The hydrogen electrode

The hydrogen electrode is considered the primary standard to which other electrodes are compared. Unlike those of many other electrodes, the characteristics of hydrogen electrodes exhibit a high degree of reproducibility. 

The basic electrode is an oxidation-reduction electrode operating under equilibrium conditions between electrons in a noble metal, hydrogen ions in solution, and dissolved molecular hydrogen. The activity of dissolved hydrogen, an+, is taken as the independent variable and is fixed by maintain- 

By universal convention, is set equal to zero, which establishes the hydrogen electrode as the standard reference. The expected exchange equilibrium for the hydrogen electrode 

Various configurations have been developed for hydrogen electrodes and these are described in the literature (Ives and Janz, 1961). Platinum, gold, and palladium as well as other metals have been proposed for the metallic element of the electrode. Frequently platinized platinum is the preferred substance. The metal substrate may be wire, mesh, or foil. Preparation of the platinized electrode proceeds somewhat differently from the method outlined in Chapter 2. The platinum surface is usually cleaned in a solution of the following composition (Bates, 1954)  

The definition of the cell potential requires that we label one electrode as the right-hand and the other as the left-hand electrode. The cell potential is defined as in Eq. (17.16) by 

It is customary, but not necessary, to place the more positive electrode in the right-hand position. As we pointed out, this cell potential is always measurable as a difference in potential between two wires (for example, Pt) having the same composition. The measurement also establishes which electrode is positive relative to the other in our example, the copper is positive relative to the zinc. It does not, however, yield any hint as to the absolute value of the potential of either electrode. It is useful to establish an arbitrary zero of potential we do this by assigning the value zero to the potential of the hydrogen electrode in its standard state. 

The hydrogen electrode is illustrated in Fig. 17.2. Purified hydrogen gas is passed over a platinum electrode which is in contact with an acid solution. At the electrode surface the equilibrium 

The Gibbs energy of the electrons in any metal is measured relative to that in the staridard hydrogen electrode. The assignment in Eq. (17.31) yields the conventional zero of Gibbs energy for ions in aqueous solution. Using Eq. (17.31) in Eq. (17.29), we obtain 

Standard Gibbs energies of other ions in aqueous solution are measured relative to that of the ion, which has a standard Gibbs energy equal to zero. 

When used as a standard electrode, the hydrogen electrode operates in a solution containing hydrogen ions at constant (unit) activity based usually on hydrochloric acid, and the hydrogen gas must be at 1 atmosphere pressure the effect of change in gas pressure is discussed in Ref. 28. 

Exact potentiometric determination of pH using the hydrogen electrode is not as easy as it might at first appear. In principle it could be coupled with a suitable reference electrode to form the cell 

However, even if the liquid junction potential is eliminated it is required to know Eref, since the value of pH obtained will be strongly influenced by its value. The value of E ef is calculated assuming that ionic activity coefficients are determined only by the total ionic strength and not by the individual chemical properties of the species. 

All that one can hope to do by such a method is to determine pH values which are consistent with those calculated from thermodynamic constants using Equation (6.38). Thus, it is necessary to reassess the values of potential adopted by reference electrodes used for this purpose and also to take appropriate account of liquid junction potentials. 

If the pH value determined by using Equation (6.54) is to identify with that used in Equation (6.38), to give the thermodynamic dissociation constant of an acid, then the following equality is necessary 

Experimentally, the hydrogen electrode is immersed in a series of solutions containing weak acids and their salts, the true dissociation constants of the acids being known. Connection of the chosen reference electrode to this solution is achieved by means of a saturated potassium chloride salt bridge. When the left-hand side of Equation (6.56) is plotted as a function of ionic strength, a reassessed value of E fgf is obtained as intercept on the n axis. No account has been taken of the liquid junction potential which exists between the salt bridge and the electrolyte solution and whose value varies with both the nature of the buffer used and its concentration. Such variations only produce uncertainties of the order of tenths of millivolts in the value of obtained and the scale of pH values thus obtained constitutes the conventional pH scale. 

Before discussing the energy diagrams for electrochemical surface reactions, we introduce a very useful concept from electrochemistry, the normal hydrogen electrode (NHE) and the related RHE. The convention is to use the equilibrium between protons and electrons at a given potential with gas-phase to define a reference potential so that at f/=0 V with respect to the NHE, the following reaction is in equiUbrium at standard conditions (p= bar, 7 =298.15 K, pH=0)  

The RHE defines the equilibrium potential of R11.4 to be 0 at any pH. It differs from the NHE scale by the free energy difference between pH=0 and any pH  

The hydrogen electrode provides a direct link between the free energies in gas-phase adsorption and those relevant in electrochemistry. 

