Electrodes reversibility

Figure Bl.28.4. Cyclic voltaimnogram for a simple reversible electrode reaction in a solution containing only oxidized species.

The effects of ultrasound-enlianced mass transport have been investigated by several authors [73, 74, 75 and 76]. Empirically, it was found that, in the presence of ultrasound, the limiting current for a simple reversible electrode reaction exhibits quasi-steady-state characteristics with intensities considerably higher in magnitude compared to the peak current of the response obtained under silent conditions. The current density can be... 

The overpotential is defined as the difference between the actual potential of an electrode at a given current density and the reversible electrode potential for the reaction. 

Turning now to the acidic situation, a report on the electrochemical behaviour of platinum exposed to 0-1m sodium bicarbonate containing oxygen up to 3970 kPa and at temperatures of 162 and 238°C is available. Anodic and cathodic polarisation curves and Tafel slopes are presented whilst limiting current densities, exchange current densities and reversible electrode potentials are tabulated. In weak acid and neutral solutions containing chloride ions, the passivity of platinum is always associated with the presence of adsorbed oxygen or oxide layer on the surface In concentrated hydrochloric acid solutions, the possible retardation of dissolution is more likely because of an adsorbed layer of atomic chlorine ... 

The standard electrode potential of M /Mcan be evaluated accurately and conveniently providing it forms a reversible electrode and can be... 

Steady-state potential comparable with Type 1 reversible electrode Metal in a solution of electrolyte in which ions are produced by a corrosion reaction in an VAf exchange that determines the potential. Zn in NaCI solution Zn in dilute HCI... 

Steady-state potential comparable with Type 2 reversible electrode Potentials of electropositive metals that react with solution to give sparingly soluble salts of the metal. Cu or Ag in NaCl or Ag in HCI giving an M/MX/X type of electrode... 

Steady-state potential comparable with Types 4 and 5 reversible electrodes Potential of metal depends on pH of solution, although the dependence is confined to a limited range of pH and does not conform precisely to the Nernst equation. Ni in H2SO4 (Ni/Hj, H + ) Cu in NaOH (Cu/CujO/OH")... 

An ideal reversible cell is characterised by an e.m.f. that remains constant irrespective of the rate of reaction in either direction, i.e. each interface constituting the cell must be so completely non-polarisable that it resists any attempt to change its potential. Although this is impossible to achieve in practice, a number of interfaces approximate to ideality providing the rate of reaction is maintained at a very low value. These reversible electrodes (or half-cells) are used as reference electrodes for determining the potential of a single electrified interface. 

Thus a reversible electrode of the type A//A/0/H must conform with the thermodynamic equilibria... 

Reversible Electrode an electrode of high exchange current density. 

It must be emphasised that standard electrode potential values relate to an equilibrium condition between the metal electrode and the solution. Potentials determined under, or calculated for, such conditions are often referred to as reversible electrode potentials , and it must be remembered that the Nernst equation is only strictly applicable under such conditions. 

Overpotential. It has been found by experiment that the decomposition voltage of an electrolyte varies with the nature of the electrodes employed for the electrolysis and is, in many instances, higher than that calculated from the difference of the reversible electrode potentials. The excess voltage over the calculated back e.m.f. is termed the overpotential. Overpotential may occur at the anode as well as at the cathode. The decomposition voltage ED is therefore ... 

In practice, for a ternary system, the decomposition voltage of the solid electrolyte may be readily measured with the help of a galvanic cell which makes use of the solid electrolyte under investigation and the adjacent equilibrium phase in the phase diagram as an electrode. A convenient technique is the formation of these phases electrochemically by decomposition of the electrolyte. The sample is polarized between a reversible electrode and an inert electrode such as Pt or Mo in the case of a lithium ion conductor, in the same direction as in polarization experiments. The... 

The detection of the AC component allows one to separate the contributions of the faradaic and charging currents. The former is phase shifted 45° relative to the applied sinusoidal potential, while the background component is 90° out of phase. The charging current is thus rejected using a phase-sensitive lock-in amplifier (able to separate the in-phase and out-of-phase current components). As a result, reversible electrode reactions yield a detection limit around 5 x 10 7m. 

Thus, the Volta potential may be operationally defined as the compensating voltage of the cell of Scheme 16. However, it should be stressed that the compensating voltage of a voltaic cell is not always the direct measure of the Volta potential. The appropriate mutual arrangement of phases, as well as application of reversible electrodes or salt bridges in the systems, allows measurement of not only the Volta potential but also the surface and the Galvani potentials. These possibilities are schematically illustrated by [15]... 

Cyclic voltammograms of the [Fe(CN)6] /Fe[(CN)g] redox couple with the bare and the DNA-modified electrodes are shown in Fig. 5 [14a]. The peak currents due to the reversible electrode reaction of the redox system on the bare Au electrode were significantly suppressed by the treatment with DNA. In contrast, the treatment with unmodified, native DNA made no suppression, and that with HEDS caused only a slight one, as seen in Fig. 

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... 

It is relevant to follow up the description given above on reversible electrodes with attention focused on the potential difference of a cell. The potential difference measured under reversible conditions, is called the electromotive force, or emf, of the cell, E. It is clear that if a cell is reversible then it is implied that the half-cells of which it is composed are also reversible. 

Before treating specific faradaic electroanalytical techniques in detail, we shall consider the theory of electrolysis more generally and along two different lines, viz., (a) a pragmatic, quasi-static treatment, based on the establishment of reversible electrode processes, which thermodynamically find expression in the Nernst equation, and (b) a kinetic, more dynamic treatment, starting from passage of a current, so that both reversible and non-reversible processes are taken into account. 

As ksh in this instance is very small, then according to the Butler-Volmer formulation (eqn. 3.5) the reaction rate of the forward reaction, K — 8,he "F(E 0)/flr, even at E = E°, is also very low. Hence Etppl. must be appreciably more negative to reach the half-wave situation than for a reversible electrode process. Therefore, in the case of irreversibility, the polarographic curve is not only shifted to a more negative potential, but also the value of its slope is considerably less than in the case of reversibility (see Fig. 3.21). In... 

So, corresponding to id for a reversible electrode reaction, Jp is a linear function of concentration the greater sensitivity of the latter permits determinations down to 10 1M(instead of 10 6 Mfor id). Apart from this advantage, the second derivative curve, by means of the difference between its maximum and minimum as a function of concentration, offers an even better check on reaction reversibility32 than the straight-line plot of E (according to eqn. 3.49) against log(tcd - i)/i (see also p. 120), especially because Ip, as a property at the halfwave potential, is more sensitive to the occurrence of irreversibility (cf., pp. 124-127). 

The above considerations concern a reversible electrodic process, ox + ne red as instead of 20-100 hz in the sinusoidal technique a fixed frequency of 225 Hz is normally used in the square-wave mode, the chance of irreversibility in the latter becomes greater, which then appears as asymmetry of the bellshaped I curve. Such a phenomenon may occur more especially when the complete i versus E curve is recorded on a single drop, a technique which has appeared useful51 in cases of sufficient reversibility. 

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