Reference electrodes, aqueous solutions bridge

Changes in the reference electrode junction potential result from differences in the composition of die sample and standard solutions (e.g., upon switching from whole blood samples to aqueous calibrants). One approach to alleviate this problem is to use an intermediate salt bridge, with a solution (in the bridge) of ions of nearly equal mobility (e.g., concentrated KC1). Standard solutions with an electrolyte composition similar to that of the sample are also desirable. These precautions, however, will not eliminate the problem completely. Other approaches to address this and other changes in the cell constant have been reviewed (13). 

In aqueous solutions, the method of measuring electrode potentials has been well established. The standard hydrogen electrode (SHE) is the primary reference electrode and its potential is defined as zero at all temperatures. Practical measurements employ reference electrodes that are easy to use, the most popular ones being a silver-silver chloride electrode and a saturated calomel electrode (Table 5.4). The magnitude of the liquid junction potential (LJP) between two aqueous electrolyte solutions can be estimated by the Henderson equation. However, it is usual to keep the LJP small either by adding the same indifferent electrolyte in the two solutions or by inserting an appropriate salt bridge between the two solutions. 

Aqueous reference electrodes, such as SCE and Ag/AgCl electrodes, are often used in noil-aqueous systems by dipping their tips into lion-aqueous solutions of the salt bridge. The tip should not be dipped directly into the solution under study, because the solution is contaminated with water and the electrolyte (usually KC1). When we use such aqueous reference electrodes, we must take the liquid junction potential (LJP) between aqueous and non-aqueous solutions (Table 6.2) into account. If we carefully reproduce the composition of the solutions at the junction, the LJP is usually reproducible within 10 mV. This is the reason why aqueous reference electrodes are often used in non-aqueous systems. However, the LJP sometimes exceeds 200 mV and it is easily influenced by the electrolytes and the solvents at the junction (Section 6.4). The use of aqueous reference electrodes should be avoided, if possible. 

For studies in aqueous solutions, the external reference electrode is often an Ag/AgCl/KCl electrode. Electrical contact with the solution is achieved using a disc-like membrane made of porous fritted glass. Because ions have a tendency to migrate across the membrane, a small potential Ej is generated by this liquid junction. This phenomenon can be minimised by inserting a saturated KC1 solution as a salt bridge. 

For most potentiometric measurements, either the saturated calomel reference electrode or the silver/silver chloride reference electrode are used. These electrodes can be made compact, are easily produced, and provide reference potentials that do not vary more than a few mV. The silver/silver chloride electrode also finds application in non-aqueous solutions, although some solvents cause the silver chloride film to become soluble. Some experiments have utilised reference electrodes in non-aqueous solvents that are based on zinc or silver couples. From our own experience, aqueous reference electrodes are as convenient for non-aqueous systems as are any of the prototypes that have been developed to date. When there is a need to exclude water rigorously, double-salt bridges (aqueous/non-aqueous) are a convenient solution. This is true even though they involve a liquid junction between the aqueous electrolyte system and the non-aqueous solvent system of the sample solution. The use of conventional reference electrodes does cause some difficulties if the electrolyte of the reference electrode is insoluble in the sample solution. Hence, the use of a calomel electrode saturated with potassium chloride in conjunction with a sample solution that contains perchlorate ion can cause dramatic measurements due to the precipitation of potassium perchlorate at the junction. Such difficulties normally can be eliminated by using a double junction that inserts another inert electrolyte solution between the reference electrode and the sample solution (e.g., a sodium chloride solution). 

The aqueous saturated calomel electrode (SCE) is generally employed as the reference electrode though in a few cases as the normal calomel electrode (NCE), connected to the solution in the non-aqueous solvent by means of a salt bridge involving the latter solvent, in order to avoid contamination of the solution to be studied with water. Otherwise, an Ag/Ag+ or an Hg/Hg22+ electrode or... 

Choice of electrolyte for salt bridges and reference electrodes. Many of the difficulties encountered in potentiometric measurements can be attributed to erratic or drifting junction potentials caused by clogged junctions. Certain elementary rules should be observed in choosing the filling solution for a salt bridge or reference electrode, particularly when they will be used in organic solvents or solutions that are only partially aqueous. 

Some Practical Considerations in the Use of Salt Bridges. Salt bridges are most commonly used to diminish or stabilize the junction potential between solutions of different composition and to minimize cross-contamination between solutions. For example, in working with nonaqueous solvents an aqueous reference electrode often is used that is isolated from the test solution by a salt bridge that contains the organic solvent. However, this practice cannot be recommended, except on the grounds of convenience, because there is no way at present to relate thermodynamically potentials in different solvents to the same aqueous reference-electrode potential furthermore, there is a risk of contamination of the nonaqueous solvent by water. 

Until recently, the most popular reference half-cell for potentiometric titrations, polarography, and even kinetic studies has been the saturated aqueous calomel electrode (SCE), connected by means of a nonaqueous salt bridge (e.g., Et4NC104) to the electrolyte under study. The choice of this particular bridge electrolyte in conjunction with the SCE is not a good one because potassium perchlorate and potassium chloride have a limited solubility in many aprotic solvents. The junction is readily clogged, which leads to erratic junction potentials. For these practical reasons, a calomel or silver-silver chloride reference electrode with an aqueous lithium chloride or quaternary ammonium chloride fill solution is preferable if an aqueous electrode is used. 

Conventional reference electrodes consist of a solid reversible electrode and an aqueous electrolyte solution. To measure the individual contributions from the anode and the cathode of a PEM fuel cell, the electrolyte solution of the reference electrode must either be in direct contact with one side of the solid proton exchange membrane or be located in a separate compartment with electrical contact between the reference electrode and the solid membrane by means of a salt bridge [66], As a result, two different types of reference electrode configurations are employed for the study of fuel cells internal and external. 

As reference electrode [211] for polarographic work, an aqueous or methanolic calomel electrode, connected to the DMSO solution with a suitable salt bridge to avoid contamination, has been used. The most stable reference electrodes in DMSO seem to be the amalgam electrodes, such as Tl(Hg)/TlCl [214] or Li(Hg)/LiCl [207]. 

Nitrobenzene has been used for electrolysis [402] it was found that certain radicals were rather stable in this solvent. Nitrobenzene has a liquid range from 5.7 to 210.9°C BU4NCIO4 may be used as supporting electrolyte. An aqueous SCE separated from the solution by a suitable bridge with porous glass has been used as reference electrode. Nitrobenzene may be purified by passing it through a column of alumina followed by a distillation in vacuo. 

This is probably the most widely used reference electrode. It is usually made with a saturated aqueous potassium chloride solution bridge, although 1 mol/dm and 0.1 mol/dm solutions also are commonly used (see Table 4.1). Calomel electrodes can have very low resistance and good performance. For this reason they are frequently used for checking other types of electrodes. 

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