The flattering of the absorption spectrum of suspensions, as compared to that of solutions

Abstract

It is proved that the absorption spectrum of a suspension of particles is flattened, as compared with that of a solution of the pigments contained in the particles. This flattening is due to the enhanced mutual shading of the pigment molecules in the suspension. A quantitative theory of this flattening is given. If the flattening is measured experimentally, the optical density of the individual particles can be computed, at least in principle (for spherical particles a nomograph is given to this purpose). This optical density may be used for the calculation of the “true” absorption spectrum of a pigment in suspension (which can be used, for example, for an improved analysis of a composite spectrum, in terms of overlapping absorption spectra of the several pigments present in the particle). Such an analysis is generally required when relative photochemical efficiencies of these pigments must be determined.