Reference Electrodes reproducibility

Fig. 12. Absorption spectra of polyaniline film on a semitransparent Pt electrode immersed in 1 M HC1. Spectra (a) to (f) were recorded at potentials of —0.2, +0.6, +0.8, + 1.0, + 1.2 and + 1.4 V relative to a silver reference electrode. Reproduced from [173b],...

Figure 4.18 — (A) Schematic diagram of a flow-injection system for potassium (1) carrier solution (2) injection valve (3,4) flow cell (5) pseudo-reference electrode (6) waste. (B) Detail of the flow-cell (3,4) CHEMFETs (5) pseudo-reference electrode. (Reproduced from [150] with permission of Elsevier Science Publishers).

Fig. 23 Cell construction of an amperometric detector for capillary LC. 1 column 2 and 5 fluoroplastic body 3 working electrode 4 reference electrode. (Reproduced with permission from Elsevier.)...

Figure 4-21 SERS in cytidine-3 -monophosphate (3 -CMP). 3 -CMP concentration 2 x 10 3 M, 0.15 M KC1, 2 x 10 3 M Tris buffer (pH 7.2). 514.5 nm excitation, laser power at the electrode 10 mW, prior activation of the Ag electrode 1x5 mCb between -0.1V and +0.2 V. (a) Adsorption potential E, - 0.1 V vs. Ag/AgCl reference electrode, (b) Adsorption potential E, -0.6 V vs. Ag/AgCl reference electrode. (Reproduced with permission from Ref. 44.)...

Fig. 7-26. Cyclic voltammograms recorded at a platinum electrode on MeCN solutions containing (a) 1 (b) ferrocene (c) ruthenocene. Scan rate 0.3Vs A Potential values refer to the Ag/AgCl reference electrode (reproduced by permission of the Chemical Society of Japan).

Fig. 2-39. The basic mode of the contact polarisation curve (CPC) field installation 1- ore body 2- borehole 3- compensation resistor 4- electric current source 5- resistance of shunt 6-potentiometer 7- recorder A- current eleetrode B- auxiliary current electrode M- measuring electrode N- non-polarisable measuring (reference) electrode (reproduced with permission from Ryss, 1983).

Fig. 2-45. Scheme of the CLPC field installation 1- transducer of compensation 2- ammeter 3-galvanic decoupling unit 4- potentiometer 5- recorder 6- electrical current source 7-measurement channels switch A, B- current electrodes M, ...Mg -measuring non-polarisable electrodes N- remote non-polarisable reference electrode (reproduced with permission from Ryss, 1983). 

Fig. 15 Linear, close to unity dependence of the open circuit voltage on the LUMO level of the acceptor (first reduction potential as determined electrochemically with respect to an Ag/AgCl reference electrode). (Reproduced from [103] with permission, 2001, Wiley-VCH)...

The salt analysis consisted of soaking the reference sample in deionised water for 8h, to redissolve all the salt, then determining the salt concentration in the deionised water by titration with silver nitrate (0-0.5 molar) using an Orion EA940 expandable ion analyser link to a single junction reference electrode. Reproducibility of the salt measurement using this equipment was 0.001 mg/cm2. 

Ag/AgCl reference electrode (reproduced by permission of the Chemical Society of Japan). 

Figure 14.2 OTTLE cells for UV-vis-NIR sp>ectroscopy (a) employing a metal mesh electrode and conventional thin layer cuvette with reservoir for counter and reference electrodes, (h) a thin layer cuvette with side arms for counter and reference electrodes, reproduced with permission of American Chemical Society, (c) a thin layer flow cell. Reprinted with permission from reference (16). Copyright 1989 and 1993, American Chemical Society.

Fig. 22 Experimental setup showing the electrochemical cell under the AFM scanning head, the samples of patterned PPy on Au-coated siheon and the eoimter and reference electrodes (Reproduced from Smela and Gadegaard (1999))...

