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| Preparation of Colloidal Sols |
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| For the preparation of lyophilic and lyophobic sols different methods are used. |
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| Since lyophilic sols are quite stable, they can be easily prepared by shaking the lyophilic substance with the dispersion medium. |
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| Examples are: Colloidal sols of gum, starch, gelatine and egg albumin. |
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| Lyophobic sols are prepared by two methods. They are: |
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| 1) Condensation methods |
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| 2) Dispersion methods. |
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| Condensation methods |
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| In condensation methods particles of atomic or molecular size are induced to combine to form aggregates of colloidal dimensions. To achieve this, chemical as well as physical methods are employed. |
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| a) Chemical methods |
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| Colloidal sols can be prepared by chemical reactions, which involve double decomposition, oxidation, reduction and hydrolysis. Examples of sols made by these methods are given below: |
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| i) Double decomposition |
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| ii) Oxidation |
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| iii) Reduction |
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| iv) Hydrolysis |
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| Sols of silver halides are prepared by mixing dilute solution of silver salts with alkali halides in equivalent amounts. Similarly, silica gel sol is prepared by mixing dilute solutions of sodium silicate and HCl. |
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| b) Physical methods |
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| i) Exchange of solvent |
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| In this method, a true solution in mixed with an excess of another solvent in which the solute is insoluble but the solvent is miscible. For e.g., a solution of sulphur in alcohol mixed with excess of water results in a colloidal sol of sulphur. |
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| ii) By excessive cooling |
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| A colloidal sol of ice in an organic solvent such as CHCl3 or ether is obtained by freezing a solution of water in the solvent. The molecules of water, which can no longer be held in solution separately come together to form particles of colloidal size. |
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| Dispersion methods |
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| In dispersion methods, colloidal particles are obtained by breaking large particles of a substance in the presence of a dispersion medium. Since the sols formed are unstable, they are stabilized by adding stabilizing agents. Some of the dispersion methods are: |
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| a) Mechanical dispersion |
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| b) Electrical disintegration |
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| c) Peptization. |
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| a) Mechanical dispersion |
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| In mechanical dispersion, the coarse suspension of the substance is ground in a colloid mill, a ball mill or an ultrasonic disintegrator. The colloid mill consists of two metal discs, close together and rotating at high speed (7000 revolutions per minute) in opposite directions. By the process of such grinding, the suspension particles are torn off to the colloidal sizes. |
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| b) Electrical disintegration or Bredig's Arc Method |
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| In this method, an electric arc is struck between electrodes of the metal immersed in the dispersion medium. |
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| fig 7.12 - Bredig's arc method |
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| The intense heat produced by the arc
vaporizes the metal, which then condenses to form particles of colloidal size. By this method, sols of metals such as gold, silver and platinum can be prepared. |
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| c) Peptization |
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| The process of converting a precipitate into a colloidal sol by shaking it with the dispersion medium, in the presence of a small amount of electrolyte, is called peptization. The electrolyte used is called the
peptizing agent. This method is used to convert a freshly prepared precipitate into a colloidal sol. |
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| In the process of peptization, the precipitate adsorbs one of the ion of the electrolyte onto its surface. The ion adsorbed on the surface is generally common with those of the precipitate. Adsorption of ion results in the development of positive or negative charge on precipitates and which ultimately break up into colloidal size particles. For e.g., a precipitate of silver iodide already formed can be dispersed by the addition of potassium iodide. Here potassium iodide is the
peptizing agent. |
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