Detection and Estimation of Vanadium

Apart from naturally occurring ores of vanadium, vanadium steels, and ferrovanadium, the commonest compounds of vanadium are those which contain the element in the pentavalent state, viz. the pentoxide and the various vanadates. The analytical reactions usually employed are, therefore, those which apply to vanadates. Most vanadium ores can be prepared for the application of these reactions by digesting with mineral acids or by alkaline fusion with the addition of an oxidising agent. When the silica content is high, preliminary treatment with hydrofluoric acid is recommended. Vanadium steels and bronzes, and ferrovanadium, are decomposed by the methods used for other steels; the drillings are, for instance, dissolved in sulphuric acid and any insoluble carbides then taken up in nitric acid, or they are filtered off and submitted to an alkaline fusion. Compounds of lower valency are readily converted into vanadates by oxidation with bromine water, sodium peroxide, or potassium permanganate.

Wet Tests

When rendered faintly acid, colourless solutions of vanadates become yellow, and, with the addition of more acid, orange- red, in consequence of the formation of polyvanadates.
Hydrogen sulphide, on being passed through an acidified solution of a vanadate, does not give rise to a precipitate, but a blue solution results in consequence of the formation of a vanadyl salt. Blue solutions are obtained also by the action of other reducing agents. This reaction is not peculiar to vanadium compounds, since salts of molybdenum also give blue solutions on being reduced. In the case of vanadium, however, by the use of zinc, cadmium or aluminium, reduction can be made to proceed still further, with the formation of a green, and finally a lavender, solution.
Ammonium sulphide, on being added to a neutral solution of a vanadate, gives rise to a brownish-red solution which contains a thiovanadate. Ammonium sulphide cannot, however, be employed to separate those metals normally precipitated by this reagent, since vanadates of zinc, manganese and nickel are thrown down when the solution is rendered alkaline. When the thiovanadate solution is acidified, partial precipitation of the vanadium takes place in the form of sulphides or oxysulphides.
Ammonium chloride solution has no action on the alkali vanadates, but if sufficient of the solid is added to form a saturated solution, colourless ammonium metavanadate is precipitated.
Ammonium hydroxide is similarly without action on acid solutions of alkali vanadates, but if added to a vanadate solution which contains cations of ferric iron, aluminium, chromium, uranium, barium, etc., vanadates of these metals may be precipitated. In the ordinary process of group analysis, the vanadyl salt which is present in the filtrate from the hydrogen sulphide group may undergo reoxidation to the vanadate when this filtrate is warmed with nitric acid previous to adding ammonium hydroxide for the precipitation of the hydroxides of iron, aluminium, and chromium. This oxidation can be carried to completion by boiling with hydrogen peroxide solution. For the separation of vanadium from iron and aluminium, the precipitate first produced on addition of ammonium hydroxide is redissolved in nitric acid, and the precipitation is repeated two or three times. If the precipitate is boiled with ammonium phosphate, the vanadium passes into solution and leaves the iron and aluminium in the residue. Iron can also be separated from vanadium by the addition of excess of sodium carbonate and sodium peroxide to an acidified solution of the salts, whereupon the iron is thrown out as ferric hydroxide and the vanadium remains in the filtrate as vanadate. Traces of vanadium occluded in the precipitate can be removed by dissolving the latter in the minimum quantity of nitric acid and repeating the precipitation. An alternative method for the separation of aluminium from vanadium depends on the fact that a boiling dilute solution of sodium aluminate and sodium vanadate precipitates aluminium hydroxide when it is treated with a large excess of ammonium nitrate in small quantities at a time; addition of barium chloride to the filtrate precipitates barium metavanadate. If chromium is present it will have been oxidised to chromate by the hydrogen peroxide or sodium peroxide treatment, and will be left in solution with the vanadate after removal of iron and aluminium. In order to separate the chromium and vanadium, the acidified chromate-vanadate solution is reduced with sulphur dioxide, warmed with bromine water (which oxidises the vanadyl salt back to the vanadate without affecting the chromium salt), and then poured into a 10 per cent, solution of caustic soda. Chromium hydroxide is precipitated, while the vanadate remains in solution. An alternative method for the separation of chromium and vanadium consists in precipitating with lead acetate; acetic acid and hydrogen peroxide are added to the vanadate-chromate solution, whereby the chromate is reduced to the chromic salt; addition of lead acetate at this stage throws down lead vanadate, the chromium salt remaining in solution.
