A Comprehensive Treatise on Inorganic and Theoretical Chemistry

242 INORGANIC AND THEORETICAL CHEMISTRY
§ 19. The Higher Oxides of Platinum
Anhydrous platinum dioxide, or platinic oxide, PtO2, was obtained by
J. J. Berzelius * by gently heating the hydrate—E. von Meyer recommended a
protracted heating at 175°, and O. Brunck, at 150°. L. Wohler added that a complete
dehydration without some decomposition is not possible. P. Laffitte and
P. Grandadam, and A. Baroni obtained the oxide by heating the metals in oxygen
at a high press.—vide platinous oxide. J. J. Berzelius' dioxide obtained by
heating the commercial nitrate for 35 days at 250° to 280°, according to
Li. Wohler, contained 13-82 per cent, of oxygen when the theoretical amount is
14-1 per cent. R. Adams and co-workers, W. F. Short, W. F. Bruce, and
V. Voorhees and R. Adams prepared it by fusing chloroplatinic acid with
sodium nitrate at 500° to 550°, and washing the product with water to eliminate
the nitrates. It is used as a catalyst in organic syntheses, and it can be reactivated
by shaking it with air or oxygen, but there is an accumulation of
poison in use which necessitates its purification. E. P. Schoch obtained the
dioxide by anodic oxidation. J. Piazza obtained a mixture of platinum and its
dioxide at the platinum cathode during an electric discharge in oxygen ; and
P. Grandadam, by heating the metal in oxygen under press.
G. P. Thomson and co-workers obtained X-radiograms of the dioxide. The
black powder, said J. J. Berzelius, loses oxygen when heated. L. Wohler said
that the oxide is completely decomposed into platinum and oxygen at 450° in an
atm. of carbon monoxide ; at 510° in an atm. of oxygen ; and above 300° in an
atm. of carbon dioxide. C. Marie also noted that the salt is decomposed at a dull
red-heat. L. Wohler said that the speed of dissociation depends on the proportion
of water of hydration. The last traces of oxygen are very difficult to remove by a
blast-flame ; in air or carbon dioxide the oxygen is not all expelled in a combustion
furnace, but it can be removed by heating in hydrogen. According to L. Wohler
and W. Frey, when platinum dioxide is heated at 510° to 515°, in vacuo, until its
oxygen-content diminished below that required for the monoxide, the residue contained
metallic platinum ; and an examination of the residue indicated that when
platinum dioxide is heated, it dissociates into the metal and a solid soln. of either
the monoxide or sesquioxide in the dioxide. The equilibrium press, are attained
too slowly for measurement. The oxides may be heated for days between 100°
and 200° above the temp, corresponding with equilibrium without losing oxygen
perceptibly. The metal takes up oxygen equally slowly. The dissociation press,
of the monoxide and sesquioxide are, however, higher than that of the dioxide.
The evolution of oxygen from the dioxide at constant temp, begins slowly, then
becomes very rapid, and finally diminishes gradually. The rapid evolution begins
at 514° to 520° when the oxygen content of the oxide has fallen to 11-6 to 12 per
cent. It is probable that a supersaturated solution of monoxide or sesquioxide
in the dioxide is first formed, and when this has reached a certain concentration
it decomposes suddenly. The subject was studied by F. Becker. E. von Meyer
observed that hydrogen reduces it energetically, and that it oxidizes hydrogen
at ordinary temp. L. Wohler showed that the dioxide oxidizes hydriodic acid ;
it is insoluble in dil. and cone, hydrochloric acid, and in sulphuric acid ; and it is
reduced to the monoxide by sulphurous acid, heated on a water-bath, and the
product slowly passes into soln. J. Landauer found that sodium thiosulphate
colours the dry dioxide black. L. Wohler observed that the oxide is insoluble in
nitric acid, and in aqua regia ; and it is not reduced by arsenic trioxide. E. von
Meyer said that it oxidizes carbon monoxide at ordinary temp. ; and L. Wohler,
that it is not reduced by ether. A soln. of stannous chloride in hydrochloric acid,
on a water-bath, slowly reduces the dioxide ^o the monoxide.
According to L. Schaffner, pure hydrated platinum dioxide cannot be prepared
since the product always contains complex salts—presumably adsorption products.
N. W. Fischer said that a hydrate is precipitated by magnesium from a soln. of