The effect of alumina on the Sn 2þ /Sn 4þ redox equilibrium and the incorporation of tin in Na 2 O/Al 2 O 3 /SiO 2 melts Darja Benne a , Christian R€ ussel a, * , Markus Menzel b , Klaus Dieter Becker b a Otto-Schott-Institut, Jena University, Fraunhoferstr. 6, 07743 Jena, Germany b Institut f€ ur Physikalische und Theoretische Chemie, Hans-Sommer-Str. 10, 38106 Braunschweig, Germany Received 11 December 2003; received in revised form 10 March 2004 Abstract Glasses with the basic compositions 10Na 2 O ® 10CaO ® xAl 2 O 3 ® (80 ) x)SiO 2 (x ¼ 0, 5, 15, 25) and 16Na 2 O ® 10CaO ® xAl 2 O 3 ® (74 ) x)SiO 2 (x ¼ 0, 5, 10, 15, 20) doped with 0.25–0.5 mol% SnO 2 were studied using square-wave-voltammetry at tem- peratures in the range from 1000 to 1600 °C. The voltammograms exhibit a maximum which increases linearly with increasing temperature. With increasing alumina concentration and decreasing Na 2 O concentration the peak potentials get more negative. M€ ossbauer spectra showed two signals attributed to Sn 2þ and Sn 4þ . Increasing alumina concentrations did not affect the isomer shift of Sn 2þ ; however, they led to increasing quadrupole splitting, while in the case of Sn 4þ both isomer shift and quadrupole splitting increased. A structural model is proposed which explains the effect of the composition on both the peak potentials and the M€ ossbauer parameters. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction The redox behavior of tin in molten glass is of great importance for glass technology. This is especially due to the float glass process which in the past few years, proved to be a versatile process enabling the processing of glass melts of various compositions. In the float glass process, metallic liquid tin is in contact with the melt and to a small extent, is oxidized and subsequently dif- fuses into the glass [1,2]. Here, an interaction with other polyvalent elements such as iron or chromium takes place.Athightemperature,tinoccursinareducedstate, Sn 2þ , and an oxidized state, Sn 4þ , which are in equi- librium with the physically dissolved oxygen of the melt [3–6]. Sn 4þ þ O 2 ¢ Sn 2þ þ 1 2 O 2 : ð1Þ An equilibrium constant, K , can be defined which de- pends on temperature as well as on the glass composi- tion. K ðT Þ¼ a Sn 2þ a 1=2 O 2 a Sn 4þ a O 2 ð2Þ with a i is the activity of the species i. If the temperature dependence of K ðT Þ or, however, that of the standard potential, E 0 , is known, the calcu- lation of the standard enthalpy, DH 0 , and the standard entropy, DS 0 , can be calculated. DG 0 ðT Þ¼ zFE 0 ¼ DH 0  T DS 0 ¼RT lnðK ðT ÞÞ ð3Þ where DG 0 ðT Þ is the standard free enthalpy and z the number of electron transferred; R, T and F have their usual meaning. To study redox equilibria in melts, two basic methods are applied. In one of them, the melt is equilibrated with an atmosphere of well defined oxygen activity, quen- ched, and subsequently the concentrations of the respective reduced and oxidized species are analyzed physically or chemically [6–9]. As a variation, the melt is not equilibrated before quenching, but the oxygen activity is measured using an oxygen sensor [10,11] (ZrO 2 -probe). Another possibility, in the past decade frequently applied, is to use electrochemical methods, especially the square-wave voltammetry [3,4,12–16]. Here, maxima in the current-potential curves directly * Corresponding author. Tel.: +49-3641 948 501/636 105; fax: +49- 3641 948 502. E-mail address: ccr@rz.uni-jena.de (C. R€ ussel). 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.04.017 Journal of Non-Crystalline Solids 337 (2004) 232–240 www.elsevier.com/locate/jnoncrysol