Effective spectrophotometric methods for resolving the superimposed spectra of Diclofenac Potassium and Methocarbamol

The UV spectra of Diclofenac Potassium DIC and Methocarbamol MET are superimposed making their analysis using direct or derivative spectrophotometric methods quite complicated. This study presents four effective spectrophotometric methods that enable simultaneous determination of both drugs without interference. The first method is based on application of simultaneous equation method on their zero order spectra where DIC has shown absorbance maxima at 276 nm and MET displays two absorbances maxima at 273 nm and 222 nm in distilled water. The second method relies on dual wavelength method, the two wavelengths (232 and 285 nm) were chosen for determination of DIC where the absorbance differences at these wavelengths are proportional to DIC concentration while the absorbance differences of MET are equal to zero. For the determination of MET, the two wavelengths (212 and 228 nm) were selected. The third method of first-derivative ratio has been applied where the derivative ratio absorbances of DIC and MET were measured at 286.1 and 282.4 nm, respectively. The fourth method utilizing ratio difference spectrophotometric (RD) method was eventually performed on the binary mixture. The amplitude difference between the two wavelengths (291and 305 nm) was calculated for DIC estimation while the amplitude difference between the two wavelengths (227and 273 nm) for MET determination. All methods show linearity range from 2.0-25 μg. mL^-1 and 6.0-40 μg. mL^-1 for DIC and MET respectively. The developed methods have been statistically compared with a reported method based on first derivative method and the results of statistical comparison confirm the accuracy and precision of the proposed methods therefore they can be effectively applied for determination of MET and DIC in pharmaceutical dosage form.