Voltametric determination of vancomycin in dosage forms through treatment with nitrous acid

Two methods are described for the determination of vancomycin (vancomycin hydrochloride, CAS 1404-93-9) in its dosage forms. The two methods involve a prior treatment with nitrous acid then measuring the formed nitroso derivative, either spectrophotometrically or polarographically. In the spectrophotometric method, the absorbance-concentration plot is rectilinear over the range of 4-32 micrograms/ml with minimum detectability of 2.7 micrograms/ml (1.8 x 10(-6) mol/l). The apparent molar absorptivity is 4.084 x 10(-4)l.mol-1.cm-1 and A (1%, 1 cm) is 275. The reaction product was also found to be polarographically reducible at the Dropping Mercury Electrode (DME) with E1/2 of-0.9 V vs. Ag/AgCl electrode and a diffusion current constant (Id) of 0.85 +/- 0.02. The cathodic current produced was found to be diffusion controlled with some adsorption contribution. The calibration plot was linear over the range of 0.015-0.06 mmol l-1 for direct current (DCt) mode and from 0.005-0.05 mmol l-1 for differential pulse polarography (DPP) mode with minimum detectability of 2.4 x 10(-7) mol l-1 using the latter technique. The results obtained were statistically compared with those given with the official B.P. method and were in good agreement.