Question

Sodium thiosulphate is used in photography to

• A. reduce AgBr to metallic Ag
• B. dissolve out Ag produced by reduction
• C. remove unaffected AgBr in the photographic film or plate
• D. none of these

Answer 1

Answer: (C)

In photography, AgBr is mainly used as the light sensitive material. The unchanged AgBr after developing the film is treated with Na₂S₂O₃ solution. Solution of sodium thiosulphate is used as fixer and forms a soluble complex with silver halide.
AgBr + 2Na₂S₂O₃ $\rightarrow$ Na₃[Ag(S₂O₃)₂] + NaBr