Question

Acidic ferric chloride solution on treatment with excess of potassium ferrocyanide gives a Prussian blue coloured colloidal species. It is:

• A. \(Fe_4[Fe(CN)_6]_3\)
• B. \(HFe[Fe(CN)_6]\)
• C. \(KFe[Fe(CN)_6]\)
• D. \(K_2Fe[Fe(CN)_6]_2\)

Hint:

"Insoluble" Prussian blue is \(Fe_4[Fe(CN)_6]_3\). "Soluble" (colloidal) Prussian blue is \(KFe[Fe(CN)_6]\). Always check if the reagent is in excess.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The reaction between ferric ions (\(Fe^{3+}\)) and ferrocyanide ions (\([Fe(CN)_6]^{4-}\)) produces Prussian Blue. The exact composition depends on the stoichiometry and presence of alkali metal ions.
Step 2: Detailed Explanation:
When \(FeCl_3\) reacts with potassium ferrocyanide \(K_4[Fe(CN)_6]\), the "insoluble" Prussian Blue is typically formed as \(Fe_4[Fe(CN)_6]_3\).
However, when the reaction is carried out with excess potassium ferrocyanide, a "soluble" or colloidal form of Prussian blue is obtained.
The reaction for the soluble/colloidal form is:
\[ Fe^{3+} + K^+ + [Fe(CN)_6]^{4-} \to KFe[Fe(CN)_6] \]
This species, Potassium iron(III) hexacyanoferrate(II), remains in a colloidal state.
Step 3: Final Answer:
The colloidal species formed in excess reagent is \(KFe[Fe(CN)_6]\).