Kinetics of the light-driven aqueous autoxidation of sulfur(iv) in the absence and presence of iron(ii)

Abstract

The photochemical autoxidation of aqueous, acidic sulfur(IV) solutions was studied in the absence and presence of iron(II) by a newly introduced technique using a diode-array spectrophotometer, in which the same light source is used to drive and detect the reaction. Based on detailed kinetic and stoichiometric data sets, a non-chain mechanism is proposed for the autoxidation of sulfur(IV). In this mechanism, excited hydrated sulfur dioxide, *H2O.SO2, first reacts with O2 to form peroxomonosulfate ion, HSO5-, which rapidly oxidizes another H2O.SO2 to give hydrogensulfate ion as a final product. In the presence of iron(II), the formation of iron(III) was detected, which can be interpreted through the simultaneous contribution of two additional pathways: some of the HSO5- formed oxidizes iron(II) instead of sulfur(iv), and *H2O.SO2 also reacts directly with iron(II) to yield iron(III). This mechanism provides a sufficient quantitative interpretation of all experimental observations.

Aqueous photochemical reactions of chloride, bromide, and iodide ions in a diode-array spectrophotometer. Autoinhibition in the photolysis of iodide ions

“…[45][46][47] UV-light, however, drives the reaction. 20 In the earlier study of this process, 20 sulfuric acid and perchloric acid were used to set the acidity. Fig.…”

“…Quantitative kinetic measurements on the photochemical reactions were performed with the HP-8543 diode-array instrument in a stoppered standard quartz cuvette (optical path length: 1.000 cm) using the method and general operating protocols described in earlier publications. 20,27 The reaction mixtures were kept homogeneous during the photochemical experiments by constant stirring with a 3 mm magnetic stirring rod using the built-in magnetic stirrer of the cell compartment of the HP-8543 instrument. …”

“…However, a number of examples have shown that carefully planned studies can actually use this effect for a highly quantitative study of photoreactions. 16,[18][19][20][21][22][23][24][25][26] These experiments sometimes afford mechanistic information not available from classical photochemical experiments. The limitation of the experiments is that the light source is necessarily polychromatic, although its spectral properties can be changed within certain limits using suitable filters or external light sources such as LEDs.…”

Kinetics of the autoxidation of sulfur(iv) co-catalyzed by peroxodisulfate and silver(i) ions

“…Diode array spectrophotometers were not used to avoid known problems of photoinitiation by the analyzing light beam. [38][39][40][45][46][47][48] Kinetic experiments were carried out in standard quartz cuvettes (optical path length = 1.000 cm). Constant temperature (25.0 ± 0.1°C) was maintained with an external thermostat and circulating thermal bath.…”

“…36,39 In addition, several autoxidation processes with chain mechanisms have been studied including the catalytic effect of iron(III), 21,35 and photocatalysis in the presence of cerium(III) 38 and iodide 40 ions. The chain carriers common to the different systems were sulfite ion radical (SO 3 −• ), sulfate ion radical (SO 4 −• ), and peroxomonosulfate ion radical (SO 5 −• ).…”