Methylene Blue: From 1876 Cotton Dye to Lab Standard

In 1876, at the BASF works in Ludwigshafen, a chemist experimenting with a new intermediate produced a pure blue dye for cotton. According to the company’s own historical record, that chemist was Heinrich Caro, its first head of research, and a year later BASF was granted what it describes as Germany’s first patent for a coal tar dye. The compound was methylene blue. Wainwright and Crossley, reviewing its career in the Journal of Chemotherapy in 2002, open with the same date.

One hundred and fifty years on it is still in routine use, and for reasons that have very little to do with colouring textiles.

What the phenothiazinium cation actually is

PubChem lists methylene blue as CID 6099, molecular formula C16H18ClN3S, molecular weight 319.9 g/mol, with the systematic name [7-(dimethylamino)phenothiazin-3-ylidene]-dimethylazanium chloride. Its CAS registry number is 61-73-4, and it also carries the Colour Index designations C.I. Basic Blue 9 and C.I. 52015.

The structure is a tricyclic phenothiazine: two benzene rings bridged by a nitrogen and a sulfur atom, each outer ring carrying a dimethylamino group at position 3 or position 7. The molecule is not neutral. The central ring nitrogen bears the positive charge of a delocalised cation, and chloride is the counter-ion.

That delocalisation is the source of the colour. The positive charge is shared across the whole conjugated system, so the electronic transition sits far into the visible region. Compiled reference spectra list peak absorption at 668 and 609 nm in water. Other figures appear elsewhere, and the discrepancy is instructive rather than contradictory: band position depends on the medium and on whether the cation is free or ion-paired in an organic solvent.

From biological stain to the first synthetic drug

The dye reached biology quickly. Guizetti, reviewing the imaging of malaria parasites in the Journal of Microscopy in 2026, records that methylene blue was discovered as a biological stain in 1881 by Paul Ehrlich, and that he demonstrated its usefulness for staining malaria parasites ten years later.

Staining a parasite and treating an infection are separate steps, and Ehrlich took both. Buchholz and colleagues, writing in Antimicrobial Agents and Chemotherapy in 2008, describe how Paul Guttmann and Paul Ehrlich reported the compound as an effective antimalarial agent in 1891, and note that Ehrlich introduced the concept of modern target-based chemotherapy using this dye as his example. The same paper calls methylene blue the very first synthetic compound to be used as a drug.

The claim needs precision. Earlier medicines existed, but they were isolated from plants or minerals. This one was a coal-tar product from a dye works, deployed against a defined pathogen on an argument about selective staining.

Two electrons, one colour change

Every modern use of the compound rests on a single reversible reaction. The blue cation accepts two electrons and is protonated to give leucomethylene blue, which is colourless. Oxidation reverses it. The colour is a direct readout of oxidation state, which is why the dye works as a redox indicator with no instrumentation.

Buchholz and colleagues give the midpoint potential of the methylene blue and leucomethylene blue pair as +10 mV, with the tabulated value in the same paper listed as +11 mV. A couple sitting essentially at zero against the standard hydrogen electrode is well placed to shuttle electrons between biological reductants and oxygen.

The same paper sets out the consequence. Disulfide reductases catalyse the reduction of the dye by NAD(P)H; the leuco form then auto-oxidises back to the blue cation, generating hydrogen peroxide. The authors describe the compound as a redox-cycling catalyst whose biochemical and pharmacological activity depends on the presence of a disulfide reductase and molecular oxygen. An antioxidant enzyme is turned into a pro-oxidant one.

That chemistry also underlies the one indication for which the compound holds a modern marketing authorisation. The United States prescribing information for the injectable product, distributed by American Regent, states that it is indicated for the treatment of paediatric and adult patients with acquired methaemoglobinaemia, and explains the mechanism in redox terms: the dye is reduced to leucomethylene blue by an NADPH-dependent reductase, and the leuco form then reduces the ferric iron of methaemoglobin back to the functional ferrous state.

Why analytical chemists still reach for methylene blue

The dye’s second career is as a reagent. It survives in standard methods because a strongly coloured cation that forms ion pairs with anions is exactly what a spectrophotometric assay for anionic analytes needs.

