Interfacial Acid-Base Reactions of Aluminum Oxide Dispersed in Aqueous Electrolytee Solutions. 1. Potentiometric Study on the Effect of Impurity and Dissolution of Solid Phase

摘要

A pyrogenic alumina (type C, Degussa.) was studied. The original product contains chlorine impurity (~1 x 10-4 mol/g), the hydrolysis of bound chlorine results in acidic species in aqueous suspension. Heat treatment at 1000 °C proved to be an effective purification. Potentiometric acid-base titration was used to characterize the surface charging of alumina in electrolyte solutions (KNO3, KCl). This method can measure only the H+/OH- depletion in bulk liquid phase. Trace amounts of impurity and additional acid- base reactions, such as dissolution of solid, affect the net H+/OH- consumption. We analyzed these effects on proton adsorption curves. The fitting (FITEQL) of titration data for purified alumina in KNO3 solutions between pH ~5 and ~9 is good enough with assuming surface charging reactions only and using any surface complexation model. In KCl solutions, however, only the triple layer model resulted in an acceptable optimization, presumably because of the specific adsorption of Cl- ions. Outside these material-specific limits of pH, asymmetric proton binding curves appear which are often interpreted as a result of specific ion adsorption or surface site heterogeneity. In fact, the dissolution of the amphoteric solid plays the governing role, especially below pH ~4.5. The experimental curves over the range ofpH 3-10 can be fitted well, if the partial dissolution of alumina and the solution equilibria ofmononucleus Al-species formation with literature log K values are inserted into the model, in addition to the surface complexation. The assumed chemical model can be supported only by an independent analysis of equilibrium liquid phase. The concentration of aluminum in the equilibrium supernatants with different pH values between ~4 and ~10 were measured by means of the inductively coupled plasma method. The measured amounts coincide strikingly well, especially in the alkaline region, with the calculated total concentration of Al species. ,. which provide indisputable evidence for the reality of the assumed chemical model.