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Section 13

Section 13. Analytical Voltammetry. There is a micro working electrode, a reference electrode, and an auxiliary electrode. Current flows between the working and auxiliary electrodes. The voltage between the working and reference electrodes is recorded.

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Section 13

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  1. Section 13 Analytical Voltammetry

  2. There is a micro working electrode, a reference electrode, and an auxiliary electrode. Current flows between the working and auxiliary electrodes. The voltage between the working and reference electrodes is recorded. Fig. 15.1. Setup for voltammetric measurements. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

  3. When the decomposition potential for reduction or oxidation of the analyte is reached, electrolysis begins. A limiting current is reached when the rate of electrolysis equals the rate of diffusion. In an unstirred solution, the diffusion layer extends futher into the solution with time, so the limiting current decreases exponentially. Stepwise electrolysis occurs for species with different oxidation states, or for mixtures. Fig. 15.2. Different types of voltammetric curves. (a) Stirred solution or rotated electrode. (b) unstirred solution, and (c) stepwise reduction (or oxidation) of analyte or of a mixture of two electroactive substances (unstirred solution). ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

  4. This is a complete cell. Oxygen diffuses through the membrane and is reduced at the platinum cathode ring. The anode coil is a Ag/AgCl reference electrode. Fig. 15.3. Construction of an oxygen electrode. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

  5.  Glucose oxidase is immobilized on the Pt electrode. This catalyzes reaction of glucose with oxygen: glucose + O2 = gluconic acid + H2O2 The H2O2 is oxidized at the Pt electrode by applying +0.6 V vs. Ag/AgCl: H2O2 = O2 + 2H+ + 2e- Fig. 15.4. Amperometric glucose electrode. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

  6. The mediator layer, Mox, acts as an electrocatalyst for the electrooxidation of an analyte with slow electron transfer. So a smaller potential needs to be applied, giving better selectitvity. Fig. 15.5. Redox modified electrode. Red is the analyte being measured, in the reduced form. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

  7. The microdisk electrode is about 5-30 mm. This smaller than the diffusion layer thickness. This results in: enhanced mass transfer independence on flow increased signal-to-noise ratio S-shaped curve in an unstirred solution can measure in nonaqueous solvents il = 2nFDCd Fig. 15.6. Ultramicro electrode. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

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