Standard electrochemical series

The considered electrochemical model has brought us to certain conclusions on the mechanism of polymer polarization in the M1-P-M2 systems. It is evident that definite electrode potentials are established on metals faces in contact with the polymer material. These potential values are affected by the work function of the electrons and metal afEnity to the corresponding structural elements of the polymer lining. These potentials are noncoincident with a standard electrochemical series of metals. This is natural, since the properties of polymer materials as electrolytes are not identical to those of salt solutions of the corresponding metals, for which the standard electrode potentials of metals are determined. The studied metals in pairs separated by the PVB lining are arranged in the following series ... 

The electrochemical series published in the literature must be differentiated as either the standard electrochemical series or the different practical electrochemical series, since the latter apply only to a certain given medium. The standard electrochemical series lists only the potentials of the pure elements in the list measured under standard conditions in a solution of their own salts against the normal hydrogen electrode. 

The tendency of metals to corrode is expressed most simply in the standard electrochemical series of Nemst potentials, shown in Table 38.10. These potentials are obtained in electrochemical measurements in which one electrode is a standard hydrogen electrode formed by bubbling hydrogen through a layer of finely divided platinum black. The potential of this reference electrode is defined to be zero. Noble metals are those which have a potential higher than that of a standard hydrogen electrode base metals have lower potentials. 

Almost all common metals and structural steels are liable to corrode in seawater. Regulations have to be followed in the proper choice of materials [16], In addition, there is a greater risk of corrosion in mixed constructions consisting of different metals on account of the good conductivity of seawater. The electrochemical series in seawater (see Table 2-4), the surface area rule [Eq. (2-44)] and the geometrical arrangement of the structural components serve to assess the possibility of bimetallic corrosion (see Section 2.2.4.2 and Ref. 17). Moreover the polarization resistances have considerable influence [see Eq. (2-43)]. The standards on bimetallic corrosion provide a survey [16,17]. 

When metals are arranged in the order of their standard electrode potentials, the so-called electrochemical series of the metals is obtained. The greater the negative value of the potential, the greater is the tendency of the metal to pass into the ionic state. A metal will normally displace any other metal below it in the series from solutions of its salts. Thus magnesium, aluminium, zinc, or iron will displace copper from solutions of its salts lead will displace copper, mercury, or silver copper will displace silver. 

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... 

The question arises as to which metal is dissolved, and which one is deposited, when combined in an electrochemical cell. The electrochemical series indicates how easily a metal is oxidized or its ions are reduced, i.e., converted into positively charged ions or metal atoms respectively. The standard potential serves for the comparison of different metals. 

Figure 5. Electrochemical series of metals and their standard potentials in volt (measured against NHE).

Each metal or metal area will develop an electrode with a measurable electrical potential. This potential can be referenced to that of a standard hydrogen electrode, which by convention is set at zero. Thus, all metals have either a higher or lower potential compared to hydrogen, and a comparative list of metals can be produced indicating their relative nobility. This list is the galvanic or electrochemical series and measured as an electromotive force (EMF). 

For transmetallations with a metal (metallo-de-metallations, Scheme 10-95) arylmercury compounds are particularly suitable due to the position of mercury as a noble metal in the electrochemical series of standard potentials (for examples see Makarova, 1970). 

We can use the electrochemical series to predict the thermodynamic tendency for a reaction to take place under standard conditions. A cell reaction that is spontaneous under standard conditions (that is, has K > 1) has AG° < 0 and therefore the corresponding cell has E° > 0. The standard emf is positive when ER° > Et that is, when the standard potential for the reduction half-reaction is more positive than that for the oxidation half-reaction. 

The positive value of the standard voltage obtained in the example indicates that the cell reaction shown is spontaneous. Thus, the standard potentials in Table 6.11 can be used to predict whether a particular reaction will occur, or not. The advantage of Table 6.11 is that it provides quantitative as well as qualitative information. It not only conveys that nickel is a stronger oxidizing agent than silver (because nickel is positioned below silver in the electrochemical series), but it also conveys how much stronger, in terms of the cell emf of+1.05 V. 

