Chemistry - Yellow stains after cleaning silver

Solution 1:

I suggest to do the following: Line the bottom of a pan with aluminum foil. Put the silver piece on top of the aluminum foil. The silver piece and aluminum foil must be in contact with each other. To about 2L of hot boiling water add about a half cup of baking soda (Be careful!). Then add the mixture to the pan. Make sure to cover the whole silver piece. The tarnish will disappear within several minutes. The chemistry underlying this treatment is the following: The silver tarnish is the chemical product of the reaction of silver with sulfur-containing substances in the air, $\ce{Ag2S}$. Silver sulfide is black. In this process, we convert silver sulfide to silver according to the following reaction: $$\ce{3Ag2S + 2Al->Al2S3 +6Ag}$$ In fact, aluminum has a greater affinity to sulfur than silver. The reaction between silver sulfide and aluminum takes place when the two are in contact while they are immersed in a baking soda solution. And it becomes faster when the solution is hot.The silver piece should touch the aluminum because a small electric current flows between them during the reaction. The formed aluminum sulfide may adhere to the aluminum foil, or it may form tiny, pale yellow flakes in the solution.

The advantage of this method of cleansing is that we do not remove the tarnish with some silver. We simply transfer silver sulfide to silver.

Solution 2:

Did find a source and the nature of reaction apparently involves some standard chemistry, surface chemistry, some radical activity and electrochemistry resulting, upon warming, in the demonstration of some yellow elemental sulfur!

Here is a description of the reaction pathways, starting in alkaline conditions, surface chemistry on the Aluminum metal (which can even very slowly be attacked by water liberating hydrogen):

$$\ce{Al + OH- -> Al(OH)3 + 3 e-}$$

Of consequence, NaHCO3 is amphoteric and a source of H+ ions. Also, in the presence of solvated electrons, the creation of the hydrogen atom radical (per the source) along with hydrogen gas:

$$\ce{H+ + e- ⇌ •H }$$

$$\ce{•H + •H -> H2 (g) }$$

Also, per a galvanic cell between the Aluminum and the Silver metal, the half-cell reaction at the anode being given by:

$$\ce{Al -> Al(3+) + 3 e- }$$

At the cathode, the proposed half-cell reaction explaining the removal of Silver sulfide:

$$\ce{Ag2S(s) + 2e- + 2H2O -> 2Ag(s) + H2S(g) + 2 OH- }$$

See same source, Equation (6).

And, for the observed yellow color, the cited reaction:

$$\ce{H2S + 2 OH- -> 2 H2O + S(s) }$$

which is apparently confirmed by the cited source to quote:

"However, after the cleaning of heavily tarnished samples (immersed for 30 and 60 min) some small quantities of sulfur (~0.3wt.%) are detected on the surface by EDS analysis, and the colour is changed to light yellowish."

In total, this is constitutes an albeit complex and advanced explanation for the cleaning of silver from sulfide stains with Baking Soda and Aluminum. Note, also this supporting source states to quote:

"The nascent hydrogen may also act as a reducing agent as it evolves from the metal surface. "

Unfortunately, the above mix of advanced chemistry is not generally suitable for all audiences acquainted with the removal of Ag2S by the Al/NaHCO3 protocol.