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COLLOIDS. 25
On comparing the concentration of various salts just sufficient for precipita-
tion, where at one time the same anion with different cations was tested and
another time the same cation with different anions, Pauli has arranged the cations
and anions in the following order in increasing precipitation ability:
CNS <I <Br <XO, <C1 <OCO.CH3 <HPO< <S04
XH4 <K<Xa<Li.
The protein used in these experiments was white of egg. According to Pauli
certain ions have a precipitating action and others a solvent action. The action
of a salt corresponds to the algebraic sum of the action of the ions. 1
Pauli has
attempted to associate the precipitation ability of the salts in relation to their
action upon the coagulation temperature, but without any positive results. 2
Nevertheless Spiro !
has shown that the kind of protein as well as its con-
centration are of importance for the precipitation action, and Hober * has recently
shown that the series I<Br<Cl<S04 and Li<Xa<K <Rb <Cs is valid in
alkaline reaction, but that the series is reversed in acid reaction. In nearly
neutral reaction irregularities in the ion series occur which can be considered as
a transition series between the two just-mentioned series. That the reaction must
be of great importance in the precipitation of proteins seems very probable in
consideration of the fact that the proteins take a decided electric charge on the
addition of acid or alkali. In regard to the precipitation by salts of the heavy
metals, the hydrophile colloids do not seem to differ essentially from the suspension
colloids. 5
On boiling a protein solution the protein suffers an irreversible change
and under certain circumstances flocks out. Boiled but not flocked
egg-white behaves with precipitating substances, like a suspension colloid.6
In regard to the precipitation of proteins see Chapter II.
Theories of Precipitation Phenomena.
At least for the suspension colloids there is no question that they
are flocked out by ions which carry an electric charge opposite to the
colloid particles, and also by other colloids having an opposite charge.
This fact follows from Hardy’s theory, according to which the flocking
out is a neutralization process in which the charge of the colloid is
just neutralized and the colloid therefore precipitates.7
The mixture
formed on precipitation has been showm to be electrically neutral (iso-
electric) as the precipitated particles show no cataphoresis. In this
manner it is easily understood that polyvalent ions have a stronger
precipitating action than monovalent, as the electrical charge in, for
1
Hofmeister’s Beitrage, 3, 225 (1902).
2
Pfluger’s Arch., 78, 315 (1899).
3
Spiro, Hofmeister’s Beitrage, 4, 300 (1903).
* Ibid., 11,35 (1908).
5
Pauli, Ibid., 6, 233 (1905).
6
Hardy, Proc. Roy. Soc, 66, 110 (1900).
7
Zeitschr. f. physik. Chem., 33, 385 (1900).
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