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CHAPTER X

ELEMENTS OF GROUP V. (FAMILY A.)

Vanadium, V = 51.2; Niobium, Nb=94; Tantalum, Ta = 183.

THE three rare metals comprising this family are closely related to each other, and also to the elements of family B of the same group, namely, the nitrogen and phosphorus series.

Vanadium occurs in a few rare minerals, as vanadite, 3Pb(VO4)2, PbCl2 (the vanadium analogue of pyromorphite); pucherite, BiVO; mottramite, (PbCu)3(VO4)2,2(PbCu)(HO)2. Small quantities also occur in certain iron ores, the vanadium ultimately finding its way into the Bessemer slag, in which it has been found concentrated to the extent of 1.5 per cent.

Metallic vanadium was first isolated by Roscoe (1867), although its existence was previously discovered by Del Rio (1801). The metal is extremely difficult to obtain, as at a red heat it combines with oxygen with great readiness, yielding the pentoxide V2O, and also with nitrogen, forming the nitride VN. The element is prepared by heating the dichloride in a stream of perfectly pure hydrogen

VC12+H2=2HC1+ V.

Vanadium is unacted upon by air at ordinary temperatures, but when heated burns brilliantly to the pentoxide.

Niobium and tantalum are found associated together in the rare mineral tantalite or columbite. The first to be discovered was tantalum (Hatchett, 1801), and was originally named columbium; and the name niobium (from Niobe, the daughter of Tantalus) was given to the allied element by Rose (1846), Niobium is obtained by heating the trichloride, NbCl3, in a stream of hydrogen.

Vanadium forms five oxides, corresponding to the oxides of nitrogen, while three oxides of niobium and two of tantalum are known :—

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The pentoxides are obtained when the metals are burned in air or oxygen. They give rise respectively to vanadates, niobates, and tantalates, corresponding to nitrates and metaphosphates, thus

Sodium nitrate, NaNO3.

Sodium metaphosphate, NaPO3.
Sodium metavanadate, NaVO3.

Sodium metaniobate, NaNbO3.
Sodium metatantalate, NaTaО3.

The closer relation of these elements to phosphorus than to nitrogen is seen in the formation of salts derived from ortho- and pyro-acids, corresponding to orthophosphates and pyrophosphates. The naturally occurring vanadium compounds above mentioned are vanadates derived from the hypothetical orthovanadic acid, HVO. Both metavanadic acid, HVO, and pyrovanadic acid, HVO7, have been obtained. Unlike the phosphorus compounds, the metavanadates are the most stable of the three classes of salts, and the orthovanadates the least stable. The most important of these salts is the ammonium metavanadate, NH,VO3, which is prepared by dissolving the pentoxide in ammonia. This salt is insoluble in ammonium chloride, and use is made of this property in the preparation of vanadium compounds from the mineral mottramite. When ammonium metavanadate is ignited, vanadium pentoxide is obtained

2NH VO3=V2O5 +2NH3+ H2O.

Vanadium acts also as a feeble base.

Thus, when the tetroxide, or hypovanadic oxide, is dissolved in sulphuric acid, hypovanadic sulphate, V2O2(SO4)2 is formed. The solution of this salt possesses a rich blue colour. Vanadium forms three chlorides, having the composition

VC12 (or V2Cl); VClg (or VýCl¿); VCI.

Niobium gives a trichloride, NьCl, and pentachloride, NbC15, while only the pentachloride of tantalum is known, TaCl.

Vanadium forms a number of compounds with oxygen and chlorine. Thus, when vanadium tetrachloride is acted upon by water, it yields hypovanadic chloride, V2OCl2, which dissolves in the water, giving a blue solution.

Vanadium oxychloride, or vanadyl trichloride, VOCl3, corresponds to phosphorus oxychloride. POCl3. From vanadyl trichloride, by treatment with zinc, vanadyl dichloride is obtained, VOC, and by the action of hydrogen at a high temperature upon this, both vanadyl monochloride, VOCI, and divanadylmonochloride, VO¿Cl, are formed.

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Occurrence.-Chromium does not occur in nature in the uncombined state. In combination with oxygen and associated with iron, it is met with in considerable quantities in the mineral chrome iron ore, or chromite, Cr2O,FeO. This ore is the chief source of chromium compounds. Other natural compounds are crocoisite, PbCrO4, and chrome-ochre, Cr2O3. Traces of chromium are present in various minerals, such as the emerald and green serpentine, and impart to them their green colour.

Modes of Formation.-Although chromium compounds are manufactured for industrial purposes, the element itself has received no technical application.

