COLLOIDS AND COLLOIDAL SOLUTIONS 375 



Variety of Solvent. 



It is now known that other solvents besides water can dissolve sub- 

 stances forming colloidal solutions. 



Cellulose dissolved in Schweitzer's reagent or in zinc chloride forms 

 a colloidal solution from which the substance is precipitated as a gelatin- 

 ous mass. Nitrocellulose dissolved in acetic acid, acetone, or alcohol- 

 ether, forms a colloidal solution. Sodium chloride can be made to form 

 a colloidal solution in petroleum ether and the alkali metals in organic 

 solvents. 



Sols and Gels. 



Graham called the apparent solutions of colloids colloidal solutions, 

 or sols, and the precipitated or gelatinous substance, gels. We can 

 further distinguish the solvent by prefixing its name, e.g. hydrosol, 

 alcoholgel, etc. 



PREPARATION OF ARTIFICIAL COLLOIDAL 

 SOLUTIONS. 1 



A. Colloidal Solutions of Metallic Sulphides. 



(a) Cadmium Sulphide. A fine suspension of cadmium sulphate, pre- 

 viously washed with distilled water, is treated with hydrogen sulphide. The 

 solution gradually becomes milky and finally has a yellow colour with a reddish 

 surface. The excess of hydrogen sulphide is removed by a current of nitrogen 

 or by boiling. 



(&) Arsenious Sulphide. About I gm. of arsenious acid is boiled 

 for a few minutes with about 75 c.c. of distilled water ; the solution is 

 filtered and allowed to cool. On passing hydrogen sulphide through 

 the cold solution, it turns a yellow-orange colour with a greenish surface. 



B. Colloidal Solution of Ferric Hydroxide. 



I c.c. of a filtered 33 per cent, solution of ferric chloride is 

 added to 100 c.c of boiling distilled water. A reddish-brown solu- 

 tion is obtained 



A colloidal solution of ferric hydroxide may also be obtained by dialysing 

 a solution of ferric chloride. 



C. Colloidal Solutions of Gold and Silver by Reduction. 



i c.c. of i per cent, gold chloride solution is diluted with 25 c.c. of 

 distilled water. 2 gm. of tannic acid are dissolved in 100 c.c. of water. 



On mixing i volume of the gold chloride solution with 3 volumes of the 

 tannic acid, a blue solution is formed. On. mixing i volume of the gold 

 chloride 'solution with i volume of the tannic acid, a red solution is formed. 



Similar solutions may be made by treating gold chloride solution with a 

 solution of i gm. of hydroquinone or pyrogallol dissolved in 500 c.c. of water. 



Ammonia is added drop by drop to 10 c.c. of silver nitrate solution 

 until the precipitate first formed just redissolves. The .solution is 

 diluted with 200 c.c. of water. On mixing equal volumes of this 

 solution with the 2 per cent tannic acid, a brown solution having 

 a greenish colour in reflected light is formed. 



1 In preparing artificial colloidal solutions the glass vessels must be absolutely clean, 

 preferably new, and washed with nitric or chromic acid. Freshly distilled water should 

 also be used. 



