Other solvents based on alkali-metal halides

7 Other solvents based on alkali-metal halides 

Inyushkina et al. investigated the solubility of MgO in individual molten alkali-metal chlorides CsCl (700-850 °C), KC1 (820-1000 °C), NaCl (850-1100 °C), and RbCl (750-900 °C) [377]. The study was performed by the method of isothermal saturation MgO was added to the melt in the form of the ordinary commercial powder, or of pills pressed at p = 80 MPa and calcined at 1250 °C. Saturation of the solutions with respect to MgO solubility was shown in Ref. [377] to be achieved after keeping the oxide in contact with the melt for 4 h. 

The solubility of the powdered MgO in molten KC1 was 140-420 times as high as that for the oxide which had been subjected to pressing and preliminary thermal treatment. This was explained by the formation of suspensions in the solutions contacting with the powdered oxide. The slope of the dependence of the solubility of MgO pressed in pills, against the inverse temperature, is three times as large as that for the powdered oxide, in molten 

The dependence of MgO solubility (log /VMg0) on the free volume of the melts studied shows that the solubility of this oxide proceeds according to the physical mechanism. In other words, the dissolution process consists, mainly, of filling voids in the melt s bulk by the oxide molecules, and only then by the following process of oxide dissociation. On the basis of the solubility data, the authors determined the enthalpy change upon dissolution of MgO as the slope of the dependence of solubility on the inverse temperature  

The values of AHs determined from the constructed plots are 54 (NaCl), 120 (KC1), 200 (RbCl) and 224 (CsCl) kJ mol-1. The experimental data show that there is no appreciable interaction of the dissolved MgO with the ionic solvent, i.e. from the viewpoint of chemical thermodynamics, the formation of Mg2+ + O2- ions in the melt is less favourable than the dissolution of the solid in the form of MgO. 

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