Direct determination of chemical oxygen demand by anodic oxidative degradation of organics at a composite 3-D electrode

Two layers of α-PbO2- and CeO2-doped β-PbO2 were anodically deposited on Ni foam substrate successively, and the prepared Ni/α-PbO2/CeO2-β-PbO2 electrode was used as a sensor to determine chemical oxygen demand (COD) by coulometric method. SEM and EDS disclose that the layers of α-PbO2 and β-PbO2 are compactly attached to the surface of the 3D structure substrate, and CeO2 is uniformly co-deposited with β-PbO2 onto the anode. Results show that when the prepared Ni/α-PbO2/CeO2-β-PbO2 is used to oxidize the organic compounds at a constant potential of + 1.45 V (vs. Ag/AgCl) in aqueous solution, the anodic charge quantity is directly proportional to the amount of the organic compounds in the range of 15–1000 mg L−1, equivalent to the quantity of COD, no matter what species of the organic compounds is detected. This means that the universal coulometric method is only decided by the total amount of COD but unrelated to the species of COD. It takes only 10~20 min to detect one sample by the well-designed electrode–cell system in this study due to the high generation rate of ·OH on β-PbO2, good electrical conductivity of CeO2-doped PbO2 layer, and the great ratio of area-to-volume (S/V) through elaborative cell–electrode system design. These properties enable the Ni/α-PbO2/CeO2-β-PbO2 electrode to be a promising sensor for coulometric determination of COD in industrial application.