Salt bridge, double-junction

In this scheme, a single vertical line represents a metal-electrolyte boundary at which a potential difference is taken into account the double vertical broken lines represent a liquid junction at which the potential is to be disregarded or is considered to be eliminated by a salt bridge. 

In gel-filled electrodes, the interior reference solution is gelatinized. As such, there is no loss of solution through the salt bridge and no contaminating solution can enter. It cannot be refilled with reference solution. In double-junction electrodes, the usual electrode is inside another glass or plastic encasement and there are two junctions for contact to the external solution. The purpose of this design is to prevent contamination in either direction. 

Interference. Most common reference electrodes contain K+ and Cl- ions, which provide good ionic transport within the junctions. Other reference electrodes are also available for those samples that are sensitive to these ions (i.e., Hg2S04 with a K2S04 salt bridge, or reference electrodes with a double salt bridge construction). 

Fig. 6.6 (a) Salt bridge and (b) double liquid junction. Composition of solutions 1 and 2 can be selected such that the contamination of the sample is prevented... 

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). 

Various attempts have been made to circumvent these problems and to eliminate junction potentials, including (1) extrapolation procedures designed to eliminate the difference between the compositions of the two solutions in the appropriate limit, (2) separation of the two solutions by means of a doublejunction salt bridge, (3) the use of double cells with dilute alkali metal amalgam connectors, and (4) the use of glass or other types of ion-specific electrodes as bridging reference electrodes. 

The extrapolation procedures used for cells with liquid junction are time-consuming, and the method is not entirely free of theoretical pitfalls.9 Because salt bridges usually involve double junctions, an important distinction needs to be made betwen the behavior of single-junction and double-junction salt... 

Thus, under equilibrium conditions, the emf of the double electrode-pair system is determined solely by electric potential differences developed at the two liquid junctions that involve KC1 salt bridges. The two Ej may differ because of the effect of soil colloids. Thus the fact that this emf can develop is known as the suspension effect.40 Only ionic transport processes across the liquid junctions need be taken into account in order to evaluate E. Ionic transport processes across the semipermeable membrane between the suspension and the solution are not germane. Moreover, since neither Ej nor Ej can be calculated by strictly thermodynamic methods, the interpretation of E must be made in terms of specific models of ionic transport across salt bridges contacting suspensions and solutions. Thus the relation between E and the behavior of ions in soil suspensions is not direct. 

Finally, chloride ions from an ordinary calomel electrode were found to interfere seriously with pH determinations, resulting in non-reproducible readings and end points [9]. Only in this one study was a special doublejunction calomel reference electrode with KN03 in the outer tube employed to prevent interference by chloride ions. Use of this double-junction electrode is labeled as KN03 salt bridge under Conditions in Table 1. 

Acetylcholineesterase A stock solution of 0.52mg/mL of the pesticide trichlorophen in lOmM phosphate buffer of pH 7.5 was diluted with buffer to various concentrations. The obtained solutions were then analyzed using an ACh biosensor based on the inhibition effect of trichlorophen on the function AChE which promotes the hydrolysis of the natural neurotransmitter, acetylcholine. The sensor was fabricated by immobilizing AChE onto the surface of an antimony disc electrode, which was then used in conjunction with a double junction Ag/AgCl (0.1 M-KC1) reference electrode with a 0.1 M lithium acetate salt bridge. 

In the cell diagrams the boundary between the electrode and the electrolyte, where the potential difference arises, is marked by a single vertical line (e. g. Zn Zn++). The line dividing two electrolytes (e. g. Zn+ f Cu++) expresses the existence of the liquid junction potential added to the electrode potentials. When this liquid junction potential is eliminated (e. g. by a salt bridge, see below), so that it can be disregarded, a double line is written instead of a single one (Zn++ Cu++). 

Ah already stated the liquid junction potential results from the different mobility of ions. Consequently no diffusion potential can result at the junction of the electrolyte solution the ions of which migrate with the same velocity. It is just this principle on which the salt bridge, filled by solutions of those salts the ions of which have approximately the same mobilities, is based (the equivalent conductivities of ions Kf and Cl- at infinite dilution at 25 °C are 73.5 and 70.3 respectively and the conductivities of ions NH+ and NOg are 73.4 and 71.4 respectively). Because ions of these salts have approximately the same tendency to transfer their charge to the more diluted solution during diffusion, practically no electric double layer is formed and thus no diffusion potential either. The effect of the salt bridge on t he suppression of the diffusion potential will be better, the more concentrated the salt solution is with which it is filled because the ions of the salt are considerably in excess at the solution boundary and carry, therefore, almost exclusively the eleotric current across this boundary. 

The double vertical line indicates that the liquid Junction potential is either ignored or kept small by a suitable salt bridge. 

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... 

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. 

Figure 1.3.4 Schematic cell connected to an external power supply. The double slash indicates that the KCl solution contacts the Cd(N03)2 solution in such a way that there is no appreciable potential difference across the junction between the two liquids. A salt bridge (Section 2.3.5) is often used to achieve that condition.

With the liquid level above the analyte solution, some contamination i>f the sample is inevitable. In most instances, the amount of contamination is too slight to be of concern. In determining ions such as chloride, potassium, silver, and mercury, however, precaution must often be taken to avoid this source of error. A common w-ay is to interpose a second salt bridge between the analyte and the reference electrode this bridge should contain a noninierfering electrolyte, such as potassium nitrate or sodiujn sulfate. Double-junction electrodes based on this design are offered by several manufacturers. 

Slant lines, vertical lines, or sometimes a semicolon, indicate phase boundaries across which there arise potential differences that are included in the measured potential of the entire cell. Conventionally, a double slant or vertical line signifies a liquid junction— the zone of contact between two electrolyte solutions. Physically, this may be a porous membrane as in Figure 2.1, or a salt bridge of some sort. The anode is written to the left, the cathode on the right. If there are several components in one electrolyte solution, the components are separated by a comma. For example, for a cell (without liquid junction) composed of a silver/silver-chloride half-cell and a hydrogen gas electrode, one could write for one set of conditions... 

The double vertical stroke after indicates a membrane junction or salt bridge. The double stroke shows the termination of one half-cell and the beginning of the second. This cell could also be written to show the salts used, as shown ... 

It may be necessary to eliminate the leakage of the potassium chloride filling solution from the junction into the sample (for example, a sample with silver ion). An auxiliary salt bridge, which is a glass body with a junction at one end, is available from some electrode manufacturers. This can be filled with a salt solution which does not contain a contaminant. The reference electrode is then placed in the salt bridge to make contact with the salt solution (see Figure 3.9). Reference electrodes which have this double junction feature built in are also available. 

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