Molecular Refraction

R, a concept relating the electronic polarizability α_E of a substance to the substance’s refractive index n. Within the range of applicability of expressions for this concept, molecular refraction, like n, characterizes the capacity of a substance to refract light but differs from n in that it is practically independent of the density, temperature, and aggregation state of a particular substance. The basic formula for molecular refraction has the form

where M is the molecular mass of the substance, ρ is the density, and N_A is Avogardro’s number. The expression (*) for molecular refraction is the equivalent of the Lorentz-Lorenz formula (with the same limits of applicability) but in many cases is more convenient for practical use. Molecular refraction is often represented as the sum of the “refractions” of certain constituents that can be either atoms or groups of atoms making a molecule of a complex compound, or bonds of the atoms in such a molecule. For example, the molecular refraction of a saturated hydrocarbon C_kH_2k+2 = kR_c + (2k + 2)7R_H. This important property of molecular refraction—additivity—permits the successful use of refractometric analysis in such physicochemical tasks as the study of the molecular structure, the determination of molecular dipole moments, the study of hydrogen bonding, and the determination of the composition of mixtures.