In this process, the positive electrode (the anode) is made of the impure copper which is to be purified. The negative electrode (the cathode) is a bar of pure copper. The two electrodes are placed in a solution of copper sulphate.
Copper ions leave the anode and are attracted to the cathode, where they are deposited as copper atoms. The pure copper cathode increases in size, while the anode dwindles away. The impurities left behind at the anode form a sludge beneath it.