Differential conductance coefficients in a cation-exchange membrane

Abstract

An experimental procedure is described for determining the six independent conductance coefficients that control the fluxes of ions and solvent in a system of a single salt and water through an ionic membrane. Some of the data required are electrical and have been published already. Additionally, measurements are needed on flows under a pressure gradient. A cell is described for measuring the volume and salt fluxes through the membrane in the pressure range up to 1 MN m -2 . Data are given at various concentrations up to 1 mol dm -3 , for NaBr, CsBr and SrBr 2 in the cation permeable membrane Zeo-Karb 315 at 25 ºC. The calculation of the non-equilibrium thermodynamic conductance coefficients is described and their values tabulated. It is indicated that the postulate of local linear behaviour is obeyed for electrical potential and osmotic gradients. For pressure gradients, the filtration coefficients L p decreased by about 1% per atmosphere across a membrane 1.4 mm thick. This effect correlated reasonably well with the observed effect of pressure on the membrane thickness if an idealized capillary model was used to describe flow. The differential conductance coefficients are strong functions of solution concentration. This dependence effectively limits use of integral linear flux equations to extremely small concentration intervals across the membrane.