Applications of Potentiometry

Although potentiometric measurements frequendy are applied as a means of endpoint detection in potentiometric titrations, the purpose of this volume is not to review the many titradon procedures that utilize potentiometric measurements. However, a brief summary of the species that can be monitored through the use of potentiometric measurements will illustrate the potential applications. 

Although all potentiometric measurements (except those involving membrane electrodes) ultimately are based on a redox couple, the method can be applied to oxidation-reduction processes, acid-base processes, precipitation processes, and metal ion complexation processes. Measurements that involve a component of a redox couple require that either the oxidized or reduced conjugate of the species to be measured be maintained at a constant and known activity at the electrode. If the goal is to measure the activity of silver ion in a solution, then a silver wire coupled to the appropriate reference electrodes makes an ideal potentiometric system. Likewise, if the goal is to monitor iron(UI) concentrations with a platinum electrode, a known concentration of 

For adverse conditions (in terms of either temperature or vibration) the antimony electrode has proved useful, particularly for industrial processes with extreme environmental problems. The electrode is not particularly reliable for precise measurements, but its simple form (consisting of antimony metal embedded in an insulating material) allows pH measurements under such adverse conditions. The principle of the electrode is based on a half-reaction whereby the metal and its metal oxide are both insoluble and the electrode s response is dependent on hydronium ion activity  

Potentiometric redox measurements are often performed in nonaqueous or mixed-solvent media. For such solvents various potentiometric sensors have been developed, which, under rigorously controlled conditions, give a Nemstian response over a wide ranges of activities, particularly in buffered solutions. There are some experimental limitations, such as with solvent purification and handling or use of a reference electrode without salt bridges, but there also ate important advantages. Solutes may be more soluble in such media, and redox 

Potentiometry has found extensive application over the past half-century as a means to evaluate various thermodynamic parameters. Although this is not the major application of the technique today, it still provides one of the most convenient and reliable approaches to the evaluation of thermodynamic quantities. In particular, the activity coefficients of electroactive species can be evaluated directly through the use of the Nemst equation (for species that give a reversible electrochemical response). Thus, if an electrochemical system is used without a junction potential and with a reference electrode that has a well-established potential, then potentiometric measurement of the constituent species at a known concentration provides a direct measure of its activity. This provides a direct means for evaluation of the activity coefficient (assuming that the standard potential is known accurately for the constituent half-reaction). If the standard half-reaction potential is not available, it must be evaluated under conditions where the activity coefficient can be determined by the Debye-Hiickel equation. 

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Wang and Taha described an interesting application of potentiometry called batch injection. As shown in the following figure, an ion-selective electrode is placed in an inverted position in a large-volume tank, and a fixed volume of a sample or standard solution is injected toward the electrode s surface using a micropipet. 

Electrode Systems. Direct Potentiometric Measurements. Potentiometric Titrations. Null -point Potentiometry. Applications of Potentiometry. 

Electrode systems. Direct poientiometric measurements. Potentio-metric titrations. Null-point potentiometry. Applications of potentiometry. 

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. 

Shinskey, F. G. pH and plan Control in Process and Waste Streams. New York John Wiley and Sons, 1973. Provides an excellent description of the application of potentiometry in process control. 

Applications of potentiometry are rather widespread, and its efficiency is high enough when operating with relative potential values. A mention should he made, first of all, of the determination of basic thermodynamic quantities, such as the equilibrium constants for coordination... 

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). 

One of the most important applications of potentiometry is the determination of [H+] or the pH of a solution, where pH is defined as the negative logarithm to the base 10 of the hydrogen ion activity (and is approximately equal to the negative logarithm to the base 10 of the hydrogen ion concentration under certain conditions discussed subsequently) ... 

A Nernst equation may be written for every galvanic cell at equilibrium. Therefore the equation is the basis of all thermodynamic applications of potentiometry (i.e. measurement of open-circuit cell potentials by means of a potentiometer or other zero-current device). Cells may be constructed and appropriate Nernst equations written to find, for example, the dissociation constant of water, and many electrolyte activity coefficients and stability and solubilit,v constants. Potentiometric titration curves are also interpieted by means of the appropriate Nernst equation. 

For the study of the solvent effect, comparable equilibrium constants have to be determined in water and in solution made with non-aqueous solvents or solvent mixtures. Potentiometric (usually pH-metric) equilibrium measurements are used for this purpose in polyfunctional systems. The solvent effect makes the application of potentiometry somewhat difficult. The substitution of water by organic solvents results in changes of the autoprotolysis constant of the solvent changing the pH scale. The lower relative permittivity of the system favours association processes which have to be considered, e.g., in the determination of the ionic strength of the solution. Diffusion potentials at the liquid junctions connecting the galvanic cell with the reference electrode may falsify the measured data. 

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 main applications of potentiometry are related to measnring the following values ... 

There are a number of applications of potentiometry. In this book, the reader will learn only two of them (1) estimation of the standard electrode potential and (2) pH measurements. 

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