Vapor-Liquid Coexistence Curves in the Critical Region and the Critical Temperatures and Densities of Difluoromethane and Pentafluoroethane

The vapor-liquid coexistence curves in the critical region of difluoromethane (HFC-32) and pentafluoroethane (HFC-125) were measured by a visual observation of the meniscus in an optical cell. Seventeen saturated-liquid and 13 saturated-vapor densities have been measured in a temperature range from 330 K to the critical temperature, corresponding to a density range from 218 to 790 kg-m⁻³ for HFC-32. Sixteen saturated-liquid and 13 saturated-vapor densities have been measured in a temperature range from 331 K to the critical temperature, corresponding to a density range from 342 to 895 kgm⁻³ for HFC-125. The experimental uncertainties of temperature and density measurements are estimated to be within ±10 mK and between ±0.10% and ±0.42%, respectively. Not only the level of the meniscus disappeared but also the intensity of the critical opalescence was considered in the determination of the critical temperature and density of HFC-32, being 351.255 ± 0.010 K and 424 ± 1 kgm⁻³, and those of HFC-125, being 339.165 ± 0.010 K and 568 ± 1 kgm⁻³.