DIAZOTIZATION (SODIUM NITRITE TITRATION)
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.
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.