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ChEBI: Methyl green is an iminium salt composed of (4-{[4-(dimethylamino)phenyl][4-(trimethylazaniumyl)phenyl]methylidene}cyclohexa-2,5-dien-1-ylidene)(dimethyl)ammonium and chloride ions in a 1:2 ratio. A histological dye used to demonstrate nucleic acids by the Unna-Pappenheim stain, in conjunction with Pyronin Y. It has a role as a fluorochrome and a histological dye. It is an organic chloride salt, an iminium salt and a quaternary ammonium salt. It contains a methyl green(2+).

Cupric oxide is heated with 8% acetic acid, As2O3 is added, and the mixture is refluxed for two hours. The product is allowed to cool for half an hour, filtered, washed and dried.Alternate method: Dilute acetic acid is allowed to react with an excess of freshly precipitated Cu(OH)2 and the product is separated by filtration. Dilute acetic acid is added to a solution of As2O3 in boiling NaOH until the color of phenolphthalein disappears. The hot solutions are mixed (mole ratio of CuO:As2O3 = 4:3) and allowed to stand for several days [S. Avery, J. Amer. Chem. Soc. 28, 1159 (1906)].