Liquid junction potentials prevention

Some commercial electrodes are supplied with a double junction. In such arrangements, the electrode depicted in Fig. 15.1(h) is mounted in a wider vessel of similar shape which also carries a porous disc at the lower end. This outer vessel may be filled with the same solution (e.g. saturated potassium chloride solution) as is contained in the electrode vessel in this case the main function of the double junction is to prevent the ingress of ions from the test solution which may interfere with the electrode. Alternatively, the outer vessel may contain a different solution from that involved in the electrode (e.g. 3M potassium nitrate or 3M ammonium nitrate solution), thus preventing chloride ions from the electrode entering the test solution. This last arrangement has the disadvantage that a second liquid junction potential is introduced into the system, and on the whole it is preferable wherever possible to choose a reference electrode which will not introduce interferences. 

The establishment of such interfacial potentials is readily envisaged for cases where the net transport of an electrolyte is prevented because one of its constituents cannot partition. What is perhaps less obvious is that such potentials arise continually within solution phases, even where there is no physical separation into distinct phases. These so-called liquid junction potentials or diffusion potentials play an important role in electrochemical experiments, but because there is no well-defined phase boundary, they are intrinsically more difficult to measure. This chapter discusses how these potentials arise, how they may be calculated, what quantities are associated with them, and how they may be minimised. Finally, interfaces between electrolytes (i.e. those interfaces between immiscible electrolyte solutions (ITIES)) and the application of some of the concepts developed earlier in the chapter to non-standard electrolyte systems, such as polymer electrolytes and room-temperature ionic liquids, will be discussed. 

Some galvanic cells contain two electrolyte solutions with different compositions. These solutions must be separated by a porous barrier or some other kind of junction in order to prevent rapid mixing. At this liquid junction in the zero-current cell, there is in general a liquid junction potential caused by diffusion of ions between the two bulk eleetrolyte phases. 

As electrochemistry moved into mixed and nonaqueous electrolytes it became of interest to compare potentials in different media. Serious problems preventing comparison are the Uquid junctimi potentials between different electrolytes. Such liquid junction potentials also occur in the measurement in aqueous systems, but they are generally suppressed by a salt bridge. Salt bridges for aqueous systems usually consist of (saturated) solutions of KCl or NH4NO3. For both KCl and NH4NO3 similar mobilities for the cation and the anion of the respective salt were measured in aqueous solutions. Thus the liquid junction potential between two aqueous electrolytes cmmected via such a bridge should be smaller than the experimental error (see Chap. 1). Data in aqueous systems without liquid junction potentials are obtained from measurements in cells without transference such as ... 

In order to prevent the zinc sulfate and the copjjer sulfate mixing, a porous disc, or a conducting salt bridge of potassium sulfate in a tube, or a wick can be used to connect the solutions. This helps to reduce the liquid junction potential, which occurs when two solutions of unequal concentration, or containing dissimilar ions are placed in contact. Because of the different rates of diffusion of the ions, a liquid junction (or diffusion) potential, E, is set up and this affects the total cell emf. For example, between a solution of 0.1 M HCl and a solution of 0.01 M HCl, E is about 38 mV. 

Both internal and external reference electrodes possess an interface between the internal solution and the external environment. This interfaee is eommonly established within a porous junction and is designed to permit electrolytic communication while preventing flow. In any event, the junction gives rise to the isothermal liquid junetion potential (ILJP), Ed(T2), which develops, because some ions diffuse faster than others, thereby generating an eleetrie field that opposes the proeess. Integration of the electric field across the junetion yields the isothermal liquid junction potential. Bard and Faulkner provide a detailed discussion of the thermodynamics of the isothermal liquid junction. For dilute solutions, the potential ean be ealeulated from Henderson s equation. In the ease of Thermoeell I, the isothermal liquid junetion potential is expressed by ... 

The fastest and most accurate reference electrode has a flowing junction. A constant small flow of electrolyte purges the junction and prevents movement of the process into the junction. The result is a small quickly established liquid junction potential that can meet the demands of high accuracy and batch applications. However, the reference flow must be kept small enough so as to not contaminate the process or excessively... 

Even in a galvanic cell with a salt bridge, there is some leakage of ions across the liquid junction, which causes the battery to lose its chemical potential over time. Commercial cells use an insoluble salt to prevent this from happening. 

The double line represents the liquid junction between two dissimilar solutions and is usually in the form of a salt bridge. The purpose of this is to prevent mixing of the two solutions. In this way, the potential of one of the electrodes will be constant, independent of the composition of the test solution, and determined by the solution in which it dips. The electrode on the left of cell 13.28 is the saturated calomel electrode, which is a commonly used reference electrode (see below). The cell is set up using the hydrogen electrode as the indicating electrode to measure pH. 

The salt bridge, an agar jelly saturated with either KCl or NH4NO3, is often used to connect the two electrode compartments. This device introduces two liquid junctions, whose potentials are often opposed to one another, and the net junction potential is very small. The physical reason f or the cancellation of the two potentials is complex. The use of a jelly has some advantages in itself It prevents siphoning if the electrolyte levels differ in the two electrode compartments, and it slows the ionic diffusion very much so that the junction potentials, whatever they may be, settle down to reproducible values very quickly. 

The membranes used in ion-selective electrodes separate two different electrolytes and are not equally permeable to all kinds of ions. At the interface between the two electrolytes, different events contribnte to the measured membrane potential. First, a diffusion potential arises from differences in mobility and concentration of ions in contact at the interface, as seen in liquid junctions. Second, a Donnan potential arises when the membrane completely prevents the diffnsion of at least one species from solution to the other. Third, the exchange equilibria between the electrolyte and the membrane interface must also be considered to adequately describe the membrane potential of ion-selective electrodes with solid or liquid electrolyte manbranes. 

Commonly, cells utilize three (working, auxiliary and reference) electrodes. It is desirable to site the reference electrode very close to the working electrode in order to control accurately the potential of the latter. The auxiliary electrode should be positioned sufficiently near to the working electrode to prevent excessive cell voltages. In practice, the reference electrode often has a gelled or microporous ceramic-polymer liquid junction for convenience of handling, while the auxiliary electrode is situated downstream so that its electrode reaction (e.g. gas evolution) does not interfere (chemically or hydrodynamically) with the working electrode reaction. 

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