Potentials, reversible electrode

The Chlorine Electrode Process. Chlorine exists in various oxidation states [5-7] as shown in Table 4.1.3. Of these, the chloride ion and elemental chlorine couple 

FIGURE 4.1.1. Potential-pH diagram for the chlorine-water system at 25°C [6]. (Wifli permission from NACE and CEBELCOR). 

The standard potential of reaction (28) is available from a number of 

FIGURE 4.1.2. Distribution of the components of active chlorine in the brine as a function of pH at 25 C. 

These authors used low-pressure chlorine during the experiments to minimize the deviation of the CP activity, caused by the formation of trichloride ions  

In the solid state the atoms in a metal are closely packed and the well-defined electron energy levels that are found in single atoms are not present. There is a continuum of levels and the available electrons fill the states from the bottom upwards, the highest level being known as the Fermi level. Hence, the electrons in metals are relatively mobile and metals are good conductors of electricity. When a metal electrode is dipped into a solution of the corresponding ions it will equilibrate  

A metal dipping into a solution of its ions has an equilibrium such as 

Other electrodes involve gases in equilibrium with ions in solution, e.g. hydrogen and chlorine electrodes function through operation of the following equilibria 

These require the gas to be bubbled over the surface of some inert electrode material dipping into a solution of the ions of the gas. Surface adsorbed gas molecules then enter into equilibrium with ions in solution and cause the electrode to adopt a potential characteristic of the position of equilibrium. For the hydrogen electrode it is seen that the oxidized form is in solution, while for the chlorine electrode the oxidized form is adsorbed at the surface. In a further type of electrode (redox electrode) both oxidized and reduced forms occur in solution, electrons being donated or accepted by an inert 

Each of the electrode systems described above constitutes what is known as a half-ceir and it is necessary to couple two such half-cells to form a complete electrochemical cell. When all the equilibrium components of a half-cell are in their standard states of unit activity, the electrode is said to be Standard Electrode and to adopt its Standard Potential. 

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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 ... 

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")... 

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 ... 

If the rates of the electrode reactions are large and the system is fairly close to equilibrium (the electrode potential is quite close to the reversible electrode potential), then the right-hand sides of Eqs (5.4.3) and (5.4.4) correspond to the difference between two large numbers whose absolute values are much larger than those of the left-hand sides. The left-hand side can then be set approximately equal to zero, kcc0x — A acRed 0, and in view of Eqs (5.2.14) and (5.2.17),... 

If A is placed in an open circuit electrochemical cell containing no A+ ions, A will donate electrons to the metal electrode, forming A+ ions. Rather quickly, the electrode will be unable to accept additional electrons and the system will reach equilibrium. This equilibrium potential is a reversible electrode potential. If the electrode potential is made more positive, the electrode will again be able to accept electrons and additional A+ will be produced. Conversely, if the electrode is made more negative, A+ will accept electrons from the electrode. The rate at which A is oxidized is proportional to the current density, i (typically in units of A/cm2), by the relation... 

Example 5.1. Calculate the reversible electrode potential for Cu immersed in a CUSO4 solution having concentrations of 0.01, 0.001, and 0.0001 mol/L at 25°C, neglecting ion-ion interaction (using concentrations instead of activities). The standard electrode potential for Cu/Cu electrode is 0.337 V. 

Example 5.3. Calculate the reversible electrode potential for a Cu electrode immersed in a CUSO4 aqueous solution with concentrations 1.0, 0.01, and 0.001 mol/L... 

In a mixed copper-zinc solution of complex cyanide, however, the Cu ion concentration can be reduced to the order of lO mol/L and the concentration ratio (zinc ion)/(copper ion) will be made very large. A detailed calculation for this case is given by Faust in the 1974 edition of Modem Electroplating (1). It is shown there, and in detail below, that the copper cyanide complex is Cu(CN)3 , for which the dissociation value is known. The dissociation constant for the zinc cyanide complex, Zn(CN)4 , is also well known. Using those values that determine the fraction concentration of the free metal ion in solution and assuming an initial specific molar concentration, it is shown below that their respective reversible electrode potentials [see also Eq. (11.1)] can be brought together. 

