Kinetics and Mechanism of the Photoinitiated Autoxidation of Sulfur(IV) in the Presence of Iodide Ion

Abstract

The kinetics and mechanism of the photoinitiated and iodide ion-catalyzed aqueous autoxidation of sulfur(IV) has been studied in a diode-array spectrophotometer using the same light beam for excitation and detection. Light absorption of both the iodide ion and sulfur(IV) contribute to the initiation of a highly efficient radical chain reaction, the overall rate of which depends on the reactant and catalyst concentrations, the pH, and the light intensity in a complex manner. To interpret all the experimental findings, an elaborate scheme is proposed, in which the chain carriers are SO3-*, SO4-*, SO5-*, I*, and I2-*. There are three termination steps, each of them is second-order with respect to the chain carriers. Model calculations and nonlinear fitting have been used to show that the proposed scheme gives an excellent quantitative interpretation of the experimental results.

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

“…It is known from previous studies that the appearance of the break point most likely refers to the total depletion of dissolved oxygen from the system. 21,38,40 This was demonstrated in this work by showing that dissolved oxygen is the limiting reagent in the first phase under the conditions used in this study ( Fig. S2 in ESI †).…”

“…38 There are more similarities with the case of iodide ion initiated photocatalysis, where decreasing acidity accelerated the reaction. 40 The findings displayed in Fig. 2-6 will be interpreted by postulating a mechanism in the next sections.…”

“…This may seem a rather interesting observation in an autoxidation process, where oxygen acts as a reactant, but it is common in similar systems. 38,40 The entire process is not zeroth-order, though, primarily because sulfur(IV) could not be used in high enough excess to be considered constant during a single kinetic run. The rate of reaction (2) as a function of sulfur(IV) concentration is plotted in Fig.…”

Light-induced multistep redox reactions: The diode-array spectrophotometer as a photoreactor

“…Although the basic concept discussed here can be used both in gaseous and liquid phase, experimental limitations and the general design of commercially available spectrophotometers make these studies feasible in liquid phase. The results covered in this paper were obtained in aqueous solutions [5,[7][8][9][10].…”

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

“…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.…”