31.16 How do I remove silver tarnish?
Description
This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
31.16 How do I remove silver tarnish?
Several commercial silver polishes use a fine abrasive ( such as calcium
carbonate ) to physically remove the tarnish, and also include in their
formulation components ( such as benzotriazole ), that form complexes or
films that slow the reappearance of tarnish. There are also a variety of
chemical tarnish removers that contain sulfuric acid, sodium thiosulfate,
or hot sodium phosphate, a chemical formulary should contain recipes.
A very popular technique for removing tarnish ( silver sulfide ), involves
an electrochemical cell that utilises the electrochemical series. In an
electrochemical cell, oxidation occurs at one electrode, whilst reduction
occurs at the other. Electrical contact between the two metals provides the
pathway for electron migration to occur from the anode to the cathode, and
when immersed in an electrolyte, charge will be balanced by transfer of
the sulfide ions.
The standard electrode potential at 25C of Ag+ + e- -> Ag is +0.799V, and if
the other metal is anodic relative to silver, the silver in the sulfide at
the cathode will gain electrons and revert to metallic silver. The metal
at the anode will be oxidized by the removal of electrons. The sulfide ions
can travel to the anode via the electrolyte. The electrochemical series
indicates metals that will work, and some readily-available and cheap metals
are iron, zinc, aluminium and magnesium. The standard electrode potential at
25C of Al3+ + 3e- -> Al is -1.66V, and aluminium foil is cheap.
When aluminium is the anode, and water is the electrolyte, the aluminium
surface will eventually be coated with an insulating film of aluminium
sulfide - which will gradually decrease the cleaning speed. Adding sodium
bicarbonate ( baking soda ) or sodium carbonate ( washing soda ) to the
electrolyte will assist in the evolution of hydrogen as the silver is
converted, and the hydrogen will combine with the sulfide ions to produce
some hydrogen sulfide gas ( rotten eggs smell ). The aluminium at the anode
will be converted to aluminium oxide.
Aluminium oxidises on contact with air, forming an insulating layer of oxide
that is present on the surface of foil, disposable alumimium dishes, or
pots that are commonly used as the anode when cleaning silver at home.
Such a layer is brittle, and can micro-crack in hot water, and the higher
temperatures can also increase the redox reaction rate. Cleaning formulations
may include sodium chloride ( table salt ), boric acid, or borax. It should
be noted that the process is not converting the silver back to the original
hard, lustrous surface, but into a soft, white powder that can be removed by
rubbing. The sulfide has actually dislocated some silver atoms from the metal
surface, and the cleaning treatment will not relocate them, but it will
eliminate the unsightly, dark, tarnish.
A typical procedure is to line a pyrex glass dish with alumimium foil so the
bright side of the foil will contact the solution. Add one litre of water,
and heat until near boiling. Add one tablespoon of sodium chloride and one
tablespoon of sodium bicarbonate, and gently stir to dissolve. Ensure that
the silverware has been washed in warm dishwashing detergents, and well
rinsed in warm water, to remove dirt and grease. Carefully add the silverware
to the dish, ensuring that each item is in contact with the aluminium foil,
and leave until the tarnish has disapppeared. Because no protective film
has been provided, the tarnish may reappear rapidly.
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