200 SOLUTION AND PRECIPITATION OF PROTEINS 



for each other. When oleic acid is placed on water the acid spreads upon the 

 water because by so doing the COOH can dissolve in the water without separating 

 the hydrocarbon chains from each other. 



"When the surface on which the acid spreads is sufficiently large the double 

 bond in the hydrocarbon chain is also drawn onto the water surface, so that the 

 area occupied is much greater than in the case of the saturated fatty acids. 



"Oils which do not contain active groups, as for example pure paraffin oil, 

 do not spread upon the surface of •:^'ater."^'* 



That the solution of isoek ':tric casein in NaOH has no connection 

 with the Donnan equilibrium is also evidenced by the fact that 

 there is no point in the concentration of NaOH where further in- 

 crease in the concentration of NaOH lowers the rate of solution, 

 as would be the case if the Donnan equilibrium influenced this 

 phenomenon. 



The stability of solutions of this kind is guaranteed by the forces 

 of chemical attraction between certain groups of the molecule of 

 Na caseinate and water; and the precipitation by electrolytes is due 

 to a diminution of these forces. 



IV. Precipitation and Solution of Gelatin in Water. 



Solutions of gelatin in water require enormous concentrations of 

 salts for precipitation and it is very probable that the forces causing 

 solution are the forces of residual valency discussed in the case of the 

 solution of Na caseinate. These cases have no connection with the 

 Donnan equilibrium, and this is borne out by the well known fact 

 that solutions of gelatin are always more readily salted out by sul- 

 fates than by chlorides, regardless of the pH of the gelatin solution. 

 This is illustrated by Table III. 



0.8 per cent solutions of gelatin were prepared at three different 

 pH; namely, 4.7 (isoelectric gelatin), 3.8 (gelatin chloride), and 6.4 

 to 7.0 (Na gelatinate). The purpose was to find the molecular con- 

 centration of different salts, namely, (NH4)2S04, Na2S04, MgS04, KCl, 

 and MgCl2, required for precipitation. Table HI shows that regardless 

 of the pH the sulfates are better precipitants than the chlorides. 

 There is, therefore, no definite relation between the sign of charge 

 of the colloidal particles and of the precipitating ion. 



^^Langmuir, I., /. Am. Chem. Soc, 1917, xxxLx, 1850. 



