Determination of Cation Exchange Capacity and Exchangeable Cations

APPENDIX 8.3 DETERMINATION OF CATION EXCHANGE CAPACITY AND EXCHANGEABLE CATIONS 

In determining the CEC. a soil saturated with a single index ion is washed free of soluble salts, often with alcohol to keep the soil flocculated and to prevent loss of the index cation by hydrolysis  

Hydrolysis would yield a low estimate of the CEC. The index cation is then extracted from the soil with still another salt solution and measured to give the CEC. 

The salt used to furnish the index cation should be relatively soluble in the alcohol used for sample washing. The low solubility of NaCl in ethanol has been a frequently overlooked source of error. The salt then dissolves during the subsequent extraction step and yields an anomalously high CEC value. Soils containing large quantities of hydroxyoxide or amorphous minerals may also retain salts in particle micropores, so that washing does not completely remove the salts. This also yields high CEC values. 

To eliminate the problems associated with the washing step, Okazaki, Smith, and Moodie proposed a CEC procedure in which salts are not removed between the index cation saturation and extraction steps. Rather, the anion of the salt providing the index cation is analyzed in the final extract. In accordance with electrical neutrality, the CEC is then equal to the total quantity of index cations removed during extraction minus the quantity of index anions removed simultaneously. The main potential source of error from this procedure arises from anion repulsion, if the quantity of index salt remaining after saturation is merely calculated from the weight of solution retained and its initial (or average) concentration. This error minimized if the index solution is lowered to approximately 0.1 M during the final two saturation washes. The error is eliminated if the quantities of index salt are analytically determined instead. 

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