Chapter XXIII: The Platinum Metals (1)

The six metals: ruthenium, Ru, rhodium, Rh, palladium, Pd, osmium, Os, iridium, Ir, and platinum, Pt, are met with associated together in nature. Platinum always predominates over the others, and hence they are known as the _platinum metals_. By their chemical character their position in the periodic system is in the eighth group, corresponding with iron, cobalt, and nickel.

The natural transition from titanium and vanadium to copper and zinc by means of the elements of the iron group is demonstrated by all the properties of these elements, and in exactly the same manner a transition from zirconium, niobium, and molybdenum to silver, cadmium, and indium, through ruthenium, rhodium, and palladium, is in perfect accordance with fact and with the magnitude of the atomic weights, as also is the position of osmium, iridium, and platinum between tantalum and tungsten on the one side, and gold and mercury on the other. In all these three cases the elements of smaller atomic weight (chromium, molybdenum, and tungsten) are able, in their higher grades of oxidation, to give acid oxides having the properties of distinct but feebly energetic acids (in the lower oxides they give bases), whilst the elements of greater atomic weight (zinc, cadmium, mercury), even in their higher grades of oxidation, only give bases, although with feebly developed basic properties. The platinum metals present the same intermediate properties such as we have already seen in iron and the elements of the eighth group.

In the platinum metals the intermediate properties _of feebly acid and feebly basic metals_ are developed with great clearness, so that there is not one sharply-defined acid anhydride among their oxides, although there is a great diversity in the grades of oxidation from the type RO_{4} to R_{2}O. The feebleness of the chemical forces observed in the platinum metals is connected with the ready decomposability of their compounds, with the small atomic volume of the metals themselves, and with their large atomic weight. The oxides of platinum, iridium, and osmium can scarcely be termed either basic or acid; they are capable of combinations of both kinds, each of which is feeble. They are all intermediate oxides.

The atomic weights of platinum, iridium, and osmium are nearly 191 to 196, and of palladium, rhodium, and ruthenium, 104 to 106. Thus, strictly speaking, we have here two series of metals, which are, moreover, perfectly parallel to each other; three members in the first series, and three members in the second--namely, platinum presents an analogy to palladium, iridium to rhodium, and osmium to ruthenium. As a matter of fact, however, the whole _group_ of the platinum metals is characterised by _a number of common properties_, both physical and chemical, and, moreover, there are several points of resemblance between the members of this group and those of the _iron_ group (Chapter XXII.) The atomic volumes (Table III., column 18) of the elements of this group are _nearly equal_ and _very small_. The iron metals have atomic volumes of nearly 7, whilst that of the metals allied to palladium is nearly 9, and of those adjacent to platinum (Pt, Ir, Os) nearly 9·4. This comparatively small atomic volume corresponds with the great infusibility and tenacity proper to all the iron and platinum metals, and to their small chemical energy, which stands out very clearly in the heavy platinum metals. All the platinum metals are very _easily reduced_ by ignition and by the action of various reducing agents, in which process oxygen, or a haloid group, is disengaged from their compounds and the metal left behind. This is a property of the platinum metals which determines many of their reactions, and the circumstance of their always being found in nature _in a native state_. In Russia in the Urals (discovered in 1819) and in Brazil (1735) platinum is obtained from alluvial deposits, but in 1892 Professor Inostrantseff discovered a vein deposit of platinum in serpentine near Tagil in the Urals.[1] The facility with which they are reduced is so great that their chlorides are even decomposed by gaseous hydrogen, especially when shaken up and heated under a certain pressure. Hence it will be readily understood that such metals as zinc, iron, &c., separate them from solutions with great ease, which fact is taken advantage of in practice and in the chemical treatment of the platinum metals.[1 bis]

[1] Wells and Penfield (1888) have described a mineral sperryllite
found in the Canadian gold-bearing quartz and consisting of
platinum diarsenide, PtAs_{2}. It is a noticeable fact that this
mineral clearly confirms the position of platinum in the same group
as iron, because it corresponds in crystalline form (regular
octahedron) and chemical composition with iron pyrites, FeS_{2}.

[1 bis] Some light is thrown upon the facility with which the platinum
compounds decompose by Thomsen's data, showing that in an excess of
water (+ Aq) the formation from platinum, of such a double salt as
PtCl_{2},2KCl, is accompanied by a comparatively small evolution of
heat (_see_ Chapter XXI., Note 40), for instance, Pt + Cl_{2} +
2KCl + Aq only evolves about 33,000 calories (hence the reaction,
Pt + Cl_{2} + Aq, will evidently disengage still less, because
PtCl_{2} + 2KCl evolves a certain amount of heat), whilst on the
other hand, Fe + Cl_{2} + Aq gives 100,000 calories, and even the
reaction with copper (for the formation of the double salt) evolves
63,000 calories.

All the platinum metals, like those of the iron group, are grey, with a comparatively feeble metallic lustre, and are very infusible. In this respect they stand in the same order as the metals of the iron series; nickel is more fusible and whiter than cobalt and iron, so also palladium is whiter and more fusible than rhodium and ruthenium, and platinum is comparatively more fusible and whiter than iridium or osmium. The saline compounds of these metals are red or yellow, like those of the majority of the metals of the iron series, and like the latter, the different forms of oxidation present different colours. Moreover, certain complex compounds of the platinum metals, like certain complex compounds of the iron series, either have particular characteristic tints or else are colourless.

The platinum metals are found _in nature associated together in alluvial deposits_ in a few localities, from which they are washed, owing to their very considerable density, which enables a stream of water to wash away the sand and clay with which they are mixed. Platinum deposits are chiefly known in the Urals, and also in Brazil and a few other localities. The platinum ore washed from these alluvial deposits presents the appearance of more or less coarse grains, and sometimes, as it were, of semi-fused nuggets.[2]

[2] The largest amount of platinum is extracted in the Urals, about
five tons annually. A certain amount of gold is extracted from the
washed platinum by means of mercury, which does not dissolve the
platinum metals but dissolves the gold accompanying the platinum in
its ores. Moreover, the ores of platinum always contain metals of
the iron series associated with them. The washed and mechanically
sorted ore in the majority of cases contains about 70 to 80 p.c. of
platinum, about 5 to 8 p.c. of iridium, and a somewhat smaller
quantity of osmium. The other platinum metals--palladium, rhodium,
and ruthenium--occur in smaller proportions than the three above
named. Sometimes grains of almost pure osmium-iridium, containing
only a small quantity of other metals, are found in platinum ores.
This _osmium-iridium_ may be easily separated from the other
platinum metals, owing to its being nearly insoluble in aqua regia,
by which the latter are easily dissolved. There are grains of
platinum which are magnetic. The grains of osmium-iridium are very
hard and malleable, and are therefore used for certain purposes,
for instance, for the tips of gold pens.

