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I found this info about Rhodium:
Standard Method of Chemical Analysis wrote:Rhodium
Rhodium is found only in platinum ores. It is a white metal, difficultly
fusible, and insoluble in acids. Rhodium, however, dissolves in aqua regia
when alloyed with platinum, to a cherry red solution. It is also soluble in
molten phosphoric acid and dissolves when fused with acid potassium sulphate with
the formation of K3Rh(SO2)3. If the metal is treated with chlorine in the presence of
sodium chloride there forms a soluble salt, Na3RhCl4.
Rhodium monoxide, RhO, is obtained by heating the hydroxide Rh(OII)3,
by cupellation of an alloy of rhodium and lead, or by igniting the finely-divided metal in a
current of air. It is a grey powder with a metallic appearance, and is not attacked by acids,
and when heated in hydrogen is reduced with evolution of light.
The oxide, Rh203, is obtained as a grey iridescent spongy mass by heating the nitrate.
It is also formed as a crystalline mass when sodium rhodochloride is heated in oxygen.
It is perfectly soluble in acids
RhO2, is obtained by repeated fusions of the metal with
KOH and KNO3. It is attacked neither by alkalies nor by acids and is reduced by hydrogen
only at a high temperature.
Hydrogen sulphide precipitates from hot solution and incompletely a brown rhodium sulphide, Rh2S4.
Potassium hydroxide precipitates at first a yellow hydroxide, Rh(OHU -
f-H2O soluble in an excess of the reagent. If boiled, a gelatinous, dark-brown hydroxide, Rh(OH)3,
separates. A solution of NasRhCU does not show this reaction immediately, but the precipitate appears
in the course of time. An addition of alcohol causes a black precipitate immediately.
Ammonium hydroxide produces a yellow precipitate which is insoluble in
Potassium nitrite precipitates from hot solutions a bright yellow precipitate
of double nitrite of potassium and rhodium.
Zinc, iron and formic acid precipitate rhodium as a black metal.
Hydrogen reduces rhodium salts.
To detect small amounts of rhodium in the presence of other metals,
evaporate the solution and displace with a fresh solution of sodium hypo-
chlorite; the yellow precipitate formed is soluble after an addition of acetic
acid. After a long agitation the solution changes to an orange-yellow color
and after a short time the color passes and finally a grey precipitate settles and the solution turns sky-blue.
Rhodium is estimated mainly in ores, thermo couples and salts.
Preparation and Solution of the Sample
When rhodium is estimated in thermo couples or other alloys of platinum
and rhodium the wire or sample is rolled to a thin ribbon and dissolved in
aqua regia. Both metals will go into solution, forming the chlorides of rhodium and platinum.
The aqua regia will have to be replaced from time to time, as the alloy dissolves slowly.
The rhodium from salts is precipitated with zinc and the black metallic
rhodium cleaned with dilute aqua regia, filtered, washed, ignited and reduced with hydrogen.
Some alloys and ores are alloyed with silver and the silver and platinum
are dissolved in HNO3. The residue is cleaned with aqua regia, dried, and
weighed as metallic rhodium. If the residue is ignited reduce with hydrogen.
The material or residue containing rhodium is fused with KHSO4 for some
time at a low red heat and the mass leached with hot water acidified with HCl.
The rose-red solution contains the rhodium.