Reversible potentials hydrogen electrode

Fig. 3. Hypothetical Evans diagram and polarization curve for a metal corroding in an acidic solution, where point A represents the current density, /q, for the hydrogen electrode at equiUbrium point B, the exchange current density at the reversible or equiUbrium potential, for M + 2e and point...

Metals in practice are usually coated with an oxide film that affects the potential, and metals such as Sb, Bi, As, W and Te behave as reversible A//A/,Oy/OH electrodes whose potentials are pH dependent electrodes of this type may be used to determine the solution s pH in the same way as the reversible hydrogen electrode. According to Ives and Janz these electrodes may be regarded as a particular case of electrodes of the second kind, since the oxygen in the metal oxide participates in the self-ionisation of water. 

Reference Electrode an equilibrium (reversible) electrochemical half-cell of reproducible potential against which an unknown electrode potential can be measured. Examples of those commonly used in corrosion are the Pt, H /H (the hydrogen electrode), Hg/Hg Clj/Cl" (the calomel electrode), Cu/CuS04/Cu, Ag/AgCl/Cl", all with fixed activities of the dissolved ions. 

According to Sato et al.,6,9 the barrier-layer thickness is about 1.5 to 1.8 nm V-1, and increases to 3 nm around the oxygen-evolution potential. In Fig. 5, the scale of the electrode potential, Vrhe, is that of the reversible hydrogen electrode (RHE) in the same solution. The electrode potentials extrapolated from the linear plots of the potentials against the film thickness suggested that the potential corresponding to the barrier thickness equal to zero is almost equal to 0.0 V on the RHE scale, independent of the pH of the solution, and approximately agrees with the equilibrium potential for the oxide film formation of Fe or Fe. Therefore it is concluded that the anodic overpotential AE applied from the equilibrium potential to form the oxide film is almost entirely loaded with the barrier portion. 

For many electrodes it is fonnd that one H+ or OH ion is involved in the reaction per electron hence, the electrode potential becomes 0.059 V more negative when the pH is raised by 1 unit this is the same potential shift as found for the hydrogen electrode. For such electrodes a special scale of electrode potentials is occasionally employed These potentials, designated as E refer to the potential of a reversible hydrogen electrode (RHE) in the same solution (i.e., at the given pH). For the electrodes of the type considered, potentials in this scale are independent of solution pH. 

It is not a trivial point that 0fj vs. E curves are practically linear. In a reversible system the electrode potential can be linked to the activities (concentrations) of the potential-determining substances. In the system being discussed, this substance is atomic hydrogen. According to the Nemst equation we have E = const - (RTIF) X In Cjj. It follows that the degree of coverage, 0, is linearly related to the logarithm of concentration c in the solution ... 

The standard electrode potential of reaction (15.20) calculated thermodynamically is 1.229 V (SHE) at 25°C. For reachons (15.21) and (15.22), these values are 0.682 and 1.776 V, respechvely. The equihbrium potenhals of all these reactions have the same pH dependence as the potential of the reversible hydrogen electrode therefore, on the scale of potentials (against the RHE), these equilibrium potenhals are... 

Some metals are thermodynamically unstable in aqueous solutions because their equilibrium potential is more negative than the potential of the reversible hydrogen electrode in the same solution. At such electrodes, anodic metal dissolution and cathodic hydrogen evolution can occur as coupled reactions, and their open-circuit potential (OCP) will be more positive than the equilibrium potential (see Section 13.7). 

OHads formation has a clear voltammetric signature on a number of surfaces, including the (lll)-oriented surfaces of platinum group metals, Pt(lll) in alkaline and acid electrolytes of non-adsorbing anions [Markovic and Ross, 2002], and Au(lll), Au(lOO), and Ag(lll) in neutral and alkaline electrolytes [Savinova et al., 2002]. On these surfaces, the reaction has a reversible character. Anderson and co-workers calculated the reversible potential of Reaction (9.1) on Pt to be 0.62 V with respect to a reversible hydrogen electrode (RHE) [Anderson, 2002]. The Pt(lll)-OH bond energy has been estimated to be about 1.4 eV in an alkaline electrolyte [Markovic and Ross, 2002]. 

A standard rotating disk electrode (RDE) setup with a gas-tight Pyrex cell was used for the experiment on CO adsorption and the HOR. A Pt wire was used as counterelectrode. A reversible hydrogen electrode, RHE(t), kept at the same temperature as that of the cell (t, in °C), was used as the reference. All the electrode potentials in this chapter will be referenced to RHE(f). The electrolyte solution of 0.1 M HCIO4... 

An important eonelusion was that the best catalyst is not the alloyed one as expected, nor the mixture of Pt/XC 72 and Ru/XC 72 powders, but one eonsisting of a dispersion of Pt colloid and Ru colloid on the same carbon support, i.e., the Pt + Ru/XC 72 eatalyst. The latter leads to higher current densities for the eleetro-oxidation of methanol than the other catalysts with the same atomic ratio for potentials lower than 0.5 V versus a reversible hydrogen electrode (RHE) (Fig. 11.3). This result... 

If two zinc electrodes are set up in opposition to one another as in Figure 6.12 (A), the difference of potential between them, measured by a potentiometer or voltmeter, is zero. If an infinitesimally small external emf is applied to the electrodes so that A is positive and B is negative, a very small current flows round the circuit, and Zn atoms pass from A into solution as Zn2+ ions, and Zn2+ ions leave the solution and are deposited as Zn atoms on B. If the small emf is reversed so that B is positive and A is negative, the current flows in the opposite direction, and zinc is dissolved from B and deposited on A. An electrode such as the zinc electrode, which reacts thus to an infinitesimal applied emf, is known as a reversible electrode. The hydrogen electrode described earlier is a reversible electrode. If two molar hydrogen electrodes are set up in opposition to one another, Figure 6.12 (B), the... 

Considerable practical importance attaches to the fact that the data in Table 6.11 refer to electrode potentials which are thermodynamically reversible. There are electrode processes which are highly irreversible so that the order of ionic displacement indicated by the electromotive series becomes distorted. One condition under which this situation arises is when the dissolving metal passes into the solution as a complex anion, which dissociates to a very small extent and maintains a very low concentration of metallic cations in the solution. This mechanism explains why copper metal dissolves in potassium cyanide solution with the evolution of hydrogen. The copper in the solution is present almost entirely as cuprocyanide anions [Cu(CN)4]3, the dissociation of which by the process... 

Two things to notice are that fc, will depend on the reference electrode chosen and secondly E° is the equilibrium potential not for the standard hydrogen electrode but rather for the electrochemical equilibrium between H+ and H ds. This can be seen by explicitly writing out the equation corresponding to the reverse reaction ... 

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.

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