Calomel electrode standard potential

Voltammetry. In a voltammetric sensor the concentration of an ion that is to be measured causes a change in the potential and thereby gets recorded as a function of output voltage. Various standard electrodes are used to generate the initial or zero potential, such as calomel electrodes, standard silver chloride electrodes, and others. If a dropping mercury electrode is used, the method is termed polarography. 

V. 19-8. In a calomel electrode, reduction potential depends on the chloride potential. Therefore, the standard reduction potential for a calomel electrode has a different chloride concentration from the saturated calomel electrode. 19-9. Dry cells and lead-acid cells "run down" as the concentrations of reactants and products eventually reach their equilibrium values, where A,G and Eceii both become 0. This does not happen in a fuel cell because fuel is continuously added. 19-10. Both AI and Zn can be used because they are more active than Fe ... 

The following data were collected for the analysis of fluoride in tap water and in toothpaste, (a) For the analysis of tap water, three 25.0-mL samples were each mixed with 25.0 mL of TISAB, and the potential was measured with an F ISE relative to a saturated calomel electrode. Five 1.00-mL additions of a standard solution of 100.0-ppm F were added to each, measuring the potential following each addition. 

The electrolysis is carried out at a reference potential of -2.4 volts vs a standard calomel electrode. An initial current density of 0.0403 amp/cm is obtained which drops to 0.0195 amp/cm at the end of the reduction, which is carried on over a period of 1,682 minutes at 15° to 20°C. The catholyte is filtered, the solid material is washed with water and dried. 430 g of the 2,3-bis-(3-pyridyl)-butane-2,3-diol is recrystallized from water, MP 244° to 245°C. 

In the common method of electro-gravimetric analysis, a potential slightly in excess of the decomposition potential of the electrolyte under investigation is applied, and the electrolysis allowed to proceed without further attention, except perhaps occasionally to increase the applied potential to keep the current at approximately the same value. This procedure, termed constant-current electrolysis, is (as explained in Section 12.4) of limited value for the separation of mixtures of metallic ions. The separation of the components of a mixture where the decomposition potentials are not widely separated may be effected by the application of controlled cathode potential electrolysis. An auxiliary standard electrode (which may be a saturated calomel electrode with the tip of the salt bridge very close to the cathode or working electrode) is inserted in the... 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

All potentials refer to a standard hydrogen electrode in the used solvent. Exemptions are stated explicitly. Potential reported versus a saturated calomel electrode, converted assuming iiscE=-0-241 V vs. SHE. 

The standard Volta potential of the calomel electroded (Hg.Cl ) was determined for the first time by Klein and Lange, but the accepted and usually used value of this potential (equal to 0.207 V) was found by Randles.48 Later measurements by Parrel and McTigue et al.21 and Antropov et al. gave 0.212 V and 0.220 V, respectively. In the methanol a value equal to 0.25 has been found. 

Expression (8) can be used to calculate the real energy of ions in any solvent, provided the standard potential of the calomel electrode, and the standard potentials of the elements and the A ° P° (Hg, Cl") under study... 

In the calculation of H the standard Volta potential of the calomel electrode found by Randles has been used (see Section V). The other estimators of (abs), for instance, -4.73 V by Corner andTryson seem to be doubtful. A detailed discussion of this topic may be found in the papers by Trasatti. ... 

FIG. 3 Temperature dependence of the standard potential of the saturated calomel electrode (1) and Ag/AgCl electrode (2). 

A method has been developed for differentiating hexavalent from trivalent chromium [33]. The metal is electrodeposited with mercury on pyrolytic graphite-coated tubular furnaces in the temperature range 1000-3000 °C, using a flow-through assembly. Both the hexa- and trivalent forms are deposited as the metal at pH 4.7 and a potential at -1.8 V against the standard calomel electrode, while at pH 4.7, but at -0.3 V, the hexavalent form is selectively reduced to the trivalent form and accumulated by adsorption. This method was applied to the analysis of chromium species in samples of different salinity, in conjunction with atomic absorption spectrophotometry. The limit of detection was 0.05 xg/l chromium and relative standard deviation from replicate measurements of 0.4 xg chromium (VI) was 13%. Matrix interference was largely overcome in this procedure. 

