DESC:FIELD OF THE INVENTION

The present invention relates to an improved process for preparation of methylene blue of Formula I, which provides process, without isolation of intermediates in high yield and quality.

I

BACKGROUND OF THE INVENTION

Methylene blue agent, which is chemically known as 3,7-bis(Dimethylamino)-phenothiazin-5-ium chloride trihydrate as represented in Formula I.

I

methylthionine chloride; tetramethylthionine chloride; 3,7-bis(dimethylamino) phenothiazin-5-ium chloride; C.I. Basic Blue 9; tetramethylthionine chloride; 3,7-bis(dimethylamino) phenazathionium chloride; Swiss blue; C.I. 52015; C.I. Solvent Blue 8; aniline violet; and Urolene Blue®) is a low molecular weight (319.86), water soluble, tricyclic organic compound.

Methylene blue, It has many uses in a range of different fields, such as biology and chemistry. At room temperature it appears as a solid, odorless, dark green powder, that yields a blue solution when dissolved in water. The hydrated form has 3 molecules of water per molecule of methylene blue. Methylene blue should not be confused with methyl blue, another histology stain, new methylene blue, nor with the methyl violets often used as pH indicators.

Methylene blue is a component of a frequently prescribed urinary analgesic/anti-infective/anti-spasmodic known as "Prosed", a combination of drugs which also contains phenyl salicylate, benzoic acid, hyoscyamine sulfate, and methenamine (aka hexamethylenetetramine and not to be confused with 'methanamine').

Methylene Blue, perhaps the most well-known phenothiazine dye and redox indicator, has also been used as an optical probe of biophysical systems, as an intercalator in nanoporous materials, as a redox mediator, and in photoelectrochomic imaging.
See, for example, Colour Index (Vol. 4, 3rd edition, 1971) and Lillie et al., 1979, and references cited therein.

Methylene Blue is currently used to treat methemoglobinemia (a condition that occurs when the blood cannot deliver oxygen where it is needed in the body). Methylene Blue is also used as a medical dye (for example, to stain certain parts of the body before or during surgery); a diagnostic (for example, as an indicator dye to detect certain compounds present in urine); a mild urinary antiseptic; a stimulant to mucous surfaces; a treatment and preventative for kidney stones; and in the diagnosis and treatment of melanoma.
Methylene blue was first described in a German Patent in 1877 (Badische Anilin- und Soda-Fabrik, 1877). In that patent, Methylene blue was synthesized by nitrosylation of dimethylaniline, subsequent reduction to form N,N-dimethyl-1 ,4-diaminobenzene, and subsequent oxidative coupling in the presence of hydrogen sulphide (H2S) and iron(III) chloride (FeCl3 ).
Bernthsen described subsequent studies of Methylene blue and methods for its synthesis (see Bernthsen, 1885a, 1885b, 1889).

F. Oxazine and Thiazine Dyes. Fierz-David and Blangley, Fundamental Process of Dye Chemistry. Inter sciences,308-314, 1949, also describes methods for the synthesis of Methylene blue from dimethylaniline, as illustrated in the following scheme

Scheme(1)
In step (a), nitrosodimethylaniline is prepared from dimethylaniline by treatment with nitrite (NaNO2) in aqueous acid (HCl) solution. In step (b), the nitroso compound is reduced to form p-aminodimethylaniline using additional aqueous acid (HCl) solution using zinc dust. The metal residue after step (b) is removed by filtration and the filtrate is oxidised in the presence of thiosulfonic acid, sulphuric acid and non-reducing zinc chloride solution, step (c). Further oxidation in the presence of dimethylaniline results in the thiosulfonic acid of Bindschedlers green, step (d). The ring is then closed using manganese dioxide or copper sulphate to form methylene blue. More specifically, a clear neutral solution of p-aminodimethylaniline is acidified (H2SO4), and a non-reducing zinc chloride solution is added (ZnCl2 with Na2Cr2O7). Aqueous aluminium sulphate (Al2(S2O3)3) and crystalline sodium thiosulphate (Na2S2O3) are added. Aqueous sodium dichromate (Na2Cr2O7) is added. The mixture is heated by dry steam. Aqueous acidic (HCl) dimthylaniline is then added. Aqueous sodium dichromate (Na2Cr2O7) is added. The mixture is heated with dry steam, and becomes dark greenish-blue in colour due to the formation of the thiosulfonic acid of Bindschedler green. An aqueous slurry of manganese dioxide or copper sulfate is added, and the mixture heated by dry steam, and the dye precipitates from the concentrated zinc chloride solution. To recover the dye from the mixture it is cooled and acidified (H2SO4) to dissolve the aluminium, manganese and chromium salts. The mixture is cooled further and the crude dye collected by filtration. Purification from water, sodium chloride and zinc chloride gives the zinc double salt of methylene blue as bronzy red crystals.

