In black and white photographic film, light converts silver chloride into metallic silver. After the film
has been developed, the unreacted silver chloride is removed by reaction with sodium thiosulfate
to produce a ‘fixed’ negative.
AgCl + 2Na2S2O3 → 4Na+ + Cl
– + [Ag(S2O3)2]3–
What is the function of the thiosulfate ion?
A to make the silver ions soluble
B to oxidise the silver ions
C to oxidise the silver metal
D to reduce the silver ions

Question

In black and white photographic film, light converts silver chloride into metallic silver. After the film
has been developed, the unreacted silver chloride is removed by reaction with sodium thiosulfate
to produce a ‘fixed’ negative.
AgCl + 2Na2S2O3 → 4Na+ + Cl
– + [Ag(S2O3)2]3–
What is the function of the thiosulfate ion?
A to make the silver ions soluble
B to oxidise the silver ions
C to oxidise the silver metal
D to reduce the silver ions

chmvijay · Answer

its called the photographic fixer 
The fixer stabilises the image, removing the unexposed silver halide remaining on the photographic film or photographic paper, leaving behind the reduced metallic silver that forms the image, making it insensitive to further action by light. Without fixing, the remaining silver halide would quickly darken and cause fogging of the image.
can u find the oxidation state of silver tin this [Ag(S2O3)2]3–