Browsing by Author "Caley, William F."

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    Application of Polymer Theory to Silicate Melts The System MO + MF2 + SiO2
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (09-12), 1978) Masson, Charles R.; Caley, William F.
    Expressions are derived for activities of MO and MF2 as functions of composition in MO + MF2+ SiO2 melts. The treatment is based on the assumption that the melts consist of M2+ cations and O2–, F– and an array of silicate and fluorosilicate anions of general formula SinO3n+1–mF(2n+2–m)–m in thermodynamic equilibrium, where 1 ⩽n⩽∞ and 0 ⩽m⩽ 2n+ 2. As in previous treatments, the equilibrium between the oxide and silicate ions is described in terms of an equilibrium constant k, the value of which is independent of the chain length n of the polyions. The equilibrium between the fluoride, silicate, fluorosilicate and oxide ions is described in terms of a second equilibrium constant k′ the value of which is assumed to be independent of the degree of substitution of O– by F on the silicate chains. Theoretical curves are shown for the activity of PbO as a function of composition in PbO + PbF2+ SiO2 melts for the simple case in which k′= 0, i.e., for the situation in which PbF2 acts solely as a diluent. Comparison with experimental data shows that the value of k′ for this system is finite. Accurate determination of k′ requires a knowledge of the activities of both MO and MF2. Knowledge of the value of k′ allows the ion fraction of any species to be evaluated approximately in the range of composition for which the theory is applicable.
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    Thermodynamics and constitution of silicate melts. The system PbO + PbF2+ SiO2
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (09-12), 1978) Caley, William F.; Masson, Charles R.
    Activities of PbF2 in PbO + PbF2 and PbO + PbF2+ SiO2 melts with up to 40 mol% PbF2 were determined at 1073–1173 K by means of an electrochemical cell with CaF2 as solid electrolyte. The results, together with previous measurements of the activity of PbO, were interpreted in terms of a theory in which the anionic constitution of the melts is represented by O2–, F– and an equilibrium array of silicate and fluorosilicate ions of formula SinO3n+1–mF(2n+2–m)–m. The equilibrium constant k′ for the formation of fluorosilicate ions by the reaction F–+ SinO3n+1–mF(2n+2–m)–m= SinO3n–mF(2n+1–m)–m+1+ O2– was estimated to be 0.4 ± 0.025. This allowed the anionic distribution to be evaluated for basic melts. In addition to O2– and F–, the main anionic constituents in melts with XPbO/XSiO2 > 2.5 and XPbF2⩽ 0.25 are SiO4–4, SiO3F3–, SiO2F2–2, SiO3F–, Si2O6–7 and Si2O6F5–, with decreasing amounts of more highly fluorinated species. The volatile constituent SiF4 is also present in small amount and becomes increasingly significant as XSiO2/XPbO is increased. Substitution of PbO by PbF2 causes a net decrease in the mean chain length of the anions.