Electrodes for potentiometry

A schematic diagram of a typical pH electrode system is shown in Fig. 10.1. The cell potential, i.e. the electromotive force, is measured between a pH electrode and a reference electrode in a test solution. The pH electrode responds to the activity or concentration of hydrogen ions in the solution. The reference electrode has a very stable half-cell potential. The most commonly used reference electrodes for potentiometry are the silver/silver chloride electrodes (Ag/AgCl) and the saturated calomel electrodes (SCE). 

Reference electrodes for potentiometry are of three main types ... 

Most important for the construction of useful electrodes for potentiometry is the design of the sensor interface with the sample solution. This interface should be designed such that the maximum possible degree of selectivity with the interesting constituent is achieved. As a result of such efforts, the field of ion-selective electrodes (ISEs) has been established. lESs are not a priori chemical sensors. Normally, they do not fulfil the condition of being small and cheap. Nevertheless, ISEs are a very important preliminary stage on the way to highly useful chemical sensors. 

This chapter will describe the iodine-iodide electrode with respect to properties that are relevant for practical applications, i.e., as a reference electrode for potentiometry as typically used in measuring chains with pH glass electrodes, which was introduced by Ross [164]. 

Used as ImM soln. in CHCI3 as carrier in Pb-selective electrode for potentiometry. Cryst. Sol. CHCI3. Shpigun, L.K. et al, Zh. Anal. Khim., 1986, 41, 617 use)... 

Electrochemical cell for potentiometry with an ion-selective membrane electrode. 

Potentiometry, which measures the open-circuit equilibrium potential of an indicator electrode, for which the substance being examined is potential determining... 

An important condition for potentiometry is high selectivity the electrode s potential shonld respond only to the snbstance being examined, not to other components in the solntion. This condition greatly restricts the possibilities of the version of potentiometry described here when metal electrodes are nsed as the indicator electrodes. The solntion shonld be free of ions of more electropositive metals and of the components of other redox systems (in particnlar, dissolved air). Only corrosion-resistant materials can be nsed as electrodes. It is not possible at all with this method to determine alkali or alkaline-earth metal ions in aqneons solntions. 

An appropriate ion-specific electrode was found to provide a convenient, precise and relatively inexpensive method for potentiometry of copper(II) ion in copper-complex azo or formazan dyes. Copper(II) ion in copper phthalocyanine dyes can be quantified after anion exchange. Twelve commercial premetallised dyes evaluated using this technique contained copper(II) ion concentrations in the range 0.007 to 0.2%. Thus many copper-complex direct or reactive dyes are likely to contribute low but possibly significant amounts of ionic copper to textile dyeing effluents [52]. 

In potentiometry, we measure the emf of a cell consisting of an indicator electrode and a reference electrode. For emf measurements, we generally use a pH/ mV meter of high input impedance. The potential of the reference electrode must be stable and reproducible. If there is a liquid junction between the indicator electrode and the reference electrode, we should take the liquid junction potential into account. 

Ion solvation has been studied extensively by potentiometry [28, 31]. Among the potentiometric indicator electrodes used as sensors for ion solvation are metal and metal amalgam electrodes for the relevant metal ions, pH glass electrodes and pH-ISFETs for H+ (see Fig. 6.8), univalent cation-sensitive glass electrodes for alkali metal ions, a CuS solid-membrane electrode for Cu2+, an LaF3-based fluoride electrode for l , and some other ISEs. So far, method (2) has been employed most often. The advantage of potentiometry is that the number and the variety of target ions increase by the use of ISEs. 

A major branch of analytical chemistry uses electrical measurements of chemical processes at the surface of an electrode for analytical purposes. For example, hormones released from a single cell can be measured in this manner. Principles developed in this chapter provide a foundation for potentiometry, redox titrations, electrogravimetric and coulometric analysis, voltammetry, and amperometry in the following chapters.1-2... 

Hg2 ion-selective electrode calibration curve from J. A. Shatkin, H. S. Brown, and S. licht, Composite Graphite Ion Selective Electrode Array Potentiometry for the Detection of Mercury and Other Relevant Ions in Aquatic Systems, Anal. Chem. 1995, 67,1147. It was not stated in the paper, but we presume that all solutions had the same ionic strength. 

Table 5), and several are now being used, or are potentially useful, for measuring key ocean elements. The most common use of direct potentiometry (as compared with potentiometric titrations) is for measurement of pH (Culberson, 1981). Most other cation electrodes are subject to some degree of interference from other major ions. Electrodes for sodium, potassium, calcium, and magnesium have been used successfully. Copper, cadmium, and lead electrodes in seawater have been tested, with variable success. Anion-selective electrodes for chloride, bromide, fluoride, sulfate, sulfide, and silver ions have also been tested but have not yet found wide application. 

Among the various possibilities that offer the EC detection, ampe-rometry and conductimetry are, in this order, the most common. Although potentiometry results are a very interesting technique in many fields of Analytical Chemistry, it has not found enough echo in the microchip technology. Its incursion in microchips is related with the employment of ion-selective electrodes for Ba2+ determination [55] or potentiometric titration of iron ferrocyanide [56], but it has not yet been associated with CE microchips. 

Akaiwa et al. [324] have used ion exchange chromatography on hydrous zirconium oxide, combined with detection based on direct potentiometry with an ion selective electrode, for the simultaneous determination of chloride and bromide in non saline waters. 

