(PROTO-) CHLORIDE OF PALLADIUM, PdCl.

This compound is produced when the metal is dissolved in as little nitrohydrochloric acid as possible; it forms a red solution from which crystals of the same color may be obtained by evaporation; it is completely decomposed by heat, yielding first an oxychloride, and afterwards the metal. Chloride of palladium is sometimes used in analysis for separating iodine from chlorine and bromine. This chloride forms double-salts with the chlorides of the alkali-metals.

The double chloride of palladium and potassium, PdCl. KCl, and that of palladium and ammonium, PdCl.NH,Cl, are slightly soluble in water, and insoluble in alcohol; they may be obtained in fine crystals.

The corresponding sodium salt, PdCl.NaCl, is deliquescent, and soluble in

water.1

BICHLORIDE OF PALLADIUM, PdCl,.

The bichloride is formed when the preceding compound is heated with an excess of aqua regia, and may be obtained as a brown crystalline mass by evaporation in vacио. It is very unstable, being decomposed when heated, even in a state of solution, into chloride of palladium and chlorine.

When solution of bichloride of palladium is mixed with chloride of potassium, or chloride of ammonium, a red precipitate of a double chloride is produced. A compound of palladium with carbon is formed when this metal is heated in the flame of a spirit-lamp.

REACTIONS OF PALLADIUM.-(Prot) oxide.-Potassa, soda, and their carbonates; brownish precipitate, soluble in excess; and reprecipitated from solution in the carbonates by boiling.

Ammonia and its carbonate; no precipitate except in the chloride, which gives a flesh-colored precipitate (ammonio-chloride of palladium) soluble in large excess, on standing.

Hydrosulphuric acid and sulphide of ammonium; black sulphide of palladium. Sulphate of iron and (proto-) chloride of tin; in concentrated solutions; a black precipitate of reduced palladium, and, in the case of tin, a green supernatant liquid.

Iodide of potassium; black precipitate of iodide of palladium.

Cyanide of mercury; yellowish-white precipitate of cyanide of palladium; produced after a time in acid solutions.

(Binoxide).-Chlorides of potassium and ammonium; brownish-red precipitate of a double salt, sparingly soluble in water and alcohol.

Solution of the bichloride of palladium evolves chlorine when heated, and is converted into the (proto-) chloride.

RHODIUM.

Sym. Rh. Eq. 52.2. Sp. Gr. 10.6.

$278. Preparation.-Rhodium is also found in the ores of platinum. It is extracted from them by dissolving the ore in aqua regia, precipitating the platinum by chloride of ammonium, neutralizing the solution with carbonate of soda, adding cyanide of mercury to separate the palladium, and evaporating the

1 A series of ammonia-compounds, similar to those of platinum, have been obtained from the chloride of palladium.

filtered liquid to dryness with excess of hydrochloric acid; the residue is treated with alcohol, which leaves the double chloride of rhodium and sodium undissolved, as a red-brown powder. This salt is heated in a bulb-tube through which a stream of pure hydrogen is passed; the rhodium is thus reduced to the metallic state, and the chloride of sodium may be washed away by water.

Properties.-Rhodium is a grayish-white ductile metal; it is exceedingly hard, and one of the most infusible of the metals; the oxyhydrogen blowpipe-flame only softens it.

This metal is unaltered in air at the ordinary temperature, but, at a red heat, is easily oxidized.

Pure rhodium is not attacked by acids, it even resists the action of aqua regia, which dissolves it, however, when alloyed with other metals.

A mixture of potassa and nitre converts rhodium into sesquioxide.

Rhodium is employed, on account of its hardness, for making the nibs of gold pens.

When heated with bisulphate of potassa, the double sulphate of rhodium and potassa is formed.

Two oxides of rhodium are known.

The Oxide, RhO, is produced when the finely divided metal is heated in air, but it then becomes partially converted into sesquioxide.

The Sesquioxide of rhodium, Rh,O,, is formed when aqua regia acts upon alloys of rhodium with other metals.

It may be prepared by fusing finely divided rhodium with a mixture of potassa and nitre, and washing the mass, first with water, then with a dilute acid. It is thus obtained as a black powder, which is not decomposed by heat. Hydrated sesquioxide of rhodium is obtained as a yellowish-brown gelatinous precipitate, when a solution of the sesquichloride is boiled with potassa.

Sesquioxide of rhodium combines with acids, forming salts which are red in concentrated, and pink in diluted solutions.

Several intermediate oxides of rhodium also exist.

It is also capable of playing the part of a weak acid, dissolving in alkalies, and forming salts termed rhodiates.

CHLORIDES OF RHODIUM.

(Proto-) chloride of rhodium, RhCl, is obtained by heating rhodium in air, and treating the product with hydrochloric acid, when the sesquioxide which is present is dissolved in the form of sesquichloride, and the (proto-) chloride is left as an insoluble reddish powder.

Sesquichloride of rhodium, Rh,Cl,, has a brownish-black color, and does not crystallize. It resists a pretty high temperature without decomposition, and dissolves in water to form a red solution.

