Double junction electrode

Some electrodes are double-junction electrodes. Such electrodes are encased in another glass tube and therefore have two junctions, or porous plugs. The purpose of such a design is to prevent contamination—the contamination of the electrode solution with the analyte solution, the contamination of the analyte solution with the electrode solution, or both, by the diffusion of either solution through the porous tip or plug. See the next section for tips concerning these problems. 

What is special about gel-filled electrodes and double-junction electrodes ... 

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. 

The half-cell enclosed by the dashed line in Figure 15-1 is called a silver-silver chloride electrode. Figure 15-3 shows how the electrode is reconstructed as a thin tube that can be dipped into an analyte solution. Figure 15-4 shows a double-junction electrode that minimizes contact between analyte solution and KCI from the electrode. The silver-silver chloride and calomel reference electrodes (described soon) are used because they are convenient. A standard hydrogen electrode (S.H.E.) is difficult to use because it requires H2 gas and a freshly prepared catalytic Pt surface that is easily poisoned in many solutions. 

If another compartment filled with, for example, NaNOj, is used in order to prevent contamination of the sample with KC1, the double-junction electrode is written as... 

Various types of reference electrodes are used for permanent embedment in concrete. Some fall into the category of double junction electrodes of the second kind and are therefore reference electrodes in the true sense. Others are simply a piece of metal or another material put into the concrete. Although stable and accurate, SCE is not used for permanent embedment in concrete, mainly because it contains a liquid metal, which makes it difficult to manufacture in a rugged form. In addition, environmental reasons make it undesirable for permanent use in the field (poisonous mercury and mercury compounds). 

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. 

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. 

The potential of interest in pH measurement is the difference between the potential developed at the outer and inner glass surfaces of the measurement electrode as defined by Equation 4-1 g. All other potential represents an error. Figure 4-la shows the physical location of the potential (Ei - E2) and many extraneous potentials for a combination double junction electrode, which has a reference electrode as a concentric ring around the glass measurement electrode. The combination electrode is popular because it reduces the installation and spare parts requirements. The internal salt bridge between the outer and inner junction of the double junction increases the time it takes for process ions to migrate to the inner chamber where the internal si lver silver chloride electrode is located. 

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. 

Commercial forms of the electrode are available and in general are similar to the calomel electrode depicted in Fig. 15.1(h) with the replacement of the mercury by a silver electrode, and calomel by silver chloride. The remarks concerning clogging of the sintered disc, and the use of ion exchange membranes and double junctions to reduce this are equally applicable to the silver-silver chloride electrode. 

The construction of these electrodes is exactly similar to that already described for the pH responsive glass electrode. They must of course be used in conjunction with a reference electrode and for this purpose a silver-silver chloride electrode is usually preferred. A double junction reference electrode is often used. The electrode response to the activity of the appropriate cation is given by the usual Nernst equation ... 

Reference electrodes are usually a calomel or a silver-silver chloride electrode. It is advisable that these be of the double-junction pattern so that potassium chloride solution from the electrode does not contaminate the test solution. Thus, for example, in titrations involving glacial acetic acid as solvent, the outer vessel of the double junction calomel electrode may be filled with glacial acetic acid containing a little lithium perchlorate to improve the conductance. 

As in normal potentiometry one uses and indicator electrode versus a reference electrode, the electrodes should, especially in pH measurements, be those recommended by the supplier of the pH meter in order to obtain a direct reading of the pH value displayed. In redox or other potential measurements any suitable reference electrode of known potential can be applied. However, a reference electrode is only suitable if a junction potential is excluded, e.g., an Ag-AgCl electrode in a solution of fixed Ag+ concentration or a calomel electrode in a saturated KC1 solution as a junction in many instances a direct contact of Cl" with the solution under test (possibly causing precipitation therein) is not allowed, so that an extra or so-called double junction with KN03 solution is required. Sometimes micro-electrodes or other adaptations of the surface are required. 

