- The thermodynamics of the Rb+-H+ exchange on zirconium phosphate have been studied. The heats of the exchange on three samples of widely different crystallinities have been determined calorimetrically. For the most crystalline exchange the reaction is exothermic with virtually constant heat changes up to 75% rubidium loading. For that reaction, the enthalpy change was found to be -2.25 kcal per mole of rubidium (1 cal = 4.184 J) and the corresponding entropy change -36 eu (1 eu = 4.184 J K-1 mol-1). In the case of the nearly amorphous exchanger, the reaction is initially exothermic, but then the heat function passes through a broad maximum and becomes progressively more endothermic. A maximum value of Hx = -3.3 kcal per mole of exchanger was obtained at 45% of exchange. For the amorphous exchanger the Rb+-H+ exchange isotherm has also been determined. The exchange reaction is reversible and the equilibrium constant was found to be 2.8 10-5. 1980 American Chemical Society.