Potentiometry, fundamentals

Definitive measurements by fundamental quantities complemented by an empirical factor, e.g. titre (titrimetry), as well as by well-known empirical (transferable) constants like molar absorption coefficient (spectrophotometry), Nernst factor (potentiometry, ISE), and conductivity at definite dilution (conductometry)... 

In this present book, we will look at the analytical use of two fundamentally different types of electrochemical technique, namely potentiometry and amper-ometry. The distinctions between the two are outlined in some detail in Chapter 2. For now, we will anticipate and say that a potentiometric technique determines the potential of electrochemical cells - usually at zero current. The potential of the electrode of interest responds (with respect to a standard reference electrode) to changes in the concentration of the species under study. The most common potentiometric methods used by the analyst employ voltmeters, potentiometers or pH meters. Such measurements are generally relatively cheap to perform, but can be slow and tedious unless automated. 

In a related application, polyelectrolyte microgels based on crosslinked cationic poly(allyl amine) and anionic polyfmethacrylic acid-co-epoxypropyl methacrylate) were studied by potentiometry, conductometry and turbidimetry [349]. In their neutralized (salt) form, the microgels fully complexed with linear polyelectrolytes (poly(acrylic acid), poly(acrylic acid-co-acrylamide), and polystyrene sulfonate)) as if the gels were themselves linear. However, if an acid/base reaction occurs between the linear polymers and the gels, it appears that only the surfaces of the gels form complexes. Previous work has addressed the fundamental characteristics of these complexes [350, 351] and has shown preferential complexation of cationic polyelectrolytes with crosslinked car-boxymethyl cellulose versus linear CMC [350], The departure from the 1 1 stoichiometry with the non-neutralized microgels may be due to the collapsed nature of these networks which prevents penetration of water soluble polyelectrolyte. 

The use of equation (3.2) to study the behaviour of catalysts is known as solid electrolyte potentiometry (SEP). Wagner38 was the first to put forward the idea of using SEP to study catalysts under working conditions. Vayenas and Saltsburg were the first to apply the technique to the fundamental study of a catalytic reaction for the case of the oxidation of sulfur dioxide.39 Since then the technique has been widely used, with particular success in the study of periodic and oscillatory phenomena for such reactions as the oxidation of carbon monoxide on platinum, hydrogen on nickel, ethylene on platinum and propylene oxide on silver. 

Just as in aqueous solutions, potentiometry is the most fundamental and powerful method of measuring pH, ionic activities and redox potentials in non-aqueous solutions. Here we deal with the basic techniques of potentiometry in non-aque-ous solutions and then discuss how potentiometry is applicable to studies of chemistry in noil-aqueous solutions. Some topics in this field have been reviewed in Ref. [1],... 

Refs. [i]KahlertH(2002) Potentiometry. In ScholzF(ed)Electroanalytical methods. Springer, Berlin, pp 227-228 [ii] Oldham KB, Myland IC (1994) Fundamentals of electrochemical science. Academic Press, San Diego, p 135 [iii] Damaskin BB, Petrii OA (1978) Fundamentals of theoretical electrochemistry. (In Russian), Vysshaya Shkola, Moscow, p 118 [iv] Brett CMA, Oliveira Brett AM (1993) Electrochemistry. Oxford University Press, p 21 [v] Petrii OA, Tsirlina GA (2002) Electrode potentials. In Bard AJ, Stratman M, Gileadi E, Urbakh M (eds) Thermodynamics and electrified interfaces. Encyclopedia of electrochemistry, vol 1. Wiley-VCH, Weinheim, pp 1-23 [vi] Erdey-Gruz T (1972) Kinetics of electrode processes, p 149 [vii] Hamann CH, Hammett A, Vielstich W (1998) Electrochemistry. Wiley-VCH, Weinheim, p 86... 

Refs. [i] Koryta / (1982) Ions, electrode and membranes. Wiley, New York [ii] Kahlert H (2005) Potentiometry. In Scholz F(ed) Electrochemical methods. Springer, Berlin [iii] Vielstich IV, Lamm A, Gasteiger H (2003) Handbook of fuel cells -fundamentals, technology, applications. Wiley-VCH, Chichester... 

Although electrochemistry has the stigma of being difficult to use, and therefore is often overlooked as an analysis option, potentiometric measurements are probably the most common technique encountered. Many analytical chemists make potentiometric measurements daily, whenever they use a pH meter. Potentiometry is based on the measurement of the potential between two electrodes immersed in a test solution. As the electrical potential of the cell is measured with no current flow between the electrodes, potentiometry is an equilibrium technique. The first electrode, the indicator electrode, is chosen to respond to the activity of a specific species in the test solution. The second electrode is a reference of known and fixed potential. The design of the indicator electrode is fundamental to potentiometric measurements, and should interact selectively with the analyte of interest so that other sample constituents do not interfere with the measurement. Many different strategies have been developed to make indicator electrodes that respond selectively to a number of species including organic ions. 

