A Comprehensive Treatise on Inorganic and Theoretical Chemistry

PLATINUM 27
mixed iridium and platinum salt ; and the second part of the wash-water was
evaporated to dryness, heated -to redness, and added to a fresh portion of ore.
Palladium, rhodium, ruthenium, osmium, and iridium are obtained from the
residues which do not dissolve in the aqua regia, and the mother-liquors after
precipitation.
C. Claus, W. von Schneider, H. Louis, W. C. Heraeus, and C. Bullman recommend
evaporating the aqua regia soln. to dryness, and heating it at say 140° to
150° in order to drive off the nitric acid, and to reduce the iridium tetrachloride
to the trichloride. When the residue is dissolved in water or dil. hydrochloric acid,
the lower chloride of iridium is not then precipitated "with the platinum. An
analogous process was used by V. V. Lebedinsky and V. G. Chlopin. A similar
remark applies to the palladium and rhodium salts. The liquid is allowed to stand
to allow silver chloride and other insoluble matters to settle. Any gold present
can be precipitated by ferrous sulphate. If a relatively high proportion of palladium
is present, it is advisable to add more ammonium chloride, and nitric acid amounting
to 2O to 30 per cent, of the soln. If the liquor is digested on the steambath,
ammonium chloropalladate may be precipitated along with much of the
platinum and iridium in the soln. The presence of nitric acid, however, greatly
retards the precipitation of the other precious metals. W. von Schneider tried to
prevent the precipitation of iridium and rhodium "with the platinum by evaporating
the hydrochloric acid soln. nearly to dryness ; diluting with "water, and making
the soln. alkaline with sodium hydroxide free from potassium. The soln. was
then mixed -with alcohol and boiled, the precipitate dissolved in hydrochloric acid,
and the soln. treated with ammonium chloride to furnish ammonium chloroplatinate.
The process -was discussed by T. WiIm, E. H. Archibald, F. Schulz,
W. Halberstadt, K. Seubert, and C. Claus.
W. H. Wollaston sometimes precipitated the palladium as cyanide by adding
mercuric cyanide to the aqua regia soln. in which the excess of acid has been
neutralized ; platinum is precipitated by adding ammonium chloride to the filtrate,
whilst the rhodium and the greater part of the iridium remain in the nitrate. To
prevent the precipitation of iridium, J. J. Berzelius recommended adding some
nitric acid to the soln. H. V. Collet-Descotils, J. Cloud, L. N. Vauquelin, and
W. von Schneider discussed the process. F. Wyatt treated the aqua regia soln.
with ammonium chloride to precipitate the platinum ; then with sodium hydroxide
and mercuric cyanide to precipitate the palladium—the rhodium remains in soln.
The residue insoluble in aqua regia is heated in a current of air "when osmium is
converted into volatile osmium tetroxide, and rhodium oxide is deposited in the
hotter parts of the tube. The residue is heated with salt in a current of
chlorine when sodium chloroiridate is formed which can be dissolved in boiling
water.
(ii) The precipitation of the platinum by other reagents.—Instead of using
ammonium chloride as precipitant for the platinum, the precipitation by potassium
chloride was discussed by J. J. Berzelius, C. Bullman, and H. Pirngruber.
V. A. Jacquelain tried a mixed soln. of ammonium and potassium chlorides as
j>recipitant. J. W. Dobereiner treated the aqua soln. with lime-water in darkness,
but the process does not give a satisfactory separation of iridium and platinum.
It was discussed by C. Claus, W. von Schneider, and H. St. C. Deville and H. Bebray.
After removing most of the platinum as ammonium chloroplatinate, the metals
remaining in soln. can be precipitated by iron or zinc, although iridium is reduced
very slowly. The washed material is treated with aqua regia (HCl 4 vols., and
HNO3 1 vol.) diluted with 4 vols, of water. Gold and palladium are quickly
dissolved, the platinum is dissolved a little more slowly, but very little iridium,
rhodium, and ruthenium pass into soln. The platinum can be precipitated from
the soln. as ammonium chloroplatinate ; the gold, if present, by ferrous sulphate ;
and the palladium can then be precipitated, as indicated above. These precipitates
contain more or less iridium, rhodium, and ruthenium. The remaining metals are