Development of an Equation of State for Gases

More than a hundred equations of state relating the pressure, volume, and temperature of gases have been proposed according to Dodge ( 7 ) , but only a very few of them have attained any practical importance as the majority do not represent the data with sufficient accuracy. In this work the significant pressure-volume-temperature (hereafter referred to as P V T ) characteristics of pure gases have been examined in detail, and an equation has been developed to fit precisely the characteristics common t o different gases. PVT data may be plotted on different types of graphs, of which probably the oldest is that of pressure vs. volume with temperature as a parameter, as shown in Figure 1. From this graph van der Waals deduced two properties of the critical isotherm namely, that at the critical point the slope is zero and an inflection occurs.' Van der Waals expressed these two properties algebraically in the following well-known manner :