Remarks on Refractive Index Mixture Rules

The most important refractive index mixture rules used for determining the refractive index of a solute are scrutinized regarding their relative merits and interrelations. The Dale-Gladstone and the Biot-Arago equation are found to follow from either the LorentzLorenz equation or the Wiener equation if the solution is dilute and if, in addition, the refractive indices of solute and solvent are nearly the same. A mixture rule proposed by the writer represents the limiting form of the Wiener equation in case of dilute solutions without restriction as to the value of the refractive indices. The performance of all equations is tested on literature data. The results are in line with expectation on the basis of theoretical analysis. The special case of solutes of colloidal dimensions, where none of the equations applies satisfactorily, is briefly discussed. (Author)