COLUMBIUM-TANTALUM-MOLYBDENUM.

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indeed the exact determination of the quantity of titanium in its compounds is a matter of considerable difficulty.

§ III. COLUMBIUM-TANTALUM.

(826) COLUMBIUM, or NIOBIUM (Nb=975) was discovered in the year 1801 by Hatchett, who found it in a black mineral from Massachusetts, termed columbite. In the following year Ekeberg obtained a new metal, which he termed tantalum (Fa=137*5), from the tantalite and yttro-tantalite of Sweden.

These two metals were asserted by Wollaston to be identical— an opinion generally received until Rose showed that the American mineral contained a metallic acid different from that furnished by tantalite: this acid he termed the niobic (NbO) ; and its metallic constituent, niobium, is the columbium of Hatchett. Rose at the same time stated that associated with this was a second metallic acid, which he termed the pelopic, but this he has since ascertained to be a compound of the metal which he called niobium.

Columbium and tantalum have been but incompletely studied; they are too rare to need a detailed description here: they have a considerable analogy with silicon,-tantalic anhydride, according to Rose, having the formula TaÐ ̧.

Hermann supposed the yttro-tantalite of Siberia to contain a new metal, analogous to columbium, to which he gave the name of Ilmenium; but he has since proved the so-called ilmenic acid to be a mixture of the tantalic and columbic anhydrides.

§ IV. MOLYBDENUM: Mo=96, or Mo=48. Sp. Gr. from 8.615 to 8.636.

(827) The principal ore of molybdenum is the bisulphide, a mineral which occurs chiefly in Bohemia and in Sweden, in appearance much resembling plumbago, and hence its name, from μodúßdawa, “a mass of lead." Molybdenum is also occasionally found oxidized, in combination with lead, as molybdate of lead. The metal may be obtained by roasting the pure native sulphide in a free current of air; the sulphur passes off as sulphurous anhydride, whilst the molybdenum also combines with oxygen, and remains behind in the form of molybdic anhydride. If this be mixed into a paste with oil and charcoal, and exposed to the heat of a smith's forge, in a crucible lined with charcoal, it is reduced to

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:

OXIDES OF MOLYBDENUM.

the metallic state. In this form molybdenum is white, brittle, and very difficult of fusion. The anhydride may also be reduced by heating it to redness in a porcelain tube in a current of hydrogen when the pulverulent metal is heated in the open air it is gradually oxidized, and finally converted into molybdic anhydride. It is readily oxidized by nitric acid; if the metal be in excess, a soluble nitrate of the binoxide is obtained; if the acid predominate, the oxidation proceeds further, and molybdic acid is formed: aqua regia produces similar results. Molybdenum is also oxidized when fused with nitre, and molybdate of potassium is produced.

(828) OXIDES OF MOLYBDENUM.-Molybdenum forms three oxides; the protoxide (Mo), and the binoxide (Mo) are both possessed of basic characters: the third (Mo) reacts energetically upon bases, and yields well characterized salts.

The protoxide (Mo✪=112, or MoO=56) is precipitated from the solution of a molybdate in hydrochloric acid which has been reduced by means of a bar of zinc, on adding ammonia in excess ; it is thus thrown down as a black hydrate which absorbs oxygen from the air: it is soluble in a solution of carbonate of ammonium, but not in those of the fixed alkalies or their carbonates. It may also be obtained in the anhydrous form, by digesting molybdic anhydride with zinc and hydrochloric acid.

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The binoxide (Mo, 128, or MoO,=64) may be prepared by igniting a mixture of 2 parts of molybdate of sodium and 1 part of sal ammoniac, and digesting the mass in a solution of potash, to remove any undecomposed molybdic acid. The residue when well washed, is the pure anhydrous oxide, which has been reduced from molybdic acid by the hydrogen of the ammonia. It is of a dark brown colour, but it becomes purple if exposed to solar light; it is nearly insoluble in acids. The hydrated binoxide may be obtained by digesting molybdic anhydride mixed with copper filings, in hydrochloric acid; an excess of ammonia precipitates the oxide of a rusty-brown colour, whilst the copper is retained in solution. Hydrated binoxide of molybdenum is soluble in pure water, but is precipitated by the addition of any salt. The solution gelatinizes on keeping. The salts which this oxide forms with acids are of a reddish-brown colour, or, if anhydrous, are nearly black.