Sesquichloride of rhodium forms crystallizable double-salts with the chlorides of the alkali-metals; these compounds are best prepared by heating, in a current of chlorine, a mixture of finely-divided rhodium with an alkaline chloride.

The name of rhodium is derived from the red color of its compounds (Sodov, a rose).

REACTIONS OF RHODIUM-(Sesquioxide).-Potassa, soda, and their carbonates; yellowish hydrate, by the former, on boiling, by the latter in the cold, after some time.

Ammonia and its carbonate; yellowish precipitate.

Hydrosulphuric acid and sulphide of ammonium (the former after some time); brown precipitate.

(Proto-) chloride of tin; dark-red brown color.

Iodide of potassium; similar reaction.

Chlorides of potassium and ammonium; pink precipitates.

Compounds of rhodium may easily be reduced to the metallic state by heating in an atmosphere of hydrogen; the reduced rhodium may be distinguished by its insolubility in aqua regia, and its solubility in fused bisulphate of potassa, to which it imparts a pink color.

IRIDIUM.

Sym. Ir. Eq. 99. Sp. Gr. 16.

§ 279. This metal, which has received the above name in consequence of the various colors of its compounds, occurs in native platinum, generally in combination with osmium, and its extraction will be described in the history of that metal. An alloy of iridium and platinum is also found in nature, crystallized in octohedra, the specific gravity of which is 22.3.

Iridium is obtained, by calcining the ammonio-chloride, in a spongy state, but it may be rendered more compact by pressure. It has a gray color, and its spec. grav. is about 16; but it is generally believed to be even heavier than platinum, from the high specific gravity of the alloy mentioned above.

Iridium is neither malleable nor ductile; it has not yet been fused, and is oxidized if heated and allowed to cool in air. Like rhodium, it is not attacked by acids, unless it be alloyed with platinum, or some other metal, when aqua regia dissolves it. It is oxidized by a mixture of potassa and nitre, and is capable of direct combination with chlorine. It is also attacked by bisulphate of potassa at a high temperature.

Finely divided iridium (iridium-black) possesses properties similar to those of platinum-black.

Iridium forms four oxides; viz: oxide, IrO; sesquioxide, Ir,O,; binoxide, IrO,; teroxide, IrO,.

The oxide is prepared by decomposing the double chloride of iridium and potassium with an alkaline carbonate, when it is obtained as a greenish-gray precipitate, which dissolves in acids.

This oxide is not decomposed by heat, but may be easily reduced by hydrogen. It is gradually oxidized when exposed to air.

Sesquioxide of Iridium, Ir,O,, is formed when the metal is oxidized by nitre, or by caustic alkalies. It may be prepared by heating the double chloride of iridium and potassium (IrCl, KCl) with carbonate of potassa, in an atmosphere of carbonic acid, and washing the residue with slightly acidulated water :

2(IrCl,.KCl)+4(KO.CO1)=Ir ̧0 ̧+6KCl+0+4CO,.

The sesquioxide is a black powder, which is reduced to the (prot-) oxide when heated; it is insoluble in acids, but combines with alkalies to form brown, unstable compounds, which are little known.

The most important oxide of iridium is the binoxide, IrO,, which is produced when solutions of the lower oxides are boiled in contact with air, or with nitric acid.

When solution of the sesquichloride is heated with potassa in contact with air, no precipitate is formed at first, but oxygen is gradually absorbed; the solution becomes blue, and deposits blue hydrated binoxide of iridium, IrO, 2HO.

This oxide resembles the binoxide of platinum in its chemical relations; its solutions have a deep red-brown color.

Teroxide of Iridium, IrO,, is obtained as a greenish precipitate when terchloride of iridium is decomposed by an alkali.

CHLORIDES OF IRIDIUM.

The chlorides of iridium correspond to the oxides.

The (Proto-) chloride is produced when chlorine is passed over finely divided iridium at a red heat; it is formed more readily, if the iridium be mixed with chloride of potassium. It has a dark-green color, and is insoluble in water. The double chlorides which it forms with the chlorides of potassium and ammonium are soluble and crystallizable.

Sesquichloride of Iridium, Ir,Cl, is formed by dissolving the sesquioxide in hydrochloric acid; it has a very dark color, is deliquescent, and uncrystallizable. It combines with the alkaline chlorides, forming soluble double-salts, which are decomposed by ebullition into soluble salts of the (proto-) chloride, and insoluble salts of the bichloride (which are precipitated).

The Bichloride (IrCl) is the product of the action of nitro-hydrochloric acid upon iridium (alloyed with other metals), or one of its oxides; it is soluble in water, and forms a yellowish-red solution.

This chloride also combines with the chlorides of the alkali-metals.

The double-salt of bichloride of iridium and chloride of potassium is soluble in water, and forms a red solution, from which very dark red octohedra may be obtained, having the composition IrCl,. KCl. HO.

