Equilibrium galvani potential

According to the exact position of the equilibrium galvani potential for Ca2+-ions on the voltage axis, stimulation or inhibition of the Na+—K+ exchange can be understood. The effect of anions (e.g., MgATP2-) may be similiar as shown in Fig. 4. 

It follows from Eq. (2.6) that the equilibrium Galvani potential depends only on the nature of the two phases (their bulk properties, which are decisive for the values of Jj), not on the state of the interphase (i.e., its size, any contamination present, etc.). 

The values of exchange current density observed for different electrodes (or reactions) vary within wide limits. The higher they are (or the more readily charges cross the interface), the more readily will the equilibrium Galvani potential be established and the higher will be the stability of this potential against external effects. Electrode reactions (electrodes) for which equilibrium is readily established are called thermodynamically reversible reactions (electrodes). But low values of the exchange current indicate that the electrode reaction is slow (kinetically limited). 

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 ... 

At a constant temperature T and pressnre p, the condition of ion transfer equilib-rinm (32.3) is given by the equality of the electrochemical potentials in both phases. This condition yields the Nemst equation for the equilibrium Galvani potential dilference. 

The equilibrium partition of ions present in the system gives rise to the equilibrium Galvani potential difference A (p = y (w) — (o) between the phases w and o (Nernst potential) [7,8]... 

Potential of zero charge Electrode potential on absolute scale Electrode potential at standard conditions Electrode potential at equilibrium Galvani potential... 

The term galvani potential denotes the potential difference between two electrically conducting phases. The equilibrium galvani potential is the galvani potential in the electrochemical equilibrium. 

Under these conditions the chemical standard potentials in both phases are equal, so that one gets for the equilibrium Galvani potential difference ... 

To conclude on the physical meaning of the activation energy, or the lack of it, is not very easy yet. The only safe conclusion is to state that the local driving force at the interface is much higher than that involved in ionic diffusion or ionic conductivity, as soon as we depart from the equilibrium Galvani potential difference. Consequently, linearized equations such as the Nernst-Planck equation do not apply outside the narrow potential range, where the charge transfer resistance can be measured. 

Considering the aforementioned definition of electrochemical potential and solving the equations for A 0 yields the equilibrium Galvani potential (= 0jnetal solution) ... 

Thermodynamics furnishes us with the relationship between the equilibrium Galvani potential at an electrode and the activity in solution of those ions which are able to penetrate the phase boundary, and hence the electric double layer. The difference in standard chemical potentials... 

Without a doubt the most interesting feature of this equation for the analyst is the possibility of being able to explore quickly and directly the activity or concentration of a particular ion i through measurement of the equilibrium Galvani potential change with a suitable electrode. This is the field of direct potentiometry. 

There are further possibilities for the thermodynamicist. At known concentrations, the activity coefficients f cm be evaluated from a=f-c (see Appendix). The standard equilibrium Galvani potential difference can be determined by choosing aj + or... 

In fact, this strong pH-dependence of the equilibrium Galvani potential difference can be observed with the above construction. Such a large pH-dependence can even be found with a suspension of Raney nickel in contact with a normal platinum electrode. Attempts with other metals, which are known not to catalyze reaction (R2), yielded irreproducible results. This example serves to illustrate the requirement of an unobstructed phase transfer, which in this case was coupled with a chemical reaction. 

As shown in Chapter 1.3, the equilibrium Galvani potential of this electrode depends upon the pH of the solution according to ... 

In view of this last possibility, care should be taken that the upper filUhole for the salt bridge electrolyte is not closed with an air-tight seal, so that the hydrostatic pressure of the column of electrolyte is sufficient to allow the solution to flow out. For the same reason, the reference electrode should not be immersed in the test solution so far that the liquid level in the sample solution is higher than that in the reference electrode. There may be ions present in the test solution which upon penetration into the reference electrode are able to alter the equilibrium Galvani potential of the reference electrode half-cell (for example, Br or 1 with an Ag/AgCl internal element). 

In view of all the difficulties associated with liquid junction potentials, it is desirable to construct cells without liquid junctions. In practice this is possible more often than one might think. All that is needed is for the sample solution to contain an ion with constant activity which does not interfere with the ion-selective indicator electrode used, and which can itself be specifically sensed with a second ion-selective electrode. This second ion-selective electrode can then function as a reference electrode, since it fulfills the primary requirement of a good reference electrode, i.e. it has a constant equilibrium Galvani potential at the interface electrode/solution. With such a set-up problems of a technical nature arise only if the resistance of the ion selective electrode connection with the reference electrode jack is too large. In this case an ion-meter with a high-ohmic differential input must be used. Such devices are already sold commercially (see Fig. 43). 

If the test solution does not contain any detectable ion in a constant, stable amount, as is the case in most analytical work, then a suitable compound which does not interfere with the indicator electrode can often be added before the EMF measurement. The amount of such an ion-pair (an ion can only be added as a cation-anion ion-pair) needed to produce a constant equilibrium Galvani potential at the second electrode can lie between 10 and 10 M, if this ion is originally present in the test solution only in traces. For cases where the individual sample solution contains vari-... 

It has already been pointed out that under suitable conditions the normal hydrogen electrode provides the most stable equilibrium Galvani potential, and is therefore to be recommended for very precise measurements. In such applications the hydrogen electrode is not always immersed in an acid solution with an = 1, since this relatively high ion concentration leads to a relatively large liquid junction potential. This cannot be fully compensated for even with the use of a suitable salt bridge (a liquid junction potential of about 5mV can be calculated for the system 0.1 M HCl sat. 

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