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CAUTION: If you choose to attempt any of the procedures or experiments mentioned on this web site, you do so entirely at your own risk. In order to use this web site you must read and agree to the Terms of Use.
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.)
Supplies:
Sweaty gym socks
Disposable gloves
Sodium bicarbonate
Plastic wrap / plastic bag
Spray bottle of warm water
Phenolphthalein paper
 |
 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 40°C.
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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