Electroanalytical chemistry reference electrodes

The reference electrode contributes heavily to the economics of electroanalytical chemistry. Companies that sell and service electroanalytical instmmentation are few in number and small in size, or they are parts of much larger companies. One suppHer of electroanalytical instmmentation is Princeton AppHed Research Corp. (PARC) of Princeton, Newjersey. PARC is a subsidiary of EG G Instmments, Inc. Among the many suppHers of ion-selective electrodes are Orion (Boston, Massachusetts), Corning (Corning, New York), and Ingold (Wilmington, Massachusetts). Brinkmann Instmments, Inc. (Westbury, New York) is a useful suppHer of titration equipment. 

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. 

Mercury is widely used in the practice of electroanalytical chemistry, both for working electrodes and for reference electrodes (in the latter case usually as an electrode of the second kind). 

Molten salts or ionic liquids (also referred to as fused salts by some authors) were among the very first media to be employed for electrochemistry. In fact, Sir Humphrey Davy describes electrochemical experiments with molten caustic potash (KOH) and caustic soda (NaOH) [1] as early as 1802 A wide variety of single molten salts and molten salt mixtures have been used as solvents for electroanalytical chemistry. These melts run the gamut from those that are liquid well below room temperature to those melting at more than 2000°C. The former present relatively few experimental challenges, whereas the latter can present enormous difficulties. For example, commercially available Teflon- and Kel-F-shrouded disk electrodes and Pyrex glass cells may be perfectly adequate for electrochemical measurements in ambient temperature melts such as the room-temperature chloroaluminates, but completely inadequate for use with molten sodium fluoroaluminate or cryolite (mp = 1010°C), which is the primary solvent used in the Hall-Heroult process for aluminum electrowinning. 

Refs. [i] Sawyer D, Sobkowiak A, Roberts ]L Jr (1995) Electrochemistry for chemists, 2nd edn. Wiley Interscience, New York [ii] Inzelt G (2006) Standard potentials (chap. 1). Platinum (chap. 17.3). In Bard A), Stratmann M, ScholzP, Pickett C (eds) Inorganic chemistry. Encyclopedia of electrochemistry, vot 7a. Wiley-VCH, Weinheim [Hi] Kahlert H (2002) Reference electrodes. In Scholz P (ed) Electroanalytical methods. Springer, Berlin, pp 263-264... 

Polarography is the term used for voltammetry with the dropping mercury electrode (DME). The technique has been discussed extensively in several textbooks and reviews [1-, 237-242] to which the reader is referred for details concerning both theoretical problems and practical applications. The electrode (Fig. 31) was developed early in the century by Heyrovsky and was the dominating tool in electroanalytical chemistry for several decades. Because of the low oxidation potential of mercury (0.3-0.4 V versus SCE), the DME has been used almost exclusively for the study of reduction processes. Compared with mercury film electrodes, the DME offers the advantage that the electrode surface is continuously renewed. This property reduces undesirable surface effects caused by adsorption. 

Figure 11.6.7 Different geometries for thin-layer electrochemical detector cells involving different placements of the working (W), auxiliary (A) and reference (R) electrodes. [Reprinted from S. M. Lunte, C. E. Lunte, and P. T. Kissinger, in Laboratory Techniques in Electroanalytical Chemistry, ...

The data in this table are mainly taken from A. J. Bard, J. Jordan, and R. Parsons, Eds., Standard Potentials in Aqueous Solutions, Marcel Dekker, New York, 1985 (prepared under the auspices of the Electrochemistry and Electroanalytical Chemistry Commissions of lUPAC). Other sources of standard potentials and thermodynamic data include (1) A. J, Bard and H. Lund, Eds., The Encyclopedia of the Electrochemistry of the Elements, Marcel Dekker, New York, 1973-1986. (2) G. Milazzo and S. Caroli, Tables of Standard Electrode Potentials, Wiley-Interscience, New York, 1977. The data here are referred to the NHE based on a 1-atm standard state for H2. See the footnote in Section 2.1.5 concerning the recent change in standard state. 

Since the comprehensive reviews published some time ago in Electroanalytical chemistry [5, 6], and a concise version of these in 1976 [7], there have been many recent reviews covering spectroelectrochemistry, using radiation other than in the visible region [8-11]. However, there are far fewer reviews exclusively covering UV-visible spectroelectrochemistry. The most recent and complete review (with 390 references) was published in 1996 [12]. Other surveys include those by Pragst [13], McCreery and coworkers [14] and Plieth and coworkers [15]. There are also the well-known biennial reviews in Analytical Chemistry within the Dynamic Electrochemistry sections [16, 17]. Various book chapters and monographs provide excellent summaries on the techniques/theory and the applications of optically transparent electrodes [18-21]. Although we shall... 

D. 0. Raleigh, "Electrode Processes in Solid-Electrolyte Systems," and his references therein, in Advances in Electroanalytical Chemistry, Vol. 6, edited by A. J. Bard,... 

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. 

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