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pure it scarcely yields in malleability to gold and silver : it is excessively
ductile and tenacious, and takes a good polish : it is harder than copper,
but softer than iron. Its hardness is increased by the presence of iridiurn.
Platinum is more ductile than malleable. It may be drawn into the finest
wire, but cannot be beaten into such thin leaves as gold and silver. It
undergoes no change by exposure to heat, and can only be melted by the
oxyhydrogen jet. It cannot be oxidized at any temperature. Its rate of
expansion and contraction by heat is so similar to that of glass, that it
admits of being welded with or fused into glass. The specific gravity of
platinum fluctuates between 21 and 22. Its extreme difficulty of fusion, and
the perfect manner in which it resists the action of almost all acids, at a
boiling or even at a red heat, render it importantly useful in many of the
arts, and indispensable in the laboratory. The curious catalytic action of
clean surfaces of platinum on wire and foil of pulverulent and spongy plati-
num, and platinum black, upon gaseous mixtures, especially in determining
the combination of oxygen and hydrogen, has rendered this metal useful in
gaseous analyses. Bibulous paper, or fibres of asbestos, saturated with a
strong solution of chloride of platinum, dried, and ignited, yield an ash
which exhibits the properties of a finely-divided platinum in perfection.
This catalytic action of platinum appears increased in proportion to the
mechanical division of the metal and the perfect cleanliness of its surface.

Platinum- Black. Platinum-black may be prepared by dissolving proto-
chloride of platinum in a strong hot solution of caustic potassa, and adding
alcohol : the hot mixture is stirred till the effervescence, arising from the
escape of carbonic acid ceases ; this is so violent as to require the use of a
capacious vessel. The platinum falls in the form of a black powder, from
which the supernatant liquor is poured off; the powder is then boiled suc-
cessively in alcohol, in hydrochloric acid, and in solution of potassa, and
lastly, in four or five portions of distilled water. If the alcohol is not
entirely removed, the powder ignites on drying, and loses its catalytic power.
It is dissolved by a solution of chlorine. When dry, it looks like lamp-
black, but acquires a metallic aspect after having been heated white-hot.
This form of platinum is also obtained by heating an aqueous solution of
.4 parts of bichloride of platinum, 10 of crystallized carbonate of soda, and 1
of grape-sugar to 212, stirring the liquor till the whole of the black pre-
cipitate has fallen, which is then well washed and dried : it is improved by



522 PROTOXIDE OF PLATINUM.

boiling it first in nitric acid, and then in solution of potassa, and finally
washing and drying:.

A much more simple method of procuring platinum-black consists in
decomposing the ammonia or potash chloride of platinum by zinc. The
yellow compound is diffused through water acidulated with diluted sulphuric
acid, and a bar of zinc is introduced. As the hydrogen is evolved the pla-
tinum is separated as a black powder, and simply requires washing. Mag-
nesium is equally effectual, and being purer than zinc, leaves, when dis-
solved, no metallic residue to contaminate the platinum. Sodium amalgam
also throws down platinum-black still more rapidly. Pure mercury only
very slowly precipitates a solution of the chloride. Iron, and some other
metals, throw down platinum in a very finely-divided state.

Spongy Platinum. Spongy platinum is readily procured by heating, on
platinum foil, the dry ammonio-chloride of platinum until the yellow color
has disappeared, and nothing but a gray spongy-looking metallic substance
remains. Care should be taken not to overheat it. It is platinum in a less
finely-divided state than platinum -black. Its properties are remarkable :
these have been elsewhere described (see CATALYSIS). It condenses certain
gases when they are in contact with it, and causes their union, although the
metal itself undergoes no change. That condensation is said to be equal to
250 times the volume of the platinum. In the presence of hydrogen it con-
denses oxygen, and vice versa; and produces water by causing the combina-
tion of the two. At a moderate heat it condenses hydrogen and deutoxide
of nitrogen, producing ammonia and water. Although it does not appear to
absorb or condense oxygen when alone, yet it acts upon the freshly precipi-
tated resin of guaiacum like the ozonides, or those bodies which easily part
with oxygen in the nascent state. It oxidizes the resin, and imparts to it a
blue color. During these combinations the platinum passes to the state of
full red heat, but undergoes no change of weight or properties.

