Electrochemistry redox titrations

In a titration the analytical utility of the measured potential Hes not in its value, which may drift or be otherwise unstable, but in the magnitude of the change of its value near an end point. In a redox titration, the potential changes from something close to the of the analyte to something close to the E of the titrant. This works fine provided the electrochemistries of both analyte and titrant are reversible. The technique may fail, however, if the electrode responds slowly to concentration changes because of irreversibiUty. 

The RDE technique has found widespread use in analytical electrochemistry because of an excellent signal-to-noise ratio resulting from the enhanced mass transport. The RDE method has also been employed for monitoring concentrations in kinetic applications [59], as described for ultramicroelectrodes [60] and in the determination of the stoichiometry for electron-transfer reactions by means of redox titration [61]. The latter procedure will be described next. 

Redox Titrations Electrochemistry Chapter 19 Standard Electrode Potentials Chapter 20 Oxidation/ReductionTitrations Chapter 21 Potentiometry Chapter 17 Using Electrode Potentials Chapter 18 Oxidation/Reduction Titrations Chapter 19 Potentiometry... 

Spectroelectrochemistry, Potential of Combining Electrochemistry and Spectroscopy, Fig. 2 Electrochemical redox titration of the cytochrome c oxidase from Paracoccus denitrificans... 

As ascribed, the EPR spectrum with g = 2.10 can be low-spin Fec(III). When the isolated enzyme is reductively titrated this signal disappears at a potential Emj -0.3 V [65]. This would seem to indicate that the putative Fec(III) form is not relevant, at least not to hydrogen-production activity. The cubane is a one-electron acceptor as it can shuttle between the 2+ and 1 + oxidation states. Therefore, if the active center were to take up a total of two electrons, then the oxidation state of the Fec would, as least formally, shuttle between II and I. Recently, a redox transition in Fe hydrogenase with an Em below the H2/H+ potential has been observed in direct electrochemistry [89]. This superreduced state has not been studied by spectroscopy. It might well correspond to the formal Fec(I) state. For NiFe hydrogenases Fec(I) has recently been proposed as a key intermediate in the catalytic cycle [90] (cf. Chapter 9). 

Use of the potential of a galvanic cell to measure the concentration of an electroactive species developed later than a number of other electrochemical methods. In part this was because a rational relation between the electrode potential and the concentration of an electroactive species required the development of thermodynamics, and in particular its application to electrochemical phenomena. The work of J. Willard Gibbs1 in the 1870s provided the foundation for the Nemst equation.2 The latter provides a quantitative relationship between potential and the ratio of concentrations for a redox couple [ox l[red ), and is the basis for potentiometry and potentiometric titrations.3 The utility of potentiometric measurements for the characterization of ionic solutions was established with the invention of the glass electrode in 1909 for a selective potentiometric response to hydronium ion concentrations.4 Another milestone in the development of potentiometric measurements was the introduction of the hydrogen electrode for the measurement of hydronium ion concentrations 5 one of many important contributions by Professor Joel Hildebrand. Subsequent development of special glass formulations has made possible electrodes that are selective to different monovalent cations.6"8 The idea is so attractive that intense effort has led to the development of electrodes that are selective for many cations and anions, as well as several gas- and bioselective electrodes.9 The use of these electrodes and the potentiometric measurement of pH continue to be among the most important applications of electrochemistry. 

Lui, S.M. and J.A. Cowan (1994). Direct reversible protein electrochemistry at a pyrol54ic graphite electrode. Characterization of the redox thermodynamics of the Fe4S4-siroheme prosthetic center in the hexameric dissimilatory sulfite reductase and the monomeric assimilatory snlflte reductase from desulfovibrio vulgaris (Hilden-hourgh). Systematic pH titration experiments and implications for active site chemistry./.Am. Chem. Soc. 116, 11538-11549. 

---