Double-junction reference electrodes

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 ... 

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.]...

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. 

Hadjidemietriou [25] used an Orion Model 93-07 nitrate ion-selective electrode with a 1 x 2 sensing module construction, and an Orion Model 90-02 double junction reference electrode fitted on a pH meter. The outer chamber was filled with 0.04 M ammonium sulfate solution and the inner chamber with Orion 90-00-02 solution. 

Direct potentiometric measurement an Agl membrane electrode with a double junction reference electrode system must be used to quantify CN-. 

Tests for S include the antimony test, silver-silver sulfide electrode test and the lead acetate paper and silver foil tests. In the antimony test, the color produced by treating a 200 ml test sample with 0.5 ml saturated potassium antimony tartrate and 0.5 ml 6 N HCl is compared with colors produced when solutions containing known amounts of S are treated in the same way. In the silver-silver sulfide electrode test, the test sample is diluted 1 1 with an alkaline solution of an oxidizing agent and the electrode potential relative to a double-junction reference electrode is measured. 

Figure 19.3 Measurement set-up of ion-specific electrodes. Schematic of a measuring cell with a ISE targeted for the fluoride ion using a double junction reference electrode. From the two sides of the membrane two equilibria are produced, LaF, - LaF -F F . To avoid osmosis of KCI into the solution, the reference electrode is surrounded by a separate chamber which contains an auxiliary non-interfering electrolyte. KNO, IM difficult to oxidize or to reduce, is often used for F , CP, P, CN , or Ag+. The measurement requires a high impedance mihivoltmeter (type pH-meter). Right, two examples of simple to construct coated wire electrodes . A wire forms an ohmic contact with the membrane (ISE for fluorine with a LaF, crystal and ISE for chloride with frit).

Controlled potential electrolysis in aqueous solution was carried out with a carbon-cloth working electrode and in the presence of a Ag/AgCl double junction reference electrode, with constant nitrogen purge, as described by us previously." The electrolysis was stopped when the current dropped to less than 1% of the original value. 

The cmc and micelle ionization degrees near the cmc (9) were determined by emf measurements using a specific bromide ion electrode (Orion 9435), in conjunction with a double junction reference electrode (Orion 9002) and a millivoltmeter (Orion 701A). Some determinations were also performed by means of conductivity. It was noted that emf measurements yielded cmc values slightly lower than those obtained from the equivalent conductivity vs. (concentration) plots. This, however, is of no importance in this work where we are only interested in relative changes. 

Figure 15-3 Double-junction reference electrode has an inner electrode identical to those in Figures 14-11 and 14-12. The outer compartment is filled with an electrolyte such as KNO3 that is compatible with the titration solution. KCI electrolyte from the inner electrode slowly leaks into the outer electrode, so the outer electrolyte should be changed periodically. [Fisher Scientific, Pittsburgh, PA.]...

The electrode that houses the ion-selective membrane, the inner electrolyte, and the irmer reference electrode is called the indicator electrode or working electrode. If the external reference electrode contains an interchangeable bridge electrolyte, it is often termed a double junction reference electrode to indicate that two separate hq-uid junction potentials are present (one between the reference electrolyte and the bridge electrolyte, and the second between the bridge electrolyte and the sample solution). The use of combination pH glass... 

It is important that the surface of a glass membrane be swollen before measurements are taken. Prior to first use the electrode should therefore be allowed to swell overnight in a 3 mol/L KCl solution. For the same reason the electrodes should also be stored in a solution of this type between measurements and after measurements have been conducted in nonaqueous solvents. Here, the choice of a double junction reference electrode with the same solvent as in the mea.suring solution and added organic salt for sufficient conductivity is important. In the case of endpoint detection a platinum/ferrocene redox element without a liquid junction could also be used. 

Some operators prefer to use a double junction reference electrode, to prevent contamination of the electrode by the surfactant solution and vice versa. This is basically a glass sheath with a frit at its lower end, which is filled with an electrolyte such as 1M sodium nitrate and in which the calomel electrode is placed. An improvised version consists simply of a small beaker containing the electrolyte and the calomel electrode, and connected to the titration vessel via a piece of string soaked in the same solution. 

The ion-sensor together with double junction reference electrode was dipped in the stirred electrochemical cell with a working volume of 15 ml. The electrode potential was monitored with a Orion pH meter model SA 520 and recorded. At the steady-state potentiometric response, varying concentrations of the ionic solution ( KCl, NH4CI, NaCl) were inj ted into the cell and the new steady-state potential was recorded. The measurements were made with the sol-gel modified electrode with and without the ionsensing membrane. 

The reference electrode normally has a slow leak into the solution at a rate of a few microlitre per hour. It is therefore advisable to ensure the filling solution (normally 3 mol/L KCl) is clean, either by using a high grade salt for its preparation, or by piuification. Double-junction reference electrodes exist and these are useful for instance if silver or sulphide are to be determined due to the presence of ionic silver in the silver reference electrode cartridge. 

The limited number of well functioning, classical or spectrophotometric methods is available for measuring fluoride ion concentration in different samples. Therefore, after the invention of lanthanum fluoride crystal-based ISE [15], its use as a detector in standardized methods becomes almost general. For example, the Environmental Protection Agency (ERA) METHOD 9214 [44] is for measuring the concentration of fluoride ions in water samples as well as in soil extracts. It is a direct potentiometric method using the ion-selective fluoride electrode and the conventional or double junction reference electrode. The lower limit of detection is 0.025 mg dm. Fluoride concentrations from 0.025 to 500 mg dm can be measured. 

EPA METHOD 9212 [52] for measuring the chloride ion concentration in water samples as well as in soil extracts. In this direct potentiometric method, the ion-selective chloride electrode and double junction reference electrodes are used. The dynamic range of the analysis is between 2 and 1000 mg/dm . ... 

EMF measurements Cell voltages were measured at room temperature in an air-conditioned laboratory at 22.5 0.5 °C with an Orion pH Meter (Model 720A) connected to an Orion model 607 manual electrode switch. As a reference electrode an Orion model 90-02 Ag/AgCl double junction reference electrode was used throughout. The measured potential values were recorded with an ABB Goertz 420 strip charge recorder. 

If the sample solution might react with chloride ions, for example silver or lead salts, then a double junction reference electrode may be used, with an additional liquid junction of KNO3. 

It is considerably easier to titrate mixed chloride and bromide in solution by potentiometry. The use of a silver ion selective electrode, or even a silver wire, together with a double junction reference electrode, since the chloride ions from a calomel electrode would react, allows the determination of the silver ion concentration. Other precipitation titrations may be followed using suitable ion selective electrodes. 

Fig. 9.2 Titration of Zonyl FSA with 0.05 N Hyamine 1622. Metrohm model 670 titrator, Orion model 9342BN surfactant-selective electrode, and model 90-02 double-junction reference electrode. (From Ref. 10.)...

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