Galvanic cells selectivity

A parameter that is convenient for said purpose is the electrode potential E it must not be confused with the concept of a potential difference between the electrode and the electrolyte. By convention the term electrode potential E is used to denote the OCV of a galvanic cell that consists of the given electrode (the one that is studied) and a reference electrode selected arbitrarily. Thus, the potential of this electrode is compared with that of a reference electrode that is identical for all electrodes being studied. In accordance with this dehnition, the electrode potential of the reference electrode itself is (conventionally) regarded as zero. Any electrode system for which the equilibrium Galvani potential is established sufficiently rapidly and reproducibly can be used as a reference electrode. We shall write the electrode system to be used as the reference electrode, generally, as M /E ... 

OCV and Discharge Voltage The OCV, of a galvanic cell depends on the electrochemical system selected for it and is somewhat affected by the electrolyte... 

It has been emphasized repeatedly that the individual activity coefficients cannot be measured experimentally. However, these values are required for a number of purposes, e.g. for calibration of ion-selective electrodes. Thus, a conventional scale of ionic activities must be defined on the basis of suitably selected standards. In addition, this definition must be consistent with the definition of the conventional activity scale for the oxonium ion, i.e. the definition of the practical pH scale. Similarly, the individual scales for the various ions must be mutually consistent, i.e. they must satisfy the relationship between the experimentally measurable mean activity of the electrolyte and the defined activities of the cation and anion in view of Eq. (1.1.11). Thus, by using galvanic cells without transport, e.g. a sodium-ion-selective glass electrode and a Cl -selective electrode in a NaCl solution, a series of (NaCl) is obtained from which the individual ion activity aNa+ is determined on the basis of the Bates-Guggenheim convention for acr (page 37). Table 6.1 lists three such standard solutions, where pNa = -logflNa+, etc. 

As mentioned previously, electroanalytical techniques that measure or monitor electrode potential utilize the galvanic cell concept and come under the general heading of potentiometry. Examples include pH electrodes, ion-selective electrodes, and potentiometric titrations, each of which will be described in this section. In these techniques, a pair of electrodes are immersed, the potential (voltage) of one of the electrodes is measured relative to the other, and the concentration of an analyte in the solution into which the electrodes are dipped is determined. One of the immersed electrodes is called the indicator electrode and the other is called the reference electrode. Often, these two electrodes are housed together in one probe. Such a probe is called a combination electrode. 

The voltage we measure is characteristic of the metals we use. As an additional example, unit activity solutions of CuCE and AgCl with copper and silver electrodes, respectively, give a potential difference of about 0.45 V. We could continue with this type of measurement for aU the different anode-cathode combinations, but the number of galvanic cells needed would be very large. Fortunately, the half-reactions for most metals have been calculated relative to a standard reference electrode, which is arbitrarily selected as the reduction of hydrogen ... 

The combination of chemistry and electricity is best known in the form of electrochemistry, in which chemical reactions take place in a solution in contact with electrodes that together constitute an electrical circuit. Electrochemistry involves the transfer of electrons between an electrode and the electrolyte or species in solution. It has been in use for the storage of electrical energy (in a galvanic cell or battery), the generation of electrical energy (in fuel cells), the analysis of species in solution (in pH glass electrodes or in ion-selective electrodes), or the synthesis of species from solution (in electrolysis cells). 

Use of the potential of a galvanic cell to measure the concentration of an electroactive species developed later than a number of other electrochemical methods. In part this was because a rational relation between the electrode potential and the concentration of an electroactive species required the development of thermodynamics, and in particular its application to electrochemical phenomena. The work of J. Willard Gibbs1 in the 1870s provided the foundation for the Nemst equation.2 The latter provides a quantitative relationship between potential and the ratio of concentrations for a redox couple [ox l[red ), and is the basis for potentiometry and potentiometric titrations.3 The utility of potentiometric measurements for the characterization of ionic solutions was established with the invention of the glass electrode in 1909 for a selective potentiometric response to hydronium ion concentrations.4 Another milestone in the development of potentiometric measurements was the introduction of the hydrogen electrode for the measurement of hydronium ion concentrations 5 one of many important contributions by Professor Joel Hildebrand. Subsequent development of special glass formulations has made possible electrodes that are selective to different monovalent cations.6"8 The idea is so attractive that intense effort has led to the development of electrodes that are selective for many cations and anions, as well as several gas- and bioselective electrodes.9 The use of these electrodes and the potentiometric measurement of pH continue to be among the most important applications of electrochemistry. 

