Vapour pressure and standard enthalpy of sublimation of KBF4 by a TG based transpiration technique

A horizontal thermobalance was adapted as a transpiration apparatus for the measurement of the vapour pressure of KBF4 (s). Attainment of equilibrium was ascertained by the invariance of the measured values of the vapour pressures over a range of flows under isothermal conditions. Measured values of the vapour pressures could be represented by the least-squares expressions: log (p/Pa) = 8.16(±0.01) – 4892(±248)/T(K)(538–560 K), log (p/Pa) = 6.85(±0.06) – 4158(±240)/T(K) (576–660 K), which correspond to the equilibria of orthorhombic and cubic KBF4 vapours, respectively. From these expressions the temperature of transformation of the orthorhombic to the cubic phase was identified to be 561 K. From the slopes of the above equations, the enthalpies of sublimation of the orthorhombic and cubic phases were found to be (93.7 ± 4.7) and (79.6 ± 4.6) kJ mol−1, respectively. These values differ by 14.1 kJ mol−1 which could be ascribed to the enthalpy of the orthorhombic to cubic phase transition of KBF4. Third-law analysis of the vapour pressure data yielded a value of (104.6 ± 1.0) kJ mol−1 for Δ H sub o of KBF4 (s) at 298.15 K.