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 approximately 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.