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

🧠 Identify the Soluble Prussian Blue

Prussian blue exists in two practical forms:

• Soluble (peptisable, colloidal): $\mathrm{KFe[Fe(CN)_6]}$
• Insoluble (coarse precipitate): $\mathrm{Fe_4[Fe(CN)_6]_3}$

The reaction with excess ferrocyanide ($\mathrm{K_4[Fe(CN)_6]}$) on acidic $\mathrm{FeCl_3}$ favours the soluble (potassium-stabilised) colloidal form:

$\mathrm{FeCl_3 + K_4[Fe(CN)_6] \rightarrow KFe[Fe(CN)_6] + 3\,KCl}$

🗺️ Why "Soluble" Prussian Blue Forms with Excess Ferrocyanide

When the limiting reagent is the iron(III) salt and ferrocyanide is in excess, the K⁺ ions remain available to stabilise the lattice. The result is a 1:1:1 mixed-iron complex where K⁺ sits in the lattice cavities, producing a colloidal blue suspension that does not precipitate easily.

With excess Fe³⁺ (limiting ferrocyanide), you get the K-free $\mathrm{Fe_4[Fe(CN)_6]_3}$ instead — the insoluble form.

⚡ The Mixed-Valence Origin of Prussian Blue's Colour

Both forms have iron in two different oxidation states:

• The bracket Fe is Fe(II) (binds CN⁻ from the carbon end → low-spin, octahedral, sits in the C-bound cavity).
• The outer Fe is Fe(III) (binds N from cyanide → octahedral, in the N-bound cavity).

The intense blue colour comes from a Fe²⁺ → Fe³⁺ intervalence charge-transfer band — strong, visible-region absorption.

⚠️ Don't Confuse with Turnbull's Blue

Both Prussian blue and Turnbull's blue are now known to be the same compound. The historical names refer only to which iron salt was added to which reagent first.

$\boxed{\text{Answer: (4) } \mathrm{KFe[Fe(CN)_6]}}$