On the Rothmund‐Kornfeld Description of Cation Exchange

Simple descriptions of cation‐exchange isotherms are required to permit interpolation and extrapolation from a limited data set. Conventional thermodynamic approaches do not result in simple expressions in terms of a small number of measurable constants. The empirical Rothmund‐Kornfeld power function approach has been found to be applicable to a large 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 derive the true exchange equilibrium constant for the exchange reaction, as well as the exchange‐phase activity coefficients used in thermodynamic approaches based on the Vanselow‐Argersinger (mole fraction) and Gaines‐Thomas (equivalent fraction) conventions. Values of the exchange equilibrium constant and the exchange‐phase activity coefficients calculated from fitted Rothmund‐Kornfeld parameters compare favorably with those calculated using conventional thermodynamic approaches. The method based on the Rothmund‐Kornfeld approach is potentially more accurate because it avoids subjective extrapolation of the data required for the thermodynamic approach.