Salt bridge, auxiliary

In the common method of electro-gravimetric analysis, a potential slightly in excess of the decomposition potential of the electrolyte under investigation is applied, and the electrolysis allowed to proceed without further attention, except perhaps occasionally to increase the applied potential to keep the current at approximately the same value. This procedure, termed constant-current electrolysis, is (as explained in Section 12.4) of limited value for the separation of mixtures of metallic ions. The separation of the components of a mixture where the decomposition potentials are not widely separated may be effected by the application of controlled cathode potential electrolysis. An auxiliary standard electrode (which may be a saturated calomel electrode with the tip of the salt bridge very close to the cathode or working electrode) is inserted in the... 

An equivalent circuit of the three-electrode cell discussed in Chapters 6 and 7 is illustrated in Figure 9.1. In this simple model, Rr is the resistance of the reference electrode (including the resistance of a reference electrode probe, i.e., salt bridge), Rc is the resistance between the reference probe tip and the auxiliary electrode (which is compensated for by the potentiostat), Ru is the uncompensated resistance between the reference probe and the working-electrode interphase (Rt is the total cell resistance between the auxiliary and working electrodes and is equal to the sum of Rc and Ru), Cdl is the double-layer capacitance of the working-electrode interface, and Zf is the faradaic impedance of the electrode reaction. 

Figure 9.5 Controlled-potential coulometry cell with a mercury pool working electrode. a, Platinum wire contact to mercury pool working electrode b, mercury pool working electrode c, reference electrode d, auxiliary electrode e, porous Vycor f, sample solution g, inert gas inlet h, stirrer i, reference electrode salt bridge j, clean mercury k, waste. [From Ref. 11, adapted with permission.]...

Figure 11.8 Cell for use of thin-film electrodes. A, Aluminum retaining plate B, OTE C, copper foil D, O-ring E, Lucite body F, glass salt bridge for reference electrode G, Hamilton valves H, auxiliary electrode I, quartz disk. [From Ref. 89, with permission. Copyright 1972 American Chemical Society.]...

The detector usually consists of a three-electrode system working electrode, reference electrode, and the auxiliary (counter) electrode (Figure 3.42). The working electrode is usually made from glassy carbon. The reference electrode can be a salt bridge silver/silver chloride electrode, but at high pH, another soUd-state reference electrode is needed. The auxiliary electrode is usually just the steel outlet tubing. 

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. 

One useful filling solution for nonaqueous measurements is methanol saturated with potassium chloride. Since potassium chloride is used as the electrolyte, the filling solution may be placed directly in the reference electrode salt bridge, and an auxiliary salt bridge is not required as in the case of most other filling solution electrolytes. 

In general, more precise selection of an electrolyte as outlined and careful choice of solvent can reduce the junction potential. For example, an auxiliary salt bridge containing 50% dimethylsulfoxide (DMSO), 50% methanol, and a tetraethylammonium perchlorate electrolyte could be used as an intermediate solution between a reference electrode containing methanolic-KCl filling solution and a sample in DMSO solvent. Figure 3.9 illustrates use of an intermediate electrolyte in a salt bridge. 

Multiple rinse with solvent or aliquot of sample Purging or blanketing sample with inert gas Add a neutral salt to the sample Connect sample or a surrounding shield to solution ground terminal Alter reference filling solution Use an auxiliary salt bridge with an intermediate electrolyte filling solution Use a reference electrode with different type of junction... 

Several steps can be taken to minimize the junction potential. These steps include selection of an appropriate type of junction, providing a compatible filling solution, and/or separating the refer ence from the sample by the use of an auxiliary salt bridge. 

A change in the filling solution or the use of an auxiliary salt bridge may be required with such viscous samples as latex. A reaction of the sample with a potassium chloride filling solution can be avoided by using an auxiliary salt bridge with an alternate electrolyte filling solution. 

Auxiliary salt bridge A glass tube open at one end to receive intermediate electrolyte filling solution, and the reference electrode tip and a junction at the other end to make contact with the sample. 

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