Acoustic determination of the helium content of carbon dioxide from he head pressure cylinders and FT-IR studies of the density of the resulting supercritical CO2: Implications for reproducibility in supercritical experiments

The speed of sound reaches a minimum value at the critical point of a fluid. An acoustic technique is used to measure the critical pressures of mixtures of He + CO2 of known composition (0.1-3 mol % He). These data are then used to establish the composition of CO2 samples from commercial He head pressure cylinders. The He content of apparently similar samples of HHPCO2 is widely different. Only one sample (out of 5) showed the 3 mol % He predicted for such cylinders. These findings can only be explained if the He and CO2 within our cylinders were not fully equilibrated. If such lack of equilibrium is common, the He content of HHPCO2 from a given cylinder will be unpredictable and largely dependent on the particular history of that cylinder (i.e., its treatment during storage and handling), ET-IR measurements of the Fermi triad absorption bands of CO2 (similar to 5000 cm(-1)) were then used to quantify the difference in density between pure CO2 and HHPCO2 containing 2.2 mol % He at the same temperature and pressure. Striking differences in density were observed close to the critical point. The implications of these results for reproducibility in supercritical experiments are discussed. It is recommended that, if possible, HHPCO2 should be avoided for all studies involving CO2 close to its critical point.