During investigations on clays and minerals, of which only small amounts were readily available, it became necessary to find an accurate micro-method for determining cation-exchange capacities. Usual methods, which are far too numerous to detail here (see, e.g., Kelley, 1948) required considerably more than the 10-50 mg. of material that could be spared. Three possible methods were tested, viz., (a) an adaptation of the manganese method of Bower and Truog (1940 a); (b) a method employing saturation with strontium and utilising spectrographic determination of the sorbed strontium; (c) an adaptation of the usual ammonium acetate method (Schollenberger and Dreibelbis, 1930; Jackson and Truog, 1939).

The results obtained by the first two methods were usually somewhat high—not surprising, as polyvalent ions would have difficulty in bridging exchange spots any distance apart and might consequently be sorbed as M(OH)+ rather than as M++ (Bower and Truog, 1940 b). Method (c) gave results in very good agreement with those obtained with the macro-method normally employed and was the one finally adopted. A complete description and assessment of the method is at the moment in the press (Mackenzie, 1951) and only a brief indication need be given here.