Ask A Scientist Chemistry Archive

From:       gottfried
Text:       I teach middle school science, and one of my colleagues, who 
thinks I know everything, asked why the following recipe works. (I was at a 
loss, so in order to continue to appear brilliant I need help!)  Tarnished 
silver can be cleaned if placed in a glass bowl which has warm water, salt, 
and aluminum foil.  I assume that in some way the Aluminum displaces the Ag, 
but, what is the role of the salt?
---------------------------------------
Author:      jchultz
Text:        The reaction is a redox reaction but not a displacement per se.  
The half reactions are   Ag+ +e-  ---) Ag and Al ---) Al3+ + 3e- 
When added together the cell potential is +2.48.  This positive number 
indicates that the reaction will be spontaneous (the number is in volts).  The 
silver oxide is reduced to silver metal and the aluminum foil is oxidized to 
aluminum ions.  The salt is present to help the transfer of the electrons 
(usually called a 'salt bridge').  If the aluminum actually displaced the 
silver, you would not have silverware after you cleaned them...they would all 
be aluminumware!  So it is the electrons that are being shuffled around, and 
not the elements themselves.
====================================================================
December 2007
Correction/Revision posted within all our silver cleaning articles.
====================================================================
These are a "mix" of answers/suggestions none of which is correct, I do not 
think. First, the stoichiometry (AgS) is wrong -- it should be Ag2S or Ag2O 
-- the former being the culprit (I think). If Ag2O were the culprit you 
could never "polish" silver since the oxygen would be so prevalent that 
it would "tarnish" almost immediately. The tarnishing of silverware is 
retarded by the formation of the ammonia complex -- Ag(NH3)xx. I am not 
sure the value of "xx". This retards the formation of the sulfide, which 
is ever-present. You can test this by sticking a silver fork/knife into 
a "hard boiled" egg -- a rich source of H2S. Certainly using hypochlorite 
would not be recommended. That would be very counter-productive since 
"bleach" and ammonia form chloramines that are more corrosive than either 
hypochlorite or ammonia. Using a mild abrasive removes the sulfide layer, 
which makes sense, but the "secret", if there is one, is to retard the 
diffusion of gases to the silver surface and stabilizing the reduced state 
of silver. I would check out the formulation of a commercial silver "polish".
 
Vince Calder
====================================================================