The Preparation and Assessment of a Series of Pure Dye Acids

High‐load paper chromatography has shown previous preparations of the free acids of C.I. Acid Red 88, 13, and 27 to have been impure, although their purity, like that of the free acids of C.I. Acid Orange 7 and 8, had seemed satisfactory according to spectrophotometric examination and titration with alkali and titanous chloride. Reasons are given for the lack of sensitivity of these latter methods when used without supporting data from high‐load paper chromatography. The preparation of pure samples of these dye acids is described, together with appropriate analytical data.