In a copper-zinc voltaic cell, one half-cell consists of a ZnZn electrode inserted in a solution of zinc sulfate and the other half-cell consists of a CuCu electrode inserted in a copper sulfate solution. These two half-cells are separated by a salt bridge.

At the zinc electrode (anode), ZnZn metal undergoes oxidation by losing two electrons and enters the solution as Zn2+Zn2+ ions. The oxidation half-cell reaction that takes place at the anode is

Zn(s)→Zn2+(aq)+2e−Zn(s)→Zn2+(aq)+2e−

The CuCu ions undergo reduction by accepting two electrons from the copper electrode (cathode) and depositing on the electrode as Cu(s)Cu(s). The reduction half-cell reaction that takes place at the cathode is

Cu2+(aq)+2e−→Cu(s)Cu2+(aq)+2e−→Cu(s)

The electrons lost by the ZnZn metal are gained by the CuCu ion. The transfer of electrons between ZnZn metal and CuCu ions is made possible by connecting the wire between the ZnZn electrode and the CuCu electrode. Thus, in the voltaic cell, the electrons flow through an external circuit from the anode to the cathode. For a voltaic cell to work, the solution in the two half-cells must remain electrically neutral. This can happen only if the flow of ions is countered with the flow of electrons. The flow of ions is made possible with the use of a salt bridge. A salt bridge is a solution of some other metal that has common ions. If a copper-zinc voltaic cell utilizes ZnSO4ZnSO4 and CuSO4CuSO4 solution, you will use a saturated Na2SO4Na2SO4 solution in the salt bridge. Thus, the salt bridge will help the migration of ions across the two compartments, or two half-cells.