Silver ion adsorption

Table 3.1 Physical properties, silver ion adsorptions, and photocataiytic activities of Ti02 powders...

Parodi, P.W. 1980. Separation of milk fat triglycerides into classes by silver ion adsorption thin-layer chromatography. Austr. J. Dairy Technol. 35, 17-22. 

Weakly marked and broad, the second anodic peak shows that the D—N carbon contains the smallest quantity of electroactive oxides, which indicates that it is resistant to oxidation during silver ion adsorption. 

After the addition of silver nitrate, potassium nitrate is added as coagulant, the suspension is boiled for about 3 minutes, cooled and then titrated immediately. Desorption of silver ions occurs and, on cooling, re-adsorption is largely prevented by the presence of potassium nitrate. 

A study518 of the mechanism of oxidation of alcohols by the reagent suggested that a reversible, oriented adsorption of the alcohol onto the surface of the oxidant occurs, with the oxygen atom of the alcohol forming a coordinate bond to a silver ion, followed by a concerted, irreversible, homolytic shift of electrons to generate silver atoms, carbon dioxide, water, and the carbonyl compound. The reactivity of a polyhydroxy compound may not, it appears, be deduced from the relative reactivity of its component functions, as the geometry of the adsorbed state, itself affected by solvent polarity, exerts an important influence on the selectivity observed.519... 

The end points of precipitation titrations can be variously detected. An indicator exhibiting a pronounced colour change with the first excess of the titrant may be used. The Mohr method, involving the formation of red silver chromate with the appearance of an excess of silver ions, is an important example of this procedure, whilst the Volhard method, which uses the ferric thiocyanate colour as an indication of the presence of excess thiocyanate ions, is another. A series of indicators known as adsorption indicators have also been utilized. These consist of organic dyes such as fluorescein which are used in silver nitrate titrations. When the equivalence point is passed the excess silver ions are adsorbed on the precipitate to give a positively charged surface which attracts and adsorbs fluoresceinate ions. This adsorption is accompanied by the appearance of a red colour on the precipitate surface. Finally, the electroanalytical methods described in Chapter 6 may be used to scan the solution for metal ions. Table 5.12 includes some examples of substances determined by silver titrations and Table 5.13 some miscellaneous precipitation methods. Other examples have already been mentioned under complexometric titrations. 

The Ag2 S ISE has Nemstian response dE/d log a( = 0.0296 V in the sulphide concentration range 10" to 10" M and silver ions from 10 to 10 M if the solutions are prepared from pure salts, as a further concentration decrease is prevented by adsorption on the glass (see p. 76 and [87, 163]). After prolonged use, the limit of the Nemstian behaviour shifts to about 10" m [130] as a result of formation of mixed potentials on accumulation of metallic silver in the membrane surface. An analogous deterioration in the membrane function in the presence of iodine results from surface oxidation [23]. Cyanide interferes only at large concentrations the equilibrium constant of the reaction... 

The variation of rate with a fractional power of the silver ion concentration can be explained if adsorption of the silver ions by the silver occurs prior to reaction and if the adsorption follows a Freundlich isotherm. The reaction rate then would be directly proportional to the concentration - of adsorbed silver ions, since the concentration of adsorbed ions would be related to that in solution by the equation... 

Direct evidence for the adsorption of silver ions by silver and gold has been obtained by several investigators (Euler, 8 Proskurnin and Frumkin, 9 Veselovsky, 10 Euler and Zimmerlund, 11 cf. James, 12). Veselovsky, who was careful to work with oxide-free surfaces, found that adsorption begins at a silver ion concentration of only 1 X 10-18. The concentration used in the kinetic experiments just discussed are greater than this by several orders of magnitude. 

Thus, the available evidence indicates that little or no adsorption of hydroquinone by silver occurs. Rabinovich s data are unacceptable because of the large experimental errors involved. The possible amount of adsorption indicated by the data of Perry, Ballard, and Sheppard does not exceed the limits of error in their analytical determination of hydroquinone and could not under any circumstances cover more than a small fraction of the silver surface. The kinetics of the reaction between hydroquinone and silver ions do not indicate adsorption of the reducing agent, although the first-order dependence of rate on concentration is not incompatible with weak adsorption. It seems unlikely, accordingly, that adsorption of hydroquinone by silver plays a role of any consequence in the silver catalysis of the reaction between hydroquinone and silver ion. 

