How can electrolysis be used to purify copper?
Electrolysis of copper transfers copper atoms from an impure copper anode to a pure copper cathode, leaving the impurities behind.
Unrefined copper is about 99 % pure. It contains traces of metals such as Fe, Zn, Ag, Au, and Pt. The purity can be increased to 99.95 % by electrolytic refining in a cell similar to that shown below.
A thin sheet of high-purity Cu serves as the cathode. The anode is the impure Cu. The electrolyte is a solution of copper(II) sulfate.
The Fe and Zn impurities are more easily oxidized than Cu. When current passes through the cell, these impurities go into solution from the anode, along with Cu.
The ions all migrate toward the cathode, but Cu²⁺(aq) is more readily reduced than Fe²⁺(aq) or Zn²⁺(aq), so it is the only ion that plates out.
The cathode increases in size, and the anode erodes away. The impurity ions remain in solution.
Other impurities, such as Ag, Au, and Pt, are less easily oxidized than Cu. These remain in metallic form and fall to the bottom of the cell, forming “anode sludge” from which they can later be recovered.