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

Determination of dissociation degrees of K₃NbF₈ and K₃TaF₈ by thermodynamic analysis of subsystems of the KF-K₂NbF₇ and KF-K₂TaF₇ systems

Ladislav Kosa and Ivo Proks

Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, SK-845 36 Bratislava, Slovakia

E-mail: uachkosa@savba.sk

Received: 20 December 2007  Revised: 6 March 2008  Accepted: 17 March 2008

Abstract: A special form of the LeChatelier-Shreder equation describing the equilibrium between the crystalline phase and the melt in system A-AB in which the substance AB partially dissociates upon melting was applied to systems KF-K₃NbF₈, K₂NbF₇-K₃NbF₈ and to KF-K₃TaF₈, K₂TaF₇-K₃TaF₈ subsystems of the binary systems KF-K₂NbF₇ and KF-K₂TaF₇ in which the additive compounds K₃NbF₈ and K₃TaF₈ are formed. Using the phase diagram of the system KF-K₂NbF₇ determined by McCawley and Barclay (1971) and the values of the fusion enthalpy of K₃NbF₈ taken from literature, the intervals of the dissociation degree values of K₃NbF₈ for both branches of the liquidus curve of K₃NbF₈ were calculated. The calculated values of the dissociation degree depend on the coordinates of the liquidus curve of K₃NbF₈ of the pertinent phase diagram, on its used branch and section, and on the value of the fusion enthalpy of K₃NbF₈. For the measured fusion enthalpy of K₃NbF₈ (57 kJ mol⁻¹), a common interval of the dissociation degree values of K₃NbF₈ for both branches of the liquidus curve of K₃NbF₈ is 0.71–0.72. Similarly, intervals of the dissociation degree values of K₃TaF₈ for both branches of the liquidus curve of K₃TaF₈ were calculated using the phase diagram of the system KF-K₂TaF₇ determined by Boča et al. (2007) and the measured fusion enthalpy of K₃TaF₈ ((52 ± 2) kJ mol⁻¹). The error of the determination of the fusion enthalpy of K₃TaF₈, the common interval of the dissociation degree values of K₃TaF₈ for both branches of the liquidus curve of K₃TaF₈ is 0.68–0.69.

Keywords: K₃NbF₈  - K₃TaF₈  - dissociation - thermodynamic analysis - binary systems

Full paper is available at www.springerlink.com.

DOI: 10.2478/s11696-008-0064-5

Chemical Papers 62 (6) 616–622 (2008)