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The Activities of Calcium and Potassium Ions as Related to Concentration and Drying in Clay Suspensions
Author(s) -
Marshall C. E.,
McLean E. O.
Publication year - 1948
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1948.036159950012000c0038x
Subject(s) - soil water , citation , potassium , chemistry , library science , soil science , geology , computer science , organic chemistry
E determination of cationic activities on hoJ_ mionic soils or on suspensions of soil constituents is now readily carried out by the use of clay membrane electrodes. Previous studies have shown how they may be used to determine potassium (4, 5), ammonium (6, 7), sodium (8, 9), and calcium (2, 3) ion activities in clay suspensions. The extension of this work to homionic soils is now known to offer no difficulty. One may, however, anticipate that the soil-water ratio and the previous history of the sample may be of much greater importance than is the case with most pH measurements. One reason, of course, is that we are now concerned with the major dissociated ions, not with a relatively minor constituent like the hydrogen ion at pH values above 5. The general electrochemical behavior might be expected to be somewhat similar to that of the pure colloidal acids as regards the suspension effect or the influence of concentration. The studies here reported, carried out on four well-characterized clay minerals, are intended to provide a guide for later work on complete soils. Two soils have also been studied in a preliminary way. In addition to studying the influence of clay concentration upon the potassium or calcium activity, it seemed important also to know something of the effect of previous drying. The fixation of potassium might be expected to show itself in a lowered K ion activity. Two drying conditions were chosen: 30% relative humidity at 35° C and oven-drying at 100° C. After drying, the clays were re-suspended in water and brought back to the original concentration. We thus have three sets of data for each clay. Each set comprised measurements at three different clay concentrations, and with two different amounts of the exchange cation corresponding, respectively, to 80% and 100% saturation as given by the inflexion on the pH titration curve. In order to bring the tabulation of results within a reasonable compass, the 80% saturation values and the determinations at the two highest concentrations of the .previously dried clays are omitted. They are. utilized in drawing the conclusions regarding the effects of drying.