Dispersion Or Disintegration Methods
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Dispersion Or Disintegration Methods
Surface Chemistry of Class 12
Dispersion Or Disintegration Methods
In these methods, large lumps, particles are broken down to a colloidal size in the dispersing phase by using some appropriate mechanical means. The resulting unstable solution are then stabilized by adding suitable stabilizers the required dispersion may bring about in any of the following methods :
(a) Electro−dispersion or Bredig’s arc method: Colloidal sols of metals like Au, Ag, Cu and Pb etc. are obtained by the condensation of vapour. The addition of a trace of alkali in water stabilizes the colloidal solution so formed.
(b) Mechanical dispersion method: Colloids like black, paints, varnishes, ointments, dyestuffs, dental creams, etc., are prepared by this method. The colloids are then stabilized by adding a suitable protective colloid like gum arabic.
(c) Peptization: When to a freshly prepared precipitate of an electrolyte having a common ion [with precipitate (or solid) to be dispersed] is added the former passes into colloidal form. The method of breaking down a precipitate into colloidal sol form by adding an electrolyte solution, is called peptization and the electrolyte used for this purpose is called peptizing agent.
(i) Substance such as glue, gelatin and egg−albumin can be dispsered in warm water, merely by mixing and in such cases, water is itself acting as a peptizing agent.
(ii) Freshly precipitated iron hydroxide can be peptized to a reddish brown sol by the additon of a small quantity of ferric chloride solution (peptizing agent)
Fe(OH)3 + Fe3+ → Fe(OH)3Fe3+
(peptizing agent) (Sol)
(iii) Al(OH)3 precipitate is peptized by the addition of a small quantity of dilute
NaOH solution.
Al(OH)3(s) + OH− → Al(OH)3OH−
(peptizing) (Sol)
(iv) Freshly precipitated AgCl suspension is peptized by shaking it with water containing a little AgNO3.
AgCl(s) + Ag+→ AgCl. Ag+
(peptizing agent) (Sol)
Although there is no general rule, it is found that the most effective peptizing agent for a substance is one which contains a common ion e.g., ferric or hydroxide ions for freshly precipitated Fe(OH)3 or chloride or Ag+ ions for AgCl precipitate.
(v) Gelatin, gum arabic, starch, egg albumin, glycogen and soap are converted into sol (lyophilic) by peptizing these substances with water.
Cause of peptization is believed to be due to the preferential adsorption of the inner (or common) ions of the added electrolyte. Thus during the peptization of Fe(OH)3 by FeCl3 the Fe3+ ions are adsorbed on the Fe(OH)3 particles. Due to similar positive charges on the particles, they get separated yielding positive colloidal particles of nearly same charge and size, Fe(OH)3. x Fe3+.
High molecular weight organic substances from sols very easily, when brought in contact with or warmed with suitable dispersion medium. For example gelatin or starch sol in water, cellulose nitrate sol in organic solvent such as ether and ethanol mixture.
(d) Dispersion by ultrasonic waves: In this technique, a high frequency current is passed through a circular quartz crystal. by connecting it to two electrodes. When an alternating current is passed, the crystal begins to oscillation, low−melting alloy sols and sols of mercury have been obtained by using this method.
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