ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

Thermodynamic Equilibrium between Melt and Crystalline Phase of a Compound AgBrwith Dystectic Melting Point. III. Application to the Systems KF-K₃TiF₇ and KCl-K₃TiF₆Cl

I. Proks and V. Daněk

Institute of Inorganic Chemistry, Slovak Academy of Sciences, SK-842 36 Bratislava

Abstract: The special form of the LeChatelier-Shreder equation describing the equilibrium between crystalline phase and melt in the system A-AB when substance AB partially dissociates upon melting was applied in the systems KF-K₃TiF₇ and KCl-K₃TiF₆Cl. Because all the three characteristic parameters, i.e. the dissociation degree of the dystectic melting binary compounds, their enthalpy of fusion, and their phase diagrams, were determined experimentally, the LeChatelier-Shreder equation was used in order to determine the thermodynamic consistency of these quantities and thus also their reliability.Using two known phase diagrams of the system KF-K₃TiF₇ and the measured enthalpy of fusion of K₃TiF₇ the value of the dissociation degree of K₃TiF₇ was calculated. The value 0.720 was calculated on the basis of the phase diagram measured by Chernov et al. while the value 0.404 resulted from that measured 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 degree the enthalpy of fusion of K₃TiF₇ was calculated. The value 62 446 J mol^-1 was obtained using the dissociation degree 0.65, while the value 60 623 J mol^-1 corresponds to the dissociation degree 0.67. The calorimetrically measured value is (57 000 +/- 2000) J mol^-1.Using the lowest measured eutectic temperature in known phase diagrams of the system KCl-K₃TiF₆Cl (925 K) and the calorimetrically determined enthalpy of fusion Of K₃TiF₆Cl, the composition of the eutectic point χ_eut(K₂TiF₆) = 0.314 was calculated, while the measured values axe 0.250, 0.297, and 0.307.

Full paper in Portable Document Format: 553a137.pdf

Chemical Papers 55 (3) 137–141 (2001)