80 COLLOIDS IN BIOLOGY AND MEDICINE 



positively charged in respect to the fluid; in an alkaline fluid it will 

 become negatively charged. If the dispersed phase itself has basic 

 or acid properties it will behave in pure water like a cation or an 

 anion respectively. If to an acid suspension, e.g., clay, an alkali is 

 added, K ions are adsorbed, OH ions are concentrated and held at 

 the outer film and the negative charge is thereby increased. The 

 reverse occurs upon adding acids; the charge is released and may 

 even take the opposite sign. According to this view, colloids wan- 

 dering to the anode are discharged only by cations; those travelling 

 to the cathode only by anions. We have seen that polyvalent ions 

 have a considerably greater discharging power, which increases with 

 their valence. This experimental fact accords, as do all the others, 

 with the theory propounded. 



In the case of hydrophile colloids, it suffices for us to assume that 

 we are dealing with very large amphoteric molecules which become 

 cations in acid solution and anions in alkaline solution. 



Salting Out. 



If large quantities of a neutral salt, for instance ammonium 

 sulphate, are added to a solution of a hydrophile colloid (albumin, 

 globulin, casein, albumose, silver protected by a protective colloid, 

 etc.), or certain inorganic hydrosols, e.g., sulphur, the colloid is thrown 

 out, but it redissolves upon dilution with water. This is the process 

 of salting out, as practised technically. If, for instance, enough com- 

 mon salt is added to an aqueous solution of phenol, the phenol 

 separates out. We might regard this as a withdrawal of water by 

 means of the electrolyte, since we know from the observations of 

 recent years that ions form hydrates in aqueous solution, i.e., every 

 ion attracts a greater or smaller number of molecules of water. I 

 have, however, been unable to discover any relation between the 

 salting out process, from the figures for the hydration of a number 

 of different ions as calculated by E. H. RIESENFELD and B. 

 REINHOLD*. 



The more closely a hydrophile colloid approaches the crystalloid 

 condition, the greater is the concentration of 'salt required for salting 

 out. Thus, for instance, the albumoses are classified in accordance 

 with concentration of salt required for their precipitation (see 

 p. 166.) 



As early as 1907, BECHHOLD had already called attention to the 

 relation between salting out, and particle size and salt concentration. 

 S. ODEN produced the experimental evidence that reversible sul- 

 phur and silver hydrosols could actually be separated in fractions, 



