Preparation Of Colloidal Solutions

Preparation Of Colloidal Solutions

As colloid represents a state between true solution and suspension, so colloidal solution may be prepared either.

(1) by coalescing or combining several particles of molecular size into bigger particles of colloidal size called condensation method, or

(2) by breaking up the big coarse particles of suspension smaller particles of colloidal dimensions called disintegration method.

Condensation methods: In these methods, the parent substance from which the colloidal particles are obtained, are present initially in the solution as small ions, atoms and molecule; and methods are used to induce the aggregation of these to form aggregates of colloidal size. Various condensation methods are used to obtain colloid.

(a) By excessive cooling: Colloidal solution of ice in an organic solvent like pentane or chloroform is prepared by freezing a solution of water in the solvent. Preparation of ice−creams is based on this principle.

(b) By exchange of solvent: Colloidal solution of a substance can be obtained by dissolving the substance in a solvent and then, pouring the solution thus−obtained into another solvent, in which the substance is insoluble or sparingly soluble. Thus, when saturated alcoholic solution of sulphur is poured into water, colloidal solution of sulphur is obtained. Alcohol is then removed by dialysis. Such sols are, however, unstable.

(c) By controlled condensation: Sols of certain insoluble substances are produced by precipitating them in the presence of some protective colloidal system (e.g., gelatin, glucose, glycerol, etc.). For example Prussian blue sol is obtained by precipitating it in the presence of starch

3K_4[Fe(CN)₆] +4FeCl₃ → Fe₄[Fe(CN)₆]₃ + 12KCl

Prussian blue

(d) By double decomposition: This is usual way of preparing sols of insoluble inorganic salts.

For example,

(i) Colloidal solution of aresenious sulphide is obtained by passing slowly H₂S gas through a cold dilute solution of As₂O₃ in water

As₂O₃ + 3H₂S → As₂S₃ + 3H₂O

The excess of H₂S is then removed by passing hydrogen gas through the solution

(ii) Silver halides (e.g., AgCl) sols can be prepared by mixing dilute solutions of silver salts
(e.g., AgNO₃) and alkali halides (e.g., NaCl) in equivalent amounts.

AgNO₃ + NaCl → AgCl + NaNO₃

(e) By hydrolysis: Colloidal solution of many oxides and hydroxides of weakly electro−positive metals like iron, aluminium, tin, thorium, etc., can be suitably obtained by this method.

FeCl₃ + 3H₂O → Fe(OH)₃ + 3HCl

Deepred colloidal

solution

(f) By reduction:

2AuCl₃ + 3HCHO + 3H₂O → 2Au + 3HCOOH + 6HCl

(Colloidal)

2AuCl₃ + 2SnCl₂ → 2Au + 3SnCl₄

(Colloidal)

(g) By Oxidation:

2H₂S + SO₂ → 3S + 2H₂O