Question

Sodium thiosulphate, ${Na}_2{S}_2{O}_3\cdot 5H_{2}O$ is used in photography to :

• A. reduce the silver bromide to metallic silver
• B. convert the metallic silver to silver salt
• C. remove undecomposed $AgBr$ as soluble thiosulphate complex
• D. remove reduced silver

Answer 1

Answer: (C)

remove undecomposed $AgBr$ as soluble thiosulphate complex

Silver bromide reacts with sodium thiosulphate solution to form sodium argentothiosulphate $N{a}_3\left[Ag\right(S_2{O}_3{)}_2]$ which is water-soluble.

$AgBr\left(s\right)+2N{a}_2{S}_2{O}_3\to N{a}_3\left[Ag\right(S_2{O}_3{)}_2]$

Thus, undecomposed AgBr can be removed as soluble thiosulphate complex.