The design, construction, and testing of a new Knudsen effusion apparatus

https://doi.org/10.1016/j.jct.2005.08.013Get rights and content

Abstract

A new Knudsen effusion apparatus, enabling the simultaneous operation of nine effusion cells at three different temperatures, is fully described. The performance of the new apparatus was checked by measuring the vapour pressures, between 0.1 Pa and 1 Pa, over ca. 20 K temperature intervals of benzoic acid, phenanthrene, anthracene, benzanthrone, and 1,3,5-triphenylbenzene. The derived standard molar enthalpies of sublimation are in excellent agreement with the mean of the literature values available for these five compounds and with the recommended values for four of them.

Introduction

The Knudsen effusion method [1], [2], [3] is one of the most widely used methods for measuring the vapour pressures of crystalline organic compounds for pressures less than 1 Pa. In a typical effusion experiment, the crystalline sample is placed at the bottom of a cylindrical cell kept at a constant temperature and the vapour (assumed to be in equilibrium with the crystalline phase) is allowed to effuse through a small orifice located at the top of the cell into an evacuated space. At the temperature T, the mass m of the sample sublimed from the effusion cell, during the time period t, is related to the vapour pressure of the crystalline compound by the following equation:p=(m/Aowot)·(2πRT/M)1/2,where M is the molar mass of the effusing vapour, R is the gas constant, Ao is the area of the effusion orifice and wo is the transmission probability factor which is usually calculated by means of equation (2) or of equation (3) where l is the length of the effusion orifice and r its radius:wo={1+(3l/8r)}-1,wo={1+(l/2r)}-1.This method has been widely used by our Research Group for measuring the vapour pressures of several organic compounds using an effusion apparatus enabling the simultaneous operation of three effusion cells at each experimental temperature [4]. As each effusion cell has a different effusion orifice area, the obtained results may be checked for deviations from the equilibrium pressure. If the areas of the effusion orifices are not very different, the pressures calculated at each temperature for each effusion cell are usually equal to within experimental error. For some compounds, however, the calculated pressures systematically decrease with the increasing size of the effusion orifice indicating that the results may be affected by a low condensation coefficient value or by a self cooling effect [5], [6]. In this case, according to the equation developed by Whitman [7] and Motzfeldt [8], the equilibrium pressure at each temperature may be derived by plotting p against (pwoAo), to obtain the intercepts of the derived straight lines at zero area as the equilibrium pressures.

The new apparatus presented in this work enables the simultaneous operation of nine effusion cells, which may be controlled at three different temperatures, during one effusion experiment. By keeping the same temperature for each group of three effusion cells with different orifice areas, deviation of results from the equilibrium pressures at three different temperatures may be checked, simultaneously. So in one experimental run the equilibrium pressures at three different temperatures are determined.

Section snippets

The experimental apparatus and procedure

Besides the possibility of the simultaneous operation of nine effusion cells instead of only three, the main differences between the new effusion apparatus and the previous one are related to the control and measurement of the effusion temperature. The previous thermostatic oil or water bath has been replaced by temperature controlled aluminium blocks enabling experimental measurements between ambient temperature and ca. 480 K. The temperatures are measured using platinum resistance probes

Results

In order to test the quality of the results obtained with the new experimental apparatus, the vapour pressures of the following five compounds were measured over temperature intervals of ca. 20 K: benzoic acid and anthracene (recently recommended as primary standards for enthalpy of sublimation measurements [9]), phenanthrene and 1,3,5-triphenylbenzene (recently recommended as tertiary standards for enthalpy of sublimation measurements [9]), and benzanthrone for which we previously obtained

Discussion

Table 5 presents 22 literature results for the enthalpy of sublimation of benzoic acid. Some values of the vapour pressures, calculated at the temperature limits of the experimental temperature range used in this work, are also presented. There is an excellent agreement between the mean of the literature results and the results obtained in the present work for both the standard enthalpy of sublimation at T = 298.15 K and the vapour pressures. In 1974, Cox [37] recommended for benzoic acid the

Acknowledgements

Thanks are due to FCT (Fundação para a Ciência e a Tecnologia), Lisboa, Portugal, for financial support granted through Centro de Investigação em Química da Universidade do Porto (Group 5).

References (56)

  • M. Radomska et al.

    Thermochim. Acta

    (1980)
  • S.A. Kudchadker et al.

    J. Chem. Thermodyn.

    (1979)
  • M.A.V. Ribeiro da Silva et al.

    J. Chem. Thermodyn.

    (1995)
  • L.A. Torres-Gomez et al.

    Thermochim. Acta

    (1988)
  • O.T. Glukhova et al.

    Thermochim. Acta

    (1985)
  • S. Murata et al.

    J. Chem. Thermodyn.

    (1982)
  • C.G. de Kruif et al.

    J. Chem. Thermodyn.

    (1982)
  • M. Colomina et al.

    J. Chem. Thermodyn.

    (1982)
  • E. Morawetz

    J. Chem. Thermodyn.

    (1972)
  • J.S. Chickos et al.

    Thermochim. Acta

    (1998)
  • C.G. de Kruif

    J. Chem. Thermodyn.

    (1980)
  • B.F. Rordorf

    Chemosphere

    (1986)
  • M.A.V. Ribeiro da Silva et al.

    J. Chem. Thermodyn.

    (1999)
  • S.P. Verevkin

    J. Chem. Thermodyn.

    (1997)
  • L. Malaspina et al.

    J. Chem. Thermodyn.

    (1974)
  • M. Knudsen

    Ann. Phys.

    (1909)
  • M. Knudsen

    Ann. Phys.

    (1909)
  • M. Knudsen

    Ann. Phys.

    (1909)
  • M.A.V. Ribeiro da Silva et al.

    Thermochim. Acta

    (1990)
  • M.J.S. Monte, Ph.D Thesis, University of Porto, Porto,...
  • E. Cater, The effusion method at age 69: current state of the art, in: J. Hastie (Ed.) 10th Materials Symposium on...
  • C.J. Whitman

    J. Chem. Phys.

    (1952)
  • K. Motzfeldt

    J. Phys. Chem.

    (1955)
  • R. Sabbah et al.

    Thermochim. Acta

    (1999)
  • G.T. Furukawa et al.

    J. Res. Nat. Bur. Stand.

    (1951)
  • D.R. Stull et al.

    The Chemical Thermodynamics of Organic Compounds

    (1969)
  • W.V. Steele et al.

    Am. Inst. Chem. Eng. Symp. Ser. (AIChE Symp. Ser.)

    (1990)
  • B.V. Lebedev et al.

    Zhur. Obshch. Khim.

    (1982)
  • Cited by (228)

    View all citing articles on Scopus
    View full text