|MadSci Network: Chemistry|
Hello, Pete! I haven't seen the ad. that you mention, but I have actually removed tarnish by a very similar method. What you need is baking soda, an aluminum pan, hot water, and tarnished silver. Begin the process by dissolving baking soda in the hot water in the aluminum pan. Now score or scratch the aluminum (to break through its native aluminum oxide layer) and suspend the tarnished silver in the solution. Most silver compounds are not as stable as aluminum compounds, and what happens here is that some of the aluminum metal of the pan is converted to ionic form by the formation of aluminum bicarbonate or carbonate, while the silver atoms are changed from ionic form in the tarnish to plain metal atoms. (The reaction is accompanied by a quite strong smell of hydrogen sulfide, as most silver tarnish is sulfides of silver; please do this in a ventilated area. From practical experience, the stink is pretty bad and you'll appreciate the removal of the hydrogen sulfide from the area.) This is not so much an electrolytic tarnish removal as it is an oxidation- reduction reaction pair, with the aluminum metal of the pan being oxidized and the silver ions in the tarnish being reduced. Such a reaction is sometimes called an "electroless" reaction; certain metals can be plated out of special solutions by this class of oxidation-reduction reaction. I have heard, but have not done it myself, that vinegar (forms aluminum acetate instead of carbonates/bicarbonates) can be used instead of the baking soda. You might try both and see which one works better. Your message has several questions in it. I'll go through that now and see what I can answer for you. Pete: I recently saw a sales ad on TV for a metal plate which you use to clean silver without polish. You put the plate in the sink, add a couple of tablespoons of ordinary salt and fill the sink with hot water. Any tarnished silver immersed in the water and making contact with the metal plate is cleaned so quickly you can see the tarnish disappearing. If this is some form of electrolysis, where is the circuit (no batteries or wires involved)? How does it work and what metal is the plate made of? Mad Scientist: I don't know what the plate is made of, but it is probably some metal that will form compounds more readily than silver. The salt is no doubt to lower the resistivity of the water and provide electrical conduction through the solution via ions. The metal in the plate is oxidized, the electrons passing into the silver and reducing the silver ions in the tarnish back to silver metal. The salty solution in essence acts to "complete" the circuit via ions. That is, as the silver reduces, it releases the remainder of the tarnish into solution, and the metal plate reacts with (I'm guessing, but it's a fairly good one) the chlorine ions in the solution to form a chloride or with the chlorine acting as a catalyst to help along the formation of an oxide or hydroxide. Pete: If this process is so great (and on the face of it, it looks much preferable to polishing), why hasn't it supplanted polish? Mad Scientist: While the silver sulfides are an "ugly, brownish stain", some people like the presence of some silver oxide, which is black. Polishing will remove the tarnish/oxide from the high spots, but leave the oxide in the recesses. This gives the silver more "contrast", if you will, and some people like that. It tends to "enhance" the patterning. The simple oxidation-reduction will remove all the silver oxide, and thus makes the silverware look "flat" or "less contrasty" and "less pleasing" to the eye. I can say that when I used the aluminum pan/baking soda technique, the silverware did come out looking entirely free of all oxide or sulfides. Whether or not the esthetics of "naked silver" please you is of course a matter of taste. Pete: I can't help feeling that it, too, removes silver with the tarnish just like ordinary polish, but would it remove more, or less? Mad Scientist: It does not remove any silver, in the ideal sense, because the silver in the tarnish is being converted back to silver metal and is not lost. Now, practically, some small amount of silver may well end up in the solution (no reaction is ever 100% perfect), but the amount should be far less than that lost via polishing.
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