Diazotization (Sodium Nitrite Titration): Assay Methods

ASSAY METHODS

A number of pharmaceutical substances can be assayed by official methods employing sodium nitrite titrations. A few typical examples are described below to get an indepth knowledge about sodium nitrite titrations.

1. PREPARATION OF 0.1 M SODIUM NITRITE SOLUTION

Materials Required : Sodium nitrite : 7.5 g.

Procedure : Weigh accurately 7.5 g of sodium nitrite and add sufficient DW to produce 1 litre in a 1000 ml volumetric flask.

2. STANDARDIZATION OF 0.1 M SODIUM NITRITE SOLUTIOlN WITH SULPHANILAMIDE

Materials Required : Sulphanilamide (previously dried at 105°C for 3 hours) : 0.5 g ; hydrochloric acid ( ₋~ 11.5 N) : 20 ml ; 0.1 M sodium nitrite.

Theory : The nitrous acid, generated on the introduction of sodium nitrite solution into the acidic reaction mixture, reacts with the primary amino group of sulphanilamide quantitatively, resulting into the formation of an unstable nitrite that decomposes ultimately with the formation of a diazonium salt. The diazonium salt thus produced is also unstable, and if the reaction mixture is not maintained between 5-10°C, it shall undergo decomposition thereby forming phenol products which may react further with nitrous acid. The reactions involving the formation of the diazonium salt may be expressed in the following manner :

NaNO₂ + HCl → HNO₂ + NaCl

At the equivalence point a slight excess of HNO₂ is present which must persist for at least 1 minute. This excess HNO₂ may be detected by employing either starch iodide strip or paste and designated by the following equation :

Procedure : Weigh accurately 0.5 g of suphanilamide and transfer to a beaker. Add to it 20 ml of hydrochloric acid and 50 ml of DW, stir until dissolved and cool to 15°C in an ice-bath. Add to it 25 g of crushed ice, and titrate slowly with sodium nitrite solution, stirring vigorously, until the tip of the glass rod dipped into the titrated solution immediately produces a distinct blue ring on being touched to starch-iodide paper. The titration is supposed to be complete when the end-point is deducible after the resulting mixture has been allowed to stand for 1 minute. Each 0.01722 g of sulphanilamide is equivalent to 1 ml of 0.1 N sodium nitrite.

3. CALCIUM AMINOSALICYLATE

Materials Required : Calcium aminosalicylate : 0.5 g ; hydrochloric acid ( ₋~  11.5 N) : 10.0 ml ; potassium bromide : 1.0 g ; 0.1 M sodium nitrite ; starch-iodide paper.

Theory : The assay of calcium aminosalicylate is based upon the reaction designated by the following equation :

Procedure : Weigh accurately about 0.5 g of calcium aminosalicylate, into a funnel placed in the mouth of a 250 ml volumetric flask. Wash through with 10 ml of hydrochloric acid and enough DW to dissolve, add 1.0 g potassium bromide and make up the volume upto 250 ml mark. Pipette 50 ml into a conical flask, cool to below 15°C (in ice-bath) and titrate gradually with 0.1 M sodium nitrite solution while shaking the contents of the flask vigorously and continuously until a distinct blue colour is achieved when a drop of the titrated solution is placed on a starch-iodide paper 5 minutes after the last addition of the 0.1 M NaNO₂ solution. Care must be taken to add NaNO₂ solution at the rate of 0.1 ml near the end of the titration. Each ml of 0.1 M sodium nitrite is equivalent to 0.01722 g of C₁₄H_l2CaN₂O₆.

4. ISOCARBOXAZID

Materials Required : Isocarboxazid : 0.5 g ; glacial acetic acid (99% w/w or 17.5 N) : 20.0 ml ; hydrochloric acid ( ~−  11.5 N) : 20.0 ml ; 0.1 M sodium nitrite ; starch-iodide paper.

Theory : The estimation is based on the fact that isocarboxazid undergoes rapid cleavage in acidic medium to produce benzylhydrazine. The latter reacts quantitatively with nitrous acid (NaNO₂ and HCl) to give rise to benzylazide. The various reactions involved are expressed as follows :

Procedure : Weigh accurately about 0.5 g of isocarboxazid and dissolve it in 20 ml of glacial acetic acid. Add to it 20 ml of hydrochloric acid and 50 ml of DW. Cool to about 15°C in an ice-bath and titrate slowly with 0.1 M NaNO₂ while shaking vigorously and continuously until a distinct blue colour is obtained on a starch-iodide paper that lasts for 5 minutes after the final addition of the 0.1 M NaNO₂ solution to the titrated solution. Add NaNO₂ solution very carefully at the rate of 0.1 ml at a time as the end-point is approached. Each mole of 0.1 M sodium nitrite is equivalent to 0.02313 g of C₁₂H₁₃N₃O₂ .

5. PHTHALYLSULPHATHIAZOLE

Materials Required : Phthalylsulphathiazole : 0.5 g ; sodium hydroxide solution (20% w/v in water) : 10.0 ml ; hydrochloric acid ( ~−  11.5 N) : 20.0 ml ; 0.1 M sodium nitrite ; starch-iodide paper.

Theory : The assay is based upon the reactions designated by the following equations :

Phthalylsulphathiazole undergoes hydrolysis to give phthalic acid and sulphathizole. The latter reacts with nitrous acid to yield the corresponding diazonium salt quantitatively.

Procedure : Weigh accurately about 0.5 g of phthalylsulphathiazole and heat on a water-bath for 2 hours after the addition of 10.0 ml of sodium hydroxide solution. Cool the contents of the flask to 15°C in an ice-bath, add to it 10.0 ml of water and 20.0 ml of hydrochloric acid and carry out the titration slowly with 0.1 M sodium nitrite solution. The contents of the flask are shaken thoroughly and continuously until a distinctly visible blue colour is obtained when a drop of the titrated solution is placed on a starch-iodide paper 5 minutes after the last addition of the 0.1 M NaNO₂ solution. Towards the approach of the end-point the addition of NaNO₂ solution must be at the rate of 0.1 ml. Each ml of 0.1 M sodium nitrite is equivalent to 0.04034 g of C₁₇H₁₃N₃O₅S₂.

6. COGNATE ASSAYS

A plethora of pharmaceutical substances that can be assayed by the help of sodium nitrite titrations are mentioned in Table 12.1.