The Vapor Pressure, Orthobaric Liquid Density, and Critical Constants of Isobutane

The vapor pressures and orthobaric liquid densities of isobutane (propane−2‐methyl) were measured from 30° to 125°C. The equation $${\log }_{10}\mathrm{p\hspace{0.17em}(}\mathrm{normal\; atmos.}{\text{)=4.31269–1126.71/T\hspace{0.17em}\hspace{0.17em}(T=t}}^{\circ }\mathrm{C}\mathrm{\hspace{0.17em}(}\mathrm{Int.}\text{)+273.13)}$$ gives a fair representation of the vapor pressures. The critical constants of isobutane are determined by the compressibility method. They are: t_c=134.98±0.05°C (Int.), p_c=36.00±0.05 normal atmos., v_c=0.263 liters per mole (4.53 ml per g), d_c=3.80 moles per liter (0.221 g per ml). The uncertainty in the critical volume and density is 2 percent.