Potassium metabisulfite

Potassium metabisulfite
An acidifier used in fixers and stop-baths.
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Potassium metabisulfite - acidifier used in fixers and stop baths.
Potassium permanganate - chemical used extensively in reducers, bleaches and toners.
Potassium persulfate - chemical sometimes used in super-proportional reducers.

Chemical (basically a solution of sodium thiosulfate plus potassium metabisulfite as acidifier). Used after development to make soluble those parts of a photographic image unaffected by the developer.

See also: Stop, Fixer, Light source, Space, Main light