Double-salt bridges

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). 

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). 

In order to prevent the contamination of the reference electrode or the mixing of the electrolytes an arrangement with double salt bridge is also applied. 

One of the first reports that can be found in the literature is a study performed by Tabushi et al. in which it was described that the quatemized DABCO with stearyl chains (1, Scheme 1) was able to transfer 5 -adenosine diphosphate (ADP) very efficiently from aqueous solutions at pHs of 3 and 5 into a chloroform phase. The selectivity found for the transfer of ADP over AMP was explained by the formation of a double salt bridge between the phosphate groups and the quatemized receptor. 

Figure 5.24 shows a device intended to facilitate the use of conventional electrodes as flow sensors. The potentiometric circuit is closed by using a reference electrode in some cases, the electrode must be of the double salt bridge type to ensure compatibility with the analyte, e.g. chloride. A simple solution involves connecting the waste tube to the output of the indicator electrode in a beaker and inserting the reference electrode in the waste liquid. Alternatively, one can use a methacrylate well to screw the output of the selective electrode on one side and to evacuate the liquid excess via an overflow trough on the other. 

Fig. 19. A Schematic diagram of a reference electrode with double salt bridge and sleeve diaphragms. A construction of Orion made of plastic, allowing measurements in acidic HF solutions. B Complete measuring cell consisting of an ion-selective indicating electrode and a reference electrode with double salt bridge and sleeve diaphragm (Philips)...

Ag/AgCl Reference Electrode with Sleeve Diaphragm and Double Salt Bridge 1 Thermostated Sample Cell... 

Cells without liquid junction are to be employed whenever possible. For measurements with salt bridges, sleeve diaphragms should be chosen for their ability to provide a stable liquid junction potential (with double salt bridge reference electrodes the pH of the outer electrolyte should be adjusted to match that of the sample). In addition, the sample and salt bridge electrolyte solutions should have similar ionic strengths. Cells with liquid junction seldom allow accuracies better than 0.01 pa units. 

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