Apparatus for determination of vapor pressures at ambient temperatures employing a Knudsen effusion cell and quartz crystal microbalance

We describe an apparatus for the measurement of vapor pressures of solids or liquids at ambient temperatures (260 K < T < 350 K) demonstrated to operate over a pressure range of 10−2–10−6 Torr (1–10−4 Pa), but capable of being extended to lower pressures by at least an order of magnitude. It employs a Knudsen cell to produce an effusive molecular beam, only a small fraction of which is deposited on a cooled (225 K) quartz crystal microbalance (QCM). Vapor pressure values are derived from the mass gain rate determined by the QCM, Knudsen cell temperature, and fixed geometric factors. The accuracy and precision of the measurements are improved by locating the apparatus within an ultrahigh vacuum chamber (base pressure <10−9 Torr) with high pumping speed (2000 l s−1). A beam flag is used to interrupt the beam and allow for the subtraction of signal caused by the deposition of background molecules. The temperatures of both the microbalance and Knudsen cell are controlled to ±0.2 K using thermoelectric heaters/coolers. Measurements of the vapor pressure of benzoic acid, used as a primary reference material, agreed with literature reports over the entire temperature range to within the expected experimental uncertainty. In addition, the vapor pressure curves as a function of temperature (∼270–340 K) were determined for four isomers of dinitrotoluene (DNT). These curves can be readily expressed using the Clausius–Clapeyron relationship: log 10 P(Torr) = a - b/T(K), where a and b coefficients are listed below. Also listed is the heat of sublimation derived from these data. (Uncertainties are quoted as 2σ.)