To prepare a pure sample of Methyl orange from sulphanilic acid.

 To prepare a pure sample of Methyl orange from sulphanilic acid.

Theory:

For the preparation of Methyl orange sulphanitic acid is first converted into sulphonyl benzene diazonium chloride by treatment with Nitrous acid in the presence of hydrochloric acid at 0 - 5 ℃. The diazonium compound so formed is coupled with dimethyl aniline in the alkaline medium to get methyl orange. The solid product formed is separated by filtration through a Buchner funnel, washed with water, and crystallized from boiling water.

Requirements:

(a) Apparatus : 

250 ml. round-bottomed flask, beaker, Buchner funnel, filtration flask, and suction pump.

(b) Chemicals :

• Sulphanilic acid = 7g
• Anhydrous sodium carbonate = 2g
• Sodium nitrite = 2g
• Dimethyl aniline = 4ml .
• Conc. hydrochloric acid = 8ml .
• Dil hydrochloric acid =20ml.
• Sodium chloride =  8g
• 20% NaOH solution = 30ml

 

Chemical Equations :

Procedure:

• Dissolve 7.0 g sulphanilic acid in a solution of 2.0 g of anhydrous sodium carbonate in 50.0 ml. of water in a 250 ml. round-bottomed flask. Warm to get a clear solution. 
• Dissolve 2.0 g of sodium nitrite in 10.0 ml. of water and add to a solution of sulphanilic acid.
• Cool the resulting solution to 0 - 5 ℃ in an ice bath.
• Dilute 8.0 ml. of conc. HCl with 20.0 ml. of water and add dropwise to the reaction mixture maintaining the temperature below 10 ℃ Shake the solution to mix thoroughly. 
• Dissolve 4.0 ml. of dimethyl aniline in 20.0 ml. of dil HCl and cool to 0 - 5 ℃. Add this cold solution to the reaction mixture. Shake thoroughly for 10 minutes and add with stirring 30.0 ml. of 20% NaOH solution.
• Warm the solution to 60°C in a water bath and add 8.0 g of sodium chloride. 
• Shake and cool the mixture in an ice bath. Separate the product formed through a Buchner funnel and wash with water.
• Crystallize the crude product from water.
• Dry in a desiccator and note the weight. Determine the melting point of the pure product.

Result :

• Colour = deep orange 
• Yield 8.0 g.

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