Quantification of iodide oxidation by trichloromethyl peroxyl radicals and I− + I2 ⇌ I3−equilibrium in alcohol/water mixtures

Abstract

The molar absorption coefficient (e) of I3− at λmax, and the equilibrium constant (Keq) for the equilibrium I2 + I− ⇌ I3− were determined for various alcohol/water mixtures by using standard analytical procedures, i.e. measurement of optical densities via UV spectrophotometry. The stability of I3− is found to increase significantly with increasing amount of alcohol with Keq ranging from 7.1 × 102 dm3 mol−1 in H2O to, e.g., 2.5 × 104 dm3 mol−1 in 70% (v/v) of 2-propanol/H2O mixture. Also, by increasing the amount of alcohol, the position of λmax for I3− is shifted to longer wavelengths (from 350 to 357 nm). This red-shift is accompanied by a drop in e from 25 400 dm3 mol−1 cm−1 in pure H2O to, e.g., 14 000 dm3 mol−1 cm−1 in a 70% (v/v) of a 2-propanol/H2O mixture. The obtained data were applied for quantitative determination of I3− by means of the time-resolved technique of pulse radiolysis, where this product was formed via oxidation of a total of three iodide ions by each CCl3OO˙. The overall process afforded formation of one equivalent each of I2 (complexed by I− to I3−) and I˙ (complexed to I2˙− and suffering as such disproportionation). An excellent agreement was obtained with the data from the standard analytical measurements. Furthermore, the radiation chemical yields of I3− generated in these pulse radiolysis experiments (e.g., G(I3−) = 0.96 μmol J−1 in 2-propanol/H2O mixtures) are in complete correspondence with a previously suggested multi-electron oxidation mechanism. Complementary steady-state product analysis of γ-irradiated solutions confirmed the formation of I3− in high yields, with limiting values of G ≅ 0.9 μmol J−1 in 2-propanol/water mixtures over wide ranges of pH, iodide, CCl4, CH2Cl2 and 2-propanol concentrations. All the results also strongly corroborate corresponding oxidation mechanisms of organic sulfides and selenides by halogenated peroxyl radicals.