A Comprehensive Treatise on Inorganic and Theoretical Chemistry

PLATINUM 171
studied the reduction of ferric salts by colloidal soln. of platinum. S. Cooke noted
the reduction of ferric salts by hydrogenized platinum. E. Muller and G. Wegelin
found that the presence of platinum favoured the reduction of ferric chloride by
zinc ; and R. Peters, W. Manchot and J. Herzog, and E. Oberer, the reduction
of CObaltic salts. C. St. Pierre noted that palladic chloride is partially reduced
by platinum, and E. Bose studied the equilibrium between platinum and platinic
Chloride. E. W. Hilgard found that spongy platinum favours the decomposition
of potassium chloroplatinate. J. A. Buchner studied the action of ammonium
nitrate on platinum. C. Claus, J. !Lang, O. KLottig, and S. Tennant noted that
fused potassium nitrate attacks platinum. The metal is also attacked by molten
barium and strontium nitrates. H. St. C. Deville and H. Debray noted that when
platinum is treated with iron disulphide in fused borax, platinum sulphide is formed.
According to A. Frumkin and A. Obrutscheva, if a hydrogen electrode is
immersed in a soln. of a neutral salt, e.g. sodium sulphate, in which the osmotic
press, of the hydrogen ions is less than the electrolytic soln. tension of the electrode,
a small amount of hydrogen ions will pass into soln. and the resulting negative
charge on the surface of the electrode will attract sodium ions from the soln.,
which will therefore become acid. A. Frumkin and A. Donde observed that
purified spongy platinum, well-washed with purified water in an atm. of hydrogen,
adsorbs alkali from a soln. of sodium sulphate ; the alkali is not completely removed
by washing with a considerable quantity of water, so that the amount of acid
liberated is always in excess of that of the alkali. It has not been found possible
to prepare spongy platinum in presence of oxygen which adsorbs acid and liberates
alkali. Charcoal treated with platinum and activated in air adsorbs acid from
potassium chloride soln. and liberates alkali, whereas the opposite effect is observed
in an atm. of hydrogen. Addition of thiocarbamide to the potassium chloride soln.
poisons the platinum, and the charcoal then adsorbs only acid independently of
the gaseous atm. I. M. KolthofF and T. Kameda also observed that platinizedplatinum,
in an atm. of hydrogen, adsorbs the cation from a neutral salt soln.,
and an eq. amount of free acid is formed in the soln. Zinc sulphate soln., in an
atm. of hydrogen, increased in acidity to an extent eq. to the amount of zinc
adsorbed by the platinum. Ammonium chloride likewise became slightly acid,
but in oxygen such soln. became very distinctly acid, and the acidity increased
the longer the oxygen was passed. This is attributed to the formation of hexaaquoplatinic
acid, which reacts with the ammonium ions present : H2Pt(OlI)6
+2NH4->(NIT4)2Pt(OH)6H-2H. Similar results were obtained with trimethylammonium
and potassium chlorides. No acid adsorption from hydrochloric acid
occurs in a hydrogen atm., but in an oxygen atm. there is eq. adsorption of hydrogen
and chlorine ions. Sodium hydroxide is strongly adsorbed in a hydrogen atm. ;
maximum adsorption occurs at a concentration of 00007iV. This adsorption is
increased by addition of sodium chloride, and in presence of large amounts of the
latter the maximum disappears. In the presence of oxygen the nexa-aquoj>latinic
acid formed neutralises some of the alkali, and only apparent adsorption of the
latter, therefore, takes place. F. Todt observed that the hydrolytic adsorption in
phosphate buffer soln. containing potassium chloride, at a platinized-platinum
surface, gives rise to acid in amount sufficient to account for the hydrogen effect
observed by L. Wolff. H. Gall and W. Manchot studied the reducing action of
hydrogen and platinum on various inorganic salts.
Reactions of platinum of analytical interest.—A soln. of hydrochloroplatinic
acid gives no precipitate with hydrochloric acid, but the soln. gives yellow precipitates
of the sparingly soluble chloroplatinates when treated with cone. soln. of
potassium or ammonium chloride. When the soln. of hydrochloroplatinic acid
is treated with hydrogen sulphide in the cold, platinum disulphide is slowly precipitated,
the precipitation is faster with a warm soln. N. W. Fischer 24 observed
that the limit of the reaction occurs with a soln. containing one part of platinum
in 30,000 parts of liquid. According to R. Gaze, the precipitation is incomplete—