If a solution of tetrachloride of molybdenum (MoCl1) be added, drop by drop, to a concentrated solution of the acid-molybdate of ammonium, a deep blue precipitate of molybdate of molybdenum (MoO2,4 MoO3) is formed. This compound is soluble in water,

MOLYBDIC ACID-THE MOLYBDATES.

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but is precipitated by the addition of any saline body. The addition of a small quantity of a stannous salt to a soluble molybdate reduces the molybdic acid, and produces this beautiful blue compound, which may serve as a test of the presence of molybdic acid care must be taken not to add the tin salt in excess. Another molybdate of the binoxide of molybdenum (Mo 2 Mo✪) has a green colour.

(829) Molybdic Anhydride (Mo✪ ̧=144, or MoO ̧=72): Composition in 100 parts, Mo, 66·6; →, 33'4.-This compound is obtained in the form of an impure anhydride by roasting the sulphide of molybdenum at a low red heat; it remains behind as a dirty yellow powder; caustic ammonia dissolves the anhydride, leaving oxide of iron and other impurities. The ammoniacal solution crystallizes on evaporation, and by a low red heat the ammonia is expelled, leaving the anhydride behind, of a pale buff colour. The anhydride reddens moistened litmus-paper, and is sparingly soluble in water, forming a yellow solution. At a red heat it fuses to a straw-coloured glass of sp. gr. 3'49: it undergoes volatilization in open vessels, and is deposited on cool surfaces in brilliant transparent needles. No definite hydrate of molybdic acid is known. When precipitated from its salts by the addition of an acid, it may be redissolved, if the acid be added in excess with concentrated sulphuric acid it forms a yellow solution. It is also freely soluble in a solution of cream of tartar. Molybdic acid forms well characterized salts, both normal and acid. Those of the alkalies are soluble. Normal molybdate of ammonium crystallizes in colourless square prisms. An acid molybdate of ammonium [(HN) H ̧ 5 Mo→], crystallizes readily in six-sided prisms. Various anhydro-molybdates of the alkalies have been formed, which contain as many as 3, 4, and even 5 equivalents of the anhydride to 1 of fixed base. Molybdate of lead (PbMoO) occurs native in crystals of a yellow colour; it is soluble in nitric acid, and in solution of caustic potash if the alkali be in large excess.

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A solution of molybdate of ammonium may be advantageously employed in certain cases to detect the presence of very small quantities of phosphoric acid in solution. The solution suspected to contain the phosphate must be acidulated with nitric acid, and the molybdate then added. The liquid becomes yellow, and on boiling, deposits a yellow crystalline precipitate, consisting of molybdic and phosphoric acids in combination with ammonia. According to Sonnenschein it contains 6.747 per cent. of ammonia, and about 3 per cent. of P. Arsenic acid forms a similar

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OTHER COMPOUNDS OF MOLYBDENUM.

compound with molybdate of ammonium when the solutions are boiled.

Sonnenschein takes advantage of the insolubility of the phosphoric compound to detect small quantities of ammonia by its means. In order to prepare the test solution, he first procures the yellow precipitate, by adding molybdate of ammonium to an acidulated solution of phosphate of sodium, ignites the precipitate to expel the ammonia, adds nitric acid to the residue, in order completely to reoxidize any reduced molybdic acid, evaporates to dryness, and expels the nitric acid by ignition. A solution of carbonate of sodium is employed to dissolve the remaining mixture of phosphoric and molybdic acids, and the solution is supersaturated with hydrochloric acid. This liquid, it is stated, will easily detect the presence of 1 part of sal ammoniac in 10,000 of water. Salts of sodium are not affected by it, but strong solutions of the salts of potassium yield a similar yellow precipitate.

