COLLOIDS 553 



stance of definite composition, being usually, if not always, a more 

 or less complex mixture of absorption products. 



3. Although not usually pure substances, it is not at all im- 

 plausible to assume that the dispersed particles may, to some extent, 

 undergo ordinary' electrochemical ionization, in which case the par- 

 ticles would partake of the nature of enormously large ions. This 

 assumption is interesting as offering a purely electrochemical ex- 

 planation of the origin of the charge which is found on such par- 

 ticles, and it is to be said that frequently the effect of foreign sub- 

 stances on the electrical charges of suspended particles is explain- 

 able on this assumption. For further information on these matters 

 reference must be had to the papers of Duclaux, 12 Jordi, 13 and 

 P. P. von Weimarn. 14 



The Flocculation of Colloids by Electrolytes 1. When neutral 

 salts are added to colloid solutions in gradually increasing amounts 

 there always follows sooner or later a precipitation of the dispersed 

 substance. If the salt solution be removed and pure water added 

 in its place, this decanted, and pure water again added, so as to 

 wash out the salt as thoroughly as possible, the final result may 

 be that the coagulum (or gel) becomes redispersed, or such redis- 

 persion may not occur. The outcome is determined both by the 

 nature of the colloid and by that of the neutral salt used. 



2. If acids and alkalies are the electrolytes used, the relation- 

 ships are somewhat different. The addition of alkali to a negatively 

 charged colloidal solution renders it more stable, while a positively 

 charged colloid is flocculated. With acids the reverse of this condi- 

 tion holds ; that is, the positive colloid is stabilized and the negative 

 one is flocculated. 



3. The concentration of the electrolyte required to flocculate a 

 given colloidal solution depends very greatly on the nature of the 

 electrolyte used. It has been generally considered that the cathion 

 of the electrolyte is the active agent in flocculating negative colloids, 

 while the anion is active in the case of positive ones. Thus acids 

 (hydrogen ions) are very effective in flocculating arsenic sulphid, 

 and alkalis (hydroxyl ions) are effective with ferric hydroxid, as 

 might be inferred from the preceding paragraph. Different cathions, 

 however, show very different degrees of precipitating or flocculating 

 power. Thus in the case of arsenic sulphid, if the flocculating power 

 of the potassium ion is taken as unity, that of calcium is about 

 twenty, while that of the aluminium ion is three hundred and fifty. 

 The flocculating power in this case increases very rapidly with the 



12 "Thesis," Paris, 1904. 



13 C. R., 136, 680, and 1,448; 137, 122; Bull. Soc. Chim., 31, 573. 



14 Articles in Ztschr. Chem. Ind. Roll, 1906-1911. Also in "Grundziige 

 der dispersoid Chemie," Dresden, 1911. 



