GOLD AND THE PLATINUM FAMILY
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| | | | | |
| | ATOMIC | | HIGHEST | HIGHEST | MELTING
| SYMBOL | WEIGHT | DENSITY | OXIDE | CHLORIDE | POINT
__________|________|________|_________|_________ |__________|_____________
| | | | | |
Ruthenium | Ru | 101.7 | 12.26 | RuO_{4} | RuCl_{4} | Electric arc
Rhodium | Rh | 103. | 12.1 | RhO_{2} | RhCl_{2} | Electric arc
Palladium | Pd | 106.5 | 11.8 | PdO_{2} | PdCl_{4} | 1500°
Iridium | Ir | 193. | 22.42 | IrO_{2} | IrCl_{4} | 1950°
Osmium | Os | 191. | 22.47 | OsO_{4} | OsCl_{4} | Electric arc
Platinum | Pt | 194.8 | 21.50 | PtO_{2} | PtCl_{4} | 1779°
Gold | Au | 197.2 | 19.30 | Au_{2}O_{3} | AuCl_{3} | 1064°
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~The family.~ Following iron, nickel, and cobalt in the eighth column of
the periodic table are two groups of three elements each. The metals of
the first of these groups--ruthenium, rhodium, and palladium--have
atomic weights near 100 and densities near 12. The metals of the other
group--iridium, osmium, and platinum--have atomic weights near 200 and
densities near 21. These six rare elements have very similar physical
properties and resemble each other chemically not only in the type of
compounds which they form but also in the great variety of them. They
occur closely associated in nature, usually as alloys of platinum in the
form of irregular metallic grains in sand and gravel. Platinum is by far
the most abundant of the six.
Although the periodic classification assigns gold to the silver-copper
group, its physical as well as many of its chemical properties much
more closely resemble those of the platinum metals, and it can he
conveniently considered along with them. The four elements gold,
platinum, osmium, and iridium are the heaviest substances known, being
about twice as heavy as lead.
PLATINUM
~Occurrence.~ About 90% of the platinum of commerce comes from Russia,
small amounts being produced in California, Brazil, and Australia.
~Preparation.~ Native platinum is usually alloyed with gold and the
platinum metals. To separate the platinum the alloy is dissolved in aqua
regia, which converts the platinum into chloroplatinic acid
(H_{2}PtCl_{6}). Ammonium chloride is then added, which precipitates the
platinum as insoluble ammonium chloroplatinate:
H_{2}PtCl_{6} + 2NH_{4}Cl = (NH_{4})_{2}PtCl_{6} + 2HCl.
Some iridium is also precipitated as a similar compound. On ignition the
double chloride is decomposed, leaving the platinum as a spongy metallic
mass, which is melted in an electric furnace and rolled or hammered into
the desired shape.
~Physical properties.~ Platinum is a grayish-white metal of high luster,
and is very malleable and ductile. It melts in the oxyhydrogen blowpipe
and in the electric furnace; it is harder than gold and is a good
conductor of electricity. In finely divided form it has the ability to
absorb or occlude gases, especially oxygen and hydrogen. These gases,
when occluded, are in a very active condition resembling the nascent
state, and can combine with each other at ordinary temperatures. A jet
of hydrogen or coal gas directed upon spongy platinum is at once
ignited.
~Platinum as a catalytic agent.~ Platinum is remarkable for its
property of acting as a catalytic agent in a large number of
chemical reactions, and mention has been made of this use of
the metal in connection with the manufacture of sulphuric acid.
When desired for this purpose some porous or fibrous substance,
such as asbestos, is soaked in a solution of platinic chloride
and then ignited. The platinum compound is decomposed and the
platinum deposited in very finely divided form. Asbestos
prepared in this way is called platinized asbestos. The
catalytic action seems to be in part connected with the
property of absorbing gases and rendering them nascent. Some
other metals possess this same power, notably palladium, which
is remarkable for its ability to absorb hydrogen.
~Chemical properties.~ Platinum is a very inactive element chemically, and
is not attacked by any of the common acids. Aqua regia slowly dissolves
it, forming platinic chloride (PtCl_{4}), which in turn unites with the
hydrochloric acid present in the aqua regia, forming the compound
chloroplatinic acid (H_{2}PtCl_{6}). Platinum is attacked by fused
alkalis. It combines at higher temperatures with carbon and phosphorus
and alloys with many metals. It is readily attacked by chlorine but not
by oxidizing agents.
~Applications.~ Platinum is very valuable as a material for the
manufacture of chemical utensils which are required to stand a high
temperature or the action of strong reagents. Platinum crucibles,
dishes, forceps, electrodes, and similar articles are indispensable in
the chemical laboratory. In the industries it is used for such purposes
as the manufacture of pans for evaporating sulphuric acid, wires for
sealing through incandescent light bulbs, and for making a great variety
of instruments. Unfortunately the supply of the metal is very limited,
and the cost is steadily advancing, so that it is now more valuable than
gold.
~Compounds.~ Platinum forms two series of salts of which platinous
chloride (PtCl_{2}) and platinic chloride (PtCl_{4}) are examples.
Platinates are also known. While a great variety of compounds of
platinum have been made, the substance is chiefly employed in the
metallic state.
~Platinic chloride (PtCl_{4}).~ Platinic chloride is an orange-colored,
soluble compound made by heating chloroplatinic acid in a current of
chlorine. If hydrochloric acid is added to a solution of the substance,
the two combine, forming chloroplatinic acid (H_{2}PtCl_{6}):
2HCl + PtCl_{4} = H_{2}PtCl_{6}.
