Determination of equilibrium state and Sn redox ratio in aluminoborosilicate glass melts by potentiometry and voltammetry

Over the last three decades many studies have reported the investigation of the redox behaviour of multivalent elements (M) in glass melts by voltammetry. In these works the redox ratio at any temperature, described as ([Mx+]/[M(x+n)+]) or the concentration %[Mx+], was determined under the assumption that the melt was equilibrated with air (PO2 = 0.21 bar). Some potentiometric studies have reported the determination of PO2 in glass melts, however no significant attempts have been made to determine the actual redox ratio using a measured PO2 combined with a corresponding voltammogram. Herein, potentiometry and square wave voltammetry of alkali-lean alumino-borosilicate E-model glass melts doped with Sn were performed sequentially at the same equilibrium state. The actual redox ratios of the main pair, [Sn2+]/[Sn4+], derived from the peak potential and oxygen activity, were higher than those at PO2 = 0.21 bar. The temperature dependence of the values converted to %[Sn2+] showed that the actual concentration of Sn2+ in the melts was 3–6% higher than expected at PO2 = 0.21 bar. These results are discussed in terms of the fining behaviour of melts. Keywords: Redox ratio, Multivalent element, Glass melts, Oxygen activity, Square wave voltammetry, Melt fining