Sorita, R. and Kawano, T. (1997) A highly selective CO sensor using LaMnOj electrode-attached zirconia galvanic cell. Sens. Actuators B, 40, 29-32. 

The entropy of Ga2Se3(cr) at 298.15 K was estimated in [74M1L] to be (179.9 + 17.0) by comparison with related compounds and a second law evaluation of galvanic cell data recorded by Mamedov [69MAM]. The value is not selected since it is... 

The enthalpy of formation has been determined from vapour pressure and galvanic cell measurements. Each determination has been re-evaluated as discussed in Appendix A using the second and third laws, the selected heat capacity, the entropy of Ag2Se(cr), the selected properties of selenium, and the CODATA [89COX/WAG] values of silver. The results are summarised in Table V-62. 

Two necessary conceptions are those of the thermodynamic system and its surroundings. A thermodynamic system may be any arbitrarily selected portion of matter or space. It may be, for instance, a volume of gas, a heat engine, or a galvanic cell. The surroundings are the immediate environment of the system, with which it may exchange energy. Important types of interaction between a thermodynamic system and its surroundings are the flow of heat from one to the other, or the action of work of one on the other, or both. 

Skelton, W. H., Patterson, J. W., Free energy determinations by solid galvanic cell measurements for selected metal, metal-fluoride reactions, J. Less-Common Met., 31, (1973), 47-60. Cited on pages 201, 545. 

The properties of electrochemical cells allow us to. use them in a variety of ways for determining the concentrations of individual ions on physical-chemical properties. We will not deal with all types of electrochemical measurements here rather we will select examples to illustrate the use of electrochemical techniques in water chemistry. Our examples will include the measurement of activity (concentration) by potentiometric methods using galvanic cells and specific ion electrodes, and the measurement of activity (concentration) by electrolytic cells using techniques such as polarography and amperometric titration. 

Galvanic corrosion is of particular concern in design and material selection. Material selection is important because different metals come into contact with each other and may form galvanic cells. Design is important to minimize differing flow conditions and resultant areas of corrosion buildup. Loose corrosion products are important because they can be transported to the reactor core and irradiated. 

The inner filling solution for the sensors is usually 0.01 M NaCl, which is also used to condition the potentiometric sensors. Electrochemical potential is measured with the following galvanic cell Ag/AgCl/bridge electrolyte/sam-ple solution/ion-selective membrane/inner filling solution/ AgQ/Ag. A high impedance pH-mV meter is used to measure the electrochemical potential. Selectivity coefficients are evaluated by the matched potential method (also known as method of mixed solutions), or via the separate solution method. 

The main attractiveness of fuel cells follows from the definitions given above. It comprises the high theoretical efficiency associated with direct conversion of chemical energy into electrical energy by means of galvanic cells [10] the selectivity of the electrochemical process and the advantage of a continuous metabolism by using the ambient air to oxidize the steadily supplied fuel. 

If both an electric and an electrolytic contact are established between different metals, a galvanic cell forms that results in an accelerated corrosion of the metal with the less noble corrosion potential (Chapter 7). With an adequate selection of materials, or by placing an insulating material between the metals, this kind of corrosion can be avoided (Figure 12.4(a)). For small objects, such as electronic components, one can prevent galvanic corrosion by completely immersing a joint of dissimilar metals into a polymer, thus avoiding that the metals come in contact with an electrolyte film that could form in presence of humidity. 

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