In photography, sodium thiosulphate is used as

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 hyposolution dissolves the unreduced silver bromide by forming a complex. AgBr+2Na2S2O3→Na3AgS2O32Sodium argentothiosulphate (soluble)+NaBrThus sodium thiosulphate acts as a complexing agent.