Predicting Redox Reactions
A chemical species may be difficult to characterize, and its derivatives obtained by reduction or by oxidation may be easier to identify than the species itself. It may also turn out that a species endowed with oxidizing or reducing properties must be qua
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Predicting Redox Reactions
A chemical species may be difficult to characterize, and its derivatives obtained by reduction or by oxidation may be easier to identify than the species itself. It may also turn out that a species endowed with oxidizing or reducing properties must be quantitatively determined. In such a case, it is logical to predict its redox titration. Therefore, we must necessarily know the direction and quantitative character of the predicted redox reactions. Predicting redox reactions is not necessarily an easy task. It involves a thorough examination of the reaction’s experimental conditions. In particular, we must examine the influence of such factors as the medium’s acidity and the complexation and precipitation of the redox species. The majority of this chapter is devoted to thermodynamic prediction. The last section mentions some kinetic considerations about redox reactions.
14.1
Redox Phenomena and Acidity
The medium’s acidity may play a part in redox phenomena in several respects. In the first instance, depending on its value, the pH can induce the formation of new redox couples. These new couples involve species that do not exist in other ranges of acidity. In such a respect, the pH may simply change the redox couple’s nature by modifying the acido-basic status of the Ox or Red form, or both. Finally, even if the redox couple is perfectly defined notably from the acido-basic standpoint, the protonic activity of the medium may greatly change its redox power. The example of redox equilibria involving chlorine with the oxidation numbers −I, 0, and +I illustrates the influence of the medium’s acidity on their course: • at pH < 1.2, the three species—hypochlorous acid, dichlorine, and the chloride ions Cl− —are present. The following two equilibria occur: HClO + H+ + 1e− 1/2Cl2(w) + H2 O,
(14.1)
1/2Cl2(w) + 1e− Cl− .
(14.2)
J.-L. Burgot, Ionic Equilibria in Analytical Chemistry, DOI 10.1007/978-1-4419-8382-4_14, © Springer Science+Business Media, LLC 2012
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14 Predicting Redox Reactions
In this range, chlorine can be under the oxidation states −I, 0, and +I. • at pH > 1.2, chlorine can take only two oxidation states: +I and −I, but two pH regions must be distinguished. In the region where 1.2 < pH < 7.5, the following half-redox equilibrium must be taken into account: HClO + H+ + 2e− Cl− + H2 O.
(14.3)
In the range pH > 7.5, it is ClO− + 2H+ + 2e− Cl− + H2 O;
(14.4)
• finally, after examination of equilibrium (14.4), it appears that the potential of the solution, in which the two species ClO− and Cl− coexist, is a function of pH. Indeed, Nernst’s equation is E = E ◦ ClO− /Cl− − 0.06pH − 0.03log Cl− / ClO− .
(14.5)
It must be emphasized that even if the oxidation states do not change in this case, the members of the couple do change with the pH value and, therefore, the nature itself of the couple changes. As a result, several standard potentials must be considered. For example, for the couple HClO/ClO− , we find E ◦ (HClO/ClO− ) = 1.51 V, and for the
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