AGAIN THE RACKETT EQUATION

A previous method for calculating the critical constants using the Rackett equation is revised. The new procedure requires (1) the introduction of an exponential term function of the reduced temperature, which is more accurate than the Rackett value of 2/7 in the case of high molecular weight compounds, and (2) a trial-and-error calculation according to which all the critical constants can be derived from two experimental data of density and a known Z(RA) value. Rules are given for calculating the Z(RA) values for several classes of compounds, including strongly polar molecules. The possibility of applying the method to strongly associated molecules such as alcohols and acids is also discussed. The new procedure is applied to the prediction of the critical constants of nearly 90 compounds, both polar and non-polar. A comparison is given with a modified Lydersen method.