Electrodes for potentiometric measurements

FIGURE 10.2 A schematic diagram of a combination glass pH electrode. A thin glass bulb with an inner Ag/AgCI electrode responds to pH changes in the test solution. A second Ag/AgCI in an outer jacket with a liquid junction serves the reference electrode for potentiometric measurement. An attached temperature probe is used to compensate for temperature effects. 

The indicator electrodes for potentiometric measurements have traditionally been categorised into three separate groups. A first group of electrodes... 

The indicator electrodes for potentiometric measurements traditionally have been categorized into three separate classes. First-class electrodes consist of a metal immersed in a solution that contains the metal ion. These electrode systems provide a direct response to the ion or species to be measured ... 

Trace amounts of cyanide are usually determined by flow injection spectrophotometric procedures. The target species is first halogenated with chlora-mine-T, after which it reacts with a mixture of pyrazolone or barbituric acid and isonicotinic acid or pyridine to form a bluish-violet polymethine dye. The implementation of gas-diffusion modules in the flow setup for hydrogen cyanide separation avoids matrix interferences and enables the adaptation of inherently nonselective detectors, such as metallic silver-wire electrodes for potentiometric measurements. Total inorganic cyanide, including free and complexed species, such as iron-cyanide complexes, may be determined by sample decomposition with UV irradiation and further photometric or amperometric analysis. 

Tymecki L, Zwierkowska E, Koncki R (2004) Screen-printed reference electrodes for potentiometric measurements. Anal. Chim. Acta 526 3-11... 

Thermodynamics describes the behaviour of systems in terms of quantities and functions of state, but cannot express these quantities in terms of model concepts and assumptions on the structure of the system, inter-molecular forces, etc. This is also true of the activity coefficients thermodynamics defines these quantities and gives their dependence on the temperature, pressure and composition, but cannot interpret them from the point of view of intermolecular interactions. Every theoretical expression of the activity coefficients as a function of the composition of the solution is necessarily based on extrathermodynamic, mainly statistical concepts. This approach makes it possible to elaborate quantitatively the theory of individual activity coefficients. Their values are of paramount importance, for example, for operational definition of the pH and its potentiometric determination (Section 3.3.2), for potentiometric measurement with ion-selective electrodes (Section 6.3), in general for all the systems where liquid junctions appear (Section 2.5.3), etc. 

An amperometric technique relies on the current passing through a polarizable electrode. The magnitude of the current is in direct proportion to the concentration of the electroanalyte, with the most common amperometric techniques being polarography and voltammetry. The apparatus needed for amperometric measurement tends to be more expensive than those used for potentiometric measurements alone. It should also be noted that amperometric measurements can be overly sensitive to impurities such as gaseous oxygen dissolved in the solution, and to capacitance effects at the electrode. Nevertheless, amperometry is a much more versatile tool than potentiometry. 

Figure 4.17 — (A) Exploded view of a tubular flow-cell integrated microconduit system. I Ag/AgCl inner reference electrode M sensitive membrane S internal reference solution. (B) Detail of the integrated microconduit shown within the dotted lines in C. (C) Integrated-microconduit FI manifold for potentiometric measurements C carrier stream R reference electrode solution P pump V injection valve I indicator electrode R reference electrode I pulse inhibitor G ground W waste. (Reproduced from [140] with permission of Pergamon Press).

Selective electrodes have a variable specificity. Precision can be increased when they are used as indicating electrodes in potentiometric measurements. The concentration of ions present in solution and the ionic strength will undergo small variations during measurement relative to the concentration of the ion being measured. When two ionic species undergo stoichiometric reaction, this property can be used for their determination. The end point in the measurement is characterised either by the total disappearance of one of the species or by the appearance of an excess of one of the species. The appearance or disappearance of a secondary species can also be used to determine the end point. 

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. 

Quasi-reference electrodes. Precise potentiometric measurements require reference electrodes that are highly reproducible, but there are many applications where this is less essential. For example, in routine analytical polaro-graphic or voltammetric measurements, the accurate measurement of the current is more important than the precise measurement of potential. For these purposes a simple quasi-reference electrode is suitable. In a halide-containing supporting electrolyte, a silver wire or mercury pool will adopt a reasonably steady potential that is reproducible to within 10-20 mV. 

