Handbook of Solvents - George Wypych - ChemTech - Ventech!

48 George Wypych
tween the solvent and the impurity. If this difference is small, the impurities, present in
small quantities, will have little influence on the reading.
2.3.5 VAPOR DENSITY AND PRESSURE
The relative vapor density of solvents is given by the following equation:
d
vp
Ms
= [2.3.2]
M
air
where:
Ms the molecular mass of solvents
Mair the molecular mass of air (28.95 Daltons)
Figure 2.3.10 shows that the vapor density has linear correlation with molecular mass and
that for both alcohols and ketones (as well as the other solvents) the relationship is similar.
The data also show that solvent vapor densities are higher than air density. This makes ventilation
a key factor in the removal of these vapors in the case of spill or emissions from
equipment. Otherwise, the heavier than air vapors will flow along floors and depressions
filling pits and subfloor rooms and leading to toxic exposure and/or risk of ignition and subsequent
explosions.
The Clausius-Clapeyron equation gives the relationship between molecular weight of
solvent and its vapor pressure:
dln p MΛ
=
2 dT RT
[2.3.3]
where:
p vapor pressure
T temperature
M molecular mass of solvent
Λ heat of vaporization
R gas constant
Figure 2.3.11 shows that the vapor pressure of alcohols increases as the number of carbon
atoms in the molecules and the molecular mass increases. A small increase in vapor pressure
is produced when branched alcohols replace normal alcohols.
Vapor pressure at any given temperature can be estimated by the use of Antoine (eq.
[2.3.4]) or Cox (eq. [2.3.5]) equation (or chart). Both equations are derived from
Clausius-Clapeyron equation:
B
logp = A−
C + T
B
logp = A−
T + 230
[2.3.4]
[2.3.5]
where:
A, B, C constants. The constants A and B are different in each equation. The equations
coincide when C = 230 in the Antoine equation.
From the above equations it is obvious that vapor pressure increases with temperature.