Why does ketchup dissolve aluminum foil?

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?

Cecil replies:


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.

Send questions to Cecil via cecil@straightdope.com.

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