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The Formation of Mixed Magnesium-Aluminum Hydroxides In Soil Materials¹

ABSTRACT

Potentiometric titration curves show a buffer region in the vicinity of pH 8.2 to 8.5 when soil materials, which contain reactive forms of Al, are titrated in a Mg solution. This buffering is due to the formation of mixed Mg-Al hydroxide. The product formed during the titration of gibbsite in 0.2N MgCl₂ solution was identified as mixed Mg-Al hydroxide by X-ray diffraction analysis. The buffer region also appears when allophane, Al sesquioxide, and clay with exchangeable Al are titrated in Mg solutions. The reaction with exchangeable Al and Al sesquioxide is rapid, but the reaction requires several hours and even days with gibbsite and allophane. It is concluded that soils containing these and possibly other forms of Al, when raised to alkaline pH levels, can remove Mg from the soil solution by precipitation of mixed Mg-Al hydroxide.

¹ Contribution of the Dept. of Soils & Plant Nutrition, Univ. of Calif., Citrus Research Center & Agri. Exp. Sta., Riverside. Part of a dissertation submitted by the senior author in partial satisfaction of the requirements of a graduate program in Soil Science Ph. D. degree. Presented in part before Div. S-9, SSSA, Nov. 1969, at Detroit, Mich.

² Former Graduate Student and Professor of Soil Science, respectively.

Received for publication November 13, 1969. Accepted for publication June 25, 1970.