Factors determining ion exchange selectivity : I. Aqueous K+-H+ exchange with [alpha]-zirconium phosphate. II. Gas-solid ion exchange reactions

Abstract

The heats of K^+ - H^+ exchange have been measured calorimetrically on three samples of zirconium phosphate of widely differing crystallinity. The more crystalline exchangers both exhibit exothermic reaction over the full range of composition. There is, however, a distinct break in the curves at a composition of approximately 50% corresponding to two exchanged phases previously identified. The nearly amorphous exchanger shows an initially exothermic reaction followed by an endothermic process. The thermodynamic equilibrium constant for the exchange process has been evaluated as 3.1 x 10^-5 resulting in a ΔG° of =6.15 kcal/mole. The overall enthalpy change, ΔH°, is found to be +1.58 kcal/mole and the resultant ΔS° = -15.3 e.u. Entropy changes in the crystalline material can largely be accounted for by consideration of differences in hydration of the cations within the lattice. Comparison of this work to the results of previous studies on Li^+ - H^+ and Na^+ - H^+ exchange on amorphous zirconium phosphate results in the selectivity sequence K > Na > Li at low loading. This sequence corresponds to the weak field sequence of Eisenman's theory of ion exchange selectivity and is a result of the predominance of hydration effects. At higher loading the strong field sequence Li > Na > K occurs and is a consequence of the dominance of electrostatic effects within the exchanger. The equilibrium constant as a function of temperature has been measured for the reaction of hydrogen chloride with the anhydrous lithium, sodium and potassium exchanged forms of crystalline zirconium phosphate at elevated temperatures. Pressure vs. composition isotherms are found to correlate well with the previously determined phase diagrams of these materials. The equilibrium constants for these reactions were determined at 25 °C by extrapolation of ln K vs 1/T plots...