Then an equation of state can be expressed in a dimensionless form for molar volume
Vm (Pc/RTc) as a universal function of reduced temperature Tr ¼ T/Tc, reduced pressure Pr ¼ P/Pc and reduced molar refraction r. Parameter r has been also used as a third parameter for the prediction of the viscosity of heavy hydrocarbons (Riazietal.,
2005). On this basis, a three-parameter corresponding-states model is proposed for the reduced speed of sound in heavy hydrocarbon liquids as follows: