A SENSITIVE CRYOGENIC REFRACTOMETER - APPLICATION TO THE REFRACTIVE-INDEX DETERMINATION OF PURE OR MIXED LIQUID METHANE, ETHANE, AND NITROGEN

A cryogenic refractometer is described. It enables preparation of liquefied samples and measurement of their refractive index at low temperatures (down to 20 K). The liquid sample can be a pure liquefied gas or a mixture. The composition of the liquid mixture can be estimated from the volume and pressure of each gas measured during the liquefying process. The determination of absolute values of the refractive index necessitates a calibration, therefore the accuracy is limited to 10(-3) index unit. Sensitivity and reproductibility are better (5 X 10(-4) and could be improved to 10(-4). The refractive index of pure liquid methane was determined at several temperatures (91-106 K) and 670 nm. The results obtained agree with the published data. The refractive index of mixtures of liquid methane and ethane, with and without dissolved nitrogen, was also determined at 94 K and 1.5 bar total pressure, for which conditions no data were previously available.