Analytical Applications of Potentiometry

Samples must be in the form of liquids or gases for analysis. Sample preparation may include buffering the sample to an appropriate pH for some ISE and gas measurements or adding ionic strength buffer to make all samples and standards of equivalent ionic strength. The detection limit for most common electrodes is about 10 M, while gas probes have detection limits in the low ppm range. 

An understanding of stoichiometry, acid-base theory, and simple equilibrium calculations is required for the following quantitative applications. Review your introductory chemistry text or a basic quantitative analysis text such as those by Harris or Enke listed in the bibliography. 

The concentration of an ion in solution may be measured directly via the potential developed by the half-cell involved and by applying the Nemst equation. For example, a silver/silver ion half-cell plus an SCE gives the following relationship  

Potentiometry is a useful way to determine the endpoint in many titrations. For example, the concentration of Ag+ ion in solution can be used to determine the equivalence point in the titration of Ag+ with Cl . In this titration, the following reaction takes place  

The concentration of the Ag in solution steadily decreases as Cl is added. At the equivalence point, [Ag+] = [Cl ]. But, for the sparingly soluble salt silver chloride, [Ag [Cl J is a constant called the solubility product, A jp. In the case of the AgCl precipitation reaction, = 1 x 10 at 25°C. Therefore, 

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Potentiometry—the measurement of electric potentials in electrochemical cells—is probably one of the oldest methods of chemical analysis still in wide use. The early, essentially qualitative, work of Luigi Galvani (1737-1798) and Count Alessandro Volta (1745-1827) had its first fruit in the work of J. Willard Gibbs (1839-1903) and Walther Nernst (1864-1941), who laid the foundations for the treatment of electrochemical equilibria and electrode potentials. The early analytical applications of potentiometry were essentially to detect the endpoints of titrations. More extensive use of direct potentiometric methods came after Haber developed the glass electrode for pH measurements in 1909. In recent years, several new classes of ion-selective sensors have been introduced, beginning with glass electrodes more or less selectively responsive to other univalent cations (Na, NH ", etc.). Now, solid-state crystalline electrodes for ions such as F , Ag", and sulfide, and liquid ion-exchange membrane electrodes responsive to many simple and complex ions—Ca , BF4", CIO "—provide the chemist with electrochemical probes responsive to a wide variety of ionic species. 

The participation of cations in redox reactions of metal hexacyanoferrates provides a unique opportunity for the development of chemical sensors for non-electroactive ions. The development of sensors for thallium (Tl+) [15], cesium (Cs+) [34], and potassium (K+) [35, 36] pioneered analytical applications of metal hexacyanoferrates (Table 13.1). Later, a number of cationic analytes were enlarged, including ammonium (NH4+) [37], rubidium (Rb+) [38], and even other mono- and divalent cations [39], In most cases the electrochemical techniques used were potentiometry and amperometry either under constant potential or in cyclic voltammetric regime. More recently, sensors for silver [29] and arsenite [40] on the basis of transition metal hexacyanoferrates were proposed. An apparent list of sensors for non-electroactive ions is presented in Table 13.1. 

Applications of potentiometry involve the use of an electrochemical cell consisting of a reference electrode of constant potential and an indicator electrode that responds to the analyte studied and sample composition. The electromotive force (e.m.f.) of this cell can be regarded as the difference of the potentials of the two electrodes (half-cells). 

In this section the theory and methodology of electro-analytical chemistry are explored. Chapter 22 provides a (general foundation for the study of subsequent chapters in this section. Terminology- and conventions of electrochemistry as well as theoretical and practical aspects of the measurement of electrochemical potentials and current s are. presented. Chapter 23 comprises the many methods and applications of potentiometry. and constant-potential coulometry and constant-current coulornetrv are discussed in Chapter 2 4. The many facets of the important and widely used technique of voltammetry are presented in ( hapter 2.5. which concludes the section. 

The trade-offs between direct calibration and standard addition are treated in Ref 103. The same recovery as is found for the native analyte has to be obtained for the spiked analyte (see Section 3.2). The application of spiking to potentiometry is reviewed in Refs. 104 and 105. A worked example for the application of standard addition methodology to FIA/AAS is found in Ref 106. Reference 70 discusses the optimization of the standard addition method. 

