Kinetics and mechanism of bromide ions oxidation by cerium(IV) in sulphuric acid solutions revisited

Kinetics of Br⁻ anion oxidation by cerium(IV) species in aqueous H₂SO₄ solutions have been reexamined. The rate of reaction was determined spectrophotometrically based on a factor analysis of the absorbance – time data collected in the wavelength range 318–390 nm – the region characteristic for the cerium(IV) sulphato complexes. The data fit very well to a pseudo-first order dependence under a large molar excess of the reductant. The rate law of the form –dCe^IV]/dt = kCe^IV]Br^–]² has been obtained at constant H₂SO₄ concentration and ionic strength I = 2 m. The pseudo-first order rate constant decreases with an H₂SO₄] increase from 0.1 to ca. 0.4 m range, then increases for higher H₂SO₄]. The apparent activation parameters have been calculated from the third order rate constants k for different H₂SO₄].