# What pressure does liquid nitrous oxide need at −45 °C?

What pressure does liquid nitrous oxide need at −45 °C to remain liquid? How can I calculate its pressure required for other temperatures?

• Rather read from phase diagram then calculate, unless you find curve fitting equation. Mar 22 '21 at 23:24
• @Mithoron Would you point me to a phase diagram to read this from? I could really not find a decent one. Mar 22 '21 at 23:43
• I also found a bad one... Mar 22 '21 at 23:55
• «Nitrous oxide: Saturation properties and the phase diagram», Ferreira, Lobo, The Journal of Chemical Thermodynamics, 41, 2009, 1394-1399, doi 10.1016/j.jct.2009.06.017, seems to cover the range you need here. Mar 23 '21 at 6:15
• If such a diagram was not available, or for other compound, one could use the knowledge of the boiling point, the evaporation molar enthalpy (or its approximation by the empirical rule ( forgot the rule name ) saying evaporation molar entropy change is 90 J/K/mol and the Clausius-Clapeyron equation.) Then it is possible to compute the approximation of the curve p_vap = f(T) Mar 23 '21 at 7:03

## 1 Answer

Using NIST Reference Fluid Thermodynamic and Transport Properties Database (REFPROP) – NIST Standard Reference Database 23, Version 9, I found the saturation point of nitrous oxide at equilibrium for a temperature of $$T=-45.000\ \mathrm{^\circ C}$$ at a pressure of $$p=783110\ \mathrm{Pa}=7.8311\ \mathrm{bar}$$; for $$T=-78.000\ \mathrm{^\circ C}$$, it is $$p=182290\ \mathrm{Pa}=1.8229\ \mathrm{bar}$$.

• Thank you very much! I was about to ask if there may be a (freely accessible) online database/calculator where one could get this info and I just found this REFPROP version 10 is selling for \$325 here: nist.gov/srd/refprop I guess you have purchased that, right? Would you mind looking up one more value - pressure for -78°C? Thanks! Mar 23 '21 at 19:48
• Looks like there is an alternative to REFPROP - it is called CoolProp. Not very user friendly though. Website: coolprop.org Mar 23 '21 at 20:06