Electrode potentials Nernst

Electrode Potential Nernst Equation and the Electrochemical Cell 441 Tableau 8.5. Redox Equilibria of Cl Species... 

Electrode Potential Nernst Equation and the Electrochemical Cell 447... 

The redox (electrode) potential for ion-ion redox systems at any concentration and temperature is given by the Nernst equation in the form... 

Activity Versus Concentration In describing metallic and membrane indicator electrodes, the Nernst equation relates the measured cell potential to the concentration of analyte. In writing the Nernst equation, we often ignore an important detail—the... 

It should be noted that the simple Nernst equation cannot be used since the standard electrode potential is markedly temperature dependent. By means of irreversible thermodynamics equations have been computed to calculate these potentials and are in good agreement with experimentally determined results. 

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. 

As shown in Section 2.28, when a metal M is immersed in a solution containing its own ions M"+, then an electrode potential is established, the value of which is given by the Nernst equation ... 

In the Nernst equation the term RT/nF involves known constants, and introducing the factor for converting natural logarithms to logarithms to base 10, the term has a value at a temperature of 25 °C of 0.0591 V when n is equal to 1. Hence, for an ion M+, a ten-fold change in ionic activity will alter the electrode potential by about 60 millivolts, whilst for an ion M2 +, a similar change in activity will alter the electrode potential by approximately 30 millivolts, and it follows that to achieve an accuracy of 1 per cent in the value determined for the ionic concentration by direct potentiometry, the electrode potential must be capable of measurement to within 0.26 mV for the ion M+, and to within 0.13 mV for the ion M2 +. ... 

The Nernst equation shows that the glass electrode potential for a given pH value will be dependent upon the temperature of the solution. A pH meter, therefore, includes a biasing control so that the scale of the meter can be adjusted to correspond to the temperature of the solution under test. This may take the form of a manual control, calibrated in 0 C, and which is set to the temperature of the solution as determined with an ordinary mercury thermometer. In some instruments, arrangements are made for automatic temperature compensation by inserting a temperature probe (a resistance thermometer) into the solution, and the output from this is fed into the pH meter circuit. 

Electrical units 503, 519 Electrification due to wiping 77 Electro-analysis see Electrolysis and Electrogravimetry Electrochemical series 63 Electro-deposition completeness of, 507 Electrode potentials 60 change of during titration, 360 Nernst equation of, 60 reversible, 63 standard 60, (T) 62 Electrode reactions 505 Electrodeless discharge lamps 790 Electrodes antimony, 555 auxiliary, 538, 545 bimetallic, 575... 

In the case of a solution with a previously known aH+ (see below), we could determine 2°H+-.H2(iatm)> provided that a reference electrode of zero potential is available however, experiments, especially with the capillary electrometer of Lippmann, did not yield the required confirmation about the realization of such a zero reference electrode16. Later attempts to determine a single electrode potential on the basis of a thermodynamic treatment also were not successful17. For this reason, the original and most practical proposal by Nernst of assigning to the standard 1 atm hydrogen potential a value of zero at any temperature has been adopted. Thus, for F2H+ H2(iatm) we can write... 

The terms EAgCl/AgjCi- and h+/h2 are designated as the electrode potentials. These are related to the standard electrode potentials and to the activities of the components of the system by the Nernst equations. By a convention for the standard Gibbs energies of formation, those related to the elements at standard conditions are equal to zero. According to a further convention, cf. Eq. (3.1.56),... 

The equilibrium electrode potential is given by the Nernst equation (cf. 3.2.17),... 

The membrane potential expressed by Eqs (6.1.3) and (6.1.4) is termed the Nernst membrane potential as it originates from the analogous ideas as the Nernst equation of the electrode potential (p. 165) and the equation of the Nernst potential at ITIES (Eq. (3.3.50)). 

The Nernst equation is used to calculate electrode potentials or cell potentials when the concentrations and partial pressures are other than standard state values. The Nernst equation using both base 10 and natural logarithms is given by ... 

Activity Dependence of Electrode Potentials - The Nernst Equation... 

Electrode and therefore cell potentials are very important analytically as their magnitudes are determined by the activities of the reactants and products involved in the electrode reactions. The relation between such activities and the electrode potential is given by the Nernst equation. For a general half-cell reaction written as a reduction, i.e. aA + bB +. .. ne = xX + yY +. . ., the equation is of the form... 

Since the absolute and the conventional electrode potentials differ only by an additive constant, the absolute potential depends on the concentration of the reactants through the familiar Nernst s equation. This dependence is implicitly contained in Eq. (2.6) the real Gibbs energies of solvation contain an entropic term, which depends on the concentration of the species in the solution. 

Equation 13 0.059, [ox] E — Eq + log n [red] This is the well known Nernst equation. Eo is the potential of the electrode when [ox] = [red]. This potential E0 is called the standard electrode potential (sep) which is a characteristic for a particular redox couple. Table 2-1 gives the sep of a number of redox couples. 

Figure 7.11 Nernst graph of the electrode potential Ag+.Ag as V against ln([Ag+]/moldm 3) as V. A value of Ag+ Ag = 0.8 V is obtained as the intercept on the y-axis...

We consider again the redox reaction Ox + ze = Red with a solution initially containing only the oxidized form Ox. The electrode is initially subjected to an electrode potential Ei where no reaction takes place. For the sake of simplicity, it is assumed that the diffusion coefficients of species Ox and Red are equal, i.e., D = D0x = DKeA. Now, the potential E is linearly increased or decreased with E(t) = Ei vt (v is a potential scan rate, and signs + and represent anodic scan and cathodic scan, respectively.) Under the assumption that the redox couple is reversible, the surface concentrations of Ox and Red, i.e., cs0x and 4ed, respectively, are always determined by the electrode potential through the Nernst equation... 

ET much faster than transport (transport control). Electrochemical equilibrium is attained at the electrode surface at all times and defined by the electrode potential E. The concentrations Cox and Cred of oxidized and reduced forms of the redox couple, respectively, follow the Nernst equation (1) (reversible ET)... 

Strategy. The procedure to adopt has two parts first, we will determine the electrode potential Epe +.Fe the emf. Secondly, the concentration will be calculated by using the Nernst equation (equation (3.8)). 

Notice how this form of the Nernst equation (equation 3.9) can be thought of as a linear equation of the form y = mx + c thus allowing calibration graphs to be drawn. Figure 3.6 represents such a calibration graph for copper ions in aqueous solution. Note the important conclusion that the electrode potential decreases (i.e. becomes more negative) as the activity decreases. 

It is a good idea, when using such simple Nernst plots as an analytical method of determining an activity, to check that the intercept at jc = 0 is indeed the standard electrode potential. (There are many compendia listed in the Bibliography at the end of this book that cite large numbers of values, as does Appendix 3.)... 

Dynamic electrochemistry is seen to alter the ratio of a(0) to a(R) for the redox couple at the surface of the working electrode (i.e. at the electrode solution interface). Note that this alteration occurs during electrolysis, such that the electrode potential Eq,r can shift according to the Nernst equation. 

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