Determination of surface charge density of α-alumina by acid-base titration

The surface charge density (sigma(o)) of colloidal alpha alumina suspended in various 1:1 electrolytes was measured using acid-base titration. An autotitrator capable of dispensing accurately 25 0.1 mu L of titrant was used. The pH and temperature in the titration cell were monitored using single junction electrodes and platinum resistance thermometers, respectively. A constant supply of nitrogen gas in the, cell was used to maintain inert conditions. The whole set up was interfaced with a computer for easy data acquisition. It was observed that the material exhibits a point of zero charge (PZC), this occurred at pH of 7.8 +/- 0.1, 7.6 +/- 0.2, 8.5 +/- 0.1, 8.3 +/- 0.1 for NaCl, NaNO3, CsCl and CsNO3 systems, respectively. It was also observed that below PZC, a. increases with increase in electrolyte concentration (C-o) whereas above PZC, sigma(o) decreases with increase in C-o. It was concluded that sigma(o) of this material is a function of pH and C-o and that its polarity can be varied through zero by varying these parameters.