In a simulated-crossover test, the catholyte is spiked with a small volume of anolyte in an isolated vessel, mimicking crossover (Figure ). Quantitative 1 H NMR is then used to monitor the changes in the organic species. Recalling that ferricyanide is paramagnetic and alters the bulk magnetic susceptibility of the solution, its concentration can be indirectly measured from the chemical shift of water (Methods S2). This technique therefore enables the simultaneous quantification of aqueous quinone and ferricyanide following simulated crossover. the final concentration of 2,6-DHAQ was within the range measured in our previous on-line NMR crossover studies (~2 mM). Both the anolyte and catholyte were prepared in 1 M KOH, same as in the full-cell experiments presented in the previous section.