A Comprehensive Treatise on Inorganic and Theoretical Chemistry

56 INOKGANIC AND THEORETICAL CHEMISTRY
B. Delachanal and A. Mermet found that the red colour—red colloidal platinum—
produced when soln. of platinum salts are treated with stannous chloride is due
to the presence of colloidal metallic platinum in a very fine state of subdivision,
and that the formation of this in place of the more usual brown colloidal metal
is due to the action of stannic chloride and its products of hydrolysis as protective
colloids. The red colloid is also formed when the reduction of platinum salts is
effected by means of a soln. of phosphorus in ether, if gelatin is added as a protective
colloid. The identity of the two red substances has been established by spectroscopic
observations. If the colloidal soln. obtained by reduction with stannous
chloride is shaken up -with ethyl ether or ethyl acetate, the organic solvents take
up the red colour, and this is found to be connected -with the solubility of stannic
chloride in these media, in which it plays the part of protective colloid. When the
aq. soln. is diluted with a large volume of water, or when the ethyl acetate soln.
is poured into water, a chocolate-brown precipitate is obtained. According to
K. A. Schneider, the precipitate has the composition PtSn3O5, but actually no
definite compound is formed because the composition of the precipitate varies
considerably with the conditions under which it is produced. It is probable that
it is analogous -with purple of Cassius, and is a mixture of colloidal platinum and
colloidal stannic acid. The properties of the purple of Cassius, and red colloidal
platinum are similar.
J. Duclaux found that the osmotic press, of the soln. is less than 2 cm. of water.
C. Thomas said that the change to white of the colour of the ultramicroscopic
particles marks the beginning of coagulation. P. LaI and P. B. Ganguly observed
that the colloid is coagulated by exposure to ultra-violet light. E. Miiller studied
the polarization, and extinctive coeff. of the colloidal soln. ; A. T. Williams, and
O. Scarpa, the spectra of colloidal soln. of platinum ; E. B. Spear and co-workers,
the coagulation of the sol in ultra-violet light ; and E. B. Spear and K. D. Kahn,
the precipitation of the colloid on metallic surfaces. H. Freundlich observed that
the hydrosol of platinum shows anodic convection like arsenic sulphide sol.
S. W. Pennycuick studied the cataphoretic velocity. L. Holla found that the
velocity of migration of the colloidal particles in an electric field with a drop of
potential of 1 volt per cm. is 24 O X 1O -5 cm. per second ; T. Svedberg gave for the
lower limit 7*6 x K) -3 cm. per second; and E. F. Burton, 2-3xl0 -5 cm. per second
for the speed of colloidal soln. in ethyl malonate. The subject was discussed by
A. Einstein, F. Evers, J. J. Bikermann, N. Bach and N. Balaschowa, W. Biltz,
and W. R. Whitney and J. C. Blake.
According to L. Liebermann, reddish-brown, colloidal soln. of platinum become
dark brown when hydrogen is passed through the liquid. The colloidal soln. of
platinum dissolves hydrogen gas roughly in proportion to the concentration.
E. O. Auerswald studied the subject. G. Kernot and F. de S. Niquessa found that
some protective colloids—e.g. gum arabic, dextrin, and albumen—reduce the proportion
of gas absorbed, but sucrose has a negligibly small effect. In virtue of the
absorbed hydrogen, colloidal platinum favours many reductions catalytieally
though the activity of the colloid decreases with use, and the chemical work done
increases, but not proportionally, with the cone, of the colloidal soln. Thus,
C. Paal and A. Schwarz found that acetylene is reduced to ethylene and ethane ;
and ethylene to ethane. C. Paal and J. Gerum observed that many organic
substances such as unsaturated oils are hydrogenized—e.g. linseed oil is hardened
to a white fat. J. Donau found that a borax bead is coloured reddish-brown by
colloidal platinum. J. Eggert found that ferric salts are reduced to the ferrous
state ; and C. Paal and H. Biittner, that ammonium molybdate is reduced.
E. C. Auerswald studied the poisoning of the catalytic activity of platinum hydrosol
by mercury.
Li. Liebermann observed that the colloidal soln. of platinum contains activated
oxygen. C. Paal observed that carbon monoxide is oxidized by oxygen to carbon
dioxide in the presence of colloidal platinum ; and C. Paal and A. Schwarz, that