In the Daniell cell, a zinc bar is placed in an aqueous ZnSO 4 solution, and a copper bar is placed in an
aqueous CuSO 4 solution. The anode of this cell is the zinc bar where Zn (s) is oxidized to Zn2+ (aq).
The cathode is the copper bar, and it is the site of the reduction of Cu2+ (aq) to Cu (s). The half-cell
reactions are written as follows:
Zn (s) → Zn2+ (aq) + 2e− (anode)
Cu2+ (aq) + 2e− → Cu (s) (cathode)If the two half-cells were not separated, the Cu2+ ions would react directly with the zinc bar and no
useful electrical work would be obtained. To complete the circuit, the two solutions must be
connected. Without connection, the electrons from the zinc oxidation half-reaction would not be
able to get to the copper ions, so a wire (or other conductor) is necessary. If only a wire were
provided for this electron flow, the reaction would soon cease anyway because an excess negative
charge would build up in the solution surrounding the cathode and an excess positive charge would
build up in the solution surrounding the anode. This charge gradient is dissipated by the presence of
a salt bridge, which permits the exchange of cations and anions. The salt bridge contains an inert
electrolyte, usually KCl or NH 4 NO 3 , whose ions will not react with the electrodes or with the ions in
solution. At the same time the anions from the salt bridge (such as Cl−) diffuse from the salt bridge of
the Daniell cell into the ZnSO 4 solution to balance out the charge of the newly created Zn2+ ions, the
cations of the salt bridge (such as K+) flow into the CuSO 4 solution to balance out the charge of the
SO 4 2− ions left in solution when the Cu2+ ions deposit as copper metal.
During the course of the reaction, electrons flow from the zinc bar (anode) through the wire and the
ammeter, toward the copper bar (cathode). The anions (Cl−) flow externally (via the salt bridge) into
the ZnSO 4 , and the cations (K+) flow into the CuSO 4 . This flow depletes the salt bridge and, along
with the finite quantity of Cu2+ in the solution, accounts for the relatively short lifetime of the cell.