The colour change produced by the addition of hydrogen peroxide to a strongly acid solution of a vanadate constitutes the commonest qualitative test for the presence of vanadium. High concentrations of vanadium give rise to an intense reddish-brown coloration, while very low concentrations give a faint rose-red tint. The reaction is sufficiently delicate to detect 1 part of vanadium in 160,000. In applying the test it is necessary to remember that chromium, titanium and iron also produce comparable colorations. In the case of chromium, however, the further addition of ether gives a blue colour, whereas the colour due to vanadium is unaffected; addition of hydrofluoric acid or of ammonium fluoride destroys the colour due to titanium, and any yellow colour due to the presence of iron is removed by addition of phosphoric acid.
Tannin yields a rich blue coloration with vanadates in concentrations down to 2 mgm. of vanadium pentoxide per litre. Gallic acid and pyrogallol also give a blue colour. Commercial ethers, which contain vinyl alcohol, give rise to a rose coloration in concentrations down to 0.1 mgm. of vanadium pentoxide per litre, or even as low as 0.02 mgm. per litre if the proportion of vinyl alcohol is increased. "Cupferron" produces a red coloration with weak solutions of vanadium salts even when the dilution is 10^-6 mgm. per c.c. A 0.2 per cent, aqueous solution of diphenylamine in the presence of hydrochloric acid gives a violet coloration with aqueous solutions of vanadium compounds; this colour is not affected by the presence of nitrates, titanates, or iron, and detects vanadium in a solution which contains 0.0002 per cent. Addition of an extremely dilute solution of potassium thio-cyanate and a trace of sulphuric acid gives a yellow coloration which becomes blue with further addition of sulphuric acid; this reaction is sensitive to 1 part of vanadium pentoxide in 5000. Vanadates also give colour changes with resorcinol, quinine, morphine, strychnine, phenol, aniline, etc., and have, therefore, been employed from time to time as analytical reagents for these organic compounds.
Solutions of vanadates which are neutral or faintly acid with acetic acid readily yield precipitates of vanadates of the heavy metals. Silver nitrate, with a carefully neutralised solution, produces a curdy, reddish-brown precipitate, soluble both in ammonium hydroxide and in nitric acid. Mercurous nitrate throws down a yellow precipitate of mercurous vanadate, which is soluble in nitric acid. Lead acetate gives a yellow precipitate, which dissolves in nitric acid and becomes white on standing. Orthovanadates can be distinguished from metavanadates by the colours of the copper salts which they throw down on the addition of copper sulphate; metavanadates yield a yellow, crystalline precipitate, while orthovanadates yield an apple-green precipitate. These colours vary, however, with the acidity of the solution.
Solutions of pyrocatechol acetate and aniline, or pyrocatechol acetate and piperazine, are sensitive micro-reagents for vanadium.

Dry Tests

When heated in a bead of microcosmic salt in the oxidising flame, vanadium compounds impart a brownish-red coloration to the bead, which becomes orange on cooling; in the reducing flame a brownish-green colour is produced,
Vanadium compounds do not colour the ordinary flame.

Volumetric Methods

The most convenient and the usual method for the estimation of vanadium is a volumetric process. The vanadium is first obtained in acid solution as vanadate, and reduced to the tetravalent state by one of several reducing agents which are available. The solution is then titrated in the presence of sulphuric acid with hot potassium permanganate solution, which quantitatively oxidises the lower vanadium salt to the vanadate. Using sulphur dioxide to effect the reduction, the following reactions take place: -

(i) 2HVO₃ + SO₂ + H₂SO₄ = 2VOSO₄ + 2H₂O.
(ii) 10VOSO₄ + 2KMnO₄ + 12H₂O = 10HVO₃ + K₂SO₄ + 2MnSO₄ + 7H₂SO₄.

