Question

Sodium Thiosulphate is used in photography
A. as $AgBr$ is reduced to non-metallic silver
B. to convert metallic silver into silver salts
C. to remove reduced silver
D. to remove undecomposed $AgBr$ in the form of $N{a_3}[Ag{({S_2}{O_3})_2}]$ (a complex salt)

Answer 1

Hint: Photographs are developed by negatives, which are reactions of silver bromide when exposed to sunlight. In presence of sunlight, silver bromide reacts to form the image on the screen. Sometimes, not all silver bromide is used up. Sodium Thiosulphate is mostly used as a fixer in photography because of its ability to complex nature.

Complete step by step answer:
$AgBr$ emulsions are used in photography for the development of films.
Since, Silver Bromide is photosensitive, which means it undergoes a chemical reaction which light falls on it.
It forms silver deposits when light is incident on areas which have silver bromide present.
Once, the photograph has been developed, all excess of silver bromide needs to be removed.
Since sodium thiosulfate has a complex forming ability, during photography, it reacts with the undecomposed silver bromide($AgBr$ ) to form a silver thiosulphate complex which is soluble in nature.
Hence we can say that Sodium Thiosulphate is used in photography to remove undecomposed $AgBr$ in the form of $N{a_3}[Ag{({S_2}{O_3})_2}]$

So, the correct answer is Option D .

Note: Sodium thiosulphate is mainly used as a photographic fixer used for both film as well as photographic paper processing methods. It is used to dissolve silver bromide in areas which have been left unaffected by the light. Sodium thiosulphate was also applied in a process called waxing , which would be essential in reducing the size of the grains in paper, but it was in no way related to photography.
It is also used in many other fields, such as cyanide poisoning due to its complexing nature. It can form bidentate complexes easily and has more affinity towards metals. This property makes it very important as a supplement in many fields.