Abstract
The Fe2+/Fe3+ and Mn2+/Mn3+ redox equilibria in the Na2O-B2O3 melts have been individually measured as a function of the melt compositions, temperatures, oxygen partial pressures, and dopant element concentrations. The mutual interaction of these two redox couples has been studied by electron spin resonance (ESR) in the Na2O·2B2O3 melts.The redox ratios of Fe and Mn in the Na2O-B2O3 melts tend to decrease with increasing the dopant concentration. A plot of log [(M2+)/(M3+)] vs. -log PO2 indicated a straight line of the slope of 8 and 4 for Fe and Mn, respectively. The values for the reduction enthalpy of Fe and Mn in Na2O-B2O3 melts were found to be 80-120 kJ/mol and 40-60 kJ/mol, respectively. The redox ratio of Fe and Mn decreased with increasing the XNa2O/XB2O3 ratio. It was shown by ESR spectroscopy that the electron exchange reaction: Fe2++Mn3+=Fe3++Mn2+ proceeded to the right.
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