Kinetics and mechanism of electron-transfer reactions of aqueous and co-ordinated thallium(III). Part X. Kinetics of reduction of hexa-aquathallium(III) by hydrogen peroxide and induction of the reaction by cerium(IV) and iron(II) ions.

Abstract

Thallium(III) and H₂O₂ react in aqueous perchloric acid solution as in equation (i). The kinetics of reaction have Tl^III+ H₂O₂→ Tl ^I+ 2H⁺+ O₂(i) been followed by estimating Tl^III iodometrically and the rate law for [HClO₄]= 0·5–2·0M and I= 0·5–2·5M is –d[Tl^III]/dt=k′[Tl^III][H₂O₂]²/(K_h+[H⁺]). Here k′ is the apparent rate constant, which probably includes a formation constant also, and K_h is the acid dissociation constant of Tl³⁺(aq); k′ has the value 6·7 ± 0·5 l mol^–1 s^–1 at I= 2·0M and 30 °C, and the energy of activation is 10·3 ± 0·7 kcal mol^–1. The reaction is independent of ionic strength in the range 0·5–2·5M(LiClO₄). The reaction is induced by cerium(IV) ions through the intermediate radicals H₂O₂⁺ and/or HO₂˙, and the induction factor is unity. Iron(II) ions act as a strong initiator for the reaction, and hydrogen peroxide can be determined in their presence using thallium(III) as a primary standard.