The Standard Potential of a Metal

The standard potential of a metal in a given solvent thus apparently depends on the sublimation energy of the metal, its ionization potential and the energy of solvation of the ions. Calculations have shown that of these factors the heat of sublimation is much the smallest, but since the other two quantities generally do not differ very greatly, all three factors must play an important part in detenninmg the actual electrode potential. 

In order to determine the standard potential of a metal forming a soluble, highly dissociated chloride, e.g., zinc, the measurements are made on cells of the type... 

The Electrochemical Reactions in the Corrosion of Aluminium The Standard Potential of a Metal... 

Since the single potential of a metal cannot be measured it is necessary to use a suitable reference elecrode such as the Hg/Hg2Cl2/KCl electrode or the Ag/AgCl/KCl electrode, and although potentials are frequently expressed with reference to the standard hydrogen electrode (S.H.E.) the use of this electrode in practice is confined to fundamental studies rather than testing. 

It is apparent that since the electrode potential of a metal/solution interface can only be evaluated from the e.m.f. of a cell, the reference electrode used for that purpose must be specified precisely, e.g. the criterion for the cathodic protection of steel is —0-85 V (vs. Cu/CuSOg, sat.), but this can be expressed as a potential with respect to the standard hydrogen electrode (S.H.E.), i.e. -0-55 V (vs. S.H.E.) or with respect to any other reference electrode. Potentials of reference electrodes are given in Table 21.7. 

First of all, the important role of platinum as the metal part of the standard hydrogen electrode (SHE), which is the primary standard in electrochemistry should be mentioned. The standard potential of an electrode reaction (standard electrode potential) is defined as the value of the standard potential of a cell reaction when that involves the oxidation of molecular hydrogen to solvated (hydrated) protons (hydrogen ions) ... 

ACTIVITY SERIES- Also referred to as the electromotive series or the displacement series, this is an arrangement of the metals (other elements can be included) in the order of their tendency to react with water and acids, so that each metal displaces from solution those below itiu the series and is displaced by those above it. See Table 1. Since the electrode potential of a metal in equilibrium with a solution of its ions cannot be measured directly, the values in the activity series are, in each case, the difference between the electrode potential of the given metal tor element) in equilibrium with a solution of its ions, and that of hydrogen in equilibrium with a solution of its ions. Thus in the table, it will be noted that hydrogen lias a value of 0.000. In experimental procedure, the hydrogen electrode is used as the standard with which the electrode potentials of other substances are compared. The theory of displacement plays a major role in electrochemistry and corrosion engineering. See also Corrosion and Electrochemistry. 

What is the standard potential of a cell that uses the Zn2+/Zn and Ag+/Ag couples When the cell is allowed to produce an electric current, which metal is deposited ... 

Any surface (typically a piece of metal) on which an electrochemical reaction takes place will produce an electrochemical potential when in contact with an electrolyte (typically water containing dissolved ions). The unit of the electrochemical potential is volt (TV = 1JC1 s 1 in SI units).The metal, or strictly speaking the metal-electrolyte interface, is called an electrode and the electrochemical reaction taking place is called the electrode reaction. The electrochemical potential of a metal in a solution, or the electrode potential, cannot be determined absolutely. It is referred to as a potential relative to a fixed and known electrode potential set up by a reference electrode in the same electrolyte. In other words, an electrode potential is the potential of an electrode measured against a reference electrode. The standard hydrogen electrode (SHE) is universally adopted as the primary standard reference electrode with which all other electrodes are compared. By definition, the SHE potential is OV, i.e. the zero-point on the electrochemical potential scale. Electrode potentials may be more positive or more negative than the SHE. 

The standard electrode potential is a quantitative expression of the readiness of the element to lose electrons. It is therefore a measure of the strength of a metal as a reducing agent. The more negative the electrode potential of a metal, the more powerful its action as a reductant. 

Other Bivalent Metals.—The standard potentials of a number of... 

The potentials of half-cells that are constructed using amalgams are important whenever one is interested in performing reactions in a media in which hydrogen evolution at the metal-solution interface might interfere with the reaction of interest. Consequently, half-cell potentials for alkali metal amalgams are important to synthetic chemists and electrochemists alike. The standard potential for a metal amalgam in contact with a solution of its monovalent cation ( °(M" /M(Hg))) can be related to that of the pure metal in contact with its monovalent cation (EJ ) as shown in Eq. (14)... 

Table 5 compares the standard potential of the electron electrode in hexamethylphosphotriamide (5 °C) with the standard potentials of alkali metals (25 °C). Data for liquid ammonia are also given. In both solvents the rubidium electrode potential serves as a reference point since it depends very little on the solvent. It is seen from the Table that in both solvents the standard equilibrium potential of the electron electrode is more positive than that of a lithium electrode and is close to the potentials of other alkali metals. In the course of experiment, cathodic production of dilute solutions (10 — 10 mol/1) of solvated electrons takes place and this makes the electron electrode equilibrium potential more positive compared to the standard value. In case of hexamethylphosphotriamide the same happens when electrons are bound in strong non-paramagnetic associates by the cations of all alkali metals except lithium (see Sect. 4). This enables one to assume that under the conditions of the experiments the electron-electrode equilibrium potential in liquid ammonia and hexamethylphosphotriamide is more positive than the equilibrium potential of all alkali metals. This makes thermodynamically possible primary cathodic generation of solvated electrons in solutions of all alkali metal salts in the two solvents. 

