Which coordination complex ion is formed during the development of black and white photographic films?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → [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(NH3)2]+ — Incorrect. This is Tollens’ reagent, the diamminesilver(I) ion, used to test for aldehydes, not in photography.
Option (3) [Au(CN)2]− — Incorrect. This complex is formed in the extraction of gold by the cyanide process.
Option (4) [Ag(CN)2]− — Incorrect. This dicyanido complex of silver is used in extracting silver from its ore, 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 | – | – | – | – |