Clausius- Clapeyron’s equation can also be used to estimate approximately the vapor pressure of a liquid at any arbitrary temperature in conjunction with a relation for latent heat of a substance, known as Trouton’s rule.

Trouton’s Rule

(2.14)

where latent heat of vaporization, kJ/kgmol
and  T_B → boiling point at 1.013 bar (N.B.P.)
From Eq. (2.10) and (2.14),

	(2.15)
	(2.16)

where p is the vapor pressure in kPa for any temperature T.

Sublimation  

It is the solid to vapor phase transformation.

(2.17)

where l_sub → latent heat of sublimation.
 and vapor pressure is low,

(2.18)

(2.19)

(2.20)

At the triple point:

lsub = lvap + lfus

(2.21)

Now,

(2.22)

and

(2.23)

Since, l_sub > l_vap at the triple point

(2.24)