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| Interaction between Colloids-Gold Number |
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| It is known that lyophobic sols are unstable. They are however stabilized by adding stabilizers. When two lyophobic sols are mixed together, they may coagulate by mutual neutralization of electric charges or by destroying each others stabilizers. A lyophobic sol may be protected by the addition of a lyophilic sol. |
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| It has been observed that lyophilic sols protect a lyophobic sol by preventing it from coagulating in the presence of an electrolyte. A possible reason might be that the lyophilic colloidal particles form a protective layer around the lyophibic particles. It is observed that gold or silver sol protected by gelatin, gum or starch do not coagulate easily. This protective power of a lyophilic sol can be expressed in terms of gold number. |
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| It is the number of milligrams of the protective colloid which just prevent coagulation of 10ml of standard gold sol when 1 ml of 10% solution of sodium chloride is added to it. |
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| Different lyophilic sols have different gold numbers. Smaller the gold number of the protective colloid, greater is the protective power of the lyophilic sol. The gold numbers of a few colloids are listed below. |
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