Sodium-Potassium Exchange Equilibria on a Clay Soil

Sodium-potassium exchange equilibria were studied at 25°C using a clay soil. The exchange behaviour and preferential adsorption of potassium over sodium were explained by Argersinger thermodynamic approach. The values of cation exchange capacity were reasonably close in all treatments and were independent of exchanger composition. The adsorption isotherm indicated a preferential adsorption of potassium relative to sodium. The selectivity coefficient values were dependent on ionic strength and exchanger composition indicating the non-ideal behaviour of the soil. The activity coefficients of adsorbed sodium were more than that of potassium due to its presence mostly in Gouy layer resulting from its larger radii in hydrated state unlike potassium which was present in Stern layer. The standard free-energy changes of exchange were positive and it meant that potassium was more strongly bound than sodium.