Thermodynamic equilibrium between melt and crystalline phase of a compoundA(q)B(r) with dystectic melting point III. Application to the systems KF-K3TiF7 and KCl-K3TiF6Cl

The special form of the LeChatelier-Shreder equation describing the equilib rium between crystalline phase and melt in the system A-AB when substance A B partially dissociates upon melting was applied in the systems KF-K3TiF7 a nd KCl-K3TiF6Cl. Because all the three characteristic parameters, i.e. the dissociation degree of the dystectic melting binary compounds, their enthal py of fusion, and their phase diagrams, were determined experimentally, the LeChatelier-Shreder equation was used in order to determine the thermodyna mic consistency of these quantities and thus also their reliability. Using two known phase diagrams of the system KF-K3TiF7 and the measured ent halpy of fusion of K3TiF7 the value of the dissociation degree of K3TiF7 wa s calculated. The value 0.720 was calculated on the basis of the phase diag ram measured by Chernov et al. while the value 0.404 resulted from that mea sured by Chrenkova et al. The experimentally determined values are 0.65 or 0.67. From the same phase diagrams and the measured values of the dissociation de gree the enthalpy of fusion of K3TiF7 was calculated. The value 62 446 J mo l(-1) was obtained using the dissociation degree 0.65, while the value 60 6 23 J mol(-1) corresponds to the dissociation degree 0.67. The calorimetrica lly measured value is (57 000 +/- 2000) J mol(-1). Using the lowest measured eutectic temperature in known phase diagrams of t he system KCl-K3TiF6Cl (925 K) and the calorimetrically determined enthalpy of fusion Of K3TiF6Cl, the composition of the eutectic point chi (eut)(K2T iF6) = 0.314 was calculated, while the measured values axe 0.250, 0.297, an d 0.307.