How does photographer's hypo work?

Is AgCl solid soluble in Na2S2O3 aqueous?, if so why? If Ag solid is present will a developer coninue to reduce to Ag ions? if so why?
Elizabeth Brown browng@babe.net.au

Converting Ag into Ag+ is oxidation, not reduction.

An emulsion of sodium thiosulfate (called hypo by photographers) is used to stop development of exposed film. Thiosulfate converts undeveloped silver bromide grains in the film into water-soluble silver thiosulfate complexes that can be removed when the film is washed.

S2O32-+ AgBr(s) AgS2O3- + Br-
S2O32-+ AgS2O3- Ag(S2O3)3-
Silver chloride behaves as silver bromide does.

See these Web sites for more detail on the chemistry of photography:

Chemistry in Black and White Photography (Shane Phillips, California State University)
A laboratory exercise showing how photographic developers work. Includes a section on toning for iron, copper, and sepia.
http://wwwchem.csustan.edu/chem1002/photog.htm (12/27/98)

Components of Photographic Materials (George L. Smyth)
The composition and preparation of film paper, and developing chemicals is described.
http://www.freeyellow.com/members6/glsmyth/photomat.htm (3/19/99)

Author: Fred Senese senese@antoine.frostburg.edu



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