The plating rules
In electroplating it is important to know the following points.
The object is the cathode
The object to be plated must be used as the cathode.
A salt of the plating metal
A solution of a salt of the plating metal must be used as the electrolyte.
The anode is the plating metal
The anode must be a plate or rod made of the plating metal.
A dilute electrolyte
For a good-quality coating the electrolyte should be dilute; the reaction is then slower, so the plating is done well.
Worked example — copper on an iron spoon
Suppose you want to copper-plate an iron spoon. What do you use as the anode and cathode of the electrolytic cell, and what electrolyte?
Cathode: the iron spoon — the object to be plated.
Anode: a copper rod.
Electrolyte: copper sulphate solution.
Why each rule is there
Each rule comes from the conventions of 12.2. Metal ions are positive, so they are reduced at the cathode — the object must be the cathode for metal to land on it. The electrolyte supplies those metal ions, so it must contain the plating metal. The anode is made of the same metal because a metal anode is oxidised itself (rule 7): as ions are taken out at the cathode, the anode dissolves and puts the same ions back, so the solution stays the same strength. And a slow deposit gives the atoms time to settle into an even, firmly stuck layer; a fast one comes out rough and flakes off.
The object is never the anode
If the object is connected to the positive terminal by mistake, it becomes the anode and dissolves instead of being plated.
Object at the cathode, plating metal at the anode
Cathode: the object. Anode: the plating metal. Electrolyte: a dilute solution of a salt of the plating metal.