All the platinum metals give compounds with the halogens, and the highest haloid type of combination for all is RX_{4}. For the majority of the platinum metals this type is exceedingly unstable; the lower compounds corresponding to the type RX_{2}, which are formed by the separation of X_{2}, are more stable. In the type RX_{2} the platinum metals form more stable salts, which offer no little resemblance to the kindred compounds of the iron series--for example, to nickelous chloride, NiCl_{2}, cobaltous chloride, CoCl_{2}, &c. This even expresses itself in a similarity of volume (platinous chloride, PtCl_{2}, volume, 46; nickelous chloride, NiCl_{2} = 50), although in the type RX_{2} the true iron metals give very stable compounds, whilst the platinum metals frequently react after the manner of suboxides, decomposing into the metal and higher types, 2RX_{2} = R + RX_{4}. This probably depends on the facility with which RX_{2} decomposes into R and X_{2}, when X_{2} combines with the remaining portion of RX_{2}.

As in the series iron, cobalt, nickel, nickel gives NiO and Ni_{2}O_{3}, whilst cobalt and iron give higher and varied forms of oxidation, so also among the platinum metals, platinum and palladium only give the forms RX_{2} and RX_{4}, whilst rhodium and iridium form another and intermediate type, RX_{3}, also met with in cobalt, corresponding with the oxide, having the composition R_{2}O_{3}, besides which they form an acid oxide, like ferric acid, which is also known in the form of salts, but is in every respect unstable. _Osmium_ and _ruthenium_, like manganese, form still higher oxides, and in this respect exhibit the greatest diversity. They not only give RX_{2}, RX_{3}, RX_{4}, and RX_{6}, but also a still _higher form of oxidation_, RO_{4}, which is not met with in any other series. This form is exceedingly characteristic, owing to the fact that the oxides, OsO_{4} and RuO_{4}, are volatile and have feebly acid properties. In this respect they most resemble permanganic anhydride, which is also somewhat volatile.[3]

[3] In characterising the platinum metals according to their relation
to the iron metals, it is very important to add two more very
remarkable points. The platinum metals are capable of forming a
sort of unstable compound with _hydrogen_; they absorb it and only
part with it when somewhat strongly heated. This faculty is
especially developed in platinum and palladium, and it is very
characteristic that nickel, which exactly corresponds with platinum
and palladium in the periodic system, should exhibit the same
faculty for retaining a considerable quantity of hydrogen (Graham's
and Raoult's experiments). Another characteristic property of the
platinum metals consists in their easily giving (like cobalt which
forms the cobaltic salts) stable and characteristic saline
_compounds with ammonia_, and like Fe and Co, double salts with the
cyanides of the alkali metals, especially in their lower forms of
combination. All the above so clearly brings the elements of the
iron series in close relation to the platinum metals, that the
eighth group acquires as natural a character as can be required,
with a certain originality or individuality for each element.

When dissolved in aqua regia (PtCl_{4} is formed) and liberated from the solution by sal-ammoniac ((NH_{4})_{2}PtCl_{6} is formed) and reduced by ignition (which may be done by Zn and other reducing agents, direct from a solution of PtCl_{4}) platinum[3 bis] forms a powdery mass, known as spongy platinum or platinum black. If this powder of platinum be heated and pressed, or hammered in a cylinder, the grains aggregate or forge together, and form a continuous, though of course not entirely homogeneous, mass. Platinum was formerly, and is even now, worked up in this manner. The platinum money formerly used in Russia was made in this way. Sainte-Claire Deville, in the fifties, for the first time melted platinum in considerable quantities by employing a special furnace made in the form of a small reverberatory furnace, and composed of two pieces of lime, on which the heat of the oxyhydrogen flame has no action. Into this furnace (shown in fig. 34, Vol. I. p. 175)--or, more strictly speaking, into the cavity made in the pieces of lime--the platinum is introduced, and two orifices are made in the lime; through one, the upper, or side orifice, is introduced an oxyhydrogen gas burner, in which either detonating gas or a mixture of oxygen and coal-gas is burnt, whilst the other orifice serves for the escape of the products of combustion and certain impurities which are more volatile than the platinum, and especially the oxidised compounds of osmium, ruthenium, and palladium, which are comparatively easily volatilised by heat. In this manner the platinum is converted into a continuous metallic form by means of fusion, and this method is now used for melting considerable masses of platinum[4] and its alloys with iridium.

[3 bis] Platinum was first obtained in the last century from Brazil,
where it was called silver (platinus). Watson in 1750 characterised
platinum as a separate independent metal. In 1803 Wollaston
discovered palladium and rhodium in crude platinum, and at about
the same time Tennant distinguished iridium and osmium in it.
Professor Claus, of Kazan, in his researches on the platinum metals
(about 1840) discovered ruthenium in them, and to him are due many
important discoveries with regard to these elements, such as the
indication of the remarkable analogy between the series Pd--Rh--Ru
and Pt--Ir--Os.

_The treatment of platinum ore_ is chiefly carried on for the
extraction of the platinum itself and its alloys with iridium,
because these metals offer a greater resistance to the action of
chemical reagents and high temperatures than any of the other
malleable and ductile metals, and therefore the wire so often used
in the laboratory and for technical purposes is made from them, as
also are various vessels used for chemical purposes in the
laboratory and in works. Thus sulphuric acid is distilled in
platinum retorts, and many substances are fused, ignited, and
evaporated in the laboratory in platinum crucibles and on platinum
foil. Gold and many other substances are dissolved in dishes made
of iridium-platinum, because the alloys of platinum and iridium are
but slightly attacked when subjected to the action of aqua regia.

The comparatively high density (about 21·5), hardness, ductility,
and infusibility (it does not melt at a furnace heat, but only in
the oxyhydrogen flame or electric furnace), as well as the fact of
its resisting the action of water, air, and other reagents, renders
an alloy of 90 parts of platinum and 10 parts of iridium (Deville's
platinum-iridium alloy) a most valuable material for making
standard weights and measures, such as the metre, kilogram, and
pound, and therefore all the newest standards of most countries are
made of this alloy.

