Why is Sodium thiosulphate used in photographic processing of a film?

Options :
  1. Because it is an oxidizing agent
  2. Because it is a reducing agent
  3. Because it reacts with light
  4. Because it helps in fixing the photograph
Answer and Explanation :-

Answer: Option 4


Sodium thiosulphate is a colourless crystalline compound that is more familiar as the pentahydrate, Na2S2O3.5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulphite or 'hypo'. Sodium thiosulphate is used in photographic processing of film as a fixer or mix of chemicals because the negative photographic plate contains an emulsion of Silver Hallides, like, Silver Bromide (AgBr) that needs to be removed for a clearer picture. Sodium thiosulphate reacts with the Silver Bromide and transforms it into a soluble complex silver salt that can be dissolved in the fixer bath, which contains a hypo clearing agent solution for the purpose.

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