Determination of alpha-lipoic acid in pharmaceutical samples using inhibitory kinetic approach in SLS micellar medium

Abstract

A novel, repeatable, and swift kinetic approach for determining alpha-lipoic acid (ALA) in sodium lauryl sulfate (SLS) micellar medium has been presented, and it has been connected to ALA determination in drug formulations. The approach is based on ALA inhibitory property. ALA (containing two sulfur atoms) forms a chelate with Pd2+, lowering the effective [Pd(II)], and ultimately, the Pd2+ catalyzed cyanide substitution rate from [Fe(CN)6]4- by 4-cyanopyridine (4-CNpy). Fixed times of 5 and 10 minutes were chosen under optimal reaction conditions with [4-CNpy] = 2.0 × 10-3 mole dm-3, pH = 3.50 ± 0.02, Temp = 298 ± 0.2 K, I = 0.1 mole dm-3 (NaClO4), [Fe(CN)64-] = 1.25 × 10-4  mole dm-3, [Pd+2] = 6.0 × 10-5 mole dm-3, and [SLS] = 7.75 × 10-3 mole dm-3 to calculate the absorbance at 477 nm associated with the final substitution product [Fe(CN)5 4-CNpy]3-. ALA's inhibiting influence on the Pd2+ catalyzed cyanide substitution with 4-CNpy from [Fe(CN)6]4-, has been represented by a modified mechanistic approach. The concentration of ALA in various water specimens can be measured at the micro-level down to 1.0 × 10-6 mole dm-3 using the established kinetic spectrophotometric approach. The suggested method is highly reproducible and has been effectively applied to accurately quantify the ALA in pharmaceutical samples. Even as much as 1000 with [ALA], typical additives used in medications do not significantly hinder the determination of ALA.