15
$\begingroup$

I know two distinct recipes for cleaning silver:

  • Put them into a pot with some club soda and a piece of aluminium foil and pour over a hot water.
  • Gently clean silverware with a fine chalk powder (obtained from a pharmacy).

I'd be happy to know:

  1. What happens chemically during these procedure - why does it work, and what are the byproducts of these reactions?
  2. How toxic (if at all) are the byproducts?
  3. How much silver is lost in such a procedure?
  4. Are these procedures "idempotent"? I mean, if I apply one of these procedures to an already cleaned item, will it consume some more silver, or will it remain intact? In other words, do the reactions consume just tarnish or do they consume pure silver too?
$\endgroup$
2
  • $\begingroup$ I like to add a few comments to the silver cleaning process. I don't dispute the overall reaction, but aluminium sulfide is unstable and reacts with water to produce Al-hydroxide and H2S which is only slightly soluble in water. It gets driven off gradually. Next, aluminium sulfide (which exists as a solid) is white, like all inorganic aluminium salts I know. I don't know what causes the aluminium to turn black. Aluminium metal doesn't react easily with acids but is dissolved more easily in basic solutions. I expect that aluminium foil contains a skin of the metal oxide which hampers the electr $\endgroup$ Apr 30, 2023 at 12:26
  • $\begingroup$ The tarnish on silver objects is often silver sulfide with some oxide. Most processes involve removing that chemically. Since tarnishing only happens on the surface, there is very little loss of silver in this process. $\endgroup$
    – matt_black
    May 1, 2023 at 10:31

1 Answer 1

10
$\begingroup$

Put them into a pot with some club soda and a piece of aluminium foil and pour over a hot water.

I am familiar with this process. I am not familiar with the other. Perhaps someone else can provide an answer for it.

What happens chemically during these procedure - why does it work, and what are the byproducts of these reactions?

This process cleans silver by a redox process. Aluminum is a more active metal than silver. This means that aluminum metal, Al(s), will reduce silver salts (salts of Ag+). The most common type of tarnish on silver is silver oxide, Ag2O. The reaction is:

$$\ce{3Ag2O(s) + 2Al(s) -> 3Ag(s) + Al2O3(s)}$$

The club soda - containing sodium bicarbonate - plays two roles. First it acts as a supporting electrolyte so that electrons can be conducted through the solution. Second, the carbonic acid in it dissolves the passivation layer of aluminum oxide that protects the aluminum foil. This produces aluminum carbonate, which is unstable in water and decomposes into amphoteric aluminum hydroxide, which is soluble through the formation of a number of complex ions.

$$\ce{Al2O3(s) + 3H2CO3(aq) -> Al2(CO3)2(s) + 3H2O(l)-> 2Al(OH)3 (aq) + 3CO2(g)}$$

The net reaction is then:

$$\ce{3Ag2O(s) + 2Al(s) +3H2CO3(aq) -> 3Ag(s) + 2Al(OH)3(aq) + 3CO2(g)}$$

The byproducts are carbon dioxide and aluminum hydroxide.

  • How toxic (if all) are the byproducts?

Not very. Aluminum salts have very low health impact on you or the environment. Additionally, you produce only very small amounts of aluminum salts this way.

  • How much silver is lost in such a procedure?

None (Almost). The process coverts solid silver oxide into solid silver metal, both of which are negligibly soluble in club soda (or any other aqueous solution). The $K_{sp}$ of AgOH (which is what the surface of Ag2 looks like when exposed to water) is $1.52\times10^{-8}$. All of the action happens at the surface of both metals. The only common silver salts of appreciable solubility in aqueous solutions are silver nitrate and silver sulfate.

  • Is this procedure "idempotent"?

Yes. This procedure converts silver oxide into metallic silver. It has no effect on metallic silver: metallic silver does not react with most acids; it cannot be reduced farther by aluminum metal; and it is not soluble in water or most aqueous solutions.

$\endgroup$
5
  • $\begingroup$ Thanks for a thorough answer. I observed that the aluminium foil gets black during the process, so I thought they are some silver compounds. But if there are no silver compounds as the result and everything reduces atomic silver, what is it then? (Just a minor thing, formatting got wrong at * How much silver is lost in such a.) $\endgroup$
    – Petr
    Oct 30, 2012 at 20:31
  • 1
    $\begingroup$ The black stuff is aluminum salts: aluminum carbonate, aluminum hydroxide, or some other salts depending on what else is in the club soda. Maybe aluminum phosphate. $\endgroup$
    – Ben Norris
    Oct 30, 2012 at 22:34
  • 2
    $\begingroup$ Or aluminum sulfide if the tarnish on the silver is silver sulfide. $\endgroup$ Oct 31, 2012 at 1:32
  • $\begingroup$ @PaulJ.Gans - Of course, if the tarnish is silver sulfide, the chemistry changes a little, but the most important parts of the answer - no silver is consumed and the procedure does nothing to untarnished silver - remain the same. $\endgroup$
    – Ben Norris
    Nov 1, 2012 at 21:15
  • $\begingroup$ Will the sodium also reduce the silver in the silver oxide/sulphide? $\endgroup$
    – golvok
    Apr 7 at 8:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.