Electroanalytical cells

This review has been restricted mainly to clarification ofthe fundamentals and to presenting a coherent view ofthe actual state of research on voltaic cells, as well as their applications. Voltaic cells are, or may be, used in various branches of electrochemistry and surface chemistry, both in basic and applied research. They particularly enable interpretations of the potentials of various interphase and electrode boundaries, including those that are employed in galvanic and electroanalytical cells. 

When a solid material has been placed in an electrolytic solution a certain electrical potential may be built up at the contact surface however, this single potential cannot be measured in the absolute sense, nor can an electrical current be forced through the electrode without the aid of a second electrode. Therefore, electrometry in an electrolyte always requires two electrodes, the poles or terminals of the electroanalytical cell, and can be carried out by means of either non-faradaic or faradaic methods. 

See also Electroanalytical techniques Electroanalytical cells, 9 567 Electroanalytical techniques, 9 567-590 active, 9 568-581 economic aspects, 9 588 passive, 9 581-586 static and dynamic measurements, 9 586-588... 

Figure 1.4 The typical electroanalytical cell consists of three electrodes in a few milliliters of solution. Electron transfer occurs at the surface of the working electrode. Reactant and product are transported to and from this surface by diffusion.

Most of the industrial cells discussed in Chapter 31 have been developed from laboratory prototypes mentioned in this chapter. Polarographic cells and other cells for electroana-lytical purposes discussed in different monographs [1,2] have been omitted however, some electron spin and some electroanalytical cells can be found in Chapter 2. The problems connected with the scale-up of organic electrode processes and electrochemical engineering have been treated [3-5] and are discussed in Chapter 31. Fuel cells have also been omitted here they are discussed in a special issue of Electrochimica Acta (nr 24, 1998). 

The remaining feature of the electroanalytical cell is the liquid junction potential. As an example, consider the junction which arises in a cell used to determine the pH of 0.01 M HNO3 solution with a saturated calomel electrode (SCE) as reference ... 

In cyclic voltammetry a redox-active molecule is placed in an electroanalytical cell and the electrode potential is raised from a starting value at which there is no electroactivity. When electron transfer occurs a current is measured, and the shape of the trace depends upon, among other factors, the size and shape of the electrode. Thus, at a disk or wire of millimeter-sized dimensions (millielectrode) under conditions of linear diffusion, an initial current increase imder the control of electron-transfer kinetics meets a current decrease under diffusion control towards an effectively planar surface, and a characteristic peak shape is observed [Fig. 4(a)]. If the electron-transfer reaction produces a relatively stable species, then on reversing the scan direction a current is observed in the opposite direction. 

A Electrochemical Cells 628 22B Potentials in Electroanalytical Cells 633 22C Electrode Potentials 635 22D Calculation of Cell Potentials from Electrode Potentials 645... 

Electrochemical cells may be used in either active or passive modes, depending on whether or not a signal, typically a current or voltage, must be actively appHed to the cell in order to evoke an analytically usehil response. Electroanalytical techniques have also been divided into two broad categories, static and dynamic, depending on whether or not current dows in the external circuit (1). In the static case, the system is assumed to be at equilibrium. The term dynamic indicates that the system has been disturbed and is not at equilibrium when the measurement is made. These definitions are often inappropriate because active measurements can be made that hardly disturb the system and passive measurements can be made on systems that are far from equilibrium. The terms static and dynamic also imply some sort of artificial time constraints on the measurement. Active and passive are terms that nonelectrochemists seem to understand more readily than static and dynamic. 

The ionic potentials can be experimentally determined either with the use of galvanic cells containing interfaces of the type in Scheme 7 or electroanalytically, using for instance, polarography, voltammetry, or chronopotentiometry. The values of and Aj f, obtained with the use of electrochemical methods for the water-1,2-dichloroethane, water-dichloromethane, water-acetophenone, water-methyl-isobutyl ketone, o-nitrotol-uene, and chloroform systems, and recently for 2-heptanone and 2-octanone [43] systems, have been published. These data are listed in many papers [1-10,14,37]. The most probable values for a few ions in water-nitrobenzene and water-1,2-dichloroethane systems are presented in Table 1. 