[4] This process has altered the technical treatment of platinum to a
considerable extent. It has in particular facilitated the
manufacture of alloys of platinum with iridium and rhodium from the
pure platinum ores, since it is sufficient to fuse the ore in order
for the greater amount of the osmium to burn off, and for the mass
to fuse into a homogeneous, malleable alloy, which can be directly
made use of. There is very little ruthenium in the ores of
platinum. If during fusion lead be added, it dissolves the platinum
(and other platinum metals) owing to its being able to form a very
characteristic alloy containing PtPb. If an alloy of the two metals
be left exposed to moist air, the excess of lead is converted into
carbonate (white lead) in the presence of the water and carbonic
acid of the air, whilst the above platinum alloy remains unchanged.
The white lead may be extracted by dilute acid, and the alloy PtPb
remains unaltered. The other platinum metals also give similar
alloys with lead. The fusibility of these alloys enables the
platinum metals to be separated from the gangue of the ore, and
they may afterwards be separated from the lead by subjecting the
alloy to oxidation in furnaces furnished with a bone ash bed,
because the lead is then oxidised and absorbed by the bone ash,
leaving the platinum metals untouched. This method of treatment was
proposed by H. Sainte-Claire Deville in the sixties, and is also
used in the analysis of these metals (_see_ further on).

To obtain pure platinum, the ore is treated with aqua regia in which only the osmium and iridium are insoluble. The solution contains the platinum metals in the form RCl_{4}, and in the lower forms of chlorination, RCl_{3} and RCl_{2}, because some of these metals--for instance, palladium and rhodium--form such unstable chlorides of the type RX_{4} that they partially decompose even when diluted with water, and pass into the stable lower type of combination; in addition to which the chlorine is very easily disengaged if it comes in contact with substances on which it can act. In this respect platinum resists the action of heat and reducing agents better than any of its companions--that is, it passes with greater difficulty from PtCl_{4} to the lower compound PtCl_{2}. On this is based the method of preparation of more or less pure platinum. Lime or sodium hydroxide is added to the solution in aqua regia until neutralised, or only containing a very slight excess of alkali. It is best to first evaporate and slightly ignite the solution, in order to remove the excess of acid, and by heating it to partially convert the higher chlorides of the palladium, &c., into the lower. The addition of alkalis completes the reduction, because the chlorine held in the compounds RX_{4} acts on the alkali like free chlorine, converting it into a hypochlorite. Thus palladium chloride, PdCl_{4}, for example, is converted into palladious chloride, PdCl_{2}, by this means, according to the equation PdCl_{4} + 2NaHO = PdCl_{2} + NaCl + NaClO + H_{2}O. In a similar manner iridic chloride, IrCl_{4}, is converted into the trichloride, IrCl_{3}, by this method. When this conversion takes place the platinum still remains in the form of platinic chloride, PtCl_{4}. It is then possible to take advantage of a certain difference in the properties of the higher and lower chlorides of the platinum metals. Thus lime precipitates the lower chlorides of the members of the platinum metals occurring in solution without acting on the platinic chloride, PtCl_{4}, and hence the addition of a large proportion of lime immediately precipitates the associated metals, leaving the platinum itself in solution in the form of a soluble double salt, PtCl_{4},CaCl_{2}. A far better and more perfect _separation_ is effected _by means of ammonium chloride_, which gives, with platinic chloride, an insoluble yellow precipitate, PtCl_{4},2NH_{4}Cl, whilst it forms soluble double salts with the lower chlorides RCl_{2} and RCl_{3}, so that ammonium chloride precipitates the platinum only from the solution obtained by the preceding method. These methods are employed for preparing the platinum which is used for the manufacture of platinum articles, because, having platinum in solution as calcium platinochloride, PtCaCl_{6}, or as the insoluble ammonium platinochloride, Pt(NH_{4})_{2}Cl_{6}, the platinum compound in every case, after drying or ignition, loses all the chlorine from the platinic chloride and leaves finely-divided metallic platinum, which may be converted into homogeneous metal by compression and forging, or by fusion.[5]