Instead of sodium dichromate, manganese dioxide and catalytic amount of copper sulfate can be used for the oxidation in description of US4212971. Another procedure for the preparation of phenothiazine dyes has been reported in which the reaction is carried out with sodium thiosulfate in the presence of zinc chloride and aluminum sulfate and the oxidation is carried out with sodium chromate in the presence of copper sulfate in sulfuric acid in description of RU63231.

Leventis et al.,Sythesis of Substituted Phenothiazines Analogous to Methylene Blue by Electrophilic and Nucleophilic Aromatic Substitutions in tandem. A Mechanistic perspective. Tetrahedron,1997, vol. 53(29), 10083-10092, describe methods for the synthesis of certain Methylene blue analogs, which employ phenothiazine as a starting material and which add the desired 3,7-substituents by halogenation followed by amination. The authors assert that Methylene blue is synthesized commercially by oxidation of N,N-dimethyl-p-phenylenediamine with Na2Cr2O7 in the presence of Na2S2O3, followed by further oxidation in the presence of N,N-dimethylamine.

F. Oxazine and Thiazine Dyes. Fierz-David et al., Fundamental Processes of Dye Chemistry. Inter-sciences,1949, 308-314, describes the synthesis of the zinc chloride double salt of Methylene blue and the removal of zinc by chelation with sodium carbonate followed by filtration to generate zinc free methylene blue. However, the authors acknowledge that this technique cannot be used on a large scale, because the yields are poor.

Methods for synthesizing high purity Methylene blue and its derivatives have been proposed in WO2006/032879. The compounds are synthesized according to the following scheme:

Methods of synthesizing and purifying certain 3,7-diamino-phenothiazin-5-ium compounds (referred to herein as "diaminophenothiazinium compounds") including Methythioninium Chloride (also known as Methylene Blue) are provided WO2015052496.

WO2015052496, US8263589 process for making methylene blue converted to Iodide (MTI) salt than coveted to hydrogen Iodide salt(MTI.HI).

US8691979B2 process for making synthesizing and purifying certain 3,7-diamino-phenothiazin-5-ium compounds (referred to herein as “diaminophenothiazinium compounds”) including Methylhioninium Chloride (MTC) (also known as Methylene Blue). In one embodiment, the method comprises the steps of, in order: a thiosulfonic acid formation (TSAF); an oxidative coupling (OC)(sodium persulfate, potassium persulfate, and ammonium persulfate); and a ring closure (RC). Also described are resulting compounds and compositions comprising them (e.g., tablets, capsules) for use in methods of medical treatment and diagnosis, etc., for example, for tauopathies, or Alzheimer's disease (AD). Methylene Blue in example as significant amount of azure B was detected in isolated sample.

WO20150021500 describe process for making methylene blue via acid addition salt, purification in cold water and acetone.

CN105399703 provides a preparation method for methylene blue, which belongs to the tech. field of fine chem. synthesis. The method of the present invention comprises: doing arom. nucleophilic substitution reaction in the presence of a base with p-nitroaniline and nitrobenzene to obtain 4,4'-dinitro diphenylamine; in the presence of paraformaldehyde, hydrogenizing 4,4'-dinitro diphenylamine with palladium/carbon to obtain 4,4'-dimethylamino diphenylamine; in the presence of sulfur and iodine, doing cyclization of 4,4 '-dimethylamino diphenylamine to obtain 4,4 '-dimethylamino thiodiphenylamine; then using oxidn. agent to obtain methylene blue crude products, and finally obtaining the methylene blue product purified by water. The product purity of the present invention is high, the present invention avoids using hazardous and noxious substances in synthesis process, the generated waste is considerably less, is friendly to the environment, and meets the requirement of environmental protection. The base is NaOH or KOH, the reaction temp. is 50-85 °C, and the molar ratio of nitrobenzene and p-nitroaniline is 1-5:1.

