Vanadium Pentoxide, V₂O₅

Vanadium Pentoxide, vanadic oxide, or vanadic anhydride, V₂O₅, is one of the commonest of the compounds of vanadium, and constitutes the starting material for the preparation of many other vanadium compounds. Its manufacture on the large scale, as a stage in the industrial production of ferrovanadium and metallic vanadium. A laboratory method for its extraction from vanadium minerals is as follows: The mineral is roasted strongly with a mixture of sodium carbonate and potassium nitrate. The aqueous extract of the fused product, which contains alkali vanadates together with the alkali salts of other acids, is first neutralised with nitric acid, to precipitate silicic acid and aluminium hydroxide, and then concentrated until most of the potassium nitrate is crystallised out. The mother-liquor is heated with barium chloride and ammonium hydroxide, whereupon barium vanadate, chromate, phosphate, arsenate, molybdate, tungstate and sulphate separate out. Treatment of these barium salts with sulphuric acid liberates the free acids, which are carefully neutralised with ammonium hydroxide and concentrated. The further addition of ammonium hydroxide at this stage yields white, crystalline ammonium metavanadate, NH₄VO₃, which is purified by repeated crystallisation. On being heated in a platinum crucible with access of air, ammonium metavanadate decomposes with formation of red, amorphous vanadium pentoxide, ammonia, and water:

2NH₄VO₃ = V₂O₅ + 2NH₃ + H₂O.

By dissolving the residue in caustic soda and repeating the precipitation and decomposition of ammonium metavanadate, a pure product can be obtained. It is necessary for the ammonium metavanadate to be quite free even from traces of organic matter, chlorides, phosphates, etc., as otherwise a mixture of the pentoxide and the lower oxides is obtained on ignition.

Chemically pure vanadium pentoxide is alternatively prepared by precipitating insoluble mercurous vanadate, HgVO₃, from a neutral solution of a vanadate, and distilling off the mercury, or by ignition of vanadium salts of volatile acids, for example, vanadium oxytrichloride, VOCl₃. The oxide also results from the oxidation of any of the lower oxides, or by the electrolysis of a solution of sodium vanadate or copper vanadate, using a divided cell; the last method yields a product of 98 per cent, purity.

By addition of mineral acids to solutions of vanadates, or by the hydrolysis of vanadium oxytrichloride, a reddish-brown, gelatinous precipitate of hydrated vanadium pentoxide is obtained. This is very similar in appearance to ferric hydroxide, Fe(OH)₃, and on examination is found to consist of very fine particles which cannot be washed free from the mother-liquor without undergoing peptisation to a colloidal solution.

Vanadium pentoxide can be obtained in two modifications: (a) red crystalline; (b) red or yellow amorphous.

(a) The red, crystalline variety is obtained from the amorphous form by fusing in a porcelain or platinum dish. After cooling, the mass is found to have decreased in volume and to have solidified to an intensely coloured, glistening mass of ruby-red, rhombic crystals, from 3 to 4 cm. long and from 2 to 3 mm. broad. Bleecker describes the solid mass as consisting of needles, arranged parallel and extending inward perpendicularly to each surface; the ends of the crystals meet at 45°, and a vertical section has the appearance of three pyramids each with its base extending outwards. The X-ray diffraction pattern has been examined.

An alternative method for the preparation of crystalline vanadium pentoxide consists in heating a mixture of the amorphous oxide and calcium fluoride to red heat in an open crucible over which is suspended another crucible, inverted, to act as a receiver. The inside of the latter becomes coated with shining, needle-shaped, yellow crystals, which also become reddish-brown on being heated.

Evaporation of the hydrochloric acid solution of the red, amorphous oxide may also give rise to crystals.

The density of the crystalline oxide at 20° C. is 3.56; other determinations gave 3.32 at 15° and 3.357 at 18° C. The oxide is not hygroscopic even after prolonged exposure under ordinary conditions. Its saturated solution in water contains 50 mgm. per litre, but the solubility is affected by the state of aggregation of the solid and by its tendency to form a hydrosol; trituration has given a sol which contained 910 mgm. per litre.

(b) The red, amorphous form of vanadium pentoxide is the form most frequently met with in the laboratory. Its preparation has been described above. It melts at 658° or 675° C. to a dark red liquid, but is not volatile even at high temperatures; it can be vaporised only in the electric furnace. The fused solid conducts electricity, with formation of hypovanadic oxide, VO₂; the electrical conductivity has been measured. The oxide absorbs water on exposure to the air, the amount taken up depending on the vapour-pressure of the surrounding atmosphere. The water of hydration can be removed by careful heating to 300° C. The dried oxide feels greasy to the touch and discolours the skin slightly. Its saturated solution contains between 0-90 and 1.25 gram per litre. The density of a solution containing 0.90 gram per litre is 0.9988 at 20° C. and 0.9978 at 26° C. The adsorptive power of vanadium pentoxide for helium, oxygen, hydrogen, carbon monoxide, and carbon dioxide at different temperatures has been measured.

