I was thoroughly entertained by this bit of hobby chemical sleuthing from Michael August Pusateri, who observed years ago, while bored on-shift, that the hot sauce at the fast food joint where he was working would clean the oxide off a penny. My immediate assumption–that the effect is due to vinegar or some other acid in the sauce–turns out to be only half right. The right answer, and the process by which Michael arrived at and verified it, make for interesting reading. You might even find a use for it.
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One missing test from the article is salt water — the only mention of the salt-only test is “rubbing salt on pennies doesn’t do it either, I tried that…” but of course rubbing salt in crystallized form won’t do anything.
I do something similar before etching copper-clad boards that have been stored for a while and have gone brown: rub with ketchup and let stand for about 15 minutes, then wash off. Much easier than the Brillo pad route (and leaves a smoother surface).
Wow. I thought cleaning pennies with salt and vinegar was in every “chemistry for kids” book ever written. Not going to comment on how’s it’s possible the author never heard of it.
Anyway, Salt + Vinegar = Sodium Acetate + Hydrochloric Acid. It’s the HCl that does the cleaning.
The pKa of acetic acid is 4.76; the pKa of HCl is generally given as -7. That means, essentially, that HCl is 12 orders of magnitude stronger, as an acid, than vinegar. The position of equilibrium always favors formation of the weaker acid, meaning that, essentially, the reaction you’ve written is statistically unfavorable, literally by a factor of 1 trillion.
When I make pasta with tomato sauce I sponge the remaining bit of sauce on the copper bottom of the pan and leave it until I’m done with the meal–leaves a nice shiney finish.
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