Question

Which of the following act as the best coagulating agent for ferric hydroxide sol?
(A)- Potassium ferrocyanide
(B)- Potassium chloride
(C)- Potassium oxalate
(D)- Aluminium chloride

Answer 1

A colloid made out of very small particles in a continuous liquid medium is known as a sol. Sols are quite stable hence they do not settle down when left undisturbed. Sol is heterogeneous mixtures in which the size of the colloid varies from 1 nm-100 nm.

Complete answer:
-The word lyophobic means ‘liquid-hating’, that is such compounds will have the least interaction between the dispersed phase and dispersed medium. lyophobic sols are also known as hydrophobic sols.
-The method of the precipitating of a colloidal solution by adding the excess of an electrolyte is referred to as coagulation or flocculation.
-The coagulation of the lyophobic sols can be carried out by following methods-
(i) By electrolysis
(ii) By mixing two oppositely charged sols
(iii) By boiling
(iv) By persistent dialysis
(v) By the addition of electrolytes
-The coagulation capacity varies with an electrolyte which in turn depends on the valency of the active ion which is known as a flocculating ion. The ion carrying charge is known as flocculating ion and is opposite to the charge on the colloidal particles.
- Hardy Schulze rule is a very important rule for the coagulation of lyophobic sols. According to Hardy Schulze rule, greater the valency of the active ion or flocculating ion, greater will be its coagulating power. According to Hardy Schulze rule,
(i) The ions carrying the charge opposite to that of sol particles are effective in causing the coagulation of the sol.
(ii) Coagulating power of an electrolyte is directly proportional to the valency of the active ions (ions causing coagulation).
-For coagulating a negative sol, different cations are used which will have their coagulating power decrease with the decrease in their valency. Similarly, for coagulating a positive sol, different anions are used which have their coagulating power decrease with their valency.
-Ferric hydroxide is a positive sol and hence will be coagulated using a negative sol. Among all the options, potassium ferrocyanide ion is having the maximum valency hence it will act as best coagulating agent for ferric hydroxide sol.

Hence the correct answer is option A.

Note: Ferric hydroxide which is obtained by hydrolysis of ferric chloride with boiling distilled water can form a lyophobic sol.
$FeC{{l}_{3}}(aq)+3{{H}_{2}}O\xrightarrow{boil}Fe{{(OH)}_{3}}(s)+3HCl(aq)$
The hydrochloric acid formed during the hydrolysis of ferric hydroxide destabilizes the ferric hydroxide sol and therefore hydrochloric acid must be removed through the process of dialysis.