The electrochemical series corresponds only to the standard condition, i.e., for unit activity of the ions, since a change to another ionic concentration can alter the order of the electrode potentials of the elements very markedly. The case of nickel plating mentioned earlier may be taken as typically illustrative of the many practical examples of the effects and the consequences of nonstandard conditions. It must also be mentioned in the context of the examples of displacement reactions provided earlier that the concentrations and the electrode potentials frequently vary during a displacement reaction. 

The redox potentials against the standard hydrogen electrode (SHE) for various reactions, usually called electrochemical series , are listed in the annually updated CRC Handbook of Chemistry and Physics, CRC Press Boca Raton, EL. On the other hand, unless otherwise indicated, the potential values used in this review are all referred to the Li+/Li reference electrode. 

From the values of standard potentials in electrochemical series... 

Note the observed phenomena. Write the molecular and net ionic equations of the reactions occurring between the metals and the salts. Arrange the metals in a series according to their activity (the electrochemical series), writing down the values of their standard electrode potentials (see Appendix 1, Table 2). What place does hydrogen occupy in this series ... 

Compare the chemical activity of the studied alkali metals. What does it depend on Why does lithium head the electrochemical series of the metals Find the values of the standard electrode potentials of the alkali metals (see Appendix 1, Table 21). 

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)... 

Referring to a list of standard electrode potentials, such as in Table 8.3, one speaks of an electrochemical series, and the metals lower down in the se-ries(with positive electrode potentials) are called noble metals. Any combination of half-reactions in an electrochemical cell, which gives a nonzero E value, can be used as a galvanic cell (i.e., a battery). If the reaction is driven by an applied external potential, we speak of an electrolytic cell. Reduction takes place at the cathode and oxidation at the anode. The reduction reactions in Table 8.3 are ordered with increasing potential or pe values. The oxidant in reactions with latter pe (or E°) can oxidize a reductant at a lower pe (or ) and vice versa for example, combining half-reactions we obtain an overall redox reaction ... 

The electrochemical series (Table 8.3) gives thermodynamic information on the so-called nobility of various metals the higher the standard electrode potential, the more noble is the metal, silver being more noble than Cu and Cu being more noble than Zn. 

The electrochemical series tabulates standard electrode potentials. Some sources call the electrochemical series oxi-dation/reduction potentials, electromotive series, and so on. The reference state of electrochemical series is the hydrogen evolution reaction, or H+/H2 reaction. Its standard electrode potential has been universally assigned as 0 V. This electrode is the standard hydrogen electrode (SHE) against which all others are compared. For example, the standard electrode potential of the Fe/Fe2+ reaction is —0.440 V and that of Cu/Cu2+ reaction is +0.337 V. The standard electrode potentials are calculated from Gibbs free energy values by Eq. (8) that is applicable only in the above-mentioned standard state. 

The potential in standard conditions ( °) of other electrochemical pairs can be obtained with respect to Eq. 3.4, permitting the compilation of a list of semireaction potentials (electrochemical series ). In this list, all the semi-reactions are written in such a way to evaluate the tendency of the oxidized forms to accept electrons and become reduced forms (positive potentials correspond to spontaneous reductions) [2]. These potentials can be correlated to thermodynamic quantities if the electrochemical system behaves in a reversible way from a thermodynamic point of view, i.e., when the electrochemical system is connected against an external cell with the same potential, no chemical reaction occurs, while any inhnitesimal variation of the external potential either to produce or to absorb current is exactly inverted when the opposite variation is applied (reversible or equilibrium potentials, Eeq)- When the equilibrium of the semi-reaction considered is established rapidly, its potential against the reference can be experimentally determined. 

Electrochemical series - An arrangement of reactions which produce or consume electrons in an order based on standard electrode potentials. A common arrangement places metals in decreasing order of their tendency to give up electrons. 

Unlike the table of the Electrochemical Series, which lists standard potentials, values for radicals are experimental values with experimental conditions given in the second column. Since the measurements leading to potentials for ion radicals are very dependent on conditions, an attempt to report standard potentials for radicals would serve no useful purpose. For the same reason, the potentials are also reported as experimental values, usually a half-wave potential in polarography) or a peak potential E in cyclic voltammetry). Unless otherwise stated, the values are reported vs. SCE (saturated calomel electrode). To obtain a value vs. 

---