The potassium and ammonium salts of this acid are nearly insoluble in
water and alcohol. The acid is therefore used as a reagent to
precipitate potassium in analytical work. With potassium chloride the
equation is
2KCl + H_{2}PtCl_{6} = K_{2}PtCl_{6} + 2HCl.
~Other metals of the family.~ The other members of the family
have few applications. Iridium is used in the form of a
platinum alloy, since the alloy is much harder than pure
platinum and is even less fusible. This alloy is sometimes used
to point gold pens. Osmium tetroxide (OsO_{4}) is a very
volatile liquid and is used under the name of osmic acid as a
stain for sections in microscopy.
GOLD
~Occurrence.~ Gold has been found in many localities, the most famous
being South Africa, Australia, Russia, and the United States. In this
country it is found in Alaska and in nearly half of the states of the
union, notably in California, Colorado, and Nevada. It is usually found
in the native condition, frequently alloyed with silver; in combination
it is sometimes found as telluride (AuTe_{2}), and in a few other
compounds.
~Mining.~ Native gold occurs in the form of small grains or larger nuggets
in the sands of old rivers, or imbedded in quartz veins in rocks. In the
first case it is obtained in crude form by placer mining. The sand
containing the gold is shaken or stirred in troughs of running waters
called sluices. This sweeps away the sand but allows the heavier gold to
sink to the bottom of the sluice. Sometimes the sand containing the gold
is washed away from its natural location into the sluices by powerful
streams of water delivered under pressure from pipes. This is called
hydraulic mining. In vein mining the gold-bearing quartz is mined from
the veins, stamped into fine powder in stamping mills, and the gold
extracted by one of the processes to be described.
~Extraction.~ 1. _Amalgamation process._ In the amalgamation process the
powder containing the gold is washed over a series of copper plates
whose surfaces have been amalgamated with mercury. The gold sticks to
the mercury or alloys with it, and after a time the gold and mercury are
scraped off and the mixture is distilled. The mercury distills off and
the gold is left in the retort ready for refining.
2. _Chlorination process._ When gold occurs along with metallic
sulphides it is often extracted by chlorination. The ore is first
roasted, and is then moistened and treated with chlorine. This dissolves
the gold but not the metallic oxides:
Au + 3Cl = AuCl_{3}.
The gold chloride, being soluble, is extracted from the mixture with
water, and the gold is precipitated from the solution, usually by adding
ferrous sulphate:
AuCl_{3} + 3FeSO_{4} = Au + FeCl_{3} + Fe_{2}(SO_{4})_{3}.
3. _Cyanide process._ This process depends upon the fact that gold is
soluble in a solution of potassium cyanide in the presence of the oxygen
of the air. The powder from the stamping mills is treated with a very
dilute potassium cyanide solution which extracts the gold:
2Au + 4KCN + H_{2}O + O = 2KOH + 2KAu(CN)_{2}.
From this solution the gold can be obtained by electrolysis or by
precipitation with metallic zinc:
2KAu(CN)_{2} + Zn = K_{2}Zn(CN)_{4} + 2Au.
~Refining of gold.~ Gold is refined by three general methods:
1. _Electrolysis._ When gold is dissolved in a solution of potassium
cyanide, and the solution electrolyzed, the gold is deposited in very
pure condition on the cathode.
2. _Cupellation._ When the gold is alloyed with easily oxidizable
metals, such as copper or lead, it may be refined by cupellation. The
alloy is fused with an oxidizing flame on a shallow hearth made of bone
ash, which substance has the property of absorbing metallic oxides but
not the gold. Any silver which may be present remains alloyed with the
gold.
3. _Parting with sulphuric acid._ Gold may be separated from silver, as
well as from many other metals, by heating the alloy with concentrated
sulphuric acid. This dissolves the silver, while the gold is not
attacked.
~Physical properties.~ Gold is a very heavy bright yellow metal,
exceedingly malleable and ductile, and a good conductor of electricity.
It is quite soft and is usually alloyed with copper or silver to give it
the hardness required for most practical uses. The degree of fineness is
expressed in terms of carats, pure gold being twenty-four carats; the
gold used for jewelry is usually eighteen carats, eighteen parts being
gold and six parts copper or silver. Gold coinage is 90% gold and 10%
copper.
~Chemical properties.~ Gold is not attacked by any one of the common
acids; aqua regia easily dissolves it, forming gold chloride (AuCl_{3}),
which in turn combines with hydrochloric acid to form chlorauric acid
(HAuCl_{4}). Fused alkalis also attack it. Most oxidizing agents are
without action upon it, and in general it is not an active element.
~Compounds.~ The compounds of gold, though numerous and varied in
character, are of comparatively little importance and need not
be described in detail. The element forms two series of salts
in which it acts as a metal: in the aurous series the gold is
univalent, the chloride having the formula AuCl; in the auric
series it is trivalent, auric chloride having the formula
AuCl_{3}. Gold also acts as an acid-forming element, forming
such compounds as potassium aurate (KAuO_{2}). Its compounds
are very easily decomposed, however, metallic gold separating
from them.
EXERCISES
1. From the method of preparation of platinum, what metal is likely to
be alloyed with it?
2. The "platinum chloride" of the laboratory is made by dissolving
platinum in aqua regia. What is the compound?
3. How would you expect potassium aurate and platinate to be formed?
What precautions would this suggest in the use of platinum vessels?
4. Why must gold ores be roasted in the chlorination process?