[5] For the ultimate purification of platinum from palladium and
iridium the metals must be re-dissolved in aqua regia, and the
solution evaporated until the residue begins to evolve chlorine.
The residue is then re-precipitated with ammonium or potassium
chloride. The precipitate may still contain a certain amount of
iridium, which passes with greater difficulty from the
tetrachloride, IrCl_{4}, into the trichloride, IrCl_{3}, but it
will be quite free from palladium, because the latter easily loses
its chlorine and passes into palladious chloride, PdCl_{2}, which
gives an easily-soluble salt with potassium chloride. The
precipitate, containing a small quantity of iridium, is then heated
with sodium carbonate in a crucible, when the mass decomposes,
giving metallic platinum and iridium oxide. If potassium chloride
has been employed, the residue after ignition is washed with water
and treated with aqua regia. The iridium oxide remains undissolved,
and the platinum easily passes into solution. Only cold and dilute
aqua regia must be used. The solution will then contain pure
platinic chloride, which forms the starting-point for the
preparation of all platinum compounds. Pure platinum for accurate
researches (for instance, for the unit of light, according to
Violle's method) may be obtained (Mylius and Foerster, 1892) by
Finkener's method, by dissolving the impure metal in aqua regia (it
should be evaporated to drive off the nitrogen compounds), and
adding NaCl so as to form a double sodium salt, which is purified
by crystallising with a small amount of caustic soda, washing the
crystals with a strong solution of NaCl, and then dissolving them
in a hot 1 p.c. solution of soda, repeating the above and
ultimately igniting the double salt, previously dried at 120°, in a
stream of hydrogen; platinum black and NaCl are then formed. The
three following are very sensitive tests (to thousandths of a per
cent.) for the presence of Ir, Ru, Rh, Pd (osmium is not usually
present in platinum which has once been purified, since it easily
volatilises with Cl_{2} and CO_{2}, and in the first treatment of
the crude platinum either passes off as OsO_{4} or remains
undissolved), Fe, Cu, Ag, and Pb: (1) the assay is alloyed with 10
parts of pure lead, the alloy treated with dilute nitric acid (to
remove the greater part of the Pb), and dissolved in aqua regia;
the residue will consist of Ir and Ru; the Pb is precipitated from
the nitric acid solution by sulphuric acid, whilst the remaining
platinum metals are reduced from the evaporated solution by formic
acid, and the resultant precipitate fused with KHSO_{4}; the Pd and
Rh are thus converted into soluble salts, and the former is then
precipitated by HgC_{2}N_{2}. (2) Iron may be detected by the usual
reagents, if the crude platinum be dissolved in aqua regia, and the
platinum metals precipitated from the solution by formic acid. (3)
If crude platinum (as foil or sponge) be heated in a mixture of
chlorine and carbonic oxide it volatilises (with a certain amount
of Ir, Pd, Fe, &c.) as PtCl_{2},2CO (Note 11), whilst the whole of
the Rh, Ag, and Cu it may contain remains behind. Among other
characteristic reactions for the platinum metals, we may mention:
(1) that rhodium is precipitated from the solution obtained after
fusion with KHSO_{4} (in which Pt does not dissolve) by NH_{3},
acetic and formic acids; (2) that dilute aqua regia dissolves
precipitated Pt, but not Rh; (3) that if the insoluble residue of
the platinum metals (Ir, Ru, Os) obtained, after treating with aqua
regia, be fused with a mixture of 1 part of KNO_{3} and 3 parts of
K_{2}CO_{3} (in a gold crucible), and then treated with water, it
gives a solution containing the Ru (and a portion of the Ir), but
which throws it all down when saturated with chlorine and boiled;
(4) that if iridium be fused with a mixture of KHO and KNO_{3}, it
gives a soluble potassium salt, IrK_{2}O_{4} (the solution is
blue), which, when saturated with chlorine, gives IrCl_{4}, which
is precipitated by NH_{4}Cl (the precipitate is black), forming a
double salt, leaving metallic Ir after ignition; (5) that rhodium
mixed with NaCl and ignited in a current of chlorine gives a
soluble double salt (from which sal-ammoniac separates Pt and Ir),
which gives (according to Jörgensen) a difficultly soluble
purpureo-salt (Chapter XXII., Note 35), Rh_{2}Cl_{3},5NH_{3}, when
treated with NH_{3}; in this form the Rh may be easily purified and
obtained in a metallic form by igniting in hydrogen; and (6) that
palladium, dissolved in aqua regia and dried (NH_{4}Cl throws down
any Pt), gives soluble PdCl_{2}, which forms an easily
crystallisable yellow salt, PdCl_{2}NH_{3}, with ammonia; this salt
(Wilm) may be easily purified by crystallisation, and gives
metallic Pd when ignited. These reactions illustrate the method of
separating the platinum metals from each other.

Metallic _platinum_ in a fused state has a specific gravity of 21; it is grey, softer than iron but harder than copper, exceedingly ductile, and therefore easily drawn into wire and rolled into thin sheets, and may be hammered into crucibles and drawn into thin tubes, &c. In the state in which it is obtained by the ignition of its compounds, it forms a spongy mass, known as spongy platinum, or else as powder (platinum black).[6] In either case it is dull grey, and is characterised, as we already know, by the faculty of absorbing hydrogen and other gases. Platinum is not acted on by hydrochloric, hydriodic, nitric, and sulphuric acids, or a mixture of hydrofluoric and nitric acids. Aqua regia, and any liquid containing chlorine or able to evolve chlorine or bromine, dissolves platinum. Alkalis are decomposed by platinum at a red heat, owing to the faculty of the platinum oxide, PtO_{2}, formed to combine with alkaline bases, inasmuch as it has a feebly-developed acid character (_see_ Note 8). Sulphur, phosphorus (the phosphide, PtP_{2}, is formed), arsenic and silicon all act more or less rapidly on platinum, under the influence of heat. Many of the metals form alloys with it. Even charcoal combines with platinum when it is ignited with it, and therefore carbonaceous matter cannot be subjected to prolonged and powerful ignition in platinum vessels. Hence a platinum crucible soon becomes dull on the surface in a smoky flame. Platinum also forms alloys with zinc, lead, tin, copper, gold, and silver.[7] Although mercury does not directly dissolve platinum, still it forms a solution or amalgam with spongy platinum in the presence of sodium amalgam; a similar amalgam is also formed by the action of sodium amalgam on a solution of platinum chloride, and is used for physical experiments.

[6] We have already become acquainted with the effect of finely-divided
platinum on many gaseous substances. It is best seen in the
so-called _platinum black_, which is a coal-black powder left by
the action of sulphuric acid on the alloy of zinc and platinum, or
which is precipitated by metallic zinc from a dilute solution of
platinum. In any case, finely-divided platinum absorbs gases more
powerfully and rapidly the more finely divided and porous it is.
Sulphurous anhydride, hydrogen, alcohol, and many organic
substances in the presence of such platinum are easily oxidised by
the oxygen of the air, although they do not combine with it
directly. The absorption of oxygen is as much as several hundred
volumes per one volume of platinum, and the oxidising power of such
absorbed oxygen is taken advantage of not only in the laboratory
but even in manufacturing processes. Asbestos or charcoal, soaked
in a solution of platinic chloride and ignited, is very useful for
this purpose, because by this means it becomes coated with platinum
black. If 50 grams of PtCl_{4} be dissolved in 60 c.c. of water,
and 70 c.c. of a strong (40 p.c.) solution of formic aldehyde
added, the mixture cooled, and then a solution of 50 grams of NaHO
in 50 grams of water added, the platinum is precipitated. After
washing with water the precipitate passes into solution and forms a
black liquid containing _soluble colloidal platinum_ (Loew, 1890).
If the precipitated platinum be allowed to absorb oxygen on the
filter, the temperature rises 40°, and a very porous _platinum
black_ is obtained which vigorously facilitates oxidation.

[7] It is necessary to remark that platinum when alloyed with silver,
or as amalgam, is soluble in nitric acid, and in this respect it
differs from gold, so that it is possible, by alloying gold with
silver, and acting on the alloy with nitric acid, to recognise the
presence of platinum in the gold, because nitric acid does not act
on gold alloyed with silver.

