The potassium ferrocyanide solution gives a Prussian blue colour, when added to :

Solution

Chemistry ma’am · 3 steps

Ferrocyanide already carries Fe²⁺, so it needs Fe³⁺ outside to give Prussian blue.

1. What the reagent holds. Potassium ferrocyanide is K₄[Fe(CN)₆]. The iron inside the complex ion is Fe²⁺.
2. What Prussian blue is. Prussian blue is ferric ferrocyanide, Fe₄[Fe(CN)₆]₃. It needs Fe²⁺ inside the complex and Fe³⁺ outside it. The intense blue comes from charge transfer between the two oxidation states of iron.

   4Fe³⁺ + 3[Fe(CN)₆]⁴⁻ → Fe₄[Fe(CN)₆]₃ v

   Salt	Cation	Result with K₄[Fe(CN)₆]
   CoCl₃	Co³⁺	no iron outside the complex, and Co³⁺ is not stable in water, so no Prussian blue
   FeCl₂	Fe²⁺	white precipitate that turns blue only slowly in air
   CoCl₂	Co²⁺	green precipitate
   FeCl₃	Fe³⁺	Prussian blue

3. So. Only FeCl₃ supplies the Fe³⁺ ions that complete ferric ferrocyanide. The answer is (d).

The idea to keep. Prussian blue needs iron in two oxidation states: Fe³⁺ salt with ferrocyanide (Fe²⁺ inside). Swap them, Fe²⁺ salt with ferricyanide, and you get Turnbull's blue.

Answer from the NTA final answer key. Solution by PrepWiser.