Although pure platinum is infusible in an ordinary wind furnace, it softens
so as to admit of welding and forging. In the arc of flame of the voltaic
current, and before the oxyhydrogen blowpipe, in a lime-furnace, it not only
admits of being fused, but when very intensely heated it is said to give off
vapor. Platinum is insoluble in nitric acid, yet when alloyed with certain
other metals soluble in this, acid, it is taken up ; as for instance, with silver.
It is attacked at high temperatures by the alkalies, especially by baryta,
lithia, and potassa, which cause its oxidation and destruction. Nitre and
the alkaline persulphides have a similar action. Platinum readily fuses with
phosphorus, but it is not affected by sulphur unless in the spongy state. It
combines with the greater number of the metals, and with many of them
such as lead, antimony, and tin forms very fusible compounds ; these actions
show the necessity of caution as to the substances which are ignited or fused
in platinum crucibles, and as to the fuel with which they are brought into
contact.

The affinity, of platinum for oxygen is, like that of gold, extremely feeble ;
it shows no disposition to become an oxide by exposure to air or oxygen at
any temperature, and although a strong electric discharge, when transmitted
through a fine platinum wire, dissipates it into black dust, this, as in the
analogous case of gold, is probably finely-divided metal, and not the result
of oxidation. Two oxides of platinum have been satisfactorily identified.

PROTOXIDE OF PLATINUM (PtO). When protochloride of platinum is
gently heated in a solution of caustic potassa, a black oxide is formed, part
of which is dissolved, and part precipitated: it may be thrown down from its
alkaline solution by diluted' sulphuric acid. It is easily reduced by heat,



OXIDES AND CHLORIDES OF PLATINUM. 523

and slowly dissolves in the acids, most of which decompose it, and resolve
it into peroxide and metal.

BINOXIDE OF PLATINUM ; PEROXIDE OF PLATINUM (Pt0 2 ) is obtained by
decomposing nitrate of platinum by carbonate of soda, so as to leave the
nitrate in excess. It falls in the form of a brown hydrate, which, when
heated, first gives out water and becomes black ; at a higher temperature it
evolves oxygen, and is reduced : it has a feeble attraction for the acids, but
combines with many salifiable bases, and dissolves in the caustic and car-
bonated alkalies. It forms a fulminating ammoniacal compound, similar to
fulminating gold.

PROTOCHLORIDE OF PLATINUM (PtCl). When perchloride of platinum is
exposed in a porcelain capsule to a temperature not exceeding that of melt-
ing tin (about 400), and stirred so long as it evolves chlorine, it is con-
verted into a gray powder, insoluble in water, and not decomposed by sul-
phuric or nitric acid, but soluble in boiling hydrochloric acid. This is the
protochloride of platinum. It forms crystallizable double salts with the alka-
line chlorides. It is decomposed at a red heat, leaving a residue of metallic
platinum.

