The coexistence curve between the solid and the gas phases is called the sublimation curve [see Fig.3.4]. The relevant Clausius-Clapeyron is
where is the molar latent heat of sublimation.
Neglecting and using the ideal gas approximation for , we have
Thus, along the sublimation curve,
Near triple point of NH3, we have
1. Find T and P of triple point.
2. Find latent heats of sublimation and vaporization.
1. The sublimation and vaporization curves meet at the triple point. Hence,
2. For the sublimation curve,
For the vaporization curve,
the molar enthalpy can be written as
Now, at low
Thus, we can neglect the pressure variation and write
on sublimation curve
where at .
The latent heat of sublimation is therefore
Eq(3.20) can thus be integrated to give
which can be used to extrapolate experimental measurements.