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!