William Hyde Wollaston discovered rhodium in 1803-4 in crude platinum ore
from South America rather soon after his discovery of another element,
palladium. He dissolved the ore in aqua regia (a mixture of hydrochloric
and nitric acids), neutralized the acid with sodium hydroxide (NaOH), and
precipitated the platinum by treatment with ammonium chloride, NH4Cl, as
ammonium chloroplatinate. Palladium was then removed as palladium cyanide
by treatment with mercuric cyanide. The remaining material was a red
material containing rhodium chloride salts from which rhodium metal was
obtained by reduction with hydrogen gas.
Rhodium metal is silvery white. Rhodium has a higher melting point and
lower density than platinum. It has a high reflectance and is hard and
durable. Upon heating it turns to the oxide when red and at higher
temperatures turns back to the element. It is a major component of
industrial catalytic systems.
The industrial extraction of rhodium is complex as the metal occurs in
ores mixed with other metals such as palladium, silver, platinum, and
gold. Sometimes extraction of the precious metals such as rhodium,
platinum and palladium is the main focus of a particular industrial
operation while in other cases it is a byproduct. The extraction is
complex because of the other metals present and only worthwhile since
rhodium is the basis of very important catalysts in industry.
Preliminary treatment of the ore or base metal byproduct is required to
remove silver, gold, palladium, and platinum. The resulting residue is
melted with sodium bisulphate (NaHSO4) and the resulting mixture extracted
water to give a solution containing rhodium sulphate, Rh2(SO4)3. The
rhodium is precipitated out as the hydroxide by addition of sodium
hydroxide, NaOH, and redissolved in hydrochloric acid, HCl, to give
H3RhCl6. This is treated with NaNO2 and NH4Cl to form a precipitate of the
rhodium complex (NH4)3[Rh(NO2)6]. Dissolution of the precipitate in HCl
gives a solution of pure (NH4)3RhCl6. Evaporation to dryness and burning
under hydrogen gas gives pure rhodium.
The following uses of rhodium are gathered from a variety of sources.
- Alloying agent to harden platinum and palladium. Such alloys are used
for furnace windings, thermocouple elements, bushings for glass fibre
production, electrodes for aircraft spark plugs, and laboratory crucibles
- Used as an electrical contact material as it has a low electrical
resistance, a low and stable contact resistance, and is highly resistant
- Plated rhodium produced by electroplating or evaporation is
exceptionally hard and is used for optical instruments
- Used for jewelry
- Industrial catalyst
- Rhodium is used as part of the catalytic system in car catalytic
converters, used to clean up exhaust gases to some extent
Fabrication and Forms
H Cross Company can provide Rhodium in commercially pure grade in wire
sizes from .005" diameter up to .100" diameter, strips and ribbons from
.0005" thick to .020" thick and from .020" to .100" wide and in sheet or
foil form from .0005" thick to .020" thick and up to 2" wide. Please
us if you have needs outside of these ranges, as we will always try to be
of assistance for your specific requests.
Each H. Cross Company product is delivered in packaging specifically
designed for the product's application. As an example, wire is furnished
on returnable plastic reels, each containing a single length of wire.
Strips, Ribbons, Sheets and foils are flat packed to prevent damage during
shipping. If required, H. Cross Company can adapt standard packaging
methods or develop new ones for your special needs.