Special electrochemical sensors that operate on the principle of the voltammetric cell have been developed. The area of chemically modified solid electrodes (CMSEs) is a rapidly growing field, giving rise to the development of new electroanalytical methods with increased selectivity and sensitivity for the determination of a wide variety of analytes [490]. CMSEs are typically used to preconcentrate the electroactive target analyte(s) from the solution. The use of polymer coatings showing electrocatalytic activity to modify electrode surfaces constitutes an interesting approach to fabricate sensing surfaces useful for analytical purposes [491]. 

Cuculic and Branica [788] applied differential pulse anodic stripping voltammetry to a study of the adsorption of cadmium, copper, and lead in seawater onto electrochemical glass vessels, quartz cells, and Nalgene sample bottles. Nalgene was best for sample storage and quartz was best for electroanalytical vessels. 

Electrogravimetry, which is the oldest electroanalytical technique, involves the plating of a metal onto one electrode of an electrolysis cell and weighing the deposit. Conditions are controlled so as to produce a uniformly smooth and adherent deposit in as short a time as possible. In practice, solutions are usually stirred and heated and the metal is often complexed to improve the quality of the deposit. The simplest and most rapid procedures are those in which a fixed applied potential or a constant cell current is employed, but in both cases selectivity is poor and they are generally used when there are... 

Potentiometry is the most widely used electroanalytical technique. It involves the measurement of the potential of a galvanic cell, usually under conditions of zero current, for which purpose potentiometers are used. Measurements may be direct whereby the response of samples and standards are compared, or the change in cell potential during a titration can be monitored. 

It should be noted here that the ultra thin-layer cells (UTLC) which result from the close approach of an STM tip to a conducting substrate may have important electroanalytical applications in studies other than STM imaging (64). This is because extremely large current densities should be attainable in such cells, and also because of the fast transit times (e.g., 50 nsec for d - 10 nm) for reactants across the cell. Thus, such UTLC s might facilitate the determination of fast heterogeneous rate constants or the study of reactive electrochemical intermediates (64). 

Electroanalytical techniques are an extension of classical oxidation-reduction chemistry, and indeed oxidation and reduction processes occur at the surface of or within the two electrodes, oxidation at one and reduction at the other. Electrons are consumed by the reduction process at one electrode and generated by the oxidation process at the other. The electrode at which oxidation occurs is termed the anode. The electrode at which reduction occurs is termed the cathode. The complete system, with the anode connected to the cathode via an external conductor, is often called a cell. The individual oxidation and reduction reactions are called half-reactions. The individual electrodes with their half-reactions are called half-cells. As we shall see in this chapter, the half-cells are often in separate containers (mostly to prevent contamination) and are themselves often referred to as electrodes because they are housed in portable glass or plastic tubes. In any case, there must be contact between the half-cells to facilitate ionic diffusion. This contact is called the salt bridge and may take the form of an inverted U-shaped tube filled with an electrolyte solution, as shown in Figure 14.2, or, in most cases, a small fibrous plug at the tip of the portable unit, as we will see later in this chapter. 

As mentioned previously, electroanalytical techniques that measure or monitor electrode potential utilize the galvanic cell concept and come under the general heading of potentiometry. Examples include pH electrodes, ion-selective electrodes, and potentiometric titrations, each of which will be described in this section. In these techniques, a pair of electrodes are immersed, the potential (voltage) of one of the electrodes is measured relative to the other, and the concentration of an analyte in the solution into which the electrodes are dipped is determined. One of the immersed electrodes is called the indicator electrode and the other is called the reference electrode. Often, these two electrodes are housed together in one probe. Such a probe is called a combination electrode. 

In electroanalytical chemistry, the unchanging reference is a half-cell that, at a given temperature, has an unchanging potential. There are two designs for this half-cell that are popular—the saturated calomel electrode (SCE) and the silver-silver chloride electrode. These are described below. 

A cell is a complete electroanalytical system consisting of an electrode at which reduction occurs, as well as an electrode at which oxidation occurs, and including the connections between the two. A half-cell is half of a cell in the sense that it is one of the two electrodes (and associated chemistry) in the system, termed either the reduction half-cell or the oxidation half-cell. The anode is the electrode at which oxidation takes place. The cathode is the electrode at which reduction takes place. An electrolytic cell is one in which the current that flows is not spontaneous, but rather due to the presence of an external power source. A galvanic cell is a cell in which the current that flows is spontaneous. 

