HARDY'S RULE 93 



This coagulation may be brought about b\' adding [a) acids or 

 alkahes, (6) suitable electrolytes, or (c) colloids of opposite sign. 

 The coagulation of suspensoids (Class IV. colloids) by the above 

 means is easily carried out and is a reversible process. On washing 

 out the adsorbed precipitant the dispersoid is re-established. 

 Hydrophilic colloids, on the other hand, are more stable than 

 hydrophobic colloids. They usually need the addition of a large 

 quantity of the coagulating substance and the resulting coagulum 

 is frequently irreversible. We have seen that they are diphasic 

 systems where the continuous phase is more or less a continuation 

 of the disperse phase. If a substance A is dispersed in water to 

 form an emulsoid, what really results is a dispersion of a solution 

 of water in A, throughout a solution of A in water. The stability 

 of such a system will depend in great measure on the viscosity of 

 the intermicellar liquid. The viscosity depends on the concen- 

 tration of the more viscous A in the less viscous water. The 

 range of viscosity making for stability will be bounded on the one 

 hand by a certain minimimi and on the other hand by a certain 

 maximum concentration of water in the continuous phase. 



Salting Out. 



There are two factors implicated in the precipitation of hydro- 

 philic colloids by salts, one of these (a) is determined by the degree 

 of hydration of the colloid (see p. 98), while the other (j8) is related 

 to the solubility of the colloid-hydrate at the isoelectric point. At 

 this pa, the two factors may be included in the equation 



log s = am -\- ^Q 



where s = solubility of the colloid and m the molecular concentra- 

 tion of the salt. If precipitation is not carried out at the isoelectric 

 point, but at some other hydrogen ion concentration, then instead 

 of /3q another constant ^^ has to be used, depending on the ^^H. 

 If the precipitations are not induced at the isoelectric point of the 

 colloid, the powers of various salts to flocculate any colloid 

 depends on the valency of the cation if the pJi of the fluid is greater 

 than the isoelectric pH, and on the valency of the anion if the 

 solution is on the acid side. That is, under these conditioiis onlv 

 those ions are effective coagvdants whose electric charge is opposite 

 to that of the colloid (Hardy's rule). 



Protective action of Hydrophilic Colloids. 



Many emulsoids when added in comparatively minute quantities 

 to suspensoids prevent the coagulation of the suspensoids by 

 electrolytes. As a matter of fact, each emulsoid which exhibits 

 this property has a characteristic protective power which may 



