The flocculating power of \([Fe(CN)_6]^{4-}\) is higher than \(PO_4^{3-}\) ion because:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → It has higher negative charge
An electrolyte coagulates a sol through the ion that carries the charge opposite to that of the sol particles. By the Hardy–Schulze rule, the greater the valence (charge) of the flocculating ion, the greater its power to cause precipitation. For a positive sol the order is:
\( \ce{[Fe(CN)6]^{4-}} \) > \( \ce{PO4^{3-}} \) > \( \ce{SO4^{2-}} \) > \( \ce{Cl^{-}} \)
\( \ce{[Fe(CN)6]^{4-}} \) carries a charge of −4 against −3 for \( \ce{PO4^{3-}} \), so it neutralises the positive charge on the particles more effectively. Its flocculating power is higher because it has a higher negative charge.
Why the other options are incorrect
Option (1) It is a complex ion — Incorrect. Being a complex ion has no bearing on coagulation; only the charge of the ion counts.
Option (3) It involves metal ion in it — Incorrect. The \( \ce{Fe^{2+}} \) inside the complex does not act on the sol; the ion acts as a whole through its net charge.
Option (4) It has a strong ligand field — Incorrect. Ligand field strength decides the splitting of d orbitals (colour, magnetism), not coagulating power.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Coagulation or Precipitation – “… The ion responsible for neutralisation of charge on the particles is called the coagulating ion. A negative ion causes the precipitation of positively charged sol and vice versa. It has been observed that, generally, the greater the valence of the flocculating ion added, the greater is its power to cause precipitation. This is known as Hardy-Schulze rule. In the coagulation of a negative sol, the flocculating power is in the order: Al3+ > Ba2+ > Na+. Similarly, in the coagulation of a positive sol, the flocculating power is in the order: [Fe(CN)6]4– > PO43– > SO42– > Cl–.”