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Electrochemical Properties of Some Oxisols and Alfisols of the Tropics

Van Raij, Bernardo, Peech, Michael
Soil Science Society of America journal 1972 v.36 no.4 pp. 587-593
A horizons, Alfisols, Oxisols, calcium chloride, electrical charges, electrochemistry, ionic strength, ions, magnesium sulfate, models, oxides, protons, titration, tropics, Brazil
A study of the distribution of the electric charges in the Ap and B2 horizons of two highly weathered Oxisols and one Alfisol from Brazil was made by means of potentiometric titration and by direct measurement of adsorption of ions in the presence of varying concentration of electrolyte. The titration curves at different ionic strengths crossed at the common point of intersection, the zero point of charge (ZPC), or the pH at which the net electric charge is zero. Thus, the electrochemical behavior of these soils was found to be similar to that exhibited by many metallic oxides in which the surface potential of a reversible double layer is determined solely by the activity of potential determining H⁺ and OH⁻ ions in the bulk solution. Direct determination of the adsorption of ions from NaCl, CaCl₂, and MgSO₄ solutions showed that pH, electrolyte concentration, and valence of the counter ion influence the magnitude of the electric charges on the soil particles. From known surface areas and ZPC of these soils, the values for the net electric charge calculated by the application of the Stern model of the double layer theory were found to be in good agreement with the experimental results.