CHAPTER III. ELECTROGRAVIMETRY AND COULOMETRY I ...

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Analyte is quantitatively deposited on a previously weighted electrode. FIGURE 3-1. “Harris” Fig. 17-5 (p. 379). Sensitivity is limited by the difficulty in determining the small difference in weight. <strong>III</strong>. Coulometry A form of chemical analysis based on counting the electrons used in a reaction. i/. Potentiostatic Coulometry (Controlled-Potential Coulometry) e.g., EXAMPLE 3-2. “Harris” Ex. (p. 380). Current decreases with time owing to: a) Depletion of sample in the bulk solution. b) Concentration polarization. FIGURE 3-3. “Bard” Fig. 10.3.1 (p. 378). FIGURE 3-4. “Skoog” Fig. 21-1a (p. 519). The total quantity of electricity Q(t) consumed in the electrolysis is given by the area under the I-t curve: Q(t) = ∫ I(t)⋅dt. With a two-electrode cell, the voltage between two electrodes is E = E(cathode) − E(anode) − IR − overpotentials As the analyte, say, Cu 2+ , is used up, the current decreases and both the ohmic and overpotentials decreases in magnitude. If E and E(anode) are constant, then E(cathode) shifts to more negative values and the solutes more easily reduced than H + will be electrolyzed. FIGURE 3-5. “Harris” Fig. 17-7 (p. 380). To prevent the cathode potential from becoming so negative that unintended ions are reduced, a three-electrode cell with a potentiostat to control the cathode potential can be used. FIGURE 3-6. “Harris” Fig. 17-3 (p. 377). Working (indicator) electrode - reaction of interest takes place. 2
Reference electrode - maintain a constant potential irrespective of changes in current. Counter (auxiliary) electrode - to complete the electrochemical cell but its potential is rarely of interest. ii/. Amperostatic Coulometry (Controlled-Current Coulometry) FIGURE 3-7. “Bard” Fig. 10.4.1 (p. 385). When applied current < limiting current, current efficiency = 100% When applied current > limiting current, current efficiency < 100% The selectivity is poorer than the controlled potential method. iii/. Coulometric Titrations Employs a titrant that is electrolytically generated by a constant current, i.e., unstable reagents such as Ag 2+ , Cu + , Mn 3+ , and Ti 3+ can be generated and used in a single vessel. FIGURE 3-8. “Harris” Fig. 17-9 (p. 382). Some external means must be used to determine the end-point of the reaction, e.g., visual indicators, potentiometry, amperometry, and photometry. e.g., 2Br − → Br 2 + 2e H 2 C=CH 2 + Br 2 → H 2 BrCCBrH 2 iv/. Applications a) Neutralization, Precipitation, and Complex-Formation Titrations TABLE 3-9. “Skoog” Table 21-1 (p. 529). b) Oxidation/Reduction Titrations TABLE 3-10. “Skoog” Table 21-2 (p. 530). v/. Comparison of Coulometric and Volumetric Titrations Advantages: a) Absolute technique needing no standard solution - minimizes errors in 3
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CHAPTER III. ELECTROGRAVIMETRY AND COULOMETRY I ...

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