The solubility of metal oxides in alkali hydroxide melts and their mixtures with salts has been studied using the method of isothermal saturation at temperate range 673 – 873 K. The effect of temperature on the oxides solubility in melts are satisfactorily described by the linear Schroder – Le Chatellier equation ln C = A + B/T, where A and B are constants, C is concentration of oxide in melt (mole. fract.) and T is temperature (K).

Effective practical used of molten system alkali hydroxide – metal oxide depends on the knowledge of the thermodynamic properties of the system as whole as well as the partial thermodynamic properties of individual components of the system.

Thermodynamic calculations for molten binary and multicomponent systems based on that for saturated solutions meet the following equation: µ_sd = µ_sol. Since the µ_sol = µ⁰_a=1(T, p) + RTln a, where is a chemical potential of supercooled molten oxide and a is the activity of oxide in the melt relative to the supercooled molten oxide as a standard state, that lna = (µ_sd = µ⁰)/RT.

Thus the temperature dependence of the activity molten oxide will be described the equation below.

On the other side ln a = ln x + ln λ and G_i^ex = RT In λ , therefore using the experimental data on the solubility of the oxides and solving the above equation we have calculated the excess relative partial molar thermodynamic quantities, which are also listed below.

The results of calculations was showed that the systems molten alkali – oxide are characterized by the deviation from the state of ideal solutions that indicate of interaction between components of the solution.