SURFACE-PROPERTIES OF ALUMINUM HYDROXIDE AT HIGH-SALT CONCENTRATION

The aim of this work was to use electroacoustic measurements to determ ine the zeta potential of precipitated aluminum hydroxide particles in NaCl solutions at concentrations up to 3 M. The commercial device use d in this study, the AcoustoSizer, is not calibrated for electrolytes above about 0.1 M, and thus the first step of this investigation invol ved the extension of the calibration procedure to concentrations up to 3 M. Having calibrated the device, we then determined the zeta potent ial for the aluminum hydroxide as a function of pH and background elec trolyte concentration. The main result of the study was that at 0.5 M NaCl, the isoelectric point of the aluminum hydroxide shifted up by ab out 2 pH units for a supposedly ''indifferent'' electrolyte such as Na Cl, and that at 3 M NaCl, there was no isoelectric point. It appears t hat the sodium ions are preferentially adsorbed on the hydroxide surfa ce at high concentrations, and at the highest concentration studied, t he surface remains positive for all pH values. (C) 1997 Academic Press .