Exchange kinetics of rubidium and caesium ions on pyridinium tungstoarsenate exchanger.

概要

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The Kinetics of exchange of Rb+ and Cs+ has been investigated on pyridinium tungstoarsenate ion exchanger using limited bath technique. The exchange rate was studied as a function of concentration, temperature and particle size of exchanger to determine the slowest step. It was found that Rb+ exchange is particle diffusion controlled whereas Cs+ exchange shows dual behaviour at low concentration level. However, at higher concentrations, particle diffusion controls Cs+ exchange but the rate is concentration dependent. This anomalous behaviour has been explained by assuming that Cs+ exchange approaches closely to a chemical reaction. Thermodynamic parameters of exchange such as effective diffusion coefficient (D), activation energy (Ea) and entropy of activation (ΔS*) were calculated. The D and Ea values were found in the same order as that of ionic size, ΔS* value being positive for Cs+ exchange and negative for Rb+ exchange. A negative ΔS* value is usual but a positive value for Cs+ exchange might be due to dehydration of the cation while passing through the channels of the exchanger. This explanation is in line with that of interaction energy for the exchange of Cs+ on pyridinium tungstoarsenate.
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