Silver chloride formation

A fundamental requirement for all coulometric methods is 100% current efficiency that is, each faraday of electricity must bring about chemical change in the analyte equivalent to one mole of electrons. Note that 100% current efficiency can be achieved without direct participation of the analyte in electron transfer at an electrode. For example, chloride ion may be determined quite easily using poten-tiostatic coulometry or using coulometric titrations with silver ion at a silver anode. Silver ion then reacts with chloride to form a precipitate or deposit of silver chloride. The quantity of electricity required to complete the silver chloride formation serves as the analytical variable. In this instance, 100% current efficiency is realized because the number of moles of electrons is essentially equal to the number of moles of chloride ion in the sample despite the fact that these ions do not react directly at the electrode surface. 

Birss V I and Smith C K 1987 The anodic behaviour of silver in chloride solutions-l. The formation and reduction of thin silver chloride films Electrochim. Acta 32 259-68... 

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 +. 

Bronze disease necessitates immediate action to halt the process and remove the cause. For a long time, stabilization was sought by removal of the cuprous chloride by immersing the object in a solution of sodium sesquicarbonate. This process was, however, extremely time-consuming, frequentiy unsuccesshil, and often the cause of unpleasant discolorations of the patina. Objects affected by bronze disease are mostiy treated by immersion in, or surface appHcation of, 1 H-henzotriazole [95-14-7] C H N, a corrosion inhibitor for copper. A localized treatment is the excavation of cuprous chloride from the affected area until bare metal is obtained, followed by appHcation of moist, freshly precipitated silver oxide which serves to stabilize the chloride by formation of silver chloride. Subsequent storage in very dry conditions is generally recommended to prevent recurrence. 

Silver Chloride. Silver chloride, AgCl, is a white precipitate that forms when chloride ion is added to a silver nitrate solution. The order of solubility of the three silver halides is Cl" > Br" > I. Because of the formation of complexes, silver chloride is soluble in solutions containing excess chloride and in solutions of cyanide, thiosulfate, and ammonia. Silver chloride is insoluble in nitric and dilute sulfuric acid. Treatment with concentrated sulfuric acid gives silver sulfate. 

Halide Complexes. Silver hahdes form soluble complex ions, AgX and AgX , with excess chloride, bromide, and iodide. The relative stabihty of these complexes is 1 > Br > Cl. Complex formation affects solubihty greatiy. The solubihty of silver chloride in 1 A/ HCl is 100 times greater than in pure water. 

Qualitative. The classic method for the quaUtative determination of silver ia solution is precipitation as silver chloride with dilute nitric acid and chloride ion. The silver chloride can be differentiated from lead or mercurous chlorides, which also may precipitate, by the fact that lead chloride is soluble ia hot water but not ia ammonium hydroxide, whereas mercurous chloride turns black ia ammonium hydroxide. Silver chloride dissolves ia ammonium hydroxide because of the formation of soluble silver—ammonia complexes. A number of selective spot tests (24) iaclude reactions with /)-dimethy1amino-henz1idenerhodanine, ceric ammonium nitrate, or bromopyrogaHol red [16574-43-9]. Silver is detected by x-ray fluorescence and arc-emission spectrometry. Two sensitive arc-emission lines for silver occur at 328.1 and 338.3 nm. 

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. 

This material can be used only in seawater or similar chloride-containing electrolytes. This is because the passivation of the silver at discontinuities in the platinum is dependent upon the formation of a film of silver chloride, the low solubility of which, in seawater, inhibits corrosion of the silver. This anode, consisting of Pt-lOPd on Ag, was tried as a substitute for rapidly consumed aluminium, for use as a trailing wire anode for the cathodic protection of ships hulls, and has been operated at current densities as high as 1 900 AmHowever, the use of trailing anodes has been found inconvenient with regard to ships manoeuvrability. 

Though both silver nitrate and sodium chloride have high solubility in water, silver chloride is very slightly soluble. What will happen if we mix a solution of silver nitrate and sodium chloride Then, we will have a solution that includes the species present in a solution of silver chloride, Ag+(aq) and Cl (ag), but now they are present at high concentration The Ag+(agJ came from reaction (8) and the Cl (aq) came from reaction (6) and their concentrations far exceed the solubility of silver chloride. The result is that solid will be formed. The formation of solid from a solution is called precipitation ... 

Consider now a somewhat different type of complex ion formation, viz. the production of a complex ion with constituents other than the common ion present in the solution. This is exemplified by the solubility of silver chloride in ammonia solution. The reaction is ... 

Generally, such a remarkable restriction of metal dissolution results not only from the formation of a thin surface oxide film but also from the formation of a comparatively thick film such as silver chloride or zinc chloride. In this chapter, however, we use the term passive film only for compact and thin oxide films. 

Complex formation removes some of the Ag+ ions from solution. As a result, to preserve the value of Ksp, more silver chloride dissolves. Formation of a complex increases the solubility of a sparingly soluble compound. 

Precipitated silver chloride dissolves in ammonia solutions as a result of the formation of Ag(NH3)2+. What is the solubility of silver chloride in 1.0 M NH.(aq) ... 

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.)... 

It was observed in those cases in which the oxidation was arrested too soon that an appreciable amount of silver chloride was deposited when the solution was concentrated. Furthermore, the mother liquor, when treated with phenyl-hydrazine in acetic acid, deposited some yellow crystalline D,L-mannose phenylhydrazone, m. p. 195-200° (Maquenne block). It is apparent that too vigorous oxidation results in the formation of hexoses. 

In Nebraska, state regulations require that the chemical makeup of animal feed sold in the state be accurately reflected on the labels found on the feed bags. The Nebraska State Agriculture Laboratory is charged with the task of performing the analytical laboratory work required. An example is salt (sodium chloride) content. The method used to analyze the feed for sodium chloride involves a potentio-metric titration. A chloride ion-selective electrode in combination with a saturated calomel reference electrode is used. After dissolving the feed sample, the chloride is titrated with a silver nitrate standard solution. The reaction involves the formation of the insoluble precipitate silver chloride. The electrode monitors the decrease in the chloride concentration as the titration proceeds, ultimately detecting the end point (when the chloride ion concentration is zero). 

A common test for chloride ions in a solution involves adding AgN03(ag) to the solution being tested. If chloride ions are present in sufficient quantity, the chemist will observe a white cloudiness that indicates the formation of a precipitate. Kgp for silver chloride is 1.8 x 10 °. 

The silver-silver chloride electrode (Ag AgCl) is easily and cheaply made. Two silver electrodes are cleaned (see Section 9.1.1 above) and immersed in aqueous KCl solution (a concentration of 0.1 mol dm is convenient). Next, a potential of about 2 V is applied across them for c. 10 min, causing a thin outer film of silver chloride to develop on the positive electrode. Solid AgCl is formed by a two-step reaction, involving first the electro-formation of silver ion ... 

The relative rate of fog formation compared to image development increases with increasing pH of the hydroxylamine solution. This is to be expected from analogy with the studies of the reduction of silver chloride and silver bromide precipitates, where the change in nitrogen yield shows that the uncatalyzed reaction becomes more and more prominent as the pH is increased. 

Silver dissolves very slowly in hot concentrated sulfuric acid forming silver sulfate, Ag2S04. Reaction with hydrochloric acid is slow and stops after initial formation of a protective layer of silver chloride on the surface. 

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