Salt bridge is a U-shaped tube containing a semi-solid paste of some inert electrolyte like KCl, KNO3, NH4Cl, etc. in agar-agar and gelatin. An inert electrolyte is one which:
(a) Does not react chemically with the solutions in either of the component.
(b) Does not interface with the net cell reaction.
Function of the salt bridge
In the electrochemical cell a salt bridge serves two very important functions:
(i) It allows the flow of current by completing the circuit
(ii) It maintains electrical neutrality.
The transference of electrons from anode to cathode leads to net positive charge around the anode due to increase in the concentration of cations and net negative charge around the cathode due to excess of anions in solutions. The positive charge around the anode will prevent electrons to flow out from it and the negative charge around the cathode will prevent the inflow of electrons at it. The reaction would then, stop, and no current will flow. The salt bridge comes to aid and restores the electrneutrality of the solutions in the two compartments. It contains concentrated solutions of an inert electrolyte the ions of which are not involved in electrochemical reactions. The anions of the electrolyte in the salt bridge migrate to the anode compartment and cations to the cathode compartment. Therefore, the salt bridge prevents the buildup of charges and preserves the flow of current. In the electrochemical cell, the salt bridge can be replaced by the porous partition which allows the migration of ions but does not allow mixing of the two solutions.