Purification of Thionin, Azure A, Azure B and Methylene Blue

Column and paper chromatography of four thiazin dyes revealed both inorganic and organic impurities. In thionin, azure A, azure B and methylene blue, sodium and other metal cations were found as inorganic impurities. The analysis for organic impurities revealed that the dyes were mixtures; specifically each dye contained one or more of the other dyes as impurities. Inorganic impurities were detected by ashing the dyes in the presence of H₂SO₄ and chromatographing the sulfate salts on paper. They were removed by filtration through ion exchange resins. Organic impurities were detected by paper chromatography and removed by column chromatography on Woelm's neutral alumina.