The Gapon and the Vanselow Selectivity Coefficients

Abstract

AbstractA general thermodynamic relation between the Gapon and the Vanselow cation exchange equilibrium constants is derived and is shown to be violated by the expressions given traditionally in the soil chemistry literature for the Gapon and the Vanselow selectivity coefficients. This result suggests that the selectivity coefficient expressions are mutually in conflict. The traditional expression for the Gapon selectivity coefficient is shown to be in error, fundamentally because the Gapon convention—according to which cations react with an exchanger in equivalents—has no direct molecular interpretation. Therefore, the thermodynamic activity of an exchanger phase cannot be equated numerically with an equivalent fraction, even in some limiting case. The correct expression for the Gapon selectivity coefficient is derived and is discussed briefly for the case of Na‐Ca exchange. It is shown that the traditional expression and the correct one will not be very different numerically if the exchangeable sodium percentage is < 20.