92 



DISPERSE SYSTEMS 



sol migrated to the cathode in acid sohitioii or to the anode in 

 alkahne sokition. 



Isoelectric Point. 



At a pH of -t-8 in Hardy's cataphoresis experiment the albumin 

 particles did not migrate to one pole or the other. This indicates 



that the protein is equally 

 ionised as an acid and as a base, 

 and is electrically neutral. The 

 isolectric point of an amphoteric 

 colloid is that pH at which 

 certain of the characteristic 

 properties of the colloid are 

 minimal, viz., osmotic pressure, 

 viscosity, imbibition and sta- 

 bility, as is shown for gelatin in 

 Fig. 21 from Loeb's results. 



The actual concentration of 

 H ions at which these minimal 

 values are reached is specific for 

 each colloid. Most animal pro- 

 teins are isoelectric on the acid 

 side of neutrality, while vege- 

 table proteins reach this point 

 on either side of pH 7. 



Coagulation of Gels and Precipita- 

 tion of Sols. 



As we have seen, at the 

 isoelectric point, the colloidal 

 particles become electrically 

 neutral, and, therefore, one of 

 the factors tending to keep 

 them apart has been removed, 

 viz., the repulsion of similarly 

 charged bodies. When, by 

 simple molecular agitation, some 

 of the particles pick up an 

 electron, as they are bound to 

 do, and so become differently 

 charged from their neighbours, 

 mutual attraction takes place ; these particles coming together form 

 larger aggregates, thus accounting for the lowering of osmotic 

 pressure and of the stability of the dispersion at this point. 



4.7 

 PH 4.1 4.i 4.2 43 44 4.S 4J H 5-4 55 5-6 5-6 S f 



Fig. 21. — Curves showing that the total 

 swelling, viscosity, osmotic pressure, and con- 

 ductivity of gelatin are minimal at the isoelectric 

 point, pn 4-7. (.After Loeb.) 



