Standard Single Electrode Reduction Potentials

Measurement of Standard Single Electrode Reduction Potentials 

FIGURE 4.1.6. Electric circuit for measurement of electrode potentials. A Voltmeter reads +0.337 V at 25°C. Copper electrode is positive with respect to SHE B Voltmeter reads —0.76 V at 25°C. Zinc electrode is negative with respect to SHE. 

The lUPAC has adopted the following conventions [2] for determining the electrode potentials. 

The cell implicit in the measurement of a standard electrode potential should be arranged so that the SHE is on the left (Fig. 4.1.6). 

The measured potential difference across such a cell gives the magnitude of the standard electrode potential. 

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The single electrode reduction potentials are measured with respect to the SHE. The SHE, which is based on the reaction, 2H+ + 2e -o- H2 on platinized platinum, is assigned the reduction potential of zero when the activity of the hydrogen ion is unity and the pressure of H2 is 1 atm. Figure 4.1.6 illustrates the schematic for measuring the standard single reduction potential of Cu /Cu and Zn /Zn systems. 

The half-cell reduction potential of the standard hydrogen electrode (SHE) was set arbitrarily to 0.000... V by international agreement. Since it is impossible to determine the potential of a single half-cell without comparing it to another, an arbitrary standard was established. 

We call S R and S electrode potentials and S R and SJR standard electrode potentials or standard reduction potentials. Notwithstanding these names, these quantities really are not the potentials of single electrodes, but rather the measured potentials of cells XII and XIII. These measured potentials contain... 

Every oxidation must be accompanied by a reduction (i.e., the electrons must have somewhere to go). So it is impossible to determine experimentally the potential of any single electrode. We therefore establish an arbitrary standard. The conventional reference electrode is the standard hydrogen electrode (SHE). This electrode contains a piece of metal electrolytically coated with a grainy black surface of inert platinum metal, immersed in a 1 MH+ solution. Hydrogen, H2, is bubbled at 1 atm pressure through a glass envelope over the platinized electrode (Figure 21-8). 

Tabulated values of standard reduction potentials, E°, refer to single electrodes. For example, for the half-cell reaction 7.6, the value of °cT+/cu = +0.34 V. However, it is impossible to measure the potential of an individual electrode and the universal practice is to express all such potentials relative to that of the standard hydrogen electrode. The latter consists of a platinum wire immersed in a solution of ions at a concentration of Imoldm (strictly, unit activity) in equilibrium with H2 at 1 bar pressure (equation 7.12). This electrode is taken to have a standard reduction potential E° = QY at all temperatures. 

The electrochemical reduction potential of a single electrode against a standard hydrogen electrode RT... 

By galvanic contact corrosion, base metals will often form anodes and decompose by oxidation while more noble metals function as cathodes. The electrochemical series specifies the reduction potential V (volt) for single electrodes in this series, base anodic metals have a low reduction potential and noble cathodic metals have a high reduction potential. The following thermodynamic standard values are given for G gg (kJ/mol)... 

Electrocatalysis is manifested when it is found that the electrochemical rate constant, for an electrode process, standardized with respect to some reference potential (often the thermodynamic reversible potential for the same process) depends on the chemical nature of the electrode metal, the physical state of the electrode surface, the crystal orientation of single-crystal surfaces, or, for example, alloying effects. Also, the reaction mechanism and selectivity 4) may be found to be dependent on the above factors in special cases, for a given reactant, even the reaction pathway [4), for instance, in electrochemical reduction of ketones or alkyl halides, or electrochemical oxidation of aliphatic acids (the Kolbe and Hofer-Moest reactions), may depend on those factors. 

Why does coulometry not require external calibration standard solutions Explain why a silver electrode can be an indicator electrode for chloride ion. What are the three processes by which an analyte in solution is transported to an electrode surface What single transport process is desired in polarography Explain how the other transport processes are minimized in polarography. Would you expect the half-wave potential for the reduction of copper ion to copper metal to be different at a Hg electrode from that at a platinum electrode Explain your answer. 

Table 7.2 Standard electrode potentials for the single- and multi-electron reduction of carbon dioxide (in an aqueous solution at pH 7) [126]...

Shortcomings of the choice of the equilibrium state as the electrical reference point in the evaluation of the temperature effect on the rate of electrode reactions, and consequently of the overpotential as an experimental substitute for A(A0) in the WE-RE cell at various temperatures, have been discussed in the previous section. Hence, another reference point should be sought. From a theoretical point of view, the choice is unambiguous—it is the zero point on the relative electrode potential scale, defined by the SHE convention. Basically, this is also an equilibrium state, but of a single reaction selected by convention, namely, the reduction of two hydrogen ions to molecular hydrogen. The value of A0 at the interface when this reaction is held at equilibrium, assuming all species involved are in standard thermodynamic states, is fixed by the SHE convention as zero. The same convention associates additional properties with this reference state temperature, solvent, and solute Independence. Formally, the properties of the SHE satisfy the principal theoretical requirements for the electrical reference point in the evaluation of the effect of temperature on the rate of electrode reactions. 

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