Visual observation of the critical temperature and density: CO2 and C2H4

The critical temperature (Tc) and density (ρc) of carbon dioxide (CO2) and ethylene (C2H4) have been determined by a modern version of the classical technique of visual observation of the disappearance of the meniscus. We discuss in some detail what a ``visual observation of the disappearance of the meniscus'' is, a subject that is relevant to the possible use of critical temperatures as secondary temperature reference points. For CO2 we find Tc = 30.977±0.004°C and ρc = 0.4678±0.0022 g/cm3. For C2H4 we find Tc = 9.194±0.004°C and ρc = 0.2146±0.0006 g/cm3. The errors quoted for ρc are two standard deviations from a fit to an assumed symmetrical scaling equation of state. The errors quoted for Tc are an ``estimated'' limit of error. A comparison of the values of Tc which we have measured with recent PVT data yields new values for the critical pressures (Pc). We find that Pc = 73.753 bar (72.789 atm) for CO2 and Pc = 50.390 bar (49.731 atm) for C2H4.