4 ADSORPTION EQUILIBRIA AT THE ELECTRIFIED SURFACE-ELECTROLYTE INTERFACE 

FIGURE 11.7 Experimental (open circles/dashed line) and calculated (full line) coverage of OH versus potential (vs. RHE) for water splitting over Pt(l 11) (light gray) and PtjNi (black). Experimental data are from Stamenkovic et al. (2007), and the theoretical data are from Rossmeisl et al. (2008a). Adapted from Rossmeisl et al. (2008a). 

---

Since it is not possible to measure a single electrode potential, one electrode system must be taken as a standard and all others measured relative to it. By international agreement the hydrogen electrode has been chosen as the reference ... 

For many purposes the hydrogen electrode is not convenient and it can be replaced by another cell of known standard electrode potential. A well-known example is the calomel cell shown in Figure 4.5. 

A particular concentration measure of acidity of aqueous solutions is pH which usually is regarded as the common logarithm of the reciprocal of the hydrogen-ion concentration (see Hydrogen-ION activity). More precisely, the potential difference of the hydrogen electrode in normal acid and in normal alkah solution (—0.828 V at 25°C) is divided into 14 equal parts or pH units each pH unit is 0.0591 V. Operationally, pH is defined by pH = pH(soln) + E/K, where E is the emf of the cell ... 

The packaging approach utilized for tliis battery is similar to that for nickel—hydrogen single cylindrical cells as shown in Figure 23. The sdv er electrode is typically the sintered type used in rechargeable sdv er—zinc cells. The hydrogen electrode is a Teflon-bonded platinum black gas difhision 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...

The hydrogen electrode consists of an electrode of platinum foil (approximately 1 X 1 X 0-002 cm) welded to a platinum wire which is fused into a glass tube. In order to increase its catalytic activity it is platinised by making it cathodic in a solution of chloroplatinic acid (2% chloroplatinic acid in 2 N HCl) frequently lead acetate is added to the solution (0-02%) and this appears to facilitate the deposition of an even and very finely divided layer... 

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. 

Electrodes and Galvanic Cells. The Silver-Silver Chloride Electrode. The Hydrogen Electrode. Half-cells Containing an Amalgam, Electrode. Two Cells Placed Back to Back. Cells Containing Equimolal Solutions. The Alkali Chlorides as Solutes. HC1 in Methanol or Ethanol Containing a Trace of Water. The Alkali Chlorides in Methanol-Water Mixtures. The Heal of Solution of HC1. Proton Transfer Equilibrium from Measurements of E.M.F. 

The Alkali Chlorides as Solutes. In order to make a similar study of the transference of KC1, NaCl, and LiCl between water and methanol-water mixtures, the hydrogen electrode was replaced by an amalgam electrode, as described in Sec. 111. The arrangement when two cells having potassium amalgam electrodes are placed back to back may be written... 

The pH at the equivalence point is thus approximately 3.7 the secondary ionisation and the loss of carbonic acid, due to any escape of carbon dioxide, have been neglected. Suitable indicators are therefore methyl yellow, methyl orange, Congo red, and bromophenol blue. The experimental titration curve, determined with the hydrogen electrode, for 100 mL of 0.1 M sodium carbonate and 0.1M hydrochloric acid is shown in Fig. 10.7. 

Electrode potentials are customarily tabulated on the standard hydrogen electrode (SHE) scale (although the SHE is never actually used experimentally because it is inconvenient in many respects). Therefore, conversion of potentials into the UHV scale requires the determination of E°(H+/H2) vs. UHV. According to the concepts developed above, such a potential would measure the energy of electrons in the Pt wire of the hydrogen electrode, modified by the contact with the solution. 

Studies of pzc in mixed solvents were also carried out by Blaszczyk etal n using the dipping method. They worked in mixtures offormamide and NMF and estimated the shift of the standard potential of the hydrogen electrode, of the surface dipole potential atHg, and of the liquid junction potential. 

Elving, P. J. Enyo, M. Critical Observations on the Measurement of Adsorption at Electrodes Mechanism of the Hydrogen Electrode Reaction as Studied by Means of Deuterium as a Tracer 7... 

If the concentrations in the cell are such that it is reported as having a positive emf (that is, the mercury/mercury(I) chloride electrode is found to be positive), then the reaction as written is spontaneous. If the concentrations were such that the emf were reported as negative (that is, the hydrogen electrode were found to be positive), then the reverse of the reaction that we have derived would be spontaneous. 

A problem with compiling a list of standard potentials is that we know only the overall emf of the cell, not the contribution of a single electrode. A voltmeter placed between the two electrodes of a galvanic cell measures the difference of their potentials, not the individual values. To provide numerical values for individual standard potentials, we arbitrarily set the standard potential of one particular electrode, the hydrogen electrode, equal to zero at all temperatures ... 

---