CN105130926 method comprises the steps of: conducting nitrosation of sodium nitrite and N,N-dimethylaniline to obtain nitroso-N,N-dimethyl aniline, conducting hydrogenation reduction on nitroso-N,N-dimethyl aniline to obtain p-amino-N,N-dimethyl aniline, oxidizing, conducting addition reaction with sodium thiosulfate to obtain 2-amino-5-dimethylaminophenyl thiosulfonic acid, conducting oxidn. condensation with N,N-di-Me aniline to obtain bis(4-dimethylaminophenyl) thiosulfonic acid, and conducting ring closure reaction to obtain methylene blue. The method has the advantages of high product purity, simple process, low cost, easily available raw materials, little environmental pollution, and is suitable for industrial production.

The prior art processes involve the use of expensive reagents and laborious process for
the preparation of Methylene blue. Hence, there is a need in the art to develop an improved process for the preparation of Methylene blue.

The present inventors have now found an improved process for the preparation of Methylene blue and its salts, which is simple, industrially applicable and yields Methylene blue or a salt thereof in high purity.

OBJECTIVE OF THE INVENTION

The main objective of the present invention is to provide an improved process for the preparation of methylene blue.
Yet another objective of the present invention is to provide an improved process, which avoids isolation of intermediates and use of reducing agent ammonium dicromate.
Yet another objective of the present invention is to provide a simple and environmentally friendly process for the preparation of methylene blue, which avoids use of hazardous and expensive reagents.
Yet another objective of the present invention is to provide a process with a good yield and purity.
Yet another objective of the present invention is to provide an improved process for methylene blue, which is simple and industrially applicable.
Yet another objective of the present invention is to provide an improved purification process for methylene blue, which is simple and industrially applicable.

SUMMARY OF INVENTION

The present invention provide an improved process for the preparation of methylene blue or hydrate of Formula (I),

I
which comprises:
a) reacting of N,N-dimethyl-p-phenylenediamine or hydrochloride Formula (II)

II
with Sodium thiosulfate pentahydrate of Formula (III)

III
in presence of ammonium dicromate and water to give 2-amino-5-dimethylaminophenyl thiosulfonic acid compound of Formula (IV);

IV
b) reacting N,N-Dimethylaniline or hydrochloride salt, water and sulfuric acid
c) Step (b) solution added into step (a) reaction mixture at 0ºC-5ºC to give compound formula(V)

V

d) Compound formula (V) react with an ammonium dicromate at temperature 0ºC-5ºC .
e) Compound formula (V) react with water, hydrochloric acid and copper sulfate pentahydrate to give title compound formula (I).
f) Compound formula (I) purified by using protic solvents preferably C1-C5 alcohols like methanol, ethanol, isopropanol..etc for the removal of Azure impurity in formula (I)

DETAILED DESCRIPTION OF THE INVENTION

Throughout this specification, including any claims which follow, unless the context requires otherwise, the word "comprise," and variations such as "comprises" and "comprising," will be understood to imply the inclusion of a stated integer or step or group of integers or steps, but not the exclusion of any other integer or step or group of integers or steps.

It must be noted that, as used in the specification and any appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a pharmaceutical carrier" includes mixtures of two or more such carriers, and the like.

Ranges are often expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent "about," it will be understood that the particular value forms another embodiment.

The present invention relates to an improved two steps process for the preparation of methylene blue.
The synthetic scheme of the present invention is as shown below:

Another aspect of the invention is to provide a process for methylene blue or its pharmaceutically acceptable salts, wherein the intermediates formed in the synthesis are not isolated.
The said process comprises of reacting N,N-dimethyl-p-phenylenediamine or hydrochloride Formula (II)

II

with Sodium thiosulfate pentahydrate of Formula (III)

III
in presence of dicromate, selected form ammonium dicromate, sodium dicromate, potassium dicromate preferably ammonium dicromate and water to give 2-amino-5-dimethylaminophenyl thiosulfonic acid compound of Formula (IV);

IV
at a temperature ranging from -5 to 15°C; preferably at 0 to 10°C; more preferably at 0 to 5°C to form a 2-amino-5-dimethylaminophenyl thiosulfonic acid compound of Formula (IV). The reaction is preferably carried out in water and water miscible solvent mixture like alcohol, ketones. The 2-amino-5-dimethylaminophenyl thiosulfonic acid intermediate thus formed is carried out for the next step with out isolation i.e. in-situ.

