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, , of H₃BO₃ was estimated to be 174.1 ± 4.7 kJ mol⁻¹ at the mean temperature of the present measurements, viz., 345 K.