Excess of sulphur dioxide is removed before the titration by boiling the reduced solution in an atmosphere of carbon dioxide. Hydrogen sulphide can be used in place of sulphur dioxide and gives slightly higher results; excess is removed in the same manner by boiling, and the precipitated sulphur is removed before titrating. Hydrogen peroxide, when added in small proportions to a concentrated sulphuric acid solution of a pentavalent vanadium salt, also brings about reduction to the tetravalent state; excess of the hydrogen peroxide is decomposed catalytically by the vanadyl sulphate formed. Other reducing agents which have been employed are bismuth amalgam, mercury, sodium thiosulphate, concentrated hydrochloric acid, and electrolytic methods. The use of hydrochloric acid has not always given good results in the hands of different chemists, since reduction has a tendency to proceed lower than the tetravalent stage; the use of alcohol and hydrochloric acid has been recommended.

The estimation of a vanadate solution by direct titration with ferrous sulphate or ferrous ammonium sulphate solution has been worked out, and is found to be specially applicable to the analysis of vanadium alloys. The vanadate is again reduced to the tetravalent state by the ferrous salt, the end point being obtained by the use of potassium ferricyanide as internal indicator; alternatively, a known excess of the ferrous salt solution is added to the vanadate, the amount unused being titrated with potassium diehromate. This method facilitates the rapid estimation of vanadium in the presence of chromium. Knop's indicator (diphenylamine in sulphuric acid) has recently been successfully employed.

Vanadium lends itself also to iodometric methods of determination. The vanadate solution is reduced with hydrobromic acid, excess of potassium iodide is added, and the iodine thereupon liberated is estimated with sodium thiosulphate solution. The reaction is:

2HVO₃ + 2HBr + 4HCl = 2VOCl₂ + Br₂ + 4H₂O.

This method is also available for the estimation of vanadium and chromium together in solution. The use of hydriodic acid as the reducing agent gives rise to inconsistent results, as reduction does not stop at the tetravalent stage, but may proceed also to the trivalent stage, according to the experimental conditions. The care necessary in the application of iodometric methods renders it unlikely that they will come into general use.

Treadwell has recently shown that vanadium in acid solution is reduced quantitatively to the divalent state by electrolytically deposited cadmium, zinc amalgam, or lead amalgam, if air is carefully excluded; the reduced solution is run into excess of potassium permanganate and titrated with oxalic acid, or it may be oxidised to the tetravalent state by the addition of excess of copper sulphate solution and then titrated with potassium dichromate, using diphenylamine as indicator. A modification of this procedure consists in running the acidified vanadate solution through a Jones reductor which contains amalgamated zinc into an excess of ferric alum solution; the quantity of ferric salt thereby reduced is determined by back titration with potassium permanganate.

Other volumetric processes which have been worked out include the use of potassium ferrocyanide, potassium ferricyanide, titanous chloride, and stannous chloride. According to Rosenheim and Yang, vanadium pentoxide is best determined in solution by addition of excess of caustic soda and back titration with sulphuric acid at 100° C., using a-naphthophthalein as indicator.

The application of any of the foregoing processes involves the previous separation of other elements which may interfere. For details of these the reader is referred to standard works on analysis.

Electrometric Methods

Electrometric Methods have been applied for the estimation of vanadium alone and alloyed with other metals, e.g. iron, chromium, uranium. The reduced solution is either gradually oxidised by means of a suitable oxidising agent (potassium permanganate, ammonium persulphate, nitric acid), or the vanadate solution is gradually reduced with ferrous sulphate solution; the changes in the E.M.F. of a suitable cell indicate the end point.

