The Oxidation Reaction of Tl(I) to Tl(III) by the Peroxodisulfate Ion. I. Kinetics and Mechanism in an Aqueous Solution

Abstract

Abstract The oxidation reaction of the thallium(I) ion to the thallium(III) ion by the peroxodisulfate ion has been studied in an aqueous acidic solution. The reaction constituted a chain reaction initiated by the thermal decomposition of the peroxodisulfate ion, the reaction involving no direct reactions between the thallium(I) and peroxodisulfate ions. At thallium(I) ion concentrations larger than 0.004 mol dm−3, the reaction mechanism was assumed to be: S2O82−\oversetk1→2SO4\ewdot; S2O82−+H+\oversetk2→HSO4−+1/2O2+SO3; Tl(I)+SO4\ewdot\oversetk3→Tl(II)+SO42−; S2O82−+Tl(II)\oversetk4→Tl(III)+SO4\ewdot+SO42−; 2Tl(II)\oversetk5\undersetk−5\ightleftharpoonsTl(I)+Tl(III). The rate of the reaction was described as −d[S2O82−]/dt=(k1+k2[H+])[S2O82−]+k4(k1⁄k5)1⁄2[S2O82−]3⁄2. The rate constants at an ionic strength of 0.16 mol dm−3 were determined to be k1=1.99×1019exp[−157 kJ mol−1/RT]s−1, k2=2.75×1012exp[−103 kJ mol−1/RT] dm3 mol−1 s−1, and k4(k1⁄k5)1⁄2=2.81×1013exp[−108 kJ mol−1/RT] dm3⁄2 mol−1⁄2 s−1 in 0.01 mol dm−3 perchloric acid, the k4 value being increased with a decrease in the hydrogen-ion concentration. The ionic strength (μ) dependence was described as logk4(k1⁄k5)1⁄2=−4.17–1.05 μ1⁄2 in 0.01 mol dm−3 perchloric acid at 40 °C. The reaction rate was completely retarded by the addition of 1% acrlyronitrile, 5×10−6 mol dm−3 cerium(III) sulfate, 1×10−3 mol dm−3 cerium(IV) sulfate, or 0.1 mol dm−3 sodium acetate, and it was also remarkably retarded by the addition of 1×10−3 mol dm−3 tetranitromethane. The copper(II) ion and molecular oxygen did not appreciably affect the reaction rate, but the iron (III) ion accelerated it greatly.