Studies of NiO dissolved in alkali silicate melts based on redox potential and visible absorption spectra

TLDR: In this paper, the half-wave potential of Ni2+Ni0 depends on the alkali species and contents, and the relative ratio of tetrahedral to octahedral species of Ni 2+ also depended on the acidity of the acid.

Abstract: The redox behavior of Ni2+ ions in alkali silicate melts, xR2O · (1 - x)SiO2, (R = Na, K and Cs and x in the range 0.175 – 0.45), was investigated using linear sweep voltammetry and differential pulse polarography. The half-wave potential of Ni2+Ni0 depends on the alkali species and contents. It shifted to the positive side when the alkali species changed from sodium through potassium and cesium at a fixed ratio of alkali oxide to silica, indicating that NiO loses its thermodynamic stability in this order. Based on visible absorption spectra, it is shown that the Ni2+ ion exists in tetrahedral and octahedral coordinations in the alkali silicate glasses. The relative ratio of tetrahedral to octahedral species of Ni2+ also depends on the alkali species, and the content of the tetrahedral species of Ni2+ in potassium or cesium silicate is greater than that in sodium silicate. From these two results, it is assumed that, while NiO dissolves in the silicate melts as a weak base, some Ni2+ ions change into a tetrahedral coordination in a strongly basic solvent such as cesium silicate.