(silver, iridium, gold, osmium, palladium, platinum, rhodium, ruthenium)
White and shiny metal element, symbol Ag and atomic number 47, belonging
to group IB (or 11) of the periodic table and to the transition elements
series.
Known since antiquity, it was used by some peoples to manufacture
ornamental objects and coins. Alchemists called it Moon or Diana, in the
goddess Moon's honour, and attributed the symbol of the waxing moon to it.
Silver is an excellent electricity conductor and, apart from gold, it's
the most ductile and malleable metal. Its hardness goes from 2.5 to 2.7 it is
therefore harder than gold but more tender than copper. It melts at about 962
°C, boils at about 2212 °C, and has a relative density par to 10.5 and atomic
weight par to 107.868.
Silver isn't very reactive; it's insoluble with dissolved acids or
bases, but it can be easily dissolved in concentrated nitric or sulphuric
acids; at room temperatures it doesn't react with oxygen or water. If exposed
to the air it doesn't oxidize but its easily etched by the sulphur and
sulphides contained in small amounts in the atmosphere; the characteristic
blackening is due to the formation of silver sulphide on the surface of the
metal. Black silver sulphide, Ag2S, is one of the less solvable in
aqueous solutions; this feature is utilized to set apart silver ions from other
cations.
Silver is relatively abundant in nature but it's often found combined
with other elements and nearly always impure: the richest prospects are in Peru
and Norway, where mines have been exploited for hundreds of years. It's often
found together with gold and can be collected in considerable quantities during
the digging out and working of this metal. It can also be found combined with
other elements in several rocks and in numerous minerals, like cerargyrite
(also called silver chloride or more often horn silver), pirargyrite,
sylvanite, and argentite. The metal can also be obtained as by-product of lead,
copper and zinc working. Nearly all the silver produced in Europe derives from
the working of lead sulphide or galena. The majority of the silver mined in the
world comes from Mexico, Peru, Canada, United States and Australia.
During the mining processing, silver minerals are bought to high
temperatures in special furnaces to allow the transformation of sulphides in
sulphates; metallic silver is collected from these compounds through chemical
precipitation. Several metallurgical processing is used to extract silver from
the argentiferous minerals of lead, copper and zinc. To make an example in the
amalgamation process the grounded minerals are added a small amount of liquid
mercury that, put together with silver makes a particular amalgam; this
material is separated from residues and then distilled in order to provide, at
the end of the process, pure silver. In the leaching methods, the silver melts
in a salt solution (usually sodium cyanide), they then make it precipitate by
brining the solution in contact with metallic zinc or aluminium. The process
used to separate silver from copper is called Parker, it's commonly used also
for the extraction of precious metals from lead. Impure silver produced
metallurgically is refined with electrolytic methods or through cupellation, a
process that removes eventual impurities through vaporisation or absorption.
The use of silver is popular in jewellery, silverware production, silver
plate and coin minting. The most valuable silver, generally used for silver
services, is constituted for the 92.5 per cent by pure metal and for the
remaining 7.5 per cent from copper. The silver used for industrial applications
instead is usually a silver alloy with good mechanical features such as hardness
and wear resistance. Silver used to be utilized for the production of mirrors
and glass coverings, but nowadays in these fields it's been substituted almost
completely by aluminium. Currently high
quantities of this metal are used for the production of electronic equipment or
electric circuits. Furthermore, solutions solved in silver nitrate (Ag NO3)
and other unsolvable compounds like potassium, are used in medicine as
antiseptics and bactericides.
The halide silver salts (silver bromide, chloride and iodide), which
darken if exposed in the light, are used in preparing emulsions for
photographic films. These salts are solvable in sodium triphosphate, which is a
compound normally used for photographic fixing processes.
White metal, extremely
hard, flaky, symbol Ir and atomic number 77; belongs to the transition elements
series of the periodic table. It was discovered in 1804 by the British chemist
Smithson Tennant and it was named after the iridescence of some of its
compounds.
Iridium is chemically totally inert, it resists to aqua regia but melted
salts etch it; in its compounds it reaches oxidation stages 3 and 4. Its atomic
weight is 192.22; it melts at approximately 2454°C, boils at approximately 5300
and its relative density is par to 22.4, the highest among all the elements.
It's an extremely rare metal; you can find it in alluviums in alloys
together with platinum, under shape of platiniridium and with osmium as
osmiridium. It's mainly used in alloys
with platinum to build precision instruments, surgical instruments, fountain
pens, weights and lengths.
GOLD
Metallic element with symbol Au (aurum in Latin) and atomic number 79,
belonging to the family of the transition elements of the periodic table.