(830) SULPHIDES OF MOLYBDENUM.-Three sulphides of molybdenum are known, MoS,, MOS, and MoS1: the last two are sulphur-anhydrides.

Bisulphide of Molybdenum (MoS,=160, or MoS,=80; Sp. Gr. 46): Comp. in 100 parts, Mo, 60; S, 40.—This sulphide is the principal ore of the metal: it is a soft solid of a leaden-grey colour and metallic lustre. The bisulphide may also be formed artificially by heating molybdic anhydride in the vapour of sulphur. It is unchanged by heat in closed vessels, but if roasted in the open air, sulphurous anhydride is formed and is volatilized, while molybdic anhydride remains. Nitric acid decomposes it, and converts the metal into molybdic acid; oil of vitriol also decomposes it when boiled upon it, forming a blue solution, whilst sulphurous anhydride escapes.

The tersulphide (MoS,=192, or MoS ̧=96) is precipitated by transmitting sulphuretted hydrogen through a solution of a molybdate, and adding hydrochloric acid. It is of a dark-brown colour, and forms sulphur salts with the sulphides of the alkaline metals. The potassium salt crystallizes in magnificent iridescent crystals (K,MOS, or KS,MOS,). The tetrasulphide of molybdenum also combines readily with the sulphides of the alkaline metals.

A molybdous chloride (MoCl,=167), or protochloride of molybdenum (MoCl=83.5) is obtained by dissolving the protoxide in hydrochloric acid. A tetrachloride (MOC) is procured by heating the metal in a current of dry chlorine: it forms a red vapour, which sublimes in deliquescent fusible crystals, in appearance re

TESTS FOR MOLYBDENUM.

sembling those of iodine.

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It may also be obtained in solution by

dissolving the binoxide in hydrochloric acid.

A chloromolybdic acid sublimes in yellowish scales when the binoxide is heated in a current of chlorine. It is soluble both in water and in alcohol, and consists of (MoCl, 2 MoO3), or (MoO,Cl). Similar compounds may be formed with many acidifiable metals, such, for example, as tungsten, chromium, and vanadium.

(831) CHARACTERS OF THE SALTS OF MOLYBDENUM :

1. Little is known of the molybdous salts, or salts corresponding to the protoxide. They yield a dark-brown precipitate with the hydrates of the alkalies and their carbonates; the precipitate is soluble in excess of carbonate of ammonium, and is deposited again on boiling the liquid; sulphuretted hydrogen slowly produces a brown precipitate of hydrated sulphide which is soluble in sulphide of ammonium.

2.-The salts corresponding to the binoxide have a dark colour, and a metallic astringent taste. Infusion of galls produces with them a brownish-yellow solution; ferrocyanide of potassium gives a dark-brown precipitate; ammonia a rusty-brown precipitate of the binoxide.

3.-The molybdates yield characteristic reactions with zinc, tin, and copper. With zinc in dilute acid solutions, the liquid becomes first blue, then green, and finally black, after which the addition of ammonia produces a deposit of hydrated protoxide of molybdenum. The addition of a small quantity of stannous chloride in solution to a liquid containing a molybdate, produces a beautiful blue molybdate of molybdenum (MoO2,4 Mo✪3), but care must be taken not to have the tin salt in excess, or the precipitate becomes of a dull green. Copper filings in similar solutions reduce the molybdic acid to the binoxide, which is precipitated as a brown hydrate by ammonia. Before the blowpipe, the compounds of molybdenum yield, in the oxidating flame, a colourless bead with borax, and with microcosmic salt; in the reducing flame they give a brownish-red bead with borax, and a green one with microcosmic salt.

Molybdenum is usually estimated in the form of the bisulphide, of which 100 parts contain 60 of the metal.