Electrophoresis of colloidal α-alumina

Measurements of the electrophoretic mobility (uE) of particles of colloidal α-alumina were made as a function of pH, electrolyte concentration and electrolyte type (NaCl, NaNO3 and KCl) using two similar instrumental techniques. Significant differences (50% or less) in the values of uE of particles in NaCl were obtained from the two instruments; however, the isoelectric points (IEPs) (the pH at which uE=0), estimated from the two sets of measurements, occurred at 7.5 ± 0.3 and 7.8 ± 0.05 and were not significantly different. The latter estimate corresponds with those for particles in KCl and NaNO3 of 8.05 ± 0.11 and 7.95 ± 0.18, respectively, made using the same instrument and indicate that the IEP was a weak function of electrolyte type. When cations acted as counterions (pH > IEP), the absolute magnitudes and the ranges of uE with electrolyte concentration were found to be significantly less than when anions acted as counterions (IEP > pH). Estimates of the zeta potential (ζ), made using various procedures, showed variations of up to 25% at low ratios of electrical-double-layer thickness (κ−1) to particle radius (a) (κa∼10) and were of a similar scale to differences in uE, but no significant variations (95% confidence) in ζ were obtained at high values (κa∼200).