Acetylcholineesterase Miniaturized multichannel transduc-tor with planar Au electrode which was first covered with a choline-selective liquid membrane made from 66% PVC-polyvinyl acetate (PVA), 33% 2-nitrophenyl octyl ether plasticizer and 1% ion-pair choline phosphotungstate. A second layer of 2% AChE in the PVA-polyethylene dispersion was spread on the top. The electrode was used as working electrode versus Ag/AgCl for potentiometric measurement of Ch and ACh in 0.1 M Tris buffer at 7.4. Optimum pH range for the sensor was 7-9. The calibration graph was linear from 0.02-10 mm ACh and detection limit was 5 pM. Response time was 3-5 min. Sensor was suitable for determination of ACh in biological fluids. [86]... 

The experimental aspects to be discussed in this chapter include cell design, electrode materials, construction and cleaning of electrodes, solution composition, and control instrumentation. Electrode materials specially designed for potentiometric measurements, which rely on the material selectivity, are discussed in Chapter 13. 

Potentiometric measurements are simple the redox potential is measured compared to a reference electrode. For pH measurements, commercially available electrodes, comprising the working (glass electrode) and reference electrodes, can be used. For redox potential measurements, the working electrode is usually an inert (e.g., platinum) electrode, and the reference electrode can be a hydrogen electrode, calomel, or other electrodes. Ion-selective electrodes are also based on potential measurements. 

The gas-sensing configuration described above forms a very useful basic unit for potentiometric measurements of biologically important species. In principle, the immobilized or insolubilized biocatalyst is placed on a conventional ion-selective electrode used to measure the decrease in the reactants or the increase in products of the biochemical reaction. The biocatalyst include... 

Fig. 4 The flow-through electrochemical cell for potentiometric measurements at temperatures up to 400°C A - flow-through Ptfhh) electrode, B - flow-through YSZ(Hg/HgO), C -flow-through external Ag/AgCl reference electrode [13, 16].

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. 

An important field of application for potentiometric measurement with the glass electrode is electrometric titration. The e.m.f. in a cell is here... 

Potentiometry is the most common method of electrochemical detection used in flow injection analysis. This methodology creates more favorable conditions for potentiometric measurements when compared to batch procedures. In FIA measurements, it is easier to avoid incidental contamination or that resulting from leakage of the solution from the reference electrode. Moreover, deactivation of the sensing surface of the indicator electrode due to adsorption, precipitation, or corrosion is minimized greatly as a consequence of the short contact time with the sample... 

Guilbault and Montalvo were the first, in 1969, to detail a potentiometric enzyme electrode. They described a urea biosensor based on urease immobilized at an ammonium-selective liquid membrane electrode. Since then, over hundreds of different applications have appeared in the literature, due to the significant development of ion-selective electrodes (ISEs) observed during the last 30 years. The electrodes used to assemble a potentiometric biosensor include glass electrodes for the measurement of pH or monovalent ions, ISEs sensitive to anions or cations, gas electrodes such as the CO2 or the NH3 probes, and metal electrodes able to detect redox species some of these electrodes useful in the construction of potentiometric enzyme electrodes are listed in Table 1. 

For potentiometric measurements (zero current), the complete measuring circuit consists of the microelectrode, the input amplifier, and a reference electrode. Sometimes a local reference electrode is needed if the electrical potential varies over short distances in the sample, for example, across a cell membrane (Figure 2). 

Nonselective, metallic indicator electrodes have been used for potentiometric measurements in complex biological media, for example, Pt electrodes have been used to monitor the redox potential of fermentation broths as cultures grow [17]. However, zero-current potentiometry more often involves ISEs based on solid membranes, composed of a sparingly soluble salt of the ion of interest or liquid membranes, in which an ion-selective reagent is dissolved, with the membrane separating reference... 

The rate of enzymatic reactions can also be established by potentiometric measurement of product formation using an ion selective electrode. The most important ion selective electrode is the glass electrode for pH measurement. Despite their outstanding selectivity for H+ ions, glass electrodes are used only seldom in enzyme electrodes because their sensitivity is affected by the buffer capacity of the sample matrix or sensor filler solution. 

---