Vapor Pressures of the Inert Gases

A single vapor‐pressure relationship applicable from the triple point to the critical point has been found to exist for the inert gases argon, krypton, and xenon when the reduced vapor pressure P_R is related to T_R the reduced temperature. This vapor‐pressure function resulted from the Frost‐Kalkwarf Eq. (12) and can be expressed as $$\log P_R{\mathrm{=\alpha +(\beta /T}}_R\mathrm{)+\gamma \hspace{0.17em}}\log T_R{\text{+0.1832\hspace{0.17em}(P}}_R{\mathrm{/T}}_{R^2}\mathrm{),}$$ where 0.1832 is a universal constant. For these inert gases β=−2.2936 and γ=−2.6786 while α can be calculated from the boundary condition, α+β+0.1832=0. The vapor‐pressure data for helium and neon produced similar reduced‐vapor‐pressure relationships having different constants. These results are not unexpected in view of the significant quantum effects associated with helium and to a lesser degree with neon. Values calculated with this reduced‐vapor‐pressure equation have been found to be in excellent agreement with experimental values reported for argon, krypton, and xenon. The average deviation for these gases was 0.63%, for neon 1.51%, and for helium 0.79%.