Pure gold is shiny yellow and is the most ductile and malleable metal:
it can be reduced in laminas 0.0001 mm thick and pulled in very fine and long
strings (29 grams of gold can be transformed in a string 100 Km long). Its
hardness goes from 2.5 until 3, it is therefore one of the most tender metals;
it's an excellent heat and electricity conductor. Gold is a black powder when
finely subdivided, in colloidal suspension the solution becomes of a colour
between ruby red and purple red.
From a chemical point of view it's a very low-reactive metal: it isn't
etched by air, humidity, or heat, but it solves in solutions containing
chloride, bromides, iodide, in oxidizing mixes, alkaline cyanides and aqua
regia, a mixture of chloride and nitric acid. Its most important compounds are
chlorides and cyanides. It melts at 1064°C, boils at 2808°C; its relative
density is 19.3 and atomic weight par to 196.967.
It isn't rare to find precious
metals like gold and platinum at a pure stage in a sac or a vein inside a
sedimentary or igneous rock mixed together with other minerals. They are
geologic veins, formed by the sediments settled by undercurrents during the
past millenniums, or due to magma injections in igneous rock fissures. Gold
particularly, is often found in quartz veins.
Even if it's very rare in nature, gold is homogenously distributed on
the earth's crust, usually in quartziferous rocks and alluviums, at its native
stage or in alloys with silver. Together with tellurium and silver it
constitutes minerals like calaverite and sylvenite. It can be in gold amalgam
together with mercury and in small quantities in pyrites; traces of it can be
found in galena. It is also contained in seawater within a concentration
between 5 and 250 parts against 1000 parts of water in weight. Altogether the
quantity of gold in seawater is believed to amount to approximately 9 billions
of tons, but the recovery costs are extremely high.
Gold was known since ancient times, not just for its beauty and
corrosion resistance, but also because the extraction and working of it were
easier than the ones of any other metal.
It's used in jewellery and coins since antiquity; first in its native
condition, then in alloys with other materials that give it the necessary
hardness; the amount of gold contained in an alloy is expressed in carats.
For gilding and inscriptions it's used under shape of very fine
laminas (leafs); Cassius' purple is a
colloidal solution of finely divided gold and tin hydroxide, formed through the
reaction of gold chloride with tin chloride and used to colour glass. The tetrachloride
auric acid is used in photography, while the potassium dicyanaurate is used for
gold plating. Gold is also used in odontology, instead its radioisotopies are
used in biological researches and in the diagnosis of cancers (isotopic
tracers).
Gold is extracted from rivers by using dredges, machines provided of a
series of excavators that scrape the bottom pouring their content in a special
screen. The recovered material gets mixed and washed with water; so that the
auriferous sand passes through the holes of the screen and deposits it's self
on the oscillating tables. This method can be also used in dry riverbeds as
long as there is water under the soil: they dig a well, then the pump water
from it in to the riverbed, after that the dredge can be put to navigation.
During the extracting process of gold from rocks, the auriferous
minerals are collected and crumbled with special machinery; an amalgam is made
on copper leafs covered with mercury (amalgam process); gold is then obtained
through distillation, while the mercury is recycled. Gold can also be recovered
by cyanidation, which consists in treating the amalgam with a sodium cyanide
solution, letting the gold precipitate. In both methods gold has then have to
be purified and separated from silver through electrolysis. Some metals,
especially those where gold is combined with tellurium have to be calcined,
i.e. warmed at high temperatures before extraction.
The rarest form of gold to be found is the nugget; the biggest nugget
that has ever been found, the Welcome Stranger, weights 71 Kg and was casually
discovered just under the earth's surface in Victoria, Australia during 1869.
Gold was already in use in ancient times when the antique populations of
the Mediterranean used to obtain it from alluvium sands and from gravel. The
same method was used in India, Central Asia and the west regions of the
Mediterranean. Following the evolution of the extraction techniques, the
auriferous veins were exploited already during the first years of Christ's era;
further progress in gold's extraction and production were made beginning from
the Medieval times.
Currently the main countries producing gold are South Africa, United
States, Russia, Australia, Canada, China and Brazil.
OSMIUM
Fragile metallic element, white-azure colour, symbols Os and atomic
number 76; belongs to the family of the transition elements of the periodic
table. Discovered in 1803 from chemist Smithson Tennant, osmium has hardness
equal to 7, melts at 2700 °C, has a very high relative density, par to 22.61,
and atomic weight 190.2. It resists to the action of acids, but it's solvable
in aqua regia or smoking nitric acid; it forms salts were its oxidation stage
goes from +1 to +8.
The metal is found in platinum minerals and under shape of alloy together
with iridium. Alloys with iridium and platinum are used for the definition of
measure and weight standards.
PALLADIUM
Relatively rare metallic element, white-silverish colour, symbol Pd and
atomic number 46; it belongs to the transition family of the periodic table.
