Charge Characteristics of Spodic Horizons

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Abstract
Surface charge properties of six Spodosol Bir horizons, enriched in sesquioxides and organic matter, were investigated by acid‐base potentiometric titrations and ion adsorption measurements. The results indicate that the horizons have electrochemical properties similar to constant surface potential colloids as the sign and magnitude of the net charge were dependent upon the pH and indifferent electrolyte concentration of the ambient solution. The pH values (pHzpc) at which the net surface charge underwent reversal ranged from 4.2 to 4.9, and the values obtained by acid‐base potentiometric titrations agreed fairly well with those obtained by ion adsorption measurements. Although sesquioxidic components in combination with organic matter apparently dominate the charge properties of these Bir horizons, there was evidence from the titration curves of small amounts of clay minerals with a permanent negative charge. The latter was confirmed by potentiometric titrations of two Bir horizons to which various quantities of montmorillonite had been added. While the electrochemical behavior of these Spodosol Bir horizons was qualitatively identical to that of reversible interface colloids, theoretical mathematical models applicable for a quantitative description of the latter, such as the Stern double layer model, were found to give an inadequate representation of the observed values of surface charge and potential.