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.

S₂O₃^2-+ AgBr(s)

AgS₂O₃^- + Br^-
S₂O₃^2-+ 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

General Chemistry Online! How does photographer's hypo work?