Hydrogen scale

Solution Potential. The standard electrode potential of aluminum (A1 + 3e) is —1.66 V on the standard hydrogen scale and —1.99 V... 

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... 

Meta Standard electrode potential, hydrogen scale (V) Corrosion potential in flowing sea-water, hydrogen scale (V)... 

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 strength of acids and bases is measured on a pH (potential of hydrogen) scale pH = -logio [H+]... 

The standard Galvani potential difference can be related to the standard electrode potentials and oxi/rediCo) oii hydrogen scale in water or organic... 

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. 

The scale of electrochemical work functions makes it possible to calculate the outer potential difference between a solution and any electrode provided the respective reaction is in equilibrium. A knowledge of this difference is often important in the design of electrochemical systems, for example, for electrochemical solar cells. However, in most situations one needs only relative energies and potentials, and the conventional hydrogen scale suffices. 

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. 

Electrode potentials are measured by coupling electrodes in a cell, the second electrode of which is a certain constant one. The one that is chosen is usually a hydrogen electrode, as described previously.21 It is then the potentials of such cells that are called relative electrode potentials or potentials of electrodes on the standard hydrogen scale. What is meant is that by an arbitrary convention, these particular cell potentials are no longer called cell potentials but relative electrode potentials, and, indeed, the word relative is often dropped because those in the know realize what is meant... 

However, at least two other reference electrodes, calomel (Hg. 7.42) and silver silver chloride electrodes, are in common use as secondary reference electrodes (they are easier to set up than die hydrogen reference electrode). Potentials of electrodes measured using one of die secondary reference electrodes can be directly converted to values on die hydrogen scale, if die potential of die secondary reference electrode with respect to the hydrogen electrode is known (see also Section 7.5.73). 

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. 7.15. When the cell is so constructed that the activity of cupric ions is unity, the voltmeter should show 0.34 V at 25 °C, which is then the standard potential of copper on the standard hydrogen scale.

Once this standard electrode potential is known by means of experimental measurements, the Nemst equation permits a calculation of what the electrode potential will be when the solution has any other cIq + titan the value of unity used in the standard potential. For example, the electrode potential of a copper electrode on the standard hydrogen scale immersed in aCu2+ solution of Oq =2 X 10-2 would be (Fig. 7.16)... 

The dissolution reaction is Pt - Pt2+ + 2e and the value of its reversible thermodynamic potential is 1.2 V on the normal hydrogen scale. The evolution of O2 in acid solution at a current density of, say, 100 mA cm, needs an overpotential on platinum of nearly 1.0 V, i.e., the electrode potential would be >2.0 V. It follows feat at these very anodic potentials platinum would tend to dissolve, although its dissolution would be slowed down by fee fact feat it forms an oxide film at fee potentials concerned. Nevertheless, fee facts stated show feat fee alleged stability of Pt may be more limited than is often thought. This is an important practical conclusion because dissolved Pt from an anode may deposit on fee cathode of fee cell, and instead of having fee surface one started wife as fee cathode, it becomes in fact what is on its surface, platinum. 

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. 

In the linear sweep technique, a recording of the current during the potential sweep (say, from 0.0 V on the normal hydrogen scale to 1.2 V positive to it in a 1 M H2 S04 solution) completes one act of the basic experiment. However, and hence the title of this part of the chapter, the electronics can be programmed so that when the electrode potential reaches 1.20 V, it begins a return sweep, going from 1.2 to 0.00 V, NHS. Completion of the two sweeps and back to the starting point is one act in what is called cyclic voltammetry.16 The current is displayed on a cathode ray oscilloscope screen on an X Y recorder, and it is normal to cany out not one but several and often many cycles. Much information is sometimes contained in the difference between the second and other sweeps in comparison with the first (Fig. 8.10). 

Knowledge of the numerical value of the entity represented by Eq. (9.1) allows one to make up cells that give the potential of an electrode "on the absolute scale, just as the Celsius scale was later shown to be expressed on the absolute or Kelvin scale of temperatures, in which there is a rationally based zero at -273 °C. Thus, to find the absolute value, VM>abs of an electrode potential expressed on the standard hydrogen scale, one writes... 

In addition, other processes may occur such as the reaction of the hydroperoxide ion with the conductor to form metal-oxygen bonds which in turn may be reduced. The hydroperoxide ion may itself decompose to reform oxygen, etc. The potential of an oxygen electrode is invariably a mixed potential with a value of about 1.0 V on the standard hydrogen scale, at zero current drain. 

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)... 

HYDROGEN SCALE. I, A thermometric scale. I See also Temperature Settles and Standards, i... 

Uranium has die four oxidation states, (III), (IV), (V), and (VI) the ions in aqueous solution are usually represented as LI +, U4+, U02+, and U02i+. The oxidation-reduction scheme, on tlie hydrogen scale (in... 

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... 

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