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26 GENERAL AND PHYSICO-CHEMICAL.
example, a trivalent ion is three times greater than in a monovalent ion.
Otherwise greater precipitation ability of polyvalent ions can also be
explained by a greater hydrolytic cleavage of the salts (page 23).
The mechanism of the precipitation of the isoelectric solution accepted
in Hardy’s theory is explained by Bredig x
as follows: At the boundary
between suspended particles and solvent a certain surface tension exists
which tries to diminish the total contact surface between the two media,
which can happen by the small particles uniting to form larger ones,
when nocking is brought about. The electrical charge of the particles
acts against the surface tension so that equally charged particles repel
each other. If the electrical charge is discharged, as takes place in the
isoelectric point, then the surface tension reaches its highest value and
the precipitation may occur.
The correctness of Hardy’s claim that precipitation occurs just in
the isoelectric fluid is disputed on special grounds by Billitzer. He
believes that the ions have a much greater charge than the colloid par-
ticles. An ion collects the oppositely charged colloid particles around
itself, and during these neutralization processes it may occur, that the
entire complex may become so large as to become visual and on
account of the gravity it precipitates out.
In general it can be stated that the stability of a colloid is greater the smaller?
cet. par., the particles are; as the probability that the number of particles sufficient
for the precipitation is then less. With equal size of particles the stability of a
colloid is dependent upon the size of the charge which the particles carry. Too
weak and very strongly charged colloids are relatively more stable; the first
because of the large number which must collect around an ion when flocking takes
place and the second because the number of particles required for the neutrali-
zation is perhaps too small, so that the necessary size of the complex for precipita-
tion is not attained. 2
The findings of Linder and Picton 3
that when colloidal AS2S3 is
precipitated with BaCk> the solution becomes acid, and a small quantity
of barium remains in the precipitate, corresponds to Billitzer’s theory.
This quantity of barium cannot be removed by water, but can be replaced
by the corresponding cation by washing with a solution of another salt.
According to Billitzer in the mutual precipitation of colloids a quan-
tity relation exists which is dependent upon the electrical charges 4
(see
also page 22).
The fact that the precipitation of colloids is a manifestation of
processes which occur in a homogeneous medium, makes the understand-
ing of these especially difficult. If, as is generally accepted, we consider
1
Anorganische Fermente (1901), 15.
Zeitechr. f. physik. Chem. Soc, 45, 327 (1904); 51, 129 (1905).
Mourn. Chem. Soc, 67, 63 (1895).
•Zeitechr. f. physik. Chem., 51, 141 (1905).
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