For biological azo dye
reduction, i.e., coupled to the oxidation of organic primary
electron donors by anaerobically incubated bacteria, the E0¢
value of NAD(P)H, the cofactor with the lowest electron
potential (-320 mV), can be taken into account; whereas for
the chemical azo dye reduction by sulfide, the E0¢ value of
HS-/S0 is -270 mV.