The invention of methylene blue.

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Methylene blue (also known as methylthionine chloride) is an organic dye of the thiazine group with the chemical formula C16H18ClN3S. It is used as a biological stain and indicator. It is also applied externally for irrigation, as an ointment for purulent skin conditions and is taken internally to treat inflammations of the urinary bladder. In addition, it is administered intravenously in glucose solution for cases of hydrocyanic acid, carbon monoxide and hydrogen sulphide poisoning. Industrially, methylene blue is used as a textile dye, in colouring paper as well as in pencils and printer’s ink[1].


The German chemist Heinrich Caro invented[2] methylene blue in 1876. Here is the method[2] he used in synthesising it:

  • He first had to prepare nitrosodimethylaniline for which he did by preparing a cold solution of 10 kilograms of dimethylaniline in 30 kilograms of hydrochloric acid and 200 litres of water. The solution was then added to a solution of 5.7 kilograms of pure sodium nitrite in 200 litres of water. The mixture then turned yellow and contained crystals of nitrosodimethylaniline.
  • This solution was then added to a solution of 500 litres of water and 50 kilograms of concentrated hydrochloric acid in a closed wooden barrel. Hydrogen sulphide was then steadily added to the solution in the barrel until the yellow colour disappeared. He noted that the solution temporarily turned red, became colourless and that when air entered the barrel, a blue foam was formed which covered the solution.
  • 200 litres of an iron chloride solution was then slowly added to the colourless solution until the odour of the hydrogen sulphide disappeared.
  • Sodium chloride was then added to the solution after which an aqueous solution of zinc chloride was added continuously until the blue dye precipitated.
  • The solution was then filtered and the dye obtained was pressed and dried by which it was then suitable for industrial use.

Here is a photograph[3] of methylene blue powder:

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Here is a photograph[4] of Heinrich Caro:

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  1. blue
  2. /2up, pages 247-249.
  3. 19415768933.html
  4. /issue-4/pioneer-thinker-then-and-now-methlyene-blue.html