Thermodynamics of various polyvalent elements in a 15Na2O · 85SiO2 glass melt

Glass melts with the basic composition of 15Na2O · 85SiO2 doped with various polyvalent elements were studied by square-wave voltammetry. In the melts, the thermodynamics of the redox pairs Sb5+/Sb3+, As5+/As3+, Sn2+/Sn0, Sb3+/Sb0, As3+/As0, Cu+/Cu0, V5+/V4+, Fe3+/Fe2+, Cr3+/Cr2+, V4+/V3+and Ti4+/Ti3+were studied quantitatively. The temperatures applied were 1000–1550°C. Two temperature ranges can be distinguished: one range from 1000°C to 1300°C and another one from 1300°C to 1550°C. In both ranges, the peak potentials depended linearly on temperature. However the slope was smaller in the high temperature range. As proved by XRD and DTA, this is caused by the crystallization of cristobalite at temperatures ⩽1300°C. This leads to an increase in the alkali content of the residual melt which then exhibits smaller standard potentials than the 15Na2O · 85SiO2 melt. This is shown by means of a direct comparison of the standard potentials with those determined in a melt with the basic composition of Na2O · 2SiO2.