The yellow, amorphous variety is unstable and its identity is a little doubtful. It is stated to be obtained sometimes from the red, amorphous form by evaporation of a solution of the latter in hydrochloric acid, and it may also result from the ignition of ammonium meta-vanadate or from the decomposition of vanadates by acids. According to Bleecker it is most conveniently prepared by the electrolytic decomposition of copper vanadate. It becomes brick red on being heated, and is similar to the red variety in its general properties, except that it appears to be less hygroscopic and less soluble; its saturated aqueous solution contains between 300 and 400 mgm. per litre.

Vanadium pentoxide dissolves in acids, both organic and inorganic, to form vanadyl or unstable vanadic salts, and in alkalis to produce ortho-, pyro-, meta-, and poly-vanadates. The physico-chemical changes involved when vanadium pentoxide is heated with various basic oxides in the powder state have been investigated by Tammann. On being digested with liquid ammonia slow absorption of ammonia takes place; the composition of the product has not been definitely established. The oxide also dissolves in alcohols to produce esters, and combines with methylamine and ethylamine to form compounds of the type 2(R.NH₂).V₂O₅, where R represents the alkyl radical.

Vanadium pentoxide is a powerful oxidising agent, and undergoes reduction in stages depending on the reducing agent employed and on other conditions of the process. In the absence of moisture it is reduced to hypovanadic oxide, VO₂, by sulphur dioxide, red phosphorus, and ammonia, while dry hydrogen, carbon monoxide, sulphur, and potassium cyanide, at varying temperatures and atmospheric pressure, yield vanadous oxide, V₂O₃. Hydrogen at 2500° C. and 75 atmospheres pressure yields hypovanadous oxide, VO. In acid solution reduction of vanadium pentoxide to the tetravalent state, which is characterised by the appearance of a blue colour, can be effected with quite a large number of reducing agents: sulphur dioxide, hydrogen chloride, hydrogen bromide, hydrogen iodide, hydrogen sulphide, nitrous acid, phosphorous acid, oxalic acid, tartaric acid, lactic acid, citric acid, hydrazine, hydroxylamine, alcohol, formalin, sugar, ferrous sulphate, sodium thiosulphate, and mercury. Sulphur dioxide is most commonly employed for the reduction; it works slowly in the cold but rapidly when the solution is heated:

V₂O₅ + SO₂ = 2VO₂ + SO₃.

Excess of sulphur dioxide can be removed by boiling in an atmosphere of carbon dioxide. In the presence of suspended carbonaceous matter reduction may proceed to the trivalent stage. With many of the above-mentioned reagents reduction proceeds quantitatively, so that vanadium pentoxide can be employed for the estimation of the reducing agent, e.g. for hydroxylamine and hydrazine; alternatively, these substances become available for the estimation of solutions of vanadium pentoxide and of vanadates. Dry hydrogen chloride in the presence of a dehydrating agent does not reduce vanadium pentoxide, but forms vanadium oxytrichloride:

V₂O₅ + 6HCl ⇔ 2VOCl₃ + 3H₂O.

Concentrated solutions of hydrochloric acid, however, dissolve vanadium pentoxide with the production of an intense brown coloration; addition of water gives a yellow solution which evolves chlorine on being warmed, the solution becoming blue:

V₂O₅ + 2HCl ⇔ 2VO₂ + Cl₂ + H₂O.

This reaction is reversible, so that for complete conversion to the tetravalent state the concentration of the hydrochloric acid must be maintained. Repeated evaporation of the solution to dryness gives a residue of hypovanadic oxide, VO₂. Volatilisation of the vanadium to a small extent as oxychlorides may also take place. Dilute hydrochloric acid has no reducing action on vanadium pentoxide. Hundeshagen has observed that the solution of vanadium pentoxide in hydrochloric acid dissolves gold and other noble metals. If the solution is neutralised, the gold is precipitated as a greyish-violet powder, which redissolves on adding more acid. The reaction is expressed:

3VOCl₃ + Au ⇔ 3VOCl₂ + AuCl₃.
Alkaline ⇔ Acid

With hydrobromic acid and hydriodic acid reduction may proceed to the trivalent state, and it has been shown that in the presence of acetic acid, hydrazine also produces vanadous oxide, V₂O₃, instead of hypovanadic oxide, VO₂:

V₂O₅ + H₂N.NH₂ = V₂O₃ + N₂ + 2H₂O.

