Reduction of silver ions from solution

II. Physical Development and the Reduction of Silver Ions from Solution. 109... 

The effect of the cyanine dye and of gelatin on the reaction rate shows that reduction of silver ions from solution is not the rate-controlling process. These influences of adsorbed components on the reaction rate speak against the concept that solution of the silver halide is the rate controlling process. Hence, the silver catalyzed reduction of silver chloride by hydroxylamine takes place substantially at the solid silver/ silver halide interface. 

The rate of development by catechol at pH 8.78 varies approximately as the square root of the catechol concentration. The rate of reduction of silver ions from solution, on the other hand, varies directly as the catechol concentration. Thus, the results obtained with catechol parallel those obtained with hydroquinone. 

The suggested mechanisms differ in detail (Mott, 66, 67 Berg, 68 Anastasevich, 69 Frank-Kamenetskii, 70 Bagdasar yan, 17, 71) but all involve the idea that electrons can be transferred to silver much more readily than to a silver halide crystal. Each mechanism can be criticized on some detail (cf. Sheppard, 15 James, 72). As a general criticism, however, none of the mechanisms has explained the fact that the rate of development under simplified conditions varies with the square root of the hydroquinone and catechol concentrations, whereas the rate of reduction of silver ions from solution by the same agents varies as the first or somewhat higher power of the concentration. 

The mechanism suggested by Bagdasar yan has been formulated in more concrete terms than the others. It is in good agreement with some experimental data, but not with all. As in his treatment of the reduction of silver ions from solution, Bagdasar yan treats the transfer of electrons from the developer to the silver nucleus as the rate-controlling... 

Under the usual conditions of commercial practice, the development reaction does not occur entirely at the silver/silver halide interface. Some reduction of silver ions from solution takes place. Such reduction presumably can occur at any point on the silver/solution interface, and the mechanism should be the same as that for post-fixation physical development. The relative extent of the physical development in comparison with that at the silver/silver halide interface will depend upon the silver halide solvent action of the developing solution and upon the rate of the direct development. 

Post-fixation physical development is simpler in mechanism than direct development. The fundamental reaction is the reduction of silver ions from a solution of silver salt. This reaction is accelerated by the presence of silver nuclei, and the mechanism of the development is the mechanism of this catalytic process. II. 

The cyanine dye, 3,3 -diethyl-9-methylthiacarbocyanine chloride, had a much greater effect than gelatin in decreasing the reaction rate of the silver chloride. The rate of reduction of silver chloride varied linearly with the amount of silver chloride surface not covered by the dye, and the rate attained at complete coverage was of the order of one-thousandth that for the undyed precipitate. The dye exerted scarcely any effect upon the reduction of silver ions from silver sulfite complex solution. 

A large part of the reduction of silver chloride by hydrazine evidently takes place by a different mechanism from that of the reduction by hydroxylamine. The effect of gelatin and dye on the process, together with the appearance of colloidal silver in the solution when gelatin is present to stabilize it, shows that the reaction involves dissolved silver chloride to a greater degree than the hydroxylamine reaction. Indeed, if the reaction rate is plotted against a silver ion concentration calculated on the assumption that a saturated solution of silver chloride is maintained, the same relation is obtained as is found for the reduction of silver ions from a solution of the sulfite ion complex. 

Electroless plating — An autocatalytic process of metal deposition on a substrate by reduction of metal ions from solution without using an external source of electrons. It is promoted by specific reductants, namely formaldehyde, sodium hypophosphide, sodium boro-hydride, dialkylamine borane, and hydrazine. Electroless deposition has been used to produce different metal (e.g., nickel, cobalt, copper, gold, platinum, palladium, silver) and alloy coatings. It can be applied to any type of substrate including non-conductors. Some substrates are intrinsic catalytic for the electroless deposition other can be catalyzed usually by sensibilization followed by Pd nucleation also, in some non-catalytic metallic substrates the electroless process can be induced by an initial application of an appropriate potential pulse. In practical terms, the evaluation of the catalytic activity of a substrate for the electroless deposition of a given metal is... 

Silver enhancement is based on the reduction of silver ions from one solution (usually the enhancer) by another (the initiator) in the presence of Au-NPs [77]. The reduction reaction causes silver to build up preferentially on the surface of the Au-NPs, giving rise to a core-shell structure. An illustrative example is the work from Bonanni etal. [78]. They used streptavidin-coated Au-NPs and silver enhancement kits to amplify the impedimetric signal generated in a biosensor detecting the DNA hybridization event. The scheme displaying the sensor preparation procedure is shown in Fig. 4.13. A good reproducibility was achieved (RSD lower than 8.5%), the detection limit being 11.8 pmol. 

Fig. 1 The electrode/electrolyte interface, iUustiatmg Faradaic chaige transfer (top) and capacitive redistribution of chaige (bottom) as the electrode is driven negative, (a) Physical representation (b) Two-element electrical circuit model for mechanisms of charge transfer at the interface. The capacitive process involves reversible redistribution of chaige. The Faradtiic process involves transfer of electrons from the metal electrode, reducing hydrated cations in solution (symbolically 0 + e R, where the cation O is the oxidized form of the redox couple O/R). An example reaction is the reduction of silver ions in solution to form a silver plating on the electrode, reaction (8a). Faradaic charge injection may or may not be reversible...

In general, silver clusters in solution are prepared by reduction of silver ions. Proper scaffolds, e.g., DNA, proteins, dendrimers and polymers, are essential to prevent the aggregation of clusters to larger nanoparticles. Although it is clear that the emission originates from few-atom silver clusters, many aspects of this exciting class of nanoscopic metals are not yet fully understood. 

