Emf series

Electromotive force series based on experimentally determined potentials or on potentials calculated from appropriate free energies have been developed for several molten solvents. Plambeck in 1966 critically reviewed and tabulated values for the LiCl-KCl eutectic (450°C), equimolar NaCl-KCl (700-900°C), NaF-KF eutectic (850°C), MgCla-NaCl-KCl eutectic (475 C), AlCla-NaCl-KCl (66-20-14 mole %) (218 C), Li2S04-K2SO4 eutectic (625°C), and Li2S04-Na2S04-K2S04 eutectic (575°C). By far the most extensive series is that for the LiCl-KCl eutectic, developed 

It should be noted that formulas such as Cd + and HgJ+ are only intended to specify the oxidation states and the number of atoms per unit. No implications should be drawn regarding coordination between central ion and surrounding ions. 

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The reversible or equilibrium potentials given in the EMF series of metals may have little significance in assessing which metal in a couple will have an enhanced corrosion rate and which will be protected. 

Table 6.11 lists, to the right of the arrows, reducing agents or disposition to electron loss or disposition to oxidation in order of increasing strength. Such a list is more popularly called the electromotive force, or emf, series. The maximum potential difference which can be measured for a given cell is called the electromotive force (abbreviated emf) and represented by the symbol Ecell. It may be recounted that the emf values reported in Table 6.11 are for those cells under specified standard conditions in which all the concentrations are 1 M and pressures are 1 atm. The emf of such a cell is said to be its standard electromotive force, and is given by the symbol E ell. 

The standard emf series based on hydrogen is obviously not applicable to molten salt electrolysis systems. No emf series similar to that for aqueous systems has been established for molten electrolytes this is due to the nonavailability of accepted standard electrodes and the use of numerous molten electrolytes involving widely differing tamperers, consequent to the widely varying melting temperatures of the salts used. In spite of these, many emf series have been compiled, using a variety of molten salts with different stand-... 

As is evident from Eq. (11.4), copper and zinc are very far apart in the standard EMF series, so alloy codeposition seems next to impossible. Fortunately, the difference can be eliminated (even reversed) by changing the values of the activities. This can be achieved by inducing a considerable change in ionic concentrations via complex ion formation, as discussed in detail below. 

There is no fundamental difference between the two half-cells or electrodes in a cell for measuring emf (electromotive force), especially in molten salts. However, it is usual to designate one of the electrodes as reference electrode if it is used for the measurement of an emf series. In many cases a diaphragm is used to separate the two half-cells. 

Two emf series [91,92] are given in Table 5, measured in the cells M/ MjClp LiCl, and KCl/AgCl, LiCl-KCl/Ag. 

Each electrochemical reaction has its own reversible potential, just as each element has its own melting temperature. A list of these reversible potentials under standard conditions is called an electromotive (emf) series. 

The standard reversible potential is that listed in the EMF series of Table 1 and represents a special case of the Nernst equation in which the second term is zero. The influence of the solution composition manifests itself through the logarithmic term. The ratio of activities of the products and reactants influences the potential above which the reaction is thermodynamically favorted toward oxidation (and conversely, below which reduction is favored). By convention, all solids are considered to be at unit activity. Activities of gases are equal to their fugacity (or less strictly, their partial pressure). 

Factors Involved in Galvanic Corrosion. Emf series and practical nobility of metals and metalloids. The emf. series is a list of half-cell potentials proportional to the free energy changes of the corresponding reversible half-cell reactions for standard state of unit activity with respect to the standard hydrogen electrode (SHE). This is also known as Nernst scale of solution potentials since it allows to classification of the metals in order of nobility according to the value of the equilibrium potential of their reaction of dissolution in the standard state (1 g ion/1). This thermodynamic nobility can differ from practical nobility due to the formation of a passive layer and electrochemical kinetics. 

Table 6.2 Emf series of some elements and classification of metals and metalloids in order of nobility23...

Reactions in liquid HF are known that also illustrate amphoteric behavior, solvolysis, or complex formation. Although HF is waterlike, it is not easy, because of the reactivity, to establish an emf series, but a partial one is known. 

Table IB Abbreviated EMF Series on the Normal Hydrogen Electrode Scale and on the Modified Normal Hydrogen Electrode Scale...

Table 2.1 Standard aqueous half-cell potentials at 25 °C (also known as standard electrode, redox, or oxidation potentials, and as the standard emf series)(a)...

By combining many pairs of half-cells into voltaic cells, we can create a list of reduction half-reactions and arrange them in decreasing order of standard electrode potential (from most positive to most negative). Such a list, called an emf series or a table of standard electrode potentials, appears in Appendix D, with a few examples in Table 21.2 on the next page. 

Describe how standard electrode potentials (Ehair-ceii values) are combined to give Ecen and how the standard reference electrode is used to find an unknown haif-ceiii explain how the reactivity of a metal is related to its Ehaif-ceih write spontaneous redox reactions using an emf series like that in Appendix D ( 21.3) (SPs 21.3,21.4) (EPs 21.24-21.40)... 

The emf series of the standard half-ceU electrode potential on the hydrogen scale are given in Table 2.2. The reactions in this table are written as reduction reactions from left to right at T=25 °C. They have the same polarity as the reduction potential, which is measured experimentally. 

Comparing Galvanic couples of tin-platinmn and tin-gold illustrates the exchange current density effect on the overall corrosion rate. The reversible potential of Au Au in the emf series is -F1.50 V vs. SHE, more positive than Pt Pt (-F1.20 V vs. SHE). 

It follows from Fig. 5.5 that a transfer from the passive state (2) to the active state (3) implies a potential decrease. The greater the difference between the real corrosion resistance of the material based upon passivation (indicated by the rest potential in the passive state) and its thermodynamic nobleness (expressed by the EMF series in Table 3.1), the larger the potential decrease following the transfer from passive to active state. That is, this potential decrease indicates to what extent the real corrosion resistance of the material depends on passivity. Table 5.1 shows examples of such a potential decrease for several metal-environment combinations. The activation is in these cases caused by continuous grinding of the metal surfaces, i.e. the materials are kept active in spite of an otherwise spontaneous passivation tendency for many of the material-environment combinations represented in flie table. 

The EMF series can be thought of as a chart that indicates how noble a metal is to oxidation. It can be noted that the lower the ranking of a metal in this EMF series, the higher its tendency to oxidize. Consider a galvanic cell consisting of two metals Mj and M2, each immersed in the solution of its own ions. Supposing that metal Mj is ranked lower in the EMF series than metal M2, we can write the individual half-cell reactions as follows. For simplicity, we consider both M, and M2 as having the same valence, equal to n . 

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