Temperature dependence of liquid density: The choice of a reference state

Abstract

Some reduced equations of state where the scaling parameters are different from the parameters for the liquid-vapor critical point were studied. The problem is pertinent for many compounds for which the critical point cannot be reached. First, calculations were made for the equation of Shelomentsev and Bolotin, both in its original form and in our modification where the scaling parameters pertain to both the normal melting point and the normal boiling point. Good results were obtained for saturated liquid densities of normal paraffins, but the inconvenience of dealing at the same time with two physical reference states remains. Second, a modified Guggenheim density equation was used, but with the parameters at the normal boiling point instead of at the critical point. Again for n-paraffins, good results were obtained both for density and for isobaric expansivity. Two characteristic parameters for each material are needed. Two ‘universal’ parameters, the same for all paraffins, can be used also, but the predictive accuracy is somewhat lower.