There are _two kinds_ of _platinum compounds_, PtX_{4} and PtX_{2}. The former are produced by an excess of halogen in the cold, and the latter by the aid of heat or by the splitting up of the former. The starting-point for the platinum compounds is _platinum tetrachloride_, _platinic chloride_, PtCl_{4}, obtained by dissolving platinum in aqua regia.[7 bis] The solution crystallises in the cold, in a desiccator, in the form of reddish-brown deliquescent crystals which contain hydrochloric acid, PtCl_{4},2HCl,6H_{2}O, and behave like a true acid whose salts correspond to the formula R_{2}PtCl_{6}--ammonium platinochloride, for example.[7 tri] The hydrochloric acid is liberated from these crystals by gently heating or evaporating the solution to dryness; or, better still, after treatment with silver nitrate a reddish-brown mass remains behind, which dissolves in water, and forms a yellowish-red solution which on cooling deposits crystals of the composition PtCl_{4},8H_{2}O. The _tendency_ of PtCl_{4} _to combine_ with hydrochloric acid and water--that is, _to form higher crystalline compounds_--is evident in the platinum compounds, and must be taken into account in explaining the properties of platinum and the formation of many other of its complex compounds. Dilute solutions of platinic chloride are yellow, and are completely reduced by hydrogen, sulphurous anhydride, and many reducing agents, which first convert the platinic chloride into the lower compound platinous chloride, PtCl_{2}. That faculty which reveals itself in platinum tetrachloride of combining with water of crystallisation and hydrochloric acid is distinctly marked in its property, with which we are already acquainted, of giving precipitates with the salts of potassium, ammonium, rubidium, &c. In general it _readily forms double salts_, R_{2}PtCl_{6} = PtCl_{4} + 2RCl, where R is a univalent metal such as potassium or NH_{4}. Hence the addition of a solution of potassium or ammonium chloride to a solution of platinic chloride is followed by the formation of a yellow precipitate, which is sparingly soluble in water and almost entirely insoluble in alcohol and ether (platinic chloride is soluble in alcohol, potassium iridiochloride, IrK_{3}Cl_{6}, _i.e._ a compound of IrCl_{3}, is soluble in water but not in alcohol). It is especially remarkable in this case, that the potassium compounds here, as in a number of other instances, separate in an anhydrous form, whilst the sodium compounds, which are soluble in water and alcohol, form red crystals containing water. The composition Na_{2}PtCl_{6},6H_{2}O exactly corresponds with the above-mentioned hydrochloric compound. The compounds with barium, BaPtCl_{6},4H_{2}O, strontium, SrPtCl_{6},8H_{2}O, calcium, magnesium, iron, manganese, and many other metals are all soluble in water.[8]

[7 bis] PtCl_{4} is also formed by the action of a mixture of HCl
vapour and air, and by the action of gaseous chlorine upon
platinum.

[7 tri] Pigeon (1891) obtained fine yellow crystals of
PtH_{2}Cl_{6},4H_{2}O by adding strong sulphuric acid to a strong
solution of PtH_{2}Cl_{6},6H_{2}O. If crystals of
H_{2}PtCl_{6},6H_{2}O be melted in vacuo (60°) in the presence of
anhydrous potash, a red-brown solid hydrate is obtained containing
less water and HCl, which parts with the remainder at 200°, leaving
anhydrous PtCl_{4}. The latter does not disengage chlorine before
220°, and is perfectly soluble in water.

[8] Nilson (1877), who investigated the platinochlorides of various
metals subsequently to Bonsdorff, Topsöe, Clève, Marignac, and
others, found that univalent and bivalent metals--such as hydrogen,
potassium, ammonium ... beryllium, calcium, barium--give compounds
of such a composition that there is always twice as much chlorine
in the platinic chloride as in the combined metallic chloride; for
example, K_{2}Cl_{2},PtCl_{4}; BeCl_{2},PtCl_{4},8H_{2}O, &c. Such
trivalent metals as aluminium, iron (ferric), chromium, didymium,
cerium (cerous) form compounds of the type RCl_{3}PtCl_{4}, in
which the amounts of chlorine are in the ratio 3:4. Only indium and
yttrium give salts of a different composition--namely,
2InCl_{3},5PtCl_{4},36H_{2}O and 4YCl_{3},5PtCl_{4},51H_{2}O. Such
quadrivalent metals as thorium, tin, zirconium give compounds of
the type RCl_{4},PtCl_{4}, in which the ratio of the chlorine is
1:1. In this manner the valency of a metal may, to a certain
extent, be judged from the composition of the double salts formed
with platinic chloride.

Platinic bromide, PtBr_{4}, and iodide, PtI_{4}, are analogous to
the tetrachloride, but the iodide is decomposed still more easily
than the chloride. If sulphuric acid be added to platinic chloride,
and the solution evaporated, it forms a black porous mass like
charcoal, which deliquesces in the air, and has the composition
Pt(SO_{4})_{2}. But this, the only oxygen salt of the type PtX_{4},
is exceedingly unstable. This is due to the fact that _platinum
oxide_, the oxide of the type PtO_{2}, has a feeble acid character.
This is shown in a number of instances. Thus if a strong solution
of platinic chloride treated with sodium carbonate be exposed to
the action of light or evaporated to dryness and then washed with
water, a sodium platinate, Pt_{3}Na_{2}O_{7},6H_{2}O, remains. The
composition of this salt, if we regard it in the same sense as we
did the salts of silicic, titanic, molybdic and other acids, will
be PtO(ONa)_{2},2PtO_{2},6H_{2}O--that is, the same type is
repeated as we saw in the crystalline compounds of platinum
tetrachloride with sodium chloride, or with hydrochloric
acid--namely, the type PtX_{4}8Y, where Y is the molecule
H_{2}O,HCl, &c. Similar compounds are also obtained with other
alkalis. They will be platinates of the alkalis in which the
platinic oxide, PtO_{2}, plays the part of an acid oxide. Rousseau
(1889) obtained different grades of combination BaOPtO_{2},
3(BaO)2PtO_{2}, &c., by igniting a mixture of PtCl_{4} and caustic
baryta. If such an alkaline compound of platinum be treated with
acetic acid, the alkali combines with the latter, and a _platinic
hydroxide_, Pt(OH)_{4}, remains as a brown mass, which loses water
and oxygen when ignited, and in so doing decomposes with a slight
explosion. When slightly ignited this hydroxide first loses water
and gives the very unstable oxide PtO_{2}. Platinic sulphide,
PtS_{2}, belongs to the same type; it is precipitated by the action
of sulphuretted hydrogen on a solution of platinum tetrachloride.
The moist precipitate is capable of attracting oxygen, and is then
converted into the sulphate above mentioned, which is soluble in
water. This absorption of oxygen and conversion into sulphate is
another illustration of the basic nature of PtO_{2}, so that it
clearly exhibits both basic and acid properties. The latter appear,
for instance, in the fact that platinic sulphide, PtS_{2}, gives
crystalline compounds with the alkali sulphides.

