Acidic ferric chloride solution on treatment with excess of potassium ferrocyanide gives a Prussian blue coloured…

Step 1: Understand the Reaction Conditions

Acidic $\ce{FeCl3}$ + excess $\ce{K4[Fe(CN)6]}$ → Prussian blue colloidal species.

Key feature: excess potassium ferrocyanide means K is present in the product.

Step 2: The Reaction

$\ce{FeCl3 + K4[Fe(CN)6] -> KFe[Fe(CN)6] + 3KCl}$

When $\ce{K4[Fe(CN)6]}$ is in excess, the Prussian blue colloidal species formed is $\ce{KFe[Fe(CN)6]}$ (potassium ferric ferrocyanide), not the simple $\ce{Fe4[Fe(CN)6]3}$.

Step 3: Distinguish from the Standard Formula

| Conditions | Product | |---|---| | Stoichiometric/standard | $\ce{Fe4[Fe(CN)6]3}$ | | Excess $\ce{K4[Fe(CN)6]}$ | $\ce{KFe[Fe(CN)6]}$ (soluble Prussian blue / Berlin blue) |

The colloidal Prussian blue in the presence of excess ferrocyanide contains K⁺ to balance charge: $\ce{Fe^{3+} + [Fe(CN)6]^{4-} + K^+ -> KFe[Fe(CN)6]}$

Answer: (a) $\ce{KFe[Fe(CN)6]}$

Key Points to Remember:

• Stoichiometric Prussian blue: $\ce{Fe4[Fe(CN)6]3}$ (Prussian blue, insoluble)
• Prussian blue with excess ferrocyanide: $\ce{KFe[Fe(CN)6]}$ (Berlin blue, colloidal/soluble form)
• Both are blue; the K+ content in the excess-K4[Fe(CN)6] reaction stabilises the colloid
• Prussian blue is used as a pigment and in the Lassaigne nitrogen test