COLLOIDS. 43 



Electrolyte Precipitation of Suspension Colloids. It must be 

 remarked that for every precipitating electrolyte a certain minimal con- 

 centration is necessary to bring about flocking. In comparing the 

 precipitation ability of various electrolytes the concentration of that 

 solution which is just sufficient to cause a visible cloudiness is given in 

 millimolls (==-J^Q gram-molecule) per liter. 



HARDY l has also found that colloids which wander to the anode 

 are chiefly flocked out by the cations of the precipitating electrolyte, 

 and colloids wandering to the cathode are chiefly flocked out by the anion?. 

 H. SCHULTZE 2 has proven that the precipitating ability is influenced 

 greatly by the valence of the precipitating ions, as the divalent ions act 

 much stronger than the monovalent and the trivalent are still more 

 active than the divalent. This rule has been substantiated by HARDY 

 and others. 3 This valence rule becomes clear by the following experi- 

 ment of FREUNDLiCH. 4 The figures give the lowest precipitation con- 

 centration expressed in millimolls per liter. The hydrosol was As 2 S 3 

 (negative) and the valence of the cations is applicable chiefly for the 

 precipitating action. 



K 2 S0 4 MgCl 2 .................. 0.717 



-~Y~ .............. b5 ' b MgS0 4 .................. 0.810 



KCl ................ 49.5 CaCl 2 ................... 0.649 



KNO, .......... 50.0 SrCl 2 ................... 0.635 



NaCl ............ 51.0 BaCl 2 ................... 0.691 



Lid ................ 58.4 Ba(NO 3 ) 3 ............... 0.687 



H 2 S0 4 ZnCl 2 ................... 0.685 



-% .............. 30.1 UO 2 (N0 3 ), .............. 0.642 



T T pi o n o A1C1 3 ................... 0.0932 



A1(N0 3 ) 3 ............... 0.0982 



The precipitating action of anions upon a positive hydrosol (Fe[OH] 3 ) 

 is shown in the following experiment of FREUNDLICH: 



KCl. ..9.03 K 2 SO 4 . . ..0.204 



KXO, .............. 11.90 H 2 SO 4 .................. 0.219 



NaCl ............... 9.25 MgSO 4 .................. 0.217 



9.64 K 2 Cr 2 O 7 ................. 0.194 



FREUNDLICH has extended the valence rule by the fact that with a negative 

 sol, H ions, the ions of the heavy metals, as well as organic cations in weaker con- 

 centration, have a greater precipitating action than other cations; OH ions as well 

 as organic anions act against the precipitating action of the cations. The reverse 

 is shown with a positive sol; OH ions and organic anions of smaller precipitation 

 concentration than corresponds to their valence; H ions and organic cations 

 act against the precipitating properties of the anions. 



1 Zeitschr. f. physik. Chem., 33, 385, 1900. 



2 Journ. prakt. Chem. (2), 25, 431, 1882. 



3 Proc. Roy. Soc., 66, 110, 1899. 



4 Zeitschr. f. Chem. u. Ind. d. Roll., 1, 323, 1907. 



