Abstract
Correlations producing thermodynamic property tables employ the concepts of scaling with increasing frequency in the vaporβliquid critical region. One of the important concepts is that the vapor pressure equation should provide infinite curvature and finite slope Ο~c~ at the critical point. The vapor pressure critical exponent ΞΈ describes the divergent curvature in a power law expression.
This paper provides an extensive study of ΞΈ. We have determined an optimal value of ΞΈ by two general approaches: a curve fit method (CFM) which employs leastβsquares analyses, and a numerical derivative method (NDM). The CFM is interpolative but requires a vapor pressure equation, while the NDM is extrapolative but is independent of the vapor pressure equation.
The vapor pressure equations, which satisfy scaling concepts most closely, exhibit a very flat minima for the CFM. As a result, the values of ΞΈ which provide reasonable correlations vary over an appreciable range (depending upon the compound, form of the equation, and the temperature range). The NDM did not present any particular difficulties. Our overall weighted average for ΞΈ is 0.199 with a standard deviation of 0.052, while the overall numerical average was 0.225 with a standard deviation of 0.045; the final recommended value of ΞΈ is 0.22 Β± 0.04.