The 2-amino-5-dimethylaminophenyl thiosulfonic acid intermediate of Formula (IV) thus formed above is using to give compound formula(V), In step-(b) preparation of homogenous solution of N,N-Dimethylaniline or salt added in water and then take ice and added sulfuric acid(98%) and then added solution to above homogeneous solution at a temperature ranging from -5 to 15°C; preferably at 0 to 10°C. After completion of the reaction thus formed homogenous solution is used for next stage

The compound formula (IV) solution addition of drop wise above homogenous solution . The compound formula(V)

V

was further treating with dicromate, selected form ammonium dicromate, sodium dicromate, potassium dicromate preferably ammonium dicromate
at a temperature ranging from -5 to 15°C; preferably at 0 to 10°C. The obtained compound formula(V)

V
thus formed is carried out for the next step with out isolation i.e. in-situ.
Compound formula (V) react with water, hydrochloric acid and copper sulfate pentahydrate to give title compound formula (I).
The step (e) the weight cake of Compound formula (V) react with water, then addition of acid, like hydrochloric acid, sulfuric acid bur preferably hydrochloric acid and copper sulfate pentahydrate to give title compound formula (I). the reaction mixture refluxed at a temperature ranging from 70 to 950C, preferably at 85 to 900C to obtain pure methylene blue title compound formula(I).

The present inventors have compared the purity of Methylene blue with the prior-art reported Methylene blue under the conditions of HPLC testing and found that the obtained Methylene blue as per the present invention has high purity, Further, the present inventors also found that the present invention yields high when compared to prior-art reported processes by considering the overall yield factor.

The present inventors also found that the content of des-methyl (Azure impurity) impurity is comparatively less (NMT 2.5 (% area) by HPLC) in the present invention when compared with the prior-art processes.

The compound of Formula (V) is prepared starting from a) reacting of N,N-dimethyl-p-phenylenediamine or hydrochloride Formula (II) with Sodium thiosulfate pentahydrate of Formula (III) in presence of ammonium dicromate and water to give 2-amino-5-dimethylaminophenyl thiosulfonic acid compound of Formula (IV); reacting N,N-Dimethylaniline or hydrochloride salt, water and sulfuric acid , solution added into step reaction mixture at 0ºC-5ºC to give compound formula(V) Compound formula (V) react with an Ammonium dicromate at temperature 0ºC-5ºC.
The present invention process for making methylene blue using the compound formula (V) react with water, hydrochloric acid and copper sulfate pentahydrate to give title compound formula (I).

The present inventors also found that the purification for the removal of Azure impurity in Methylene Blue, Methylene Blue was suspended in protic solvents preferably C1-C5 alcohols like methanol, ethanol, isopropanol etc ( preferable solvent is ethanol 500 ml, 10 vol) reaction temperature is 45-50°C and time for 1hr or 30 min. reaction cooling temperature is 0-5°C and time for 1 hr or 2hrs. Filter and wash with same solvent and dry at 20-30 °C under vacuumed to yield the title compound with Azure impurity NMT 2.5%. Yield: 34 gm (Solid) Chromatographic Purity (by HPLC): Azure impurity is 1.97% & Methylene Blue 97.73%

The present inventors also found that the purification Methylene Blue was suspended in water (2.5 vol) and time 30 min. Reaction at temperature 15-5°C and time for 1 hr. Filter and wash with water and dry at 20-30 °C under vacuumed to yield the title compound with Azure impurity NMT 2.0%,Yield: 12 gm (Solid), Chromatographic Purity (by HPLC): Azure impurity is 1.76% & Methylene Blue 98.23%.

The following examples illustrate the nature of the invention and are provided for illustrative purposes only and should not be construed to limit the scope of the invention.

Working example :
Example-1
Preparation of 2-amino-5-dimethylaminophenyl thiosulfonic acid from N,N-Dimethyl-p-phynylenediamine
N,N-Dimethyl-p-phenylene diamine (100 gm, 0.73 mole) was suspended in Water (2000 ml, 20 vol) at temperature 5-10°C. Add solution of Sodium thiosulfate pentahydrate (199.68 gm, 0.80 mole) in water (400 ml). Add dropwise solution of Sodium persulphate (174.76 gm, 0.73 mole) in water (800 ml, 8 vol) at temperature 5±5°C. Stir for 1 hr at 5-10°C. Temperature raise to room temperature and stir for 3 hrs. Filter the material and purified by using Ethyl acetate to yield the title compound as solid.
Yield: 60 gm

