NOTES A SPECTROPHOTOMETRIC METHOD FOR DETERMINAT ION OF CAT ION-EXCHANGE CAPACITY OF

The micro-Kjeldahl method for the determination of cation-exchange capacity of clays (Mackenzie, 1951) is widely used. However, it can be time-consuming and requires considerable experience in estimating the end=point in the acid titration of liberated NHa. Many alternative methods of determining liberated NHs are available, but the photometric method described by Searcy, Reardon & Foreman (1967) for the assay of serum urea as NHa is rapid, free from any subjective stimation, and is very sensitive. Its application to the determination of exchange capacities of clay minerals requires the following preliminary procedure: a 5 mg sample of NH,-saturated clay mineral is digested in 1 ml concentrated HC1 for 30 min in a covered vitreosil basin. A blank consisting of the clay mineral in its K-saturated form is run in parallel. The contents of the basin are then transferred to a 100 ml standard flask with NHs-free distilled water. After addition of 3 ml of 4 N NaOH solution, and dilution to the mark with water the solution is neutral or very slightly alkaline. Clay minerals are not completely dissolved by this procedure, silica gel remaining insoluble. From clays containing iron, hydrated iron oxide gels are als