In this chapter, the fundamental electrochemical principles of potentiometry, voltammetry and/or amperometry, conductance, and coulometry will be summarized and clinical apphcations presented. Next, optodes and biosensors will be discussed. The chapter concludes with a discussion of in vivo and minimally invasive sensors. 

W e now turn our attention to several analytical methods that are based on oxidation/reduction reactions. These methods, which are described in Chapters 18 through 23, include oxidation/reduction titrimetry, potentiometry, coulometry, electrogravimetry, and voltammetry. Fundamentals of electrochemistry that are necessary for understanding the principles of these procedures are presented in this chapter. 

Although one of the more complex electrochemical techniques [1], cyclic voltammetry is very frequently used because it offers a wealth of experimental information and insights into both the kinetic and thermodynamic details of many chemical systems [2], Excellent review articles [3] and textbooks partially [4] or entirely [2, 5] dedicated to the fundamental aspects and apphcations of cyclic voltammetry have appeared. Because of significant advances in the theoretical understanding of the technique today, even complex chemical systems such as electrodes modified with film or particulate deposits may be studied quantitatively by cyclic voltammetry. In early electrochemical work, measurements were usually undertaken under equilibrium conditions (potentiometry) [6] where extremely accurate measurements of thermodynamic properties are possible. However, it was soon realised that the time dependence of signals can provide useful kinetic data [7]. Many early voltammet-ric studies were conducted on solid electrodes made from metals such as gold or platinum. However, the complexity of the chemical processes at the interface between solid metals and aqueous electrolytes inhibited the rapid development of novel transient methods. 

Fundamental Considerations. Within the context of this chapter potentiometry is understood to mean the measurement of potential differences across an indicator electrode and a reference electrode under conditions of zero net electrical current. Such a measurement can be used either for determining an analyte ion directly (direct potentiometry) or for monitoring a titration (see below). 

Electrochemical Impedance Spectroscopy (EIS) is a powerful technique for the characterization of electrochemical systems. The fundamental approach of all impedance methods is to apply a small-amplitude sinusoidal excitation signal to the system xmder investigation and measure the response (current or voltage or another signal of interest). An advantage of EIS compared to amperometry or potentiometry is that labels are no longer necessary, thus simplifying sensor preparation. However, the use of labels (like enzymes or nanoparticles) increases a lot the sensitivity of the method [23-26]. 

The fundamentals of gas titration with solid electrolyte cells are described in detail within the entries titration and coulometry, and special aspects are also treated within solid electrolyte, potentiometry, and amperometry. Therefore, the focus is set here to the most important errors of gas titration with SE cells. These errors are related mainly to the peripheral parameters as... 

Non-equilibrium processes at the sample/membrane interface and across the bulk membrane bias the selectivity and detection limits of the electrodes. Elimination of these nonequilibrium effects by operating the electrodes under complete equilibrium conditions will be of both practical and fundamental significance. While non-equilibrium responses are useful for potentiometric polyion-selective electrodes, it is not obvious whether potentiometry based on mixed ion-transfer potentials is a better transduction mechanism than amperome-try/voltammetry based on selective polyion transfer (65, 66). Ion-transfer electrochemistry at polarized liquid/liquid interfaces is introduced in Chapter 17 of this handbook. 

The potentiostatic multi-pulse potentiometry described here allows the dynamic measurement of potentials. The advantages of this method are the short time required for the analysis and the low noise of the signal. The "ancestor" of this technique, enzyme chronopotentiometry [7 ], posed problems of reproducibility when it was applied to the immobilized redox polymer. The excellent reproducibility of our method is clearly shown in fig. 3b. These techniques were the fundamental developments to conceive redox-FETs for the first time. After immobilization of NAD -dependent dehydrogenases covalently on the surface of the transducer the enzymatically produced NADH would be catalytically oxidized in situ by the polymeric mediator. To this very compact combination the substrate and NAD+ as cosubstrate have to be applied externally. The coimmobilization of the coenzyme NAD+ would lead to reagentless sensors. This is a subject of forthcoming investigations... 

There are specific surfactant electrodes available for analysing surfactant solutions. Here, the surfactant selectively penetrates the membrane of the electrode, thus causing an electrical potential. One fundamental problem of potentiometry is that the measured potential is not linearly dependent on the concentration of the species, but rather proportional to the logarithm of its concentration, as depicted in Figure 22.1. Thus, in measuring adsorption the accuracy is low since the adsorbed... 

Contents Introduction. - Fundamentals of Potentiometry. -Electrode Potential Measurements. - Ion-Selective Electrodes. - Measuring Techniques with Ion-Selective Electrodes. - Analysis Techniques Using Ion-Selective Electrodes. - implications of Ion-Selective Electrodes. - Outlook. - Appendix. - Literature. - Subject Index. - Index of Symbols Used. 

R. P. Buck, Analytical reviews 1972/Fundamentals Ion-selective electrodes, potentiometry, and potentiometric titrations. Anal. Chem. 44(5), 270R-295R... 

---