Electron mediators successfully used with oxidases include 2,6-dichlorophenolindophol, hexacyanoferrate-(III), tetrathiafulvalene, tetracyano-p-quinodimethane, various quinones and ferrocene derivatices. From Marcus theory it is evident that for long-range electron transfer the reorganization energies of the redox compound have to be low. Additionally, the redox potential of the mediator should be about 0 to 100 mV vs. standard calomel electrode (SCE) for a flavoprotein (formal potential of glucose oxidase is about -450 mV vs SCE) in order to attain rapid vectrial electron transfer from the active site of the enzyme to the oxidized form of the redox species. 

Additionally, other reference electrodes are used which are easier to maintain at standard conditions. These include the silver/silver chloride electrode and the saturated calomel electrode (SCE). The voltage difference between the working electrode and the reference electrode is proportional to the electrochemical potential difference between them. This is written... 

The reaction is considered to proceed via a silyl anion mechanism, although the possibility of a radical-based mechanism has also been discussed.115,125 In order to clarify the mechanism, coupling experiments on a 1 1 mixture of chlorotrimethylsilane, 27 (reduction potential <—3.0 V),126 and chlorotriphenylsilane, 28 (reduction potential vs. standard calomel electrode (SCE) < —3.0 V),120 were performed, in which the mixed coupling product 1,1,1-trimethyl-2,2,2-triphenyldisilane, 29, and the homocoupling product hexaphenyldisilane, 30, only, were found,125 as indicated in Scheme 15. 

Examples (a) Titration of Mg2+ with 8-hydroxyquinoline. In this particular instance, a diffusion current for 8-hydroxyquinoline is normally achieved at - 1.6 V Vs Standard Calomel Electrode (SCE), whereas Mg2+ ion is more or less inert at this potential. 

Amino acids enhance the oxidation peak of Cu(0) obtained with a carbon paste electrode incorporating Cu(II) cyclohexylbutyrate. The increased current is proportional to the amino acid concentration at trace levels in the pM range373. The behavior of such electrodes was investigated for cysteine (115). On scanning potentials in the positive direction, the amino acid is accumulated on the electrode as the Cu(I) complex at +0.90 V vs a standard calomel electrode (SCE), in acetate buffer at pH 4.5 linear range is 2 x 10 9 to 1 x 10-7 M, 1 min accumulation, RSD 3% (n = 5)374,375. 

This type of counter electrode is defined as a reference electrode. As we will see in Chapter 3, Section 1.2, at 25° C the saturated calomel electrode (SCE) has a potential of +0.2415 V with respect to the standard hydrogen electrode (NHE), which, although difficult to use, is the internationally accepted standard for the potential scale, having conventionally E° — 0.000 V. 

Figure 3.5 A grid which allows the interconversion of potentials between the standard calomel electrode (SCE) and the standard hydrogen electrode (SHE). In addition, the standard electrode potentials of two couples, i.e. Zn " ", Zn and Fe +, Fe, are shown for comparative purposes.

The cell emf comprises two half-cell potentials. One of these half cells will be a standard reference, such as a saturated calomel electrode (SCE), while the other will be an inert electrode such as platinum or gold. ... 

In these relationships, og and qc are, respectively, the standard potentials of the glass and calomel electrodes, is the Faraday constant, and Aci is the activity of solvated Cl". 

The standard potential of the 02/02 pair is equal to -0.15 V in water and -0.60 V in DMF. Usually, dioxygen easily captures two electrons in the stepwise reaction O2 + e —> O2 , then O2 + e 02 . In DMSO, dioxygen reductions into the superoxide ion and then into the dioxygen dianion are characterized by Ey2 = -0.5 V and Ey = -1.5 V in regard to the saturated calomel electrode (Sawyer and Gibian 1979). The superoxide ion occupies an intermediate position in the following redox triad O2 —> 02 —> In accordance with such a position, the superoxide ion... 

Potentials of some reference electrodes relative to either the standard hydrogen electrode or the saturated calomel electrode. Further data in ref. [17]. 

Fig. 4.1 Skeleton structure of a photoelectrochemical cell (PEC) comprised of a photoanode and cathode. Potentials of both are measured with reference to a third electrode, the standard calomel electrode.

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