Plot C in Figure 5 shows Ea vs. I relationship when DAD was used as a mediator in the anodic reaction. As expected, the positive shift in Ea with increasing I was smaller than that in the case of DMBQ (Figure 6B). Eeq,a (=Ecell at I = 0) of the DAD system was –0.05 V, which is more negative than E°9pH 7 of DAD (0.02 V).