Kinetics of the reaction between titanium(III) and vanadium(V)

Abstract

The kinetics of the one-equivalent reaction between titanium (III) and vanadium(V), Ti^III+ V^V→ Ti^IV+ V^IV, have been studied bystopped-flow spectrophotometry. The rate is independent of chloride-ion concentration over the range up to 0·5M investigated (replacement of perchlorate by chloride). Subsequently hydrochloric acid reactant solutions adjusted to I= 0·5M with NaCl were used, therby avoiding the use of titanium(III) in perchlorate solutions. At temperatures 2·6–35° and with hydrogen-ion concentrations in the range 0·04–0·49M the rate law (i) gives a satisfactory fit to the experimental data. At 25°, k₀=(7·8 ± 0·5)× 10³| mol^–1 s^–1 and –d[V^V]//_dt=(k₀+k_–1[H⁺]^–1)Ti^III[V^V](i)k_–1=(8·3 ± 0·4)× 10² s^–1 and activation parameters are ΔH^‡₀= 7·2 ± 0·2 kcal mol^–1, ΔS^‡₀=–16·6 ± 0·8 cal K^–1 mol^–1, ΔH^‡_-1= 14·7 ± 0·3 kcal mol^–1, and ΔS^‡_–1= 4·1 ± 1·1 cal K^–1 mol^–1. With the titanium(III) reactant in an excess a much slower second stage involving titanium(III) reduction of vanadium(IV) is observed. Use of stainless-steel syringe needles resulted in serious contamination of reactant solutions, and it was necessary to use Teflon needles at all times.