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3.4 Vapour pressures

The plot of the logarithm of vapour pressure p against the reciprocal of Kelvin temperature T for a single substance is usually nearly linear and may be used for interpolation and, with caution, for extrapolation. A value of log p₂ for a temperature T₂ may be obtained from values of log p₁ and log p₃ for temperatures T₁ and T₃ as follows:

   log p₂ = log p₁ + (log p₃ − log p₁)(T₂ − T₁)T₃/((T₃ − T₁)T₂)

In this expression T is an abbreviation of T_x where x indicates one of the International Temperature Scales. Temperatures in sections 3.4.1 and 3.4.2 are expressed as Celsius temperatures on ITS-90; T in the corresponding equations signifies T₉₀, and the Celsius temperatures t₉₀ are given by t/°C = (T/K) − 273.15. For substances other than water, vapour pressures are either not known with sufficient accuracy or are not given here with sufficient precision for specification of the temperature scale to be significant.

The vapour pressure of a substance may be affected by the presence of other substances. For example, the pressure of water vapour over ice or liquid water is enhanced by the presence of atmospheric air, and this enhancement is significant in hygrometry (see section 2.1.4)

3.4.1 Vapour pressure of ice at temperatures between —100 and 0 °C

The table has been calculated from the equation following which was fitted to values given by Wexler (1977) after correction of the temperatures from IPTS-68 to ITS-90:

where p is the pressure, T = T₉₀, and subscript t indicates the value at the triple point; τ = 1 − T/T_t. The values for substitution in the equation are:

Pressures are tabulated in Pa for Celsius temperatures t₉₀.

Reference

A. Wexler (1977) J. Res. Nat. Bar. Stds, 81A, 5–20.

D.Ambrose

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