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Evaluating the Use of Na⁺, Ca²⁺, and Divalent Cation Electrodes in Some Soil Extracting Solutions¹

ABSTRACT

The performances of sodium activity, calcium activity, and divalent cation activity electrodes were determined and compared with predicted theoretical responses in unbuffered systems and in buffered systems at concentrations of 0.2 m, 0.5 m, and 1.0 m. In support of previous findings on a less selective sodium electrode, results show that the Na-electrode exhibits a response which is nearly Nernstian, allowing good estimates of Na⁺ ion activities in soil extracts. In contrast, the usefulness of the calcium and divalent-electrodes in buffered systems (and subsequent usefulness for determining exchangeable ions) was found to be limited particularly at high levels of total salt concentration. Although better response was observed for these two electrodes in unbuffered systems, the determination of Ca²⁺ and Mg²⁺ in water-soil extracts by their combined use is limited by the apparent reversibility of the calcium electrode to Mg²⁺ ions present in calcium-magnesium salt mixtures.

¹ Published with the approval of the Director of Hawaii Institute of Geophysics as Contribution no. 215 and the Director of Hawaii Agricultural Experiment Station as Technical Paper no. 953. Work supported by OWS Contract No. 14-01-0001-673, U.S. Dep. of the Interior.

² Assistant Soil Scientist and Graduate Student, respectively.

Received for publication February 21, 1969. Accepted for publication April 21, 1969.