Kinetic and spectrophotometric determination of thioctic acid in bulk and pharmaceutical formulations

Three simple and sensitive spectrophotometric methods were developed for the determination of thioctic acid in bulk and in its pharmaceutical preparations using iron(III) as an oxidizing agent. Method A is based on kinetic investigation of oxidation reaction of the drug with iron(III) and a Subsequent chelation of the produced iron(II) with ferricyanide to form prussian blue colored product at room temperature for a fixed time of 15 minutes at 750 nm. Methods B and C are based on oxidation of the studied drug with iron(III). The equivalent iron(II) produced is allowed to react with either o-pherianthroline or bipyridyl to give colored species measurable at 5 10, 522 nm, respectively. Regression analysis of Beer-Lambert plots showed a good correlation in the concentration ranges of 0.4-4 mu g/mL with a detection limit of 0.095 mu g/mL for method A and 0.5-5 mu g/mL with detection limits 0.137 and 0.127 for method B and C, respectively. The three methods were successfully applied for the determination of the drug in its dosage forms. The percentage recoveries were 99.88 +/- 1.40, 99.98 +/- 1.26 and 100.64 +/- 1.07, respectively.