A METHOD FOR THE DETERMINAT ION OF THE CATION EXCHANGE CAPACITY OF CLAY MINERALS AND SOILS*

For many purposes it is desirable to have a method for the determination f the cation exchange capacity of clay minerals based on principles as free from ambiguity as pos-sible. Most of the many methods previously proposed have the drawback of entailing a great deal of work if one is to make sure that the mineral is completely saturated with the reagent in use, or that an excess of the reagent is absent. What might be called the complete chromatographic method by its very nature does not suffer from this diffi-culty. A knov)n weight of clay is suspended on an appropriate inert filter aid (such as Gooch asbestos), packed into a glass tube, and the exchangeable ions eluted with a solution of cesium chloride traced with radioactive Cs 13.7. The effluent is collected in a series of appropriately sized samples, and the specific radioactivity of each of these determined. When the activity of the effluent has risen to that of the influent solution, one is certain that complete xchange has taken place. A simple computation gives the total cesium holdup of the column, and if the column was initially dry, the exchange capacity of the clay sample. As a routine method, particularly when results of the highest precision are not needed, the effluent may be collected in 0nly two samples, one large sample and finally one small sample to check for complete xchange. This procedure requires a minimum of work and attention but suffers from the disadvantage that the result depends upon the differ-ence of the two counting rates measured and is thus subject o greater uncertainty. These methods have been compared with the Bower-Truog method (Ind. and Eng