The Galvanic Cell

A simple galvanic cell has two electrodes the anode and the cathode. The anode is marked with a negative sign and the cathode is marked with a positive sign, The oxidation half reaction takes place at the anode, and the reduction half reaction takes place at the cathode. Depending upon the text, electrodes may refer to only a strip of metal or both a strip of metal and the electrolyte solution in which it is submerged. The strip of metal and solution together may also be called a half ceil. 

Only potential differences between chemically identical forms of matter are easily measurable, so the two terminals of a galvanic cell must be made of the same material. The cell potential E, also called the electromotive force (emf), is the potential difference between the terminals when they are not connected. Connecting the terminals reduces the potential difference due to internal resistance within the galvanic cell. The drop in the emf increases as the current increases. The current from one terminal to the other through the load (or resistance) flows in the direction opposite the electron flow. Since electrons in tire anode have higher potential energy than those in the cathode, electrons flow through the load from the anode to the cathode. 

The standard state cell potential is simply the sum of the standard state potentials of the corresponding half reactions. The cell potential for a galvanic cell is always positive a galvanic cell always has chemical energy that can be converted to work. The real cell potential depends upon the half reactions, the concentrations of the reactants and products, and the temperature. 

Below is an example of a simple galvanic cell with the standard hydrogen electrode. Hydrogen gas is bubbled over the platinum plate. The platinum acts as a catalyst in the production of H+ ions. The half reaction is shown. The platinum plate carries an electron through the wire to the silver strip. Ag+ accepts the electron converting it to solid silver and allowing a chloride ion to solvate into the aqueous solution. 

by convention, the oxidation potential of hydrogen is zero, the cell potential of any electrode used in conjunction with the SHE will be exactly equal to the reduction potential of the half reaction occurring at the other electrode. Thus, some half reaction reduction potentials can be measured using the SHE. 

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The galvanic cell studied (shown in Fig. 5.24) utilizes a highly porous solid electrolyte that is a eutectic composition of LiCl and KCl. This eutectic has a melt temperature of 352 °C and has been carefully studied in prior electrochemical studies. Such solid electrolytes are typical of thermal battery technology in which galvanic cells are inert until the electrolyte is melted. In the present case, shock compression activates the electrolyte by enhanced solid state reactivity and melting. The temperature resulting from the shock compression is controlled by experiments at various electrolyte densities, which were varied from 65% to 12.5% of solid density. The lower densities were achieved by use of microballoons which add little mass to the system but greatly decrease the density. 

Fig. 5.24. The electrochemical properties of the galvanic cell shown have been studied under high pressure shock compression. The cell is composed of anode, electrolyte, and cathode materials studied in independent applications of thermal batteries.

The galvanic cell invented by Volta in 1800 was composed of two dissimilar metals in contact with mois-... 

Italian physicist Alessandro Volta demonstrates the galvanic cell, also known as the voltaic cell. 

Conditions necessary for the onset of corrosion are quite often provided by heterogeneities. These heterogeneities may very well exist within the metal or alloy or may be imposed by external factors. These heterogeneities can give rise to variations in potential on a metal surface immersed in an electrolytic fluid. The galvanic cell thus formed gives rise to flow of current that accompanies corrosion [188]. 

As seen from Tables 23 and 21 the ion pair (K+ + Cl") increases the viscosity of methanol but diminishes that of water. We recall that the values for the entropy of solution in Table 29 show a parallel trend in the galvanic cells of Sec. 112 placed back to back, this difference in ionic entropy between aqueous and methanol solutions would alone be sufficient to give rise to an e.m.f. We must ask whether this e.m.f. would be in the same direction, or in the direction opposite to the e.m.f. that would result from a use of (199). 

From the chemical viewpoint, the galvanic cell is a current source in which a local separation of oxidation and reduction process exists. This is explained below by the example of the Daniell element (Fig. 3). Here the galvanic cell contains copper as the positive electrode, zinc as the nega-... 

In the predominantly electronically conducting electrodes it is the chemical diffusion of the ions which controls the electrical current of the galvanic cell. This includes the internal electric field which is built up by the simultaneous motion of ions and electrons to establish charge neutrality [14] ... 

