A similar plot of the HCO 3 -ion concentration was shown in Figures and. The protons produced by the acetate oxidation reaction has a maximum concentration of 10.8 mM over the anode, whereas its maximum concentration on membrane was 6.8 mM. However, the nutrient medium has a calculated buffer capacity of 12.9 mM pH -1 , which limited the solution acidification at the membrane surface to only pH values between 6.8 to 6.