engleski

Quantification of iodide oxidation by trichloromethyl peroxyl radicals and I(-) + I(2)-- I(3) equilibrium in alcohol/water mixtures

The molar absorption coefficient (e) of I3- at lmax, 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 lmax for I3- is shifted to longer wavelengths (from 350 to 357 nm). This red-shift is accompanied by a drop of e from 25400 dm3 mol-1 cm-1 in pure H2O to, e.g., 14000 dm3 mol-1 cm-1 in a 70% (v/v) of 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 mmol 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 g-irradiated solutions confirmed the formation of I3- in high yields, with limiting values of G @ 0.9 mmol 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.

peroxyl radicals; pulse radiolysis

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