This is due to highly reversible characteristics of the electrode reaction of ABTS2– . The equilibrium potential (Eeq,c) at open circuit (and at [BOD]t 5 0.1 mM, [ABTS2–]t 5 0.5 mM) was 0.57 V, which is close to E°p9H 7 of the O2/H2O couple (0.618 V). Since Eeq,c . E°p9H 7(ABTS•–/ABTS2–), the most of ABTS2– are oxidized in the bulk phase.