The salt bridge

As is well-known the salt bridge is a device which has the purpose of eliminating or at least reducing the diffusion or liquid junction potential. The set-up is simple. The two half cells from which the galvanic cell is to be constructed, are not brought into direct contact, rather they are connected via an electrolyte phase which in most cases consists of a concentrated KCl solution. Experimental details need not be given here. The conventional explanation of the observed effect, namely a drastic reduction of the diffusion potential, is based on the fact that the transport numbers in the bridge electrolyte, t_Cl and t_k, are almost equal. As a consequence of the equal mobilities of the K+ and Cl− ions an electrical potential difference presumably inherent in any ionic diffusion process does not arise. In the present chapter we shall give the theory of the salt bridge within the framework of our treatment. It will be seen that the physical reason for the strong reduction of the liquid junction potential differs from that given in the classical treatment.