Diazotization TITRATION FOR PG NOTES VERY USEFUL FOR MPHARM

1. 1. Introduction
2. Definition
3. Principle of Diazotization
4. Determination of end point
5. Types of Diazotization titration
6. Preparation and standardization of 0.1M NaNO2 solution
7. Assay of Benzocaine
8. Assay of Dapsone
9. Assay of Isocarboxazid
10. Application of Diazotization titration
11. Reference

2.  Introduction:
 In general, aromatic primary amino moiety(ie Ar-NH2 ) as present in host
of sulpha drurgs viz., succinyl sulphathiazole, sulphamethoxazole,
sulphaphenazole and other potent pharmaceutical substances for instance
sodium or calcium aminosalicylate, isocarboxazid etc react with sodium
nitrite in an acidic medium to yield the corresponding diazonium salts as
expressed below
DIAZOTIZATION
TITRATION

3. The formation of diazonium compound by the
interaction of sodium nitrite, an inorganic
acid, and a primary aromatic amine, in ice-
cold solution is known as diazotization.
Definition:

4. Nitrous acid is formed by the interaction of sodium Nitrite and
hydrochloric acid as follows.
NaNO2 + HCl NaCl + HNO2
Sodium Nitrite Nitrous acid
The end point in the sodium Nitrite titration is determined by
the liberation of iodine from iodide, which is expressed in the
following equation
KI + HCl HI + KCl
2HI + 2HNO2 I2 + 2NO + 2H2O
In other words, the small excess of HNO2 present at the end-
point can be detected visually by employing either starch-iodide paper
or paste as an external indicator. Thus, the liberated iodine reacts with
starch to form a blue-green color which is a very sensitive reaction.
Theory/principle:

5. Various Methods for the determination of end point of the
diazotization titration.
1. Dead stop point technique
In this method a potential of 30-50 mv is applied across
two platinum electrodes an automatic pipette is used for the
delivery of nitrite solution. At the end point, the delivery of
Nitrite solution stops automatically.
2. By potentiometry
In this method calomel platinum electrode system is used.
By noting the E.M.F values at a fixed time interval after the each
addition of nitrite, considerably more precise results may
obtained than visual end point is employed. But this method
gives satisfactory results with 0.001M solution of sodium nitrite.

6. 3. Amperometric Method
Scholten and Stone have described an amperometric
method of detecting the end point by using electric circuit of
wernimont and Hopkinson.
A potential of 0.4volt is applied across 2 platinum
electrode. No current flows during the diazotization until an
excess of nitrite has been added. The end point is indicated by the
first flow of current.
At the end point in the diazotization titration there is a
sharp change in the potential of the solution the nitrous acid-
diazo equilibrium is not a reversible system, and the E.M.F
values produced are not entirely reproducible, but the potential
change around the equivalence point is from about + 0.80 - +
0.95 volt.

7. Types of Diazotisation titration
1. Direct titration:
Direct titration are carried out by treating 1 mole of the drug with 3 moles of
acid solution. Ice can be used to lower the temperature to about 0-150C. 0.1M sodium
nitrite solution is added in small amounts and the titration is carried out. The end point
is determined by any one of the technique.
2. Reverse Method:
In this method, a solution of amine and sodium nitrite are run into a solution of
acid. The method is used when the diazonium salt are insoluble.
Eg: Naphthylamine sulphonic acid forms in soluble diazonium salts due to the
formation of zwitter ions. Hence reverse methods is used in such cases.
3. Special methods:
Amino phenols are readily oxidized by nitrous acid to quinones. For such
substances the titration is carried out in the presence of copper sulphate which forms
diazo-oxide. These diazo-oxides are more stable and undergo coupling reaction.

8. PREPARATION OF 0.1 M SODIUM NITRITE
SOLUTIONS
Materials Required: Sodium nitrite
Procedure:
Weigh accurately 7.5 g of Sodium nitrite and add sufficient
Distil water to produce 1 litre in a 1000ml volumetric flask.
Standardisation of 0.1M Sodium Nitrite solution
Materials required:
Sulphanilamide (Previously dried at 1050c for 3 hrs)-0.5g,
HCl( 11.5N)-20ml,
0.1M Sodium Nitrite.