_Platinous chloride_, PtCl_{2}, is formed when hydrogen platinochloride, PtH_{2}Cl_{6}, is ignited at 300°, or when potassium is heated at 230° in a stream of chlorine. The undecomposed tetrachloride is extracted from the residue by washing it with water, and a greenish-grey or brown insoluble mass of the dichloride (sp. gr. 5·9) is then obtained. It is soluble in hydrochloric acid, giving an acid solution of the composition PtCl_{2},2HCl, corresponding with the type of double salts PtR_{2}Cl_{4}. Although platinous chloride decomposes below 500°, still it is formed to a small extent at higher temperatures. Troost and Hautefeuille, and Seelheim observed that when platinum was strongly ignited in a stream of chlorine, the metal, as it were, slowly volatilised and was deposited in crystals; a volatile chloride, probably platinous chloride, was evidently formed in this case, and decomposed subsequently to its formation, depositing crystals of platinum.

The properties of platinum above-described are repeated more or less distinctly, or sometimes with certain modifications, in the above-mentioned associates and analogues of this metal. Thus although palladium forms PdCl_{4}, this form passes into PdCl_{2} with extreme ease.[9] Whilst rhodium and iridium in dissolving in aqua regia also form RhCl_{4} and IrCl_{4}, but they pass into RhCl_{3} and IrCl_{3}[9 bis] very easily when heated or when acted upon by substances capable of taking up chlorine (even alkalis, which form bleaching salts). Among the platinum metals, ruthenium and osmium have the most acid character, and although they give RuCl_{4} and OsCl_{4} they are easily oxidised to RuO_{4}, and OsO_{4} by the action of chlorine in the presence of water; the latter are volatile and may be distilled with the water and hydrochloric acid, from a solution containing other platinum metals.[9 tri] Thus with respect to the types of combination, all the platinum metals, under certain circumstances, give compounds of the type RX_{4}--for instance, RO_{2}, RCl_{4}, &c. But this is the highest form for only platinum and palladium. The remaining platinum metals further, _like iron, give acids_ of the type RO_{3} or hydrates, H_{2}RO_{4} = RO_{2}(HO)_{2} (the type of sulphuric acid); but they, like ferric and manganic acids, are chiefly known in the form of salts of the composition K_{2}RO_{4} or K_{2}R_{2}O_{7} (like the dichromate). These salts are obtained, like the manganates and ferrates, by fusing the oxides, or even the metals themselves, with nitric, or, better still, with potassium peroxide. They are soluble in water, are easily deoxidised and do not yield the acid anhydrides under the action of acids, but break up, either (like the ferrate) forming oxygen and a basic oxide (iridium and rhodium react in this manner, as they do not give higher forms of oxidation), or passing into a lower and higher form of oxidation--that is, reacting like a manganate (or partly like nitrite or phosphite). Osmium and ruthenium react according to the latter form, as they are capable of giving _higher forms of oxidation_, OsO_{4} and RuO_{4}, and therefore their reactions of decomposition may be essentially represented by the equation: 2OsO_{3} = OsO_{2} + OsO_{4}.[10]

[9] In comparing the characteristics of the platinum metals, it must be
observed that palladium in its form of combination PdX_{2} gives
saline compounds of considerable stability. Amongst them _palladous
chloride_ is formed by the direct action of chlorine or aqua regia
(not in excess or in dilute solutions) on palladium. It forms a
brown solution, which gives a black insoluble precipitate of
_palladous iodide_, PdI_{2}, with solutions of iodides (in this
respect, as in many others, palladium resembles mercury in the
mercuric compounds HgX_{2}). With a solution of mercuric cyanide it
gives a yellowish white precipitate, palladous cyanide,
PdC_{2}N_{2}, which is soluble in potassium cyanide, and gives
other double salts, M_{2}PdC_{4}N_{4}.

That portion of the platinum ore which dissolves in aqua regia and
is precipitated by ammonium or potassium chloride does not contain
palladium. It remains in solution, because the palladic chloride,
PdCl_{4}, is decomposed and the palladous chloride formed is not
precipitated by ammonium chloride; the same holds good for all the
other lower chlorides of the platinum metals. Zinc (and iron)
separates out all the unprecipitated platinum metals (and also
copper, &c.) from the solution. The palladium is found in these
platinum residues precipitated by zinc. If this mixture of metals
be treated with aqua regia, all the palladium will pass into
solution as palladous chloride with some platinic chloride. By this
treatment the main portion of the iridium, rhodium, &c. remains
almost undissolved, the platinum is separated from the mixture of
palladous and platinic chlorides by a solution of ammonium
chloride, and the solution of palladium is precipitated by
potassium iodide or mercuric cyanide. Wilm (1881) showed that
palladium may be separated from an impure solution by saturating it
with ammonia; all the iron present is thus precipitated, and, after
filtering, the addition of hydrochloric acid to the filtrate gives
a yellow precipitate of an ammonio-palladium compound,
PdCl_{2},2NH_{3}, whilst nearly all the other metals remain in
solution. _Metallic palladium_ is obtained by igniting the
ammonio-compound or the cyanide, PdC_{2}N_{2}. It occurs native,
although rarely, and is a metal of a whiter colour than platinum,
sp. gr. 11·4, melts at about 1,500°; it is much more volatile than
platinum, partially oxidises on the surface when heated (Wilm
obtained spongy palladium by igniting PdCl_{2},2NH_{3}, and
observed that it gives PdO when ignited in oxygen, and that on
further ignition this oxide forms a mixture of Pd_{2}O and Pd), and
loses its absorbed oxygen on a further rise of temperature. It does
not blacken or tarnish (does not absorb sulphur) in the air at the
ordinary temperature, and is therefore better suited than silver
for astronomical and other instruments in which fine divisions have
to be engraved on a white metal, in order that the fine lines
should be clearly visible. The most remarkable property of
palladium, discovered by Graham, consists in its capacity for
_absorbing_ a large amount of _hydrogen_. Ignited palladium absorbs
as much as 940 volumes of hydrogen, or about 0·7 p.c. of its own
weight, which closely approaches to the formation of the compound
Pd_{3}H_{2}, and probably indicates the formation of _palladium
hydride_, Pd_{2}H. This absorption also takes place at the ordinary
temperature--for example, when palladium serves as an electrode at
which hydrogen is evolved. In absorbing the hydrogen, the palladium
does not change in appearance, and retains all its metallic
properties, only its volume increases by about 10 p.c.--that is,
the hydrogen pushes out and separates the atoms of the palladium
from each other, and is itself compressed to 1/900 of its volume.
This compression indicates a great force of chemical attraction,
and is accompanied by the evolution of heat (Chapter II., Note 38).
The absorption of 1 grm. of hydrogen by metallic palladium (Favre)
is accompanied by the evolution of 4·2 thousand calories (for Pt
20, for Na 13, for K 10 thousand units of heat). Troost showed that
the dissociation pressure of palladium hydride is inconsiderable at
the ordinary temperature, but reaches the atmospheric pressure at
about 140°. This subject was subsequently investigated by A. A.
Cracow of St. Petersburg (1894), who showed that at first the
absorption of hydrogen by the palladium proceeds like solution,
according to the law of Dalton and Henry, but that towards the end
it proceeds like a dissociation phenomenon in definite compounds;
this forms another link between the phenomenon of solution and of
the formation of definite atomic compounds. Cracow's observations
for a temperature 18°, showed that the electro-conductivity and
tension vary until a compound Pd_{2}H is reached, and namely, that
the tension _p_ rises with the volume _v_ of hydrogen absorbed,
according to the law of Dalton and Henry--for instance, for

