Types of liquid junctions

There are three ways in which this physical separation can be achieved  

Under these conditions the current is carried in solution by ions migrating in the solutions and also through the narrow tube, or the porous pot, or through the salt bridge. Such cells do not operate reversibly in a rigorous manner because the processes occurring at the junctions will contribute to the thermodynamic quantities. Such cells are called cells with liquid junctions. 

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Fig. 2.2. Various types of liquid junction with restrained diffusion ...

Fig. 6.11 Outflow of internal filling solution from various types of liquid junctions stabilizes the Ej, but contaminates the sample...

Flowing junction — is a special kind of - liquid junction made in such a way that the two electrolyte solutions forming the junction are flowing in a vertical tube so that the upward flow of the heavier electrolyte meets the downward flow of the lighter electrolyte solution. Lamb and Larson have developed this type of liquid junction and they have discovered that the flowing junction gives... 

The solution is given for the case of a smeared-out boundary and linear spatial distributions of concentrations. Generally, Henderson and Planck equations yield similar results however, for junctions with a pronounced difference in ion mobilities (like HCl-LiCl), the deviation can reach about 10 mV. A specific feature of the Planck equation is the existence of two solutions, the firstbeing close to that of Henderson, and the second one being independent of the solution concentration and of no physical meaning [iv]. Two particular types of liquid junctions are (a) two solutions of the same electrolyte at different concentrations and (b) two solutions at the same concentration with different electrolytes having an ion in common. For type (b) junctions, the simplification of the Henderson model results in the Lewis-Sargent equation ... 

There is some confusion in the literature concerning the expression "cells with transference" and "cells with liquid junction. In this book the first of these will be reserved for cells of the type represented by (10) and (11) which contain only one solute, and the second expression will include these and also cells containing more complicated types of liquid junctions. 

A second example of this type of liquid junction is 0.1 M KCI/0.01 M KCl. This situation is completely analogous to that above, except that in this case the K and Cr ions migrate at nearly the same rate, with the chloride ion moving only about 4% faster. So a net negative charge is built up on the right side of the junction, but it will be relatively small. Thus, Ej will be negative and is equal to —1.0 mV. 

The diffusion potential results from a tendency of the protons in the inner part of the gel layer to diffuse Toward the dry membrane, which contains —SiO Na, and a tendency of the sodium ions in the dry membrane to diffuse to the hydrated layer. The ions migrate at a different rate, creating a type of liquid-junction potential. But a similar phenomenon occurs on the other side of the membrane, only in the opposite direction. These in effect cancel each other, and so the potential of the membrane is determined largely by the boundary potential. (Small differences in boundary potentials may occur due to differences in the glass across the membrane—these represent a part of the asymmetry potential.)... 

Figure 2.3.2 Types of liquid junctions. Arrows show the direction of net transfer for each ion, and their lengths indicate relative mobilities. The polarity of the junction potential is indicated in each case by the circled signs. [Adapted from J. J. Lingane, Electroanalytical Chemistry, 2nd ed., Wiley-Interscience, New York, 1958, p. 60, with permission.]...

Measurements were made of the emf of cells of the type (without liquid junction) for the first dissociation step,... 

A third type of semiconductor junction uses a conducting liquid to contact the semiconductor. Generally, the solvent and electrolyte do not participate in the charge-transfer process. Therefore, to control the Fermi level of the liquid phase, a donor/acceptor pair (a redox couple ) must be added to the solution. The electrochemical potential, or Fermi level, of the solution phase is then given by the Nemst equation (equation 8) ... 

Measurement of Liquid Junction Potentials with Different Electrolytes.—If the same assumption is made as on page 209, that the potential of an electrode reversible with respect to a given ion depends only on the concentration of that ion, then in cells of the type... 

Elimination of Liquid Junction Potentials.—Electromotive force measurements are frequently used to determine thermodynamic quantities of various kinds in this connection the tendency in recent years has been to employ, as far as possible, cells without transference, so as to avoid liquid junctions, or, in certain ca.ses, cells in w hich a junction is formed between two solutions of the same electrolyte. As explained above, the potential of the latter type of junction is, within reasonable limits, independent of the method of forming the boundary. 

Concentration Cells Involving Mixtures of Electrolytes. There is a type of concentration cell, in addition to those already discussed, the study of which yields data of value. The data are useful, for instance, in the computation of liquid junction potentials as described in Chapter 13. These can be represented by the example ... 

There are two reasons for the instability of liquid-junction narrow band gap n-type semiconductors in water under irradiation (a) degradative corrosion (e.g., n-GaAs and n-CdS) and (b) inactive oxide film formation (e.g., n-Si) under photoanodic conditions. It was reported that liquid-junction n-GaAs, which is a typical narrow band gap semiconductor, can be stabilized against corrosion under irradiation by... 

The liquid junction may consist of a bridging solution in a salt bridge. A commonly used kind of salt bridge is a glass tube filled with gel made from agar and concentrated aqueous KCl or KNO3 this type of Uquid junction is believed to reduce the liquid junction potential to several millivolts or less. 

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. 

However, in the case of stress-corrosion cracking of mild steel in some solutions, the potential band within which cracking occurs can be very narrow and an accurately known reference potential is required. A reference half cell of the calomel or mercury/mercurous sulphate type is therefore used with a liquid/liquid junction to separate the half-cell support electrolyte from the process fluid. The connections from the plant equipment and reference electrode are made to an impedance converter which ensures that only tiny currents flow in the circuit, thus causing the minimum polarisation of the reference electrode. The signal is then amplifled and displayed on a digital voltmeter or recorder. 

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