Hydrogen also reduces pentavalent and tetravalent vanadium salts to the trivalent state in the presence of spongy platinum.

With mercury the following reaction takes place:

V₂O₅ + 2Hg + 3H₂SO₄ = 2VOSO₄ + Hg₂SO₄ + 3H₂O.

The equilibria for tri-, tetra-, and penta-valent vanadium in sulphuric acid solution have been studied by Auger.

Concentrated acid solutions of vanadium pentoxide are reduced to the tetravalent state by hydrogen peroxide, the peroxides of sodium, barium, magnesium, and by persulphates of potassium and ammonium. Acid solutions of vanadium pentoxide give rise to pervanadic acid with hydrogen peroxide.

Reduction of acid solutions of vanadium pentoxide to the tetravalent state also takes place with bismuth amalgam; magnesium gives the trivalent salts of vanadium, while by using zinc, zinc coated with cadmium, electrolytically deposited cadmium, or sodium amalgam in the absence of air, divalent vanadium salts are obtained in solution. Vanadous salts and hypovanadous salts are, however, much more conveniently prepared by electrolytic reduction of acid solutions of vanadium pentoxide in an atmosphere of carbon dioxide.

Vanadium pentoxide becomes markedly photo-sensitive when immersed in glycerol, benzaldehyde, cinnamic aldehyde, cuminol, or aqueous mannitol solution, and exposed to light. It blackens and undergoes reduction, giving rise, initially, to hypovanadic oxide, VO₂. With aqueous solutions of citric acid or tartaric acid carbon dioxide is evolved during the change.

Molten vanadium pentoxide is a corrosive substance and attacks most containers even when made of platinum, fused silica, or graphite.

When a soluble vanadate is treated with mineral acids, a red, curdy form of vanadium pentoxide is precipitated, which, on being shaken with water, appears to dissolve to a red liquid. This reaction gives rise to the following usual method for making a colloidal solution: Ammonium metavanadate, NH₄VO₃, is made into a paste with 10 per cent, hydrochloric acid of 10 per cent, concentration, and the resulting gel of vanadium pentoxide is washed repeatedly on the filter with distilled water until it assumes the colloidal form, i.e. until it is peptised, and in consequence passes through the filter. On shaking the residue at this stage with a large quantity of water, a beautiful, clear red hydrosol is obtained. Vanadium pentoxide hydrosols have been prepared also: (a) By hydrolysing the organic esters of vanadic acid; (b) by pouring fused vanadium pentoxide into water; (c) by passing the vapours of vanadium oxytrichloride, VOCl₃, into boiling water. The flakes of vanadium pentoxide produced are treated with a large quantity of distilled water. Sols prepared by these methods vary in colour from blood red to reddish-brown, but Wegelin also prepared a canary-yellow sol by prolonged trituration of the reddish-brown crystals of vanadium pentoxide obtained by slow cooling from the molten state.

Vanadium pentoxide sols may contain up to 1.25 gram of vanadium pentoxide per litre. They do not undergo change on being boiled with water, and are not precipitated by addition of alcohol. The particles are negatively charged, and on being electrolysed move towards the anode; the hydrosol behaves like solutions of vanadates and vanadium pentoxide in that it yields the ions of poly vanadic acids. The sol is very sensitive to the action of electrolytes, relatively low concentrations of which are sufficient to produce clouding within a few minutes. The flocculation value as a rule is lower the greater the value of the precipitating ion. Addition of several drops of mineral acid or of other electrolyte is followed by immediate adsorption of the positive ion with formation of vanadium pentoxide gel. If nitric acid has been added the gel can be reconverted into the sol with water - that is, the gelation is reversible; if sodium chloride or calcium chloride has been used the gel cannot be resolated. When treated with suitable concentrations of electrolytes a vanadium pentoxide sol sets to a stiff jelly. In these sols there is always a small amount of the oxide in molecular solution. This portion is not thrown down by electrolytes, and passes through a dialysing membrane. The relative flocculation values with some inorganic salts have been determined, and the cataphoresis at small electrolyte concentrations has been measured. Stiff, transparent gels of vanadium pentoxide have been prepared from its hydrosols by coagulation with a suitable electrolyte, or by dialysis; hydrosols of the vanadates of manganese, iron, aluminium and zinc can also be used.

Vanadium pentoxide sols can be employed to bring about coagulation of positively charged colloids; for example, ferric hydroxide and aluminium hydroxide. The amount necessary for the coagulation of a given quantity of the positive colloid is very small in comparison with the required quantities of arsenic trisulphide, antimony trisulphide, and other negative colloids. It appears, therefore, that the colloidal particles of vanadium pentoxide carry a relatively much larger electrical charge. The pentoxide sols on being treated with reducing agents furnish the sols of lower oxides of vanadium, which are also found to be negatively charged. The viscosity of vanadium pentoxide sols has been investigated.

