pH-induced difference spectrophotometric methods for drug analysis. Part 2: Determination of mefenamic and flufenamic acid.

On the basis of the spectral changes induced by changing the solvent medium from HCl (0.01 mol . l-1) to NaOH (0.01 mol . l-1), coefficient-difference spectrophotometric methods using the six-points quadratic order of the orthogonal polynomials have been developed. The optimum wavelength ranges were 326 to 386 nm for mefenamic acid and 322 to 382 nm for flufenamic acid, both measured at 12 nm intervals. The respective mean percentage recoveries were 100.1 +/- 0.89 over a concentration range of 0.6-2.6 mg/100 ml of mefenamic acid and 100.7 +/- 0.61 over a range of 0.2-2.4 mg/100 ml for flufenamic acid (p = 0.05). Both methods gave precise results with relative standard deviation less than 1%. Trials to adapt single wavelength difference technique as well as the cubic order of the orthogonal polynomials were also performed; their results were discussed on a statistical basis. The developed procedures have been applied to the analysis of some randomly collected market preparations and the results obtained proved suitability for application in routine analysis.