Figure 5.43. UP-spectra of Ag YSZ electrodes for (a) cathodic and (b) anodic polarization of the galvanic cell Ag YSZ Pd,PdO at 547°C. In (b), the shift of the Fermi edge of the small silver particles on YSZ under anodic polarization is shown enlarged (5x).24 Reprinted with permission from Wiley-VCH.

FIGURE 12.5 The cell potential is measured with an electronic voltmeter, a device designed to draw negligible current so that the composition of the cell does not change during the measurement. The display shows a positive value when the + terminal of the meter is connected to the cathode of the galvanic cell. The salt bridge completes the electric circuit within the cell. 

C19-0016. Use tabulated thermodynamic data to verily that the galvanic cell of Section Exercise is spontaneous in the direction written. 

C19-0111. For the galvanic cell in Problem, which solution concentration would have to be reduced, and to what concentration, to reduce the cell potential to 0.0 V ... 

A nonzero OCV of a galvanic cell implies that the potential of one of the electrodes is more positive than that of the other (there is a positive and a negative electrode). For the galvanic cell without transference, the OCV can be written as... 

FIGURE 2.2 Directions of current flow when the galvanic cell functions as a battery (a) and... 

One of the features found at interfaces between two electrolytes (a) and ( 3) is the development of a Galvani potential, between the phases. This potential difference is a component of the total OCV of the galvanic cell [see Eq. (2.13)]. In the case of similar electrolytes, it is called the diffusion potential and can be determined, in contrast to potential differences across interfaces between dissimilar electrolytes. 

When analyzing hot gases for their hydrogen snlfide content, for instance, a solid electrolyte sensor consisting of the galvanic cell... 

This can be accomplished by applying an electrical potential in the external circuit in such a manner that an emf occurs in opposition to that of the galvanic cell. The opposing emf is varied by means of a potentiometer until the current flow from the cell is essentially zero. Under these conditions, the cell may very well approach reversibility. This is readily tested by changing the direction of the current and allowing an infinitesimally small current flow in the opposite direction. If the cell is reversible, the cell reaction will proceed in the reverse direction with the same efficiency as in the forward direction. For a reversible reaction... 

In case (a), the galvanic cell under non-faradaic conditions, one obtains an emf of 0.34 - (-0.76) = 1.10 V across the Cu electrode ( + pole) and the Zn electrode (- pole). In case (b), the galvanic cell with internal electrolysis, the electrical current flows in the same direction as in case (a) and the electrical energy thus delivered results from the chemical conversion represented by the following half-reactions and total reaction, repsectively ... 

In case (c), a voltage opposite to and higher than the emf of the galvanic cell is imposed as a consequence, the current flow and hence also the electrochemical reactions are reversed, which means that half-reaction 1 becomes an anodic oxidation and half-reaction 2 is a cathodic reduction, so that Zn is deposited instead of Cu. 

The electrode is considered to be a part of the galvanic cell that consists of an electronic conductor and an electrolyte solution (or fused or solid electrolyte), or of an electronic conductor in contact with a solid electrolyte which is in turn in contact with an electrolyte solution. This definition differs from Faraday s original concept (who introduced the term electrode) where the electrode was simply the boundary between a metal and an electrolyte solution. 

It should be noted that the reversibility of the galvanic cell has so far been considered from a purely thermodynamic point of view. Reversible electrode processes are sometimes considered in electrochemistry in a rather different sense, as will be described in Chapter 5. 

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. 

Figure 16.2 shows a comparison of a galvanic and electrolytic cell for the Sn/Cu system. On the left-hand side of Figure 16.2, the galvanic cell is shown for this system. Note that this reaction produces 0.48 Y But what if we wanted the reverse reaction to occur, the nonsponta-neous reaction This can be accomplished by applying a voltage in excess of 0.48 V from an external electrical source. This is shown on the right-hand side of Figure 16.2. In this electrolytic cell, electricity is being used to produce the nonspontaneous redox reaction.

Calculate the standard cell potential for the galvanic cell in which the following reaction occurs. 

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