A Revised Equation for Estimating the Vapour Pressure of Low-Volatility Substances from Isothermal TG Data

Author: Focke W.  

Publisher: Springer Publishing Company

ISSN: 1388-6150

Source: Journal of Thermal Analysis and Calorimetry, Vol.74, Iss.1, 2003-10, pp. : 97-107

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Abstract

Previous investigators have used the Langmuir vaporisation relation to estimate the vapour pressures of low-volatility compounds from thermogravimetric data. However, this equation is strictly valid for evaporation into a vacuum only. For measurements conducted at finite pressures, molecular diffusion must be taken into account. A revised equation is proposed: dm A/dt8m A P A D A B/t. It is also shown that the proportionality between vapour pressure and vaporisation rate is very general. It arises from the assumptions of ideal gas behaviour, Raoult's law and a negligible concentration of the sample compound far from the sample surface.