Reference silver chloride

There are two procedures for doing this. The first makes use of a metal probe coated with an emitter such as polonium or Am (around 1 mCi) and placed above the surface. The resulting air ionization makes the gap between the probe and the liquid sufficiently conducting that the potential difference can be measured by means of a high-impedance dc voltmeter that serves as a null indicator in a standard potentiometer circuit. A submerged reference electrode may be a silver-silver chloride electrode. One generally compares the potential of the film-covered surface with that of the film-free one [83, 84]. 

Silvcr/Silvcr Chloride Electrodes Another common reference electrode is the silver/silver chloride electrode, which is based on the redox couple between AgCl and Ag. 

When the potential of an electrode of the first kind responds to the potential of another ion that is in equilibrium with M"+, it is called an electrode of the second kind. Two common electrodes of the second kind are the calomel and silver/silver chloride reference electrodes. Electrodes of the second kind also can be based on complexation reactions. Eor example, an electrode for EDTA is constructed by coupling a Hg +/Hg electrode of the first kind to EDTA by taking advantage of its formation of a stable complex with Hg +. 

Reference Electrodes and Liquid Junctions. The electrical cincuit of the pH ceU is completed through a salt bridge that usually consists of a concentrated solution of potassium chloride [7447-40-7]. The solution makes contact at one end with the test solution and at the other with a reference electrode of constant potential. The Hquid junction is formed at the area of contact between the salt bridge and the test solution. The mercury—mercurous chloride electrode, the calomel electrode, provides a highly reproducible potential in the potassium chloride bridge solution and is the most widely used reference electrode. However, mercurous chloride is converted readily into mercuric ion and mercury when in contact with concentrated potassium chloride solutions above 80°C. This disproportionation reaction causes an unstable potential with calomel electrodes. Therefore, the silver—silver chloride electrode and the thallium amalgam—thallous chloride electrode often are preferred for measurements above 80°C. However, because silver chloride is relatively soluble in concentrated solutions of potassium chloride, the solution in the electrode chamber must be saturated with silver chloride. 

Measuring electrodes for impressed current protection are robust reference electrodes (see Section 3.2 and Table 3-1) which are permanently exposed to seawater and remain unpolarized when a small control current is taken. The otherwise usual silver-silver chloride and calomel reference electrodes are used only for checking (see Section 16.7). All reference electrodes with electrolytes and diaphragms are unsuitable as long-term electrodes for potential-controlled rectifiers. Only metal-medium electrodes which have a sufficiently constant potential can be considered as measuring electrodes. The silver-silver chloride electrode has a potential that depends on the chloride content of the water [see Eq. (2-29)]. This potential deviation can usually be tolerated [3]. The most reliable electrodes are those of pure zinc [3]. They have a constant rest potential, are slightly polarizable and in case of film formation can be regenerated by an anodic current pulse. They last at least 5 years. 

Table 2.24 Breakdown potentials (mV) for 316 stainless steel, titanium and cobalt-chromium-molybdenum alloy in oxygen-free 0.17 m NaCl solution at 37°C using a silver/ silver chloride reference electrode.

Various types of reference electrodes have been considered in Section 20.3, and the potentials of these electrodes and their variation with the activity of the electrolyte are listed in Table 21.7, Chapter 21. It is appropriate, however to point out here that the saturated calomel electrode (S.C.E.), the silver-silver chloride electrode and the copper-copper sulphate electrode are the most widely used in corrosion testing and monitoring. 

This electrode is perhaps next in importance to the calomel electrode as a reference electrode. It consists of a silver wire or a silver-plated platinum wire, coated electrolytically with a thin layer of silver chloride, dipping into a potassium chloride solution of known concentration which is saturated with silver chloride this is achieved by the addition of two or three drops of 0.1M silver nitrate solution. Saturated potassium chloride solution is most commonly employed in the electrode, but 1M or 0.1 M solutions can equally well be used as explained in Section 15.1, the potential of the electrode is governed by the activity of the chloride ions in the potassium chloride solution. 

