Galvanic cell with transport

So far, a cell containing a single electrolyte solution has been considered (a galvanic cell without transport). When the two electrodes of the cell are immersed into different electrolyte solutions in the same solvent, separated by a liquid junction (see Section 2.5.3), this system is termed a galvanic cell with transport. The relationship for the EMF of this type of a cell is based on a balance of the Galvani potential differences. This approach yields a result similar to that obtained in the calculation of the EMF of a cell without transport, plus the liquid junction potential value A0L. Thus Eq. (3.1.66) assumes the form... 

Another way of determining the solvent transport by emf measurements has been proposed by C. Wagner The two half cells contain two solvent mixtures of similar composition which are both saturated with a sparingly soluble salt, e.g. a silver salt AgX. Though the chemical potential of is the same throughout the galvanic cell with transport, the emf is different from zero since the chemical potential of the solvent is different in the two half cells and A moles of the non-aqueous solvent component are transported into the cathode compartment per Faraday. 

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. 

Galvanic cells that include at least one electrolyte-electrolyte interface (which may be an interface with a membrane) across which ions can be transported by diffusion are called cells with transference. For the electrolyte-electrolyte interfaces considered in earlier sections, cells with transference can be formulated, for example, as... 

A solid state galvanic cell consists of electrodes and the electrolyte. Solid electrolytes are available for many different mobile ions (see Section 15.3). Their ionic conductivities compare with those of liquid electrolytes (see Fig. 15-8). Under load, galvanic cells transport a known amount of component from one electrode to the other. Therefore, we can predetermine the kinetic boundary condition for transport into a solid (i.e., the electrode). By using a reference electrode we can simultaneously determine the component activity. The combination of component transfer and potential determination is called coulometric titration. It is a most useful method for the thermodynamic and kinetic investigation of compounds with narrow homogeneity ranges. For example, it has been possible to measure in a... 

Chemelec cell — An electrolytic cell with electrodes (both anode and cathode) made from expanded metal or simple metal plates. The cell is filled with inert particles, which are floated when the electrolyte solution enters the cell from the bottom. This causes effectively turbulence with increased - mass transport. This cell is employed in wastewater treatment especially from galvanic operations. 

In voltaic cells, it is possible to carry out the oxidation and reduction halfreactions in different places when suitable provision is made for transporting the electrons over a wire from one half-reaction to the other and to transport ions from each half-reaction to the other in order to preserve electrical neutrality. The chemical reaction produces an electric current in the process. Voltaic cells, also called galvanic cells, are introduced in Section 17.1. The tendency for oxidizing agents and reducing agents to react with each other is measured by their standard cell potentials, presented in Section 17.2. In Section 17.3, the Nernst equation is introduced to allow calculation of potentials of cells that are not in their standard states. 

Figure 18-3 shows the movement of various charge carriers in a galvanic cell during diseharge. The electrodes are connected with a wire so that the spontaneous cell reaction occurs. Charge is transported through such an electrochemical cell by three mechanisms ... 

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. 

In Eqs. (122) and (123), M(Hg) is an alkali metal amalgam electrode, MX the solvated halide of the alkali metal M at concentration c in a solvent S, and AgX(s)/Ag(s) a silver halide-silver electrode. Equation (124) is the general expression for the electromotive force " of a galvanic cell without liquid junction in which an arbitrary cell reaction 0)1 Yi + 0)2Y2 + coiYi + , takes place between k components in v phases. In Eq. (124) n is the number of moles of electrons transported during this process from the anode to the cathode through the outer circuit, F the Faraday number, and the chemical potential of component Yi in phase p. Cells with liquid junctions require the electromotive force E in Eq. (124) to be replaced by the quantity E — Ej), where Ey> is the diffusion potential due to the liquid junction. The standard potential E° for the cell investigated by Eq. (122) is given by the relationship... 

The kinetic properties of galvanic cell I with doped zirconia as solid electrolyte arise from the fact that the flux of current through a cell such as cell I is a measure of the reaction rate by which oxygen is passed from one side of the cell to the other. Only oxygen ions can flow through the electrolyte when the electrical circuit is closed. For the rate of transport of the 0 particles in moles per unit time J through the electrolyte we can write ... 

The transport of mass in the form of ions through the electrolyte can often be attributed to a chemical reaction or a transport process at an electrode. In this way reaction rates can be measured electrically. It is often possible to analyze reaction mechanisms in detail by a combination of rate measurements by means of the electrical current with measurements of thermodynamic quantities— in particular, chemical potentials—by means of the emf of the galvanic cell. More details on kinetic investigations using galvanic cells will be given in Section V.B. 

Standard galvanic cells are independently sealed, whereas the Eloflux cell is completely diflerent The main difference is that one electrode is used for transporting both the gas and the electrolyte. Therefore, this cell design reduces costs by simplification of the ceU components. There are fewer parts with less degradation and corrosion. Eloflux was described in 1965 by Winsel and Wendtland [17-20]. 

That is, we set AG j(H+, aq) = 0. Then the thermodynamic properties of anions can be found by measuring the chemical potentials of ionic solutions containing H (as, for example, by the technique of galvanic cell electromotive forces (emfs), described below) in combination with different anions, and then using Eq. (4.1.3b). The anionic chemical potentials so determined can be employed as secondary standards in solutions containing different cations, and this matching process is continued as needed. Extensive tabulations constructed in this manner are available. However, this convention becomes inapplicable for processes where H" " ions are transported across the phase boundaries of the aqueous solutions. 

As explained, there are endoelectrogenic soiu-ces in the cell membranes. However, it is quite likely that some macroscopic membranes around organs are also the sites of electricity sources. Nordenstr0m (1983) proposed that there are closed DC circuits in the body with the well-conducting blood vessels serving as cables (e.g., a vascular-interstitial closed circuit). These DC currents can cause electro-osmotic transport through capillaries. Dental galvanism is the production of electricity by the metals in the teeth. 

Previous efforts relevant to oxygen transport have used two-electrode amperometric sensors such as the Clark and Mackereth cells (Clark, 1959). These sensors are inexpensive, accurate, and small, but they require frequent calibration, consume oxygen, and suffer from long-term drift. Other options include cyclic voltammetric sensors with remote three-electrode cells, and variations on galvanic techniques to monitor limiting currents at steady state (Fan et al., 1991 Haug and White, 2000 Utaka et al., 2009). 

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