Adsorption of silver ion

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, 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-... 

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). 

FIGURE 6.44 Adsorption of Silver ions (Ag(I)) on modified activated carbons as a function of equilibrium time. (Source Rivera-Utrilla, J. and Ferro-Garcia, M. A., Adsorp. Sci. TechnoL, 3, 293, 1986. With permission.)... 

The solution to be measured is acidified to allow a smooth reaction at the reference electrode, and also to prevent adsorption of silver ions on the precipitate and to prevent the precipitation of other silver salts, like phosphate and carbonate. 

A current example investigating the mechanism of this method was employed by Borguet and coworkers. Utilizing small concentrations of silver ions and an electrochemical scanning tunnelling microscope, they were able to visualize the electrodeposition of Ag utilizing a sodium dodecyl sulfate (SDS) surfactant. In this case, the SDS adsorbs in parallel lines on the Au(lll) current collector surface. The lines appear wavy due to the adsorption of silver ions. After the application of... 

Song X, Gunawan P, Jiang R, Leong SSJ, Wang K, Xu R (2011) Surface activated carbon nanospheres for fast adsorption of silver ions from aqueous solutions. J Hazard Mater 194 162-168... 

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. 

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. 

Idzikowski, S. (1973) Adsorption of inorganic ions on iron(III) oxide from mixtures of strong electrolytes. IV. Adsorption of silver, cupric and aluminium sulphate in presence of indifferent electrolyte. Rocz. Chem. 231-238... 

Multicomponent vC—C bands were also observed in the SER spectra of phenylacetylene adsorbed on copper, silver (114), and gold (83) electrodes. The principal components characterizing the species on Ag and Au are at ca. 2017 and 1985 cm"1 (An —93 and —125 cm"1), respectively. The higher wavenumber band is displaced by the adsorption of chloride ions when the potential was changed from —0.6 to 0.0 V. Four nC—C components were observed for the species on the copper electrode, centered... 

Unlike rhodium, iridium can be an effective sensitizer for latent image formation. Mitchell and associates (187), and West and Saunders (188) used chloroiridite to sensitize sheet crystals of silver bromde. Silver chloroiridite, formed by adsorption of chloroiridite ions by the crystal surface, provides an effective hole trap. However, gelatin interferes with its use in this way in photographic emulsions. 

The dissolution process begins with an atom in a half crystal position at the surface. In the case of silver, it can be shown that two steps can be experimentally distinguished (3,4) (1) moving of half crystal atoms into adsorption positions on the surface and (2) transfer of silver ions from adsorption positions through the inner double layer into the solution. 

It has been shown that Cu and Ag dissolve at essentially the same transport-controlled rate in acidified ceric sulfate solution (37). The authors postulate that the larger enthalpy of this reaction facilitates desorption of the product ions. The equilibrium adsorption of silver salts on silver is dependent on the anion and is much less than a monolayer at low concentrations (38). 

The adsorption of cyanate ions was studied on silver [113, 135] and gold electrodes. A bipolar band can be observed for adsorption on a silver electrode [135] (Fig. 45). Corrigan and Weaver [135] have attempted a deconvolution of this band after observing that the intensity of the positive-going part was lower than expected... 

Points of zero charge can only be defined for surfaces that can be either positively or negatively charged. For silver halides and oxides this means that at least two charging reactions must be considered. The point of zero charge of an oxide cannot be determined by, say, only adsorption of protons. As a second process the desorption of protons or adsorption of hydroxyl ions presents itself. Thermodynamically the sum effect of these two charging mechanisms, or rather their competition, is observed. Therefore, points of zero charge are related to a... 

The adsorption of alkali Ions (and of earth alkali ions, not shown) differs from that of the anions SO and HPO In that the latter adsorb specifically on uncharged silver Iodide, with the concomitant change In p.z.c. (sec. 3.8. fig. 3.23 -25), whereas the former do not shift the p.z.c. For alkali ions, specificity starts only when there Is already 1 on the surface. This Is an example of specific adsorption of the second kind, as defined in sec. 3.6d. Apparently, the alkali Ions only adsorb on 1 sites, so that there will be some analogy with water structure-originating alkali lon-iodlde Ion interaction In solution. We will come back to this in sec. 3.10g. 

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