Example – 2
Preparation of 2-amino-5-dimethylaminophenyl thiosulfonic acid from N,N-Dimethyl-p-phynylenediamine dihydrochloride
N,N-Dimethyl-p-phenylene diamine (100 gm, 0.73 mole) was suspended in Water (2000 ml, 20 vol) at temperature 5-10°C. Add solution of Sodium thiosulfate pentahydrate (199.68 gm, 0.80 mole) in water (400 ml, 4 vol). Add dropwise solution of Ammonium dichromate (135 gm, 0.73 mole) in water (800 ml, 8 vol) at temperature 5±5°C. Stir for 1 hr at 5-10°C. Temperature raise to room temperature and stir for 3 hrs. Filter the material and purified by using Ethyl acetate to yield the title compound as solid.
Yield: 60 gm

Example – 3
Preparation of Methylene blue crude from 2-amino-5-dimethylaminophenyl
thiosulfonic acid
Add Methanol (400 ml, 4 vol), water (600 ml, 6 vol), charge 2-amino-5-dimethylaminophenyl thiosulfonic acid (100 gm, 0.4 mole) and cool to -5 to 0°C. Add solution of N,N-dimethyl aniline and sulfuric acid (Preparation of solution of N,N-dimethyl aniline and sulfuric acid: Take N,N-dimethylaniline (48.8 gm, 0.4 mole) and D.M.Water (100 ml, 1 vol) at temperature 25-30°C. Cool to 0-5°C. Take ice (100 gm) and add Sulfuric acid (98%) (40 gm, 0.4 mole) and then add this solution to above solution of N,N-Dimethylaniline at temperature 0-5°C) at temperature -5 to 0°C. Add dropwise sol of Ammonium dichromate (101.6 gm, 0.4 mole) in D.M.Water (700 ml, 7 vol) at temperature -5 to 0°C. Stir for 2 hrs at temperature -5 to 0°C. Filter and wash with cold D. M. Water (200 ml, 2 vol). Obtained wet compound S-5-(dimethylamino)-2-(4-(dimethyliminio)cyclohexa-2,5-dienylideneamino)phenyl sulfothioate. Meanwhile charge D.M. Water (1080 ml, 10.8 vol) in another RBF. Adjust pH 1.1 by using Conc HCl. Charge Copper (II) sulphate pentahydrate (10 gm). Charge above wet compound S-5-(dimethylamino)-2-(4-(dimethyliminio)cyclohexa-2,5-dienylideneamino)phenyl sulfothioate to solution. Stir for 1 hr at temperature 85-90°C. Cool to 60-65°C. Filter and wash with D.M.Water (200 ml, 2 vol). Charge NaCl (930 gm) to filtrate. Stir for 5 hrs at temperature 25-30°C. Filter and well suck dried. Purified using Methanol and Acetone to yield the title compound as solid.
Yield: 95 gm (Solid)

Example – 4
Purification of Methylene Blue (Crude) to prepare Methylene blue trihydrate.
Add water (600 ml) and Methylene blue (100 gm) and heat to 55°C. Stir for 10-15 min. Cool to 25-30°C. Stir for 1 hr. Filter and wash with water to yield the title compound as solid.
Yield: 75 gm (Solid)

Example-5
Preparation of Methylene blue crude from N,N-Dimethyl-p-phynylenediamine dihydrochloride

N,N-Dimethyl-p-phenylene diamine dihydrochloride (100 gm, 0.478 mole) was suspended in Methanol (400 ml, 4 vol), water (600 ml, 6 vol) at temperature 0-5°C. Add solution of Sodium thiosulfate pentahydrate (129.4 gm gm, mole) in water (145 ml, 1.45 vol). Add dropwise solution of Ammonium dichromate (48 gm, 0.4 mole) in water (115 ml, 1.15 vol) at temperature 5±5°C. Stir the reaction mass for 1 hr at temperature 0-5°C. Add solution of N,N-dimethyl aniline and sulfuric acid (Preparation of solution of N,N-dimethyl aniline and sulfuric acid: Take N,N-dimethylaniline (48.8 gm, 0.4 mole) and D.M.Water (100 ml, 1 vol) at temperature 25-30°C. Cool to 0-5°C. Take ice (100 gm) and add Sulfuric acid (98%) (40 gm, 0.4 mole) and then add this solution to above solution of N,N-Dimethylaniline at temperature 0-5°C) at temperature -5 to 0°C. Add dropwise sol of Ammonium dichromate (126.5 gm, 0.5 mole) in D. M. Water (200 ml, 2 vol) at temperature 0-5°C. Stir for 2 hrs at temperature 0-5°C. Filter and wash with cold D. M. Water (4200 ml, 42 vol). Obtained wet compound S-5-(dimethylamino)-2-(4-(dimethyliminio)cyclohexa-2,5-dienylideneamino)phenyl sulfothioate. Meanwhile charge D.M. Water (2500 ml, 25 vol) in another RBF. Add Conc. Hcl (57.5 ml). Charge Copper (II) sulphate pentahydrate (11.8 gm). Charge above wet compound S-5-(dimethylamino)-2-(4-(dimethyliminio)cyclohexa-2,5-dienylideneamino)phenyl sulfothioate to solution. Stir for 1 hr at temperature 85-90°C. Cool to 60-65°C. Filter and wash with D.M.Water (1800 ml, 18 vol). Charge NaCl (2300 gm) to filtrate. Stir for 5 hrs at temperature 25-30°C. Filter and well suck dried. Purified using Methanol and Acetone to yield the title compound as solid.
Yield: 156 gm (Solid)

