Electrode potentials constant

For large-scale processes, which can be run continuously, there is no need for potential control since the substrate concentration can be kept constant all the time. This in effect serves to keep the electrode potential constant. 

To keep the solution saturated with H2(g). Only then is the hydrogen activity constant and the electrode potential constant and reproducible. 

As mentioned, the potential keeps the electrode potential constant at a set value, i.e. it keeps a constant voltage on the cell consisting of the working electrode and the reference electrode by delivering the necessary current between the counter electrode and the working electrode. For the working electrode this is experienced as an external current. 

Electrochemical reactors can be operated under conditions of constant electrode potential, constant current, or constant cell voltage. The first two are referred to as potentiostatic and galvanostatic modes of operation, respectively. The potenti-static method is characterized by constant values of the kinetic parameters and hence enables integration of the dififerential equations describing the different reactors. On the other hand, galvanostatic operation is characterized by an inevitable change of electrode potential with time, leading to variations in the kinetic parameters. Hence we restrict our treatment to potentiostatic operation. 

Electrochemical methods may be classified into two broad classes, namely potentiometric metiiods and voltannnetric methods. The fonner involves the measurement of the potential of a working electrode iimnersed in a solution containing a redox species of interest with respect to a reference electrode. These are equilibrium experiments involving no current flow and provide themiodynamic infomiation only. The potential of the working electrode responds in a Nemstian maimer to the activity of the redox species, whilst that of the reference electrode remains constant. In contrast, m voltannnetric methods the system is perturbed... 

In Section 8, the material on solubility constants has been doubled to 550 entries. Sections on proton transfer reactions, including some at various temperatures, formation constants of metal complexes with organic and inorganic ligands, buffer solutions of all types, reference electrodes, indicators, and electrode potentials are retained with some revisions. The material on conductances has been revised and expanded, particularly in the table on limiting equivalent ionic conductances. 

The SCE has the advantage that the concentration of Ck, and, therefore, the potential of the electrode, remains constant even if the KCl solution partially evaporates. On the other hand, a significant disadvantage of the SCE is that the solubility of KCl is sensitive to a change in temperature. At higher temperatures the concentration of Ck increases, and the electrode s potential decreases. Eor example, the potential of... 

In this manner, a current efficiency of 100% is maintained. Furthermore, since the concentration of Ce + remains at its initial level, the potential of the working electrode remains constant as long as any Fe + is present. This prevents other oxidation reactions, such as that for liiO, from interfering with the analysis. A species, such as Ce +, which is used to maintain 100% current efficiency, is called a mediator. 

Two methods are used to measure pH electrometric and chemical indicator (1 7). The most common is electrometric and uses the commercial pH meter with a glass electrode. This procedure is based on the measurement of the difference between the pH of an unknown or test solution and that of a standard solution. The instmment measures the emf developed between the glass electrode and a reference electrode of constant potential. The difference in emf when the electrodes are removed from the standard solution and placed in the test solution is converted to a difference in pH. Electrodes based on metal—metal oxides, eg, antimony—antimony oxide (see Antimony AND ANTIMONY ALLOYS Antimony COMPOUNDS), have also found use as pH sensors (8), especially for industrial appHcations where superior mechanical stabiUty is needed (see Sensors). However, because of the presence of the metallic element, these electrodes suffer from interferences by oxidation—reduction systems in the test solution. 

Reference Electrodes and Liquid Junctions. The electrical cincuit of the pH ceU is completed through a salt bridge that usually consists of a concentrated solution of potassium chloride [7447-40-7]. The solution makes contact at one end with the test solution and at the other with a reference electrode of constant potential. The Hquid junction is formed at the area of contact between the salt bridge and the test solution. The mercury—mercurous chloride electrode, the calomel electrode, provides a highly reproducible potential in the potassium chloride bridge solution and is the most widely used reference electrode. However, mercurous chloride is converted readily into mercuric ion and mercury when in contact with concentrated potassium chloride solutions above 80°C. This disproportionation reaction causes an unstable potential with calomel electrodes. Therefore, the silver—silver chloride electrode and the thallium amalgam—thallous chloride electrode often are preferred for measurements above 80°C. However, because silver chloride is relatively soluble in concentrated solutions of potassium chloride, the solution in the electrode chamber must be saturated with silver chloride. 

The diffusion coefficient of oxygen in solid silver was measured with a rod of silver initially containing oxygen at a conceim ation cq placed end-on in contact with a calcia-zirconia electrolyte and an Fe/FeO electrode. A constant potential was applied across dre resulting cell... 

In addition, the temperature dependence of the diffusion potentials and the temperature dependence of the reference electrode potential itself must be considered. Also, the temperature dependence of the solubility of metal salts is important in Eq. (2-29). For these reasons reference electrodes with constant salt concentration are sometimes preferred to those with saturated solutions. For practical reasons, reference electrodes are often situated outside the system under investigation at room temperature and connected with the medium via a salt bridge in which pressure and temperature differences can be neglected. This is the case for all data on potentials given in this handbook unless otherwise stated. 

The standard electrode potentials , or the standard chemical potentials /X , may be used to calculate the free energy decrease —AG and the equilibrium constant /T of a corrosion reaction (see Appendix 20.2). Any corrosion reaction in aqueous solution must involve oxidation of the metal and reduction of a species in solution (an electron acceptor) with consequent electron transfer between the two reactants. Thus the corrosion of zinc ( In +zzn = —0-76 V) in a reducing acid of pH = 4 (a = 10 ) may be represented by the reaction ... 

Fig. 20.7 Differential capacitance/mercury electrode potential relationships for potassium chloride at different concentrations showing (a) how minima are obtained only at low concentrations and (6) the constant capacitance at negative potentials (after Bockris and Drazic )...

Thus the equilibrium constant K can be evaluated from standard electrode potential or from the standard chemical potentials x . 

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 +. ... 

E0 Standard electrode potential, V R The gas constant F Faraday constant... 

Continuous conductivity measurement controlled with the electrode placed in the boiler. This method is not recommended because of potential safety and liability issues. In addition, there are difficulties with cleaning and maintaining the electrode, and the intense heat to which the electrode is constantly subjected may cause failure. FT boiler installations generally provide for the electrode to be placed above the first set of tubes but 4 to 6 inches below the waterline. 

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 ... 

Equation (28) shows that the constant term is eliminated. Nevertheless, A

derive information about AX. There is no way to avoid this it is a consequence of the nature of the electrode potential [see Section I.2(ii)]. 

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