Reference Electrodes and Liquid Junctions. The electrical cincuit of the pH ceU is completed through a salt bridge that usually consists of a concentrated solution of potassium chloride [7447-40-7]. The solution makes contact at one end with the test solution and at the other with a reference electrode of constant potential. The Hquid junction is formed at the area of contact between the salt bridge and the test solution. The mercury—mercurous chloride electrode, the calomel electrode, provides a highly reproducible potential in the potassium chloride bridge solution and is the most widely used reference electrode. However, mercurous chloride is converted readily into mercuric ion and mercury when in contact with concentrated potassium chloride solutions above 80°C. This disproportionation reaction causes an unstable potential with calomel electrodes. Therefore, the silver—silver chloride electrode and the thallium amalgam—thallous chloride electrode often are preferred for measurements above 80°C. However, because silver chloride is relatively soluble in concentrated solutions of potassium chloride, the solution in the electrode chamber must be saturated with silver chloride. 

It is fundamental that a reference electrode should have a stable and reproducible potential. Not all reference electrodes are suitable for all... 

Electrodes such as Cu VCu which are reversible with respect to the ions of the metal phase, are referred to as electrodes of the first kind, whereas electrodes such as Ag/AgCl, Cl" that are based on a sparingly soluble salt in equilibrium with its saturated solution are referred to as electrodes of the second kind. All reference electrodes must have reproducible potentials that are defined by the activity of the species involved in the equilibrium and the potential must remain constant during, and subsequent to, the passage of small quantities of charge during the measurement of another potential. 

Reference Electrode an equilibrium (reversible) electrochemical half-cell of reproducible potential against which an unknown electrode potential can be measured. Examples of those commonly used in corrosion are the Pt, H /H (the hydrogen electrode), Hg/Hg Clj/Cl" (the calomel electrode), Cu/CuS04/Cu, Ag/AgCl/Cl", all with fixed activities of the dissolved ions. 

FIGURE 3-14 Stripping voltammograms for 2 x 10 7 M Cu2+, Pb2+, In3+ and Cd2+ at the mercury film (A) and hanging mercury drop (B) electrodes. (Reproduced with permission from reference 21.)... 

FIGURE 3-23 Schematic of a carbon-fiber amperometric detector for capillary electrophoresis A, fused silica capillary B, eluent drop C, stainless steel plate RE, reference electrode WE, working electrode, AE, auxiliary electrode. (Reproduced with permission from reference 58.)... 

FIGURE 5-7 The alkaline and acid errors of several glass pH electrodes. A, Corning 015/H2SO4 B, Corning 015/HC1 C, Coming 015/1 M Na+ D, Beckman-GP/1 M Na+ E, L N BlackDot/lM Na+ F, Beckman E/1M Na+ G, Ross electrode. (Reproduced with permission from reference 16.)... 

FIGURE 5-18 Flow-through potentiometric cell-cap design. A, reference electrode B, iodide electrode C, flow-through cap D, inlet E, outlet. (Reproduced with permission from reference 49.)... 

FIGURE 6-20 Configuration of a penicillin sensor based on an microarray electrode coated with a pH-responsive polypyrrole. Vq = gate voltage VD = drain voltage ID = drain current PS = potentiostat CE and RE = counter and reference electrodes, respectively. (Reproduced with permission from reference 76.)... 

While there are no problems in the definition of the configuration leading to 0, difficulties are encountered in the procedure to reproduce the electrochemical situation. In fact, Eq. (17) has meaning only if the M/S interface has exactly the same structure during the measurement of E (relative to a reference electrode-electrochemical configuration) as well as during the measurement of 0. ... 

Figure 10.6. Electrochemical cell (1) reference electrode, (2) molten catalyst, (3) porous Pyrex membrane, (4) counter electrode, (5) gas inlet Pyrex tube, (6) working electrode.12 Reproduced by permission of the Electrochemical Society.

Indicator electrodes are used both for analytical purposes (in determining the concentrations of different substances from values of the open-circuit potential or from characteristic features of the polarization curves) and for the detection and quantitative characterization of various phenomena and processes (as electrochemical sensors or signal transducers). One variety of indicator electrode are the reference electrodes, which have stable and reproducible values of potential and thus can be used to measure the potentials of other electrodes. 

