Galvanic cells concentration effects

This effect appears to be of importance in the case of normal galvanic cells, the electromotive forces of which depend on the concentration of solutions in equilibrium with depolarising solids such as calomel or mercurous sulphate. The exact relationships are, unfortunately, not yet wholly elucidated. 

Having introduced matters pertaining to the electrochemical series earlier, it is only relevant that an appraisal is given on some of its applications. The coverage hereunder describes different examples which include aspects of spontaneity of a galvanic cell reaction, feasibility of different species for reaction, criterion of choice of electrodes to form galvanic cells, sacrificial protection, cementation, concentration and tempera lure effects on emf of electrochemical cells, clues on chemical reaction, caution notes on the use of electrochemical series, and finally determination of equilibrium constants and solubility products. 

From this equation it follows that addition of substances to the solutions in a galvanic cell can only affect its e.m.f., if they produce alterations in the concentrations of the substances taking part in the cell reaction. This is the case in a very marked degree when they form complex salts with the dissolved metallic salts. Thus the addition of cyanide of potassium to a Daniell cell diminishes its e.m.f., as the cyanide removes the copper ions from the solution to a much greater extent than the zinc ions. Under certain conditions this diminution may be so great as to reverse the sign of the e.m.f., namely, when the second term of the equation is more negative than the first is positive. Cells of this kind have been described by Hittorf. The addition of substances such as sodium chloride, sulphuric acid, and other substances which do not form complexes has no effect (in dilute solutions at least) on the e.m.f. of the cell. 

The Importance of Concentration Polarization As noted earlier, concentration polarization occurs when the effects of diffusion, migration, and convection are insufficient to transport a reactant to or from an electrode surface at a rate that produces a current of the magnitude given by Equation 22-2. Concentration polarization requires applied potentials that are larger than calculated from Equation 22-2 to maintain a given current in an electrolytic cell (see Figure 22-2). Similarly, the phenomenon causes a galvanic cell potential to be smaller than the value predicted on the basis of the theoretical potential and the IR drop. 

Two subtle corrosion effects can occur when a single metal is in contact with an electrolyte -differential aeration and crevice corrosion. Differential aeration can cause corrosion when no obvious galvanic cells are in evidence. To illustrate this effect, suppose we have a cell with a copper anode and cathode. If the concentration of the electrolyte and the temperature of each cell compartment is the same, no potential is generated and no corrosion occurs. However, bubble O2 into the one compartment, which becomes the cathode compartment, and corrosion will occur in the other, which forms the anode compartment. Differential aeration is, in fact, a concentration effect, and can be understood by using the Nernst equation. Electrons will flow from anode to cathode and the anode will corrode. 

Impurities have a tolerance limit, below which their detrimental effect is insignificant. For an impurity, its tolerance limits can be associated with its solubility in a Mg alloy. When these impurities are below a critical concentration (e.g. their solubility in the matrix phase), they are present in the form of solutes in Mg solid solutions. No micro-galvanic cells between the impurities and Mg matrix can be formed and hence they have no significant detrimental effect. It has been claimed that there is a rough correspondence between critical concentrations and the solubility of some elements in Mg alloys (Sheldon Roberts, 1960). Liu et al. (2008) calculated that Fe tolerance limit in Mg corresponds well to the solubility of Fe in Mg. 

Welded joints, because they can have elevated stress levels (from residual stress effects and stress concentrations), geometrical discontinuities, complex metallurgical structures, and possible galvanic cells (preferential weld corrosion)... 

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 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 Gibbs in the 1870s provided the foundation for the Nernst equation. The latter provides a quantitative relationship between potential and the ratio of effective thermodynamic concentrations [activities] for a redox couple [ox]l[red ) and is the basis for potentiometry and poten-tiometric titrations. 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. Another milestone in the development of potentiometric measurements was the introduction of the hydrogen electrode for the measurement of hydronium-ion concentrations 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. 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. The use of these electrodes and the potentiometric measurement of pH continue to be among the most important applications of electrochemistry. 

In Fig. 48 we sketched a suspension and its equilibrium liquid separated by a membrane. In both the solutions a reversible electrode, H (for instance a hydrogen electrode) and a salt bridge, S, with a calomel electrode are inserted. The Donnan potential is measured between the two salt bridges. The sol-conccntration effect is usually given as the difference in E.M.F. of the galvanic cells with the sol and the equilibrium liquid. As the two reversible electrodes have necessarily the same potential, the sol-concentration effect and the Donnan potential are identical, and what is true for one of them is as true for the other. We have treated the two effects separately, because the sol-concentra- = 0) is equal to Donnan potential. tion effect is often met in cases where one would... 

The distribution of cation concentration across the cut-edge is in agreement with studies of a Fe/Zn galvanic couple [13], Diffusional effects are in evidence in figure 2(b) where the highest iso-contour of cation concentration can be seen to exist within a pit formed by dissolution from a much smaller dendrite. A reduced zinc loss can be seen to be offset by the more tortuous route from the pit, impeded further still by closer proximity of cells containing corrosion product. 

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