Methylene Blue Protonation States and pKa Values

Chemical diagram of methylene blue showing its protonation states, pKa values, and isoelectric point.

This educational diagram depicts the acid-base behavior of methylene blue, a thiazine dye. At the top is the cationic methylene blue (MB) structure with pKa = 2.6 labeled near the central nitrogen. Arrows show reversible protonation equilibria leading to multiple species: the triprotonated MBH₃²⁺…