Chemistry > Surface Chemistry > 7.0 Colloidal System
Surface Chemistry
1.0 Introduction
2.0 Adsorption
3.0 Factors affecting adsorption of gases by solids
4.0 Adsorption Isotherms
5.0 Applications of Adsorption
6.0 Types of Solutions
7.0 Colloidal System
7.1 Different Colloidal Systems
7.2 Classification of Colloidal System
7.3 Preparation of Colloidal System
7.4 Purification of Colloidal System
7.5 Properties of colloidal system
8.0 Coagulation of colloidal solutions
9.0 Emulsions
10.0 Catalysis
11.0 Zeolites as shape-selective catalysts
12.0 Enzyme as catalysts
12.1 Characteristics of Enzymes
12.2 Mechanism of enzyme catalysis
12.3 Autocatalysis
12.4 Induced catalysis
7.3 Preparation of Colloidal System
7.2 Classification of Colloidal System
7.3 Preparation of Colloidal System
7.4 Purification of Colloidal System
7.5 Properties of colloidal system
12.2 Mechanism of enzyme catalysis
12.3 Autocatalysis
12.4 Induced catalysis
Colloidal solution of lyophillic colloids can be prepared readily by simple treating the substance with the dispersion medium. On the other hand, the preparation of colloidal solution of lyophobic colloids requires special methods which are grouped under the two headings, viz. dispersion methods and condensation methods.
1. Dispersion methods: In these methods the coarse particles are broken to smaller particles of colloidal size.
- By mechanical disintegration
This involves mechanical grinding of substance particles into the size of the colloidal particles using colloid-mill which consists of two metal discs revolving at high speed in opposite direction. The materials to be converted into colloidal sol is fed in between the two discs in the form of a wet slurry. The particles get broken to colloidal dimensions by the operating shearing force.
- By Bredig's arc method
If an electric arc is struck between two electrodes of a metal, like gold, silver, platinum or
copper, in water having traces of an alkali, the metal is found to be converted into colloidal solution. Alkali acts as a stabilizer as shown in figure.
- Peptisation
The process of converting a precipitate into colloidal particles by adding suitable electrolyte is known as peptisation and the stabilizing agent (or electrolyte) as peptizing agent. The cause of peptisation is the adsorption of the ions of the electrolyte by the particles of the precipitate. Important peptizing agents are sugar, gum, gelatin and electrolytes.
Question 1. Explain the preparation of the following.
1. Colloidal solution of ferric hydroxide
2. Sol of stannic oxide
3. Indian ink
4. Colloidal solution of $Al(O{H_3})$ and $AgCl$
Solution: The preparation steps are:
1. Freshly prepared ferric hydroxide can be converted into colloidal state by shaking it with water containing $F{e^{3 + }}$ or $O{H^ - }$ ions, viz, $FeC{l_3}$ or $N{H_4}OH$ respectively. $$Fe{(OH)_3} + FeC{l_3} \to {[Fe{(OH)_3}Fe]^{3 + }} + 3C{l^ - }$$
Here, ${[Fe{(OH)_3}Fe]^{3 + }}$ is the colloidal solution of ferric hydroxide.
2. A stable sol of stannic oxide is obtained by adding a small amount of dilute hydrochloric acid to stannic oxide precipitate.
3. Lamp black is peptised by gums to form Indian ink.
4. A colloidal solution of $Al(O{H_3})$ and $AgCl$ are obtained by treating the corresponding freshly prepared precipitate with very dilute $HCl$ and $AgN{O_3}$ or $KCl$ respectively.
2. Condensation methods: In these methods, molecules condense or aggregate together to the particles of colloidal size. All chemical changes giving rise to insoluble products can be used for the formation of sol, provided a suitable stabiliser is present.
- By oxidation
A colloidal solution of sulphur can be obtained by bubbling oxygen (or any other oxidising agent like $HN{O_3}$, $B{r_2}$, etc.) through a solution of hydrogen sulphide in water.
$$2{H_2}S + {O_2} \to 2{H_2}O + 2S$$
- By reduction
A number of metals, such as silver, gold and platinum, have been obtained in colloidal state by treating the aqueous solution of their salts with a suitable reducing agent, such as formaldehyde, phenyl hydrazine, hydrogen peroxide, stannous chloride, etc.
$$2AuC{l_3} + 3SnC{l_2} \to 3SnC{l_4} + 2Au$$
- By hydrolysis
Many salt solutions are rapidly hydrolysed by boiling dilute solutions of their salts. Ferric hydroxide sol is the best example.
- By double decomposition
A sol of arsenic sulphide is obtained by passing hydrogen sulphide through a cold solution of arsenic oxide in water. $$A{s_2}{O_3} + 3{H_2}S \to A{s_2}{S_3} + 3{H_2}O$$
