Diazotization (Sodium Nitrite Titration)

 

INTRODUCTION

In general, aromatic primary amino moiety (i.e., Ar-NH2), as present in a host of sulphadrugs viz., succinyl sulphathiazole, sulphamethoxazole, sulphaphenazole and other potent pharmaceutical substances, for instance sodium or calcium aminosalicylate, isocarboxazid, primaquine phosphate, procainamide hydro-chloride, procaine hydrochloride and dapsone react with sodium nitrite in an acidic medium to yield the corresponding diazonium salts as expressed below :

It is interesting to observe here that the above reaction is absolutely quantitative under experimental parameters. Therefore, it forms the basis for the estimation of pharmaceutical substances essentially contain-ing a free primary amino function as already illustrated earlier.

 THEORY

Nitrous acid is formed by the interaction of sodium nitrite and hydrochloric acid as follows :

NaNO2 + HCl  ------à NaCl + HNO2

 

The end-point in the sodium nitrite titration is determined by the liberation of iodine from iodide which may be expressed by the following equations :

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 employ-ing either starch-iodide paper or paste as an external indicator. Thus, the liberated iodine reacts with starch to form a blue green colour which is a very sensitive reaction. Besides, the end-point may also be accomplished electrometrically by adopting the dead-stop end-point technique, using a pair of platinum electrodes immersed in the titration liquid.