ON THE ROTHMUND-KORNFELD DESCRIPTION OF CATION-EXCHANGE

Simple descriptions of cation-exchange isotherms are required to permi t interpolation and extrapolation from a limited data set. Conventiona l thermodynamic approaches do not result in simple expressions in term s of a small number of measurable constants. The empirical Rothmund-Ko rnfeld power function approach has been found to be applicable to a la rge number of data sets. It permits description of an entire exchange isotherm with two parameters obtained directly from measured points on the isotherm. It is shown here that these parameters can be used to d erive the true exchange equilibrium constant for the exchange reaction , as well as the exchange-phase activity coefficients used in thermody namic approaches based on the Vanselow-Argersinger (mole fraction) and Gaines-Thomas (equivalent fraction) conventions. Values of the exchan ge equilibrium constant and the exchange-phase activity coefficients c alculated from fitted Rothmund-Kornfeld parameters compare favorably w ith those calculated using conventional thermodynamic approaches. The method based on the Rothmund-Kornfeld approach is potentially more acc urate because it avoids subjective extrapolation of the data required for the thermodynamic approach.