As an example we take the titration of AgN03 with NaCl by means of an Ag electrode as an indicator and a double junction calomel electrode (the external junction being filled with a KN03 solution) as a reference. 

For a complex-forming metal ion detectable by its own metal electrode, e.g. in the titration of Cu2+ with EDTA by means of a Cu electeode and a double junction calomel electrode, p/ curves are obtained of a nature comparable to those in Fig. 2.20 and with a Cu range of about 20 (cf., stability constant... 

Cyclic voltammetry was performed on precursor polymer thin films cast on platinum electrodes in order to assess the possibility of electrochemical redox elimination and consequently as an alternative means of monitoring the process. All electrochemical experiments were performed in a three-electrode, single-compartment cell using a double junction Ag/Ag+(AgN03) reference electrode in 0.1M... 

The commercial silver-silver chloride electrode is similar to the SCE in that it is enclosed in glass, has nearly the same size and shape, and has a porous fiber tip for contact with the external solution. Internally, however, it is different. There is only one glass tube (unless it is a double-junction design—see Section 14.5.3) and a solution saturated in silver chloride and potassium chloride is inside. A silver wire coated at the end with a silver chloride paste extends into this solution from the external lead. See Figure 14.5. The half-reaction that occurs is... 

H+] is measured potentiometrically with a glass electrode. Briefly, the method involves the use of a glass electrode and a double-junction calomel reference electrode in the titration cell ... 

Hamada et al. used a poly (vinyl chloride) matrix membrane ion-selective electrode for the analysis of procaine [76]. Procaine flavianate (10 mg, prepared by precipitation from an equimolar mixture of procaine hydrochloride and flavianic acid), was mixed with PVC powder (150 mg), dioctyl phthalate (370 mg), and tetrahydrofuran (4 mL). This mixture was used to produce membranes (3 cm diameter), from which discs were cut to prepare ion-selective electrodes. The electrodes were used in conjunction with a double-junction Ag-AgCl (KNO3) reference electrode for the potentiometric determination of procaine hydrochloride at 25°C. 

Pt/IJ, 1 Electrode This electrode, proposed by Coetzee and Gardner [7], is of a double-junction type, as shown in Fig. 6.1(b). It is easy to construct in many solvents and the potential is stable and reproducible. Because the electrode reaction... 

Figure 18.3—Principle of ISE measurement of fluoride ions in solution using a double junction reference electrode. The reference electrode is inserted into a separate chamber that contains the auxiliary electrolyte in order to avoid osmosis of KC1 into the sample solution. Also, 1 M KN03 can be used for F , Cl, CN or Ag+ determination. The measurement involves the use of a high impedance millivoltmeter (pH meter type). A version of an all-solid fluoride electrode is shown on the right.

Figure 15-4 Double-junction reference electrode. The inner electrode is the same as the one in Figure 15-3. The solution in the outer compartment is compatible with analyte solution. For example, if you do not want Cl to contact the analyte, the outer electrode can be filled with KN03 solution. The inner and outer solutions slowly mix, so the outer compartment must be refilled periodically with fresh KNO3 solution. (Courtesy Fisher Scientific, Pittsburgh, PA.]...

Figure 15-6 Use of Ag and calomel electrodes to measure [Ag ]. The calomel electrode has a double junction, like that in Figure 15-4. The outer compartment of the electrode is filled with KN03, so there is no direct contact between Cl in the inner compartment and Ag1 in the beaker.

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). 

All potentials vs. screen-printed Ag/AgCl pseudo-reference, except values marked with asterisk ( ), which are vs. Ag/3M AgCl double-junction reference electrode, and values marked with dagger CfO, which are vs. saturated calomel. Abbreviations CoPC cobalt phthalocyanine, SPCE screen-printed carbon electrode, GOD glucose oxidase, MWCNT multi-walled carbon nanotubes, NAD nicotinamide adenine dinucleotide, PQQ pyrroloquinoline quinone, FIA flow injection analysis. 

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