|MadSci Network: Chemistry|
This is a very good question. I have used it in the final examination for my junior level analytical chemistry class at Portland State University.
The black "tarnish" on silver objects is silver sulfide (Ag2S). It is formed when silver comes into contact with almost any sulfur containing compound (air that has oxides of sulfur from fossil fuel power plants, air from marshy areas, eggs or other high sulfur foods, or even fingerprints).
Often silver is cleaned by using a polish and a polishing cloth. This removes the tarnish and exposes a new, clean, shining silver surface--at the expense of the silver removed with the tarnish (the polishing cloth turns black). After time, if the object was silver plated, enough silver can be removed to expose the base metal under the silver plate. Or if it is solid silver (sterling silver), the loss might be visually undetectable. Wouldn't a better way be to take the silver out of the silver sulfide and put it back on the object?
That is what happens with the method you used.
Aluminum is a very reactive metal. When it is formed, or made into an object such as a foil or pot, it immediately forms a clear, hard coat of aluminum oxide. Some of the best cooking ware made has a chemically enhanced coating of this hard aluminum oxide. In order for the reaction with the tarnished silver object to occur, the object has to come into contact with the Al metal, not the oxide Al2O3--note the reaction requires Al, and forms Al2O3 (this is the jelly-like material you may find in the dish after the cleaning has finished--it is a different form of aluminum oxide).
Enter baking soda, stage right! The baking soda reacts with water to make a basic solution--one having more OH- ions.
The basic solution "softens" the hard oxide layer, and allows contact to be made between the aluminum and the silver.
The silver metal that is formed is deposited on the surface of the object, so that in the end, no silver is lost. And the object has its shine restored.
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