Palladium was discovered in 1804 from the British chemist William Hyde
Wollaston. It's ductile, malleable, corrosion resistant, and has almost the
same physical features of platinum, but it melts more easily and can be welded
easily. If finely grinded, palladium can absorb different gasses on its
surface; if it's bought to the temperature of approximately 100 °C, it absorbs
quantities of hydrogen and acetylene par to 1000 and up to 3000 times its
volume. It's etched by concentrated acids and rapidly corroded by aqua regia.
It forms divalent and tetravalent compounds and has the same chemical behaviour
of platinum. It melts at approximately 1554 °C, boils at about 2970 °C, has
relative density par to 12,02, and atomic weight 106.4. Quite rare on the
earth's surfaces, it can be found, under pure stage, in the platinum minerals
and combined with other elements in nickel minerals.
It is mainly used in the communication fields, to coat electric contacts
in automatic switches. Its also used in odontology; non-magnetic springs
manufacturing for watches and clocks; as coating for some kinds of mirrors. In
jewellery, in gold alloys, it forms white gold.
PLATINUM
Very rare metallic element, symbol Pt and atomic weight 78; it belongs
to the VIIIB (or 10) group of the periodic table; it's therefore a transition
element.
It's the most important among the platinoids group, which also includes
ruthenium, rhodium, palladium, osmium, and iridium. These elements were already
used in alloys in ancient Greece and Romans' times, but they are only mentioned
in European literature beginning from the sixteenth century. The distinction
between platinum and others platinoids dates back to the beginning of the
nineteenth century.
Platinum is a grey metal with 4.3 hardness; it has e very high melting
point, it's ductile and malleable, it dilates very little and has particularly
high electric resistance. Chemically it's relatively inert and it resists to
air, water, acids and normal reagents etchings; it solves slowly in aqua regia
forming hexchloroplatinic acid (VI), formula H2PtCl6;
halogens etch it and it reacts by heating with hydroxide, nitrate and sodium
cyanide. It melts at 1772 °C, boils at 3827 °C; its relative density is equal
to 21.45 and its atomic weight par to 195.08.
In nature platinum can be found in its elementary stage, but it's also
in some not common minerals, some of them are sperrylites, copperites and
braggites. Spheres weighting about 9 Kg were found.
The scars chemical reactivity and the high melting point make platinum
ideal for the production of instruments like melting pots, pliers, funnelsands
and capsules. Usually it's added small quantities of iridium that give it much
better hardness and resistance. It is also used to make electrical contacts for
instruments that take measures in high temperatures; under shape of sponge or
platinum black, it's used as catalyser in chemical industry. Considerable
quantities of Platinum are also used in jewellery, often in alloys with gold,
and in odontology.
RHODIUM
Silvery metallic element, symbol Rh and atomic number 45; it's a
transition element of the periodic table. Discovered in 1803 from chemist
William Hyde Wollaston, it's a very resistant metal, hardness par to 4, un
solvable in acids; melts at 1996°C, boils at 3727 °C, relative density equal to
12.4 and atomic weight par to 102.906.
When part of a compound rhodium manifests all the oxidation stages from
+1 to +6. It's name, which rises from the Greek rhodon, "pink",
alludes to the colour of the solution of many of its salts. Rhodium is found in
alloys inside platinum, osmium-iridium and gold rhodium (rhodites) minerals.
Rhodium is mainly used together with platinum: the resulting alloy is
mainly used in thermocouples for high temperature measurements; it combines the
excellent chemical resistance of platinum with the hardness and mechanical
resistance of rhodium. In its pure condition, instead, this metal is used for
the manufacturing of mirror glasses and in jewellery for plating and
silverware. Rhodium's black, a finely subdivided metal containing oxides and
hydrides, is utilized as a catalyser and as pigment for china production.
RUTHENIUM
White-grey metallic element, chemically inert, symbol Ru and atomic
number 44; it belongs to the transition elements of the periodic table.
It was discovered in 1844 by the Russian chemist Karl Karlovič
Klaus and its name alludes to the Ruthenian region, nowadays part of Ukraine.
It's quite rare on earth's surface and can be found in platinum minerals in its
metallic stage. Ruthenium melts at approximately 2250 °C, boils at 3900 °C; its
relative density is equal to 12.3 and its atomic weight to 101,07.
Together with platinum and palladium it forms
high hardness alloys, very wear-resistant and widely used in jewellery, for
metal-china dental prosthesis, for fountain pens nibs and for non-magnetic
pins. The ruthenium-molybdenum alloy, at temperatures lower than -263 °C,
behaves as a superconductor. The metal in its pure condition is extremely
resistant to acid and aqua regia etches.