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

The surface charge density (so) 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 mL 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, so increases with increase in electrolyte concentration (Co) whereas above PZC, so decreases with increase in Co. It was concluded that so of this material is a function of pH and Co and that its polarity can be varied through zero by varying these parameters. KEY WORDS: Alumina, Surface charge density, Acid-base titration, Point of zero charge Bull. Chem. Soc. Ethiop. 2007, 21(1), 117-128.