Thermal Expansion of Liquid Normal Hydrogen Between 18.8 and 22.2 K

The thermal expansion coefficient α of liquid normal hydrogen (n-H₂) was measured between 18.8 and 22.2 K in the pressure range 5 to 70 bar. The results are compared with those derived from PVT measurements by others on both normal and para (p-H₂) hydrogen. Our analysis of the earlier normal data includes fitting an empirical equation of state, and expansion coefficients are derived from this equation by differentiation. We discuss the effects on α and the compressibility β from molecular quadrupole interactions; both theoretical and empirical results suggest these to be on the order of 2% or less for the normal spin mixture. We conclude that our thermal expansion data are consistent with earlier results on both n-H₂ and p-H₂ in this range of pressures and temperatures.