154. Explain the function of salt bridge in an electrochemical cell.

The safe bridge is an inverted U-shaped glass tube filled with strong electrolytes (NH₄NO₃, KCl, KNO₃ etc.) dissolved in a gelatinous substance. The functions of the salt bridge are:
(i) To complete the circuit.
(ii) To maintain electrical neutrality of the two half-cell solutions and ensure a continuous production of electric current in a galvanic cell.

Let us consider Daniell cell,


In the oxidation half-cell, Zn will lose electrons and would change to Zn²⁺ ions. So positive charge may accumulate in this half cell due to an excess of Zn²⁺ ions. This prevents the release of electrons from zinc plate and flow of electricity stops. Similarly, the cathodic half cell, Cu²⁺ ions would gain electrons and deposited as Cu on the cathode. So the negative charge would accumulate due to an excess of  ions in the half cell. This prevents the flow of electrons to the copper plate. This also stops the flow of electric current. The salt bridge then supplies negative ions to the oxidation half-cell in order to neutralise excess of Zn²⁺ ions. Similarly, it supplies positive ions to the reduction half-cell in order to neutralise the excess of sulphate ions. Thus, salt bridge maintains the electrical neutrality in both the half cells.