Colorimetrie Methods

Colorimetrie Methods are used only for the estimation of very small percentages of vanadium, e.g. in vanadium steels and alloys. The most important depend on the intensity of the reddish-brown colour produced by the action of hydrogen peroxide on an acid vanadate solution. If chromium is present, an equal amount must be introduced into the standard vanadium solution under the same conditions of temperature, acid concentration, etc. Phosphoric acid is added to destroy any yellow colour due to ferric iron, and either hydrofluoric acid or ammonium fluoride to destroy any colour produced by titanium. A colorimetric method for the simultaneous estimation of small quantities of titanium and vanadium has also been worked out. Other colorimetric processes are based on (a) the formation of a yellow to black coloration, due to aniline black, in the presence of aniline hydrochloride and potassium chlorate or other oxidising agent, and (b) the orange coloration finally produced when an acid solution of a vanadate is brought into contact with strychnine sulphate.

Electrolytic Method

Truchot has developed an electrolytic process for the estimation of small quantities of vanadium in solution as vanadate; the solution is rendered faintly alkaline with ammonium hydroxide, and on passing the electric current a lower oxide is deposited, which is collected, converted to the pentoxide by heating in air, and weighed.

Arc Spectrum

Vanadium has been estimated in ores with fair accuracy by comparative measurement of the intensity of the lines in the arc spectrum.

Gravimetric Methods

The vanadium compound is converted into sodium vanadate by fusion or other method, and after separation from other salts (e.g. arsenate, molybdate, phosphate, chromate, tungstate) is precipitated from nearly neutral solution either as (a) mercurous vanadate or (b) basic lead vanadate. In (a), mercurous nitrate solution is added to the vanadate solution drop by drop until no further precipitation takes place; the mercurous vanadate so obtained is heated under a hood, whereupon the mercury is volatilised; the residue of pure vanadium pentoxide is weighed; hydrochloric acid should not be present. The results have a tendency to be high. In (b), lead acetate solution is added to a solution of the vanadate rendered faintly acid with acetic acid, whereupon all the vanadium is precipitated as a basic lead vanadate of variable composition. The precipitate is dissolved in nitric acid, the lead removed by boiling with sulphuric acid, and the filtrate, which contains vanadic acid, is then either evaporated to dryness and the residue weighed as V₂O₅, or the vanadic acid in it is estimated by a volumetric process. For the application of either of these methods, the removal of arsenic is effected by reducing the acidified solution of vanadate and arsenate with sulphur dioxide; the arsenic is then precipitated as sulphide with hydrogen sulphide, and the vanadium which remains in solution as the vanadyl salt is reoxidised to the vanadate for estimation. Molybdenum is separated by a similar process, except that the hydrogen sulphide is used under pressure. For the estimation of vanadium in the presence of a phosphate, mercury vanadate and phosphate are precipitated together, ignited, and the residue of vanadium pentoxide, after being weighed, converted into sodium vanadate and phosphate by fusion with sodium carbonate. The vanadate is converted into the vanadyl salt by reduction with sulphur dioxide and the phosphate determined by means of ammonium molybdate. Deduction of the equivalent quantity of phosphorus pentoxide from the weight of mixed oxides gives the vanadium content.

A general method for the separation of vanadium from arsenic, molybdenum, phosphorus, chromium, uranium, tungsten, and silicon, consists in precipitating these metals as their respective lead salts and digesting the precipitate with potassium carbonate, whereupon all the lead salts are decomposed with the exception of the lead vanadate.

The gravimetric estimation of vanadium in alkaline vanadate solutions has also been effected by precipitating as ammonium metavanadate in the presence of ammonium chloride. Precipitation is incomplete, however, unless the solution is quite saturated with ammonium chloride; the addition of alcohol is recommended. Other gravimetric processes which have been investigated include the precipitation of barium pyrovanadate, precipitation of silver metavanadate, precipitation of manganese pyrovanadate, and the use of cupferron.

The analysis of vanadium steels is effected by the application of one of the foregoing methods. Blank determinations on a steel free from vanadium but otherwise of the same approximate composition are used as a control. Iron and molybdenum are removed from hydrochloric acid solution by Rothe's ether separation method; chromium, nickel, copper, etc., are then precipitated as hydroxides by caustic soda, the filtrate containing the vanadium as vanadate. The method is modified for the simultaneous estimation of both vanadium and chromium in a vanadium-chromium steel.