The hydrogen scale

In the last section we considered variation of the potential of an electrode with respect to some reference electrode. It is impossible to determine the absolute potentials which electrodes adopt all that can be done is to measure the e.m.f. of the cell obtained by coupling the electrode in question with another electrode. If, however, this latter maintains an almost constant potential whatever the applied potential difference between the two electrodes, electrode potentials can be measured with respect to an arbitrarily chosen standard and given physical meaning. The standard is the hydrogen electrode which, with hydrogen gas at 1 atmosphere pressure, in contact with platinized platinum in a solution of hydrogen ions of unit activity is assigned a potential of zero at all temperatures. 

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The redox potential is determined with a probe consisting of a platinum electrode and a Hg/Hg2Cl2 Cl or Ag/Ag Cl—Cl reference electrode. If Ep is the potential of the platinum probe, Ej the potential of the reference electrode and tne redox potential of the soil (in mV on the hydrogen scale) then... 

Fig. 21.1 Practical galvanic series of metals and alloys showing potentials on the hydrogen scale. (Note that the potentials shown are typical values that will vary according to the nature of the solution.) (after Butler, G. and Ison, H. C. K., Corrosion and its Prevention in Water, Leonard Hill, London (1966))... 

It may be noted that the standard hydrogen electrode is rather difficult to manipulate. In practice, electrode potentials on the hydrogen scale are usually... 

Table of the amounts of licat required to warm 1 gram of water from (0 )° to (0 + j)° on the hydrogen scale. 

In these reactions, (2) is the process taking place at the reference electrode which therefore determines the potential scale. In practice other reference electrodes, such as the saturated calomel electrode are frequently used but the data are normally expressed on the hydrogen scale. 

The expression for the potential of electrodes of the second kind on the hydrogen scale can be derived from the affinity of the reaction occurring in a cell with a standard hydrogen electrode. For example, for the silver chloride electrode with the half-cell reaction... 

These quantities can be used to rearrange the equation for the EMF of a cell in non-aqueous medium into the form of a difference between the electrode potentials on the hydrogen scale for aqueous solutions with additional terms. 

Then, knowing F H2, it was relatively easy to determine values of electrode potentials for any other couple. With this methodology, they devised the standard electrode potentials ° scale (often called the E nought scale , or the hydrogen scale ). 

On the hydrogen scale, the range of values for standard reduction potentials varies from about + 3 0 V to —3.0 V for solutions in which the activity of the hydrated hydrogen ion is 1. It is also conventional to quote values for standard reduction potentials in basic solutions with a unit activity of hydrated hydroxide ion, Typical values of range between about +2.0 V to —3.0 V. 

When one refers to the relative electrode potentials of a number of systems on the hydrogen scale and then equates these with the potentials of the cells made up of the systems (the electrode-solution systems concerned) in combination with a hydrogen electrode, the situation is that one is arbitrarily taking the constant... 

Figure P6.4 shows the logarithmic dependence of the adsorption of chloride ions, log(T - r/r, on time, /(min). The adsorbing metal is aluminum, the concentration of chloride ions in solution is 1(T3 mol dm-3 at pH 12, and the potential in the hydrogen scale is -0.9 V. On the assumption that the adsorption is diffusion controlled, determine the standard free energy of adsorption of chloride ions on the surface concerned. 

Fig. 7.176. Since each electrode potential is referred to the hydrogen scale, the cell voltage, which is equal to the difference in the two electrode potentials, is given by the separation Ve.

At the outset, consider a self-driving, or energy-producing, cell with two interfaces 1 and 2, and let the equilibrium electrode potentials on the hydrogen scale be Ee j and Ee 2. Suppose Ee t is more positive than Ee Then, if an external load is provided, electrode 2 will taxi to be an electron sink for a net deelectronation reaction and electrode 1 will tend to be an electron source for a net electronation reaction. 

Berkelium is known to exist 111 aqueous solution in two oxidation states, the (III) and the (IV) states, and the ionic species presumably correspond to Bk+3 and Bk+4. The oxidation potential for the berkelium(lll)-berkehum(IV) couple is about —1.6 V on the hydrogen scale (hydrogen-hydrogen ion couple taken as zero)... 

In order to avoid confusion in reporting of electrochemical data, the Standard Hydrogen Electrode (SHE) has been selected as the primary reference electrode, which means that its standard potential has been arbitrarily set to zero, thus establishing the hydrogen scale of standard potentials. It is based on the equilibrium... 

To determine the potential of the SCE on the hydrogen scale, the same problems arise as discussed for the silver-silver chloride reference electrode, and E0 and (E° + Ej) are determined in the same manner. The recommended values31 are tabulated for the calomel electrode in Table 5.3. 

The mercury-mercurous sulfate electrode. Several commercial suppliers offer the mercury-mercurous sulfate electrode with a saturated potassium sulfate electrolyte. The potential (E° + E ) of this electrode system is 0.658 V on the hydrogen scale at 22°C.34 The electrode constitutes one-half of the Weston standard cell,35 an international secondary voltage standard, and is outstanding in reproducibility,36 in spite of the slight tendency of mercurous sulfate to hydrolyze and its rather high solubility. 

As stated in the previous chapter, to determine the reversible potential of any electrode in an arbitrary state, it is first of all necessary to know its standard potential. The required values of these potentials, stated in terms of the hydrogen scale and valid for a temperature of 25 °C, arc tabulated. Such data do not express the absolute potentials but the electromotive force of the combination of the given half coll and the standard hydrogen electrode. This fact must be remembered, when making calculations based on these potentials. 

On the hydrogen scale, according to the most reliable data, namely those of Chapprus, Travaux Mem. Bureau intemat. Poids et Mesures, 1904,13,40 and of Thiesen, Scheel, and Diesselhorst, IVt.ss. Abhandl Physiledlisch-Tech. Reichanstalt, 1900,3,68. Numerous other determinations have been made of the temperature of maximum density, the results ranging from 2 22° C (Dalton) to 4-08° G. (Kopp) The more important of these data are as follow ... 

Relative Heats of Solvation of Ions on the Hydrogen Scale... 

Arbitrary Potential Zero The Hydrogen Scale.—Since the single electrode potential [cf. equation (10)] involves the activity of an individual ionic species, it has no strict thermodynamic significance the use of such potentials is often convenient, however, and so the difficulty is overcome by defining an arbitrary zero of potential. The definition widely adopted, following on the original proposal by Nernst, is as follows ... 

According to this definition the standard potential of the hydrogen electrode is the arbitrary zero of potential [cf. equation (7a)] electrode potentials based on this zero are thus said to refer to the hydrogen scale. Such a potential is actually the e.m.f. of a cell obtained by combining the given electrode with a standard hydrogen electrode it has, consequently, a definite thermodynamic value. For example, the potential (E) on the hydrogen scale of the electrode M, M (aM+), which is reversible with respect to the 2-valent cations M, in a solution of activity aM is the E.M.F. of the cell... 

If Ezn,zn and Ecu,cu represent the standard electrode potentials on the hydrogen scale of the zinc and copper electrodes, as recorded in Table XLIX, then Eza.zn is actually the e.m.f. of the cell... 

Although equations (34) and (35) are exact, the qualitative conclu -sions drawn from them are not always strictly correct for example, siiiCv copper has a standard potential of — 0.340 on the hydrogen scale, i( would be expected, as is true in the majority of cases, that copper should be unable to displace hydrogen from solution. It must be recorded. 

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