Correcting “Static” Measurements of Vapor Pressure for Time Dependence Due to Diffusion and Decomposition

Abstract

The static method for measuring vapor pressure assumes that the sample is pure and that its temperature is steady and uniform. In practice, the measured pressure may be time dependent due to evaporative cooling after pumping on the sample, transpiration of the sample in a temperature gradient, or diffusion of an impurity out of the sample. An impurity cannot be avoided if the sample is decomposing. This article identifies and quantifies various causes of time dependence, and it includes an analysis that can obtain the vapor pressure from the time-dependent pressure of a decomposing sample. The analysis was applied to measurements of TEMAH (tetrakisethylmethylaminohafnium), whose decomposition continuously generated a volatile impurity. The corrected vapor pressures obtained for three TEMAH samples at 39 °C agreed to within ±1 %, even though the partial pressure of the impurity was as much as 7 times larger.

Apparatus To Measure the Vapor Pressure of Slowly Decomposing Compounds from 1 Pa to 105 Pa

“…For naphthalene, ferrocene, and diethyl phthalate the vapor pressure P v was defined to be the pressure measured when the condensed sample was in approximate equilibrium with its vapor. This occurred after the sample temperature had recovered from evaporative cooling, as discussed in [17], and before a significant amount of impurity had diffused out of the sample into the vapor. The P v values for these three compounds are tabulated in Section 5.2.…”

“…The stirrer was a stainless steel ball bearing inside the sample tube that was periodically lifted and dropped by an external magnet attached to a gearhead motor. Stirring caused only a small improvement of the degassing, perhaps because boiling was already stirring the sample [17]. …”

“…Table 2 gives example values of D and τ D for various liquid mixtures. See the companion article [17] for details.…”

“…The sample’s time constant for thermal equilibration was expected to be only about 100 s [17]. However, that estimate, which assumed good contact with the container wall, may have been too small for a solid sample.…”

“…As discussed in [17], transpiration will cause the difference between the measured pressure P and the vapor pressure P V ( T 0 ) at the container’s nominal temperature T 0 to be approximately…”