Purification for the removal of Azure impurity in Methylene Blue
Example – 6

Methylene Blue (50 gm) was suspended in protic solvents preferably C1-C5 alcohols like methanol, ethanol, isopropanol etc (ethanol 500 ml, 10 vol). Heat to 45-50°C and stir for 30 min. Cool to 0-5°C and stir for 1 hr at temperature 0-5°C. Filter and wash with same solvent and dry at 20-30 °C under vacuumed to yield the title compound with Azure impurity NMT 2.5%.
Yield: 34 gm (Solid)
Chromatographic Purity (by HPLC): Azure impurity is 1.97% & Methylene Blue 97.73%

Example – 7

Methylene Blue (15 gm) was suspended in water 40 ml (2.5 vol) stir for 30 min. Cool to 15-5°C and stir for 1 hr at temperature 15-5°C. Filter and wash with water and dry at 20-30 °C under vacuumed to yield the title compound with Azure impurity NMT 2.0%.
Yield: 12 gm (Solid)
Chromatographic Purity (by HPLC): Azure impurity is 1.76% & Methylene Blue 98.23% .
,CLAIMS:1) An improved process for the preparation of methylene blue or hydrate of Formula (I),

I
which comprises:
a) reacting of N,N-dimethyl-p-phenylenediamine or hydrochloride Formula (II)

II
with Sodium thiosulfate pentahydrate of Formula (III)

III
in presence of ammonium dicromate, sodium persulfate and water to give 2-amino-5-dimethylaminophenyl thiosulfonic acid compound of Formula (IV);

IV

b) Reacting N,N-Dimethylaniline or hydrochloride salt, water miscible solvents like alcohols , ketone, water and sulfuric acid

c) Step(b) solution added into step(a) reaction mixture at 0ºC-5ºC to give compound formula(V)

V
d) Compound formula (V) react with an ammonium dicromate at temperature 0ºC-5ºC .

e) Compound formula (V) react with water, hydrochloric acid and copper sulfate pentahydrate to give title compound formula (I).

g) Compound formula (I) purified by using protic solvents preferably C1-C5 alcohols like methanol, ethanol, isopropanol..etc for the removal of Azure impurity in formula (I)

2) The process according to claim 1 step-(a);(d), wherein the reaction carryout temperature range -5ºC to 10ºC and most preferably temperature range 0ºC-5ºC.

3) The process according to claim 1, step-(b), wherein the reaction carryout temperature range -5ºC to 15ºC and most preferably temperature range 0ºC-10ºC.

4) The process according to claim 1 step-(a),(d), where in the reduction is carried out without isolating compound formula (IV) by using reducing agents ammonium dicromate.

5) The process according to claim 1 step-(e), wherein the reaction carryout temperature range 75ºC to 95ºC and most preferably temperature range 85ºC-90ºC.

6) The process according to claim 1 step-(e), wherein purification of methylene blue or its pharmaceutically acceptable using a solvent selected from alcohols , water or mixture thereof, preferably solvent a solvent selected from alcohol such as methanol, ethanol, n-propanol or n-Butanol most preferably solvent is methanol.

7) The process according to claim 1, wherein the methylene blue obtained after purification using C1-C5 alcohols with water most preferably solvents is water, Ethanol & 1 to 20% aqueous alcohols used for the purification

8) The process according to claim 1, wherein the pure methylene blue drying temperature is NMT 30oC under high vacuum

9) A process for purification of methylene blue comprising methlene blue treating with water or a protic solvents preferably C1-C5 alcohols like methanol, ethanol, isopropanol, butanol for the removal of Azure impurity in formula (I)

10) A process for the preparation of methylene blue substantially as herein described with reference to the examples.