Total cell potential

An increase in conductivity usually increases T because it increases the proportion of polarisation in the total cell potential difference and lowers the ratio ( V A )/( V - AEj). Changing the conductivity of an acid copper bath with sulphuric acid produced the following result (291 A/m average c.d., P = 5) ... 

There is a good economic reason for this. Look back at the Butler Volmer equation (Eq. 7.24) the larger the ifl (Le., the better the catalysis), the smaller the overpotential needed to get a given rate of reaction. However, the smaller the overpotential, the less the total cell potential, and hence the kilowatt hours, to produce a given amount of a substance in an electrochemical reactor. 

Therefore the fraction of the total cell potential due to the junction potential cannot be unambiguously assigned. However, it is possible to estimate junction potentials indirectly or to make calculations based on assumptions about the geometry and distribution of ions in the region of the junction. For a junction between two dilute solutions of the same univalent electrolyte (concentrations C] and C2), the liquid-junction potential is described by... 

We summarize the findings in this section as follows In any redox reaction, the metal-solution potential difference is a function of the metal used, whereas the total cell potential is independent of it. 

The total cell potential E for reaction can be resolved into the respective cell components, as given in Table 1., as follows (35) (36) ... 

Traditionally, electrochemical equilibria are explained in terms of thermodynamic cell potentials. However, in electro analytical applications, such a description is of little use, because one almost always uses a non-thermodynamic measurement, with a reference electrode that includes a liquid junction. It is then more useful to go back to the basic physics of electrochemistry, i.e., to the individual interfacial potential differences that make up the total cell potential. This is the approach we will use here. 

A pure metal can be anodic only if its equilibrium half-cell potential, E M, is less than the half-cell potential of some cathodic reaction, E x, such that the total cell potential (Ex - E" ) causes current to flow as in Fig. 1.6, that is, current away from the anode area as ions in the solution. A few representative anodic reactions are listed in Table 1.3 along with their standard equilibrium half-cell potentials. 

We see from this equation that the actual potential produced by an electrochemical cell involving a hydrogen (or hydronium) ion depends linearly on the pH of the solution. The total cell potential also depends on the activity pf the metal ion in the other halfcell, which usually would be approximately constant, and the activity of molecular hydrogen, which can be controlled by its partiarpressure. Consequently, the primary variation of the cell potential is with pH.-This result suggests that an electrochemical cell can be used to measure the pH of a solution. This is actually done in the laboratory, but by using specially chosen liquid electrodes rather than a hydrogen gas electrode, which is not convenient to use.. ... 

Although we can measure the total potential of this cell (0.76 V), there is no way to measure the potentials of the individual electrode processes. Thus, if we want potentials for the half-reactions (half-cells), we must arbitrarily divide the total cell potential. For example, if we assign the reaction... 

Compare this equation with Eqs. (15.7) and (15.15). By convention, the reference electrode is connected to the negative terminal of the potentiometer (the readout device). The common reference electrodes used in potentiometry are the SCE and the silver/silver chloride electrode, which have been described. Their potentials are fixed and known over a wide temperature range. Some values for these electrode potentials are given in Table 15.3. The total cell potential is measured experimentally, the reference potential is known, and therefore the variable indicator electrode potential can be calculated and related to the concentration of the analyte through the Nemst equation. In practice, the concentration of the unknown analyte is determined after calibration of the potentiometer with suitable standard solutions. The choice of reference electrode depends on the application. For example, the Ag/AgCl electrode cannot be used in solutions containing species such as halides or sulfides that will precipitate or otherwise react with silver. 

OCV, denoted by Eqcv is the voltage between anode and cathode when the MFC is at open circuit. Note that the OCV is different from total cell potential discussed above generally OCV is lower than the total cell potential, due to different sources resulting in overpotentials. 

Compare this equation with Equations 15.7 and 15.15. By convention, the reference electrode is connected to the negative terminal of the potentiometer (the readout device). The common reference electrodes used in potentiometry are the SCE and the silver/silver chloride electrode, which have been described. Their potentials are fixed and known over a wide temperature range. Some values for these electrode potentials are given in Table 15.3. The total cell potential is measured... 

Using the same approach with two flames, Caruana et al. have studied the overall cell potential in the case where the ionic species introduced into the two flames are different [18, 21]. In this case, Li, K and Cs were used [18] as well as Cu [21]. The total cell potential is composed of contributions from the mixed potentials at the two electrodes and the diffusion potential at the junction of the two dissimilar flames. The mixed potential at each electrode surface is determined by the two surface reactions ... 

The potentials of the standard zinc-copper and copper-silver voltaic cells are 1.100 volts and 0.462 volt, respectively. The magnitude of a cell s potential measures the spontaneity of its redox reaction. Higher (more positive) cell potentiak indicate greater driving force for the reaaion as written. Under standard conditions, the oxidation of metallic Zn by Cu ions has a greater tendency to go toward completion than does the oxidation of metallic Cu by Ag+ ions. It is convenient to separate the total cell potential into the individual contributions of the two half-reactions. This lets us determine the relative tendencies of particular oxidation or reduction half-reactions to occur. Such information gives us a quantitative... 

The seawater will be involved in the oxidation half-cell reduction (i.e., it acts as the reductant) with Eox -(0.600 V)= -0.600 V. When the seawater is in equilibrium with the iron system Eceii = rcd + ox = 0 or red = 0.600 V. For nonstandard concentrations at 298K, the total cell potential developed by the iron system when paired with the hydrogen half-cell is given by Eq. (6.26). However, the hydrogen half-cell generates zero electrode potential. Therefore, the electrode potential developed by the iron system is, from Eq. (6.26)... 

Eq for anode and cathode are —0.284 and 0.361 V, respectively (Logan, 2008). Through these formula, with reaction quotient measured, the total cell potential can be calculated. 

The weighting factors are the selectivity coefficients of the membrane toward ions j relative to ion i. The smaller the /fl", the smaller are the contributions of the interfering ions to the total cell potential. The selectivity coefficients can be determined potentiometrically and are often listed in textbooks. Their value is used as a guideline when designing ISE-based potentiometric experiments. 

Total cell potential. The total potential that can be produced by any fuel cell is the difference in the anode and cathode potentials, or E f = Eca, Ea . For the adjusted standard conditions of pH = 7, this is... 

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