Background:

Sudan III is a hydrophobic bisazo dye[1].

Sudan III changes its color from orange to blue against a small volume of sulfuric acid, and the acetonitrile solution of Sudan III is the most suitable for observing the color-change phenomenon. H-NMR and UV-Vis spectroscopic studies show that the color-change mechanism of Sudan III against sulfuric acid is due to the protonation of the dye by sulfuric acid[1].

[1]. Sakurai T, et al. Color Change of Sudan III against Concentrated Sulfuric Acid in Acetonitrile and Quantification for a Small Amount of Concentrated Sulfuric Acid. Anal Sci. 2016;32(2):129-33.