In the aged sample of 2,6-DHAQ-spiked ferricyanide, a set of new peaks is also observed to the right of the 2,6-DHAQ HB resonance (Figure , shaded blue box). The appearance of these new peaks is another indicator that side reactions have taken place between ferricyanide and 2,6-DHAQ. Total correlation spectroscopy (TOCSY) was used to determine if these new proton environments were coupled to the 2,6-DHAQ protons (Methods S5). TOCSY uses through-bond interactions to reveal cross-peaks between all protons in a given spin system, provided there are couplings between every intervening proton. No cross-peaks were observed between the new peaks and the 2,6-DHAQ protons (Figure ); however, diffusion-ordered NMR spectroscopy (DOSY) experiments, revealed that the diffusion coefficient of the new proton environment at 8.6 ppm matches that of 2,6-DHAQ (Methods S6 and Figure , blue shaded box). This indicates that the new peak corresponds to a species with the same hydrodynamic radius as 2,6-DHAQ, or a species that is strongly interacting with 2,6-DHAQ (Figure ). To gain further insight into how 2,6-DHAQ influences ferricyanide losses, on-line 1 H NMR simulated-crossover experiments were performed (Figure and Methods S4). Here, a flow NMR tube was used so that a quantitative 1 H NMR spectrum of the catholyte could be acquired every 5 min throughout the simulated-crossover experiment (Methods S4 and Figure ). First, the ferricyanide loss rate was measured in the absence of 2,6-DHAQ. The measured ferricyanide loss rate was constant at 6 x 10 -4 mol dm -3 h -1 , which was consistent between repeats when performed in D2O and H2O (Figure and Figure ). Notably, this ferricyanide loss rate is higher than the ferricyanide loss rate measured in the ex situ NMR simulated-crossover experiments (1.3 x 10 -5 mol dm -3 h -1 ), possibly reflecting the influence of sample flow or the shape of the reaction vessel on reaction rate (Methods S3 and S4).