Spectrophotometric determination of certain local anaesthetics using 3-methylbenzothiazolin-2-one hydrazone

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Abstract
A rapid spectrophotometric procedure for the determination of benzocaine, procaine hydrochloride and amethocaine hydrochloride in pure and dosage forms has been developed. It is based on coupling the local anaesthetic drug with 3-methylbenzothiazolin-2-one hydrazone in the presence of ammonium iron(III) sulphate in an acidic medium. The resulting stable blue product has a λmax. of either 575 nm (for benzocaine and procaine hydrochloride) or 615 nm (for amethocaine hydrochloride). The molar absorptivities range from 0.6 × 103 to 5.5 × 104 l mol–1 cm–1. Job plots of absorbance versus molar ratio of amine to reagent indicate a 1 : 1 ratio for benzocaine and procaine hydrochloride and a 1 : 2 ratio for amethocaine hydrochloride. Results of analyses of pure drugs and their dosage forms by the proposed method are in good agreement with those obtained by the USP XXI method.

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