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22 GENERAL AND PHYSICO-CHEMICAL.
cent rat ion 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 ( = roVir gram-molecule) per liter.
Hardy 1
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 anions.
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.3
This valence rule becomes clear by the following experiment of Freundlich. 4
The figures give the lowest precipitation concentration expressed in millimolls
per liter. The hydrosol was As2 S3 (negative) and the valence of the cations is
applicable chiefly for the precipitating action.
KgSOj __ _ MgCl2 0.717
~2~ b5b MgSO-4 0.810
KC1 49.5 CaCl2 0.649
KNO, 50.0 SrCl2 0.635
NaCl 51.0 BaCl2 .0.691
LiCl 58.4 Ba(N03 ) 2 0.687
H*SOi ^"Cl2 0.685
—2~ 30.1 U02 (N03 ) 2 0.642
HC1 30.8
AICI3 0.0932
A!(NOs ) 3 0.0982
The precipitating action of anions upon a positive hydrosol (Fe[OH]3 ) is shown
in the following experiment of Freundlich :
KCJ 9.03 K2SO4 0.204
KNO, 11.90 T12 S04 0.219
NaCl 9.25 MgS04 0.217
BaGk
.9.64 K2 Cr2 7 0.194
Freundlich has extended the valence rule by the fact that with a negative
sol, II 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
SB organic anion- act against the precipitating action of the cations. The reverse
own with a positive sol; OH ions and organic anions of smaller precipitation
concentration than corresponds to their valence; H ions and organic cations
gainst the precipitating properties of the anions.
Certain above-mentioned suspensions (mastic), as well as other particles
suspended in water, act the same as suspension colloids. Schulze 5
has found
thai cloudiness due to ’lav particles on the addition of clarifying bodies (alum,
lime) give a voluminous deposition. Schloessing • found that clay suspensions
1
Zeitschr. f. physik. Chem., 33, 385 (1900).
’Journ. prakt. Chem. (2),. 25, 431 (1882).
Proc. Roy. 80c., 66, 110 (1899).
4
Zeitschr. f. Chem. u. Ind. d. Koll., 1, 323 (1907).
•Ann. Phys. (2), 12’.), 366 (1866). ,
•Compt. rend., 70, 1345 (1870).
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