Correct Option
Sodium thiosulphate (Na₂S₂O₃) is commonly employed as a 'fixer' in the photographic development process. Its primary function is to dissolve and remove any unexposed silver halide crystals, such as silver bromide (AgBr), that remain on the film or paper after the development stage. This is crucial for stabilizing the image and preventing it from further reacting with light.
The chemical reaction involved is:
AgBr + 2 Na₂S₂O₃ → Na₃[Ag(S₂O₃)₂] + NaBr
By forming a soluble complex, sodium thiosulphate ensures that the developed silver image becomes permanent and insensitive to subsequent light exposure.
Incorrect Options
Sodium sulphate (A): This compound is not utilized in the photographic fixing process. Its primary applications include use in detergents and the Kraft process for paper manufacturing.
Ammonium persulphate (C): This is a strong oxidizing agent primarily used in polymerization reactions and etching processes. It does not function as a photographic fixer.
Borax (D): Borax serves as a buffering agent and a mild antiseptic. It does not play a role in photographic development or fixing.