_p_ = 2·1 3·2 5·5 7·7 mm.
_v_ = 14 20 34 47

The maximum tension at 18° is 9 mm. At a temperature of about 140°
(in the vapour of xylene) the maximum tension is about 760 mm., and
when _v_ = 10-50 vols. the tension (according to Cracow's
experiments) stands at 90-450 mm.--that is, increases in proportion
to the volume of hydrogen absorbed. But from the point of view of
chemical mechanics it is especially important to remark that
Moutier clearly showed, through palladium hydride, the similarity
of the phenomena which proceed in evaporation and dissociation,
which fact Henri Sainte-Claire Deville placed as a fundamental
proposition in the theory of dissociation. It is possible upon the
basis of the second law of the theory of heat, according to the law
of the variation of the tension _p_ of evaporation with the
temperature T (counted from -273°), to calculate the latent heat of
evaporation L (_see_ works on physics) because 424L = T(1/_d_ -
1/D)_dp_/_dt_, where _d_ and D are the weights of cubic measures of
the gas (vapour) and liquid. (Thus, for instance, for water, when
_t_ = 100°, T = 373, _d_ = 0·605, D = 960, _dp_/_dt_ = 0·027 m.,
13,596 = 367, L = 536, whence 424L = 227,264, and the second
portion of the equation 226,144, which is sufficiently near, within
the limits of experimental error, _see_ Chapter I., Note 11.) The
same equation is applicable to the dissociation of Na_{2}H and
K_{2}H--(Chapter XII., Note 42)--but it has only been verified in
this respect for Pd_{2}H, since Moutier, by calculating the amount
of heat L evolved, for _t_ = 20, according to the variation of the
tension (_dp_/_dt_) obtained 4·1 thousand calories, which is very
near the figure obtained experimentally by Favre (_see_ Chapter
XII., Note 44). The absorbed hydrogen is easily disengaged by
ignition or decreased pressure. The resultant compound does not
decompose at the ordinary temperature, but when exposed to air the
metal sometimes glows spontaneously, owing to the hydrogen burning
at the expense of the atmospheric oxygen. The hydrogen absorbed by
palladium acts towards many solutions as a reducing agent; in a
word, everything here points to the formation of a definite
compound and at the same time of a physically-compressed gas, and
forms one of the best examples of the bond existing between
chemical and physical processes, to which we have many times drawn
attention. It must be again remembered that the other metals of the
eighth group, even copper, are, like palladium and platinum, able
to combine with hydrogen. The permeability of iron and platinum
tubes to hydrogen is naturally due to the formation of similar
compounds, but palladium is the most permeable.

