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Appendix Estimation method

United Kingdom

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INTRODUCTION

Estimated values of the vapour pressure can be used:

  • for deciding which of the experimental methods is appropriate,

  • for providing an estimate or limit value in cases where the experimental method cannot be applied due to technical reasons.

ESTIMATION METHOD

The vapour pressure of liquids and solids can be estimated by use of the modified Watson correlation (a). The only experimental data required is the normal boiling point. The method is applicable over the pressure range from 105 Pa to 10–5 Pa.

Detailed information on the method is given in ‘Handbook of Chemical Property Estimation Methods’ (b). See also OECD Environmental Monograph No.67 (c).

CALCULATION PROCEDURE

The vapour pressure is calculated as follows:

ln P vp ≈ Δ H vb Δ Z b RT b 1 –  3 – 2 T T b m T T b – 2 m 3 – 2 T T b m  – 1 ln T T b

where:

T

  • temperature of interest

    • Tb

  • normal boiling point

    • Pvp

  • vapour pressure at temperature T

    • ΔHvb

  • heat of vaporisation

    • ΔZb

  • compressibility factor (estimated at 0,97)

    • m

  • empirical factor depending on the physical state at the temperature of interest

    • Further,

    Δ H vb T b = K F (8,75 + R ln T b )

    where, KF is an empirical factor considering the polarity of the substance. For several compound types, KF factors are listed in reference (b).

    Quite often, data are available in which a boiling point at reduced pressure is given. In such a case, the vapour pressure is calculated as follows:

    ln P vp ≈ ln P 1 + Δ H v1 Δ Z b RT 1 1 –  3 – 2 T T 1 m T 1 T  – 2 m 3 – 2 T T 1 m  – 1 ln T T 1

    where, T1 is the boiling point at the reduced pressure P1.

    REPORT

    When using the estimation method, the report shall include a comprehensive documentation of the calculation.

    LITERATURE

    1. (a)

      Watson, K.M. (1943). Ind. Eng. Chem, 35, 398.

    2. (b)

      Lyman, W.J., Reehl, W.F., Rosenblatt, D.H. (1982). Handbook of Chemical Property Estimation Methods, McGraw-Hill.

    3. (c)

      OECD Environmental Monograph No.67. Application of Structure-Activity Relationships to the Estimation of Properties Important in Exposure Assessment (1993).