Journal
CHEMELECTROCHEM
Volume 8, Issue 24, Pages 4762-4773Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/celc.202101232
Keywords
oxidation of S2O32- to S4O62-; reduction with TCNQF(4)(0) and TCNQF(4)(1-); thermodynamics; kinetics; voltammetry; UV-visible spectroscopy
Categories
Funding
- Australian Research Council [DP120101066, DP170103477]
- Umm Al-Qura University scholarship
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The oxidation reaction of thiosulfate occurs in two distinct steps with different kinetics, where the excess of TCNQF(4)(1-) significantly influences the reaction rate. The use of experimental techniques such as voltammetric analysis and UV-visible spectrophotometry provide insight into the time-dependent nature of the reaction and the identity of the products.
The oxidation of thiosulfate has been of interest for more than a century, including the famous oscillating iodine clock reaction. From a thermodynamic perspective, calculations based on the reversible potentials reveal that both neutral TCNQF(4)(0) (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) or its radical anion (TCNQF(4)(1-)) can oxidize thiosulfate to tetrathionate to form TCNQF(4)(2-). The reaction of S2O32- with TCNQF(4)(0) in a 2:1 or greater concentration ratio occurs in a step-wise fashion to initially (and rapidly) form S4O62- and TCNQF(4)(1-) (reaction 1), followed by a slow step involving the oxidation of residual S2O32- by TCNQF(4)(1-) to give more S4O62- and TCNQF(4)(2-) (reaction 2). Thus, this reaction occurs in two, temporally well-resolved, steps with dramatically different kinetics. If S2O32- and TCNQF(4)(1-) are mixed in a 1 : 1 ratio, then S4O62- and TCNQF(4)(2-) are formed quantitatively on the same (hours) timescale, as for the second (slow) step described above. There is no evidence of further oxidation of S4O62- to SO42-. A slight increase in the rate of reaction 2 was observed in the presence of Bu4NPF6 and attributed to ion-pairing effects. Interestingly, during voltammetric experiments, if the Pt counter electrode is not separated from the reaction solution via a salt bridge, reaction 2 is catalyzed. The rates of reaction 2 have been studied over a wide range of conditions. Intriguingly, the mechanisms are dependent on whether S2O32- or TCNQF(4)(1-) (much faster) are in excess. With S2O32- in excess, the rate of reaction is first-order in both S2O32- and TCNQF(4)(1-). With TCNQF(4)(1-) in excess, the reaction is more complicated with indications of Li+ catalysis through electrostatic shielding of the TCNQF(4)(1-) and/or S2O32- reactants. The large difference in rate of reaction of TCNQF(4)(0)/S2O32- (rapid) compared with TCNQF(4)(1-)/S2O32- (sluggish) is in part attributable to the much larger driving force in the former case. All experimental studies were undertaken in the mixed solvent system, acetonitrile (MeCN) containing 5 % water, v/v (95 % MeCN : 5 % water) where all the reactants and products are soluble and stable. Voltammetric analysis using steady-state (microdisc) or transient cyclic voltammetry (macrodisc) with electrolyte (Bu4NPF6) and UV-visible spectrophotometry with or without electrolyte were used to monitor the time dependence and establish the identity of the products.
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