Electrochemical Investigation of the Oxidation of Thiosulfate by 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane and Its Anion Radical

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.

Automated summary

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.