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Metallic Indicator Electrodes
The potential of the indicator electrode in a potentiometric electrochemical cell is proportional to the concentration of analyte. Two classes of indicator electrodes are used in potentiometry: metallic electrodes, which are the sub- ject of this section, and ion-selective electrodes, which are covered in the nextsection.
The potential of a metallic electrode is determined by the position of a redox reaction at the electrode–solution interface. Three types of metallic electrodes are commonly used in potentiometry, each of which is considered in the following discussion.
When a copper electrode is immersed in a solution containing Cu2+, the potential of the electrode due to the reaction
If the copper electrode is the indicator electrode in a potentiometric electrochemical cell that also includes a saturated calomel reference electrode
SCE || Cu2+ (unk) | Cu(s)
then the cell potential can be used to determine an unknown concentration of Cu2+ in the indicator half-cell
Metallic indicator electrodes in which a metal is in contact with a solution con- taining its ion are called electrodes of the first kind. In general, for a metal M, in a solution of Mn+, the cell potential is given as
where K is a constant that includes the standard-state potential for the Mn+/M redox couple, the potential of the reference electrode, and the junction potential. For a variety of reasons, including slow kinetics for electron transfer, the existence of surface oxides and interfering reactions, electrodes of the first kind are limited to Ag, Bi, Cd, Cu, Hg, Pb, Sn, Tl, and Zn. Many of these electrodes, such as Zn, cannot be used in acidic solutions where they are easily oxidized by H+.
An electrode of the first kind involving an Mn+/M redox couple will respond to the concentration of another species if that species is in equilibrium with Mn+. For example, the potential of a silver electrode in a solution of Sg+ is given by
where K is a constant that includes the standard-state potential for the Ag+/Ag redox couple, the solubility product for AgI, the potential of the reference electrode, and the junction potential.
When the potential of an electrode of the first kind responds to the potential of another ion that is in equilibrium with Mn+, it is called an electrode of the second kind. Two common electrodes of the second kind are the calomel and silver/silver chloride reference electrodes. Electrodes of the second kind also can be based on complexation reactions. For example, an electrode for EDTA is constructed by cou- pling a Hg2+/Hg electrode of the first kind to EDTA by taking advantage of its for- mation of a stable complex with Hg2+.
Electrodes of the first and second kind develop a potential as the result of a redox reaction in which the metallic electrode undergoes a change in its oxidation state. Metallic electrodes also can serve simply as a source of, or a sink for, electrons in other redox reactions. Such electrodes are called redox electrodes. The Pt cathode in Example 11.1 is an example of a redox electrode because its potential is determined by the concentrations of Fe2+ and Fe3+ in the indicator half-cell. Note that the potential of a redox electrode generally responds to the concentration of more than one ion, limiting their usefulness for direct potentiometry.
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