Silver coulometry anode

The determination of chloride using an instrument known as a chloride meter is probably the most common application of coulometry in biochemistry. The instrument is designed to generate silver ions electrolytically from a silver anode. These ions are removed from the solution as undissociated silver chloride, which is either deposited on the anode or precipitated in the solution. A low concentration of carrier electrolyte (nitrate ions) permits a small current... 

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. 

Two series of linear silver salts of imides and anilides, formulated as [Ag(L)2]NEt4 (amide = 44a-h) and [Ag(L)2]Ag (L = 44a-e), have been studied using linear voltammetry, cyclic voltammetry and coulometry at Pt and vitreous electrodes in acetonitrile53,54. The linear voltammograms obtained for [Ag(L)2]Ag show two cathodic waves and one anodic wave (see Table 11). The first cathodic wave at ca —0.2 V vs Ag/Ag+ for [Ag(44a)2]Ag corresponds to reduction of the loosely bound silver ion (equation 10) and the second wave at ca —1.3 V vs Ag/Ag+ corresponds to reduction of the tightly bound silver ion (equation 11). As expected, there is only one reduction wave at —1.35 V for [Ag(44a)2]NEt4 which corresponds to reduction of the tightly bound silver ion. 

Controlled-potential coulometry may be applied to the analysis of a wide variety of substances. Clearly, metals like copper could be deposited and determined without the necessity of weighing the electrode. More importantly, mercury electrodes can be used and thus most of the metals more difficult to reduce can be determined. Also, it is possible to apply coulometry to systems in which both oxidized and reduced forms are soluble, such as determining iron by reducing iron(III) to iron(II). Anions such as chloride or bromide may be converted to AgCl or AgBr by deposition on a silver anode. 

The thickness of a pure silver plate on a base metal is to be determined by controlled-potential coulometry. The metal sheet is masked except for a circular area 0.50 cm in diameter electrical connection is made to the metal, the sheet is clamped in a cell so that the unmasked area is covered with electrolyte, and the silver plate is anodically stripped. Calculate the average thickness of the silver... 

Rees NV, Zhou Y-G, Compton RG (2011) The aggregation of silver nanoparticles in aqueous solution investigated via anodic particle coulometry. ChemPhysChem 12 1645-1647... 

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