Calomel electrodes, reference electrode

It must be emphasised that in evaluating the limiting cathode potential to be applied in the separation of two given metals, simple calculation of the equilbrium potentials from the Nernst Equation is insufficient due account must be taken of any overpotential effects. If we carry out, for each metal, the procedure described in Section 12.2 for determination of decomposition potentials, but include a reference electrode (calomel electrode) in the circuit, then we can ascertain the value of the cathode potential for each current setting and plot the current-potential curves. Schematic current-cathode potential... 

As in normal potentiometry one uses and indicator electrode versus a reference electrode, the electrodes should, especially in pH measurements, be those recommended by the supplier of the pH meter in order to obtain a direct reading of the pH value displayed. In redox or other potential measurements any suitable reference electrode of known potential can be applied. However, a reference electrode is only suitable if a junction potential is excluded, e.g., an Ag-AgCl electrode in a solution of fixed Ag+ concentration or a calomel electrode in a saturated KC1 solution as a junction in many instances a direct contact of Cl" with the solution under test (possibly causing precipitation therein) is not allowed, so that an extra or so-called double junction with KN03 solution is required. Sometimes micro-electrodes or other adaptations of the surface are required. 

Lu et al. have described a single sweep voltammetric method for the determination of chlorpromazine at carbon paste electrodes [176]. Powdered tablet was dissolved in and diluted to 100 mL with water, whereupon a 10 mL portion of the solution was mixed with 10 mL of acetate buffer solution (pH 5.3). This solution was then diluted to 100 mL. The solution was analyzed using an electrolytic cell equipped with a carbon paste working electrode, a reference saturated calomel electrode, and a platimun counter-electrode. The voltammogram was recorded by single sweep scanning from 0 to 1 V, with a scanning rate of 10 mV/sec. 

However, at least two other reference electrodes, calomel (Hg. 7.42) and silver silver chloride electrodes, are in common use as secondary reference electrodes (they are easier to set up than die hydrogen reference electrode). Potentials of electrodes measured using one of die secondary reference electrodes can be directly converted to values on die hydrogen scale, if die potential of die secondary reference electrode with respect to the hydrogen electrode is known (see also Section 7.5.73). 

EDTA). In this application the M(OH2)l+ cation that is to be titrated forms a soluble EDTA complex that is appreciably less stable than the HgEDTA complex.141 The mercury electrode is constructed in a form like that of Figure 5.40a or b, and is used in conjunction with a saturated calomel reference electrode. The electrode is immersed in a solution that contains the ion to be titrated [M(OH2) +], and a few drops of 0.01 M HgY2- are added (where Y4-represents the EDTA anion). This establishes the potential of the mercury electrode according to the half-cell reaction... 

As reference electrode any electrode whose potential is well defined and constant may be used by far the most widely used reference electrodes in aqueous and partly aqueous solution are the calomel (SCE, saturated calomel electrode) and the silver/silver chloride electrodes, both of which are electrodes of second kind. In non-aqueous solutions quite a few other reference electrodes have been used besides the calomel electrode. A discussion of reference electrodes is included in standard monographs on electroanalytical techniques, and comparisons between the diiferent types of electrode have been made.45-48... 

Most pH determinations are made by electrometric methods, the pH of the unkown solution (X) being calculated from that of a known standard (S) and the emf ( x and s) of a cell composed of a hydrogen ion-responsive electrode (for example, a glass electrode or a hydrogen gas electrode) coupled with a reference electrode (calomel, silver-silver chloride). This cell is filled successively with the standard solution S and with the unknown solution X. A liquid junction potential j exists where these solutions make contact with the concentrated KCl solution of the reference electrode. From the Nernst equation for the cell reactions and assuming an ideal hydrogen ion response ... 

The Standard Potential of Chlorine. Measurements of the potentials of galvanic cells without liquid junctions from which the standard potential of chlorine may be deduced have been made by Lewis and Ruppert40 who used, as one electrode, platinum over which a mixture of chlorine and nitrogen was bubbled, and, as reference, a calomel electrode and hydrochloric acid as electrolyte. The arrangement may be represented by... 

TABLE 12. Half-wave reduction potential of some organic halides (after Reference 117) and halosilanes (after Reference 116) using a reference saturated calomel electrode... 

The most important reference systems (calomel electrode, silver/silver chloride electrode) contain chloride ions. Here, mostly potassium chloride solutions are used, because the mobility of potassium ions (76.2x10 cm V... 

Reference electrode calomel or silver-silver chloride. 

