Determination of Rayleigh Factors

The vertical, Rv(0), and horizontal Rh(0), Rayleigh factors are calculated from Eq. (2), and the excess Rayleigh factor, corrected for anisotropy, is finally obtained from Eq. (7). 

Accurate values of the specific refractive index increment are required in light scattering equation because dn/dc appears in a quadratic form in Eq. (4). 

They are determined at constant temperature with a differential refractometer employing the same wavelength as used in the light scattering measurement (A = 633 nm). Usually the lignin concentration varies from 0.2 X 10-3 to 3 x 10 3gml 1. 

The differential refractometer is generally calibrated with aqueous salt solutions, e.g., KC1, NaCl, NH4N03 of known concentration. The difference in refractive index at 633 nm between the salt solutions and distilled or deionized water is represented by the following equations  

Furthermore, the solvent refractive index, n , required in Eq. (4) must be determined under the same wavelength and temperature conditions. 

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