Sodium thiosulfate (Na2S203) solution is used in photography to

Sodium thiosulfate (Na2S2O3), commonly known as 'hypo', is used in the photographic fixing process to stabilize the image [t2, t3]. During development, light-exposed silver bromide (AgBr) is reduced to metallic silver, but unexposed AgBr remains on the film [t2, t8]. If left untreated, this residual AgBr would darken upon further light exposure, ruining the photograph [t2, t9]. Sodium thiosulfate acts as a complexing agent, reacting with the undecomposed AgBr to form a soluble silver thiosulfate complex, specifically sodium argento thiosulfate (Na3[Ag(S2O3)2]) [t2, t7, t10]. This soluble complex, along with sodium bromide, is then easily washed away with water, leaving only the stable metallic silver image on the film or paper [t2, t8, t10]. Thus, its primary role is the removal of unreacted silver halides through complexation [t6, t8].