Kinetics of the bromate–iodide–ascorbic acid clock reaction: different mechanism of the molybdenum and vanadium catalysis

Abstract

The kinetics of the molybdenum(VI)- and vanadium(V)-catalysed bromate–iodide–ascorbic acid clock reaction has been studied by means of spectrophotometry and pH-metric measurements. Two different types of catalysis were detected in this clock reaction. It was shown that Mo^VI catalyses the component process bromate + iodide of the clock system, while V^V catalyses the direct reaction between bromate and ascorbic acid. The rate equation r=k₂[BrO₃^–][I^–][H⁺]²+k′₂[BrO₃^–][I^–][H⁺]²[Mo^VI] describes the kinetics of the Mo-catalysed system, where k₂= 44 ± 2 dm⁹ mol^–3 s^–1 and k′₂=(4.3 ± 1)× 10⁶ dm¹² mol^–4 s^–1. In the vanadium-catalysed clock reaction the equation r=k′₁[BrO₃^–][V^V]+k₂[BrO₃^–][I^–][H⁺]² was found, where k′₁= 9.6 ± 1 dm³ mol^–1 s^–1 and k₂= 42.6 ± 2 dm⁹ mol^–3 s^–1. At lower acidities (pH > 3) more complex kinetics appeared and further study was abandoned.