Information Capability of Potential pH – Diagrams and Speciation of Metal Ions

Abstract

Some limitations of potential – pH diagrams were demonstrated on a simple Zn – H₂O system. Inaccuracies arise due to the fact that the concentration of only one (prevailing) ion was predetermined for calculation of the solid-solution equilibria. As a result, the E – pH diagrams differ from the distribution diagrams as the former consider all kinds of Zn species. For example, the E – pH diagram of the Zn – H₂O system (total concentration of Zn species 10^–6 mol dm^–3) predicts an area of a stable passive layer of amorphous Zn(OH)₂ in the pH range 8.85–10.86, whereas the distribution calculations of individual Zn species exclude such a possibility. The presence of some substances (hydrated metal oxides, minerals, humic substances) may crucially affect the interpretation of the E – pH diagrams. It was shown that the presence of even low concentrations of humic acids changed the pH range of the existence of passive layer, and at their higher concentrations (e.g. 0.025 g dm^–3), the passive layer was not formed at all.