THERMODYNAMIC STUDY OF M/H EXCHANGES ON A CRYPTOMELANE-TYPE MANGANIC ACID

Ion-exchange selectivity for alkali cations on a cryptomelane-type manganic acid (CMA) was studied by plotting the corrected selectivity coefficient vs. charge fraction, XM, which is defined by the ratio of uptake in meq/g to the theoretical capacity of the CMA (2.70meq/g). The vanʹt Hoff plots for "ideal" alkali cation /H exchanges with the least steric hindrance showed a straight line, indicating that the standard enthalpy changes are zero or positive constants. Thermodynamic quantities of "ideal" and hypothetical alkali cation exchanges were evaluated using the selectivity plot and thermodynamic quantities for hydration of alkali cations, deltaY Hypothetical entropy changes, AS, for less selective Li+, Na+, and Cs+ were smaller than detalS , while the order was reversed in highly selective K and Rb. The H values for the latter two cations were extremely large in comparison with those of other cations. These findings suggest that K+ and Rb were strongly exchange-adsorbed in the tunnel site and possibly broke the hydration structure around the exchange site.