CHANGES IN PH VALUE CAUSED BY PRESSURE-DRIVEN TRANSPORT OF STRONG ELECTROLYTES ACROSS CHARGED MEMBRANES

The pressure-driven transport of small additions of strong acids (base s) to strong electrolytes across charged membranes has been studied th eoretically. The basic equations have been formulated in terms of irre versible thermodynamics. Approximate thermodynamic solutions have been obtained under the assumption that the concentration of H+ ions in th e membrane phase is substantially higher than that of OH- ions or vice versa. The phenomenological coefficients in the basic equations have been further specified within the scope of the homogeneous membrane mo del and the resulting equations have been solved numerically. It has b een shown that in the case of negatively charged membranes in the acid ic region up to a certain pH value, Hf ions can be considered merely h ighly mobile cations of a strong electrolyte; the reaction of water di ssociation plays no role. Within that region pH changes are independen t of the feed pH value and may be very large. At higher feed pH values there occurs a ''buffer'' region where the permeant pH value is pract ically independent of the feed pH value. Here the reaction of water di ssociation plays a substantial role. At still higher feed pH values th ere is a rather sharp transition to a ''basic'' region where the H+ io n rejection is again almost independent of the feed pH value but, in c ontrast to the acidic region, is rather small. It has been shown that at sufficiently high membrane electrochemical activity and Peclet numb er, changes in pH value may be described by approximate thermodynamic solutions within the whole range of validity of the small addition app roximation. (C) 1995 Academic Press, Inc.