Iodine and disinfection: Theoretical study on mode of action, efficiency, stability, and analytical aspects in the aqueous system

Although they have been in use for nearly 170 years, the mode of action of iodine-based disinfectants is not yet clearly understood, as is manifested, for example, in diverging judgements about the relevance of the individual iodine species. Although studies based on calculated equilibrium concentra tions in pure iodine solutions have already been done, there is a lack of k nowledge about iodine solutions in the presence of additional iodide which would be of intrinsic importance for disinfection practice. Therefore, a re -calculation was undertaken considering variations of this parameter pH ran ge 0-14. The presented calculations concern fresh iodine solutions not affe cted by disproportionation (iodate formation) provide information about the equilibrium concentrations the species I-, I-2, I-3(-), I-5(-), I-6(2_), H OI, HI2O-, OI-, IO2-, and H2OI+, Additional iodide and the pH value have a very pronounced been on the individual equilibrium concentrations (several Bf ten); hence, conditions can be indicated where the number of species of virtual importance is drastically reduced. In the most common case with iod ine in the presence of additional iodide at pH < 6, only I-, I-2, and I-3(- ) play a role. In the absence of additional iodide, at pH 8-9 and at high d ilution (c(I-2) < 10(-5) M), on the other hand, HOI accounts for over 90% o f the oxidation capacity. At high iodide concentration (e.g. Lugol's soluti on) the species I-5(-) and I-6(2-) make up 8.2% of the oxidation capacity. The iodine cation H2OI+, frequently quoted as an active agent in disinfecti on, is without any relevance under the conditions occuring in practice, as are IO- and HI2O- which become important only at pH > 10. The stability pro blem (i.e. rate of iodate formation) arising at pH > 6 can be reduced to hy poiodous acid, as in the simple rate law d[IO3]/dt = 0.25[HOI](3)/[H+] ws a n estimation of stability under weakly alkaline conditions. The results of this study allow us to deduce general of aqueous iodine solutions, such as reactivity, stability, a analytical aspects, and to estimate major disinfec tion-orientated properties such as microbicidal activity, irritation, and i ncorporation effects. Though the calculations consider primarily preparatio ns devoid of polymeric organic compounds capable of of complexing iodine sp ecies, the results can be largely transferred to idophoric preparations.