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Decay rate; Sodium dithionite; Temperature dependent stoichiometry




A homemade instrument for the measurement of oxygen concentration in aqueous solutions measures the decay rate of the phosphorescence of a Pd-porphyrin complex (phosphor) dissolved in the solution, which is flashed every 0.1 s with 630 nm light. The concentration of O 2 is a linear function of the decay rate. The instrument is used to study the reaction of dithionite (S 2O 4 2-) with O 2 at 25°C and 37°C. It is found that the ratio of dithionite to oxygen consumed in the reaction is 1.2 ± 0.2 at 25°C and 1.7 ± 0.1 at 37°C, suggesting a temperature-dependent stoichiometry. At both temperatures, the initial rate of O 2 consumption, -d[O 2]/dt, is found to be 1/2 order in S 2O 4 2- and first order in O 2. This finding is consistent with a previously proposed mechanism: S 2O 4 2- ↔ 2SO 2 - comes to a rapid equilibrium, and SO 2 - reacts with O 2 in the rate-determining step.


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