Prompted by the observation that recent literature displays marked disagreement as to the proper treatment of data on the refractive indices of binary liquid mixtures, this paper clarifies the relationships among refractive index, molar volume and molar refraction and the corresponding mixing properties. It is shown that the molar refraction deviation function must be calculated on a mole fraction basis and the refractive index deviation function on a volume fraction basis, which makes it directly interpretable as a sign-reversed measure of the deviation of reduced free volume from ideality.

Refractive indices for the binary liquid mixtures: {hexane or heptane + ethanol}, {o-xylene + benzene} and {cyclohexane + benzene, toluene or hexane} were measured at 298.15 K. Excess refractive indices were calculated and fitted to a Redlich-Kister function. Using some mixture rules (Lorentz-Lorenz, Dale -Gladstone, Eykman, Oster, Arago-Biot and Newton), predictions for v^E have been made and compared with experimental data taken from the literature. Furthermore the Sugden equation has been used in two different ways for predicting excess surface tensions: the first one starting from densities and the other one from refractive indices. Results are plotted together with the literature data.