Molecular properties of methylene blue, a common probe in sorption and degradation studies: a review

Methylene blue is a textile dye widely used as a reference probe in laboratory studies to set optimal removal conditions, yet reported physical properties of methylene blue are often erroneous. Here we review methylene blue properties with emphasis on erroneous or confusing literature data. We present molecular, biological, water solubility, spectroscopic, physicochemical and degradation properties, with focus on medicinal effects, lipophilicity, sorption, X-Ray diffraction, computed molecular structure, specific surface area, ultraviolet–visible, molar absorptivity, solvatochromism, pH, infrared, self-association, acid/base and redox behaviours, photocatalytic degradation, oxidative degradation, and biodegradation.