Isoelectric points and points of zero charge of metal (hydr)oxides: 50years after Parks' review

The pH-dependent surface charging of metal (hydr)oxides is reviewed on the occasion of the 50th anniversary of the publication by G.A. Parks: "Isoelectric points of solid oxides, solid hydroxides, and aqueous hydroxo complex systems" in Chemical Reviews. The point of zero charge (PZC) and isoelectric point (IEP) became standard parameters to characterize metal oxides in aqueous dispersions, and they define adsorption (surface excess) of ions, stability against coagulation, rheological properties of dispersions, etc. They are commonly used in many branches of science including mineral processing, soil science, materials science, geochemistry, environmental engineering, and corrosion science. Parks established standard procedures and experimental conditions which are required to obtain reliable and reproducible values of PZC and IEP. The field is very active, and the number of related papers exceeds 300 a year, and the standards established by Parks remain still valid. Relevant experimental techniques improved over the years, especially the measurements of electrophoretic mobility became easier and more reliable, are the numerical values of PZC and IEP compiled by Parks were confirmed by contemporary publications with a few exceptions. The present paper is an up-to-date compilation of the values of PZC and IEP of metal oxides. Unlike in former reviews by the same author, which were more comprehensive, only limited number of selected results are presented and discussed here. On top of the results obtained by means of classical methods (titration and electrokinetic methods), new methods and correlations found over the recent 50years are presented.