Two methods for the determination of the mean size of polydispersions may be based on measurement of optical scattering cross section. The first requires a single spectral transmittance together with a knowledge of particle concentration obtained independently. This method is applicable for any ratio of particle size to wavelength. Both mean size and concentration can be measured by a spectrophotometric method, which is based on measurement of spectral transmittance at two suitably selected wavelengths. The spectrophotometric method is applicable to the measurement of mean particle size of either stationary or moving polydispersions and is particularly useful for the immediate evaluation of the mean size of a polydispersion which is inaccessible except to optical observation. These two methods of size measurement are demonstrated on polydispersions of known mean diameter in an aqueous suspension.
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