CH. I.] 



ISO-ELECTRIC POINT. 



11 



(b) By changing the reaction of the fluid. 



In acid solution most colloids " adsorb " the positively 

 charged and readily diffusible hydrogen ions and acquire a 

 positive charge. In alkaline solutions they adsorb hydroxyl 

 ions and become negative. Many proteins are therefore 

 soluble both in acids and alkalies, but at some particular 

 reaction of the fluid they adsorb equal numbers of H and 

 OH ions, lose their charge, and are precipitated. The 

 exact reaction at which this takes place varies with different 

 colloids, and is the above-mentioned iso-electric point. 

 Another way of explaining this phenomenon will be found 

 on p. 31. 



6. The determination of the iso-electric point of casein. 



Into a 50 cc. measuring flask place 0-3 gm. of pure casein 

 (Hammersten's). Add about 25 cc. of distilled water, previously 

 warmed to about 40 C. and exactly 5 cc. of N. sodium hydroxide. 

 Agitate till the casein dissolves, taking care to prevent frothing. 

 Rapidly add 5 cc. of N. acetic acid, mix, cool, and make up to 50 cc. 

 with distilled water. A faintly opalescent solution of casein in 

 o-i N. sodium acetate is thus obtained. 



Make up the following series of tubes, using clean dry test-tubes. 



Place the casein solution in the tubes first, then the water, and 

 mix. Now add the acetic acid to the first tube and shake immedi- 

 ately. Then add the acid to the second tube and shake this, and 

 so on. Examine the tubes at intervals and record observations as 

 below. 



o = no change. + = opalescence. x = precipitate. 



