General Pathway of Sulfur-Chain Breakage of Polythionates by Iodine Confirmed by the Kinetics and Mechanism of the Pentathionate–Iodine Reaction

Kinetics and Mechanism of Selenium(IV) Oxidation by Aqueous Bromine Solution

The bromine-selenite reaction at strongly acidic conditions was investigated by monitoring the absorbance-time traces at the isosbestic point of the bromine-tribromide system at a constant ionic strength (0.5 M adjusted by sodium perchlorate) and temperature. Despite the simplicity of the stoichiometry, the kinetics was found to be very complex. Although the formal kinetic orders of the reactants bromine and selenite are strictly 1, that of the hydrogen ion varies from -2 to less than -3 and notably depends on the initial bromide concentration as well. The bromide ion also inhibits the reaction, making the whole system as a sound example of efficient autoinhibition. We have clearly shown that the inhibitory effect of the bromide ion cannot be explained quantitatively by either exclusively considering the unreactivity of the tribromide ion over elemental bromine or driving the reaction via hypobromous acid formed from the well-known hydrolysis of bromine in aqueous solutions. Instead of that, bromonium ion transfer initiating equilibrium is suggested between the selenium(IV) and bromine species to produce bromide ion and SeO₃Br^- followed by the hydrolysis of this short-lived intermediate. This hydrolytic transformation was found to be catalytic with respect to hydroxide and bromide ions as well. We have also demonstrated that, among the wide variety of selenium species present in the acidic aqueous solution, the best result can be obtained by considering HSeO₃ ^- as the kinetically active species toward bromine. The proposed mechanism containing 10 acid-base equilibria with known equilibrium constants, the above-mentioned initiating equilibrium, and the hydrolysis of SeO₃Br^- is able to fit all 49 kinetic absorbance-traces simultaneously, taking into account properly the most important characteristics of the measured data at strongly acidic conditions. Furthermore, this kinetic model was further extended by the direct reactions of hypobromous acid with selenium(IV) species suggested previously with reasonably modified rate coefficients to describe the pH dependence of the apparent second-order rate coefficients over the pH = 1-13 range, providing a useful tool to predict more accurately the kinetic behavior of selenium(IV) species in water treatment process conditions.

Autoinhibition by Iodide Ion in the Methionine-Iodine Reaction

The methionine–iodine reaction was reinvestigated spectrophotometrically in detail monitoring the absorbance belonging to the isosbestic point of iodine at 468 nm, at T = 25.0 ± 0.1 °C, and at 0.5 M ionic strength in buffered acidic medium. The stoichiometric ratio of the reactants was determined to be 1:1 producing methionine sulfoxide as the lone sulfur-containing product. The direct reaction between methionine and iodine was found to be relatively rapid in the absence of initially added iodide ion, and it can conveniently be followed by the stopped-flow technique. Reduction of iodine eventually leads to the formation of iodide ion that inhibits the reaction making the whole system autoinhibitory with respect to the halide ion. We have also shown that this inhibitory effect appears quite prominently, and addition of iodide ion in the millimole concentration range may result in a rate law where the formal kinetic order of this species becomes −2. In contrast to this, hydrogen ion has just a mildly inhibitory effect giving rise to the fact that iodine is the kinetically active species in the system but not hypoiodous acid. The surprisingly complex kinetics of this simple reaction may readily be interpreted via the initiating rapidly established iodonium-transfer process between the reactants followed by the subsequent hydrolytic decomposition of the short-lived iodinated methionine. A seven-step kinetic model to be able to describe the most important characteristics of the measured kinetic curves is established and discussed in detail.

Kinetics and Mechanism of the Oxidation of Thiourea Dioxide by Iodine in a Slightly Acidic Medium

The thiourea dioxide (TDO)-iodine reaction was investigated spectrophotometrically monitoring the consumption of total amount of iodine at 468 nm, at T = 25.0 ± 0.1 °C, and at 0.5 M ionic strength in buffered slightly acidic medium. The nitrogen- and carbon-containing products were found to be ammonium ion and dissolved carbon dioxide, respectively, while from sulfur part sulfate ion was exclusively detected, when fresh TDO solution was used. The stoichiometry of the reaction was established as 2I₂ + TDO + 4H₂O → SO₄^2- + 2NH₄⁺ + 4I^- + CO₂ + 4H⁺ indicating a strict 2:1 stoichiometric ratio. However, using aged TDO solution this stoichiometric ratio is shifted to lower values suggesting the formation of elementary sulfur augmented by the 2TDO + I₂ + 4H₂O → S + SO₄^2- + 4NH₄⁺ + 2I^- + 2CO₂ hypothetical limiting stoichiometry. We also confirmed experimentally that in aqueous solution TDO slowly rearranges into an unindentified species. This species then produces elementary sulfur at a later stage of the aging process via subsequent reactions accounting for a loss of reducing power. The direct reaction between TDO and iodine was found to be relatively rapid and completed within seconds in absence of initially added iodide ion. Formation of the latter ion, however, strongly inhibits the oxidation process; hence, the system is autoinhibitory with respect to iodide ion. Furthermore, increase of pH markedly accelerates the reaction as well. These observations suggest that a short-lived steady-state intermediate (iodinated TDO) is produced in a rapid pre-equilibrium, where iodide and hydrogen ions are also involved. A nine-step kinetic model, to be able to describe the most important characteristics of the experimental curves with four fitted parameters, is proposed and discussed.

Solid–Liquid Reaction Synthesis of High-Purity Potassium Hexathionate and Its Antibacterial Properties

Hexathionate, an intermediate in the oxidation process of inorganic sulfur compounds, plays important roles in different fields of chemistry, chemical technology, and biology. To achieve innovative results in different fields related to polythionates, the use of hexathionate is crucial, however, it is not yet commercially available. Herein, we report a simple and convenient method to prepare high-purity potassium hexathionate. Hexathionic acid was easily synthesised through a solid–liquid reaction between sodium thiosulfate (solid) and disulfur dichloride. The potassium salt crude product was obtained by neutralising hexathionate acid with potassium hydroxide, and was purified successively using 0.1 M sulfuric acid and 0.5 M hydrochloric acid. A high product purity of 99.8 % (titrimetric analysis, 98.81 %) was obtained. The antibacterial activities of potassium hexathionate against Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative Pseudomonas aeruginosa (P. aeruginosa) were determined by minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) analyses and the cup diffusion method. Potassium hexathionate displayed more effective antibacterial activities against P. aeruginosa than S. aureus. The MICs against P. aeruginosa and S. aureus were 1.95 and 125 µg mL–1, respectively. The corresponding MBC value against P. aeruginosa was 1000 µg mL–1. Furthermore, the duration of the antimicrobial activity, determined by the cup diffusion, shows the potential of potassium hexathionate as a sustained antibacterial candidate. However, there is no bactericidal activity against S. aureus in the concentration range tested.