Thus, an equation in agreement with the experimental data for the hydroquinone-silver ion reaction can be derived either on the basis of the assumption that adsorption of silver ions by the silver is a prelude to the reaction, or on the basis of the assumption that the rate-controlling step in an electrode process is the rate of transfer of electrons to the silver electrode. The first mechanism carries with it the assumption that a silver ion adsorbed by silver is more easily reduced than an ion in solu-... 

The data for the p-phenylenediamine-silver ion reaction are not accounted for by Bagdasar yan s treatment. On the basis of an adsorption mechanism, the data would suggest that the important phase for the catalyzed reaction is adsorption of the p-phenylenediamine by the silver catalyst. The extent of the adsorption would depend upon the surface conditions of the catalyst, which apparently depend on changes in the protective colloid or in the salt concentration. A catalytic mechanism involving activation of the p-phenylenediamine by the catalyst would be consistent with the observation of Weissberger and Thomas that colloidal silver markedly catalyzes the oxygen oxidation of p-phenylenediamine. 

In some cases it has been found that the maximum on saturation adsorption of a solute from a solution corresponds to the formation of an adsorption layer one molecule thick. Thus Euler Zeit. Elehtrochem. xxviii. 446,1922) found that a maximum adsorption of silver ions by silver and gold leaf was attained in a 0 03 A solution. It was found that 5 5 and 8 5 to 9 mgm. of silver ions were adsorbed by a square metre of metallic silver and gold respectively, such a surface concentration is practically unimolecular. The adsorption of silver ions by silver bromide (K. Fajans, Zeit Phys. Ohem. cv. 256, 1928) was found on the other hand to be not complete, for only every fourth bromide ion in a silver bromide surface was found to adsorb a silver ion. Similar conclusions as to the unimolecular character of the adsorbed film in the case of chemical charcoal as an adsorbing agent for fatty and amino acids may be drawn from the data of Foder and Schonfeld Koll. Zeit xxxi. 76, 1922). 

In all these studies, there is general agreement that silver is the source of the electrons, but the identity of the adsorption site of the oxygen is not clear. Since electron transfer mechanisms of the type described earlier (Fig. 16) can occur, silver ions are not necessarily the adsorption site and the gzz values would also be consistent with adsorption on the support. However, the degree of dispersion of the silver will be very dependent on the method of preparation and clusters of Ag atoms or ions of different sizes are possible. This leads to an alternative explanation of the low gzz values, since these... 

Figure 27-2 shows a colloidal particle of AgCI growing in a solution containing excess Ag+, H+, and N03. The surface of the particle has excess positive chaige due to the adsorption of extra silver ions on exposed chloride ions. (To be adsorbed means to be attached to the... 

The structure of and possible cation location in these materials is fairly well known (2, 8, 4, )> and their ion-exchange behavior toward a multitude of pairs of ions, mostly including sodium, has been measured and interpreted in terms of basic properties of ions, crystal structures, and pore dimensions. The major part of these studies is with alkali- and alkaline-earth cations, alkylammonium ions, rare-earth cations, and silver and thallium ions (1). In contrast, the ion adsorption of transition metals in faujasite has received little attention. 

Sensitization by compounds that contain a labile sulfur atom, such as thiosulfate ion and thiourea, was established by Sheppard in 1925 (121), although Newton (122) obtained this form of sensitization in 1874 without recognizing it. The steps in the sensitizing process involve (a) adsorption of the sensitizer by the silver halide, (b) reaction of the adsorbed sensitizer with silver ions to form Ag2S, and (c) probably rearrangement of the Ag2S molecules on the grain surface to form nuclei. 

The adsorption of trace silver ion (2 x 10-6 M) has been studied by use of a silver ion-selective electrode the results are shown in Figure 6.6. For periods of up to a day, silver losses on Teflon were less than 2% at the end of 30 days the losses (28%) were least for Vycor glass. The study indicates that none of the materials is suitable for long-term storage of solutions containing low levels of silver unless a complexing ligand such as thiosulfate is present. 

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