The information of the specific refractive index increment of a solution, dn/dc, is mainly required to the determination of molecular weights of polymer by light scattering, which is defined as (n-n_1)/c, where n, n_1, and c are the refractive indices for the solution, the solvent and the concentration of solute (in grams per milliliter), respectively. A precise knowledge of the specific increment is, moreover, a prerequisite not to incur an uncertainty in molecular weights, because it enters as a square term in the constant K of the basic equation of light scattering.
However, the value is not a characteristic constant of a given polymer, but depends on the temperature, the pressure, the wavelength of light and the nature of solvent, and their measurements are delicate and mistakable. Though R. Chiang also emphasized an importance of the precision of dn/dc values in his book, few discussions have been made about these problems till now.
Conventionally it is known that the empirical relation of Gradstone-Dale is held among the natures of solvent and solute, and their increments, which may be convenient to calculate the value of dn/dc in other solvents and at other temperatures from data already known, provided the heighest accuracy is not required.
In this paper, the application of the relation to poly (2-methy-5-vinyl pyridine) - solvent systems and its validity were discussed briefly, considering the condition under which this rule is established.