Question

Hypo is used in photography to

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

Answer 1

Answer: (C)

After the developing, the sensitive emulsion of silver bromide is still present on the plate in the parts unaffected by light. Therefore, it is necessary to remove it in order to get the permanent image. This process is called fixing of image. The negative plate after washing is dipped in a fixing solution of sodium thiosulphate (hypo). It dissolves unaffected silver bromide but leaves metallic silver unaffected.
$2N{a}_2{S}_2{O}_3+AgBr⟶N{a}_3[Ag(S_2{O}_3{)}_2]+NaBr$