Surface Chemistry of Some Constant Potential Soil Colloids

Abstract

A survey was conducted to determine the extent to which the mineral compositions of soils of Hawaii consist of colloids of the constant potential type or colloids of the constant charge type. Seven soils of diverse mineralogical composition were selected for the study. They included soils with kaolinitic, oxidic, and montmorillonitic mineralogies. Three other soils which lack mineralogical designations were also examined. They included an organic soil (Histosols) and two soils developed from volcanic ash (Andepts). Soils which contain colloids of the constant potential type, were distinguished from soils which contain colloids of the constant charge type through potentiometric titration and determination of the existence or nonexistence of a well-defined zero point of charge. Soils with pocentiometric titration curves which intersected at a common point were judged to be colloids of the constant potential type. The point of intersection of the titration curves was used as the zero point of charge. Organic matter, montmorillonite and a certain variety of non-crystalline alumina-silicate gave curves which did not intersect. These soils have materials which are predominantly of the constant charge type. All other soil minerals including the oxides and hydrous oxide of iron and aluminum, kaolinite, halloysite and non-crystalline hydrous oxides showed well defined zero points of charge. On the basis of this survey, it was concluded that the majority of soils in the State of Hawaii have minerals which behave as colloids of the constant potential type. The implication of this finding to the soils of Hawaii and other tropical areas is discussed.