A parameter that is convenient for said purpose is the electrode potential E it must not be confused with the concept of a potential difference between the electrode and the electrolyte. By convention the term electrode potential E is used to denote the OCV of a galvanic cell that consists of the given electrode (the one that is studied) and a reference electrode selected arbitrarily. Thus, the potential of this electrode is compared with that of a reference electrode that is identical for all electrodes being studied. In accordance with this dehnition, the electrode potential of the reference electrode itself is (conventionally) regarded as zero. Any electrode system for which the equilibrium Galvani potential is established sufficiently rapidly and reproducibly can be used as a reference electrode. We shall write the electrode system to be used as the reference electrode, generally, as M /E ... 

One distingnishes practical and standard reference electrodes. A standard RE is an electrode system of particnlar confignration, the potential of which, nnder specified conditions, is conventionally taken as zero in tfie corresponding scale of potentials (i.e., as the point of reference nsed in finding tfie potentials of otfier electrodes). Practical REs are electrode systems having a snfficiently stable and reproducible value of potential which are nsed in the laboratory to measure the potentials of other electrodes. The potential of a practical reference electrode may difier from the conventional zero potential of the standard electrode, in which case the potential of the test electrode is converted to this scale by calculation. 

Figure 1 Electrochemical detection of catechol, acetaminophen, and 4-methyl catechol, demonstrating the selectivity of differential pulse detection vs. constant potential detection. (A) Catechol, (B) acetaminophen, and (C) 4-methylcatechol were separated by reversed phase liquid chromatography and detected by amperometry on a carbon fiber electrode. In the upper trace, a constant potential of +0.6 V was used. In the lower trace, a base potential of +425 mV and a pulse amplitude of +50 mV were used. An Ag/AgCl reference electrode was employed. Note that acetaminophen responds much more strongly than catechol or 4-methylcatechol under the differential pulse conditions, allowing highly selective detection. (Reproduced with permission from St. Claire, III, R. L. and Jorgenson, J. W., J. Chromatogr. Sci. 23, 186, 1985. Preston Publications, A Division of Preston Industries, Inc.)...

E°)/6 = 1.371 V hence the voltage curve is high and far from symmetrical. If a glass pH electrode is used as the reference electrode the determination of the equivalence point becomes even more reproducible. 

The described electrodes, and especially the silver chloride, calomel and mercurous sulphate electrodes are used as reference electrodes combined with a suitable indicator electrode. The calomel electrode is used most frequently, as it has a constant, well-reproducible potential. It is employed in variously shaped vessels and with various KC1 concentrations. Mostly a concentration of KC1 of 0.1 mol dm-3, 1 mol dm-3 or a saturated solution is used (in the latter case, a salt bridge need not be employed) sometimes 3.5 mol dm-3 KC1 is also employed. The potentials of these calomel electrodes at 25°C are as follows (according to B. E. Conway) ... 

Fig. 2.4j is a simplified diagram of an amperometric detector. Three electrodes are used, called working, auxiliary and reference electrodes (we ae and re). The we is the electrode at which the electroactivity is monitored, and the re, usually a silver-silver chloride electrode, provides a stable and reproducible voltage to which the potential of the we can be referenced. The ae, usually stainless steel, is a current-carrying electrode. 

A baseline potential pulse followed each current pulse in order to strip extracted ions from the membrane phase and, therefore, regenerated the membrane, making it ready for the next measurement pulse. This made sure that the potentials are sampled at discrete times within a pulse that correspond to a 6m that is reproducible from pulse to pulse. This made it possible to yield a reproducible sensor on the basis of a chemically irreversible reaction. It was shown that the duration of the stripping period has to be at least ten times longer than the current pulse [53], Moreover the value of the baseline (stripping) potential must be equal to the equilibrium open-circuit potential of the membrane electrode, as demonstrated in [52], This open-circuit potential can be measured prior to the experiment with respect to the reference electrode. 

---