MadSci Network: Chemistry

Re: An electrolytic silver tarnish remover?

Date: Mon Jun 4 16:12:37 2001
Posted By: Matthew Buynoski, Senior Member Technical Staff,Advanced Micro Devices
Area of science: Chemistry
ID: 985595894.Ch

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

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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