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

Step 1: Understand Potassium Ferrocyanide

Potassium ferrocyanide = $\ce{K4[Fe(CN)6]}$

• Ferrocyanide anion: $\ce{[Fe(CN)6]^{4-}}$ (iron in +2 oxidation state)
• It is a reducing agent (can reduce $\ce{Fe^{3+}}$ to $\ce{Fe^{2+}}$)

Step 2: The Prussian Blue Test

$\ce{FeCl3 + K4[Fe(CN)6] -> Prussian blue precipitate}$

$\ce{Fe^{3+}}$ ions react with $\ce{[Fe(CN)6]^{4-}}$ (ferrocyanide) to form the characteristic Prussian blue precipitate: $\ce{4Fe^{3+} + 3[Fe(CN)6]^{4-} -> Fe4[Fe(CN)6]3}$

This reaction is the basis of:

1. Lassaigne's test for nitrogen (NaCN → FeCN complex → Prussian blue with FeSO₄)
2. Qualitative analysis of $\ce{Fe^{3+}}$ ions

Step 3: Evaluate Each Option

| Salt | Cation | Gives Prussian Blue with $\ce{K4[Fe(CN)6]}$? | |---|---|---| | $\ce{CoCl3}$ | $\ce{Co^{3+}}$ | ❌ — gives different colour | | $\ce{FeCl3}$ ✅ | $\ce{Fe^{3+}}$ | ✅ — Prussian blue | | $\ce{FeCl2}$ | $\ce{Fe^{2+}}$ | ❌ — Turnbull's blue uses $\ce{Fe^{2+}}$ + $\ce{K3[Fe(CN)6]}$ | | $\ce{CoCl2}$ | $\ce{Co^{2+}}$ | ❌ — no Prussian blue |

Answer: (b) $\ce{FeCl3}$

Key Points to Remember:

• $\ce{FeCl3}$ + $\ce{K4[Fe(CN)6]}$ → Prussian blue (Lassaigne test basis)
• $\ce{FeCl2}$ + $\ce{K3[Fe(CN)6]}$ → Turnbull's blue (same compound as Prussian blue!)
• Prussian blue formula: $\ce{Fe4[Fe(CN)6]3}$
• $\ce{Co^{2+}}$ + $\ce{K4[Fe(CN)6]}$ → olive green to greenish precipitate (not blue)