376 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



D. Colloidal Solutions of Platinum and Silver by Disintegration. 



On forming an arc between two pieces of platinum or silver wire under 

 distilled water, i.e. by separating the two poles when a suitable current is passed, 

 a colloidal solution of the metal is formed. The larger particles which are 

 formed settle and can be separated by decantation or nitration. 



The metal, e.g. bismuth or chromium, is ground finely in a ball mill and 

 treated for several days alternately with concentrated alkali and acid. On 

 treating with water a colloidal solution results. 



Colloidal copper and silver solutions are obtained if distilled water be 

 boiled in copper and silver vessels. 



Colloidal lead solutions are formed when water, free from oxygen and from 

 which oxygen is excluded, is kept in contact with lead. 



A suspension of fine particles of kaolin is obtained by shaking 

 some kaolin vigorously with water and pouring off from the larger 

 particles which settle rapidly. 



E. Colloidal Solution of Lead Chromate Using Viscous 



Media. 



Sols of lead chromate and barium sulphate can be prepared if the 

 reactions leading to their formation be carried out in a solution containing a 

 colloid such as a solution of caseinogen. 



Colloidal solutions of some inorganic salts may be prepared by 

 dissolving them in glycerol and pouring the solution into water (Craw). 



E.g. if some potassium chromate and lead nitrate be dissolved 

 separately in glycerol, the solutions mixed and poured into water, a 

 colloidal solution of lead chromate is formed. 



Detection of Colloidal Solutions. 



I. Dialysis. 



The simplest and most convenient way of showing the presence 

 of a colloid in solution is that of dialysis as used by Graham. 



As dialyser Graham employed a piece of parchment paper fastened 

 between two hoops forming a sort of tray which could be immersed in 

 water or other liquids. As the object is to obtain as large a surface as 

 possible the parchment paper is conveniently made in the form of a 

 sausage skin. The colloidal solution is introduced into the sausage skin, 

 the ends may be tied tightly and the skin is immersed in water or 

 other liquid, or it may be bent into U shape and suspended in a large 

 tall vessel. 



Thimbles made of parchment paper are useful for small quantities 

 of solution, and " soufflet " cases may also be used, especially for testing 

 solutions. Fish-bladder is another material frequently employed. 



Collodion thimbles or tubes prepared by coating surfaces of test 

 tubes, etc., with a solution of collodion in acetic acid, followed by im- 

 mersion in water and removal of the tough membrane from the glass, 

 form excellent dialysers. 



