Standard electrode potentials definition

The values of Hn and E are zero for water, by virtue of the constants 1.74 and 2.60. In these definitions, pKa refers to the acid ionization constant of the conjugate acid of the nucleophile, and E° to the standard electrode potential for the two-electron half-reaction ... 

Several descriptions of electrode reaction rates discussed on the preceding pages and the difficulty to standardize electrode potential scales with respect to different temperatures imply several definitions of activation energies of electrode reactions. The easiest way to determine this quantity, for example, for an irreversible cathodic process, employs Eqs (5.2.9), (5.2.10) and (5.2.12) at a constant electrode potential,... 

Stanski, electrodeposition, 1301,1303 Standard hydrogen electrode, 1108 see also hydrogen electrode potential, definition, 840, 1060, 1061 Steady state, 1147, 1212 current, 1248... 

The thermodynamic information is normally summarized in a Pourbaix diagram7. These diagrams are constructed from the relevant standard electrode potential values and equilibrium constants and show, for a given metal and as a function of pH, which is the most stable species at a particular potential and pH value. The ionic activity in solution affects the position of the boundaries between immunity, corrosion, and passivation zones. Normally ionic activity values of 10 6 are employed for boundary definition above this value corrosion is assumed to occur. Pourbaix diagrams for many metals are to be found in Ref. 7. 

Because the silver electrode is on the right, the measured potential is, by definition, the standard electrode potential for the silver half-reaction, or the silver couple. Note that the silver electrode is positive with respect to the standard hydrogen electrode. Therefore, the standard electrode potential is given a positive sign, and we write... 

When we look carefully at Equations 18-11 and 18-12, we see that the constant is the electrode potential whenever the concentration quotient (actually, the activity quotient) has a value of 1. This constant is by definition the standard electrode potential for the half-reaction. Note that the quotient is always equal to 1 when the activities of the reactants and products of a half-reaction are unity. 

The standard electrode potential shown in the first entry for Br2(at ) is hypothetical because the solubility of Br2 at 25°C is only about 0.18 M. Thus, the recorded value of 1.087 V is based on a system that—in terms of our definition of —cannot be realized experimentally. Nevertheless, the hypothetical poten-... 

In the above equation, if a = 1, then E = eP. The standard potential of an electrode eP is the potential of an electrode in contact with a solution of its ions of unit activity. The standard potentials are always expressed against the standard hydrogen electrode (SHE), the potential of which is zero by definition. The standard potentials are a function of temperature they are usually tabulated for 25° C. Standard electrode potential is also called normal electrode potential. 

As already discussed, the standard hydrogen electrode (SHE) is the chosen reference half-cell upon which tables of standard electrode potentials are based. The potential of this system is zero by definition at all temperatures. Although this reference electrode was often used in early work in electrochemistry, it is almost never seen in chemical laboratories at the present time. It is simply too awkward to use because of the requirement for H2 gas at 1 bar pressure and safety considerations. 

To determine the standard electrode potential of an element M we set up a cell as illustrated in Fig. 7,6. The element is placed in a solution of its ions at unit activity (standard state, based on the unit-molality definition) and coupled to a standard hydrogen electrode.f The potential of element M with respect to the platinum of the hydrogen electrode is called the standard electrode potential of M. (If the element M is positive with respect to the hydrogen electrode then the standard electrode potential of M is positive and vice versa.) If the metal in the cell is zinc we find... 

LT is the standard cell potential difference, which is determined only by the reactants in definited standard states. This quantity results as the difference of standard electrode potentials. The power term Ila contains the corrected composition quantities a, (fugacities and activities) with the stoichiometric coefficients v, of the gases and condensed substances taking part in the cell reaction [10,12]. If a sensor at equilibrium delivers signals in agreement with Equation (25-7) then we have a reaction celt. In this case at solid electrolytes with oxide ion vacancies Vo> two reactions can be found besides... 

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 ... 

For solutions in protic solvents (water is the most important one) the universal reference electrode for which, under standard conditions, the standard electrode potential is zero by definition all temperatures is the hydrogen electrode (HVH2). In Table 4.7 are the absolute electrode potentials for hydrogen, which can be interpreted in the following way. A redox couple more negative than 0.414 V should liberate hydrogen from water and a couple more negative than 0.828 V should liberate H2 from 1 mol OEI solution. 

Physicochemical Property Definitions Standard electrode potential Standard reduction-oxidation potential... 

Various standard electrode potentials can be calculated for a half-cell after the definition of standard electrode potential. Currently, most of standard electrode potentials can be obtained from the text. 

Definitions. Define briefly (a) difference of potential, (b) electromotive force, (c) salt bridge, (d) anode, (e) positive electrode, (f) reference half-cell, (g) standard electrode potential, (h) decomposition potential, (i) overvoltage, (j) sacrificial anode. 

Since the measured cell potential difference is actually the potential difference between two electrodes, it immediately comes to mind to assimilate each of the bracketed terms into the potential of each of the electrodes. They are called electrode potentials. E° and °2, which are in the two subgroups, exhibit characteristic values of both couples Oxi/Redi and Oxa/Reda. These constants are called standard potentials of both couples and are symbolized (Oxi/Redi) and °(Ox2/Red2). Assigning numerical values to and E°2 has been a problem since the experimental determination of absolute electrode potentials hence, assigning those to standard electrode potentials is impossible (see the electrochemistry part). It was solved by assigning relative values to them. The strategy was based on the fact that if absolute electrode potentials are not measurable, the difference between them can be. Thus, an electrode standard potential has been chosen conventionally for the couple H+w/H2(g) (hydrogen electrode). Its standard electrode has been set definitively to the value 0.0000 V at every temperature ... 

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 ... 

We consider first equilibrium (a) and begin by noting that definition of the standard electrode potential of the Sn/Sn " couple imphes that for the following cell,... 

An important conclusion to be made here is related to the definition of the standard electrode potential given in the lUPAC manual [1] The standard potential of an electrochemical reaction, abbreviated as standard potential, is defined as the standard potential of a hypothetical cell, in which the electrode (half-cell) on the left of the cell diagram is the SHE and the electrode at the right is the electrode in question. Note that E of a half-reaction (or a total electrochemical reaction) as an intensive variable does not depend on the number of electrons nsed in the half-reaction. 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

---