ince the classical work of van der Waals in 1873, a
S' large number of equations of state have appeared in the literature. Their degree of complexity, as indicated by
the number of constants, has been deliberately increased to better represent the PVT behavior of substances over wide ranges of temperature and pressure. In this respect, the equation of state proposed by Martin and Hou (78)requires nine constants to predict the PVT behavior of a substance. Similarly, the BenedictWebb-Rubin equation (6) requires eight constants, while the Beattie-Bridgeman equation (7, 2) uses five constants to predict the PVT behavior of 3 substance for conditions for which the constants are calculated. More recently, an equation of state has been suggested by Pings and Sage (23), in the form of an orthogonal polynomial. These equations have received considerable attention over recommended ranges of temperature and pressure. For instance, the Benedict-Webb-Rubin equation is claimed to be reliable for twice the critical density (6). Although these equations are logically developed from well known thermodynamic principles, their application is frequently limited by the inaccessibility of the constants for the PVT conditions of interest. Establishing these constants is frequently a lengthy process, requiring curve fitting and trial-anderror procedures. For instance, in establishing the constants of the Benedict-Webb-Rubinequation, ten distinct s t e p are required, two of which in
