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 λmax= 420 nm and Iµmax= 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.