Silver halide, thiosulfate complex

The thiosulfate ion, S2 O2- "3, is a structural analogue of the sulfate ion where one oxygen atom is replaced by one sulfur atom. The two sulfur atoms of thiosulfate thus are not equivalent. Indeed, the unique chemistry of the thiosulfate ion is dominated by the sulfide-like sulfur atom which is responsible for both the reducing properties and complexing abilities. The ability of thiosulfates to dissolve silver halides through complex formation is the basis for their commercial application in photography (qv). 

Silver Chloride. Silver chloride, AgCl, is a white precipitate that forms when chloride ion is added to a silver nitrate solution. The order of solubility of the three silver halides is Cl" > Br" > I. Because of the formation of complexes, silver chloride is soluble in solutions containing excess chloride and in solutions of cyanide, thiosulfate, and ammonia. Silver chloride is insoluble in nitric and dilute sulfuric acid. Treatment with concentrated sulfuric acid gives silver sulfate. 

A list of more common silver complexing agents includes thiosulfates, thiocyanates, sulfites, cyanides, ammonia and other amines, thiols, thiourea, thioacids, thioethers and alkali halides. Most are not practical as fixing agents because they fail to satisfy all the above criteria. Many find use as silver halide stabilizers (Section 59.2.1.8). These include thiocyanate, thiourea and other thioorganic compounds. These and other complexing agents such as cyclic imides, phosphines and arsines find use in silver halide diffusion transfer processes (Section 59.2.1.6). 

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. 

Thiosulfate ion is a good coraplexing ligand, and it forms stable complexes with silver. This reaction is the basis for removing the unreacted silver halide from photographic film. 

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 halides 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) 2> higl thiosulfate concentrations, species such as Ag2(S203) g are present. Silver sulfide dissolves in alkaline sulfide solutions to form complex ions such as Ag(S 2 Ag(HS) 4. These ions are... 

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. 

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 ... 

Silver Thiosulfate. Silver thiosulfate [23149-52-2], Ag2S203, 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 halides before the addition of the thiosulfate solution. In the presence of excess thiosulfate, the very soluble Ag2(S203)4-3 and Ag2(S203)<5-5 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). 

Only three simple silver salts, ie, the fluoride, nitrate, and perchlorate, are soluble to the extent of at least one mole per liter. Silver acetate, chlorate, nitrite, and sulfate are considered to be moderately soluble. AH other silver salts are, at most, sparingly soluble the sulfide is one of the most insoluble salts known. Silver(I) also forms stable complexes with excess ammonia, cyanide, thiosulfate, and the halides. Complex formation often results in the solubilization of otherwise insoluble salts. Silver bromide and iodide are colored, although the respective ions are colorless. This is considered to be evidence of the partially covalent nature of these salts. 

Ammonia complexes.. EflFect of complex formation on solubility. Cyanide complexes. The cyanide process of treating gold and silver ores. Complex halides and other complexes. Sodium thiosulfate as photographic fixer. Hydroxide complexes. Amphoteric hydroxides. Sulfide complexes. Equilibrium expressions for complex formation. Structural chemistry—tetrahedral, octahedral, square complexes. Existence of isomers. 

Most silver(I), compounds can easily be obtained by ligand exchange with AgNOs. A partial list of the salts includes halides, cyanide, azide, acetylide, carbonate, chromate, oxide, sulfide, sulfite, permanganate, phosphate, thiocyanate, and thiosulfate, as well as the soluble salts mentioned previously. Silver(I) linear two-coordinate complexes are known with amines, cyanide, nitrile, halide. 

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