Quantitative Estimation of Dimenhydrinate in Pharmaceuticals Using Redox Reaction with Oxone

The kinetics and mechanism of dimenhydrinate (DMH) oxidation by potassium hydrogen peroxomonosulfate (KHSO₅) in aqueous medium were investigated iodometrically at 20°C. The reaction was found to have a stoichiometry of 1:1. The reaction is first order in KHSO5 and diphenhydramine (DPH) concentrations. An increase in the pH from 7.7 to 10.4 causes an increase in the reaction rate. It is shown that the kinetics of the reaction are subject to the mechanism of acid-base catalysis—the proposed reaction kinetic equation. The oxidation product of DPH is DPH N-oxide (pDPH). Two analytical methods were developed using KHSO₅ as a reagent: a titrimetric method involving N-oxidation with iodometric back titration (method A) and a spectrophotometric method based on the formation of a triiodide chromogen with maximum absorption at 350 nm (method B). Method A was effective for concentrations of 0.5–5.0 mg in 10 mL, while Method B obeyed Beer’s law over 0.1–4.1 μg/mL, with a molar absorptivity of 24.4×10³ L/mol cm. The limits of quantification were 0.05 mg/mL (method A) and 0.11 μg/mL (method B). Both methods were unaffected by common excipients and showed reliability in analyzing bulk DMH s as well as API in Vomacur® and Dramina® tablets of 50 mg and Vomex A® i.v. injection solution of 62 mg/10 mL. Both methods yielded results comparable to official methods, demonstrating their applicability and reliability for quantitatively determining DMH in various pharmaceutical formulations.