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Charge Characteristics of Soils with Variable and Permanent Charge Minerals: I. Theory¹

ABSTRACT

The net surface charge density of soil colloids consisting of mixtures of variable and permanent charge minerals is treated as the sum of the two types of surface charge. The permanent charge is assumed to be constant in magnitude, and negative, zero, or positive in sign, and the variable charge is permitted to vary in magnitude and sign with pH and electrolyte concentration. Based on these assumptions, relations are developed which show that there are two zero points of charge. The zero point of charge of the mixture as a whole is designated ZPNC and the zero point of charge of the variable charge component is designated pH₀. The ZPNC can be determined by ion adsorption measurements, and pH₀ by potentiometric titration. In mixtures, ZPNC is lower than pH₀ when permanent charge is negative and higher than pH₀ when the permanent charge is positive. The magnitude of the permanent charge can be determined by measuring cation and anion retention at the pH corresponding to pH₀. Specific surface of mixed systems can be estimated by measuring negative ion adsorption at pH values sufficiently different from pH₀.

¹ Journal Series no. 2481 of the Hawaii Agric. Exp. Stn.

² Professor, Dep. of Agronomy & Soil Science, Univ. of Hawaii, Honolulu, HI 96822, and Senior Research Scientist, Commonwealth Scientific & Industrial Research Organization, Davies Lab., Townsville, Australia.

Received for publication May 29, 1979. Accepted for publication November 26, 1979.

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