Re: How do silver cleaners/polishers work?

Dear Lee,

It would appear that in order to answer this question, we must ask at 
least two other questions.
A.)  Composition of Silver “Tarnish”?
B.)  Distinction between “Cleaners” and “Polishes”?
C.)  Chemistry of tarnish removal?

A.)  
First a little background on silver.  Silver is a very soft metal, so 
malleable that utensils made from pure silver would be too soft to be of 
very much practical use.  Ordinarily, silver is alloyed with other metals 
to change the mechanical properties.  
The most common one is Sterling Silver which typically contains 92.5% 
silver and 7.5% copper.  Silver is also relatively active chemically.  

The dark brown or black tarnish that is most often seen in cooking 
utensils is composed of silver sulfide.  Sulfides are present from a 
number of sources in the kitchen.  The most recognizable would be from 
eggs.  The “rotten egg odor” is due to hydrogen sulfide.  
Other sources include garlic and onions, as well as amino acids, namely 
cysteine and methionine, derived from the cooking of protein.  Other types 
of tarnish could include hard water spots, food residues, and surface 
oxidation.  Of course, all of these could be combined into
some truly ugly stuff on your silverware.

B.)  
There is quite a significant distinction between cleaners and polishes.
Polishes work by mechanical action.  Similar to sandpaper, a very small
amount of the surface is abraded and clean, shiny surface is exposed.
The ground off material is flushed away by solvents such as water.
Unfortunately, the new surface is still subject to tarnishment by the same
materials that caused the mess in the first place.
Cleaners may use a variety of mechanisms, such as acid dissolution, 
changes in solubility, and chemical oxidation or reduction.  
Acids are useful for decomposing hard water spots which are commonly 
formed from carbonate salts of magnesium and calcium that are left behind 
when water is evaporated.  The same example also works for changes in 
solubility.  By substituting a chloride or a sulfate ion for the carbonate 
ions, the calcium and magnesium salts are now more soluble.
An example of chemical oxidation cleaning would be the self-cleaning oven 
in your kitchen.  At high temperatures, the oxygen in the air combines 
with the carbon contained in the food that spilled in the oven.  By 
trading electrons, the carbon and oxygen are literally combusted to form 
carbon dioxide.

C.)  
The Chemistry of Silver Cleaner.
I am assuming that the silver cleaner you are thinking of is the one shown 
on infomercials on late night TV.  
The “Magic” pan and solution that dissolves the tarnish like magic 
in just a few seconds.  

Right??

Anyway, this can be replicated in the home by taking a glass dish, putting 
a piece of heavy aluminum foil in the bottom, then filling the dish with 
hot tap water and adding a teaspoon of baking soda.  Stir well, and place 
the silver object in the solution, making sure that the object
touches the aluminum foil at the bottom.  Presto!!   Tarnish Disappears!!

This is actually quite a complicated series of oxidation/reduction 
reactions.  The baking soda mixed with water will form a basic solution.  
An excess of OH-1 (hydroxide) ions.  These hydroxide ions will attack the 
aluminum foil and cause it to dissolve.
  
2 Al(solid) + 6 H2O + 2 OH-1  ==>  2 Al(OH)4-1 (aluminate ion)  + 3 H2(gas)

Since we have something touching the aluminum foil that is easier to 
decompose than the water (the silver object with tarnish), the electrons
freed up will do the easiest thing, skipping the formation of the hydrogen 
gas, and will react with the silver sulfide and convert the tarnish to a 
non colored form.
The overall reaction: (leaving out several intermediate steps)

2 Al(solid)  +  3 Ag2S  + 6 H2O 

                           <====>

                             6 Ag(solid) + 2 Al(OH)3  + 3 H2S

It has always kind of amazed me that the final, overall reaction
could look so simple, but actually be a complex series of steps.

Good Question Lee!!

You may wish to look into some the electrochemistry chapters of some 
chemistry textbooks.  I would recommend starting with a general college
chemistry text, and after reading those chapters, proceed with a sophomore
text in quantitative analysis to learn more about the application of
electrochemistry.

Dr. Mike Gallagher
Senior Research Chemist
J.R. Simplot Co.