Question

Which of the following will be the most effective in the coagulation of Al (OH)₃ sol?
1. BaCl₂
2. Mg₃(PO₄)₂
3. KCN
4. NaCI

Answer 1

Correct Answer - Option 2 : Mg₃(PO₄)₂

Concept:

Coagulation or precipitation:

• The stability of the colloidal state is due to the presence of charge on colloidal particles.
• If somehow, the charge is removed, the particles will come nearer to each other to form aggregates (or coagulate) and settle down under the force of gravity.
• The process of settling of colloidal particles is called coagulation or precipitation of the sol.

Hardy-Schulze rule:

• The coagulation capacity of electrolytes depends on the valency of the active ion or flocculating ion.
• Hardy-Schulze rule states that "the greater the valency of the flocculating ion added, the greater is its power to cause precipitation.
• The ions carrying the charge opposite to that of sol particles are effective in causing coagulation.
• The coagulating power of an electrolyte is directly proportional to the valency of the active ions.

Explanation:

• 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– > PO₄^3– > SO₄^2– > Cl^–.

The given sol is Al (OH)3.

• Al (OH)3 is a positive sol.
• For its coagulation (ppt formation), the anion is added.
• The coagulating power of positive sol is directly proportional to the charge and valency of the anion.

The ionization of given negative sols are as follows:

1. BaCl₂ → Ba²⁺ + 2Cl^- ; charge of anion is 1
2. Mg₃(PO₄)₂  → 3Mg²⁺ + 2PO₄^3-; charge of the anion is 3
3. NaCl → Na⁺ + Cl^-  ; charge of anion is 1
4. KCN → K⁺ + CN^-;  charge of the anion is 1

The higher charge is for Po₄^3-. So, Mg3(PO4)2 has higher coagulating power.

Hence, Mg3(PO4)2 is most effective in the coagulation of Al (OH)3 sol.