The Formation of Mixed Magnesium‐Aluminum Hydroxides In Soil Materials

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 0.2 N MgCl 2 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.