Colorimetric assay of cimetidine in the presence of its oxidative degradates.

Three simple, accurate, and sensitive colorimetric methods for the determination of cimetidine (Cim) in pure form, in dosage forms, and in the presence of its oxidative degradates were developed. These methods are indirect, involve the addition of excess oxidant [N-bromosuccinimide (NBS) for method A; cerric sulfate [Ce(SO4)2] for methods B and C] of known concentration in acid medium to Cim, and the determination of the unreacted oxidant by measurement of the decrease in absorbance of amaranth dye for method A, chromotrope 2R for method B, and rhodamine 6G, for method C at a suitable maximum wavelength, lambda max: 520, 528, and 525 nm, for the 3 methods, respectively. Regression analysis of the Beer plots showed good correlation in the concentration ranges of 0.2-4.4 microg/mL for method A, and 0.2-3.4 microg/mL for methods B and C. The apparent molar absorptivity, Sandell sensitivity, and detection and quantitation limits were evaluated. The stoichiometric ratio between the drug (Cim) and the oxidant (NBS or Ce4+) was estimated. The validity of the proposed methods was tested by analyzing pure and dosage forms containing Cim with relative standard deviation < or = 1.18. The proposed methods could successfully determine the studied drug with varying excess of its oxidative degradation products, with recovery between 99.2 and 101.8, 100.2 and 102.8, and 99.8 and 102.0% for methods A-C, respectively.