Technical Note: Revised EMF-pH Diagram for Nickel Available to Purchase

Thermodynamic EMF-pH diagrams currently used for the nickel-water system contain most of the same nickel compounds that are in Pourbaix’s atlas.¹  Thermodynamic, spectroscopic, and electrochemical characterization studies^2-10  conducted since the original compilation have suggested that some of the compounds, especially those containing nickel with valence greater than two, either do not exist or exist in a different form. The purpose of this note is to present an EMF-pH diagram which contains those nickel compounds which are consistent with the results of the characterization studies.

Table 1 shows the nickel compounds used in the original diagrams.¹  Hydrated NiO, Ni₃O₄, Ni₂O₃, and NiO₂ were noted to correspond to Ni(OH)₂, Ni₃O₄·2H₂O, Ni₂O₃·H₂O, and NiO₂·2H₂O.¹  Only two ionic species, Ni⁺² and HNiO...