- Email: [email protected]
The air-oxidation of vanadiumIV in alkaline solution
Talenta, 1963. Vol. 10, pp. 793 to 799. Pergatnm Prem Ltd. Printed in Northern Ireland
THE AIR-OXIDATION ALKALINE Chemistry Department,
OF VANADIUMIV IN SOLUTION
G. A. DEAN and J. F. HERRINGSHAW Imperial College of Science and Technology, London S.W.7 England
(Received 2 March 1963. Accepted 21 March 1963) Summary-The air-oxidation of vanadiumrv in alkaline solution proceeds via the reduction of the aerial oxygen to peroxide. The initial rate of oxidation is proportional to the concentration of sodium hydroxide; ironr*r catalyses the reaction and chromiumx11 inhibits it. At concentrations of vanadium not less than O.O02N, the rate of oxidation is controlled by the rate of diffusion of aerial oxygen. Under suitable conditions, quantitative recovery of vanadium as vanadiumV is obtained.
No air-oxidation of vanadiumrv occurs in strongly acid solution,r but in alkaline solution the standard redox potential of the vanadiumIV-vanadiumV system becomes sufficiently less positive (-0.74 and -0*85v in IN and 3N sodium hydroxide, respectively? for such oxidation to occur. Jackson3 found that below pH 2.45 oxidation at 14” was negligible over a period of 1 week: above this pH, the rate of oxidation rapidly increased. EXPERIMENTAL Regents Solutions of vanadiumrV in O*lN-sulphuric acid were prepared from GPR vanadium pentoxide by reduction with sulphur dioxide,& and those of vanadiumV in 1N sulphuric acid from AnalaR ammonium metavanadate by the method of Rae, Murty and Rao .I All other reagents were of AnalaR quality. Anumber of reagents are known6 to contain small amounts of impurities that can reduce vanadiumP Here the chief source of this error is the sodium hydroxide; the content of reducible impurity of several specimens of GPR and AnalaR grade was about0*2peq/g (compared with about0.005 peq/g for AnalaR sulphuric acid). The concentration of this impurity was reduced to less than 0.01 req/g in the following way: 40 g of sodium hydroxide pellets were swirled with 70-80 ml of water; when dissolution was complete, the solution was heated to boiling and 1 g of sodium peroxide was added, in small portions at a time, with swirling. The peroxide decomposed rapidly, but not violently, and no frothing occurred. The solution was boiled gently for 30 min more and then allowed to cool, and was diluted to 250 ml to give 4N sodium hydroxide. Method The reaction between vanadiumrv and oxygen in alkaline solution was studied by adding sodium hydroxide solution to a stirred standard solution of vanadium Iv. After the desired time had elapsed, the solution was aciditied. Amperometric (dead-stop) determinations were then made of either the residual vanadiumxv by means of permanganate or the vanadiumv produced by means of iron”. Tests showed that the distilled water and stock solutions were saturated with oxygen. In distilled water the concentration of dissolved oxygen at saturation is 0.0012N at 15”; in solutions of electrolytes it is leas, being about 0.0006N in 2N sodium hydroxide at 15’.’ In a typical reaction mixture of 5-6 ml of 1N sodium hydroxide and 10 ml of 0*005N vanadiumlv in 0.1 N sulphuric acid at 15”, the concentration of dissolved oxygen was found to be 0.0011N. MECHANISM (A)
AND
KINETICS
Dissolved Oxygen in Excess
Table I gives a qualitative summary of many consistent determinations made with aliquots of the same solutions of vanadiumv. The results in (d) were the same 193
G. A. DEAN and J. F. HERR~NGSHAW
794
whether the chromiumrrr was added before or after the air-oxidation, but before acidifying. The differences between (c) and (b) or (d) and (b) increased with the concentration of vanadiumrv present at the beginning of the air-oxidation. TABLE I.-TITRATIONS BY IRONII IN ACID SOLUTION Titre compared with the direct titration of the vanadium”
Treatment (a) Chromiumm
added
the same
(6) VanadiumV alone, or with chromiumrrr added, and air-oxidation in 1N sodium hydroxide (c) Vanadium reduced to vanadiumIv and airoxidation in 1N sodium hydroxide
the same slightly high
(d) As (c), but chromiumrr* added (e) Chromiumnl
alone and air-oxidation
high as in (6) and (c)
nil
