Spectroscopic determination of the critical temperatures and pressures of H₂O, CO₂, and C₂H₅OH

The transmittance of 500-800 nm light was shown to change significantly at the critical points of three pure fluids. Spectroscopic measurements of H₂O, CO₂, and C₂H₅OH were made in a visible-type autoclave at high temperature and pressure. Each fluid showed significantly decreased transmittance as its critical point was approached; transmittance reached a local minimum at the critical temperature and pressure. The experimentally determined critical temperature and pressure of H₂O are 374 °C and 22.07-22.09 MPa, respectively, similar to reference values of 374.15 °C and 22.12 MPa; C₂H₅OH showed values of 243 °C and 6.30-6.31 MPa (versus reference values of 241 ± 7 and 243.4 °C and 6.3 ± 0.4 and 6.14 MPa); and CO₂ showed values of 32 °C and 7.36-7.43 MPa (versus reference values of 31.04 ± 0.2 °C and 7.37 ± 0.04 MPa). The good agreement between the experimental results and the reference values indicates that the spectroscopic method used here could be applied to other fluids, including mixed geofluids.