The design and performance of an absolute deviation type of differential refractometer is described. The apparatus comprises mercury and sodium lamps, monochromatic filters, slit, differential cell, projection lens, and micrometer microscope, all accurately aligned on an optical bench. The cell is square with a thin partition separating solution and solvent, such that the angle of incidence is about 69° on the interface. The cell and its holder, in a jacketed housing, can be turned through 180° in order to interchange solvent and solution and approximately double the deviation. By ray tracing it is shown that the distance between the slit images in the eyepiece field, Δd, is accurately proportional to the difference in refractive index, Δn, of solution and solvent, and that the factor of proportionality can be evaluated from accurately measurable geometrical quantities: the cell partition angle, the virtual distance from slit to cell center, and the magnification of the system up to the field of the micrometer eyepiece. The range of the instrument is approximately 0.01 unit and the limiting sensitivity about 3 × 10−6 unit of refractive index difference. The accuracy in determination of Δn is about 0.5 percent. The instrument is useful for determination of small differences in refractive index or concentration, and for determination of refractive index increments needed in the light-scattering method of evaluating high molecular weights.
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