The Significance of the Points of Zero Charge of Zirconium (Hydr)Oxide Reported in the Literature

Abstract

ABSTRACT The points of zero charge (PZC) of zirconium (hydr)oxide reported in the literature range from 4 to 8.6. A set of seventy-six PZC of zirconium (hydr)oxide was used to explore the effect of the origin and crystalline structure on the one hand and of the choice of the experimental method on the other. The average and median PZC was found at pH 6.5 when the entire data set was taken into account. Among the synthetic samples, monoclinic zirconium oxide has substantially higher PZC than other crystallographic forms of zirconium oxide, mixtures of different forms, and amorphous zirconium (hydr)oxide (or samples of unknown structure). But with commercially available samples, the effect of the crystalline form on the PZC was rather insignificant. The PZC obtained by means of titration (the common intersection point [CIP] of titration curves for three or more ionic strengths is identified with the PZC) and the PZC obtained by means of electroacoustic methods are substantially higher than the IEP from classical electrokinetic measurements. Interestingly, a few PZC of zirconium oxide obtained as matching CIP and IEP are very scattered, thus their significance as the “most reliable PZC” is rather limited.