Methylene blue
Description
Dark green crystals that form a deep blue aqueous solution. Methylene blue was first prepared in 1876 by Heinrich Caro as a zinc double chloride salt. This synthetic aniline dye was used a lightfast colorant for cotton, silk, and wool with the zinc salt acting as a mordant.
In recent years, the zinc-free form of the compound has been used medicinally under the same name. In this form, methylene blue is used as a biological stain for bacteria and as an indicator in oxidation-reduction reactions. Methylene blue is also used as a disinfectant and an antidote for cyanide poisoning.
Synonyms and Related Terms
Basic Blue 9; CI 52015; Solvent Blue 8; azul de metileno (Esp.); azul de metileno (Port.); methyl blue; solvent blue; Basic Lake Blue; Swiss blue
Zinc double salt of methylthionine chloride
Risks
- Toxic by ingestion and inhalation.
- Contact may cause irritation.
- Potential teratogen.
- Flammable. Flash point = 178 C
- Fisher Scientific: MSDS
- Chemical Book (Double zinc salt): SDS
Physical and Chemical Properties
- Soluble in water, and less readily soluble in ethanol (bleu)
- Soluble in concentrated sulfuric acid (yellowish green) and dilute acid (blue)
- Insoluble in ether
- pH of aq. solution = 3 - 4.5
- Composition = 2(C16H18N3SCl)ZnCl2-xH2O
- CAS = 26283-09-0
- Melting Point = 190 C (dec)
Resources and Citations
- SDC Enterprises: Colour-Index 2.0
- Wikipedia: Methylene blue Accessed April 2026
- Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
- Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997
- The Merck Index, Martha Windholz (ed.), Merck Research Labs, Rahway NJ, 10th edition, 1983 Comment: entry 6137
- The American Heritage Dictionary or Encarta, via Microsoft Bookshelf 98, Microsoft Corp., 1998
- A.Scharff, 'Synthetic dyestuffs for textiles and their fastness to washing', ICOM-CC Preprints Lyon, Getty Conservation Institute, Los Angeles, 1999