Process for the purification of methylene blue

A method for producing methylene blue comprising:

• a) providing a reaction mixture comprising one or more methylene blue intermediates;
• b) reducing the metal content therein by precipitation during solvent extraction under basic conditions wherein the basic condition is between about pH 9 and about pH 11;
• to form a reaction mass;
• c) producing crude methylene blue by further adjusting the pH of the reaction mass to an acidic pH with acid;
• d) purifying the crude methylene blue by treatment with polar solvent to remove the impurity at 0.94 RRT;
• e) producing methylene blue having purity greater than about 97% by repeating steps (b) and (c) followed by purification using water and THF;
• wherein, providing the reaction mixture comprises,
• I. reacting N,N-dimethylaniline
• N,N-dimethylaniline
• with sodium nitrite in hydrochloric acid to obtain an in-situ nitroso compound;
• nitroso compound
• II. reducing the in-situ nitroso compound
• nitroso compound
• in the presence of zinc and hydrochloric acid to obtain in-situ p-amino-N,N-dimethyl aniline;
• p-amino-N,N-dimethyl aniline
• III. reacting the in-situ p-amino-N,N-dimethyl aniline
• p-amino-N,N-dimethyl aniline
• with sulfuric acid and zinc chloride in the presence of sodium thiosulfate pentahydrate, sodium dichromate, and aluminium sulphate octahydrate to obtain an in-situ thiosulfonic acid compound;
• thiosulfonic acid compound
• IV. treating the in-situ thiosulfonic acid compound
• thiosulfonic acid compound
• with another molecule of N,N-dimethylaniline in the presence of sodium dichromate and zinc chloride to yield an in-situ di-cyclic compound;
• Di-cyclic compound
• and
• V. allowing the in-situ di-cyclic compound
• Di-cyclic compound
• to undergo a cyclisation reaction in the presence of manganese oxide to yield methylene blue crude;
• Methylene blue crude.