Surface properties of aluminum hydroxide at high salt concentration

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