9. 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 in
formation of an unstable nitrite that decomposes ultimately with the
formation of a diazonium salt. The diazonium salt thus produced is
also unstable, if reaction mixture is not maintained between 5-100c it
shall under go decomposition thereby forming phenolic products
which may react further with nitrous acid.
The reaction is as follows:
NaNO2 + HCl HNO2 + NaCl

10. Each ml of 0.1M NaNO2 0.01722g C6H8N2O2S
At equivalence point a slight excess of HNO2 is present which
must persist for at least 1 minute. The excess HNO2 may be detected by
employing either starch iodide strip or paste.
2I- + HNO2 + 2H+ I2 + 2NO + 2H2O
Procedure:
Weigh 0.5g of sulphanilamide and transfer to a beaker, Add to it
20ml of HCl and 50ml of distil water, and cool to 150c in an ice-bath.
Add to it 20ml 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.

11. Assay of Benzocaine
Materilas required:
Benzocaine - 0.4g, Hydrochloric acid ( 11.5N) - 25ml,
water - 50ml, 0.1M sodium nitrite, Starch - iodide paper.
Principle
Aromatic compounds such as Benzocaine reacts with NaNo2
in acid solution to for diazonium salt.
KI + HCl HI + KCl
2HI + 2HNO2 I2 + 2NO + 2H2O

12. Iodine formed reacts with starch mucilage to give the blue
colour
Starch Iodide paper is prepared by immersing a filter paper
in starch mucilage and Potassium iodide solution
Each ml of 0.1M NaNO2 0.01652g of C9H11NO2
Procedure:
Weigh 0.4g of Benzocaine in a beaker add 25ml of
HCl and 50ml of H2O to dissolve. Cool to 150C and titrate
slowly with 0.1M NaNo2 distinct blue color is obtained on a
starch-iodide paper .

13. Assay of Dapsone
Materials required:
Dapsone - 0.5g, Hydrochloric acid ( 11.5 N) - 20ml, 0.1M
Sodium Nitrite, starch-iodide paper.
Principle:
Aromatic primary amine reacts with NaNO2 in acid solution to
form diazonium salt.

14. Assay of Isocarboxazid
Materials required:
Isocarboxazid - 0.5g, Glacial acetic acid (99% w/w or
17.5N) - 20ml, Hydrochloric acid ( 11.5N) - 20ml, 0.1M 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 reaction
involved are expressed as follows.

15. +
Isocarboxazid Benzylhydrazine
+ HNO2 + 2H2O
Benzylazide
H2O

16. Procedure:
Weigh 0.5g isocarboxazide and dissolve in
20ml glacial acetic acid. Add 20ml of hydrochloric
acid and 50ml of distill water cool to 150C and titrate
slowly with 0.1M NaNo2 until a distinct blue color is
obtained on a starch-iodide paper.
Each ml of 0 .1M NaNO2 0.02313g C12H13N3O2

17. Applications of Diazotisation titration:
1. Benzocaine
2. Dapsone
3. Primaquine phosphate tablets
4. Procainamide HCl injection
5. Procaine HCl
6. Sodium aminosalicylate tablets, granules
7.Suramin
8. All sulphadrugs containing free amino groups life
Sulphacetamide Sodium, Sulphadimethoxide,
Sulphadoxine, Sulphadiazine, Sulphaguanidine,
Sulphapyridine, Sulphamethiazole etc.

18. 2. Conversion to amino group by chemical reaction
a. By reduction
I. Metronidazole
II. Secnidazole
III. Chloramphenicol
b. Hydrolysis
I. Paracetamol (Acetyl derivative)
II. Phthalyl Sulphathiazole (Phthalyl derivative)
III. Succinyl Sulphathiazole (Succinyl derivative)

19. References:
1. Pharmaceutical Analysis, Takeru Higuchi, Wiley-Intersciences
publication,
Pg No.141-147
2. Pharmaceutical Drug Analysis, Ashutosh kar, New Age
International publishers,
2nd Edition, Pg 207-210
3. Text book of pharmaceutical Analysis, S.Ravi Shankar, 2nd Edition,
Rx publication,
Pg 21-1 – 21-4
4. The fundamental principles organic chemistry, I.L. Finar, Volume-1,
6th Edition,
Pg-670
5. Indian Pharmacopoeia 2007,Volume 1&2, Pg-784, Appendix 2.3.31,
pg 85.