Propriétés chimiques et électrochimiques en solution dans les hydroxydes alcalins fondus-IV. Comportement électrochimique de quelques métaux utilisés comme électrodes indicatrices

Abstract

The electrochemical behaviour of metals used as indicator electrodes-nickel, platinum, gold and silver-in a melt of sodium and potassium hydroxides at 227°C has been studied by voltammetry. The curves obtained have been related to the redox and acid-base properties of the oxidation products of the metals, in solution in the melt. Nickel in acid (hydrated) medium oxidizes successively to Ni2+ (solvated by OH-) and NiO↓, then to NiO2↓. In basic medium, oxidation gives NiO22-, then NiO2↓. Analysis of the electrochemical curves permits the determination of the acidity constants of Ni2+ and NiO, and thus the establishment of an approximate potential/pH2O diagram showing the zones of existence of each species of nickel. Platinum behaves analogously to nickel, but is less reducing. Gold gives evidence of two oxidation states: Au(I) in basic medium, Au(III) in all media. An approximate potential/pH2O diagram is established. Silver is difficult to oxidize; the Ag+ formed is a feebly acid cation, but very oxidizing. At high temperatures it oxidizes the solvent ions; silver is then unoxidizable. © 1967.