A three-electrode system with a platinum working (H), a platinum auxiliary (K) and a reference saturated calomel electrode (I was placed in a U-type compartment (M) in an isothermal water bath (E). Two platinum wires jointing with the working and auxiliary electrodes, respectively, straight reached to electrolyte outside and connected to two copper wires which linked with the electrochemical workstation (B). The reference electrode was used to track the change in potential of electrode (H). A thermo-sensitive resistor (L)... 

Calomel Reference Electrode An electrode that has a potential dependent on chlorine anions. 

The most important reference systems (calomel electrode, silver/silver chloride electrode) contain chloride ions. Here, mostly potassium chloride solutions are used, because the mobility of potassium ions (76.2 x 10 cm V" s in infinitely diluted aqueous solution at 25 C) and chloride ions (79.1 x 10 cm V" s in infinitely diluted aqueous solution at 25°C) are quite similar [1]. Considering two... 

Similar to the Ag/ACl reference electrode, the electrode potential of the calomel electrode at a concentration of KCl(aq) equal (1) 0.1 mol (2) 1.0 mol L", (3) saturated with respect to KCl(s) can be calculated and compared with values given in [Chapter 10, Table 10.13]. The calomel and silver/silver chloride electrodes have similar precision and stability. A disadvantage of the calomel reference electrode is the use of Hg, which is a particularly hazardous chemical. 

Figure 16. Current and mass with respect to potential for a Prussian blue film in a KCl solution during a linear potential scan. Experimental conditions were as follows pH 2.8, KCl concentration 0.5 M, scan rate 20 mV s and reference saturated calomel electrode (SCE) (potential limits from +0.6 to —0.2 V and from —0.2 to +0.6 V). From Gabrielli et al. ...

Incidentally, a quantity called the rational potential is defined as E for the mercury-water interface (no added electrolyte) so, in general, = E + 0.480 V if a normal calomel reference electrode is used. 

Calomel Electrodes Calomel reference electrodes are based on the redox couple between Hg2Cl2 and Hg (calomel is a common name for Hg2Cl2). 

If the copper electrode is the indicator electrode in a potentiometric electrochemical cell that also includes a saturated calomel reference electrode... 

When the potential of an electrode of the first kind responds to the potential of another ion that is in equilibrium with M"+, it is called an electrode of the second kind. Two common electrodes of the second kind are the calomel and silver/silver chloride reference electrodes. Electrodes of the second kind also can be based on complexation reactions. Eor example, an electrode for EDTA is constructed by coupling a Hg +/Hg electrode of the first kind to EDTA by taking advantage of its formation of a stable complex with Hg +. 

Determine the parts per million of F in the tap water, (b) For the analysis of toothpaste a 0.3619-g sample was transferred to a 100-mL volumetric flask along with 50.0 mL of TISAB and diluted to volume with distilled water. Three 20.0-mL aliquots were removed, and the potential was measured with an L ion-selective electrode using a saturated calomel electrode as a reference. Live separate 1.00-mL additions of a 100.0-ppm solution of L were added to each, measuring the potential following each addition. 

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. 

Measuring electrodes for impressed current protection are robust reference electrodes (see Section 3.2 and Table 3-1) which are permanently exposed to seawater and remain unpolarized when a small control current is taken. The otherwise usual silver-silver chloride and calomel reference electrodes are used only for checking (see Section 16.7). All reference electrodes with electrolytes and diaphragms are unsuitable as long-term electrodes for potential-controlled rectifiers. Only metal-medium electrodes which have a sufficiently constant potential can be considered as measuring electrodes. The silver-silver chloride electrode has a potential that depends on the chloride content of the water [see Eq. (2-29)]. This potential deviation can usually be tolerated [3]. The most reliable electrodes are those of pure zinc [3]. They have a constant rest potential, are slightly polarizable and in case of film formation can be regenerated by an anodic current pulse. They last at least 5 years. 

Potential control with zinc reference electrodes presented a problem because deposits of corrosion products are formed on zinc in hot water. This caused changes in the potential of the electrode which could not be tolerated. Other reference electrodes (e.g., calomel and Ag-AgCl reference electrodes) were not yet available for this application. Since then, Ag-AgCl electrodes have been developed which successfully operate at temperatures up to 100°C. The solution in the previous case was the imposition of a fixed current level after reaching stationary operating conditions [27]. 

The electroreductive cyclization of the furanone 118 (R = -(CH2)4CH=CH— COOMe Scheme 36) using a mercury pool cathode, a platinum anode, a saturated calomel reference electrode, and a degassed solution of dry CH3CN containing -Bu4NBr as the electrolyte, gave the spirocyclic lactones 119 and 120 in a ratio 1.0 1.1 (Scheme 37)(91T383). 

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