Formation, liquid junction potential

The only process occurring in a Hquid junction is the diffusion of various components of the two solutions in contact with it. The various mobilities of the ions present in the Hquid junction lead to the formation of an electric potential gradient, termed the diffusion potential gradient. A potential difference, termed the liquid-junction potential, A0x,. is formed between two solutions whose composition is assumed to be constant outside the Hquid junction. 

When a constant ionic strength of the test solution is maintained and the reference electrode liquid bridge is filled with a solution of a salt whose cation and anion have similar mobilities (for example solutions of KCl, KNO3 and NH4NO3), the liquid-junction potential is reasonably constant (cf. p. 24-5). However, problems may be encountered in measurements on suspensions (for example in blood or in soil extracts). The potential difference measured in the suspension may be very different from that obtained in the supernatant or in the filtrate. This phenomenon is called the suspension (Pallmann) effect [110] The appearance of the Pallmann effect depends on the position of the reference electrode, but not on that of ISE [65] (i.e. there is a difference between the potentials obtained with the reference electrode in the suspension and in the supernatant). This effect has not been satisfactorily explained it may be caused by the formation of an anomalous liquid-junction or Donnan potential. It... 

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. 

Izutsu et al. studied the compiexing of Na" in acetonitrile solution with various protic and aprotic solvents using an ion-sensitive glass electrode. Parker s assumption of negligible liquid junction potential with an tetraethylammonium picrate salt bridge was adopted and found to be valid, even when water was added. The formation constants increased in the order methanol < H2O < DMF < NJ -dimethylace-tamide DMSO < HMPA. 

Three sources of the formation of the liquid junction potential Ey may be distinguished ... 

In order to use Eq. (6) in electrochemical studies of ion solvation, the problems related to the liquid junction potential have been presented in Sec. 2.2.2. Equation (11) may also be used in such studies, but the measured potentials should be expressed versus the same solvent-independent reference electrode. Such electrodes, which give a basis for the formation of a uniform scale of electrode potentials in different solvents, are available. A scale of this kind is also needed for a correlation of equilibrium potentials (E° 1/2) of electrode systems in various solvents. 

In this case the formal potential includes correction factors for activity coefficients, acid-base phenomena (hydrolysis of Fe " to FeOH " ), complex formation (sulfate complexes), and the liquid junction potential used between the reference electrode and the half-cell in question. Although the correction is strictly valid only at the single concentration at which the potential has been determined, formal potentials may often lead to better predictions than standard potentials because they represent quantities subject to direct experimental measurement. 

Note, however, that the half-wave potential Ey is usually similar but not exactly equivalent to the thermodynamic standard potential First, the product of reduction may be stabilized by amalgam formation in metal ion reductions second, there will always be a small liquid junction potential in electrochemical cells of this type that should be corrected for and hnally, it can be shown that the potential Ey is the sum of two terms ... 

It is not a simple task to present precise pH and concentration dependence measurements, because changes in the pH and concentration of electrolyte refer to the solution phase, and thermodynamic relationships with respect to the film can only be obtained if an equilibrium exists between the film and solution phases. Constant ionic strength and constant pH should be maintained. Similarly, a liquid junction potential can distort observations. The formation of gas bubbles at extreme potential limits may cause ohmic drop and may also destroy the film. The pH values and ionic strength further assumes importance in light of experimental observations by Asturias et al. [73]. That polyaniline acts as an anion exchanger at pH 2 and 3 in its conductive state and its Donnan potential becomes... 

The hydrated layer has finite thickness, therefore the exchanging ions can diffuse inside this layer, although their mobility is quite low compared to that in water (n 10-11cm2s-1 V-1). As we have seen in the liquid junction, diffusion of ions with different velocities results in charge separation and formation of the potential. In this case, the potential is called the diffusion potential and it is synonymous with the junction potential discussed earlier. It can be described by the equation developed for the linear diffusion gradient, that is, by the Henderson equation (6.24). Because we are dealing with uni-univalent electrolytes, the multiplier cancels out and this diffusion potential can be written as... 

The silver-silver chloride electrode. The silver chloride reference electrode is not generally suitable as an electrode of the second kind because of the large solubility of AgCl in many aptotic solvents from formation of anionic complexes with chloride ion. In many cases the silver chloride solubility will essentially be that of the added chloride. This contributes significantly to the junction potential in cells with liquid junction and makes the electrode unsuitable for precise potentiometric work. 

The electrochemical cell is completed by the external reference electrode, an Ag/AgCl or calomel electrode, which is in contact with the specimen by a liquid/liquid junction or salt bridge of KCl or sodium formate. The potential difference across the cell is logarithmically related to the activity of free calcium ions in the sample by Nernsfs equation. By convention, free calcium is converted from activity to concentration with its activity coefficient, which is itself dependent on ionic strength. 

Reproducible and constant potentials of cells involving liquid junctions may be obtained with "flowing junctions, which have been studied by Lamb and Larson,0 and Maclnnes and Yeh10 and others. The method of formation of flowing junctions is shown in Fig. 4. The boundary which forms at A results from the meeting of two slowly flowing streams of solutions from tubes B and C, These streams also pass by the tubes containing the electrodes. The heavier solution must obviously enter the boundary from below. The nature of the boundary... 

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