Exchange Equilibria of Sodium versus Calcium and Potassium in Some Aridisols

Sodium-calcium and sodium-potassium exchange equilibria were established in three illite dominated soils of semi-arid region. The normalized exchange isotherms for Na were drawn with 12 different equivalent fractions of competing cations at a total electrolyte concentration of 50 m.e./l in the equilibrium solution. In Ca-Na system, the exchange isotherms of Na lay below the diagonal and the values of Gibbs free energy of exchange (ΔG′_r) were positive, indicating higher preference of Ca to Na by the soil exchanger. The activity coefficients of the adsorbed Na were higher than those of Ca. The values of Gapon coefficient (K_G), Vanselow coefficient (K_V) and Krishnamoorthy-Davis-Overstreet coefficient (K_KDO) increased with sodium saturation. The values of correction factor for surface charge density in the diffuse double layer equation to match the experimental and calculated data of ESP for Na-Ca exchange were >1. In Na-K system the shape and position of the exchange isotherms of Na and the sign and magnitude of ΔG′_r suggested considerably higher preference for K to Na. The adsorbed phase activity coefficients of Na and K varied between 0 to 1 at different cationic saturation of the exchange complex.