BICHLORIDE OF PLATINUM; Perchloride of Platinum (PtCl 2 ). A solution
of chlorine has no action on ordinary platinum foil or wire, but when in the
finely-divided state of sponge or as platinum black, chlorine, especially by
the aid of heat, slowly combines with the metal to form a bichloride. The
usual process for making this salt consists in digesting fine platinum wire foil or
grains in one part of nitric and three of hydrochloric acid with three parts of
water (to keep down iridium.) The solution is accelerated by heat and is
evaporated to two-thirds of its volume after saturation with platinum. When
so evaporated, it affords a deep-brown liquid which shoots into prismatic crys-
tals, consisting of hydrated perchloride of platinum and hydrochloric acid ;
on further evaporation it yields a brown saline mass, which becomes deeper
colored upon the expulsion of its combined water. It is then a perchloride
of platinum, yielding a deep yellow solution in water, and soluble in alcohol
and in ether. If still further heated, it loses all its chlorine, and metallic
platinum remains as a dull gray film which acquires a metallic lustre by bur-
nishing. It was by this method that earthenware and china were at one
time platinized. The bichloride of platinum combines with the alkaline
chlorides forming an extensive class of double salts known as Platino-
chlorides. More frequently the name of the alkali is used as a prefix.
Among these may be mentioned the compounds with potassium, rubidium,
cesium, ammonium, and thallium. These are all more or less insoluble in
water. The salts of potassium, rubidium, caesium and ammonium are isomor-
phous and crystallize in cubes. The sodium salt is quite soluble in water
and crystallizes in prisms.

The Ammonio- Bichloride of Platinum (NH 4 Cl + PtCl 2 ) is the yellow
powder which falls when solutions of bichloride of platinum and sal-ammoniac
are mixed. When exposed to heat, it loses a little water, and at a red heat
it evolves nitrogen, hydrochloric acid, and sal-ammoniac, without undergoing
fusion, and the platinum remains in the peculiar state known as spongy or
reduced platinum. It requires 150 parts of water at 60 and 80 parts of
boiling water to dissolve one part of this salt. This ammonio-chloride is
insoluble in alcohol and in cold hydrochloric acid, but it falls as a crystalline
powder from its solution in hot hydrochloric acid. It is almost entirely in-
soluble in solution of sal-ammoniac.

The action of ammonia on the two chlorides of platinum gives rise to a
series of compound bases which have much theoretical, but little practical in-
terest, and which have hitherto been only imperfectly examined as to pro-



524 SALTS OF PLATINUM.

perties and preparation. Any condensed notice of these, compatible with
the limits of this work, would be useless to the student.

Potassio- Bichloride of Platinum (KCl,PtCI 2 ) This is thrown down in the
form of a yellow powder, when concentrated solutions of chloride of potassium
and of bichloride of platinum are mixed. It is soluble in 108 parts of water at
60 and in 19 parts of boiling water, and is deposited from its boiling solu-
tion in small octahedral crystals. It is insoluble in alcohol. The difficult
solubility of this compound renders bichloride of platinum a useful test of
the presence of the salts of potassa, as well as of the salts of rubidium and
caesium. The platino-chlorides of these two metals, however, are much less
soluble than the platino-chloride of potassium : hence a solution of the latter
gives a dense yellow precipitate in a salt of caesium or rubidium. Sodio-
Bichloride of Platinum (NaCl,PtCl a ). Chloride of sodium occasions no
precipitate with bichloride of platinum, but the mixed solutions yield on
evaporation prismatic crystals, of a deep orange color, soluble in water and
in alcohol, and which, when heated, lose 6 atoms of water of crystallization,
leaving the anhydrous double salt. A variety of other analogous double
salts have been described ; they are generally made by mixing the two
chlorides in atomic proportions. The Bibromide and Biniodide of platinum
are sparingly soluble in water, and are obtained by the decomposition of the
bichloride, by bromide and iodide of potassium.

PROTOSULPHIDE OF PLATINUM (PtS) may be formed by heating finely-
divided platinum with sulphur, or by the decomposition of protochloride of
platinum by sulphuretted hydrogen. It is a gray or black powder, unaltered
by air or water, scarcely attacked by the boiling acids, but decomposed when
heated in the air. Bisulphide of 'Platinum (PtS 2 ) falls in the form of a
brownish powder, when the sodio-bichloride of platinum is precipitated by
sulphuretted hydrogen : at a red heat it is decomposed, and leaves metallic
platinum.

