Methylene blue
![Absorption spectrum of methylene blue, in terms of the molar extinction coefficient (base 10 logarithm). In this dataset a peak absorbance of 1.7 (i.e. 98% of transmitted light absorbed) was observed with 665 nm light passing through 1 cm of 10 micromolar methylene blue solution.](/Images/godic/202501/27/Methylene_blue_absorption_spectrum0927.png")
![A volumetric flask of a methylene blue solution](/Images/godic/202501/27/Reflections_in_a_flask_of_Methylene_Blue0927.jpg")
Methylene blue (CI 52015), also known as methylthioninium chloride, has many uses in biology and chemistry; for example, it can be used as a stain and as a medication.
It is a heterocyclic aromatic chemical compound with the chemical formula C16H18N3SCl. At room temperature it appears as a solid, odorless, dark green powder, that yields a blue solution when dissolved in water. The hydrated form has 3 molecules of water per unit of methylene blue. Methylene blue should not be confused with methyl blue, another histology stain, new methylene blue, nor with the methyl violets often used as pH indicators.