Cordelia R. Chinake

Experimental studies of spatial patterns produced by diffusion–convection–reaction systems

The reactions between chlorite ions and a series of sulfur compounds are bistable and autocatalytic in hypochlorous acid. Unstirred solution mixtures of chlorite ions and thiourea, for example, can generate a travelling wave of chemical reactivity at the surface from a point of initial perturbation These reactions are highly exothermic and exhibit a sharp temperature jump at the wave front (ΔT ≈ 3–5 °C). In stoichiometric excess of chlorite ions and in unstirred solutions the travelling wave is followed by spatial patterns in the bulk of the solution. The spatial patterns, which show areas of varying acid concentrations, can be sustained for up to 15 min. Formation of the travelling wave is due to thermocapillary effects. The transition to patterns is fuelled by the coupling of buoyancy forces with thermocapillary convection.

Kinetics and mechanism of the reaction between Thymol Blue and bromate in acidic medium

The reaction between Thymol Blue (TB) and bromate in the presence of sulfuric acid has been studied by monitoring the depletion kinetics of TB. The reaction exhibits complex kinetic behaviour. Under excess bromate and acid conditions, the reaction was initially fast and was followed by a kinetically controlled slow reaction. After an induction period, the reductant again underwent a rapid depletion. The kinetic data were analysed by determining the reaction rates under the two rapid depletion conditions and by estimating the induction time between the two rapid depletion stages. Under the initial conditions the rate-limiting step was found to be first order with respect to H+, bromate and TB. The overall potential of the reaction increased continuously with the progress of the reaction. The concentration of bromide dropped initially and built up slowly reaching a maximum before decreasing rapidly with corresponding fast depletion of Thymol Blue. At low bromate concentration TB reacted with bromate with a stoichiometric ratio of 3 : 2, but with excess bromate the ratio increased to 4 : 5. The kinetics of the reaction beween Thymol Blue and aqueous bromine were monitored by a stopped-flow technique. The probable reaction mechanism with bromide acting as an autocatalyst is discussed.

Oxyhalogen–sulfur chemistry: Kinetics and mechanism of the oxidation of a Bunte salt 2-aminoethanethiolsulfuric acid by chlorite

Spectrophotometric and 1H NMR methods have been used to study the kinetics and mechanism of oxidation of the Bunte salt, 2-aminoethanethiol sulfuric acid, H2NCH2CH2S-SO3H (AETSA) by chlorite in mildly acidic media. The reaction is characterized by a long quiescent induction period followed by rapid and autocatalytic production of chlorine dioxide. The formation of chlorine dioxide is much more pronounced in stoichiometric excess of chlorite. The stoichiometry of the reaction in excess chlorite just before formation of chlorine dioxide was determined to be: 2ClO2− + H2NCH2CH2S–SO3H → ClNHCH2CH2SO3H + SO4−2 + Cl− + H+, while in excess AETSA the stoichiometry was: 3ClO2− + 2H2NCH2CH2S–SO3H + 2H2O → 2NH2CH2CH2SO3H + 2SO4−2 + 3Cl− + 4H+. Although the products in excess chlorite also included pure taurine and dichlorotaurine, monochlorotaurine was the dominant species at pH 1–3. This Bunte salt showed a facile S–S bond cleavage after a single S-oxygenation step on the inner sulfur atom. The sulfoxide is quite stable but there was no experimental evidence for the existence of the sulfone-sulfonic acid. Sulfate production was almost quantitative for the oxidation of only one of the sulfur atoms. Further reaction of the taurine occurred only on the nitrogen atom with no cleavage of the C–S bond. A 21-reaction kinetics scheme model gave reasonable agreement with experiment.