PROTOSULPHATE OF PLATINUM (PtO,S0 3 ) is obtained when a solution of
protoxide of platinum in caustic potassa is saturated with sulphuric acid, the
liquid poured off, and the precipitate dissolved in diluted sulphuric acid;
the concentrated solution is black ; diluted with water it becomes red, and
passes into persulphate. Persulphate of Platinum (Pt0 3 ,2S0 3 ) is obtained
hy acidifying the sulphur of the sulphides of platinum by nitric acid. It is
deep brown, and soluble in water, alcohol, and ether: with soda, potassa,
and ammonia, it forms double salts.

CYANIDE OF PLATINUM forms a series of double cyanides, some of which are
extremely beautiful. The platino-cyanide of potassium (KCy,PtCy) or
(K,PtCy 2 ) formed by dissolving protochloride of platinum in a solution of
cyanide of potassium, forms prismatic crystals, yellow by transmitted, and
blue by reflected light. They contain 3 atoms of water. The platino-
cyanide of magnesium forms crystals which exhibit various shades of red,
blue, and green (p. 33). When chlorine is passed through a solution of the
platino-cyanide of potassium, crystals are deposited which are green by trans-
mitted light, but of a copper color by reflected light. This salt, termed
sesquiplatino-cyanide of potassium, has the formula (K 3 ,Pt a Cy 3 ,6Aq).

ALLOYS OF PLATINUM. Iron and platinum in equal parts form a crystal-
line alloy, which takes a fine polish. Platinum dissolves in fused zinc ; the
alloy is bluish-white, brittle, and hard. Zinc heated in platinum-foil before
the blowpipe burns vividly and even with explosion. Tin and platinum
form alloys more or less brittle and fusible. When tin-foil and platinum are
wrapped together and heated by the blowpipe, they combine with incan-
descence. With its weight of nickel platinum forms a pale yellow alloy,
susceptible of a high polish. Copper and platinum form alloys, the ductility



TESTS FOR THE SALTS OF PLATINUM. 525

and color of which vary with the proportions : platinum easily destroys the
color of copper : an alloy of 7 platinum, 16 copper, 1 zinc, resembles gold
in color. Lead and platinum form brittle alloys. Platinum and lead-foil
folded together and heated before the blowpipe, combine with elevation of
temperature. Antimony and platinum readily enter into ignition in the
flame of a spirit lamp when they combine, in the same manner as tin and
zinc. Arsenic and platinum form a dark-gray brittle alloy. When particles
of arsenic are placed upon red hot platinum-leaf, they immediately fuse a hole
in it. Mercury amalgamates difficultly with platinum : spongy platinum
forms the readiest combination, especially when rubbed with the mercury in
a hot mortar. Silver and platinum form ductile alloys. Gold and platinum
require a strong heat for combination, and the color of the gold is greatly
deteriorated.

The perfection with which vessels of platinum resist the action of heat,
and of most acids, renders them peculiarly valuable in many of their applica-
tions ; but its high price is against its general adoption. In the employ-
ment of platinum-vessels, the following precautions must be attended to : 1.
They must not be subjected to the action of compounds which evolve chlo-
rine. 2. Nitre, and the alkalies, must not be fused in them. 3. No metallic
reductions must be performed in them ; nor compounds of phosphorus de-
composed, so as to evolve that substance. 4. When metallic oxides are
heated in a platinum crucible, the heat must not be raised to redness if the
oxide is easily decomposed. 5. The immediate contact of the fuel (charcoal
should always be used) with the crucible should be avoided, especially at
very high temperatures ; for by combining with silicon platinum is rendered
brittle and unsound.

TESTS FOR THE SALTS OF PLATINUM. The color and the difficult solubility
of the ammonio and potassio-chlorides of platinum, and the solubility of the
corresponding soda-compound, are very characteristic of this metal. All the
metals which reduce the chloride of gold, with the exception of palladium,
act similarly upon chloride of platinum, but its complete separation in the
metallic state is slow : iron, zinc, cadmium, and copper, are its most effective
precipitants ; they separate it as a black powder, which sometimes adheres in
films to the glass. Protosulphate of iron, tincture of galls, oxalic, sulphur-
ous, and arsenious acids occasion no precipitates in a solution of perchloride
of platinum, a circumstance which distinguishes this metal from gold, silver,
and palladium.

