Reporting molar refractions

It has become common practice to report the deviation of molar refraction from a volume fraction average of the molar refractions of the components for binary liquid mixtures in anticipation of assessing intermolecular electronic interactions. It is demonstrated here that this method does not yield information about such interactions but, in effect, reveals information almost exclusively related to the difference between volume fraction and mole fraction. The deviation of molar refraction from a mole fraction average of the refractions of the components, i.e., the excess molar refraction, provides the proper basis for assessing intermolecular electronic interactions.