Metallic indicator electrode

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 subject of this section, and ion-selective electrodes, which are covered in the next section. 

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. 

Reference electrode based on the reduction of AgCl to Ag that is, AgCfs) + Ag(s) + Cl (aq). 

Electrodes of the First Kind When a copper electrode is immersed in a solution containing Cu +, the potential of the electrode due to the reaction 

A metallic electrode whose potential is a function of the concentration of M + in an M +/M redox half-reaction. 

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Metallic indicator electrodes in which a metal is in contact with a solution containing its ion are called electrodes of the first kind. In general, for a metal M, in a solution of M"+, the cell potential is given as... 

The metal indicator electrodes for the potentiometric titrations respond to the M+/M couple prior to the equivalence point, and to the MOH/M, "OH couple after the equivalence point... 

Two main groups of indicator electrodes are considered here. In one case, metal indicator electrodes that exhibit a potential difference as a consequence of a redox process occurring at the metal surface are examined. Later, ISEs that can respond to ionic species based on the principles of ion extraction across an active sensing membrane will be studied in detail. 

Metal indicator electrodes develop a potential which is usually determined by the equilibrium position of a redox half-reaction at the electrode surface. These are further classified into the following three types, namely ... 

The most common metal indicator electrode is platinum, which is relatively inert—it does not participate in many chemical reactions. Its purpose is simply to transmit electrons to or from species in solution. Gold electrodes are even more inert than Pt. Various types of carbon are used as indicator electrodes because the rates of many redox reactions on the carbon surface are fast. A metal electrode works best when its surface is large and clean. To clean the electrode, dip it briefly in hot 8 M HN03 and rinse with distilled water. 

It is convenient to classify metallic indicator electrodes as electrodes of the first kind, electrodes of the second kind, or inert redox electrodes. 

Electrode systems of the first kind are not widely used for potentiometric determinations for several reasons. For one, metallic indicator electrodes are not vety selective and respond not only to their own cations but also to Other more easily reduced cations. For example, a copper electrode cannot be used for the determination of coppeifll) ions in the presence of silverfl) ions because the electrode poten-... 

Nonselective, metallic indicator electrodes have been used for potentiometric measurements in complex biological media, for example, Pt electrodes have been used to monitor the redox potential of fermentation broths as cultures grow [17]. However, zero-current potentiometry more often involves ISEs based on solid membranes, composed of a sparingly soluble salt of the ion of interest or liquid membranes, in which an ion-selective reagent is dissolved, with the membrane separating reference... 

A General Principles 659 23B Reference Electrodes 660 23C Metallic Indicator Electrodes 662 23D Membrane Indicator Electrodes 664 23E Ion-Selective Field-Effect Transistors 675 23F Molecular-Selective Electrode Systems 677 23G Instruments for Measuring Cell Potentials 684 23H Direct Potentiometric Measurements 686 231 Potentiometric Titrations 691 Questions and Problems 692... 

An ideal indicator electrode responds rapidly and reproducibly to changes in activity of the analyte ion. Although no indicator electrode is absolutely specific in its response, a few are now available that are remarkably selective. There are two types of indicator electrodes metallic and membrane. This section deals with metallic indicator electrodes. 

Table 23-2 lists ihe various types of ion-selective membrane electrodes that have been developed. These differ in the physical or chemical composition of the membrane. The general mechanism by which an ion-sclective potential develops in these devices depend.s on the nature of the membrane and is entirely different from the source of potential in metallic indicator electrodes. We have seen that the potential of a metallic electrode arises from the tendency of an oxidation-reduction reaction to occur at an electrode surface. In membrane electrodes, in contrast. Ihe observed potential is a kind of junction potential that develops across a membrane thal separates the anidyte solution from a reference solution. 

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