Silver tarnishes in air to form a black coating of silver sulphide, which has the formula Ag2S. The chemical equation for the formation of silver tarnish following reaction with hydrogen sulphide (H2S) is as follows:

2 Ag(s) + H2S(g) → Ag2S(s) + H2(g)

There’s speculation that, in the present day, tarnishing of silver actually occurs more quickly due to the increased amounts of hydrogen sulphide being released into the atmosphere by the combustion of fossil fuels and the like. This silver sulphide tarnish can be quite easily removed using polishes that contain abrasives to rub off the silver sulphide, but these have the disadvantage of also removing small amounts of silver. Handily, chemistry provides a method which sidesteps this problem.

This simple method merely uses aluminium foil, boiling water, bicarbonate of soda (sodium hydrogencarbonate) and table salt (sodium chloride). Line bowl with the foil, add a teaspoon or so of bicarbonate of soda and a sprinkling of salt, then just add boiling water. The tarnished silver object can then be placed into the water, making sure there is contact between it and the aluminium foil, and the silver sulphide tarnish will swiftly disappear.

The chemistry behind this reaction is relatively simple, but it’s surprisingly difficult to find a full explanation online that also fully describes the chemical reactions occurring and the reasons for the inclusion of baking soda and salt, so this is my attempt to redress that.

Aluminium has a stronger affinity for sulphur than silver, so in this reaction the aluminium simply displaces the silver from the silver sulphide compound, to free the silver metal and form the aluminium sulphide compound:

3 Ag2S(s) + 2 Al(s) → 6 Ag(s) + Al2S3(s)

The reaction itself is actually an electrochemical reaction – essentially, a tiny electric current flows between the silver and the aluminium when they are in contact, and the silver in silver sulphide is reduced (gains electrons) to form silver metal, whilst the aluminium is oxidised (loses electrons) to form Al3+ ions:

3 Ag+ + 3 e → 3 Ag

Al → Al3+ + 3e

Combining these two half equations gives us the full redox equation for the reaction:

 Al + 3 Ag+ → Al3+ + 3 Ag

This explanation is fine, but doesn’t explain the need for the bicarbonate of soda or the salt during the reaction – and both are vital components. The sodium bicarbonate is required to remove the thin layer of aluminium hydroxide that forms on the aluminium foil; without this, the reaction would be unable to obtain a ready supply of aluminium ions, and as such could not proceed. The reaction between these two also produces hydrogen, which plays no part in the removal of the silver tarnish and is just given off as a gas. The salt, meanwhile, acts as a ‘salt bridge’ – this aids in the transfer of electrons as the reaction progresses, preventing an imbalance in charge and allowing the oxidation and reduction reactions to proceed.

You might also notice a faint aroma of eggs when carrying out this procedure – this is due to the further reactions that can occur. Aluminium sulphide can go on to react with the water as follows:

Al2S3(s) + 6 H2O(l) → 2 Al(OH)3(aq) + 3 H2S(g)

Hydrogen sulphide, H2S, is actually the same gas that rotting eggs give off – hence the smell!

24 CommentsClose Comments


  • Bruce Wellman
    Posted March 23, 2014 at 4:30 am 0Likes

    Would it be possible to have a “PDF” version option for your great postings? I teach in a public high school and we are still functioning in the paper world since we are not a 1:1 computer school.

    Thanks for your great visuals to teaching chemistry!

    • Posted March 23, 2014 at 9:37 am 0Likes

      Hi Bruce,

      Thanks for the suggestion – great idea! Obviously PDFs are up for the graphics already. As far as the articles go, I can see why a PDF version would be useful – I tried exporting the page as a PDF straight from my browser, and interesting (not aesthetically pleasing) things happened!

      One thing I can recommend using, if you haven’t already heard of it, is Readability. It pulls the pictures and text from an article, and places them in a printable format. You can then print them as a PDF straight from your browser. Here’s an example from this article:

      I will look into putting up PDF versions of the articles as well, it just might take a little time 🙂

  • Vinh Dango
    Posted July 4, 2014 at 3:10 am 0Likes

    What’s great about this page is the chemical explanation for everything! Although there’s a language barrier ( I’m vietnamese btw), but I still find it fascinating. Thank you very much, Compound Interest.

    • Compound Interest
      Posted July 5, 2014 at 2:09 am 0Likes

      You’re very welcome 🙂 Thanks for taking the time to leave a comment!

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  • Pedro
    Posted January 27, 2015 at 11:31 pm 0Likes

    Great explanation about the chemistry of the process!
    Meanwhile, I’m thinking about the importance of the sodium bicarbonate: it’s important to remember that the sodium bicarbonate in water will cause the increase of the pH due to the hydrolisis reaction.
    Maybe the hydroxyl ions could combine with the newly produced aluminium ions (to form insoluble aluminium hydroxide – that probably will be visible – or soluble species like tetrahydroxoaluminate ions), removing these ions from the solution, favouring the reaction Al + 3 Ag+ → Al3+ + 3 Ag .
    Another possible explanation is the possibility of the reaction be more favourable in alkaline medium.
    Oh, and I have a question: when you talked about the thin film of aluminium hydroxide…would not be the aluminium oxide?
    Sorry for the bad English!
    Thanks a lot!

  • William Hawkes
    Posted April 27, 2015 at 10:27 am 0Likes

    Hi there im doing my masters in the cleaning of sivler using acidified thiourea and saponin…. so the chemistry you have discussed is very important for me just now… are youa ble to provide any references? I dont mean to be rude but we have to reference everything we do… are you able to help?

    • Compound Interest
      Posted May 2, 2015 at 6:22 pm 0Likes

      Hi Bill, sorry, this is one of the earliest posts on the site from back when I first started it – I’ve since gotten into the habit of providing a comprehensive list of references used for the posts! I’ll see if I can dig up the papers/sources I referenced for this piece and add them in. Will keep you posted!

      • William Hawkes
        Posted May 3, 2015 at 10:20 am 0Likes

        Thanks ever so much for this, I will of course reference you in my dissertation! This sort of down to earth information is superb!

  • Tom Elmore
    Posted August 18, 2015 at 7:33 pm 0Likes

    Man, talk about late to the party. I got a C in inorganic chem in 1967, so it’s pretty much greek to me. Last night I was looking at the equations for computing speaker cone displacement as a fxn of frequency. Between the two experiences, I’m feeling out of it, but I understand at some macro level. I really appreciate your site.

  • Cizee
    Posted November 26, 2015 at 4:27 pm 0Likes

    Thank you for this great explanation! I used this method last night to clean the tarnish off a silver dish I acquired, and was fascinated by the chemistry taking place before my eyes. The miraculous vanishing of the tarnish, the sulphurous smell, the mysterious ingredients…I felt a little like a mad scientist! I’ve been poking around in the Internet trying to find a “why” behind the silver polishing magic, and the answer has been surprisingly hard to find despite the numerous sites extolling the virtues of this method. Most folks seem content with the simple explanation, “it’s chemistry!”, but my inner science nerd was not satisfied.

    So thank you for going into such detail about the reaction taking place. Now I have a shiny dish to serve my sweet potato casserole in, *and* an acceptably scientific conversation starter this thanksgiving.

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  • Alex
    Posted January 26, 2018 at 5:42 pm 0Likes

    Nice article, but chemical reactions are so wrong! Silver can’t react with H2S without oxygen and forming Al2S3 in water solution by reaction Ag2S+Al… is just a nonsense.

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