A solution of a salt of platinum has the following special characters: 1.
Potash and ammonia throw down yellow precipitates. 2. The Chlorides of
potassium and ammonium also throw down yellow precipitates (platino-chlo-
rides). 3. The solution evaporated to dryness, and heated, yields metallic
platinum.

The chloride of ammonium from the insolubility of the platino-chloride
formed, has been usually selected as the test for this metal, the precipitation
being aided by the addition of a little alcohol or diluted hydrochloric acid.
The discovery of the salts of thallium, however, by Dr. Crookes has made
known a still more delicate test for the compounds of platinum. A solution
of nitrate of thallium will throw down a platino-chloride of that metal, in a
solution in which chloride of ammonium produces no change. The platino-
cliloride of thallium requires 15,585 parts of water to dissolve it.

In a mixture of gold and platinum, the gold may be precipitated and sepa-
rated by boiling the solution with sulphate of iron or arsenious acid, or the
platinum may be precipitated by chloride of ammonium or nitrate of thallium.
Freshly-precipitated guaiacum resin, when added to a solution of gold, .pro-



526 SALTS OF PALLADIUM.

duces a greenish-blue color; with chloride of platinum it produces no parti-
cular effect. On boiling the solutions the gold is completely reduced and
deposited, but the platinum solution remains unchanged. The salts of plati-
num, unlike those of gold and silver, are not affected by light.



CHAPTER XLI.

PALLADIUM. RHODIUM. RUTHENIUM. OSMIUM. IRIDIUM.
PALLADIUM (Pd=54).

PALLADIUM was discovered by Wollaston in 1803 : it is associated with
the other metals mentioned in the last section as constituting the ore of pla-
tinum. It has also been found alloyed with gold.

Palladium is separated from the ore of platinum by the following process.
Digest the ore in nitrohydrochloric acid, neutralize the redundant acid by
soda, throw down the platinum by sal-ammoniac, and filter : to the filtered
liquor add a solution of cyanide of mercury : a yellow flocculent precipitate
of cyanide of palladium is soon deposited, which yields palladium on expo-
sure to heat. This metal has a dull-white color, is malleable and ductile, but
hard. It fuses at a temperature above that required for the fusion of gold,
and when intensely heated by the oxyhydrogen blowpipe, it is dissipated in
sparks. When heated over the flame of a spirit-lamp, it acquires various
shades of blue upon its surface, in consequence of superficial oxidation. It
is acted on by the greater number of the acids when aided by heat, and also
by potassa and nitre : it has a strong affinity for cyanogen.

PROTOXIDE OF PALLADIUM (PdO). This oxide is the base of the salts of
the metal. Thus, when nitrate of palladium is precipitated by an alkali, the
red or dark-orange powder which falls is a hydrated oxide. In this state, it
is soluble in acids, yielding red and brown salts of an astringent taste. It
becomes black and anhydrous at a dull red heat. Binoxide of Palladium
(PdO a ) is obtained as a brown hydrate, by the action of a solution of potassa
on the potassio-chloride of palladium*

PEOTOCHLORIDE OF PALLADIUM (PdCl) is obtained by digesting palladium
in hydrochloric acid with a little nitric acid, and evaporating to dryness : it
forms a brown powder, which is nearly black when anhydrous. It forms
double salts with the basic metallic chlorides, which are soluble in water and
in alcohol. With ammonia it forms a series of compounds resembling those
of platinum. BICHLORIDE OF PALLADIUM ; PERCHLORIDE OF PALLADIUM
(PdCl a ) is only known in solution. It forms red double salts with the alka-
line chlorides.

