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282 ELECTROCHEMISTRY

What is the simplest way to clean a tarnished silver
spoon?

Electrochemistry: the chemistry of electron transfer

Cutlery or ornaments made of silver tarnish and become black;
Oxidation is loss of this is a shame, because clean, shiny silver is very attractive. The
electrons. Reduction is ‘tarnish’ comprises a thin layer of silver that has oxidized following
gain of electrons.
contact with the air to form black silver(I) oxide:

In fact, the tarnish on 4Ag (s) + O 2(g) −−→ 2Ag O (s) (7.5)
2
silver usually comprises
both silver(I) oxide Such silver can be rather difficult to clean without abrasives
and a little silver(I) (which wear away the metal). The following is a simple electro-
sulphide.
chemical means of cleaning the silver: immerse the tarnished silver
in a saucer of electrolyte, such as salt solution or vinegar, and wrap
it in a piece of aluminium foil. Within a few minutes the silver is cleaner and bright,
whereas the aluminium has lost some of its shininess.
The shine from the aluminium is lost as atoms on the surface of the foil are
oxidized to form Al 3+ ions (Equation (7.1)), which diffuse into solution. Because
the aluminium touches the silver, the electrons generated by Equation (7.1) enter the
silver and cause electro-reduction of the surface layer of Ag O (Equation (7.2)).
2
In summary, we construct a simple electrochemical cell in which the silver to be
cleaned is the cathode (Equation (7.2)) and aluminium foil as the anode supplies the
electrons via Equation (7.1).
The salt or vinegar acts as an electrolyte, and is needed since the product Al 3+
requires counter ions to ensure electro-neutrality (so aluminium ethanoate forms).
The oxide ions combine with protons from the vinegar to form water. Figure 7.2
illustrates these processes occurring in schematic form.

Aluminium metal Silver metal

− electrons + electrons
(oxidation) (reduction)

Aluminium ions Silver ions

Aluminium foil Electrolyte Silver spoon
(vinegar)

Figure 7.2 Illustration of the electron cycles that allow for the trouble-free cleaning of silver: we
immerse the tarnished silver in an electrolyte, such as vinegar, and touch the silver with aluminium
foil