The Effects of Association on the Beer's Law Behaviour of Dye Solutions

Apparent deviations from Beer's law that exist with many dyes in aqueous solution have been measured over an extended concentration range. Three types of ‘curve’ can be distinguished: A, linear, generally found in solutions <10 ‐1 M; B, concave to the concentration axis; C , a new and composite type having a lower portion of type B and an upper of type A (positive slope). C is observed with some surface‐active dyes and appears to be caused by micellisation. Where the size distribution of the absorbing species does not change over the concentration range under study, e.g. as for aqueous dispersions of pigments, no deviations from Beer's law are observed. Dyes that show marked deviations from the law in aqueous solution due to aggregation phenomena may exhibit ideal behaviour over a thousand‐fold concentration range in organic solvents.