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New Titrimetric And Spectrophotometric Methods For The Assay Of Promethazine In Pharmaceuticals Using N-Chlorosuccinimide And Two Dyes

K.Basavaiah, B.C.Somashekar, U.R.Anilkumar, V.Ramakrishna

Titrimetric and spectrophotometric methods, two each, are described for the assay of promethazine hydrochloride (PH) in bulk drug and in dosage forms using N-chlorosuccinimide (NCS) and two dyes, methyl orange and indigo carmine as reagents. In direct titrimetry (method A), aqueous solution of PH is titrated directly with a standard solution of NCS in acid medium and in the presence of potassium bromide (KBr). Indirect titration (method B) involves the addition of a measured excess of standard NCS to PH solution in acid medium followed by iodometric back titration of unreacted oxidant. Spectrophotometric methods entail the addition of a known excess of NCS to a solution of PH in acid medium and in the presence of KBr followed by determination of residual bromine by reaction with a fixed amount of either methyl orange and measuring the absorbance at 520 nm (method C) or indigo carmine and measuring the absorbance at 610 nm (method D). In all the methods, the amount of NCS reacted corresponds to the amount of PH. In both titrimetric methods, the reaction stoichiometry was found to be 1:1 (PH : NCS) and are applicable over 2-20 mg (method A) and 1.5- 10 mg (method B) ranges. In spectrophotometric methods, the absorbance was found increase linearly with the correlation coefficients of 0.9998 and 0.9995 for method C and method D, respectively. The systems obey Beer’s law for 0.5-6.0 μg/ml (method C) and 1-10 μg/ml (method D). The calculated apparent molar absorptivity values are found to be 3.92 ×104 and 1.69 ×104 l/mol/cm for method C and method D, respectively, and the corresponding Sandell sensitivity values are 0.0082 and 0.0019 μg/cm2. The limits of detection and quantification are reported for both spectrophotometric methods. Intraday and inter-day precision and accuracy of the methods were evaluated as per ICH guidelines. The methods were successfully applied to the determination of PH in tablets, injections and syrup, and the results were statistically compared with those of a reference method by applying Student’s t- and F-tests. No interference was observed from common additives and excipients found in dosage forms. The accuracy and reliability of the methods were further ascertained by performing recovery test via standard addition method.