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Rate of Cation Exchange on Clay Minerals as Determined by Specific-Ion Electrode Techniques¹

ABSTRACT

A cationic electrode (used as a potassium specific-ion electrode and a sodium specific-ion electrode were used to measure rates of cation exchange on clay minerals. The Ba-K exchange rates for kaolinite, illite, and montmorillonite were rapid with >75% exchange being measured within 3 sec. Because this time interval represents response time of the specific-ion electrode, the Ba-K exchange on these clay minerals occurs at a faster rate than this and may be an instantaneous process. Ba-K exchange on vermiculitic materials was characterized by both a rapid and a slow rate of exchange. A major part of the Ba-K exchange in silt- and clay-size vermiculite was found to be diffusion controlled.

Ca-K, Mg-K, Ca-Na, and Mg-Na exchange rates were rapid for all vermiculites and other clay minerals used in the experiment. Only Poole vermiculite exhibited two rates of Ba-Na exchange similar to those shown by all vermiculites during Ba-K exchange. Ba-Na exchange rates on all vermiculites except Poole were rapid.

¹ Publication authorized by the Director, US Geological Survey.

² Research Hydrologists, US Geological Survey, Denver, Colo.

Received for publication August 12, 1968. Accepted for publication October 31, 1968.