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Interactions of pH-Dependent and Permanent Charges of Clays: I. Use of Specific Ion Electrodes for Measuring Ca and Rb Activities in Bentonite and Illite Suspensions¹

ABSTRACT

Recent development of selective electrodes for many ions is revitalizing analytical potentiometry. Examination of recent evidence appeared to justify their use for measuring ionic activities in clay suspensions. Calcium and Rb ion activities in Fithian illite and Panther Creek bentonite were measured with the Orion divalent and the Beckman monovalent ion electrodes. The clays were saturated reciprocally with Ca-Rb, Ca-H, and Rb-H. Electrode-measured Ca and Rb activities were generally valid indexes of tracer-measured root uptake and clay retention of cations, details of which are given in companion reports. Activities and fractions active of Ca and Rb reached minimal or near minimal values when sufficient H was added to equal the pH-dependent CEC of each clay, a fact which suggested interaction of pH-dependent and permanent charges.

¹ Published with the permission of the Director of the Ohio Agricultural Research & Development Center as Journal Article no. 29-68. Presented before Div. S-2, Soil Science Society of America, Washington, D.C., Nov. 7, 1968. The results are contained in a dissertation submitted by the junior author in partial fulfillment of the requirements for the Ph.D. degree, The Ohio State University, 1967. Supported in part by grant no. GB1101, the National Science Foundation.

² Professor and former Research Assistant, Dept. of Agronomy, The Ohio State University and Ohio Agr. Res. & Devel. Center. The junior author is now Assistant Professor, Everglades Exp. Sta., Belle Glade, Fla.

Received for publication December 2, 1968. Accepted for publication January 16, 1969.