Vapour pressure and standard enthalpy of sublimation of H3BO3

A horizontal thermal analysis instrument was adapted as a transpiration apparatus for the measurement of vapour pressure of solid boric acid, H3BO3. The experimental parameters necessary for establishing a dynamic isothermal congruent vapourisation equilibrium of H3BO3 were identified. Using these optimized transpiration experiments, the vapour pressures were measured in the temperature range 326–363 K. The temperature dependence of the measured values of vapour pressures could be expressed using the expression, log(p/Pa) = 26.83(±0.09) − 9094(±246)/T   (K). The standard enthalpy of sublimation, ΔHsub0, of H3BO3 was estimated to be 174.1 ± 4.7 kJ mol−1 at the mean temperature of the present measurements, viz., 345 K.