Use of the potential of a galvanic cell to measure the concentration of an electroactive species developed later than a number of other electrochemical methods. In part this was because a rational relation between the electrode potential and the concentration of an electroactive species required the development of thermodynamics, and in particular its application to electrochemical phenomena. The work of J. Willard Gibbs1 in the 1870s provided the foundation for the Nemst equation.2 The latter provides a quantitative relationship between potential and the ratio of concentrations for a redox couple [ox l[red ), and is the basis for potentiometry and potentiometric titrations.3 The utility of potentiometric measurements for the characterization of ionic solutions was established with the invention of the glass electrode in 1909 for a selective potentiometric response to hydronium ion concentrations.4 Another milestone in the development of potentiometric measurements was the introduction of the hydrogen electrode for the measurement of hydronium ion concentrations 5 one of many important contributions by Professor Joel Hildebrand. Subsequent development of special glass formulations has made possible electrodes that are selective to different monovalent cations.6"8 The idea is so attractive that intense effort has led to the development of electrodes that are selective for many cations and anions, as well as several gas- and bioselective electrodes.9 The use of these electrodes and the potentiometric measurement of pH continue to be among the most important applications of electrochemistry. 

Measurements can be done using the technique of redox potentiometry. In experiments of this type, mitochondria are incubated anaerobically in the presence of a reference electrode [for example, a hydrogen electrode (Chap. 10)] and a platinum electrode and with secondary redox mediators. These mediators form redox pairs with Ea values intermediate between the reference electrode and the electron-transport-chain component of interest they permit rapid equilibration of electrons between the electrode and the electron-transport-chain component. The experimental system is allowed to reach equilibrium at a particular E value. This value can then be changed by addition of a reducing agent (such as reduced ascorbate or NADH), and the relationship between E and the levels of oxidized and reduced electron-transport-chain components is measured. The 0 values can then be calculated using the Nernst equation (Chap. 10) ... 

Indicator electrode — In -> potentiometry the electrode for which the -> potential is recorded, in -> amperometry the electrode for which the -> current is recorded. 

Most ion-selective electrodes, however, cannot be used for the direct determination of functional groups in organic compounds unless they are converted into ionic species. Thus, direct potentiometry is performed rarely with commercial ion-selective electrodes. For example, the CN -selective electrode gives an almost Nernstian response in the determination of substituted phenylacetonitriles and benzonitriles. Thiols can be determined by direct measurement of the voltage of a cell with an Ag2S-based electrode. Such examples are, however, not typical for applications of ion-selective electrodes. 

An example of an electrochemical cell for potentiometry is shown schematically in Fig. 1. The cell consists of an external reference electrode and an indicator electrode immersed in a test solution of analyte with some activity, (aOsampie- The indicator electrode is constructed of a reference electrode contained within a membrane and an internal reference electrolyte of fixed activity, (aOmtemai- The potential (F ceii) is measured by a pH/mV meter, and is equal to the sum of the potential between the internal (Fref.int) and external ( ref,ext) reference electrodes, the membrane potential (i memb), plus the liquid junction potential (Ey) that exists at the junction between the external reference electrode and the sample solution. 

At last, the forth dataset for nitrate was obtained by YSI Multiparameter robe 6900 supplied with a specific electrode for nitrate measurements based on the principles of potentiometry (Roig et al., 2006b). 

The sign convention for potentiometry is consistent with the convention described in Chapter 18 for standard electrode potential. In this convention, the indicator electrode is always treated as the right-hand electrode and the reference electrode as the left-hand electrode. For direct potentiometric measurements, the potential of a cell can then be expressed in terms of the potentials developed by the indicator electrode, the reference electrode, and a junction potential, as described in Section 21 A ... 

The first group, which is developed in this chapter, use ion selective electrodes (ISE). The principle of these chemical sensors is to create an electric cell in which the analyte behaves in such a way that the potential difference obtained relates to its concentration. Measurement of pH, probably the most common and best known electroanalytical method, is part of this group. Most of the measurements concern the determination of ions in aqueous solution, though particular electrodes with selective membranes also allow the determination of molecules. The sensitivity of these methods is very great for certain ions but matrix is sometime responsible for lack of reliability in these measurements. In such cases, complexometric or titrimetric methods must replace direct potentiometry. It remains however for potentiometry multiple applications in which the instruments range from low-cost pH meters to automatic titrators. 

Figure 19.6 Typical ccdibration curve of an ISE by direct potentiometry. The calibration curve of the specific electrode for the chloride ion has almost an ideal slope value. The range of linear response for the different ISE extends over 4 to 6 orders of magnitude depending on the ion. Expression 19.5 leads to the estimation that an uncertainty of 0.2 mV on E leads to an inexactness of 0.8 per cent in the concentration (for a monovalent ion). Here, the TISAB consists of NaCl 1 M for adjusting the ionic force, a complexing agent for metals and a hufler mixture of acetic acid/sodium acetate.

Cyanides can also be determined by membrane ion-selective electrodes using direct potentiometry (argentoiodide electrode or an electrode with a mixture of Agl-AgjS). The principle of the response to cyanides in the case of the most frequently used ion-selective electrodes for CN ions consists in the dissolution of Agl according to the following reaction ... 

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. 

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