Potentiometric mass titrations: Experimental and theoretical establishment of a new technique for determining the point of zero charge (PZC) of metal (hydr)oxides

In this paper, we present a novel methodology, called the potentiometric mass titration (PMT) technique, for determining the point of zero charge (pzc) of mineral hydr(oxides) immersed in electrolytic solutions. Following PMT, the pzc is identified as the common intersection point (CIP) of the potentiometric curve of the blank solution with the corresponding curves of the impregnating suspensions containing different amounts of the immersed mineral (hydr)oxides. Full experimental results related to the determination of pzc using the PMT technique and four traditional techniques (potentiometric titrations, mass titrations, immersion, and micro-electrophoresis (for determining the isoelectric point, equal to pzc in cases where no specific adsorption takes place)) are presented for four oxides, namely, MgO, gamma-Al2O3, TiO2, and SiO2. The comparison of the pzc values determined by PMT, with the corresponding ones determined using the traditional methodologies, strongly suggested that the PMT technique can be used to determine the pzc of oxides. A simulation procedure of the PMT technique has been developed and applied to model oxides with properly selected acid-base characteristics and to various combinations of models related to the charging mechanism of the oxide surface (1 site/1 pK, 1 site/2 pK, multisite models) and to the description of the interfacial region (diffuse double layer, constant capacitance, basic Stern models). The intensive application of this simulation procedure offered a quantitative interpretation of the methodology. Specifically, it was demonstrated that (a) the application of the quick scan version of the PMT technique, realized by recording the titration curve of the blank solution (pH vs V-added (acid)) and the corresponding curve of a suspension of a given amount of the immersed oxide, indeed results in the determination of the pzc, provided that this is greater than a value of about 4; (b) the application of the typical version of PMT, realized by recording the titration curves of three different suspensions (pH vs V-consumed (acid)) containing different masses of the immersed oxide, provides the pzc value of this oxide over the whole pH range; and (c) the CIP that is determined, using PMT, corresponds to the pzc irrespective of the charging mechanism of the oxide surface and the structure of the double layer developed between the oxide surface and the solution. However, in the case where the basic Stern model is used to describe the interfacial region, the pzc value determined by PMT deviates slightly from the true value when the value of the affinity constants of the ion pairs formed between the positive counterions and the surface is different than the corresponding value of the negative counterions.