A kinetic study of rearrangement and degradation reactions of tetrathionate and trithionate in near-neutral solutions.

The kinetics of reactions of trithionate and tetrathionate via different reaction pathways were studied in near-neutral solutions. In the case of trithionate, the predominant degradation reaction is hydrolysis to thiosulfate and sulfate. The pseudofirst-order rate constant is independent of pH and measured to be (6.2 ± 0.2) × 10(-7) s(-1) for the pH range of 5.5-10.5. With tetrathionate, the reaction in both neutral and alkaline solutions occurs via a thiosulfate catalyzed rearrangement reaction to trithionate and pentathionate followed by their further reactions. The data suggest that when thiosulfate is completely absent, this first step will not occur, and in the presence of thiosulfate, the rate of the first step is independent of pH in the pH range 6-8. The secondary reactions include the hydrolysis of trithionate and the further rearrangement and degradation of pentathionate. This mechanism explains the dominant reaction products for both neutral and alkaline solutions. In the presence of thiosulfate and at near-neutral pH, the rearrangement reactions dominate over the degradation reactions and exhibit second-order kinetics, with the rate constants in the pH range of 6-8 determined to be (4.24 ± 0.26) and (1.89 ± 0.18) × 10(-4) M(-1) s(-1) for tetra- and pentathionates, respectively. At extremely low thiosulfate concentrations, the main secondary reaction is the hydrolysis of trithionate at pH < 7, and at pH > 7, the degradation of pentathionate.