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Thermodynamics of redox reactions between oxides of glass melt and oxygen : I. Principles of thermodynamic analysis of redox reactions in melts of oxide glasses

The paper discusses some principles of the thermodynamic approach to analysis of oxidation-reduction reactions between polyvalent element oxides in a glass melt and oxygen. Consideration is given with due regard for the well-known concept according to which the structure of substances involved in different chemical transformations plays no part in the thermodynamic analysis of these processes. The equation of redox reactions x(Red) m +zO 2 =y(Ox) m is applicable to all redox reactions between oxides of a polyvalent element in the lowest (Red) and highest (Ox) valence states and oxygen. It is demonstrated that the redox reactions in glass melts can be classified into three groups depending on the value of x and y: (1) x=y=1, (2) x=1 and y=2, and (3) x=2 and y=1. The equation of reaction is treated within the framework of the mass action law. The equations that describe the dependences of the ratio q=x Ox y /x Red x (where x Ox and x Red are the mole fractions of Ox and Red oxides, respectively) on temperature, partial pressure of oxygen P O2 , and total concentration of oxides x M =x Ox +x Red are obtained from the equations of the mass action law and formulas that relate the activity coefficient γ Ox and γ Red to the enthalpies of dissolution of the above oxides in a melt (Δh Ox and Δh Red ) and also the enthalpy of reaction ΔH T to the standard enthalpie ΔH T 0 and to the enthalpies Δh Ox and Δh Red . These equations are deduced taking into account that the activity coefficients γ Ox and γ Red depend not only on temperature, but on P O2 and x M as well. The equations relating the ratio RC=x Ox /x Red to the quantities q and x M are also derived. It is shown that, at q=const, an increase in the total concentration of oxides x M causes the RC ratio to decrease for the second group of redox reactions and to increase for the third group of reactions. Applications of the equations obtained to the thermodynamic analysis of particular redox reactions in oxide glass melts will be reported in separate papers.