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25-Jun-1982
Dear Cecil:
I have recently experienced a phenomenon that a friend of mine declares has also
happened to her. It's rather ghastly. I covered a meatloaf with a ketchup glaze and stored
the thing in the refrigerator covered with aluminum foil. Where the foil touched the meat
I found that it was eaten away, dissolved somehow, leaving a gray aluminum puddle deposit
on the glaze. Thinking it was a fluke I re-covered the meat loaf with another piece of
foil and the same thing happened. What happened? --Ms. T., Dallas
Madam:
Aluminum has what we scientists call a "highly negative standard reduction
potential," which means, if I may be permitted to bowdlerize a few pertinent
scientific concepts, that it readily loses electrons and oxidizes. Ketchup, on the other
hand, is highly acidic, having a pH of 3.85 (7.0 is neutral), and like all acids likes to
oxidize obliging metals. The result, therefore, of a conjunction of foil and ketchup is,
as you can attest, a grayish-black mush of aluminum oxide.
Ketchup is by no means the most potent product in your pantry in this respect. I note on
my list of food acid levels that Coca-Cola, the all-American beverage, has a pH of 2.7. I
guess if you spill a Coke aboard one of those aluminum naval vessels so popular these
days, you'd better hope you can swim.
But this is no time for idle speculation. Standard reduction potentials also explain why
it's painful for people with silver tooth fillings to chew aluminum spitballs. Silver, it
turns out, has a highly positive standard reduction potential, which means it has a
craving for electrons. In the presence of an appropriate catalyst, such as your mildly
acidic saliva, we have what amounts to a crude electric battery, in which electrons flow
from the aluminum to the silver. This current is transmitted to the nerves of your teeth,
producing the unpleasant sensation familiar to all.
--CECIL ADAMS
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