Adsorption silver

The procedures of experiments were the following [15, 26]. After deposition of a specific quantity of silver on substrate the heating of a tray with silver was turned off, the shutter 7 was opened and the sensor was positioned opposite to the substrate in such a manner that the surface of the sensor was parallel to the surface of substrate. In these experiments we detected an irreversible donor signal of the sensor which can be related to adsorption silver atoms on the sensor made of a zinc oxide film. It is known [27] that silver atoms are donors of electrons. Note that the signals of the sensor were observed only when the sensor was positioned in front of a substrate. There were no signals detected in any other arrangement between sensor and substrate. 

Since there is an enormous density of such equivalent sites on the surface of a silver halide microcrystal, it would be surprising if a single speck of Ag2S were formed on each grain even under optimum conditions of sulfur sensitization. The sensitizer forms an adsorption silver complex which reacts to form an adsorbed molecule of Ag2S. 

Burshtain, D., L. Zeiri, and S. Efrima. 1999. Control of colloid growth and size distribution by adsorption-silver nanoparticles and adsorbed anisate. Langmuir 15 (9) 3050-3055. 

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. 

Figure Bl.22.6. Raman spectra in the C-H stretching region from 2-butanol (left frame) and 2-butanethiol (right), each either as bulk liquid (top traces) or adsorbed on a rough silver electrode surface (bottom). An analysis of the relative intensities of the different vibrational modes led to tire proposed adsorption structures depicted in the corresponding panels [53], This example illustrates the usefiilness of Raman spectroscopy for the detennination of adsorption geometries, but also points to its main limitation, namely the need to use rough silver surfaces to achieve adequate signal-to-noise levels.

Heiland W, Iberl F, Taglauer E and Menzel D 1975 Oxygen adsorption on (110) silver Surf. Sc/. 53 383-92... 

Tarazona A, Kreisig S, Koglin E and Schwuger M J 1997 Adsorption properties of two cationic surfactant classes on silver surfaces studied by means of SERS spectroscopy and ab initio calculations Prog. Colloid Polym. Sol. 103 181-92... 

Harder P, Grunze M, Dahint R, Whitesides G M and Laibinis P E 1998 Molecular conformation in oligo(ethylene glycol)-terminated self-assembled monolayers on gold and silver surfaces determines their ability to resist protein adsorption J. Rhys. Chem. B 102 426-36... 

Horanyi G and Rizmayer E M 1984 Radiotracer study of anion adsorption at silver electrodes in acidic medium J. Electroanal. Chem. 176 339-48... 

Franke C, Piazza G and Kolb D M 1989 The influence of halide adsorption on the electronic surface states of silver electrodes Electrochim. Acta 34 67-73... 

Lutzenkirchen-Flecht D and Strehblow FI-FI 1998 Bromide adsorption on silver in alkaline solution A surface analytical study Ber. Bunsenges. Phys. Chem. 102 826-32... 

Sensitizers as well as desensitizers form a reversal oxidoreduction system with silver halides, according to both pH and pAg of the photographic emulsion. But besides the specific influence of the emulsion, the efficiency of a sensitizing dye depends on many other factors such as its adsorption, its spectral absorption, the energetic transfer yield, the dye aggregate to the silver halide, and finally on its desensitizing property in... 

Fig. 2.6 Adsorption of gases on silver foil. (a) BET plots h) adsorption isotherms. (Solid symbols are desorption points.) (Courtesy Davis, Dc Witt...

There are several requirements for a good sensitizing dye. A good dye is adsorbed strongly to silver haUde. The dye molecules attach themselves to the surface of the silver haUde crystals, usually up to monolayer coverage. This amount can be deterrnined by measuring the adsorption isotherm for the... 

Because silver, gold and copper electrodes are easily activated for SERS by roughening by use of reduction-oxidation cycles, SERS has been widely applied in electrochemistry to monitor the adsorption, orientation, and reactions of molecules at those electrodes in-situ. Special cells for SERS spectroelectrochemistry have been manufactured from chemically resistant materials and with a working electrode accessible to the laser radiation. The versatility of such a cell has been demonstrated in electrochemical reactions of corrosive, moisture-sensitive materials such as oxyhalide electrolytes [4.299]. 

Fig. 4.59. Raman spectrum of methyl mercaptan (a) and SERS spectrum of methyl mercaptide (b) formed by adsorption ofthe mercaptan on a silver surface. The surface reaction is proven by the disappearance ofthe S-H stretching and bending bands at 2575 cm" and 806 cm", respectively. The Raman shift ofthe C-S stretching band at approximately 700 cm" is reduced during adsorption by withdrawal of electron density from the C-S, because of bonding to the silver. The symmetric methyl stretching appears above 2900cm" [4.303].

Thus an almost complete separation is theoretically possible. The separation is feasible in practice if the point at which the iodide precipitation is complete can be detected. This may be done (a) by the use of an adsorption indicator (see Section 10.75(c)), or (b) by a potentiometric method with a silver electrode (see Chapter 15). 

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)). 

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 disadvantage of adsorption indicators is that silver halides are sensitised to the action of light by a layer of adsorbed dyestuff. For this reason, titrations should be carried out with a minimum exposure to sunlight. When using adsorption indicators, only 2 x 10-4 to 3 x 10 3 mol of dye per mol of silver halide is added this small concentration is used so that an appreciable fraction of the added indicator is actually adsorbed on the precipitate. 

Other dyestuffs have been recommended as adsorption indicators for the titration of halides and other ions. Thus cyanide ion may be titrated with standard silver nitrate solution using diphenylcarbazide as adsorption indicator (see Section 10.44) the precipitate is pale violet at the end point. A selection of adsorption indicators, their properties and uses, is given in Table 10.8. 

Discussion. Very pure silver can be obtained commercially, and a standard solution can be prepared by dissolving a known weight (say, 10.787 g) in nitric acid in a conical flask having a funnel in the neck to prevent mechanical loss, and making up to a known volume (say, 1 L for a 0.1 M solution). The presence of acid must, however, be avoided in determinations with potassium chromate as indicator or in determinations employing adsorption indicators. It is therefore preferable to employ a neutral solution prepared by dissolving silver nitrate (relative molecular mass, 169.87) in water. 

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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