Colorimetric determination of benzocaine, lignocaine and procaine hydrochlorides in pure form and in pharmaceutical formulations using p-benzoquinone

A simple, accurate and sensitive method for the microdetermination of benzocaine, lignocaine and procaine hydrochlorides in pure forms and in pharmaceutical formulations is described. The procedure is based on the reaction of those drugs in an aqueous acidic medium with p-benzoquinone to form charge-transfer complexes. The method has been used for the determination of 5.0 - 70, 5.0 - 60 and 5.0 - 90 mug ml(-1) of benzocaine, lignocaine HCl and procaine HCl, respectively. The complexes have apparent molar absorptivities of 1.70 x 10(3), 2.79 x 10(3) and 2.42 x 10(3) L mol(-1) cm(-1) and Sandell sensitivities of 9.72, 10.34 and 11.25 ng cm(-2), respectively. The proposed procedure of analysis is as accurate as the British Pharmacopoeial method (2003). The method was successfully used for the determination of those drugs in the presence of their degradation products, additives and excipients, which were normally encountered in pharmaceutical formulations.