Question

In photography sodium thiosulphate is used as.

• A. Complexing agent
• B. Oxidising agent
• C. Reducing agent
• D. None of these

Answer 1

Answer: (A)

Complexing agent

Answer 2

In order to make the image permanent, it is necessary to remove the unreduced silver bromide from the surface of the developed film. This operation is called fixing of image. Fixing is done by dipping the developed film or plate in sodium thiosulphate (hypo) solution. the hypo solution dissolves the unreduced silver bromide by forming a complex.

$AgBr + 2Na_{2}S_{2}O_{3} \rightarrow \underset{(\text{soluble})}{\underset{\text{Sodium argentothiosulphate}}{Na_{3}\lbrack Ag(S_{2}O_{3})_{2}\rbrack}} + NaBr$

Thus sodium thiosulphate acts as a complexing agent.