The corresponding compound of ammonium is obtained as a very dark brown precipitate when chloride of ammonium is added to solution of bichloride of iridium; it may be dissolved in boiling water, and crystallized in octehedra. The red color of the ammonio-chloride of platinum is often due to the presence of this salt, which is not materially detrimental to an analysis, since the equivalent of iridium is nearly the same as that of platinum.

The precipitate of ammonio-chloride of iridium dissolves in solution of sulphurous acid, being converted into the double-salt of the (proto-) chloride.

Terchloride of Iridium, IrCl,, is produced when an oxide of iridium is dissolved, at a gentle heat, in very concentrated aqua regia.

It is a very dark brown, deliquescent substance, soluble in water, and capable of forming double-salts with the alkaline chlorides.

The compounds of iridium with sulphur correspond to the oxides and chlorides. A carbide of iridium also exists.

REACTIONS OF IRIDIUM.-(Binoxide).-The alkalies, when added in excess to solutions of iridium, produce à greenish color, becoming blue on exposure to air. The carbonates of soda and of ammonia, and the alkaline bicarbonates; similar reaction.

Carbonate of potassa; a brownish-red precipitate, redissolved spontaneously after some time; the color of the liquid alters as above, upon exposure to air. Hydrosulphuric acid, and sulphide of ammonium; brown precipitate, soluble in sulphide of ammonium.

(Proto-) sulphate of iron discolors the solution; after some time, a greenish precipitate.

(Proto-) chloride of tin; a light brownish precipitate.

Chlorides of potassium and ammonium; dark brown precipitates.

OSMIUM.

Sym. Os. Eq. 99.6. Sp. Gr. 10.

$280. This metal is prepared from the alloy of iridium and osmium (osmiridium), which is found in company with native platinum.

Preparation. The alloy is mixed with three parts of nitre, and strongly heated for about an hour. The fused mass (containing osmic acid, OsO,, and teroxide of iridium) is heated in a retort with a large excess of nitric acid; a considerable quantity of osmic acid distils over, and condenses in white crystals in the receiver. When no more osmic acid passes off, which may be known by the odor, the contents of the retort are mixed with water, the oxides of iridium and osmium collected on a filter, and dissolved in aqua regia. The solution is then treated with chloride of ammonium, which precipitates the two metals as double chlorides; these are suspended in water and subjected to a current of sulphurous acid, which reduces the bichloride of iridium to the state of soluble (proto-) chloride, whilst the double chloride of osmium and ammonium is left, and may be reduced by a heating in a current of hydrogen.

The solution containing the double chloride of iridium and ammonium yields crystals on evaporation, and by igniting these, metallic iridium is obtained.

Properties.-Osmium is a grayish, brittle metal, of spec. grav. about 10. When precipitated from its solutions, it has often a bluish color. It cannot be fused, nor volatilized, if air be excluded.

Osmium, in a finely divided state, absorbs oxygen from the air, and is converted into osmic acid; it takes fire in oxygen, even at 212° F. (100° C.).

When heated on platinum foil in the flame of a spirit-lamp, it is converted into osmic acid, which is volatile, and has a characteristic odor, whereby we are enabled to recognize small quantities of osmium.

Osmium dissolves in nitric acid, being converted into osmic acid.

The alkaline hydrates and nitrates, attack osmium, osmiates being produced. Five compounds of osmium with oxygen are known, viz: OsO, Os,O,, 030, OsO1, 080.

Oxide of Osmium (OsO), is obtained as a dark green precipitate when a solution of the double chloride of osmium and potassium is decomposed by potassa.

It is easily reduced by hydrogen, and dissolves in acids, forming green solutions. Sesquioxide of Osmium (0,0), is only known in combination with ammonia, and may be obtained by gently heating osmic acid with that reagent; the compound has a dark brown color, and, when boiled with solution of potassa, and subsequently washed, is very explosive. It dissolves in acids, yielding yellow compounds which do not crystallize.

Binoxide of Osmium (OsO,).—In order to obtain this oxide, a current of chlorine is passed over a mixture of osmium with chloride of potassium, when a compound of bichloride of osmium with chloride of potassium is obtained, which yields the binoxide when heated with carbonate of potassa.

Binoxide of osmium is black; it dissolves in acids, when freshly prepared, forming salts which are little known.

Osmious Acid (OsO,) is only known in combination; when an attempt is made to isolate it, it is decomposed into osmic acid and binoxide of osmium:

Osmite of potassa is obtained by decomposing a solution of the osmiate with reducing agents (alcohol or nitrite of potassa). It forms rose-colored crystals, soluble in water, but insoluble in alcohol; its aqueous solution absorbs oxygen from the air, and yields osmiate of potassa.

Osmite of soda is prepared in the same manner, and is more soluble than the osmite of potassa.

OSMIC ACID, OS0,-This acid may be prepared by heating osmium with nitric acid, in a retort, when the osmic acid condenses in the receiver in colorless prismatic crystals, which fuse and volatilize below the boiling point of water, yielding a vapor of a very peculiar, irritating odor, which is dangerous to the operator. Osmic acid should not be handled, for it destroys the skin. It is