The standard potential of a cell or half-reaction is obtained under conditions where all species are in their standard states (10). For solids, like Ag in cell 2.1.32 or reaction 2.1.33, the standard state is the pure crystalline (bulk) metal. It is interesting to consider how many atoms or what particle size is needed to produce bulk metal and whether the standard potential is a function of particle size when one deals with metal clusters. These questions have been addressed (11-13) and for clusters containing n atoms (where n < 20), indeed turns out to be very different from the value for the bulk metal (n 20). Consider, for example, silver clusters, Agn- For a silver atom n = 1), the value of can be related to for the bulk metal through a thermodynamic cycle involving the ionization potential of Ag and the hydration energy of Ag and Ag. This process yields... 

The equilibrium potential of a metal-ion electrode in conjunction with a standard hydrogen electrode to form a complete cell can be shown to be given by (Castellan 1983, Koryta Dvorak 1987)... 

The Emf Series is an orderly arrangement of the standard potentials for all metals. The more negative values correspond to the more reactive metals (Table 3.2). Position in the Emf Series is determined by the equilibrium potential of a metal in contact with its ions at a concentration equal to unit activity. Of two metals composing a cell, the anode is the more active metal in the Emf Series, provided that the ion activities in equilibrium are both unity. Since unit activity corresponds in some cases to impossible concentrations of metal ions because of restricted solubility of metal salts, the Emf Series has only limited use for predicting which metal is anodic to another. 

The most acceptable method of obtaining standard electrode potentials is by comparing tbe electrode potential of metals with the standard hydrogen electrode. Since the SHE has zero electrode potential at all temperatures by definition, the electrode potential of a metal is numerically equal to the emf of the cell formed by SHE and the metal electrode. In other words, the emf of the cell represents the electrode potential of the half cell formed by the metal with respect to the standard hydrogen electrode. In such a cell, reaction on the hydrogen electrode is oxidation and reaction on the other electrode is reduction. Such a cell can be expressed as ... 

The first two terms of the right-hand side of the equation are sometimes combined and expressed as E which is called the standard oxidation potential for the chelate system. If the chelation is strong and the ligand is in excess, the metal would be almost entirely in the chelated forms, and [M L] and [M g L] would essentially be equal to the total concentrations of the oxidized and reduced forms of the metal. If, as is usual, the oxidized form is the more strongly chelated K > ), the oxidation potential of a system is increased by the addition of the chelant. 

The thermodynamic driving force behind the corrosion process can be related to the corrosion potential adopted by the metal while it is corroding. The corrosion potential is measured against a standard reference electrode. For seawater, the corrosion potentials of a number of constructional materials are shown in Table 53.1. The listing ranks metals in their thermodynamic ability to corrode. Corrosion rates are governed by additional factors as described above. 

It is possible to titrate two substances by the same titrant provided that the standard potentials of the substances being titrated, and their oxidation or reduction products, differ by about 0.2 V. Stepwise titration curves are obtained in the titration of mixtures or of substances having several oxidation states. Thus the titration of a solution containing Cr(VI), Fe(III) and V(V) by an acid titanium(III) chloride solution is an example of such a mixture in the first step Cr(VI) is reduced to Cr(III) and V(V) to V(IV) in the second step Fe(III) is reduced to Fe(II) in the third step V(IV) is reduced to V(III) chromium is evaluated by difference of the volumes of titrant used in the first and third steps. Another example is the titration of a mixture of Fe(II) and V(IV) sulphates with Ce(IV) sulphate in dilute sulphuric acid in the first step Fe(II) is oxidised to Fe(III) and in the second jump V(IV) is oxidised to V(V) the latter change is accelerated by heating the solution after oxidation of the Fe(II) ion is complete. The titration of a substance having several oxidation states is exemplified by the stepwise reduction by acid chromium(II) chloride of Cu(II) ion to the Cu(I) state and then to the metal. 

The standard potential of an electrode is the standard emfofa cell in which the electrode on the left in the cell diagram is a hydrogen electrode. A metal with a negative standard potential has a thermodynamic tendency to reduce Irydrogen ions in solution the ions of a metal with a positive standard potential have a tendency to be reduced by hydrogen gas. 

A student was given a standard Fe(s) Fe2+(aq) half-cell and another half-cell containing an unknown metal M immersed in 1.00 M MNO,(aq). When these two half-cells were connected at 25°C, the complete cell functioned as a galvanic cell with E = +1.24 V. The reaction was allowed to continue overnight and the two electrodes were weighed. The iron electrode was found to be lighter and the unknown metal electrode was heavier. What is the standard potential of the unknown MT/M couple ... 

The standard equUibrium potential of a metal/metal ion electrode is shifted to cathodic potentials with decreasing particle size. Taking a compact metal electrode as reference, the change in potential for a microelectrode of radius r is... 

Figure 4 illustrates that our present knowledge about the dependence of the standard potential of very small silver microelectrodes on the agglomeration number is rather fragmentary. Even less is known about this dependence for other metals. The experiments of Fig. 5 prove that the rate of an electrochemical reaction in which a small microelectrode is involved, may strongly depend on the size of the microelectrode. 

An interesting correlation exists between the work function of a metal and its pzc in a particular solvent. Consider a metal M at the pzc in contact with a solution of an inert, nonadsorbing electrolyte containing a standard platinum/hydrogen reference electrode. We connect a platinum wire (label I) to the metal, and label the platinum reference electrode with II. This setup is very similar to that considered in Section 2.4, but this time the metal-solution interface is not in electronic equilibrium. The derivation is simplified if we assume that the two platinum wires have the same work function, so that their surface potentials are equal. The electrode potential is then ... 

In order to estimate the standard electrode potential of a metal ion a Born-Haber cycle consisting of the following three steps may be considered, 19> 12°1 (Fig. 21) ... 

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