BACKGROUND ELECTROLYTE CORRECTION FOR ELECTROKINETIC SONIC AMPLITUDE MEASUREMENTS

Electroacoustics has recently been used to measure electrokinetic prop erties of colloidal systems. When an alternating electric field is app lied to a colloidal suspension, charged particles in the liquid will m ove electrophoretically and create an alternating pressure wave. The e lectrokinetic sonic amplitude (ESA), which is the pressure amplitude p er unit electric field, is related to the electrophoretic mobility and zeta potential. For a solid suspension in an electrolyte solution, th e measured ESA signal is a combination of two signals: one for the sol id and the other for the background electrolyte. Under certain operati ng conditions, the contribution from the background electrolyte signal is not negligible and must be subtracted from the measured value to a rrive at the particle ESA value. Background electrolyte corrections we re 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 sig nal for the solid decreases and the background signal increases with i ncreasing ionic strength. For 0.1 M ionic strength, the measured ESA c hanges sign around pH 4.7 for the Geltech silica and near pH 5.6 for n o. 40 quartz. This suggests that silica particles were positively char ged below pH 4.7 and 5.6, respectively. However, the background-correc ted ESA data correctly show that the solids do not change sign at leas t down to a pH of 2.