It was obtained by Wöhler by the reduction of fused chromium chloride with metallic zinc, beneath a layer of fused sodium and potassium chlorides. The regulus, or alloy of zinc and chromium, was then treated with dilute nitric acid, whereby the zinc was dissolved, and the chromium was obtained in the form of a powder. Chromium may also be obtained as bright metallic scales, by the electrolysis of a solution of chromous chloride containing chromic oxide. The metal may be prepared by the reduction of the oxide, Cr2O3, by means of carbon at the high temperature of the electric furnace, or by heating the oxide with metallic aluminium.

Properties. Chromium is a hard, steel-grey metal, which is not oxidised in dry air. When heated in the oxyhydrogen flame it burns brilliantly. It dissolves in hydrochloric acid with evolution of hydrogen. The metal is not magnetic. The presence of

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2 T

minute quantities of chromium in steel imparts to the latter great hardness and tenacity.

Oxides of Chromium.-Two oxides of chromium are definitely known, namely

Chromium sesquioxide (chromic oxide)
Chromium trioxide (chromium anhydride)

.

Cr2O3.
CrO3.

The first is a basic, and the second an acidic oxide. Besides these two compounds, a hydrated oxide, derived from the unknown chromous oxide, also exists, having the composition CrO,H,O, or Cr(HO). It is obtained as a yellowish precipitate by adding potassium hydroxide to a solution of chromium dichloride (chromous chloride), with the exclusion of air. It rapidly absorbs oxygen, turning dark brown. When heated out of contact with air it is converted into the sesquioxide, with evolution of hydrogen-

2CrO,H2O=Cr2O3+ H2O+H2.

Other compounds of chromium and oxygen are described, whose composition, however, is not definitely established; thus, the product obtained as a brown powder, either by the partial reduction of the trioxide or the oxidation of the sesquioxide, is regarded by some chemists as chromium dioxide, CrO2, and by others as chromium chromate, Cr2O,CrOg. It is readily obtained by passing nitric oxide into a solution of potassium dichromate.

Chromium Sesquioxide, CrO3, is obtained as a grey-green powder, when either the hydroxide, or the trioxide, or ammonium dichromate is ignited (see page 230).

When the vapour of chromyl dichloride, CrOCl is passed through a red-hot tube, chromic oxide is deposited in the form of dark-green hexagonal crystals. Chromic oxide which has been strongly ignited is nearly insoluble in acids. It is used under the name of chrome green as a pigment, and for giving a green colour to glass.

Chromic Hydroxides.-Chromic oxide yields a number of hydrated compounds. When ammonia is added to a solution of chromic chloride, or other chromic salt, free from alkali, a light blue compound is precipitated, which, when dried over sulphuric acid, has the composition Cr.(HO)6,4H2O (or Cr.O,,7H,O). When this is dried in vacuo it loses 3H2O, and becomes Cr2(HO), H2O (or Cr2O3,4H2O); and on being heated at 200°, it again parts with 3H2O, and has the composition Cr2O, H2O.

When potassium dichromate and boric acid are heated to dull redness, and the mass treated with water, a rich green residue is obtained, having the composition Cr,O,2H,O. This compound, known as Guignet's green, is employed as a pigment.

The first two of these compounds, which may be looked upon as consisting of the hydroxide Cr2(HO), in a hydrated condition, namely, Cr,(HO),4H2O and Cr(HO), H2O, are readily soluble in acids, yielding the chromic salts.

Chromium Trioxide (chromic anhydride) CrO3.-When strong sulphuric acid is added to a cold saturated solution of potassium dichromate, the trioxide separates out in long, red, needle-shaped crystals

K2Cr2O7+H2SO4=K2SO4+H2O+2CrO3.

The liquid is decanted from the crystals, which are drained upon porous tiles, and the adhering sulphuric acid and potassium sulphate washed away by strong nitric acid. The crystals are finally heated upon a sand-bath, whereby the nitric acid is evaporated.

Chromium trioxide dissolves in water to the extent of 62 parts in 100 parts of water at 26°. It melts at a temperature about 192°. At 250° it begins to give off oxygen, and is ultimately converted into the sesquioxide

2CrO3 Cr2O3+30.

Chromium trioxide is a powerful oxidising agent, and in contact with most organic substances it is reduced. In the preparation of the compound, therefore, the liquid cannot be filtered through paper in the usual way. Warm alcohol dropped upon the trioxide at once takes fire, while in a more diluted condition it is oxidised to acetic acid; and the reduction of the chromium trioxide is made evident by the change of colour of the liquid, from red or yellow to olive green.

Gaseous ammonia reduces the trioxide to the sesquioxide, with formation of water and nitrogen

2NH3+2CrO3=Cr2O3+N2+3H,O,

the reaction being accompanied with the evolution of so much heat that the chromic oxide produced becomes incandescent.

When hydrogen peroxide is added to a dilute solution of

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