Hydrogen electrode, potential measurement

Fig. 4. Catalytic activities of metals (as potentials measured at 10-4 A.cm-2) for anodic oxidation of different reductants. Er thermodynamic oxidation-reduction potentials of reductants. H2 reversible hydrogen electrode potential in solution used to study oxidation of each reductant. Adapted from ref. 38.

The basicity of a number of reagents 1, 5 and related compounds in THF solutions was determined voltammetrically by Chevrot and coworkers . Using platinized platinum and hydrogen electrodes, they measured the cathodic-anodic current as a function of potential for overall electrode reactions (equations 43a and 43b)... 

Thus, a more appropriate question to ask is Is it possible to measure the absolute potential of the hydrogen reaction, /H+(abs) Actually it is possible. Remembering the definition of a standard hydrogen electrode potential (see Section 6.3.4), this was defined as the potential obtained when a metal comes in contact with a solution containing H+ under thermodynamically reversible conditions at unit activity, and H2 at 1 atm, at 298 K. As to the identity of the metal base, it can in principle be any metal at which it is possible to observe the reaction H2 H+ + e taking place at equilibrium. In practice, the metals used as substrates can only be noble metals because most other metals enter into equilibria with their own species in solution. Usually platinum is the metal chosen. 

In electrochemistry, it is usual to measure potentials with respect to a stable and reproducible system, known as - reference electrode. For the vast majority of practical electrochemical problems there is no need to determine - absolute potentials. However, this is necessary in cases where one wants to connect the relative electrode potential with the absolute physical quantities of the system, like electronic energies, as is the case of the work function. It is possible to convert all relative values of electrode potential to absolute-scale values and to electronic energies. For aqueous systems the - standard hydrogen electrode potential corresponds to -4.44 V in the physical scale taking electrons at rest in vacuum as reference and the absolute potential is given by the relation T(abs) = T(SHE) + 4.44 [vii]. 

The standard electrode potential measures the relative force tending to drive the half-reaction from a state in which the reactants and products are at unit activity to a state in which the reactants and products are at their equilibrium activities relative to the standard hydrogen electrode. 

Plots of = f(log [SeOj ])pn and E = f(pH)se(iv) at 293 K were linear and had the slopes 14.5 and 87.5 mV, respectively. These values are close to the Nemstian slopes required by Reaction (A.36) and the authors concluded that the half-cell was reversible. The potential, recalculated to the standard hydrogen electrode scale, measured at [SeOj ] = 1 M and [OH ] = 1 M was - 0.364 V and assigned as the standard potential of the redox-couple in alkaline solution. The authors noted this value Is close to the value in [56LAT], but they did obviously not know that this datum is in error by about 0.03V. 

The extent of reducing ion species present can be collectively measured as the Eh. Oxidation involves the loss of electrons, while the process of reduction can be viewed as the gaining of electrons. In order to maintain electrical neutrality overall, it is clear that the oxidation of one species must be accompanied by the reduction of another somewhere in the system hence, the concept of redox - the simultaneous occurrence of reduction and oxidation. In practice, a chemically inert electrode such as platinum or gold is used, which can transfer electrons to or from the environment. The potential developed is measured relative to a standard cell which itself is calibrated relative to the standard hydrogen electrode. Although measurement of Eh can be problematic, relatively stable measurements can be obtained using permanently installed electrodes of platinum or gold whereas spot measurements should be avoided due to unstable and... 

Figure 6.48 Comparison of electrode potentials (measured vs. standard hydrogen electrode) to electron energies for various redox couples (MV methylviologen, CBM conduction band minimum, VBM valence band maximum Elsevier 2004).

At the heart of electrochemistry is the electrochemical cell. We will consider the creation of an electrochemical cell from the joining of two half-cells. When an electrical conductor such as a metal strip is immersed in a suitable ionic solution, such as a solution of its own ions, a potential difference (voltage) is created between the conductor and the solution. This system constitutes a half-cell or electrode (Fig. 15.1). The metal strip in the solution is called an electrode and the ionic solution is called an electrolyte. We use the term electrode to mean both the solid electrical conductor in a half-cell (e.g., the metal strip) and the complete half-cell in many cases, for example, the standard hydrogen electrode, the calomel electrode. Each half-cell has its own characteristic potential difference or electrode potential. The electrode potential measures the ability of the half-cell to do work, or the driving force for the half-cell reaction. The reaction between the metal strip and the ionic solution can be represented as... 

Since all electrode potential measurements are relative, it is convenient to have an "ultimate reference point". By convention, the electrode potential of the hydrogen-hydrogen ion electrochemical reaction, i.e.. 

The normal hydrogen electrode (NHE) is the primary reference electrode and is used to define the accepted scale of standard potentials in aqueous media. It is also one of the most reproducible electrodes that are available. The hydrogen electrode has been successfully employed in dipolar aprotic solvents however, it is not frequently used. The aqueous saturated calomel electrode (SCE), connected to the electrolyte under study by a non-aqueous salt bridge, has become the reference electrode of choice for most investigators. Whether it is the SCE that is used, or any other suitable reference electrode for a given solvent, junction potentials will exist between the reference electrode and the electrolyte under study. These junction potentials will affect electrode potential measurements and will vary from one solvent/electrolyte system to another. In addition, the instability of the SCE in non-aqueous solvents has been noted. ... 

The hydrogen electrode is used as a reference for electrode potential measurements. Theoretically, it is the most important electrode for use in aqueous solutions. The reversible hydrogen electrode in a solution of hydrogen ions at unit activity exhibits a potential, which is assumed to be zero at all temperatures. 

Standard electrode potentials measured with respect to hydrogen are only of theoretical interest. They are relevant only to pure metals, not to alloys, and do not take into account... 

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