The best-known example is the methylene blue active substances (MBAS) determination of anionic surfactants in water, standardised internationally as ISO 7875-1. Wyrwas and Zgola-Grzeskowiak, describing a continuous-flow version of the method in the Journal of Surfactants and Detergents, summarise the principle exactly: the method is based on the formation of ion pairs between anionic surfactants and the cationic dye, and their transfer from the aqueous phase into chloroform. The standard procedure uses a triple extraction and measures the absorbance of the extract at 650 nm. Their continuous-flow variant reported a quantification limit of 16 micrograms per litre for anionic surfactants, with relative standard deviations of 1.3 per cent for a model sample and 3.8 per cent for river water.

A second family of methods uses adsorption rather than extraction. Because the cation binds strongly to negatively charged surfaces, the quantity of dye a material takes up is used as a proxy for accessible surface and exchange capacity. ASTM C837, Standard Test Method for Methylene Blue Index of Clay, is the long-established example.

The redox reporter in DNA-based sensors

The most active current use is electrochemical. Batchelor, Dief, Bonham and Gooding published a review of the dye’s interactions with DNA and their relevance for DNA-based sensors in ACS Sensors in 2025, covering exactly this ground.

The appeal is structural. A flat, aromatic, positively charged molecule associates with duplex DNA, and its redox couple is fast and reversible. Tethered to a probe strand, it turns a binding event into a measurable current: the strand changes conformation, the reporter-to-electrode distance changes, and the peak height follows. No optical path is required.

Hydrates, dye content, and what a certificate has to say

Sourcing this compound has a trap that catches people who have only ever read the molecular weight. The prescribing information for the injectable product gives the chemical name as 3,7-bis(dimethylamino)phenothiazin-5-ium chloride hydrate, with the formula written as C16H18ClN3S times x H2O and an anhydrous molecular weight of 319.86 g/mol. The variable x is not a typographical convenience. Commercial material is routinely hydrated and the water content is not fixed.

Two consequences follow for anyone weighing out a standard. Gravimetric preparation from a hydrated solid without a stated water content introduces an error that careful pipetting cannot recover. And dye-grade material has historically been specified by dye content rather than by assay; the two numbers are not interchangeable. Work that depends on an accurate concentration should start from a methylene blue reference standard whose certificate of analysis states the assay method, the water determination and the batch identity, and spectrophotometric standardisation against a known absorptivity is worth doing as a cross-check.

Frequently asked questions

What is methylene blue chemically?

Methylene blue is a phenothiazinium salt. PubChem gives the molecular formula C16H18ClN3S, a molecular weight of 319.9 g/mol and the systematic name [7-(dimethylamino)phenothiazin-3-ylidene]-dimethylazanium chloride, with CAS registry number 61-73-4. The cation is a tricyclic system bridged by nitrogen and sulfur, carrying dimethylamino groups at positions 3 and 7, and the charge is delocalised across the conjugated framework.

Why is methylene blue called the first fully synthetic drug?

Because it was made in a dye works rather than extracted from a natural source, and was then deliberately applied against a defined pathogen. Buchholz and colleagues describe it as the very first synthetic compound to be used as a drug, noting that Paul Guttmann and Paul Ehrlich reported it as an effective antimalarial agent in 1891 and that Ehrlich used it to introduce the idea of target-based chemotherapy.

What happens when methylene blue is reduced?

It accepts two electrons and is protonated to form leucomethylene blue, which is colourless, and the reaction reverses on oxidation. Buchholz and colleagues report a midpoint potential of +10 mV for the couple, listing +11 mV in their tabulated data. Because the leuco form re-oxidises in air, the dye can act as a redox-cycling catalyst in the presence of a disulfide reductase and oxygen.

What is the MBAS method?

MBAS stands for methylene blue active substances, a spectrophotometric determination of anionic surfactants in water standardised as ISO 7875-1. The anionic analyte forms an ion pair with the cationic dye, the pair is extracted into chloroform, and the absorbance of the extract is measured at 650 nm. A continuous-flow version reported a quantification limit of 16 micrograms per litre.

References

Research use only. Nonsensia supplies analytical reference standards for laboratory and research applications. This article is published for scientific and educational purposes. It is not medical advice, it does not describe any use in humans, and nothing in it should be read as a recommendation to administer any substance to a person or animal.

Filed under: Chemistry & Discovery

This article is part of our guide to Where Research Compounds Come From: Discovery, Synthesis and Structure.

The compound discussed in this article is available as an analytical reference standard: Methylene Blue – Analytical Reference Standard, 1000 mg | CAS 61-73-4.

Methylene Blue: The First Fully Synthetic Drug and Its Second Life in the Laboratory