The high results in (c) and (a) cannot be caused by impurities in the reagents because there is no corresponding error in (b). The high result in (a) cannot be due to air-oxidation of chromiumrn [c$ (e)] or to the induced air-oxidation of chromiumrn because the same result is obtained by adding the chromiumr* immediately before acidifying. It is postulated that the high results in (c) and (d) are caused by the formation of peroxide during the air-oxidation of vanadium Iv. The reaction can be formulated as follows : 0, + 2VO,*- + 2H,O + 2VO,- + 20H’ + H,O,
(1)
HsO, + 2VOS2- + 2VO,- + 20H’
(2)
followed by :
Reaction (1) is undoubtedly the result of several successive reactions; 0,’ and HO, are probably formed as intermediates. Direct tests with vanadiumrv and hydrogen peroxide in alkaline solution showed that the two overall reactions are comparable in rate, but that reaction (2) is somewhat slow at the concentrations prevailing during the air-oxidation. Thus, provided an excess of oxygen is initially present, peroxide should accumulate during the airoxidation and some peroxide should remain when all the vanadiumrv has been oxidised. This is in agreement with the experimental results. The higher results in (d> compared with (c) are caused by the oxidation of the chromiumrrr by the peroxide formed. In (c) the peroxide formed reacts only very slowly with the ironrr in the titration (see below). Under the optimum conditions for the production of peroxide, i.e., in the presence of a large excess of dissolved oxygen, the amount of free peroxide remaining corresponds to not more than one quarter of the maximum allowed by reaction (1). As the initial concentration of vanadiumrV is increased, this proportion of free peroxide finally present decreases, and, with an excess of vanadiumrv, the peroxide remaining becomes negligible.
Air oxidation of vanadiumlv in alkaline solution
A detailed examination equation of the type
795
of the kinetics was not attempted.
-d[vanadi~IV]~d~
As expected, an
= ~~oxyge~][vanadiumrv]
does not describe the results satisfactorily. Effect of hydrogen peroxide
on the titration by iron”’
The peroxide formed in the alkaline oxidation is of less consequence because at the low concen~ations encountered here it reacts only slowly with iron*1 in acid solution. The results given in Table II were obtained for titrations of 20 ml of 0*~08~ vanadiumv by appro~mately 0=02N iron= at 20°C in a solution 1N to suiphuric acid and 0*5M to orthophosphoric acid. TABLEII
~nditions No addition of peroxide
Solution lo-‘N to peroxide Solution 10-sN to peroxide
Titre,
Titre calculated if the peroxide reacted quanti~tively with ironu,
mi
ml
0.880 0.883 O-9
0.995 2.05
The observed increases in the titres correspond to only a few per cent of the peroxide present. The presence of hydrogen peroxide at the end-point causes a drift in the values of the amperometric current because of the slow oxidation of irorP. Such a drift was detected when, in the experiments summarised in Table I, relatively large concentrations of vanadiumXv had been air-oxidised and this is additional evidence for the fo~ation of peroxide. The peroxide formed is usually too dilute to be detected by the colour of pervanadium. In the last titration mentioned above, the vanadium was initially present as pervanadium and the solution was distinctly red. The end-point coincided with the discharge of this colour. Eflect of some vor~ab~~~ The rate of oxidation by dissolved oxygen is greatly increased as the concentration of sodium hydroxide is increased, the initialrate being proportional to the concentration of sodium hydroxide. The results in Table III are for solutions initially 0.0008N to vanadiumIV at 15”. TABLEIII.-EFFECX OFCONFLATION OFSODIUM HYDROXIDE Normality of sodium hydroxide
OGO6
Percentage of vanadiumXv oxidised after 1 set after 10 set
’
0.056
0.18
o-37
0.92
1.9
3.8
-
-
12
18
30
65
90
26
48
76
95
100
100
100
Traces of iron catalyse the reaction, whilst chromium~ inhibits it (Table IV). The air-oxidation of chromiumur IS . very slow, e.g., 0401 equivalents- % per day for 0.05&f chromiumrrr in 1N sodium hydroxide. s
G. A. DEAN and J. F. HERRINGSHAW
796
TABLE IV.--EFFecr OF IRON AND CHROMIUM.
(All solutions initially OGOOSNto vanadium3 Percentage of vanadiumxv oxidised after 1 set in 0*3N-sodium hydroxide at 18” Added ironm, M Chromium absent Solution 0.031 M to chromiumru
nil
10-S
104
10-a
20
34
85
*
3
4
10
17
* When the concentration of ironrn is greater than 10-aM, precipitation occurs and oxidation of the vanadium is incomplete. In the presence of chromiumrrr, this precipitation is inhibited.
