CH. I.] 



COLLOIDS. 



13 



(c) By the addition of neutral salts. 



If a suspensoid carries a negative charge it exerts an 

 attraction for positively charged ions (kations). The 

 adsorption of these by the colloid may cause a neutralisa- 

 tion of the charge, and therefore precipitation. In such 

 cases it is found that a bi- or tri-valent ion is very much 

 more potent than a monovalent ion. Thus, if a colloid is 

 negatively charged it may be readily precipitated by 

 BaCLj ; if it carries a positive charge it may be readily 

 precipitated by Na 2 SO 4 . The precipitation of an emulsoid by 

 a large excess of neutral salt, such as by saturation with 

 ammonium sulphate, is probably a different phenomenon. 



7. The precipitating effect of various ions on colloids. 



Prepare a solution of casein in o-i N. sodium acetate as de- 

 scribed in Ex. 6. 



To 2 cc. add 17-5 cc. of distilled water, and then 0-5 cc. of 

 0*1 N. acetic acid and mix quickly. A solution of casein is thus 

 obtained, alkaline to the isoelectric point, and therefore carrying a 

 negative charge. Divide the solution into four equal parts and 

 place them into four clean tubes labelled -i, -2, -3 and -4. To 

 another 2 cc. of the original solution of casein add 10 cc. of distilled 

 water and 8 cc. of o-i N. acetic acid, and mix quickly. An acid 

 solution of casein is thus obtained. Divide into four parts and 

 place them into four clean tubes labelled -fi, +2, +3 and +4. 



To the tubes marked i add 3 drops of N. KC1 (7-45 per cent.). 



To the tubes marked 2 add i drop of N. BaQ 2 (10-4 per cent.). 



To the tubes marked 3 add i drop of N. K 2 SO 4 (8*7 per cent.). 



Mix the contents of each tube and place the set of 8 tubes in a 

 water bath at about 35 C. Examine them after 15 minutes, record- 

 ing the results as in the previous exercise. 



X 



o or + 

 o 



X X 



o 



