| 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 (DELTA)G(DEGREES) of +6.15 kcal/mole. The overall enthalpy change, (DELTA)H(DEGREES), is found to be +1.58 kcal/mole and the resultant (DELTA)S(DEGREES) = -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(DEGREES)C by extrapolation of ln K vs. 1/T plots. Transformation of these reactions by means of a thermodynamic cycle to reactions similar to those used by Eisenman involving exchange of gaseous ions results in the strong field sequence Li > Na > K. The selectivity sequence was found not to change with loading.; Enthalpy changes for the gas-solid reactions were determined from the slopes of the ln K vs. 1/T plots and are found to correlate well with lattice changes which occur during the exchange process.; Comparison of the magnitudes of the free energy changes occurring in anhydrous exchange reactions and ion exchange reactions in solution substantiate the predictions of Eisenman concerning the relative effects of hydration energy and electrostatic energy changes during the exchange process. In addition, since no selectivity sequences were observed which were not as predicted by Eisenman, we conclude that Eisenman's theory is a valuable model for explaining general ion exchange selectivity. |
