Expert: Kristin - 6/9/2008

Question
I was just wondering about the chemistry involved in the process of cleaning silver tarnish using silver in a redox reaction. I understand the process of the sulfur being more attracted to the Al and therefore leaving the Ag. However i dont understand what role the NaCO3 plays or why there is a difference between that and the NaHCO3. Also what products are formed from these reactions, i am assuming some sort of Sulfur gas and an aluminium precipitate from reasearch on the internet however all equations i have found vary!


Answer
Hi Tamlyn,

I think you are looking at two different ways of removing silver tarnish.  I'll start with the carbonate - it is used as a mild abrasive to physically scrape off the tarnish.  The difference between sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) is the following reaction:

HCO3- <-> H+ + (CO3)2-

In other words, the bicarbonate can lose a proton to form carbonate, bicarbonate is the acid and carbonate is the conjugate base (or you could write it the other way with carbonate absorbing a proton and carbonate would be the base and bicarbonate would be the conjugate acid).  The carbonates (both HCO3- and (CO3)2- ) can also be used in the electrochemical technique described below.

The electrochemical method of removing tarnish relies on the fact that silver and aluminum have different standard electron potentials.  Aluminum is anodic relative to aluminum.  That means that when the two are connected electronically (electrons can travel between the two) the silver will be reduced (gain electrons) to form pure silver and the aluminum will be oxidized (lose electrons) to form aluminum oxide.  In this situation water acts as the electrolyte that allows the electrons to flow from the cathode (silver) to the anode (aluminum).

Adding bicarbonate or carbonate helps to increase the evolution of H+ which can then form hydrogen sulfide (H2S), which is the stinky sulfur gas.  So here are the reactions that you have:

(Primary reaction)
silver sulfide + aluminum <-> silver + aluminum sulfide
3 Ag2S + 2 Al <-> 6 Ag + Al2S3

(Half reactions and secondary reactions)
Ag2S + 2 e- <-> S2- + 2Ag
Al <-> Al3+ + 3 e-
Al3+ + S2- <-> Al2S3
2H+ + S2- <-> H2S

You may find that the reason that the equations your found online do not all look alike is because some are balanced, some aren't balanced, and some are balanced incorrectly.  I believe that all of these equations are properly balanced - but everyone makes mistakes occasionally!