Reference electrodes description

Description of the cell composition is based - as far as possible - on the Stockholm convention (1953), i.e. the left-hand electrode constitutes the negative terminal of the cell. Cells are listed according to the metallic constituent of the electrode mentioned first which is involved in the electrode reaction establishing the respective electrode potential. Contact materials and conductive additives may be mentioned first before the actual element of interest only for the sake of correct materials sequence. The sequence of electrode components is stated as reported in the original publications. When an oxygen electrode is used as reference electrode an oxygen partial pressure of 0.21 atm is assumed. 

The electrodes and electrode protective coating of the oxygen sensor play a crucial role in determining the performance characteristics and durability (2). The electrodes used are the inner or air-reference electrode and the outer or exhaust gas electrode. The protective coating goes over the outer or exhaust electrode. While a complete discussion of the requirements and properties of the electrodes and electrode protective coating is beyond the scope of this paper, a brief description will be given. 

Undoubtedly, the mercury/aqueous solution interface, was in the past, the most intensively studied interface, which was reflected in a large number of original and review papers devoted to its description, for example. Ref. 1, and in the more recent work by Trasatti and Lust [2] on the potentials of zero charge. It is noteworthy that in view of numerous measurements of the double-layer capacitance at mercury brought in contact with NaF and Na2S04 solutions, the classical theory of Grahame [3] stiU holds [2]. According to Trasatti [4], the most reliable PZC value for Hg/H20 interface in the absence of specific adsorption equals to —0.433 0.001 V versus saturated calomel electrode, (SCE) residual uncertainty arises mainly from the unknown liquid junction potential at the electrolyte solution/SCE reference electrode boundary. 

Reference system complex metal cyanides, 20 ml. of a 0.03M aqueous solution and 200 ml. 15W S0rensen buffer containing organic solvents (for detailed description see Experimental), apparent pH 7.45. Electrodes combined platinum electrode with Ag/AgCl in saturated KC1 as reference electrode. Reaction temperature 20°C. Abbreviations Reference systems named as oxidants—Fe, potassium ferricyanide Mo, potassium molybdicyanide. Mv.—millivolts. 

The first documented description of an ISFET contained the sensational claim that such a device could be used for the measurement of ion activities without a reference electrode. 

Engstrom and Carlsson already introduced in 1983 an SLPT [119] for the characterisation of MIS structures, which was extended to chemical gas sensors by Lundstrom et al. [26]. Both SLPT and LAPS base upon the same technique and principle. However, due to the different fields of applications in history, one refers to LAPS for chemical sensors in electrolyte solutions and for biosensors, and the SLPT for gas sensors. A description of the development of a hydrogen sensor based on catalytic field-effect devices including the SLP technique can be found, e.g., in Refs. [120,121]. The SPLT consists of a metal surface as sensitive material which is heated by, for instance, underlying resistive heaters to a specific working-point temperature, and a prober tip replaces the reference electrode (see Fig. 5.10). 

Reference Electrodes for Use in Polar Aprotic Solvents. The increased use of polar aprotic solvents for electrochemical studies has inspired a search for suitable reference electrodes. Although the description of an aprotic solvent is somewhat ambiguous (see Chapter 6), we include in this class those solvents... 

Redox potential is measured potentiometrically with electrodes made of noble metals (Pt, Au) (Fig. 12). The mechanical construction is similar to that of pH electrodes. Accordingly, the reference electrode must meet the same requirements. The use and control of redox potential has been reviewed by Kjaergaard [218]. Considerations of redox couples, e.g. in yeast metabolism [47], are often restricted to theoretical investigations because the measurement is too unspecific and experimental evidence for cause-effect chains cannot be given. Reports on the successful application of redox sensors, e.g. [26,191], are confined to a detailed description of observed phenomena rather than their interpretation. 

It is a point peculiar to electrochemical reaction kinetics (77), however, that the rates of charge-transfer processes at electrodes measured, as they have to be, at some well-defined potential relative to that of a reference electrode, are independent of the work function of the electrocatalyst metal surface. This is due to cancellation of electron-transfer energies, O, at interfaces around the measuring circuit. In electrochemistry, this is a well-understood matter, and its detailed origin and a description of the effect may be found, among other places, in the monograph by Conway (77). 

Figure 13. Representation of the potential variations in the electrochemical cell and associated electronic schemes (a-d). ( = cat + Ri + an see text). W, working electrode Ref, reference electrode A, auxiliary electrode, (e) Schematic description of the electrochemical cell with a potentiostat.

Values of the PZC at the Hg solution interface are shown as a function of electrolyte concentration in fig. 10.6. In the case of NaF, the PZC with respect to a constant reference electrode is independent of electrolyte concentration. However, in the cases of the other halides, the PZC moves to more negative potentials as the electrolyte concentration increases. The latter observation is considered to be evidence that the anion in the electrolyte is specifically adsorbed at the interface. Specific adsorption occurs when the local ionic concentration is greater than one would anticipate on the basis of simple electrostatic arguments. Anions such as Cl , Br , and 1 can form covalent bonds with mercury so that their interfacial concentration is higher than the bulk concentration at the PZC. Furthermore, the extent of specific adsorption increases with the atomic number of the halide ion, as can be seen from the increase in the negative potential shift. A more complete description of specific adsorption will be given later in this chapter. 

