolves around at least two different enzymatic reactions: (1.) the hydrolysis of esterified fatty-acids, and (2.) the conversion of urea to ammonia.
"Fresh" gym socks have an unpleasant enough smell (reaction 1), but shifting the pH toward alkalinity causes urease to produce ammonia (reaction 2)-- which not only has a more irritating odor, but is also toxic inside living systems. Hence, the body must immediately convert ammonia into a relatively nontoxic form (urea) as soon as it is created, such as in the deamination of amino acids in protein metabolism.

Update (4 Aug. 2005):  A study that appears in the Chinese Journal of Physiology (Huang et al. 2002) suggests the urea concentration of sweat to be about 22 millimolar (0.022 moles per liter).  The study subjects were sixteen males who had performed one hour of vigorous exercise. 
Undoubtedly there will be some variation in urea content due to diet, genetics, water intake, disease, and other factors.

Work Cited:

Huang, C T, M L Chen, L L Huang, & I F Mao. "Uric acid and urea in human sweat".  Chinese Journal of Physiology 45(3): 109-115 (2002).

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