The ion exchange equilibria of Na +/K + in nonaqueous and mixed solvents on a strong acid cation exchanger

Abstract

Na +/K equilibrium ion exchange isotherms on Amberlite IR-120 (known as a strong acid resin) in pure methanol, ethanol, 2-propanol, 1-butanol and 1-penthanol and in mixtures of methanol+water, ethanol+water, 2-propanol+water, 1-butanol+methanol and 1-penthanol+methanol at 313 K have been determined. In this paper two models for the description of ion exchange equilibrium in nonaqueous media are compared: the heterogeneous model (HMAM), based on the mass action law in which ideal behaviour for both the solution and the solid phase and the heterogeneity of ion exchange sites has been assumed and the homogeneous model (LAM), based on the same mass action law in which nonideal behaviour for both the solution and the solid phase has been taken into account. In the LAM model, Wilson and Debye–Hückel equations have been used to calculate activity coefficients in the solid and the liquid phase, respectively. It has been observed that the properties of the solvents that compound the solution phase exerts a strong influence not only on the useful capacity of the resin, but also on the equilibrium parameters of the system. This influence can be, at least, qualitatively explained based on the changes of dielectric constant of the liquid phase and on the solvation of ions. For systems with a behaviour close to ideality, the HMAM model shows that the energy distribution was practically symmetric, with similar equilibrium constants for both kinds of sites. In those systems with a behaviour far from the ideality, the energy distribution is clearly asymmetric.