Q: (Initially posted on January 13, 2003) FROM 3 MENTEES, TRINAYIA A., BRANDY S., and KIMMIKKA C. ALL IN MO
When you mix 1half jar of vinegar, a teaspoon of salt, and 10 dirty pennies together why do the pennies turn clean? Can you use salt and vinegar to clean jewelry?

January 15, 2003
A: FROM MENTOR JOAN LUSK IN RI
The "dirt" is not just garden variety dirt, but copper oxide that
forms slowly from reaction of the copper metal with the air. There's
a good explanation at
http://www.exploratorium.edu/science_explorer/copper_caper.html
The acid reacts with CuO to make CuCl2 and H20, and the CuCl2 is
soluble, leaving just shiny copper metal - but some of the copper has
been removed from the pennies. Actually, in discussion with one of
my colleagues, Prof. Joeseph Steim, who knows an amazing amount of
science, I learned that a major function of the NaCl is to add more
chloride ion, and thereby create [CuCl4]-2, which is more soluble
than CuCl2. I.e., CuCl2 + 2 NaCl = [CuCl4]-2 + 2Na+ Mixing NaCl
and vinegar doesn't turn the acetic acid into a stronger acid.

The copper should dissolve further because the Cu+2 and Cu metal
react to give 2 Cu+; in the salty solution, that will be mainly the
soluble CuCl. William Blake, the 19-century poet and artist, used
this method to etch his copper plates for engraving. The "verdigris"
in his recipe is Cu(acetate)2 or possibly half copper acetate, half
CuO http://www.vu.union.edu/~blake/artisan.html

Copper jewelry should brighten up the same way pennies do - but I
would test this on a small area rather than dunk my whole favorite
bracelet! The jewelry might be a copper alloy different from pennies
and might not react in the same way.

I think the same trick won't work with tarnished silver - if it did,
there would be lots of recipes for this simple cleaner, and there
aren't. The black "tarnish" is silver sulfide. You might think that
acid would react similarly to give silver chloride and H2S, the gas
that smells like rotten eggs. Possibly the silver sulfide is just
too stable to react with this relatively mild acid. Instead, the
homemade cleaner for silver is based on using aluminum metal to
reduce the Ag+ in Ag2S back to metallic silver. Possibly the silver
sulfide is just too stable to react with this relatively mild acid.
http://scifun.chem.wisc.edu/HOMEEXPTS/TARNISH.html

Gold and platinum don't tarnish, so we don't need to clean them of
anything but ordinary dirt!