One of the most fruitful uses of potentiometry in analytical chemistry is its application to titrimetry. Prior to this application, most titrations were carried out using colour-change indicators to signal the titration endpoint. A potentiometric titration (or indirect potentiometry) involves measurement of the potential of a suitable indicator electrode as a function of titrant volume. The information provided by a potentiometric titration is not the same as that obtained from a direct potentiometric measurement. As pointed out by Dick [473], there are advantages to potentiometric titration over direct potentiometry, despite the fact that the two techniques very often use the same type of electrodes. Potentiometric titrations provide data that are more reliable than data from titrations that use chemical indicators, but potentiometric titrations are more time-consuming. 

Vol. 66 Solid Phase Biochemistry Analytical and Synthetic Aspects. Edited by William H. Scouten Vol. 67 An Introduction to Photoelectron Spectroscopy. By Pradip K. Ghosh Vol. 68 Room Temperature Phosphorimetry for Chemical Analysis. By Tuan Vo-Dinh Vol. 69 Potentiometry and Potentiometric Titrations. By E. P. Serjeant Vol. 70 Design and Application of Process Analyzer Systems. By Paul E. Mix Vol. 71 Analysis of Organic and Biological Surfaces. Edited by Patrick Echlin Vol. 72 Small Bore Liquid Chromatography Columns Their Properties and Uses. Edited by Raymond P.W. Scott... 

All analysts are familiar with the principles of potentiometry and potarography and indeed, most analytical laboratories will contain a pH meter and a polarograph. However, electrochemical methods arc, in general, not very important in modern analysis. In contrast, there arc spccifiG applications such as trace metal ion analysis in water and effluents and also some other aspects of environmental analysis for which electrochemical methods are particularly attractive. This is because (1) some methods, especially anodic stripping voltammetry, have a very high sensitivity for heavy-metal ions and the lowest detection limit of from 10 to mol dm is well below that of other available methods (2) electrochemical methods are well suited for modification to on-line and/or portable devices for analysis in the held. Whether the analysis is based on current, conductivity or the response of an ion-selective electrode, both the cell and the control electronics are readily miniaturized and operate on low power Hence, this chapter considers the principles of the electroanalytical methods important in environmental and on-line analysis, together with biochemical applications of electrochemical sensors. 

Lindner E, Pendley BD (2013) A tutorial on the application of ion-selective electrode potentiometry an analytical method with unique qualities, uuexplraed opportunities and potential pitfalls. Anal Chim Acta 762 1-13... 

In the end, the further applications of this analytical technique, which can be thought of as a renaissance in potentiometry, will depend on the skill and experimental innovation of the user. To this end the author hopes to have made some small contribution. 

When first developed, potentiometry was restricted to redox equilibria at metallic electrodes, limiting its application to a few ions. In 1906, Cremer discovered that a potential difference exists between the two sides of a thin glass membrane when opposite sides of the membrane are in contact with solutions containing different concentrations of H3O+. This discovery led to the development of the glass pH electrode in 1909. Other types of membranes also yield useful potentials. Kolthoff and Sanders, for example, showed in 1937 that pellets made from AgCl could be used to determine the concentration of Ag+. Electrodes based on membrane potentials are called ion-selective electrodes, and their continued development has extended potentiometry to a diverse array of analytes. 

If metallic electrodes were the only useful class of indicator electrodes, potentiometry would be of limited applicability. The discovery, in 1906, that a thin glass membrane develops a potential, called a membrane potential, when opposite sides of the membrane are in contact with solutions of different pH led to the eventual development of a whole new class of indicator electrodes called ion-selective electrodes (ISEs). following the discovery of the glass pH electrode, ion-selective electrodes have been developed for a wide range of ions. Membrane electrodes also have been developed that respond to the concentration of molecular analytes by using a chemical reaction to generate an ion that can be monitored with an ion-selective electrode. The development of new membrane electrodes continues to be an active area of research. 

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