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)

remove unaffected AgBr in the photographic film or plate

Answer 2

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