New titrimetric and spectrophotometric methods for the determination of promethazine theoclate using metavanadate

Abstract

Four new methods (titrimetric and spectrophotometric, two each) which are accurate, precise and rapid are described for the determination of promethazine theoclate in bulk form and in tablets. The titrimetric methods are based on the oxidation of the drug by metavanadate in acidic condition followed by determination of the unreacted oxidant by titration with iron(II) ammonium sulphate (Method A) or titration of the vanadium(IV) formed, with cerium(IV) sulphate (Method B). The spectrophotometric procedures are also based on the oxidation of the drug with metavanadate followed by the estimation of unreacted oxidant after complexing with chromotropic acid and measuring the absorbance at 420 nm (Method C) or estimation of the vanadium(IV) formed, by reacting it with ferriin and measuring the resulting ferroin at 510 nm (Method D). Method A and Method B are applicable over the ranges 1-20 and 5-12 mg, respectively. Calculations in both methods are based on the reaction stoichiometry of 1:2

(drug: metavanadate). In spectrophotometry, Beer's law is obeyed over the ranges 0.0-125.0 and 10.0-60.0 μg mL^-1 for method C and method D, respectively and the corresponding molar absorptivity values are 2.64×10³ and 5.33×10³ L mol^-1 cm^-1, respectively. The methods were successfully applied to the determination of promethazine theoclate in avomine tablets, and the results obtained were in the range of 97.73-102.24% of declared content with a relative

standard deviation of 1.51-2.22%.