Oxidation of thiocyanate and iodide ions by hydrogen atoms in acid solutions. A pulse radiolysis study

Abstract

The kinetics and mechanism of the oxidation of SCN^– and I^– by H atoms in acidic aqueous solution have been studied using pulse radiolysis by following the formation of (SCN)^˙–₂ and I^˙–₂. The formation of HSCN^˙– has been observed as a precursor of (SCN)^˙–₂; it has λ_max= 420 nm and ε_max= 4.2 × 10² m² mol^–1 and its formation rate constant k(H + SCN^–)=(2.3 ± 0.1)× 10⁸ dm³ mol^–1 s^–1. Kinetic measurements are consistent with the following mechanism: [graphic omitted] where R = reduction products. With SCN^– R included volatile —SH compounds (probably H₂S) but not H₂, and with I^– R = H₂. For X^–= SCN^–, K= 0.81 ± 0.08 dm³ mol^–1, k₁= 3.0 × 10⁵ and k₂= 2.8 × 10⁶ dm³ mol^–1 s^–1; and for X^–= I^–, K= 0.82 ± 0.13 dm³ mol^–1, k₁⩽ 10⁵ and k₂=(2.3 ± 0.4)× 10⁷ dm³ mol^–1 s^–1. The spectrum of HI^˙– could not be detected. HSCN^˙– reacts rapidly with O₂ with a rate constant of (7.6 ± 0.6)× 10⁸ dm³ mol^–1 s^–1, but H(SCN)^˙2–₂ appears to be unreactive. No evidence for oxidation of Br^– and Cl^– by H could be detected.