258 THEORY OF COLLOIDAL BEHAVIOR 



CaCl 2 on the swelling of Na gelatinate is more than 10 times 

 as great as that of NaCl. While these data are only semiquan- 

 titative, they enable us to form an approximate estimate as 

 to whether or not the precipitating action of a salt on gelatin 

 can be due to a diminution of the osmotic pressure (or P.D.) 

 between the coalescent ions of gelatin in conformity with the 

 Donnan equilibrium. 



The procedure in our experiments was as follows : A stock solu- 

 tion of 5 per cent gelatin chloride of pH 3.0 was prepared; 2 

 c.c. of this solution were heated to about 45C. to bring about 

 complete liquefaction, and 50 c.c. of absolute alcohol were added 

 while the gelatin was still warm and liquid. This concentration 

 of alcohol was in excess of that required for the critical limit, and 

 the gelatin solution was slightly opalescent. Ten cubic centi- 

 meters of this mixture of gelatin-alcohol-water were titrated with 

 different salt solutions until a precipitate was found. The con- 

 centration of the salt solution was selected in such a way that 

 not less than 0.3 and not more than 0.8 c.c. of solution were 

 required for precipitation to avoid the addition of too large a 

 volume of water to the solution. The difference in the relative 

 efficiency of the different electrolytes is therefore expressed chiefly 

 in the concentration of the solution required for precipitation. 

 The reader should bear in mind that the pH of the gelatin chloride 

 solution after the alcohol and the salt solution were added could 

 not be measured, and that it was probably higher than 3.0 and 

 about the same in all solutions. In order to be able to compare 

 the relative flocculating efficiency of different salts the flocculat- 

 ing concentration is expressed in equivalents of cubic centimeters 

 of M/1,024. 



Table XL VIII shows that all salts with monovalent anion have 

 a lower flocculating power on gelatin chloride than salts with 

 divalent anion. 



Salts with monovalent anion require a molecular concentration 

 of about 100/1,024, i.e., about M/10 concentration for precipita- 

 tion, while those of the second group require a molecular concen- 

 tration of about 10/1,024, i.e., about M/100 or less. This shows 

 that the difference in the flocculating power of monovalent and 

 bivalent anions has roughly the ratio of about 1:10, i.e., that it 

 corresponds to the ratio to be expected from Fig. 70 within the 



