METHODS OF COLLOID RESEARCH 



117 



Flocculation. 



Observations of the flocculation of a suspension or colloidal solu- 

 tion determine whether it behaves as a hydrophile or a hydrophobe 

 colloid; furthermore, they show the electric charge and, under cer- 

 tain conditions, the presence of a protective colloid. The method of 

 the experiment is very simple: a suspension or colloidal solution is 



'Gas 

 Cylinder 



Swelling Substance- | 



FIG. 26. 



Cla 



thirnbte 



divided among a large number of test tubes, diminishing quantities 

 of electrolytes (NaCl, CaCl 2 , FeCl 3 ) are added and the test tubes are 

 filled to equal volumes with a solvent (water, physiological salt solu- 

 tion, etc.). After the test tubes have been exposed at uniform tem- 

 perature for a given time (1 to 24 hours), they are examined for 

 flocculation. 



In comparative experiments it is necessary that suspensions or solutions have 

 the same concentration. On account of the small amount of solid substance 

 little is to be accomplished by determinations of total solids. It is frequently de- 

 sirable to prepare a large quantity of a standard solution to last a long time, and 

 frequently the determinations may be made colorimetrically. I am accustomed 

 to prepare mastic suspensions by dropping 1 per cent alcoholic mastic solutions 

 into water which is being energetically stirred. The suspension is filtered through 

 rather dense filter paper and tested for transparency in a beaker having parallel 

 sides, to one side of which various printed lines have been glued. The suspen- 

 sion is diluted until, with a definite illumination, a certain size type can just be 

 read. 



