Potentiometry practice

Potentiometry (discussed in Chapter 5), which is of great practical importance, is a static (zero current) technique in which the information about the sample composition is obtained from measurement of the potential established across a membrane. Different types of membrane materials, possessing different ion-recognition processes, have been developed to impart high selectivity. The resulting potentiometric probes have thus been widely used for several decades for direct monitoring of ionic species such as protons or calcium, fluoride, and potassium ions in complex samples. 

Another galvanic cell of highly practical and theoretical importance is the so-called standard cell (see Section 2.2.2), use of which has to be made as a calibration standard in non-faradaic potentiometry. For this purpose, the saturated Weston cell is the most accepted as its emf is reproducible, precisely known, only slightly temperature dependent in the region around 25° C (1.01832 V) and insensitive to unexpected current flows, if any. 

In analytical practice, some methods using definitive measurements, in principle, are also calibrated by indirect reference measurements using least squares estimating to provide reliable estimates of b (spectrophotometry, potentiometry, ISE, polarography). 

Potentiometry deals with the electromotive force (EMF) generated in a galvanic cell where a spontaneous chemical reaction is taking place. In practice, potentiometry employs the EMF response of a galvanostatic cell that is based on the measurement of an electrochemical cell potential under zero-current conditions to determine the concentration of analytes in measuring samples. Because an electrode potential generated on the metal electrode surface,... 

Culberson, C.H. 1981. Direct potentiometry, in Marine Electrochemistry A Practical Introduction, M. Whitfield and D. Jagner (eds.)., John Wiley Sons, New York, pp. 187-261. 

The use of dual-polarized electrodes was first suggested more than 70 years ago 2 the subject has been reviewed thoroughly by two more recent publications.3,4 Almost all modem commercial pH meters have provision for imposing a polarizing current of either 5 or 10 nA to make possible measurements by dual-polarized electrode potentiometry. Such a provision is included because dual-polarized potentiometry is by far the most popular endpoint detection method for the Karl Fischer determination of water. For this titration a combination of reagents is used, including iodine the response curve is similar to that of Figure 4.3b. In practice the response is many times more sensitive than... 

The goal of this volume is to provide (1) an introduction to the basic principles of electrochemistry (Chapter 1), potentiometry (Chapter 2), voltammetry (Chapter 3), and electrochemical titrations (Chapter 4) (2) a practical, up-to-date summary of indicator electrodes (Chapter 5), electrochemical cells and instrumentation (Chapter 6), and solvents and electrolytes (Chapter 7) and (3) illustrative examples of molecular characterization (via electrochemical measurements) of hydronium ion, Br0nsted acids, and H2 (Chapter 8) dioxygen species (02, OJ/HOO-, HOOH) and H20/H0 (Chapter 9) metals, metal compounds, and metal complexes (Chapter 10) nonmetals (Chapter 11) carbon compounds (Chapter 12) and organometallic compounds and metallopor-phyrins (Chapter 13). The later chapters contain specific characterizations of representative molecules within a class, which we hope will reduce the barriers of unfamiliarity and encourage the reader to make use of electrochemistry for related chemical systems. 

Both direct potentiometry and the potentiometric titration method (see next Section) require the measurement of emf between an indicator electrode system and a reference electrode system, the two comprising a cell system. Despite the fact that potentials are usually referred to the standard hydrogen electrode (SHE) for tabulation purposes, they can be expressed relative to any acceptable reference system. In practice, the common reference electrodes are saturated KCl calomel (SCE), 3M KCl calomel or 3M KCl Ag/AgCl. In many present-day applications, the indicator and reference electrodes are combined in a single electrode system called a "combination electrode". 

Delahay P (1963) Chronoamperometry and chrono-potentiometry. In Kolthoff IM, Elving PJ and Sandell EB, eds. Treatise on analytical chemistry. Part I (Theory and practice), Vol 4, section D-2, Electrical methods of analysis (Reilley CN, section advisor), pp. 2233—2268. John Wiley Sons, New York. 

The shift in electrode potential caused by the complexing agent is contained in the lecond term of Equation 2.18. In this case, it amounts to a shift of —0.526 V. The important practical consequences of chelation and complexation will be discussed in more detail later. Fdr example, one can determine copper ion by direct potentiometry using a copper-ion-selective electrode, or via a potentiometric titration with EDTA using the electrode as an endpoint detector. 

In practice, electrochemistry not only provides a means of elemental and molecular analysis, but also can be used to acquire information about equilibria, kinetics, and reaction mechanisms from research using polarography, amperometry, conductometric analysis, and potentiometry. The analytical calculation is usually based on the determination of current or voltage or on the resistance developed in a cell under conditions such that these are dependent on the concentration of the species under study. Electrochemical measurements are easy to automate because they are electrical signals. The equipment is often far less expensive than spectroscopy instrumentation. Electrochemical techniques are also commonly used as detectors for LC, as discussed in Chapter 13. 

Compare this equation with Eqs. (15.7) and (15.15). By convention, the reference electrode is connected to the negative terminal of the potentiometer (the readout device). The common reference electrodes used in potentiometry are the SCE and the silver/silver chloride electrode, which have been described. Their potentials are fixed and known over a wide temperature range. Some values for these electrode potentials are given in Table 15.3. The total cell potential is measured experimentally, the reference potential is known, and therefore the variable indicator electrode potential can be calculated and related to the concentration of the analyte through the Nemst equation. In practice, the concentration of the unknown analyte is determined after calibration of the potentiometer with suitable standard solutions. The choice of reference electrode depends on the application. For example, the Ag/AgCl electrode cannot be used in solutions containing species such as halides or sulfides that will precipitate or otherwise react with silver. 

In electrochemistry an electrode is an electronic conductor in contact with an ionic conductor. The electronic conductor can be a metal, or a semiconductor, or a mixed electronic and ionic conductor. The ionic conductor is usually an electrolyte solution however, solid electrolytes and ionic melts can be used as well. The term electrode is also used in a technical sense, meaning the electronic conductor only. If not specified otherwise, this meaning of the term electrode is the subject of the present chapter. In the simplest case the electrode is a metallic conductor immersed in an electrolyte solution. At the surface of the electrode, dissolved electroactive ions change their charges by exchanging one or more electrons with the conductor. In this electrochemical reaction both the reduced and oxidized ions remain in solution, while the conductor is chemically inert and serves only as a source and sink of electrons. The technical term electrode usually also includes all mechanical parts supporting the conductor (e.g., a rotating disk electrode or a static mercury drop electrode). Furthermore, it includes all chemical and physical modifications of the conductor, or its surface (e.g., a mercury film electrode, an enzyme electrode, and a carbon paste electrode). However, this term does not cover the electrolyte solution and the ionic part of a double layer at the electrode/solution interface. Ion-selective electrodes, which are used in potentiometry, will not be considered in this chapter. Theoretical and practical aspects of electrodes are covered in various books and reviews [1-9]. 

Practically all commonly employed electroanalytical methods can be used in environmental analysis the choice of method depends on the character of the compound to be determined and of the matrix in which it occurs, as well as on sensitivity and selectivity requirements. The principal methods are voltammetry and polarography, potentiometry, coulo-metry and conductometry. 

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