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COLLOIDS. 21
solution wander to the anode in the case when the experiment is so
carried out that the formation of acid or alkali is prevented at the anode
or cathode respectively. If the neutral protein solution is treated with
a trace of acetic acid then the particles wander to the cathode. With
a certain very slight degree of acidity the direction of the wandering of
the particles is reversed. With this reaction no wandering or a double-
sided wandering of the protein bodies can be detected. This so-called
isoelectric point has been determined by Michaelis, Rona and their
collaborators for different protein substances.1
Michaelis and Rona
claim to have found in the isoelectric point the most favorable reaction
for the heat coagulation of the protein substances, while Sorensen and
Jurgensen consider the reaction which the pure protein substance
gives to pure water as the optimal precipitation reaction.2
According to Gatin-Grtjzewska3
pure glycogen wanders distinctly
to the anode.
Precipitation of the Colloids.
The colloids can be separated from their solutions in various ways.
Many colloidal solutions are so unstable that they flock out after a
time without the addition of anything (silicic acid, metallic hydroxides).
Certain colloids appear as flocculent precipitates on heating their solu-
tions (certain proteins, see Chapter II). Others solidify on cooling
from hot concentrated solutions, as semisolid forms, so-called jellies
or hydrogels, containing considerable water (glue, starch, agar).
On evaporating the hydrosols at ordinary temperature we obtain
a residue which Zsigmondy divides into reversible and irreversible col-
loids, according whether they are again soluble in water or not.4
Accord-
ing to this definition starch, dextrin, agar, gum, and proteins belong to the
reversible colloids while colloidal silicic acid, stannic acid, colloidal metallic
hydroxides and sulphides, and the pure colloidal metals belong to the
irreversible colloids. The former are relatively non-sensitive toward
the addition of electrolytes, while the latter flock out on the addition
of the smallest quantity of electrolyte, and indeed again in an irreversible
form. This classification stands in accord with what was given above
(page 15), as the reversible colloids coincide in a measure with the
hydrophile colloids and the irreversible with the suspension colloids.
Electrolyte Precipitation of Suspension Colloids. It must be
remarked that for every precipitating electrolyte a certain minimal con-
1
See page 74.
* See Ergebnisse d. Physiologie, 12, 506 which also gives the literature.
J
Pfliiger’s Arch., 403, 287 (1904).
4
Zur Erkenntnis d. Koll., page 21.
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