According to the Buder-Volmer equation, the dependence of q on 7 is linear in a range of few millivolts around the reversible electrode potential, whereas it becomes logarithmic at q> 50-100 mV away from equilibrium conditions, depending on the degree of reversibility of the specific electrode reaction ... 

Except for the Fea+-Fe2+ couple at concentrations greater than about 10-r>M and perhaps the Mn(1V)-Mn2+ couple (3) the over-all redox systems important in natural waters are not electroactive. No reversible electrode potentials are established for the NCV-NCV-NH., S042"-H2S, or CH4-... 

Reaction (13.42) has a reversible electrode potential under standard conditions of 0.4033 V. This potential was calculated from standard Gibbs energy data, and its value indicates that iron can be used as the reducing agent. 

The two cathodic partner reactions in corrosion are hydrogen evolution and oxygen reduction. Consider the Electrochemical Series (for a full list, see The Handbook of Chemistry and Physics) and work out a rule that gives the standard reversible electrode potential, less negative than which (pH 7 and aMzt = lO 6 M) a metal will no longer have a tendency to corrode (a) in 1 M acid and (b) in 1 M alkali. (Bockris)... 

It follows from equation (VI-7) and (VI-8) that the reversible electrode potential in an arbitrary state can be calculated, when its standard potential (e° or 7T°) and the activity of each component taking part in the reaction at the electrode is known. 

Theoretical decomposition voltage oan also be calculated from the reversible electrode potentials. Because the potential difference E mu, equals, according... 

Butler-Volmer equation — The Butler-Volmer or -> Erdey-Gruz-Volmer or Butler-Erdey-Gruz-Volmer equation is the fundamental equation of -> electrode kinetics that describes the exponential relationship between the -> current density and the -> electrode potential. Based on this model the -> equilibrium electrode potential (or the reversible electrode potential) can also be interpreted. 

Taking the polarographic half-wave potential (Ex 2 - - 1.45 volts, versus standard caloriel electrode hence ECil 2 - - 1.18 volts) in l.OAf CN as the measure of the reversible electrode potential (64). 

There are thus at least two electrochemical reactions occurring at a corroding electrode. The potential of such an electrode will lie between the reversible electrode potentials of the two electrochemical reactions at a point where the magnitude of the cathodic current of one is equal to the anodic current of the other. 

The fundamental causative factor for the dissolution of metals and the mechanism of the process are identical to those pertinent to the establishment of an electrode potential. We may begin consideration of die dissolution process with a discussion of the thermodynamically reversible electrode potential, Eeq, of a metal, M, and proceed to show that the dissolution reaction is a departure from the equilibrium conditions. With this approach we can appreciate the role played by the electrode potential in corrosion and gain an insight into the true nature of the process. 

The experimentally measured reversible electrode potential, E q, includes not only the above emf but also the potential difference at the metal-platinum contact. The electrons are the electromotively active particles at this junction, and it may be assumed that at equilibrium an electrical potential difference exists between the two metals which equalizes the electrochemical potential of the electrons in the two phases. As is well known, it is equivalent to the Volta potential difference and is given by the following ... 

The single thermodynamically reversible electrode potential as measured against the standard hydrogen electrode is equal to the emf of the cell,... 

It should be noted that in the case where only electrons are exchanged with the metal the value of the reversible electrode potential, Eeq, is independent of the electrode material. The value of the exchange current, however, depends on the metal in question. Although this kind of a reversible potential can only be measured on an inert electrode, in principle the potential and the exchange current may be established on any electron conductor. 

The transition of electrons across the metal-solution interface will actually occur whenever there is an electron acceptor in the solution with energy levels of appropriate value. This transition will be true, for example, when the reversible electrode potential for the oxidizing system Eeq is more noble than Egq. hi this case electrons will make transitions predominately from the metal to the solution at a rate given by... 

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