Large Amplitude pH Oscillations in the Hydrogen Peroxide−Dithionite Reaction in a Flow Reactor

Kovács, Klára; Rábai, Gyula

doi:10.1021/jp012007o

Cited by 18 publications

(12 citation statements)

References 17 publications

(34 reference statements)

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“…In batch, these pH-regulated reactions generally display an induction period with a high pH, followed by a rapid (or damped (oligo) oscillatory) transition to a lower pH. In flow, they are capable of displaying sustained large-amplitude oscillations in pH …”

Section: Introductionmentioning

confidence: 99%

Complex Behavior in the Formaldehyde−Sulfite Reaction

et al. 2004

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The formaldehyde-sulfite reaction is an example of an "acid-to-alkali" clock. It displays an induction period, during which the pH varies only slowly in time, followed by a reaction event, during which the pH increases rapidly by several units. When the reaction is performed in a closed (batch) reactor, the clock time is found to increase with a decrease in initial concentrations of formaldehyde and sulfite and an increase in the total initial concentration of S(IV). At long times, following the clock event, there is a slow decrease in pH. In an open (flow) reactor, bistability between a low-pH steady state (pH approximately 6-8) and a high-pH steady state (pH approximately 11) is observed. Additionally, we report the existence of sustained, small-amplitude oscillations in pH in this system. An extended kinetic mechanism reproduces the batch behavior but fails to account for the complex behavior observed in the flow reactor. Possible additional reaction steps are discussed.

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Section: Introductionmentioning

confidence: 99%

Complex Behavior in the Formaldehyde−Sulfite Reaction

et al. 2004

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“…Dithionite is a powerful reducing agent and very easily oxidized. ,, It undergoes rapid disproportionation in aqueous solution to form more stable sulfur compounds in different oxidation states ranging from HS – to SO 4 2– . The main products of the decomposition are sulfite and thiosulfate: 2 normalS 2 normalO 4 2 − + normalH 2 normalO → 2 HSO 3 − + normalS 2 normalO 3 2 − …”

Section: Resultsmentioning

confidence: 99%

Detailed Mechanistic Studies into the Reactivities of Thiourea and Substituted Thiourea Oxoacids: Decompositions and Hydrolyses of Dioxides in Basic Media

et al. 2014

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Dioxides of methylthiourea (methylaminoiminomethanesulfinic acid, MAIMSA) and dimethylthiourea (dimethylaminoiminomethanesulfinic acid, DMAIMSA) were synthesized and, together with thiourea dioxide (aminoiminomethanesulfinic acid, AIMSA), were studied with respect to their decompositions and hydrolyses in basic aqueous media. All three were stable in acidic media and existed as zwitterions with the positive charge spread out on the 4-electron 3-center N-C-N skeleton and the negative charge delocalized over the two oxygen atoms. All three are characterized by long and weak C-S bonds that are easily cleaved in polar solvents through a nucleophilic attack on the positively disposed carbon center, followed by cleavage of the C-S bond. The sulfur moiety leaving groups are highly unstable, reducing, and rapidly oxidized to S(IV) as hydrogen sulfite in the presence of oxidant. In aerobic conditions, molecular oxygen is a sufficient and efficient oxidant that can oxidize, at diffusion-controlled limits, the highly reducing sulfur species in one-electron steps, thus opening up a cascade of possibly genotoxic reactive oxygen species, commencing with the superoxide anion radical. Radical formation in these decompositions was confirmed by electron paramagnetic resonance techniques. In strongly basic media, decomposition of the dioxides to yield sulfoxylate (SO2(2-), HSO2(-)) is irreversible and, in anaerobic environments, will disproportionate to yield more stable sulfur species from HS(-) to SO4(2-). Decomposition products were dependent on concentrations of molecular oxygen. Solutions open to the atmosphere, with availability to excess oxygen, gave the urea analogue of the thiourea and sulfate, while in limited oxygen conditions hydrogen sulfite and other mixed oxidation states sulfur oxoanions are obtained. DMAIMSA has the longest C-S bond at 0.188 nm and was the most reactive. MAIMSA, with the shortest at 0.186 nm, was the least reactive. Electrospray ionization-mass spectrometry data managed to detect all of the formerly postulated intermediates.

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“…The so-called hydrogen peroxide/dithionite (HPD) pH oscillator reported in the literature is a good choice. 12 As shown in Fig. 2a, the solution displayed continuous and cyclic pH changes with period of about 100 seconds and amplitude as large as 7 pH units ranging from 3 to 10.…”

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confidence: 87%