The extraction of silver and the removal of undecomposed AgBr from a developed black and white photographic film involve, respectively, which complexes?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → [Ag(CN)2]− and [Ag(S2O3)2]3−
Extraction of silver: as for gold, the metal is dissolved by cyanide in the presence of oxygen and water as a soluble complex, \( \ce{[Ag(CN)2]^-} \); zinc then frees the metal from the solution.
\( \displaystyle \ce{4Ag + 8CN^- + 2H2O + O2 -> 4[Ag(CN)2]^- + 4OH^-} \)
Fixing a black and white film: hypo (sodium thiosulphate) dissolves the undecomposed AgBr as the complex ion \( \ce{[Ag(S2O3)2]^3-} \).
\( \displaystyle \ce{AgBr + 2Na2S2O3 -> Na3[Ag(S2O3)2] + NaBr} \)
Why the other options are incorrect
Option (1) [Ag(S2O3)2]3− in both — Incorrect. The thiosulphate complex is right only for the film; extraction uses cyanide.
Option (2) [Ag(CN)2]− in both — Incorrect. The cyanide complex is right only for extraction; the film is fixed with hypo, which gives the thiosulphate complex.
Option (3) [Ag(S2O3)2]3− and [Ag(CN)2]− — Incorrect. Both complexes are named, but in the reverse order.
NCERT (2026-27 edition):
5.7 Importance and Applications of Coordination Compounds – “… Some important extraction processes of metals, like those of silver and gold, make use of complex formation. Gold, for example, combines with cyanide in the presence of oxygen and water to form the coordination entity [Au(CN)2]– in aqueous solution. Gold can be separated in metallic form from this solution by the addition of zinc.”
“In black and white photography, the developed film is fixed by washing with hypo solution which dissolves the undecomposed AgBr to form a complex ion, [Ag(S2O3)2]3–.”