Silver thiosulfate complex ions

When the film paper is exposed to light, silver ions are reduced to silver atoms. Next excess silver ions are removed by combining them with thiosulfate ions to form a silver thiosulfate complex ion. This happen when the exposed film paper is placed in a hypo solution. To completely stop the developing process, the film paper is next placed in a fixer such as acetic acid to neutralize the basic hypo solution. 

Silver Thiosulfate. Silver thiosulfate [23149-52-2], Ag 2 y is an insoluble precipitate formed when a soluble thiosulfate reacts with an excess of silver nitrate. In order to minimize the formation of silver sulfide, the silver ion can be complexed by haUdes before the addition of the thiosulfate solution. In the presence of excess thiosulfate, the very soluble Ag2(S203) 3 and Ag2(S203) 3 complexes form. These soluble thiosulfate complexes, which are very stable, are the basis of photographic fixers. Silver thiosulfate complexes are oxidized to form silver sulfide, sulfate, and elemental sulfur (see Thiosulfates). 

Silver compounds having anions that are inherently toxic, eg, silver arsenate and silver cyanide, can cause adverse health effects. The reported rat oral LD values for silver nitrate, silver arsenate [13510-44-6] and silver cyanide are 500—800 (29), 200—400 (29), and 123 mg/kg (30), respectively. Silver compounds or complexes ia which the silver ion is not biologically available, eg, silver sulfide and silver thiosulfate complexes, are considered to be without adverse health effects and essentially nontoxic. 

The impact that a silver compound has in water is a function of the free or weaMy complexed silver ion concentration generated by that compound, not the total silver concentration (3—5,27,40—42). In a standardized, acute aquatic bioassay, fathead minnows were exposed to various concentrations of silver compounds for a 96-h period and the concentration of total silver lethal to half of the exposed population (96-h LC q) deterrnined. For silver nitrate, the value obtained was 16 )-lg/L. For silver sulfide and silver thiosulfate complexes, the values were >240 and >280 mg/L, respectively, the highest concentrations tested (27). 

In secondary wastewater treatment plants receiving silver thiosulfate complexes, microorganisms convert this complex predominately to silver sulfide and some metallic silver (see Wastes, INDUSTRIAL). These silver species are substantially removed from the treatment plant effluent at the settling step (47,48). Any silver entering municipal secondary treatment plants tends to bind quickly to sulfide ions present in the system and precipitate into the treatment plant sludge (49). Thus, silver discharged to secondary wastewater treatment plants or into natural waters is not present as the free silver ion but rather as a complexed or insoluble species. 

The undeveloped grains of siher halide are next removed, by treatment with a fixing bath, which contains thiosulfate ion, S0O3— (from sodium thiosulfate, hypo, NaoS. 03 5H >0). The soluble silver thiosulfate complex is formed ... 

Other complex ions formed by silver, such as the silver cyanide complex Ag(CN)2 and the silver thiosulfate complex Ag(S20a)2, have... 

Sulfur Complexes. Silver compounds other than sulfide dissolve in excess thiosulfate. Stable silver complexes are also formed with thiourea. Except for the cyanide complexes, these sulfur complexes of silver are the most stable. In photography, solutions of sodium or ammonium thiosulfate fixers are used to solubilize silver hahdes present in processed photographic emulsions. When insoluble silver thiosulfate is dissolved in excess thiosulfate, various silver complexes form. At low thiosulfate concentrations, the principal silver species is Ag2(S203) 2j high thiosulfate concentrations, species such as Ag2(S203) 3 are present. Silver sulfide dissolves in alkaline sulfide solutions to form complex ions such as Ag(S 2 Ag(HS) 4. These ions are... 

From studies of the reaction between silver and thiosulfate ions, a large number of complex ions have been suggested to exist in solution. The available thermodynamic data for these species are collected in Table 33.239,240... 

In practical systems, no solvent is as universally effective as sodium thiosulfate, Na2S203. Itis inexpensive, stable and produces few undesirable side effects such as stains, odors or toxicity. It rapidly forms stable, soluble complex ions with silver halide (for example [Ag(S203)2]3 ), which are rapidly diffusible. Several other silver complexing agents are proposed as being useful in diffusion transfer processes, however, and some of these are used either singly or in combination with other, more common solvents. Some are used with certain developers. 

Using the same general formula that we used for K, we can examine constants for the formation of complex ions and determine what woulcfbe the best substance to use as a film fixer in other words, what substance can remove a large number of silver ions from the unexposed portion of the film. We use the symbol Kf for the formation constant of a complex ion. We know that thiosulfate ions are combined with silver ions to form the Ag(S203) 23- complex ... 

The Kf value for the Ag(CN) 2 complex ion is5.6xl018. Cyanide ions (CN ) would be better removers of silver ions than thiosulfate ions. However, most of us know that cyanide is a deadly poison if inhaled or ingested. In the play Arsenic and Old Lace, it was used very effectively to rid the cast of unwanted characters. Refining the process of film developing, which included not only the discovery of proper fixing solutions but also the development of film surfaces and developers that were reliable, helped to plunge us into the age of modern photography. 

This complex ion is sufficiently stable to cause silver chloride and bromide to be soluble in thiosulfate solutions, and this is the reason that sodium thiosulfate solution ( hypo ) is used after development of a photographic film or paper to dissolve away the unreduced silver halide, which if allowed to remain in the emulsion would in the course of time darken through long exposure to light. 

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