Chemical oxygen demand by coulometry. An electrochemical device incorporating photooxidation on a TiO₂ surface could replace refluxing with Cr₂O₇^2- to measure chemical oxygen demand (Box 15-2). The diagram below shows a working electrode held at + 0.30 V versus Ag | AgCl and coated with nanoparticles of TiO₂. Upon ultraviolet irradiation, electrons and holes are generated in the TiO₂. Holes oxidize organic matter at the surface. Electrons reduce H₂O at the auxiliary electrode in a compartment connected to the working compartment by a salt bridge. The sample compartment is only 0.18 mm thick with a volume of 13.5 μL. It requires 1 min for all organic matter to diffuse to the TiO₂ surface and be exhaustively oxidized.

The blank curve in the graph below shows the response when the sample compartment contains just electrolyte. Before irradiation, no current is observed. Ultraviolet radiation causes a spike in the current, followed by a decrease to a steady level near 40 μA. This current arises from oxidation of water at the TiO₂ surface under ultraviolet exposure. The upper curve shows the same experiment, but with wastewater in the sample compartment. The increased current arises from oxidation of organic matter. When the organic matter is consumed, the current decreases to the blank level. The area between the two curves tells us how many electrons flow from oxidation of organic matter in the sample.

a) Balance the oxidation half-reaction that occurs in this cell:

where X is any halogen. Express the stoichiometry coefficients A, B, C, D, E, and F in terms of c, h, o, n, and x.
(b) How many molecules of O₂ are required to balance the halfreaction in part (a) by reduction of oxygen (O₂ + 4H- + 4e^- → 2H₂O)?
(c) The area between the two curves in the graph is This is the number of electrons O₂ would be required for the same oxidation?

(d) Chemical oxygen demand (COD) is expressed as mg of O₂ required to oxidize 1 L of sample. Find the COD for this sample.
(e) If the only oxidizable substance in the sample were C₉H₆NO₂ClBr₂, what is its concentration in mol/L?

Transcribed Image Text:

Ultraviolet radiation Organic matter Working electrode held at +0.30 V vs Ag AgCl Oxidation products TiO2 nanoparticle electrode surface coatng on 240 180 120- Sample 60 Blank 0 0 20 40 6 80 100 120 140 Time (s)