Handbook of Solvents - George Wypych - ChemTech - Ventech!

5.1 Solubility parameters 245
ln P = A+ B / T + ClnT [5.1.7]
In more complicated dependancies, the number of constants may further increase.
Another convenient method is based on empirical relation of ΔH p at 25 o C with the normal
boiling point, T b, of non-polar liquids: 1
2
ΔH = T + 23. 7T −2950
[5.1.8]
p b b
Methods of evaluation of vapor pressure may be divided into static, quasi-static, and
kinetic methods.
5.1.1.3 Static and quasi-static methods of evaluation of pair pressure
Manometric method 10 consists of thermostating with a high precision (0.01K) and vapor
pressure measurement by a level of mercury with the help of a cathetometer or membrane
zero-manometer. The accuracy of measurement is 0.1-0.2 mm Hg.
Ebulliometric method 11 is used for a simultaneous measurement of the boiling and
condensation temperature that is required for evaluation of purity of a substance and its molecular
mass.
5.1.1.4 Kinetic methods
These methods were developed based on the molecular kinetic theory of gases. The
Langmuir method is based on the evaporation of substance from a free surface into a vacuum.
The Knudsen method is based on the evaluation of the outflow rate of a vapor jet from
a mesh.
The basic expression used in Langmuir method 12 is:
m RT
P =
St M
⎛ ⎞
⎜ ⎟
⎝ ⎠
⎛ 2π
⎞
⎜ ⎟
α ⎝ ⎠
12 /
[5.1.9]
where:
m mass of evaporated substance
S surface of evaporation
t time of evaporation.
The Knudsen method 13 is based on a measurement of the mass rate of the vapor outflow
through a hole. Knudsen proposed the following expression:
P
k
m RT
=
St h M
⎛ ⎞
⎜
⎟
⎝ ⎠
⎛ Δ 2π
⎞
⎜ ⎟
β ⎝ ⎠
12 /
[5.1.10]
where:
Δm mass output of substance
Sh surface area of the hole
t time of vaporization
β Clausing parameter
M molecular mass
The method uses special effusion cameras with holes of a definite form, maintaining
high vacuum in the system. The method is widely applied to the measurements of a vapor
pressure of low volatile substances.