Spectrophotometric determination of etilefrine hydrochloride and ritodrine hydrochloride through nitrosation and subsequent cobalt chelation.

A simple and rapid method for the determination of etilefrine hydrochloride and ritodrine hydrochloride, either in pure form or in pharmaceutical formulations, is described. The method is based on the development of red product in presence of sodium nitrite and cobalt(II) salt in acid medium. The reaction is thought to proceed via preliminary nitrosation of the phenolic nucleus followed by formation of the chelate in presence of cobalt(II) salt. The highly colored chelates showed wavelengths of maximum absorption of 570 and 560 nm for etilefrine hydrochloride and ritodrine hydrochloride, respectively. The reaction product showed apparent molar absorptivities of 938 and 2930 L mol-1 cm-1 for etilefrine hydrochloride and ritodrine hydrochloride, respectively. A linear correlation was found between absorbance (at the lambda max) and concentration in ranges of 0.08-0.20 and 0.04-0.10 mg ml-1 for etilefrine hydrochloride and ritodrine hydrochloride, respectively. At the same time, the resulting colors were well developed within 25 min at boiling water bath temperature and were stable for more than 1 hr. Results of analysis of pure drugs and their dosage forms by the proposed method were in good agreement with either a reported derivative spectrophotometric procedure or the USP XXII method for etilefrine hydrochloride and ritodrine hydrochloride, respectively. The validity of the method was further confirmed using the standard addition method. The proposed method demonstrate high percentage of recovery with good accuracy and precision.