A Comprehensive Treatise on Inorganic and Theoretical Chemistry

PLATINUM 373
bromic acid, and crystallizing from the aq. soln. of the residue. The heat of formation
is (Pt, Br2, 2NH4Br, Aq.) =31-84 OaIs. J. Thomsen obtained potassium bromoplatinite,
K2PtBr4.2H2O, by boiling a soln. of a mol. of potassium chloroplatinite
with 4 mols. of sodium bromide in so little water that a large proportion of the
sodium chloride which is formed separates out. By repeated evaporation, filtering,
and cooling, the potassium salt can be freed from most of the sodium chloride, and
the salt can then be re-crystallized from its aq. soln. E. Biilmann and A. C. Anderson
obtained it by evaporating on a water-bath a mixture of 115*5 grms. of hydrobromoplatinic
acid with four times its weight of water with 28*3 grms. of potassium
oxalate until the weight is about 218 grms. Allow the liquid to cool overnight,
separate the product by suction, and dry it in air—yield 44 grms. N. Demassieux
and J. Heyrovsky studied the dissociation of the salt in soln.
The prismatic crystals of the dihydrate are almost black, and, according to
O. B. Boggild, they are rhombic bipyramids with the axial ratios a : b : c
=06058 : 1 : O7050. The optical character is negative. R. Klement gave 3-747
for the sp. gr. at 25°/4°, and 167*9 for the mol. vol. The crystals remain bright in a
cool place, or in a closed vessel at ordinary temp. The water is given off when the
crystals are exposed to sunlight, or kept in a desiccator. When the dihydrated
crystals are allowed to stand over water, the vapour is absorbed and a dark red
soln. is formed. The heat of formation of the anhydrous salt was found by
J. Thomsen to be (Pt, Br2, 2KBr) =32*31 CaIs. ; (Pt, Br2, 2KBr, Aq. =31-84 CaIs. ;
and the heat of solution, —10*63 CaIs. E. Biilmann and A. C. Anderson observed
that the salt is freely soluble in water, and when the soln. is boiled for some time,
it is decomposed.
L. Wohler and F. Miiller prepared platinum tribromide, PtBr3, by heating the
tetrabromide at 370° ; at 405°, it decomposes into the dibromide. R. Klement
obtained platinum tribromide analogous with the trichloride, and found its sp. gr.
at 22°/4° to be 6*504, and its mol. vol. 66*9.
A. J. Balard, and P. A. von Bonsdorff dissolved platinum in a mixture of hydrobromic
and nitric acids, evaporated the soln. at about 70°, and obtained platinum
tetrabromide, or platinic bromide, PtBr4. H. Topsde added that if the nitric acid
is in excess some tetrabromonitrosylbromide is formed, and if the hydrobromic
acid is in excess, hydrobromoplatinic acid. C. F. Rammelsberg observed that some
platinic bromide is formed when a soln. of platinic sulphate is treated with barium
bromate (bromide ?), and the filtered soln. evaporated. L. Pigeon employed a
process analogous to that used in the preparation of platinic chloride. V. Meyer
and H. Zublin employed the process indicated in connection with platinous bromide.
W. Halberstadt evaporated to dryness the soln. of hydrobromoplatinic acid,
obtained in V. Meyer and H. Ziiblin's process, in order to drive off the excess of
bromine, extracted the mass with water, evaporated the soln. again to dryness,
and then heated the brownish-red product to 180° to 200° with vigorous stirring
until the vapour of hydrogen bromide was no longer perceptible. The product is
boiled with water, and the soln. evaporated to dryness ; the residue is again heated
to 180° and the sequence of operations repeated so that finally the filtered soln.
is evaporated. L. von Miiller observed that the nature of the product depends on
the time occupied in drying the mass, and A. Gutbier and co-workers observed
that the product is impure, and L. von Miiller, and A. Gutbier and co-workers
recommended drying the product in bromine at 180°. The product dried at different
temp, contained the following percentage proportions of platinum :
100° 110° 120° 120° to 130°
Platinum . 26-75 27.59 31-13 to 32-96 33-05 to 3412 per cent.
130° to 140° J 40° to 150° 150° to 175° 180°
Platinum . 36-59 to 37-89 37-14 39-19 to 40-31 41-34 per cent.
IJ. Wohler and F. Miiller obtained the anhydrous tetrabromide by heating
hydrobromoplatinic acid in a current of bromine at 300°. C. Nogareda studied the