On the “concentration-driven” methylene blue dimerization

In this work a theoretical and experimental analysis of the spectrum of methylene blue is made in order to clarify the nature of the shoulder appearing at ∼620 nm. This shoulder has been several times attributed to the existence of an hypothetical methylene blue dimer. The results here obtained do not agree with the existence of such a dimeric specie, and point out differences in the ionic strength of the solution as the phenomenon responsible for the variations observed in these spectra.