NITRATE OF PALLADIUM. Palladium, when aided by heat, dissolves slowly
in nitric acid, forming a brown solution which leaves a brown subnitrate on
evaporation. Nitrate of protoxide of palladium forms a double salt with
ammonia.

SULPHIDE OF PALLADIUM (PdS) is formed by .fusing sulphur with palla-
dium ; it is white, hard, and fusible, and when long exposed to heat and air,
it loses its sulphur. It is thrown down in the form of a black powder by
the action of sulphuretted hydrogen upon the salts palladium.

PROTOSULPHATE OF PALLADIUM (PdO,S0 3 ) is obtained by boiling the pro-
tonitrate to dryness with sulphuric acid ; or by boiling the metal in sulphuric



SALTS OF RHODIUM. 52t

acid, when sulphurous acid is evolved, and a brown solution is obtained,
which deposits the sulphate in red crystals. This salt, dissolved in aqueous
ammonia, yields two ammonio-sulphates = NH 3 ,PdO,S0 3 and 2(NH 3 ,)PdO,
SO,

CARBIDE OF PALLADIUM. Palladium acquires brittleness when fused in
contact with charcoal. When a plate of palladium is long held in the flame
of alcohol, carbonaceous excrescences gradually form upon it, which, when
burned, leave palladium. This property of precipitating charcoal from flame,
and combining with it, is peculiar to palladium ; platinum and iron only show
slight indications of it.

CYANIDE OF PALLADIUM (PdCy) is formed when a solution of cyanide of
mercury is added to a neutral solution of palladium: it falls in olive-colored
or dingy yellow flakes; this furnishes a method of separating palladium from
other metals which are incapable of decomposing the cyanide of mercury.
It dissolves in cyanide of potassium, and forms palladio-cyanide of potassium.
There is also a corresponding amraoniacal salt.

TESTS FOR THE SALTS OF PALLADIUM. The fixed alkalies throw down red
or orange precipitates from the solutions of palladium, sparingly soluble in
excess of the alkali. Ferrocyanide of potassium gives an olive-green preci-
pitate; and sulphuretted hydrogen one of a dark-brown color. Protochloride
of tin occasions a brown precipitate in the neutralized solutions of palladium;
when dilute, the mixture becomes green. Protosulphate of iron throws down
metallic palladium. Cyanide of mercury forms a precipitate in all the salts
of palladium, when the acid is not in excess. Iodide of potassium occasions
a black precipitate of iodide of palladium in very diluted solutions. Chloride
of palladium added to a solution of 1 part of iodide of potassium in 400,000
of water produces a brown tint. It is therefore a delicate test for an alkaline
iodide.

As the iodide of palladium is slightly soluble in iodide of potassium the
latter must not be in excess. Chloride of palladium is not only a delicate
test for an alkaline-iodide, but it enables the chemist to separate iodide from
chlorine and bromine. The palladium salt gives no precipitate in solutions
of an alkaline-chloride or bromide unless in the latter case the solution is
very concentrated.

RHODIUM (R=52).

After the platinum and palladium have been separated from the nitrohy-
drochloric solution of the crude ore, by sal-ammoniac and cyanide of mercury,
and any excess of the cyanide has been decomposed by the addition of hydro-
chloric acid, chloride of sodium is added, and the liquor evaporated to dry-
ness, the residue is then digested in alcohol, which leaves a red insoluble
double chloride of sodium and rhodium. When this is dissolved in water,
and a plate of zinc immersed, metallic rhodium is thrown down ; or the
double chloride may be at once decomposed by heating it in a current of
hydrogen. Rhodium, discovered by Wollaston in 1108, is a white metal
very difficult of fusion, and extremely hard and brittle. When pure, the
acids do not dissolve it, but they act upon several of its alloys. It may be



Online LibraryWilliam Thomas BrandeChemistry → online text (page 80 of 124)