Galvanic cells limitations

If, however, solid electrolytes remain stable when in direct contact with the reacting solid to be probed, direct in-situ determinations of /r,( ,0 are possible by spatially resolved emf measurements with miniaturized galvanic cells. Obviously, the response time of the sensor must be shorter than the characteristic time of the process to be investigated. Since the probing is confined to the contact area between sensor and sample surface, we cannot determine the component activities in the interior of a sample. This is in contrast to liquid systems where capillaries filled with a liquid electrolyte can be inserted. In order to equilibrate, the contacting sensor always perturbs the system to be measured. The perturbation capacity of a sensor and its individual response time are related to each other. However, the main limitation for the application of high-temperature solid emf sensors is their lack of chemical stability. 

The list of error sources continues, just to mention a few the ionic strength of the sample, the liquid-junction and residual liquid-junction potentials, temperature effects, instabilities in the galvanic cell, carryover effects, improper use of available corrections (e.g., for pH-adjusted ionized calcium or magnesium). An error analysis goes beyond the limited scope of this paper more details are presented elsewhere [10]. 

For the case where diffusion of the corrosive ions is the rate controlling reaction, it has been found that P = po (1 + Ac/Aa) where p is the penetration that is proportional to the corrosion rate and p0 is the corrosion rate of the less noble uncoupled metal Ac and Aa are the areas of the more noble and active metal respectively (Uhlig and Revie, pp. 101-103).7 If a galvanic cell is not avoidable, a large anode and a limited size of cathode are recommended. Stagnant conditions and weak electrolytes may lead to pitting in spite of the large area of the exposed active metal. 

The electromotive force is the limiting value of the -> electric potential difference of a galvanic cell when the current through the external circuit of the cell becomes zero, all local charge transfer equilibria across phase boundaries, except at the electrolyte - electrolyte junctions, and local chemical equilibria within phases being established. 

Concentration overpotential is also observed when the surface concentration is increased over the bulk concentration. The most common example is the anodic dissolution of a metal. Suppose that, after part of the metal ion has been plated out from the solution in the above example, the applied emf is decreased to a value below the reversible back emf. The ceU now will operate as a galvanic cell, with the metal-plated electrode acting as the anode. The metal ion concentration at the anode surface becomes greater thah the bulk concentration of metal ion. As anodic polarization is increased, however, there is no limit to the surface concentration of metal ion except that imposed by the solubility of a salt. Since the surface concentration would have to be 10 times the bulk concentration to produce a concentration overpotential of 0.059/n V, the anodic concentration overpotential for metal dissolution is generally small unless the bulk concentration is low. 

A concentration cell is a limited form of a galvanic cell with a reduction half re-action taking place in one half cell and the exact reverse of that half reaction taking place in the other half cell. 

Transference numbers will also be found useful in obtaining precise values of the activities of ion constituents. It was another of Arrhenius tacit assumptions that ion concentrations may be used without error in the law of mass action. To investigate the limits of validity of that assumption, and to lay a foundation for the modern interionic attraction theory of solutions, it is necessary to consider the thermodynamics of solutions, and of the galvanic cell, subjects which are discussed in Chapters 5 and 6. 

Results of the determination of CO, SO, NO, and other gases with carbonates, sulfates and nitrates have been already reported by the inventors of the method [29]. The lower limit for measurements of traces with solid oxoanionic electrolytes in galvanic cells is given by the decomposition pressures of the solid electrolytes at the cell temperature. Trace measurements however are also influenced largely by adsorption equilibria on the surface of the used materials. Installations made of unused annealed materials have to be used for such measurements. The lowest SO, concentrations measured at 840 °C run to 4 10 " [94]. 

Each of the above three methods employs a different data base. Most of the property values required for the evaluation of in Equations 7-9 have been experimentally determined for III—V systems and these three relationships can be used as a test for thermodynamic consistency. The first method, Equation 7, is most reliable at or near the binary compound melting temperature. As the temperature is lowered below the melting one, uncertainties in the extrapolated stoichiometric liquid heat capacity and component activity coefficients become important. The second method, Equation 8, is limited to the temperature range in which an experimental determination of AG. is feasible (e.g., high temperature galvanic cell). Method II is also valuable for "pinning down" the low temperature values of 0yp. Method III is the preferred procedure when estimating solution model parameters from liquidus data. Since the activity coefficients of the stochiometric liquid... 

Both resistance of the electrolyte and polarization of the electrodes limit the magnitude of current produced by a galvanic cell. For local-action cells on the surface of a metal, electrodes are in close proximity to each other consequently, resistance of the electrolyte is usually a secondary factor compared to the more important factor of polarization. When polarization occurs mostly at the anodes, the corrosion reaction is said to be anodically controlled (see Fig. 5.7). Under anodic control, the corrosion potential is close to the thermodynamic potential of the cathode. A practical example is impure lead immersed in sulfuric add, where a lead sulfate film covers the anodic areas and exposes cathodic impurities, such as copper. Other examples are magnesium exposed to natural waters and iron immersed in a chromate solution. 

These few examples of the application of solid state galvanic cells in the field of solid state reactions can only present a very limited view of this important area of solid state science. The examples were chosen primarily in order to demonstrate the principles according to which solid state research in thermodynamics and kinetics should be conducted with the use of electrochemical tools and methods. Such measurements are only possible because of the existence of suitable solid electrolytes. The most important of these are Zr02(-f CaO) and Th02(+Y2 03) for oxygen, silver halides and Ag4Rbl5 for silver, copper halides for copper, some glasses in which certain ions are dissolved, and p — Al2 03(-hNaO) for sodium. 

Solid electrolytes can withstand higher temperatures than liquids which is important for the Carnot efficiency of a thermo-galvanic cell. In the case of devices featuring a liquid electrolyte and a redox couple, the electroactive species diffuse from one electrode to the other. To have high steady state current, diffusion gradient should be as steep as possible which means bringing the electrodes close to each other. There results an increase in thermal loss by conduction. On the other hand, with a solid electrolyte such as 3-A 20s the electroactive species migrate in the electrolyte where it is the only possible current carrier. Consequently the current will not be limited by mass diffusion but by heat diffusion in metallic electrodes or by the electrical resistance of the solid electrolyte. 

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