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Potentiometric Titration of Acid Montmorillonite¹

ABSTRACT

Two buffer ranges representing the titration of H and Al ions are generally observed in the potentiometric and conductometric titration curves of acid montmorillonite. Recently, a third buffer range has been reported by two groups of investigators. The weak acid neutralized in the third range has not been fully characterized, however, and hence was investigated further.

Aging of acid montmorillonite prepared by passing the lay through a resin column increased both the second and third buffer ranges while it decreased the first range. In addition, the ratio of the third to the second buffer range increased continuously. Titration curves of acid clay prepared by treatment with different concentrations of HCl contained different amounts of the third range: the higher the concentration of acid, the greater was the third range. The acidic component titrated in this range was held very strongly by the clay and could not be extracted with 1N HCl or 1N NaCl.

These observations led to the conclusion that the weak acid titrated in the third buffer range of acid montmorillonite is a complex alumino-silicate resulting from the decomposition of the clay by acid action.

¹ Contribution from The Connecticut Agr. Exp. Sta., New Haven. Presented before Div. S-2, Soil Science Society of America at Columbus, Ohio, Nov. 3, 1965. Received June 28, 1965. Approved Dec. 21, 1965. This study was supported in part by the US Atomic Energy Commission under Contract AT(30-1)-2955.

² Associate Soil Chemists. The authors wish to express their appreciation to Mr. F. C. C. Pedersen for technical assistance during this investigation.