Problems with the iodometric titration method used to evaluate the disinfecting activity of sodium hypochlorite

Sodium hypochlorite (NaOCl) is widely used as a disinfectant and antiviral agent because of its strong oxidative properties. The principal antimicrobial species in NaOCl solutions are hypochlorite ion (OCl^-) and hypochlorous acid (HOCl), whose relative proportions vary with pH. The iodometric titration method (KI method) is commonly used to evaluate disinfecting efficacy by measuring available chlorine; however, it cannot distinguish specific oxidizing species. In this study, we investigated the NaOCl-derived oxidizing species quantified by the KI method using absorption spectroscopy, pH measurements, and electron spin resonance (ESR) analysis to determine whether the KI method appropriately reflects disinfecting activity. NaOCl solutions were prepared by dissolving solid NaOCl in ultrapure water, and portions were acidified with sulfuric acid (H₂SO₄). Absorption spectra showed peaks corresponding to OCl^- in NaOCl solutions and HOCl in acidified NaOCl solutions, indicating that most OCl^- was converted to HOCl after acidification. However, the KI method yielded nearly identical oxidizing capacities for both solutions, contradicting reports that HOCl has much higher bactericidal activity than OCl^-. Because the KI method enforces strongly acidic conditions through H₂SO₄ addition, OCl^- is likely converted to HOCl during measurement, meaning the original chemical composition is not preserved. ESR analysis during the KI procedure further suggested the generation of highly reactive hydroxyl radicals (•OH) while HOCl concentrations remained largely unchanged. These findings indicate that OCl^- and HOCl may not be directly measured by the KI method. Our results suggest that the conventional KI-based evaluation of disinfecting efficacy may need to be reconsidered.