Galvani potential difference electrolyte

In addition to the case of a metal in contact with its ions in solution there are other cases in which a Galvani potential difference between two phases may be found. One case is the innnersion of an inert electrode, such as platinum metal, into an electrolyte solution containing a substance S that can exist m either an oxidized or reduced fomi tlirough the loss or gain of electrons from the electrode. In the sunplest case, we have... 

The maximum photocurrent occurs simultaneously with the light perturbation within the millisecond time scale, except at rather positive Galvani potential difference where phase shift associated with photo-oxidation of the supporting electrolyte (TPBCP) and i uCdi attenuation is observed. 

Figure 11 analyzes the effect of the electrolyte concentration in the aqueous phase, for fixed electrolyte concentrations in the organic phase c" = 10 mM and 50 mM, and a Galvani potential difference/= 5. These theoretical results are in agreement with the observations in Ref. 13. Moreover, it is shown that a deerease in the eleetrolyte eoneen-tration in the organic phase has a similar effeet to a deerease in the eleetrolyte eoneentra-tion in the aqueous phase. 

If two electrolyte solutions in immiscible solvents are in contact and contain a common ion Bz+ and the anions are not soluble in the opposite phase, then the relationship for the Galvani potential difference follows from Eq. (3.1.5) ... 

When a solid electrolyte is in contact with an electrolyte solution, with common anion Az (z <0), then the Galvani potential difference between the solid electrolyte and the solution is given by the equation... 

So far, a cell containing a single electrolyte solution has been considered (a galvanic cell without transport). When the two electrodes of the cell are immersed into different electrolyte solutions in the same solvent, separated by a liquid junction (see Section 2.5.3), this system is termed a galvanic cell with transport. The relationship for the EMF of this type of a cell is based on a balance of the Galvani potential differences. This approach yields a result similar to that obtained in the calculation of the EMF of a cell without transport, plus the liquid junction potential value A0L. Thus Eq. (3.1.66) assumes the form... 

The procedure described in the preceding section can form a basis for unambiguous determination of the Galvani potential difference between two immiscible electrolyte solutions (the Nernst potential) considering Eqs (3.1.22) and (1.4.34), e.g. for univalent ions,... 

Potential differences at the interface between two immiscible electrolyte solutions (ITIES) are typical Galvani potential differences and cannot be measured directly. However, their existence follows from the properties of the electrical double layer at the ITIES (Section 4.5.3) and from the kinetics of charge transfer across the ITIES (Section 5.3.2). By means of potential differences at the ITIES or at the aqueous electrolyte-solid electrolyte phase boundary (Eq. 3.1.23), the phenomena occurring at the membranes of ion-selective electrodes (Section 6.3) can be explained. 

The overall Galvani potential difference between the bulk of the semiconductor and the bulk of the electrolyte solution can be separated into three parts ... 

Fig. 5.29 ITIES polarogram obtained with a dropping electrolyte electrode. Curve 1 in 0.05 M LiCI + 1 M MgS04 (water) and 0.05 M Bu4NBPh4 (NB) curve 2 in 1 +0.5 mM Me4NCI (water). Dashed curve curve 2 - curve 1. The potential E is referred to the Galvani potential difference between 0.05 M Bu4NCI (water) and 0.05 M Bu4NBPh4 (NB).

Electrolyte junction — A liquid junction is the region of contact of two different -> electrolyte solutions kept apart by a porous -> diaphragm, such as sintered glass or ceramic. At the contact a -> Galvani potential difference appears, which is called -> liquid junction potential (Ej). In the case of two solutions of the same electrolyte, but with different concentrations (c(a) and c(/S)), the potential Ej is defined by the equation Ej = (t+-t-) ln ry, where t+ and t are - transport numbers of the cation and anion, respectively. If the concentration of one of the ions is the same in both solutions, but the other ion differs (e.g., NaCl and KC1), the potential Ej is given by the Henderson equation, which is reduced to the Lewis-Sargent relation for a 1 1 electrolyte Ej = ln, where A (/3) and A (a) are molar conductivities of the electrolytes in the com-... 

Rl/+X from w to o, a s are the mean activities, y s are the mean activity coefficients, and cRX s are the equilibrium electrolyte concentrations. A non-thermodynamic assumption has to be introduced to construct the scale of the inner (Galvani) potential differences from the partition measurements. The most commonly used hypothesis is that the tetraphenylarsonium cation (Ph4As+) and the tetraphenylborate anion (PI14 B ) have equal standard Gibbs energies of transfer for any combination of two solvents [iv]. This assumption enables the evaluation of AG 1 and AG j from the partition mea-... 

The Galvani potential difference may not be found experimentally because single ion quantities are not subject to experimental determination. However, because of the requirement of electroneutrality, the concentration of cations and anions in each phase is equal. Furthermore, in the limit of dilute solutions, the activities of these ions are equal in each phase. Thus, by equating (8.9.5) and (8.9.6), one obtains for dilute electrolyte concentrations... 

In the study of the interface with two immiscible electrolyte solutions (ITIES), considerable attention has been focused on the estimation of the Galvani potential difference at the water oil interface on the basis of a reasonable extra-thermodynamic assumptions. The discussion of these estimates is often made in terms of the ionic distribution coefficient, which is defined on the basis of equations (8.9.5) and (8.9.6). Generalizing this equation for the ot P interface at which ion i with charge z,- is transferred, one may write... 

An ITIES is formed between water and nitrobenzene (NB) using electrolytes with the picrate (Pic ) anion at the same activity (0.1 M). The electrolyte in water is Li Pic and that in NB, tetraphenylarsonium (TA" ") picrate. Given that the standard Gibbs energy of transfer of Pic is -4.6 kJmoU [18], estimate the Galvani potential difference at the interface. Use the data in table 8.11 to estimate the activity of Li" " in NB. 

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