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Graphiteelectrodes are often used to investigate the electrolysis of molten salts, and of aqueous solutions of ionic compounds. Graphite electrodes are inert electrodes because they do not take part in the electrolysis reactions. Instead, they provide a surface on which these reactions can happen.

Electroplating uses electrolysis to put a thin layer of a metal onto a metal object. The electrodes used in electroplating are non-inert - they do take part in the electrolysis reactions.

Improving properties

One use of electroplating is to improve the resistance of metal objects to corrosion. It also improves their appearance and may be used to produce gold-plated jewellery.

To carry out electroplating:

• the negatively charged cathode is the object to be plated
• the positively charged anode is the plating metal
• the electrolyte contains ions of the plating metal

For example, steel cutlery can be electroplated with silver using a silver anode and silver nitrate solution.

Purifying copper

Another use of electroplating is to purify copper on an industrial scale for use in electrical wiring:

• the cathode is pure copper
• the anode is impure copper, produced from copper ore
• the electrolyte is copper(II) sulfate solution

When the dc power supply is turned on:

• at the cathode, copper ions from the electrolyte gain electrons and become copper atoms:
  • Cu²⁺(aq) + 2e^- → Cu(s)
• at the anode, copper atoms lose electrons and become copper ions in the electrolyte:
  • Cu(s) → Cu²⁺(aq) + 2e^-

Over time, the pure copper cathode increases in mass and the impure copper anode decreases in mass. Impurities collect underneath the anode.