Between 15 and 50”, a rise of 10” increased the rate of reaction in 0*3N sodium hydroxide by a factor of l-4. (B) D@usion-controlled Conditions If the vanadiumvr is in excess of the dissolved oxygen, initially the vanadiumrv reacts rapidly with the oxygen in solution, and this is followed by a slower oxidation of the remaining vanadium Iv at a rate equal to the rate of diffusion of atmospheric oxygen into the solution. Experiments were made with solutions stirred without perturbation of the surface, in which the concentration of vanadiumrv was varied between O-003 and 0.008N, the liquid-air interface between 7.5 and 70 cm*, and the time between 3 and 30 min; consistent results were obtained for the rate of oxidation of vanadiumrv per cm8 of interface. The presence of iron rrr did not increase the rate of this oxidation. Between 15 and 50”, a rise of 10” increased the rate of oxidation by a factor of about l-15. The rate of air-oxidation of vanadiumrr in 2N sulphuric acid was the same as that of vanadiumIv in 0.3N sodium hydroxide. At higher concentrations of sodium hydroxide, the rate of oxidation was decreased as shown in Table V. TABLEV
Normality of sodium hydroxide
0.06
0.31
0.88
3.7
6.6
Pate of oxidation at 15”, peg. mir+. cm-p
0.30
0.31
0.31
0.16
0.05
Presumably the lower solubility of oxygen in the more concentrated accompanied by a reduced rate of diffusion. ANALYTICAL
solutions is
APPLICATION
As the oxidation of vanadium rv by oxygen in alkaline solution has been shown to be rapid and complete, and as the oxidant is inactivated on acidification, the reaction may be of use in the determination of vanadium. The oxidation of vanadiumrv to vanadiumv before titrating with ironI sulphate is a common operation in vanadium determinations and requires not only quantitative oxidation but also easy and complete removal of the excess of oxidising agent. The problem is unexpectedly difficult, as is evidenced by the continuous attention that it has received, and various reagents, namely nitric acid,s permanganate, persulphate,B perborate,lO bromate,ll perchloric
191
Air oxidation of vanadiumrv in alkaline solution
acid12 and ozone,13 have been proposed; all of these leave something to be desired, particularly when the concentration of vanadium is small. The recovery of vanadiumv after air-oxidation of vanadiumrv was determined as follows. For 40-mg quantities of vanadium, lO-ml aliquots of 0~073N vanadiumv solution were treated with sulphur dioxide in the presence of IN sulphuric acid, this treatment being followed by expulsion of the excess of sulphur dioxide in a stream of TABLEVI.-RECOVERY OF 40 mg OF VANADIUM*
0*02N iron” sulphate, ml Method Direct titre, ml
Titre after applyingmethod, ml
Air-oxidation
34.79 f 0*003 (3) 34.80 f 0.003 (3)
34.77 + 0.02 (3) 34.82 f 0.014 (4)
-0.06 f 0.07 +0.06 3 0.05
Permanganate-azide
33.76 f 0.003 (4)
33.67 f 0.016 (4)
-0.27
Silver-persulphate
3364 f 0.006 (4)
33.77 f 0.09 (6)
$0.39 f 0.28
Difference
* The f values are standard deviations from the mean ; the figures in parentheses of determinations.
%
f 0.05
are the number
carbon dioxide. The solution was made 0.5N to sodium hydroxide, which gave a total volume of about 35 ml. As the vanadiumrv IS . in excess of the dissolved oxygen, the solution was aerated until oxidation was complete. The solution was then made 1N to sulphuric acid and 05 M to phosphoric acid, and was titrated amperometrically by 0~02N ferrous sulphate (Table VI). For 2*4-mg quantities of vanadium, 10 ml of TABLE VII.-RECOVERY OF 2.4 mg OF VANADIUM
Direct titre
4.142 4.741
4.141 -
4.812 4.813
4.755 4.155
4.142 -
4.149 4.750
4.814 -
4.757 4.756
nil
+0.05
+0.03
$0.03
0.01 N ironn sulphate, ml
Titre after applying air-oxidation method, ml Difference,
%
0.004753N vanadiumv solution were diluted with 70 ml of 0.2N sulphuric acid. An excess (15 ml) of 20-vol hydrogen peroxide was added, and the solution was boiled until the decomposition of the pervanadium was complete and for a further 5 min. (This procedure converts vanadium, whatever its initial valency, into a mixture of -98 o/oof vanadiumv and ~2 o/oof vanadium Iv; all traces of peroxide are decomposed.l* The air-oxidation of the remaining vanadiumrv produces insufficient peroxide to affect the titration). The solution was allowed to cool to 20”, and 8 ml of 4N sodium hydroxide were added with swirling. After 5 min, the solution was acidified and titrated as above by O*OlNiron” sulphate (Table VII). The titres thus obtained were compared with the “direct titre” of an untreated aliquot of vanadiumv solution. Air-oxidation of the vanadiumv solution showed that less than 0.02% of the vanadium was present as vanadiumIv. In Table VI, the recovery of vanadiumv by two well-known method@ is presented for comparison.
