Aqueous chemistry of thallium(II). Part I. Kinetics of reaction of thallium(II) with cobalt(II) and iron(III) ions and oxidation–reduction potentials of thallium(II)

Abstract

Reactions of Tl²⁺ ions [generated by flash photolysis of thallium(III) solutions] with cobalt(II) and iron(III) have been studied. For the reaction Tl²⁺+ Co²⁺⇌ Tl⁺+ Co³⁺ the forward rate constant is (6·2 ± 0·5)× 10³ l mol^–1 s^–1(25 °C, [H⁺]= 0·25, I= 0·75M) and the equilibrium constant calculated from this rate constant and that of the reverse reaction is 2·0 × 10⁶. The forward rate constant for the reaction Tl²⁺+ Fe³⁺⇌ Tl³⁺+ Fe²⁺ is (1·1 ± 0·15)× 10⁶ l mol^–1 s^–1(25 °C, [H⁺]= 0·25, I= 0·30M) and the equilibrium constant calculated from this value and that of the reverse reaction is 2·4 × 10⁷. Standard reduction potentials for the reactions Tl³⁺+e -⇌ Tl²⁺(E₁⁰) and Tl²⁺+ e^–⇌ Tl⁺(E₂⁰) are +0·33 ± 0·05 and +2·22 ± 0·05 V respectively. The thermal electron-exchange reaction between thallium(I) and thallium(III) is discussed and a mechanism involving intermediate formation of Tl²⁺ ions is shown to be inconsistent with equilibrium and kinetic data. The disproportionation reaction Tl²⁺+ Tl²⁺→ Tl⁺+ Tl³⁺ has a rate constant of (5·5 ±0·5)× 10⁸ l mol^–1 s^–1(25 °C, [H⁺]= 0·25, I= 0·25M) and is diffusion controlled with an activation energy of 7·9 ± 1·5 kJ mol^–1.