Reference electrode solution

The commercial SCE depicted in Fig. 18a.4 is generally an H-cell. One arm contains mercury covered by a layer of mercury(II) chloride (calomel). This is in contact with a saturated solution of potassium chloride a porous frit is used for the junction between the reference electrode solution and the sample solution at the end of the other arm. Similar to the silver/silver chloride reference system, a calomel electrode also warrants precautionary measures to maintain the chloride concentration in the reference electrode. 

In the cell used for measuring the electrode potential, in which the two electrodes are immersed in a single phase of electrolyte solution, the outer potential, tps, ofthe test electrode-solution is equal to the outer potential, ips, of the reference electrode-solution as shown in Fig. 4—24. The difference in the Fermi level of electrons, CFtu)- between the test electrode M and the reference electrode M , then, is represented by the difference in the real potential of electrons, M/aw) - .(M0/ V). tuid hence by the difference in the electrode potential (absolute electrode potential), AE = E-E°, between the two electrodes. This difference also equals the difference in the work function, 4>no/3/v - 4>ji/s/v> between the two electrodes. Thus, the potential E of the test electrode relative to the reference electrode is the difference in the electrode potential (absolute electrode potential) between the two electrodes as indicated in Eqn. 4-35 . 

Ideal potentiometric measurements, especially in analytical chemistry, would require that the potential of the reference electrode be fixed and known, and that the composition of the studied solution affect only the potential of the indicator electrode. This would occur only if the liquid-junction potential could be completely neglected. In practice this situation can be attained only if the whole system contains an indifferent electrolyte in a much larger concentration than that of the other electrolytes, so that the concentration of a particular component in the analysed solution, which is not present in the reference electrode solution, has only a negligible effect on the liquid-junction potential Such a situation rarely occurs, so that it is necessary to know or at least fix the liquid junction potential... 

Another less precise but frequently used method employs a liquid bridge between the analysed solution and the reference electrode solution. This bridge is usually filled with a saturated or 3.5 m KCl solution. If the reference electrode is a saturated calomel electrode, no further liquid bridge is necessary. Use of this bridge is based on the fact that the mobilities of potassium and chloride ions are about the same so that, as follows from the Henderson equation, the liquid-junction potential with a dilute solution on the other side has a very low value. Only when the saturated KCl solution is in contact with a very concentrated electrolyte solution with very different cation and anion mobilities does the liquid junction potential attain larger values [2] for the liquid junction 3.5 M KCl II1 M NaOH, A0z, = 10.5 mV. 

Figure 4.17 — (A) Exploded view of a tubular flow-cell integrated microconduit system. I Ag/AgCl inner reference electrode M sensitive membrane S internal reference solution. (B) Detail of the integrated microconduit shown within the dotted lines in C. (C) Integrated-microconduit FI manifold for potentiometric measurements C carrier stream R reference electrode solution P pump V injection valve I indicator electrode R reference electrode I pulse inhibitor G ground W waste. (Reproduced from [140] with permission of Pergamon Press).

The determination of absolute or ideal energies of activation is handicapped by the fact that the variation of the cell temperature introduces undertainties either at the reference electrode—solution interface, when both working and reference electrodes are at the same temperature, or from the thermal liquid junction potential in a non-isothermal measurement [5,36]. 

The working electrode-solution interface is that corresponding to the electrode process under study and the reference electrode-solution interface is needed to close the current flow through the cell. The term IRcen denotes the Ohmic drop, with / ceii being the resistance of the cell which can be calculated for certain geometries (see below) although, when important enough, it is usually measured and... 

Reference electrode Reference electrode solution Potential mV vs SHE Reference... 

Reference electrode potentials change with temperature. Both electrochemical reactions (Nernstian thermodynamics) and chemical solubilities, e.g. of the inner reference electrode solution, are affected. Accordingly, the temperature coefficient, dE/dT (mV °C4), varies from one type of reference electrode to another. To minimise errors in potential readings the coefficient should be low and at least known. Examples of temperature coefficients are given in Table 2.2. 

In a cell with two different electrolyte solutions in contact, as in the application of a reference electrode, there is an additional source of potential difference across the interface of the two miscible electrolytes, inner reference electrode solution test solution. As noted above, the vertical dashed line is used in a cell scheme to denote such an interface and indicates the source of so-called liquid-junction potential, A< )l. For example. 

In 1977 Koryta et al. [61] reported the Avalues of several common ions at the NB/W interface, which were calculated from the extraction data using an extra thermodynamic assumption. Afterward, a newly developed electrochemical technique (so-called ion-transfer voltammetry) with a polarizable O/W interface was employed to determine AGtr° w for a variety of ions [33,62-71]. In Table 4 the reliable values of AG ° >w are compiled. Regarding the ions whose AGj r° w values are available for both electrochemical and extraction measurements, the electrochemical data, which seem to be more accurate, have been chosen preferentially. For several ions, somewhat different AGf, 0 w values from electrochemical measurements have been reported, as also seen in the database provided by Girault on a website [72], In this study, however, we have carefully chosen reliable values for the respective ions, which were determined under well-defined conditions (reference electrodes, solution compositions, etc.). 

Figure 2, Cyclic voltammogram of TcO/ on bare ITO electrode, 5.0 x 1(T M Tc04 in pH 7 phosphate buffer as supporting electrolyte, 25 mV/s scan rate, Ag/AgCl reference electrode, solution deoxygenated for 30 min prior to scan.

Electrode requirements The calomel reference electrode should be of the sleeve tip rather than the fiber or ceramic plug type and care should be taken with the replacement of the reference electrode solution. The fluoride ion-selective electrode consists of a europium-doped lanthanum fluoride crystal. The emf response of the electrode to standard solutions should not be less than 55 mV over the range of concentrations of 0.2-200 mg dm Values of the emf response lower than this can arise from poor matching of the sensor and reference electrodes, deterioration of the sensor electrode membrane, poor maintenance of the reference electrode, or poor maintenance of the sensor electrode. The analysis should be carried out in plastic vessels. 

Interface (Junction) in an electrochemical cell, it represents the location where two distinct phases come in contact with each other solid-liquid (electrode-solution), two liquids of different concentrations and/or compositions (reference electrode-solution), etc. Nernstian a reversible redox process that follows equilibria equations. 

Fig. 12.6 Combination pH probe, (a) Conventional design, (b) Pressurized reference electrode solution. (Courtesy Hach Co.)...

FIGURE 10.1 Cyclic voltammetric response i-E) for (A) Au and (B) Pt rotating disc electrode (RDE). Conditions Rotation speed, 900 rpm scan rate, 200 mV/s Ag/AgCl reference electrode. Solutions (—) 0.1 M NaOH, deaerated (.) 0.1 M NaOH. 

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