In black and white photography, the developed film is fixed by washing with hypo solution which helps in dissolving the remaining AgBr by forming a complex ion. What is the correct formula of the complex ion?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → [Ag(S2O3)2]3−
In fixing, the unexposed AgBr is dissolved by hypo (sodium thiosulphate) as a soluble complex:
\( \displaystyle \ce{AgBr + 2Na2S2O3 -> Na3[Ag(S2O3)2] + NaBr} \)
The charge on the complex ion follows from \( \ce{Ag^{+}} \) and two \( \ce{S2O3^{2-}} \) ligands:
\( \displaystyle (+1)+2(-2)=-3 \)
So the complex ion is [Ag(S2O3)2]3−, the dithiosulphatoargentate(I) ion.
Why the other options are incorrect
Option (1) [Ag(SO4)2]3− — Incorrect. Hypo, \( \ce{Na2S2O3} \), supplies thiosulphate ions, \( \ce{S2O3^{2-}} \), not sulphate ions; the ligand in the complex is thiosulphate.
Option (2) [Ag(SO3)2]3− — Incorrect. Sulphite, \( \ce{SO3^{2-}} \), is a different ion with one sulphur atom; the ligand from hypo is thiosulphate, \( \ce{S2O3^{2-}} \).
Option (4) [Ag(CN)4]3− — Incorrect. Hypo contains no cyanide. The cyanido complex of silver, [Ag(CN)2]−, is met in the extraction of silver, not in photography.
NCERT (2026-27 edition):
Importance and Applications of Coordination Compounds – …
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 30-05 S1 15-05 S1 | – | – | – | – |