Liquid Junctions and Salt Bridges

As the electrolyte ions from the reference cell must pass through some form of liquid junction before coming in contact and entering the sample liquid. the junction must be constructed such that ions are allowed to pass across the junction but there is little or no flow of liquid.

There are two basic forms of junction; those that only allow ions to pass across the junction and those that allow a liquid flow across the junction, which is kept sufficiently small so as not to effect the measurement. There are a number of materials used to form junctions, sintered glass, ground glass joints, ceramic plugs, porous TeflonÒ, polymer fibres and very small holes.

Frits, both glass and ceramic, are normally used with glass electrodes however, the pores of both types of frit easily become contaminated and blocked and, thus, need frequent cleaning or replacing. The ceramic frits probably have the smallest pore diameters and thus, take time to come into equilibrium after any pressure change.  Ground glass joints or joints with ground plastic sleeves provide very stable liquid junctions and are commonly used for ion selective electrode systems. The small hole and ground glass sleeve junctions are very porous and permit relatively large bulk liquid flows and which results in frequent electrolyte replenishment.