Vanadium pentoxide sols display peculiar optical phenomena. A freshly prepared sol is quite clear; after ageing, however, the sol, on being stirred and observed by reflected light, appears to be filled with yellow, shining streaks, as though fine crystals were floating in it. When viewed by transmitted light the aged sol appears to be clear, although peculiar dark streaks can be clearly seen. On being further examined between crossed Nicols, the aged sol exhibits the striking property of double refraction; the field remains dark so long as the sol is not disturbed, but stirring, or placing the sol in a magnetic or an electric field, causes it to become bright at once. The double refraction produced is so strong that if a concentrated sol is caused to flow through a tube of triangular cross-section which is used as a prism, it is able to split up spectral lines; the red hydrogen line, for instance, is resolved in this manner into two oppositely polarised lines. Plates showing the appearance of vanadium pentoxide sols under the ultramicroscope and in polarised light are given by Zocher.

A possible explanation of the cause of the double refraction is afforded by examination of the freshly prepared and aged sols under the ultramicroscope. The freshly prepared sol contains only sub- microns below the limits of ultramicroscopic visibility; on ageing, however, the concentration of the molecularly dispersed vanadium pentoxide decreases, and there begin to grow in the sol elongated, rod-like needles the length of which is approximately thirty times the diameter. These new particles are unquestionably crystalline, and possess slow Brownian movement, but their formation is not the ordinary process of crystallisation, because the ageing of the sol is also accompanied by changes in the dielectric constant, the specific inductive capacity, the electrical conductivity, the sensitivity towards electrolytes, and the viscosity. The growth of the rods is due to the aggregation of non-spherical primary particles in parallel layers. With further ageing the red colour of the sol finally changes to yellow, and yellow solutions are devoid of colloidal particles. It is generally thought that the phenomenon of double refraction is due to the appearance of these ultramicroscopic needles, the longitudinal axes of which become orientated so as to be coincident with the optical axes when the sol is disturbed. A similar bi-refringenee in sols of several slightly soluble substances which ordinarily form microscopic crystals, e.g. mercurous chloride, Hg₂Cl₂, and lead iodide, PbI₂, has been demonstrated by Reinders. Freundlich does not agree, however, that these rod-like structures are necessarily responsible for the double refraction; he attributes the chief cause to the presence of aggregates of amicronic non-spherical particles, not discernible under the ultramicroscope and separated from one another by amicronic distances. It is adduced in support of this view that double refraction can be produced in and removed from sols of benzopurpurin without any change being visible in the ultramicroscope. Double refraction is also shown, although not so strongly, by aged- ferric hydroxide sols, aluminium hydroxide sols, clay suspensions, soap solutions, alizarin, aniline-blue, p-azoxyanisole, p-azoxyphenetole. Red gold sols and silver sols also become doubly refracting under the influence of an alternating current.

Several hydrates of vanadium pentoxide have been obtained by the action of mineral acids on solutions of alkali vanadates. In many cases their composition agrees with that of one of the free acids corresponding to the vanadates, but it has been shown that they are not true acids; the amount of water present depends only on the conditions of drying. Von Hauer and Fritzsche both obtained an insoluble dihydrate, V₂O₅.2H₂O, which was supposed to be free pyrovanadic acid, H₄V₂O₇. By continuing the drying of this hydrate over concentrated sulphuric acid von Hauer obtained the monohydrate, V₂O₅.H₂O, which similarly was supposed to be free metavanadic acid, HVO₃. Ditte obtained several other hydrates, V₂O₅.8H₂O, V₂O₅.2H₂O, V₂O₅.H₂O, the composition of which also depended on the vapour-pressure of the atmosphere. A hydrate corresponding to orthovanadic acid, H₃VO₄ or V₂O₅.3H₂O, has not been obtained.

When hydrated vanadium pentoxide is precipitated under special conditions it forms a yellow or orange substance known as vanadium bronze. Boiling a solution of sulphur dioxide with copper vanadate gives scales of a golden or orange colour. The precipitate obtained may, however, be a partial reduction product of the vanadate, in which case its composition would be analogous to that of the tungsten bronzes. A vanadium bronze is also obtained when a solution of ammonium metavanadate is added to a solution of copper sulphate in excess of ammonium chloride until a permanent precipitate forms, the suspension then being heated to 75° C. The more slowly the precipitation takes place the more brilliant is the colour of the bronze. Guyard states that the bronze is not a hydrate of vanadium pentoxide, but an acid ammonium vanadate.