Potential liquid-junction phase

In fact, some care is needed with regard to this type of concentration cell, since the assumption implicit in the derivation of A2.4.126 that the potential in the solution is constant between the two electrodes, caimot be entirely correct. At the phase boundary between the two solutions, which is here a semi-pemieable membrane pemiitting the passage of water molecules but not ions between the two solutions, there will be a potential jump. This so-called liquid-junction potential will increase or decrease the measured EMF of the cell depending on its sign. Potential jumps at liquid-liquid junctions are in general rather small compared to nomial cell voltages, and can be minimized fiirther by suitable experimental modifications to the cell. 

Heterogeneous ET reactions at polarizable liquid-liquid interfaces have been mainly approached from current potential relationships. In this respect, a rather important issue is to minimize the contribution of ion-transfer reactions to the current responses associated with the ET step. This requirement has been recognized by several authors [43,62,67-72]. Firstly, reactants and products should remain in their respective phases within the potential range where the ET process takes place. In addition to redox stability, the supporting electrolytes should also provide an appropriate potential window for the redox reaction. According to Eqs. (2) and (3), the redox potentials of the species involved in the ET should match in a way that the formal electron-transfer potential occurs within the potential window established by the transfer of the ionic species present at the liquid-liquid junction. The results shown in Figs. 1 and 2 provide an example of voltammetric ET responses when the above conditions are fulfilled. A difference of approximately 150 mV is observed between Ao et A" (.+. ... 

In initial ET rate measurements, both the NB and aqueous phases contained 0.1 M TEAP, enabling measurements to be made with a constant Galvani potential difference across the liquid junction. In these early studies, the concentration of Fc used in the organic phase (phase 2) was at least 50 times the concentration of the electroactive mediator in the aqueous phase which contained the probe UME (phase 1). This ensured that the interfacial process was not limited by mass transport in the organic phase, and that the simple constant-composition model, described briefly in Section IV, could be used. 

The membrane phase m is a solution of hydrophobic anion Ax (ion-exchanger ion) and cation Bx+ in an organic solvent that is immiscible with water. Solution 1 (the test aqueous solution) contains the salt of cation Bx+ with the hydrophilic anion A2. The Gibbs transfer energy of anions Ax and A2 is such that transport of these anions into the second phase is negligible. Solution 2 (the internal solution of the ion-selective electrode) contains the salt of cation B with anion A2 (or some other similar hydrophilic anion). The reference electrodes are identical and the liquid junction potentials A0L(1) and A0L(2) will be neglected. 

Em, the corresponding liquid junction potential, is called the membrane potential or Donnan potential. Ideally Em changes in a Nernstian fashion with the activity of the ion in one of the phases, the activity in the other phase being held constant. This is the basis of the functioning of ion-selective electrodes (Chapter 13) and, to a good approximation, of biomembranes (Chapter 17). 

The functioning of an ion-selective electrode (ISE)4 6 is based on the selectivity of passage of charged species from one phase to another leading to the creation of a potential difference. The fundamental theoretical formulation is the same as that developed for liquid junction potentials (Section 2.11). In the case of ISEs one phase is the solution and the other a membrane (solid or liquid in a support matrix). The membrane potential, Em, for an ion, i, of charge zf is... 

The further exact application of (22) to the experimental data really requires consideration of the effects of changes in composition on the liquid-liquid junction potential between phase j8 and the reference electrode. Neglecting any changes at this junction, however, so that dE is still equal to —d(A ), and assuming that the solution is ideal, so that = RTdlogcP and remem-... 

A single vertical bar ( ) should be used to represent a phase boundary, a dashed vertical bar ( ) to represent a junction between miscible liquids, and double, dashed vertical bars ( ) to represent a liquid junction, in which the liquid junction potential has been assumed to be eliminated (see also -> concentration cells). 

Interface between two liquid solvents — Two liquid solvents can be miscible (e.g., water and ethanol) partially miscible (e.g., water and propylene carbonate), or immiscible (e.g., water and nitrobenzene). Mutual miscibility of the two solvents is connected with the energy of interaction between the solvent molecules, which also determines the width of the phase boundary where the composition varies (Figure) [i]. Molecular dynamic simulation [ii], neutron reflection [iii], vibrational sum frequency spectroscopy [iv], and synchrotron X-ray reflectivity [v] studies have demonstrated that the width of the boundary between two immiscible solvents comprises a contribution from thermally excited capillary waves and intrinsic interfacial structure. Computer calculations and experimental data support the view that the interface between two solvents of very low miscibility is molecularly sharp but with rough protrusions of one solvent into the other (capillary waves), while increasing solvent miscibility leads to the formation of a mixed solvent layer (Figure). In the presence of an electrolyte in both solvent phases, an electrical potential difference can be established at the interface. In the case of two electrolytes with different but constant composition and dissolved in the same solvent, a liquid junction potential is temporarily formed. Equilibrium partition of ions at the - interface between two immiscible electrolyte solutions gives rise to the ion transfer potential, or to the distribution potential, which can be described by the equivalent two-phase Nernst relationship. See also - ion transfer at liquid-liquid interfaces. 

As for semiconductor/metal contacts, a change in the Fermi level of the liquid phase should result in a different amount of charge transferred across the semicondnctor/liqnid junction. For semiconductor/liquid junctions, the important energetic trends for a series of different liqnid contacts can thns be determined by measuring the solntion redox potential relative to a standard reference electrode system. Within this model, solutions with more positive redox potentials shonld indnce greater charge transfer in contact with n-type semicondnctors. 

Reviewing all the results, we may say that there was reasonable agreement between theory and experiment except when the electrolyte concentration in the oil phase was very low. It was concluded that the variations in liquid junction potential had been made negligible, and when the same oil phase was used throughout the cell, the liquid-junction potentials themselves were probably very small. 

When the current does not flow through battery the measurable diflerence in electric potential between the terminals of the two electrodes is the result of all the equilibrium potential differences at the interphase between the conducting phases in contact. In the example of the Daniell cell, with both electrodes having copper terminals, there are three interfacial potential differences (apart from the small liquid junction potential difference at the contact between the two electrolyte phases) one potential difference at the contact between the zinc rod and the copper terminal (Zn/Cu) and two potential differences at the metal-solution interphases (Zn/Zn + and Cu/Cu +), which are mainly due to the charge transfer processes. 

In a diagram of a cell a single vertical line conventionally represents a phase boundary at which a potential difference is taken into account. A double vertical line represents a liquid junction at which the potential difference is ignored or is considered to be eliminated by an appropriate salt bridge. For example, a cell consisting of zinc and copper half-cells can be expressed by... 

By convention, a single vertical line indicates a phase boundary, or interface, at which a potential develops. For example, the first vertical fine in this schematic indicates that a potential develops at the phase boundary between the copper electrode and the copper sulfate solution. The double vertical line represents two phase boundaries, one at each end of the salt bridge. A liquid-junction potential develops at each of these interfaces. The junction potential results from differences in the... 

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