Interfacial acid-base reactions of aluminum oxide dispersed in aqueous electrolyte solutions. 1. Potentiometric study on the effect of impurity and dissolution of solid phase

A pyrogenic alumina (type C, Degussa) was studied. The original product con tains chlorine impurity (similar to1 x 10(-4) mol/g), the hydrolysis of bou nd chlorine results in acidic species in aqueous suspension. Heat treatment at 1000 degreesC proved to be an effective purification. Potentiometric ac id-base titration was used to characterize the surface charging of alumina in electrolyte solutions (KNO3, KCl). This method can measure only the H+/O H- depletion in bulk liquid phase. Trace amounts of impurity and additional acid-base reactions, such as dissolution of solid, affect the net H+/OH- c onsumption. We analyzed these effects on proton adsorption curves. The fitt ing (FITEQL) of titration data for purified alumina in KNO3 solutions betwe en pH similar to5 and similar to9 is good enough with assuming surface char ging reactions only and using any surface complexation model. In KCI soluti ons, however, only the triple layer model resulted in an acceptable optimiz ation, presumably because of the specific adsorption of Cl- ions. Outside t hese material-specific limits of pH, asymmetric proton binding curves appea r which are often interpreted as a result of specific ion adsorption or sur face site heterogeneity. In fact, the dissolution of the amphoteric solid p lays the governing role, especially below pH similar to4.5. The experimenta l curves over the range of pH 3-10 can be fitted well, if the partial disso lution of alumina and the solution equilibria of mononucleus A1-species for mation with literature log K values are inserted into the model, in additio n to the surface complexation. The assumed chemical model can be supported only by an independent analysis of equilibrium liquid phase. The concentrat ion of aluminum in the equilibrium supernatants with different pH values be tween similar to4 and similar to 10 were measured by means of the inductive ly coupled plasma method. The measured amounts coincide strikingly well, es pecially in the alkaline region, with the calculated total concentration of A1 species, which provide indisputable evidence for the reality of the ass umed chemical model.