MadSci Network: Chemistry |
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.
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