Silver halide, thiosulfate

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. 

Silver halide salts are used in black and white photography. During the developing process, excess silver halide is removed from the film by using a solution of sodium thiosulfate (commonly called hypo ). 

How can one verify, just by looking at the Latimer diagram of silver, that sodium thiosulfate (hypo) is useful in photographic processes that require the removal of excess, wrested silver halide Is this process (fixing) actually a redox reaction Explain... 

Sensitization by compounds that contain a labile sulfur atom, such as thiosulfate ion and thiourea, was established by Sheppard in 1925 (121), although Newton (122) obtained this form of sensitization in 1874 without recognizing it. The steps in the sensitizing process involve (a) adsorption of the sensitizer by the silver halide, (b) reaction of the adsorbed sensitizer with silver ions to form Ag2S, and (c) probably rearrangement of the Ag2S molecules on the grain surface to form nuclei. 

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. 

Uses In photography as a hardener in acid fixing baths that use ammonium thiosulfate as a solvent for silver halides. 

Uses To make rapid fixing baths, as an accelerator in sodium thiosulfate-based fixing baths also used in chloride and salted albumen papers. Mild silver halide solvent. 

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). 

Uses. The principal use for sodium thiosulfate continues to be as fixative in photography (qv) to dissolve undeveloped silver halide from negatives or prints. In applications where rapid processing is required, such as the processing of x-ray film, sodium thiosulfate has been largely replaced by ammonium thiosulfate. 

The principal use of photochemical-grade ammonium thiosulfate continues to be in photography, where is dissolves undeveloped silver halides from negatives and prints. It reacts considerably faster than sodium thiosulfate, and the fixing solutions can be used about twice as long as sodium thiosulfate solutions the washing period to remove residual thiosulfate is shorter. 

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. 

A square of photographic film 2.0 cm on an edge was suspended in a 5% solution of Na2S203 to dissolve the silver halides. After removal and washing of the film, the solution was treated with an excess of Br2 to oxidize the iodide present to IO3 and destroy the excess thiosulfate ion. The... 

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