Question

Give reason for the following:
For ferric hydroxide sol the coagulating power of phosphate ions is more than chloride ions.

Answer 1

Hint: Here, we will proceed by defining the process of coagulation. Then, we will state the Hardy Schulze law along with the discussion of the affinity of various types of anions with a sol.
Complete step-by-step answer:

Coagulation is a mechanism involving the combination of colloidal particles to turn them into large particles that gradually settle down on the surface or float as a precipitate.
In general, coagulation is caused by the addition of electrolytes. If an electrolyte is applied to a colloidal solution, the sol particles pick up the opposite charged ions and thus get neutralized. Then the neutral particles begin to accumulate to form larger-sized particles which settle down.
The amount of the electrolyte required to coagulate a definite quantity of a colloidal solution depends on the ion's valence having a charge opposite to that of the colloidal particles. This is known as the Hardy Schulze principle or Hardy Schulze law.
Greater is the valence of the electrolyte being added with the opposite charged ion, the faster is the coagulation.
The concept behind that is the attractive electrostatic forces of opposing charges between ions. The greater the ion charge, the greater the appealing electrostatic forces between the ions.
Therefore, trivalent cations are much more efficient for the coagulation of negatively charged arsenic sulfide sol. than divalent cations, which in turn are more efficient than monovalent cations.
Similarly, tetravalent anions are more efficient for coagulation of positively charged ferric hydroxide sol than trivalent anions which are more efficient than divalent anions which are, in addition, more effective than monovalent anions.
Since, phosphate ion (${\text{PO}}_4^{3 - }$) is a trivalent anion having charge of -3 whereas chloride ion (${\text{C}}{{\text{l}}^ - }$) is a monovalent anion having charge of -1.
Therefore, for ferric hydroxide sol the coagulating power of phosphate ions is more than chloride ions.

Note- The minimum quantity of an electrolyte to be supplied to a litre of a colloidal solution to induce complete coagulation or flocculation is called the electrolyte's coagulation or flocculation value. Smaller value of the flocculation value of an electrolyte means higher coagulating or precipitating power.