BACKGROUND ELECTROLYTE CORRECTION FOR ELECTROKINETIC SONIC AMPLITUDE MEASUREMENTS

Electroacoustics has recently been used to measure electrokinetic properties of colloidal systems. When an alternating electric field is applied to a colloidal suspension, charged particles in the liquid will move electrophoretically and create an alternating pressure wave. The electrokinetic sonic amplitude (ESA), which is the pressure amplitude per unit electric field, is related to the electrophoretic mobility and zeta potential. For a solid suspension in an electrolyte solution, the measured ESA signal is a combination of two signals: one for the solid and the other for the background electrolyte. Under certain operating conditions, the contribution from the background electrolyte signal is not negligible and must be subtracted from the measured value to arrive at the particle ESA value. Background electrolyte corrections were performed on a Geltech silica and a US Silica no. 40 quartz at two ionic strengths (0.01 and 0.1 M NaCl) covering the pH range 2-8. These corrections are important at high ionic strengths because the ESA signal for the solid decreases and the background signal increases with increasing ionic strength. For 0.1 M ionic strength, the measured ESA changes sign around pH 4.7 for the Geltech silica and near pH 5.6 for no. 40 quartz. This suggests that silica particles were positively charged below pH 4.7 and 5.6, respectively. However, the background-corrected ESA data correctly show that the solids do not change sign at least down to a pH of 2.