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Chapter: 3 Chemistry
    Section: 3.4 Vapour pressures
        SubSection: 3.4.1 Vapour pressure of ice at temperatures between -100 and 0 C

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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)(T2T1)T3/((T3T1)T2)

In this expression T is an abbreviation of Tx 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:

    ln(p/pt) = (a1τ + a2τ2 + a3τ3 + a4τ4)Tt/T,

 
 


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

   Tt = 273.16 K    pt = 611.657 Pa     a1 = −22.4948    a2 = −0.227    a3 = 0.502    a4= 0.562

Pressures are tabulated in Pa for Celsius temperatures t90.

t/°C

  −100

    −90

   −80

   −70

   −60

 −50

   −40

   −30

   −20

   −10

p/Pa

0.0014

0.0097

0.055

0.261

1.080

3.94

12.84

38.00

103.2

259.9




Reference

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




D.Ambrose

 

 

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