Conventions, galvanic cell

A second source of standard free energies comes from the measurement of the electromotive force of a galvanic cell. Electrochemistry is the subject of other articles (A2.4 and B1.28). so only the basics of a reversible chemical cell will be presented here. For example, consider the cell conventionally written as... 

Heat of Precipitation. Entropy of Solution and Partial Molal Entropy. The Unitary Part of the Entropy. Equilibrium in Proton Transfers. Equilibrium in Any Process. The Unitary Part of a Free Energy Change. The Conventional Standard Free Energy Change. Proton Transfers Involving a Solvent Molecule. The Conventional Standard Free Energy of Solution. The Disparity of a Solution. The E.M.F. of Galvanic Cells. 

Electrons always flow spontaneously downhill from higher electrical potential to lower electrical potential. In a galvanic cell, the electrode with the higher potential is designated by convention as the negative electrode. The electrode with the lower potential is designated as the positive electrode. 

In any galvanic cell that is under standard conditions, electrons are produced by the half-reaction with the more negative standard reduction potential and consumed by the half-reaction with the more positive standard reduction potential. In other words, the half-reaction with the more negative E ° value occurs as the oxidation, and the half-reaction with the more positive E ° value occurs as the reduction. Figure 19-15 summarizes the conventions used to describe galvanic cells. 

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 ... 

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. 

The negative sign of U in this equation is caused by the convention on the potential scale for galvanic cells while we consider in the absolute scale electron... 

A standard shorthand notation is used to describe the construction of galvanic cells and avoid the necessity of drawing pictures. It is based on the convention that the negative electrode is shown at the left. The notation for the galvanic cell shown in Figure 17-1 is... 

In this section, we describe time-resolved, local in-situ measurements of chemical potentials /, ( , f) with solid galvanic cells. It seems as if the possibilities of this method have not yet been fully exploited. We note that the spatial resolution of the determination of composition is by far better than that of the chemical potential. The high spatial resolution is achieved by electron microbeam analysis, analytical transmission electron microscopy, and tunneling electron microscopy. Little progress, however, has been made in improving the spatial resolution of the determination of chemical potentials. The conventional application of solid galvanic cells in kinetics is completely analogous to the time-dependent (partial) pressure determination as explained in Section 16.2.2. Spatially resolved measurements are not possible in this way. 

When drawing a galvanic cell, by convention, the anode is drawn on the left and the cathode on the right. In Fig. 5, an electrochemical cell using a hydrogen electrode as the anode and an AgCl/Ag electrode as the cathode is shown. The electrolyte is aqueous HC1 and both terminals are Pt. 

We next describe the operation of galvanic cells in mathematical terms, again taking the Daniell cell as our representative example. Consider Fig. 4.6.1 for electrons to flow through the external circuit left to right the electric field E points in the direction of the conventional positive current flow, i.e., to the left, whereas the electrostatic potential gradient Vfi = — points to the right. Under spontaneous... 

It is desirable to introduce a systematic nomenclature and a set of conventions that permit a unified description of galvanic cell operations. Consider as a representative example a cell at temperature T that consists of (i) a Pt electrode that is surrounded by gaseous hydrogen at pressure P and that is dipped into an HCl solution of molality m/. The latter is connected via a salt bridge to a second compartment containing a saturated HCl solution in equilibrium with AgCl(s), at molality into which a silver electrode is immersed. The cell is at uniform temperature T. This cell is represented by the scheme... 

Define the terms anode and cathode and give the convention that is used to represent a galvanic cell (Section 17.1, problems 1-2). 

Convention Write galvanic cells with anode on the left and cathode on the right. This arrangement gives a positive cell potential. 

Potentiometry is the measurement of an electrical potential difference between two electrodes (half-ceUs) in an electrochemical cell (Figure 4-1) when the cell current is zero (galvanic cell). Such a cell consists of two electrodes (electron or metallic conductors) that are connected by an electrolyte solution (ion conductor). An electrode, or half-cell, consists of a single metallic conductor that is in contact with an electrolyte solution. The ion conductors can be composed of one or more phases that are either in direct contact with each other or separated by membranes permeable only to specific cations or anions (see Figure 4-1). One of the electrolyte solutions is the unknown or test solution this solution may be replaced by an appropriate reference solution for calibration purposes. By convention, the cell notation is shown so that the left electrode (Mi,) is the reference electrode the right electrode (Mr) is the indicator (measuring) electrode (see later equation 3). ... 

The lUPAC convention is consistent with the signs that the electrodes actually develop in a galvanic cell. That is, in the Cu/Ag cell shown in Figure 18-4, the Cu electrode becomes electron rich (negative) owing to the tendency of Cu to be oxidized to Cu while the Ag electrode becomes electron deficient (positive) because of the tendency for Ag" to be reduced to Ag. As the galvanic cell discharges spontaneously, the silver electrode is the cathode, while the copper electrode is the anode. 

In this book the following conventions will be used in representing galvanic cells (a) a semicolon ( ) will be used to indicate a metal electrolyte boundary, such as Zn ZnSO (b) a liquid junction or electrolyte-electrolyte boundary will be shown by a colon ( ), thus, for instance, hydrochloric acid at the concentrations Ci and C2 may form the liquid junction... 

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