Information Capability of Potential pH Diagrams and Speciation of Metal Ions
Department of Technical Sciences, J. E. Purkyně University, Ústí nad Labem
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 106 mol dm3) predicts an area of a stable passive layer of amorphous Zn(OH)2 in the pH range 8.8510.86, 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 dm3), the passive layer was not formed at all.
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