As mentioned in the introduction, the electrical nature of a majority of electrochemical oscillators turns out to be decisive for the occurrence of dynamic instahilities. Hence any description of dynamic behavior has to take into consideration all elements of the electric circuit. A useful starting point for investigating the dynamic behavior of electrochemical systems is the equivalent circuit of an electrochemical cell as reproduced in Fig. 1. The parallel connection between the capacitor and the faradaic impedance accounts for the two current pathways through the electrode/electrolyte interface the faradaic and the capacitive routes. The ohmic resistor in series with this interface circuit comprises the electrolyte resistance between working and reference electrodes and possible additional ohmic resistors in the external circuit. The voltage drops across the interface and the series resistance are kept constant, which is generally achieved by means of a potentiostat. 

Traditionally, electrochemical equilibria are explained in terms of thermodynamic cell potentials. However, in electro analytical applications, such a description is of little use, because one almost always uses a non-thermodynamic measurement, with a reference electrode that includes a liquid junction. It is then more useful to go back to the basic physics of electrochemistry, i.e., to the individual interfacial potential differences that make up the total cell potential. This is the approach we will use here. 

At each well and depth, concentrations of dissolved oxygen, Fe(II), NO " and NO2 were measured. ORP s at Pt and WIG electrodes were measured against a Ross reference electrode. A Ross combination electrode was used to determine pH. Temperature and conductivity were also recorded. The detection limits for Fe(II) and N were 0.05 mg Fe or N per liter, and accuracies for each were +/- 10%. Sulfide was not measured, but no H2S odor was discernable in any sample. More detailed descriptions of the sampling and analytical methods will be published elsewhere. 

A reference electrode is used in both two- and three-electrode electrochemical sensors. The three-electrode configuration is used mainly for voltammetric or amperometric modes of sensing. The first design considerations for an electrochemical sensor should include the size, geometric shape, relative location, and material used for the electrode elements. A description of each of these design parameters is given below. 

The thermodynamic description of an electrified solid-liquid interphase is similar to that of nonionic systems with one important difference—the description requires the introduction of electrochemical parameters the thermodynamic charge and the electrical potential difference between the solid phase considered and a reference electrode. 

Section 23A gives more detailed descriptions of the silver-silver chloride and the calomel reference electrode systems. Both reference electrodes can be purchased from suppliers of electrochemical equipment. 

Chloride content. By embedding the combined chloride/resistivity sensor elements mentioned above (Figure 17-2), the activity of the free chloride ions in the pore solution of concrete can be monitored over time at different depths. The potential of the embedded chloride sensors is measured versus a Mn02 reference electrode and converted by Nernst s law to chloride concentration. In several field applications, hundreds of chloride sensors worked well over several years [15]. A more detailed description of the chloride sensor, its calibration and long-term stability is given in references [20,22]. 

The following procedures involve performing PEC characterizations in a three-electrode configuration using a reference electrode. A complete description of... 

The electrolyte solution of reference electrodes may serve two functions (a) to provide a constant potential of the reference electrode, and (b) to serve as the electrolyte bridge to the analyte solution, ideally with negligible diffusion potentials. The possibilities to contact the reference electrode to the adjacent solution vary greatly. The most common arrangements are shown in Fig. III.2.6. A detailed description of the diaphragms is given below. When such arrangement is used, the levels of the analyte and the reference electrode electrolytes should be balanced to prevent any contamination, either of the analyte or the reference electrode compartment. 

In the description given outlining electrochemical systems in which a current flows, key parameters include the variations of the anodic and cathodic polarisations (or overpotentials if applicable) as a function of current and time. These relationships are generally represented in the form of current-potential curves of an electrode, /= f(E), where E is the voltage between the electrode in question and a reference electrode . The experimental results can also be presented in the form of current density-potential curves. However, when the study concerns the whole electrochemical system and is not just focused on the working electrode, it is best to keep the current-potential representation I... 

Generally speaking, these characteristics are time-dependent. Here, we will only look at the steady-state current-potential curves, so called because they are obtained in steady-state conditions. However the shapes presented remain valid in qualitative terms at all times. Moreover, the voltage that is accessible in experimental conditions includes an ohmic drop term between the electrode in question and the reference electrode. However, this simplified description will not take into account this ohmic drop. Yet this type of curve, once corrected for the ohmic drop, can be determined in most cases in experimental conditions. Here we will discuss their shape in qualitative terms but not the detail of the curves... 

The electrochemical cell would need quartz windows. A typical three-electrode system, with WE, CE, and reference electrode, is used. Eor IR work, the source and spectrometer would have to be suitable and the windows of the cell would have to be IR transparent, for example, CaFj. Suitable materials have been discussed for both techniques in Chapters 4 and 5. Commercial systems are available from BioLogic Science Instruments, Claix, Erance (www.bio-Iogic.info), and from ZAHNER-Elektrik GmbH Co., KG, Kronach, Germany (www.zahner.de). Their websites contain pictures and detailed instrument descriptions as well as a number of application notes and technical notes. 

Let us adopt the Guggenheim description for the interphase, and apply these concepts to a specific experimental cell consisting of a dropping mercury electrode (Hg ) in contact with an aqueous electrolyte (KCl) and calomel reference electrode ... 

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