At least four distinct blue cationic phthalocyanine dyes sold as “alcian blue” raises the question: what is alcian blue?

We investigated physicochemical characteristics of dye lots sold as “alcian blue” using the Biological Stain Commission (BSC) precipitation test, differential scanning calorimetry, high performance liquid chromatography, thin layer chromatography and UV/visible spectroscopy. Four blue phthalocyanine dyes were detected in 11 commercial dye lots. These four included the original ingrain blue 1 CI 74000 dye and the dye sold with the name “alcian blue pyridine variant”; we discuss also the possible identity of the additional two dyes. A proposed extension to the BSC analytic scheme is presented that could distinguish three categories of commercial alcian blue dyes from each other and from the original alcian blue 8G.