Silver chloride, activated

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. 

The silver ions involved are derived from the silver chloride, and by the solubility product principle (Section 2.6), the activity of these ions will be governed by the chloride-ion activity... 

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. 

Indicator electrodes for anions may take the form of a gas electrode (e.g. oxygen electrode for OH- chlorine electrode for Cl-), but in many instances consist of an appropriate electrode of the second kind thus as shown in Section 15.1, the potential of a silver-silver chloride electrode is governed by the chloride-ion activity of the solution. Selective-ion electrodes are also available for many anions. 

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 ... 

If the pellet contains a mixture of silver sulphide and silver chloride (or bromide or iodide), the electrode acquires a potential which is determined by the activity of the appropriate halide ion in the test solution. Likewise, if the pellet contains silver sulphide together with the insoluble sulphide of copper(II), cadmium) II), or lead) II), we produce electrodes which respond to the activity of the appropriate metal ion in a test solution. 

G. A. Linhart, The Applicability of the Precipitated Silver-Silver Chloride Electrode to the Measurement of the Activity of Hydrochloric Acid in Extremely Dilute Solutions", J. Am. Chem. Soc., 41, 1175-1180 (1919). 

Thioglycosides containing free OH groups readily tolerate silver triflate-pro-moted glycosylations with glycosyl bromides and chlorides, and also tin(II) chloride-silver perchlorate-activated glycosylation, and several examples have... 

A schematic diagram of a typical pH electrode system is shown in Fig. 10.1. The cell potential, i.e. the electromotive force, is measured between a pH electrode and a reference electrode in a test solution. The pH electrode responds to the activity or concentration of hydrogen ions in the solution. The reference electrode has a very stable half-cell potential. The most commonly used reference electrodes for potentiometry are the silver/silver chloride electrodes (Ag/AgCl) and the saturated calomel electrodes (SCE). 

The cell consists of an indicator and a reference electrode, the latter usually being the calomel or silver-silver chloride type. The potential of the indicator electrode is related to the activities of one or more of the components of the solution and it therefore determines the overall cell potential. Ideally, its response to changes of activity should be rapid, reversible and governed by the Nernst equation. There are two types of indicator electrode which possess the desired characteristics - metallic and membrane. 

Metals which form sparingly soluble salts will also respond to changes in the activity of the relevant anion provided the solution is saturated with the salt, e.g. for silver in contact with a saturated solution of silver chloride and containing solid silver chloride the electrode reaction is AgCl + e = Ag + Cl, and the electrode potential is given by ... 

In some applications, silver/silver chloride or calomel electrodes are considered cumbersome to use and maintain. More importantly, they are extremely difficult to miniaturize particularly with regard to their combined use with potentiometric membrane electrodes (see Section 18a.4.5.4) that have been fabricated into highly miniaturized and compact screen-printed sensor arrays for clinical use. Thus, several reference electrodes are manufactured with the same polymeric materials that are needed to design the responsive ion-selective membranes [7]. Incorporation of suitable active agents into such membranes leads to potentiometric responses that are ideally independent of the sample... 

Figure 4.12 A solid-state electrode showing a second-order response. The electrode shown in Figure 4.11 can be modified by the incorporation of silver chloride into the membrane to enable the activity of chloride ions in a sample to be measured. A surface reaction between the test chloride ions and the membrane silver ions alters the activity of the latter, resulting in a change in the potential difference across the membrane.

Silver chloride is the active component within black and white photography. When AgCl (in a suitable emulsion matrix ) is exposed to light, it decomposes rapidly, as follows ... 

The action of an active intermediate oxidation product would explain another feature of the reaction. The reduction of silver ions by hydrazine is extremely sensitive to the presence of small amounts of copper. For example, a solution containing a mixture of silver nitrate, sodium sulfite and hydrazine which normally showed no sign of reduced silver for several minutes underwent almost immediate reaction when merely stirred with a clean copper rod. In the presence of gum arabic as stabilizer, streamers of colloidal silver passed out from the copper surface. Similarly, the addition of small amounts of cupric sulfate to a hydrazine solution eliminated the induction period of the reaction with silver chloride. 

The pH dependence of the rate of development by hydroxylamine indicates that the monovalent ion is the active species. The rate varies as about the 0.65 power of the hydroxylamine concentration at pH 12.7 and the 0.75 power at pH 10.8. These results suggest adsorption of the hydroxylamine ion, and are in complete agreement with previous findings for the catalyzed reduction of silver chloride precipitates. 

Optically Active Dodecammino-hexol-tetra-cobaltic Salts.3— These salts may be resolved by means of bromo-camphor sulphonic acid. Dodecammino-hexol-tetracobaltic chloride is mixed with silver bromo-camphor sulphonate, whereby all the chlorine is precipitated as silver chloride and the solution treated with dilute acetic acid. The ilrst crop of d-bromo-camphor sulphonate is laevo-rotatory, that from 1-bromo-eamphor sulphonate dextro-rotatory. 

This oxidation is unique, since only silver is capable of epoxidizing ethylene, and silver is active only in the oxidation of ethylene. A low-surface-area a-alumina is usually applied to support about 10-15% silver. Organic halides (1,2-dichloro-ethane, ethyl and vinyl chloride) are added as moderators, and additives (Cs, Ba) are also used to increase selectivity. At present selectivity in industrial oxidations is about 80%. 

The potential difference between inner and outer silver-silver chloride electrodes in Figure 15-9 depends on the chloride concentration in each electrode compartment and on the potential difference across the glass membrane. Because [Cl-] is fixed in each compartment and because [H+] is fixed on the inside of the glass membrane, the only variable is the pH of analyte solution outside the glass membrane. Equation 15-3 states that the voltage of the ideal pH electrode changes by 59.16 mV for every pH-unit change of analyte activity at 25°C. 

In potentiometric measurements, the indicator electrode responds to changes in the activity of analyte, and the reference electrode is a self-contained half-cell with a constant potential. The most common reference electrodes are calomel and silver-silver chloride. Common indicator electrodes include (1) the inert Pt electrode, (2) a silver electrode responsive to Ag+, halides, and other ions that react with Ag+, and (3) ion-selective electrodes. Unknown junction potentials at liquid-liquid interfaces limit the accuracy of most potentiometric measurements. 

In previous sections of this chapter, magnesium anodes have been considered as replacements for zinc in Leclanche and air-depolarized cells. In sea water-activated reserve batteries, magnesium anodes are coupled with either silver chloride, lead chloride, manganese dioxide or, occasionally,... 

Fig. 3.33 Discharge curve for sea water-activated magnesium-silver chloride reserve cell...

Researchers at Solvay reported in a patent that aluminum trichloride can be activated with metal halides [e. g., silver chloride or iron(III) chloride] and acid salts NaF nllF.33 Thus, 1,2-dichloro-l.l,2-trifluoroethane (19) has been converted into 2,2-dichloro-l.l.Ttrifluoroethane (20), and l,1.2-trichloro-l,2.2-trifluoroethane (1) into l.1.l-trichloro-2.2.2-trifluoroethane (2) with reduced byproduct formation. 

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