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Buffer Curves of Acid Clays as Affected by the Presence of Ferric Iron and Aluminum¹

ABSTRACT

Buffer curves of Al- or Fe-montmorillonites and hectorite in 1.0N KCl had appreciably smaller slopes between pH 5 and 8.5 than did curves for corresponding H-clays. Aluminum or ferric hydrous oxide-clay complexes gave buffer curves indicative of almost no exchangeable Al, Fe, or H on permanent-charge sites, but rather showed a regular increase in pH with additions of NaOH. The behavior of the clay-sesquioxide complexes is thought due to the existence, at pH below about 5, of hydroxy-Al or Fe-ions on exchange sites, with the progressive conversion of these to the corresponding hydroxides upon the addition of base. That such a reaction can result in apparent pH-dependent cation-exchange capacity (CEC) was shown by the reasonably good correspondence between CEC's measured at various pH values and the amounts of base necessary to titrate the complexes to those pH's. It is suggested that the pH-dependent CEC's of some soils may be due to the presence of clay-sesquioxide complexes. In other cases, apparent pH-dependent exchange capacity can be "created" during the neutralization of Al- or Fe-clay, through the interaction of positively charged metal ion-hydroxy complexes with exchange sites. Aluminum- or Fe-hydroxy complexes with montmorillonite were not stable upon digestion in dilute salt solutions, but underwent dismutation to produce the corresponding metal ion and hydroxide. While sesquioxide-montmorillonite complexes resembled many acid soils so far as titration behavior and pH-dependent CEC are concerned, the chlorite-like complexes were far less stable than those found in soils.

¹ Paper No. 1498, University of California Citrus Research Center and Agricultural Experiment Station, Riverside.

² Professor of Soils and Plant Nutrition, University of California, Riverside, and Visiting Associate Professor on leave from Virginia Polytechnic Institute, respectively. The work was supported by the National Science Foundation under Research Grant NSF-GP-323-Coleman.

Received for publication July 15, 1963. Accepted for publication November 25, 1963.