A literature survey of the differences between the reported isoelectric points and their discussion

Different aspects of discrepancies in the pristine point of zero charge PZC and pristine isoelectric point IEP of materials having the same chemical formula are discussed. A multiinstrument study of the IEP of alumina gave a consistent IEP. The values of electrokinetic potential of two silicas at pH>4 determined by six different zetameters were also relatively consistent. On the other hand, different zetameters produce an IEP of silica ranging from <2 (if any) to about 4. The instruments producing high IEP for fumed silica give also a high value for quartz. Analysis of the PZC and IEP of titanium, aluminum and iron oxides and hydrous oxides reported in the literature gave a nearly Gaussian distribution for each type of materials. There was no systematic effect of the crystalline form (e.g. rutile vs. anatase) and degree of hydration (e.g. Fe2O3 vs. FeOOH) on the position of the IEP, and the IEP confirmed by other methods (e.g. potentiometric titration) fall in relatively narrow pH range for titanium, aluminum and iron oxides. This suggests that the outstanding results reported in some publications are due to insufficient purity of materials or other experimental errors rather than to actual differences in the surface properties of the samples studied. In contrast, the distribution of PZC/IEP reported in the literature for various samples of zirconium oxide is rather broad, and it does not assume a Gaussian shape. Even the PZC obtained as matching values of IEP and common intersection point of titration curves are very scattered for zirconia. Apparently the crystalline form for commercially available zirconias has no systematic effect on their PZC/IEP, but synthetic monoclinic zirconia has a substantially higher PZC/IEP than other samples of zirconia. The shift in the IEP of oxides at high ionic strengths (>0.1 mol dm−3) to high pH was detected using two different instruments based on quite different principles of operation. The shift in the IEP is accompanied by the shift in the maximum in the viscosity versus pH curves. The salt specificity, namely, substantial shift in the IEP for small cations and large anions, and rather insignificant shift for large cations and small anions is observed for different solids. Probably specific adsorption of small cations is caused by interactions in solution rather than ion-surface interactions.