preface to fifteenth edition

5.30 SECTION 5
5.2.1 Vapor-Pressure Equations
Numerous mathematical formulas relating the temperature and pressure of the gas phase in equilibrium
with the condensed phase have been proposed. The Antoine equation (Eq. 1) gives good
correlation with experimental values. Equation 2 is simpler and is often suitable over restricted
temperature ranges. In these equations, and the derived differential coefficients for use in the Haggenmacher
and Clausius-Clapeyron equations, the p term is the vapor pressure of the compound in
pounds per square inch (psi), the t term is the temperature in degrees Celsius, and the T term is the
absolute temperature in kelvins ( tC 273.15).
Eq. Vapor-pressure equation dp/dT [d(ln p)/d(1/T)]
1
B
log p A t C
2.303pB
(t C) 2
2.303BT 2
(t C) 2
2
3
B
log p A T
B
log p A C log T
T
2.303pB
T 2
p
2.303B C
T2
T
2.303B
2.303B CT
Equations 1 and 2 are easily rearranged to calculate the temperature of the normal boiling
point:
B
t C (5.1)
A log p
B
T (5.2)
A log p
The constants in the Antoine equation may be estimated by selecting three widely spaced data
points and substituting in the following equations in sequence:
y3 y2 t2 t1 t3 t1
1
y y t t t C
In these equations, yi
log p i.
2 1 3 2 3
1
3 1
B
y3 y1
B (t C)(t3
C)
t t
A y 2
t C
2