Glass electrodes are now available as combination electrodes which contain the indicator electrode (a thin glass bulb) and a reference electrode (silver-silver chloride) combined in a single unit as depicted in Fig. 15.2(h). The thin glass bulb A and the narrow tube B to which it is attached are filled with hydrochloric acid and carry a silver-silver chloride electrode C. The wide tube D is fused to the lower end of tube B and contains saturated potassium chloride solution which is also saturated with silver chloride it carries a silver-silver chloride electrode E. The assembly is sealed with an insulating cap. 

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 ... 

Prepare an approximately 0.1 M silver nitrate solution. Place 0.1169 g of dry sodium chloride in the beaker, add 100 mL of water, and stir until dissolved. Use a silver wire electrode (or a silver-plated platinum wire), and a silver-silver chloride or a saturated calomel reference electrode separated from the solution by a potassium nitrate-agar bridge (see below). Titrate the sodium chloride solution with the silver nitrate solution following the general procedure described in Experiment 1 it is important to have efficient stirring and to wait long enough after each addition of titrant for the e.m.f. to become steady. Continue the titration 5 mL beyond the end point. Determine the end point and thence the molarity of the silver nitrate solution. 

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. 

Before use, condition the new electrode by employing it in the titration of a sodium lauryl sulfate solution with a solution of Hyamine 1622 or benzetho-nium chloride solution, 0.04 M using a silver-silver chloride reference electrode. During this conditioning, a layer of an insoluble cation-anion complex becomes firmly attached to the plasticized PVC coating rendering it sensitive to both anionic and cationic surfactants. 

Instead of using an indicator, an ion-sensitive electrode can be used. An aqueous solution is titrated potentiometrically against 0.04 N hyamine 1622 solution using a nitrate ion-selective electrode and a silver/silver chloride reference electrode [106]. 

Salts such as silver chloride or lead sulfate which are ordinarily called insoluble do have a definite value of solubility in water. This value can be determined from conductance measurements of their saturated solutions. Since a very small amount of solute is present it must be completely dissociated into ions even in a saturated solution so that the equivalent conductivity, KV, is equal to the equivalent conductivity at infinite dilution which according to Kohlrausch s law is the sum of ionic conductances or ionic mobilities (ionic conductances are often referred to as ionic mobilities on account of the dependence of ionic conductances on the velocities at which ions migrate under the influence of an applied emf) ... 

The described electrodes, and especially the silver chloride, calomel and mercurous sulphate electrodes are used as reference electrodes combined with a suitable indicator electrode. The calomel electrode is used most frequently, as it has a constant, well-reproducible potential. It is employed in variously shaped vessels and with various KC1 concentrations. Mostly a concentration of KC1 of 0.1 mol dm-3, 1 mol dm-3 or a saturated solution is used (in the latter case, a salt bridge need not be employed) sometimes 3.5 mol dm-3 KC1 is also employed. The potentials of these calomel electrodes at 25°C are as follows (according to B. E. Conway) ... 

In addition to their use as reference electrodes in routine potentiometric measurements, electrodes of the second kind with a saturated KC1 (or, in some cases, with sodium chloride or, preferentially, formate) solution as electrolyte have important applications as potential probes. If an electric current passes through the electrolyte solution or the two electrolyte solutions are separated by an electrochemical membrane (see Section 6.1), then it becomes important to determine the electrical potential difference between two points in the solution (e.g. between the solution on both sides of the membrane). Two silver chloride or saturated calomel electrodes are placed in the test system so that the tips of the liquid bridges lie at the required points in the system. The value of the electrical potential difference between the two points is equal to that between the two probes. Similar potential probes on a microscale are used in electrophysiology (the tips of the salt bridges are usually several micrometres in size). They are termed micropipettes (Fig. 3.8D.)... 

The membrane of the glass electrode is blown on the end of a glass tube. This tube is filled with a solution with a constant pH (acetate buffer, hydrochloric acid) and a reference electrode is placed in this solution (silver chloride or calomel electrodes). During the measurement, this whole system is immersed with another reference electrode into the test solution. The membrane potential of the glass electrode, when the internal and analysed... 

Fig. 2.4j is a simplified diagram of an amperometric detector. Three electrodes are used, called working, auxiliary and reference electrodes (we ae and re). The we is the electrode at which the electroactivity is monitored, and the re, usually a silver-silver chloride electrode, provides a stable and reproducible voltage to which the potential of the we can be referenced. The ae, usually stainless steel, is a current-carrying electrode. 

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