The reduction of silver ions by hydroxylamine from acid or slightly alkaline solution in the presence of colloidal silver proceeds with virtually a quantitative yield of nitrogen, and the reaction rates measured in terms of the amount of silver formed are identical within the limits of experimental error with those measured in terms of the amount of nitrogen evolved. The reaction rate varies as approximately the two-thirds power of the silver ion concentration at pH 4.16 and as approximately the half power at pH 8.54, in good agreement with the results... 

The action of an active intermediate oxidation product would explain another feature of the reaction. The reduction of silver ions by hydrazine is extremely sensitive to the presence of small amounts of copper. For example, a solution containing a mixture of silver nitrate, sodium sulfite and hydrazine which normally showed no sign of reduced silver for several minutes underwent almost immediate reaction when merely stirred with a clean copper rod. In the presence of gum arabic as stabilizer, streamers of colloidal silver passed out from the copper surface. Similarly, the addition of small amounts of cupric sulfate to a hydrazine solution eliminated the induction period of the reaction with silver chloride. 

The solvent action of sulfite on silver bromide and the resultant tendency to isolate latent image nuclei from the grain accounts for the failure of sulfite itself to act as a direct developer in spite of the autocatalytic character of its reduction of silver ion. The active nuclei simply are isolated from the grain before development gets under way. The same phenomenon enters to prevent sulfite-containing hydroquinone solutions of low pH (e.g., 8.5) from developing readily even though the thermodynamic conditions are suitable for reaction and the hydroquinone develops readily at the same pH when sulfite is absent. 

In the classical theory of Ostwald, Abegg, and Schaum [96] the homogeneous reduction of silver ion is assumed to be rapid and is followed by the physical deposition of silver on a latent image nucleus from a supersaturated solution of silver. The term physical development arises from this description and developers used at this time often deliberately contained soluble silver ion. It is now considered that physical and chemical development are both chemical, or electrochemical, processes in which silver ion reduction occurs at the latent image surface. 

The latent image catalyzes the reduction of silver ion either from the solid silver halide phase, as in chemical development, or from a soluble source of silver ion, as in physical development (Figure 21). One view of chemical development is that interstitial silver ions move through the silver halide crystal and are reduced on the underside of the latent image speck. In purely physical development complexed silver ion moves through the solution and is reduced on the nucleus. In this sense physical development and the early stages of chemical development are similar. 

Carbon monoxide can be determined by reduction of silver in alkaline solutions of Ag p-sulphanoylbenzoate and measuring the absorbance of the coloured Ag sol obtained [50— 52], The method has been used for determining CO in air and in blood. Carbon monoxide has also been determined (in the presence of NO, SO2, and CO2) by reduction of Ag(I) from its salt with sulphanilic acid [53]. The reduction of Pd(II) by CO followed by reaction with KIO3 to form icin ions which form ion-pairs with Pyrronine Y (extraction with benzene) has also been used for determination of CO [54]. The yellow cacotheline is reduced to the violet dihydroxycacotheline by Pd formed in the reduction of PdCU complex by CO [55]. 

Reduction of silver ions onto cheaper metals forms silver-plate. The object to be plated is made the cathode. At the pure silver anode, oxidation of silver metal to silver ions replaces the silver ions removed from the solution by plating at the cathode. ... 

Y. L. Sandler Westinghouse Research Laboratories)-. In view of the magnitude of the optical gap in ZnO ( 3 e.v.), it seems very unlikely that illumination by means of an incandescent lamp as used in Professor Schwab s experiments (Lecture 24) would cause any appreciable electronic excitation from the valance band to the conduction band in a pure ZuO crystal. It seems more likely that the electrons come from impurity levels due to the presence of water. We have recently demonstrated that the reduction of silver ions in aqueous solution can be photocatalyzed in presence of pure Ti02 or Si02 by light of wavelengths not absorbed by these oxides when in a dry state. 

If metal ions in solution are reduced to atoms by an appropriate redudng agent, they normally will combine into colloids or into microcrystalline particles which finely predpitate out. Such processes can occur within seconds, as is known for instance from the formation of silver mirrors on glass by the reduction of silver ions. If the growth of the metal particles in solution can be retarded and if appropriate ligands are additionally offered, it may happen that metal rich ligand stabilized transition metal clusters of uniform rize are formed. 

Many of the electrode reactions studied in electrochemistry involve the formation of a new phase. This may be a metal resulting from the reduction of an ion in solution, an oxide formed by corrosion or by oxidation of a solution species (e.g. Pb02 from a Pb(N03)2 bath), other metal salts from oxidation of an electrode (e.g. PbS04 from lead or AgCl from silver), or by the oxidation or reduction of another phase (e.g. PbS04 from the reduction of Pb02). 

An example of the simplest (in the sense of the number of kinetic parameters) electrochemical reaction is reduction of silver ions (Ag+) from a dilute aqueous solution of a well soluble silver salt (e.g., nitrate) in the presence of excess of an indifferent salt (e.g., potassium nitrate) on a liquid silver-mercury alloy (also called amalgam) electrode. Besides the transfer of a single electron, only diffusion steps are involved in this process. The entire reaction can be very well modeled and the kinetic parameters are determined experimentally with high level of accuracy. The information gleaned while analyzing the mechanism of silver ion reduction can be used in elucidating more complex, multi-step, multiphase processes, such as the electrochemical reaction in a lithium-ion cell. 

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