A Comprehensive Treatise on Inorganic and Theoretical Chemistry

3O INORGANIC AND THEORETICAL CHEMISTRY
water, filtered, and washed until the washings are free from copper and iron.
Nitric acid is first added to the precipitated sulphides of the platinum metals, and
then hydrochloric acid. The soln. is boiled, and the lead chloride filtered from the
cold liquid- The soln. is then treated with ammonium chloride in the usual way.
E. Wichers and co-workers observed that the residue contains mainly rhodium,
iridium, ruthenium, and osmiridium, and can be treated by one of the following
processes : (i) If rhodium is predominant, the dried residue can be intimately
mixed with 2*5 times its weight of sodium chloride, and heated to dull redness in
a current of chlorine. This converts rhodium into a soluble sodium rhodium
chloride, and some of the iridium is likewise converted into a soluble double chloride,
but iridium is less readily attacked than rhodium, (ii) If iridium is predominant,
the residue is fused at 600° to 700° with 3 parts of sodium hydroxide and 1 part
of sodium dioxide in a silver, nickel, or iron dish. Some iridium forms a basic
iridate, but most remains insoluble in water, but soluble in hot, cone, hydrochloric
acid. The ruthenium remains in the aq. soln. of the fused mass. Rhodium is
not rapidly attacked by the alkali fusion, and that treatment may be alternated
with the sodium chloride and chlorine method until all the insoluble material is
converted into a soluble form. The mixed soln. of iridium, ruthenium, and osmium
can be neutralized with hydrochloric or sulphuric acid and boiled with a little
alcohol, and the small amount of metal remaining in soln. can be recovered by the
hydrochloric acid-zinc reduction.
When the mixed iridium and rhodium chlorides contain more iridium than
rhodium, the soln. is treated with chlorine to oxidize tervalent iridium to the
quadrivalent stage, and concentrated by evaporation until over 50 grms. of the
two metals are present per litre. Enough ammonium chloride is added to precipitate
ammonium chloroiridate, which may also contain rhodium. A large excess of
ammonium chloride subsequently interferes with the precipitation of rhodium.
The iridium still present in the mother-liquor can be recovered by evaporating the
soln. to dryness, to eliminate an excess of acid which would interfere with the
subsequent separation of rhodium. The residue is extracted with water, and
filtered from the impure ammonium chloroiridate. The filtrate containing not
more than 50 grms. of rhodium per litre is boiled and treated with sodium nitrite.
This reagent first neutralizes the acid present and reacts with ammonium chloride
to form ammonium nitrite, which decomposes in the hot solution. Rhodium and
the other platinum metals, as well as certain base metals, are converted to soluble
double nitrites, while other base metals, notably iron and tin, are precipitated as
hydroxides. Heating is continued and more sodium nitrite added until the colour
of the solution becomes yellow or light brown. The precipitate is filtered oft* and
treated for the recovery of the small amounts of platinum metals which it may
contain. Ammonium chloride is added to the well-cooled filtrate to precipitate
ammonium rhodium nitrite. The granular, white or yellowish salt is separated
by filtration, washed with water and dried, or dissolved in hydrochloric acid for
further purification. It is not suitable for direct ignition to sponge. Residual
metals are recovered from the filtrate by means of hydrogen sulphide.
The recovery of platinum from parted gold and silver.—M. Pettenkofer, 9
L. Opificius, J. W. KIever, and T. Ulke discussed methods for recovering the
platinum from parted gold—8. 23, 4. M. Pettenkofer recommended the following
process for recovering the platinum from the slags produced by melting the parted
gold with nitre—or may be with potassium hydrosulphate. The platinum collects
in the slag as potassium platinate.
The slag (8 parts) is intimately mixed with galena (2 parts), sodium tartrate (1 part),
dry sodium carbonate (4 parts), and powdered glass (2 parts), and added in small portions
at a time to a red-hot crucible. The button of lead which is formed collects the platinum
metals and gold. The lead alloy is dissolved in hot aqua regia, and the soln. is heated on
a sand-bath to drive off the nitric acid. The cold soln. is filtered and the precipitated lead
and silver are washed with water. The gold is precipitated by ferrous chloride or sulphate ;