MECHANISM OF ION-EXCHANGE IN ZIRCONIUM-PHOSPHATES .16. CALORIMETRIC DETERMINATION OF HEATS OF LI+-H+ EXCHANGE
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The heats of Li+-H+ exchange have been measured calorimetrically on zirconium phosphates of different crystallinities. For the most crystalline exchanger the reaction is exothermic up to about 50% Li+ uptake. This is followed by a mildly endothermic reaction to about 77% lithium loading and a more highly endothermic reaction at still higher loading. These heat effects correspond to the formation of three separate exchange phases identified earlier by x-ray methods. In the case of the nearly amorphous exchanger the reaction is initially very slightly exothermic but soon becomes endothermic. The overall enthalpy is H = 2.57 kcal/mol and the entropy, S = -10.6 eu. Qualitative explanations for the enthalpy and entropy effects are presented and the results compared to those obtained with other alkali metal cations.