Silver chloride, adsorption

Fig. 9-6. Adaorption coverage of a reaction intermediate of hydroxo-complezes in anodic dissolution of a metallic iron electrode as a function of electrode potential in acidic sulfate solutions at pH 1.0, 2.0 and 3.0 solution is 0.5 M (Ns2S04 + H2SO4) at room temperature. Oqh adsorption coverage of reaction intermediates FeOH,4 and PeOH Vss = volt referred to the saturated silver-silver chloride electrode. [Prom Tsuru, 1991.]...

The reduction of silver chloride, precipitated in the presence of excess chloride ion, yielded the S-shaped curve typical of an autocatalyzed reaction (James, 25). The initial reaction rate, measured in terms of the reciprocal of the time required to complete 5 % of the total reaction, varied directly as the hydroxylamine concentration and inversely as the chloride ion concentration when the latter was relatively large. The specific surface of the freshly prepared precipitate, as measured by dye adsorption, decreased with aging, and the reaction rate decreased proportionately. 

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. 

The combination of foreign ions with the surfaces of colloidal particles is, in general, called adsorption. It goes without saying that this behaviour is not limited to small particles and that a large crystal of silver chloride can equally well adsorb positive or negative particles. 

Colloidal dispersions owe their stability to a surface charge and the resultant electrical repulsion of charged particles. This charge is acquired by adsorption of cations or anions on the surface. For example, an ionic precipitate placed in pure water will reach solubility equilibrium as determined by its solubility product, but the solid may not have the same attraction for both its ions. Solid silver iodide has greater attraction for iodide than for silver ions, so that the zero point of charge (the isoelectric point) corresponds to a silver ion concentration much greater than iodide, rather than to equal concentrations of the two ions. The isoelectric points of the three silver halides are ° silver chloride, pAg = 4, pCl = 5.7 silver bromide, pAg = 5.4, pBr = 6.9 silver iodide, pAg = 5.5, pi = 10.6. For barium sulfate the isoelectric point seems to be dependent on the source of the product and its de ee of perfection. ... 

FIQ. Ifl.H. Representation of silver chloride colloidal particle and adsorptive layers when Cl" is in excess. 

Polyaniline salts may also be deposited by in situ adsorption polymerization" in a few minutes, as strongly adhering films on a variety of substrates/ such as natural and synthetic fibers and textiles,8 plastic, glass, silver chloride pellets etc.,9 by immersing the substrate in a freshly mixed acidic aqueous solution of aniline and oxidizing agent, such as ammonium peroxydisulfate. It is believed that a reactive intermediate, possibly an oligomeric radical cation of aniline, is first adsorbed, which subsequently polymerizes.8... 

The reaction takes place in very low silver ion concentration, so that even silver chloride can be detected by the dark reaction product. The latter is probably not a mixture of metallic silver and manganese dioxide, but rather an adsorption complex because its color is much more intense than that of a mere mixture of these materials. 

Surface heterogeneity may be inferred from emission studies such as those studies by de Schrijver and co-workers on P and on R adsorbed on clay minerals [197,198]. In the case of adsorbed pyrene and its derivatives, there is considerable evidence for surface mobility (on clays, metal oxides, sulfides), as from the work of Thomas [199], de Mayo and co-workers [200], Singer [201] and Stahlberg et al. [202]. There has also been evidence for ground-state bimolecular association of adsorbed pyrene [66,203]. The sensitivity of pyrene to the polarity of its environment allows its use as a probe of surface polarity [204,205]. Pyrene or ofter emitters may be used as probes to study the structure of an adsorbate film, as in the case of Triton X-100 on silica [206], sodium dodecyl sulfate at the alumina surface [207] and hexadecyltrimethylammonium chloride adsorbed onto silver electrodes from water and dimethylformamide [208]. In all cases progressive structural changes were concluded to occur with increasing surfactant adsorption. 

Precipitation of silver bromide will occur when the concentration of the bromide ion in the solution is 2.0 x 103 times the iodide concentration. The separation is therefore not so complete as in the case of chloride and iodide, but can nevertheless be effected with fair accuracy with the aid of adsorption indicators (Section 10.75(c)). 

Either the Mohr titration or the adsorption indicator method may be used for the determination of chlorides in neutral solution by titration with standard 0.1M silver nitrate. If the solution is acid, neutralisation may be effected with chloride-free calcium carbonate, sodium tetraborate, or sodium hydrogencarbonate. Mineral acid may also be removed by neutralising most ofthe acid with ammonia solution and then adding an excess of ammonium acetate. Titration of the neutral solution, prepared with calcium carbonate, by the adsorption indicator method is rendered easier by the addition of 5 mL of 2 per cent dextrin solution this offsets the coagulating effect of the calcium ion. If the solution is basic, it may be neutralised with chloride-free nitric acid, using phenolphthalein as indicator. 

Similar remarks apply to the determination of bromides the Mohr titration can be used, and the most suitable adsorption indicator is eosin which can be used in dilute solutions and even in the presence of 0.1 M nitric acid, but in general, acetic (ethanoic) acid solutions are preferred. Fluorescein may be used but is subject to the same limitations as experienced with chlorides [Section 10.77(b)], With eosin indicator, the silver bromide flocculates approximately 1 per cent before the equivalence point and the local development of a red colour becomes more and more pronounced with the addition of silver nitrate solution at the end point the precipitate assumes a magenta colour. 

Discussion. The method is applicable to the determination of a mixture of two salts having the same anion (e.g. sodium chloride and potassium chloride) or the same cation (e.g. potassium chloride and potassium bromide). For example, to determine the amount of sodium and potassium chlorides in a mixture of the two salts, a known weight (Wj g) of the solid mixture is taken, and the total chloride is determined with standard 0.1 M silver nitrate, using Mohr s method or an adsorption indicator. Let w2 g of silver nitrate be required for the complete precipitation of Wj g of the mixture, which contains xg of NaCl and yg of KC1. Then ... 

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