[9 bis] _Rhodium_ is generally separated, together with iridium, from
the residues left after the treatment of native platinum, because
the palladium is entirely separated from them, and the ruthenium is
present in them in very small traces, whilst the osmium at any rate
is easily separated, as we shall soon see. The mixture of rhodium
and iridium which is left undissolved in dilute aqua regia is
dissolved in chlorine water, or by the action of chlorine on a
mixture of the metals with sodium chloride. In either case both
metals pass into solution. They may be separated by many methods.
In either case (if the action be aided by heat) the rhodium is
obtained in the form of the chloride RhCl_{3}, and the iridium as
iridious chloride, IrCl_{3}. They both form double salts with
sodium chloride which are soluble in water, but the iridium salt is
also partially soluble in alcohol, whilst the rhodium salt is not.
A mixture of the chlorides, when treated with dilute aqua regia,
gives iridic chloride, IrCl_{4}, whilst the rhodium chloride,
RhCl_{3}, remains unaltered; ammonium chloride then precipitates
the iridium as ammonium iridiochloride, Ir(NH_{4})2Cl_{6}, and on
evaporating the rose-coloured filtrate the rhodium gives a
crystalline salt, Rh(NH_{4})_{3}Cl_{6}. Rhodium and its various
oxides are dissolved when fused with potassium hydrogen sulphate,
and give a soluble double sulphate (whilst iridium remains unacted
on); this fact is very characteristic for this metal, which offers
in its properties many points of resemblance with the iron metals.
When fused with potassium hydroxide and chlorate it is oxidised
like iridium, but it is not afterwards soluble in water, in which
respect it differs from ruthenium. This is taken advantage of for
separating rhodium, ruthenium, and iridium. In any case, rhodium
under ordinary conditions always gives salts of the type RX_{3},
and not of any other type; and not only halogen salts, but also
oxygen salts, are known in this type, which is rare among the
platinum metals. Rhodium chloride, RhCl_{3}, is known in an
insoluble anhydrous and also in a soluble form (like CrX_{3} or
salts of chromic oxides), in which it easily gives double salts,
compounds with water of crystallisation, and forms rose-coloured
solutions. In this form rhodium easily gives double salts of the
two types RhM_{3}Cl_{6} and RhM_{2}Cl_{3}--for example,
K_{5}RhCl_{6},3H_{2}O and K_{2}RhCl_{5},H_{2}O. Solutions of the
salts (at least, the ammonium salt) of the first kind give salts of
the second kind when they are boiled. If a strong solution of
potash be added to a red solution of rhodium chloride and boiled, a
black precipitate of the hydroxide Rh(OH)_{3} is formed; but if the
solution of potash is added little by little, it gives a yellow
precipitate containing more water. This yellow hydrate of rhodium
oxide gives a yellow solution when it is dissolved in acids, which
only becomes rose-coloured after being boiled. It is obvious a
change here takes place, like the transmutations of the salts of
chromic oxide. It is also a remarkable fact that the black
hydroxide, like many other oxidised compounds of the platinoid
metals, does not dissolve in the ordinary oxygen acids, whilst the
yellow hydroxide is easily soluble and gives yellow solutions,
which deposit imperfectly crystallised salts. Metallic rhodium is
easily obtained by igniting its oxygen and other compounds in
hydrogen, or by precipitation with zinc. It resembles platinum, and
has a sp. gr. of 12·1. At the ordinary temperature it decomposes
formic acid into hydrogen and carbonic anhydride, with development
of heat (Deville). With the alkali sulphites, the salts of rhodium
and iridium of the type RX_{3} give sparingly-soluble precipitates
of double sulphites of the composition R(SO_{3}Na)_{3},H_{2}O, by
means of which these metals may be separated from solution, and
also may be separated from each other, for a mixture of these salts
when treated with strong sulphuric acid gives a soluble iridium
sulphate and leaves a red insoluble double salt of rhodium and
sodium. It may be remarked that the oxides Ir_{2}O_{3} and
Rh_{2}O_{3} are comparatively stable and are easily formed, and
that they also form different double salts (for instance,
IrCl_{3},3KCl_{3}H_{2}O, RhCl_{3},2NH_{4}Cl_{4}H_{2}O,
RhCl_{3},3NH_{4}Cl1-1/2H_{2}O) and compounds like the cobaltia
compounds (for instance, luteo-salts RhX_{3},6NH_{3}, roseo-salts,
RhX_{3}H_{2}O_{5}NH_{3}, and purpureo-salts IrX_{3},5NH_{3}, &c.)
_Iridious oxide_, Ir_{2}O_{3}, is obtained by fusing iridious
chloride and its compounds with sodium carbonate, and treating the
mass with water. The oxide is then left as a black powder, which,
when strongly heated, is decomposed into iridium and oxygen; it is
easily reduced, and is insoluble in acids, which indicates the
feeble basic character of this oxide, in many respects resembling
such oxides as cobaltic oxide, ceric or lead dioxide, &c. It does
not dissolve when fused with potassium hydrogen sulphate. Rhodium
oxide, Rh_{2}O_{3}, is a far more energetic base. It dissolves when
fused with potassium hydrogen sulphate.

From what has been said respecting the separation of platinum and
rhodium it will be understood how the compounds of _iridium_, which
is the main associate of platinum, are obtained. In describing the
treatment of osmiridium we shall again have an opportunity of
learning the method of extraction of the compounds of this metal,
which has in recent times found a technical application in the form
of its oxide, Ir_{2}O_{3}; this is obtained from many of the
compounds of iridium by ignition with water, is easily reduced by
hydrogen, and is insoluble in acids. It is used in painting on
china, for giving a black colour. Iridium itself is more
difficultly fusible than platinum, and when fused it does not
decompose acids or even aqua regia; it is extremely hard, and is
not malleable; its sp. gr. is 22·4. In the form of powder it
dissolves in aqua regia, and is even partially oxidised when heated
in air, sets fire to hydrogen, and, in a word, closely resembles
platinum. Heated in an excess of chlorine it gives iridic chloride,
IrCl_{4}, but this loses chlorine at 50°; it is, however, more
stable in the form of double salts, which have a characteristic
_black_ colour--for instance, Ir(NH_{4})_{2}Cl_{6}--but they give
iridious chloride, IrCl_{3}, when treated with sulphuric acid.

[9 tri] We have yet to become acquainted with the two remaining
associates of platinum--ruthenium and osmium--whose most important
property is that they are oxidised even when heated in air, and
that they are able to give _volatile_ oxides of the form RuO_{4}
and OsO_{4}; these have a powerful odour (like iodine and nitrous
anhydride). Both these higher oxides are solids; they volatilise
with great ease at 100°; the former is yellow and the latter white.
They are known as _ruthenic_ and _osmic anhydrides_, although their
aqueous solutions (they both slowly dissolve in water) do not show
an acid reaction, and although they do not even expel carbonic
anhydride from potassium carbonate, do not give crystalline salts
with bases, and their alkaline solutions partially deposit them
again when boiled (an excess of water decomposes the salts). The
formulæ OsO_{4} and RuO_{4} correspond with the vapour density of
these oxides. Thus Deville found the vapour density of osmic
anhydride to be 128 (by the formula 127·5) referred to hydrogen.
Tennant and Vauquelin discovered this compound, and Berzelius,
Wöhler, Fritzsche, Struvé, Deville, Claus, Joly, and others helped
in its investigation; nevertheless there are still many questions
concerning it which remain unsolved. It should be observed that
RO_{4} is the highest known form for an oxygen compound, and RH_{4}
is the highest known form for a compound of hydrogen; whilst the
highest forms of acid hydrates contain SiH_{4}O_{4}, PH_{3}O_{4},
SH_{2}O_{4}, ClHO_{4}--all with four atoms of oxygen, and therefore
in this number there is apparently the limit for the simple forms
of combination of hydrogen and oxygen. In combination with
_several_ atoms of an element, or several elements, there may be
more than O_{4} or H_{4}, but a molecule never contains more than
four atoms of either O or H to one atom of another element. Thus
the simplest forms of combination of hydrogen and oxygen are
exhausted by the list RH_{4}, RH_{3}, RH_{2}, RH, RO, RO_{2},
RO_{3}, RO_{4}. The extreme members are RH_{4} and RO_{4}, and are
only met with for such elements as carbon, silicon, osmium,
ruthenium, which also give RCl_{4} with chlorine. In these extreme
forms, RH_{4} and RO_{4}, the compounds are the least stable
(compare SiH_{4}, PH_{3}, SH_{2}, ClH, or RuO_{4}, MoO_{3},
ZrO_{2}, SrO), and easily give up part, or even all, their oxygen
or hydrogen.