The equipment for electroanalytical methods usually includes the required cells, but standardized preparative scale electrochemical cells are scarcely available (some equipment is offered, for example, by the Electrosynthesis Company Inc., Lancaster, USA). Most of the electrochemical cells, used for the investigations in the literature, are made in the facilities of the institutes, especially by glassblowers. This... 

Electrosyntheses of heterocycles from nitroso derivatives prepared in a batch cell according to Scheme 34 need two conditions. The first one is a good stability of the hydroxylamine intermediate and the second one is a very fast cyclization of the nitroso compound to avoid the formation of an azoxy compound by condensation of the generated nitroso and the hydroxylamine. Electroanalytical studies using cyclic voltammetry can give information on the rate of cyclization. 

Electroanalytical chemists and others are concerned not only with the application of new and classical techniques to analytical problems, but also with the fundamental theoretical principles upon which these techniques are based. Electroanalytical techniques are proving useful in such diverse fields as electro-organic synthesis, fuel cell studies, and radical ion formation, as well as with such problems as the kinetics and mechanisms of electrode reactions, and the effects of electrode surface phenomena, adsorption, and the electrical double layer on electrode reactions. 

In everyday chemical usage, the word equilibrium means that a reaction has stopped, e.g. because it has reached its position of minimum chemical potential or because one reactant has been consumed completely. In this electroanalytical context, however, we say that we are making a measurement of potential at equilibrium , yet the system has clearly not reached a true equilibrium because as soon as the voltmeter is replaced with a connection having zero resistance, a cell reaction could commence. What then do we mean by equilibrium in this electroanalytical context ... 

Having established (i) that a meaningful cell emf can only be obtained at equilibrium, (ii) that the emf comprises two electrode potentials, and (iii) that the potential of one half cell can be defined in relation to a reference electrode, we are finally in a position to extract electroanalytical data from cells by using a potentiometric approach. 

Before electroanalytical data can be obtained from a cell, it is essential to ensure that the cell is in a state of frustrated equilibrium The measured emf represents the separation in potential between the electrode potentials of two half cells. One half cell is conveniently a reference electrode, so o,r for the analyte is the only unknown. From o,r. the central relationship behind the electroanalytical determination of concentrations by using the potentiometric approach is the Nemst equation (equation (3.8)), which is written in terms of activities. 

An electrode of surface area 100 pm or less is called a microelectrode and provides a means of decreasing the double-layer capacitance which can affect our coulometry experiments so badly. Microelectrodes are also useful when the cell considered is also tiny, as, for example, is the case when performing in vivo voltammetry (see next chapter) with biological samples. For example, a nerve ending is typically 10-100 pm in diameter, so electroanalytical experiments using a conventional electrode would be impossible. 

Voltammetry and polarography are dynamic electroanalytical techniques, that is, current flows. A three-electrode cell is needed to allow accurate and simultaneous determination of current and potential. The electrode of interest is the working electrode, with the other two being the reference and counter electrodes. 

Figure 7.6 Schematic representation of a typical flow cell used for electroanalytical measurements. Note the way in which the counter electrode (CE) is positioned downsteam, i.e. the products from the CE flow away from the working electrode.

Figure 7.8 Schematic representation of a typical wall-jet electrode used for electroanalytical measurements (a) contact to Pt disc electrode (the shaded portion at the centre of the figure) (b) contact to ring electrode (c) AgCl Ag reference electrode (d) Pt tube counter electrode (e) cell inlet (f) cell body (made of an insulator such as Teflon), (b) A typical pattern of solution flow over the face of a wall-jet electrode, showing why splash back does not occur. Part (a) reproduced from Brett, C. M. A. and Brett, A. M. O., Electroanalysis, 1998, 1998, by permission of Oxford University Press.

While introducing this new way of obtaining electroanalytical data, we will need to rely on the analogies between an electrochemical cell (or sample) and an electrical circuit made up of resistors and capacitors assembled in order to mimic the current-voltage behaviour of the cell. All the time, though, we need to bear in mind that the ideas and attendant mathematics are for interpretation only, although they are fundamentally very simple. 

Schmidt, T. J., Paulus, U. A., Gasteiger, H. A. and Behm, R. J. 2001. The oxygen reduction reaction on a Pt/carbon fuel cell catalyst in the presence of chloride anions. Journal of the Electroanalytical Society 508 41-47. 

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