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490 Noble Metals (Chemistry)

silver and low in copper). As discussed earlier, gold can be 6. Rhodium
plated by several methods for art and decorative purposes,
The main use of rhodium is as a component in alloys for
(e.g., china decoration). Colored gold in the form of purple
hardening and as a coating to whiten the alloys. It is used
of Cassius is used as a coloring agent for glass. A high
to coat prongs and claws to hold diamonds because of its
gold:glass ratio (1:50,000) gives a ruby red color and a
highlighting effect. It is also used to coat costume jewelry,
low gold:glass ratio (1:100,000) gives a rose-red color.
cigarette lighters, and low-karat white gold, which is sen-
sitive to corrosion caused by perspiration. Small amounts
2. Silver
of rhodium are also used in the manufacture of liquid gold.
The most common form of silver in the decorative area
is sterling silver, Ag–Cu, which is used in silverware and 7. Osmium
was also widely used in coinage because of its hardness
Osmium is not used in jewelry or decorative applications.
and wear resistance. Sterling, 92.5% silver, will tarnish,
In fact, pains are taken to eliminate it from gold because
but work has been done to find a nontarnishing sterling
small amounts can streak a polished gold surface.
silver. A rhodium coating will protect the silver but it is
cost-prohibitiveformostuses.Platedsilveris99.99%pure
and is usually put on a blank, such as steel or nickel sil- 8. Ruthenium
ver, which is a Ni–Cu alloy containing no silver. Finely Ruthenium is used as a hardening agent, which is its only
powered silver metal is used as a pigment. Polychromic significant use in jewelry or decoration.
glass contains AgCl and Ag 2 MoO 4 , and Ag 2 SO 4 imparts
a yellowish-red color to glass bulbs.
L. Photography
3. Platinum Photography, the largest single use for silver, is too broad
a topic to be covered in any depth here so the discussion is
Prior to 1974, the greatest use of platinum was in jewelry,
limited to the mechanism of its use and does not include
but since then, its use in automobile catalytic converters
details such as positive and negative imaging.
has taken the lead. It is popular in settings for the finest
Silver salts are used because they are photochemically
jewelry because of its high intrinsic worth and the way it
reduced under mild conditions. The most common salts
sets off diamonds. Platinum with either 5–15% iridium,
are the halides: chloride, bromide, and iodide, but other
5% ruthenium, or 5% rhodium is used to harden the metal.
compounds such as mercaptides can be used. The film is
These alloys are used for small delicate pieces such as
prepared by mixing AgNO 3 and the appropriate mixture of
small chains. Rhodium is also used to finish the platinum
sodium or potassium halide to form a silver halide emul-
piece for additional whiteness as well as wear resistance.
12
sion of crystals containing approximately 10 atoms. The
The Pt–Pd alloys are commonly used in jewelry in Europe,
emulsion is spread on a backing and kept from coalescing
but the biggest outlet is in Japan.
by locking the crystals in a gel. There is a trade-off: the
larger the crystals, the faster the speed of the film or the
4. Palladium
better the photosensitivity; but because of the large crystal
Palladium has characteristics similar to platinum and is size, the resolving power or sharpness of the photograph
finding increased use. It is less costly, and because of decreases. Silver halides are sensitive only to blue and
its lower density, it is used where weight is important ultraviolet light, which are responsible for the reduction
(e.g., earrings). Palladium pieces, like platinum, are of- of the silver. Several options are available to increase the
ten finished with rhodium for the same reasons. The al- sensitivity of the film to visible light. The easiest change
loy 95Pd–4Ru–1Rh is popular for jewelry use because of is to shift from low-sensitive AgCl to the more photo-
its mechanical strength and hardness. The “white” alloy sensitive AgI. Going from the chloride to iodide requires
95.5Pd–4.5Ru sets off diamonds well and is a standard less energy to reduce the silver, which results in a shift
for palladium use in the U.S. Palladium is used as a pro- in film sensitivity to the lower-energy visible light. For
tective and decorative coating on ceramics, leather, metal, low-sensitivity film, AgCl, AgCl/AgBr, or AgBr are used;
and wood. and for high-sensitivity film, AgBr with up to 10% AgI
is used. Polymethine-type dyes are used to make the salts
more sensitive to the green and red regions. This is termed
5. Iridium
spectral sensitization and there are even dyes available
Iridium, at less than a 20% level, is used in diamond set- that will sensitize the film to the infrared region. Another
tings with platinum. Iridium black is used as a black pig- method, chemical sensitization, does not alter the light ab-
ment to decorate fine porcelain or china. sorbing properties of the silver halide but it does improve