798
G. A. DEANand J. F. HERRINGSHAW
Small differences between groups of direct titres are due to small differences in the concentrations of the iron” solutions used. The recovery of vanadiumv by air-oxidation is evidently quantitative under the conditions used. The reaction is thus a useful one where rapid and complete oxidation of vanadium in pure solution is required to be followed by a titration with a reducing agent. It is specially recommended for very dilute standard vanadiumv solutions which, under acid conditions, are often slightly reduced because of the presence of dust or organic impurities. The method has proved to be as satisfactory at vanadium concentrations of 3 x 10-6N. It cannot be used in the presence of ions that are precipitated by alkali. Small amounts of iron (i.e., insufficient to be precipitated by the alkali, p. 796) do not interfere. ChromiumIn interferes by being partially oxidised to chromiumvr. Thus, in the determination of 2.4 mg of vanadium, initially present as vanadiumrv, the presence of 2.4 mg of chromiumrrr caused a positive error of 2.5 ‘A; pre-treatment with hydrogen peroxide in dilute acid reduced the error to f l-1.5 %,* most of which is due to oxidation of chromium rrr during the pre-treatment rather than during air-oxidation. For smaller amounts of chromiumrrr, the error decreases approximately proportionally, but the exact value is somewhat dependent upon the conditions. With 2.4 mg of vanadium and O-1 mg of chromium, no error was discernible. Acknowledgement-We thank Imperial Chemical Industries Ltd. (Billingham Division) for awarding a bursary to one of us (G. A. D.), during the tenure of which this work was carried out. Zusammenfasstmg-Bei der Luftoxydation von Vanadin(IV) in alkalischer Lasung wird Luftsauerstoff zu Peroxyd reduziert. Die Anfangsgeschwindigkeit der Oxydation ist der NaOH-Konzentration proportional; Eisen(II1) beschleunigt die Reaktion, Chrom(II1) hemmt sie. Bei Vanadinkonzentrationen urn 0,002N ist die Oxydationsgeschwindigkeit durch die Andiffusion von Luftsauerstoff bestimmt. Unter geeigneten Bedingungen verliiuft die Oxydation zu Vanadin(V) quantitativ. R&urn&-L’oxydation a Pair du vanadiumn en solution alcaline s’effectue par la conversion de l’oxygene de Pair en peroxyde. Le taux initial d’oxydation est proportionnel a la concentration de la soude; le fern1 catalyse cette &action tandis que le chromern l’inhibe. Pour une concentration de vanadium de O,OO2N, le taux d’oxydation est soumis au taux de diffusion de l’oxygene.de pair. En op6rant dans des conditions convenables on peut recup&er dune man&e quantitative du vanadium sous forme de vanadiumv. REFERENCES 1 N. H. Furman, J. Amer. Chem. Sot., 1928,50,1675. * H. H. Willard and G. D. Manalo, Analyt. Chem., 1947,19,462. s 0. E. A. Jackson, Thesis, London University, 1934, p. 83. d F. P. Treadwell and W. T. Hall, Analytical Chemistry, Ninth English Edition. John Wiley and Sons, Inc., New York, 1942. Vol. II, p. 567. 5 V. P. Rao, B. V. S. R. Murty and G. G. Rao, 2. analyt. Chem., 1955,147,161. BL. Meites, Analyt. Chem., 1952,2&l, 1058; G. A. Dean, Canad. J. Chem., 1962,40,545. TInternational Critical Tables, Vol. III, p. 271. * G. L. Kelley, J. A. Wiley, R. T. Bohn and W. C. Wright, Znd. Erg. Chem., 1919, 11, 632. * We have foundlB that this error can be eliminated by keeping the solution hot for 10 min at pH 7 At this pH, the residual vanadiumIv after the peroxide treatment and before the air-oxidation. reduces any chromium v1 formed during the pre-treatment with hydrogen peroxide.
Air oxidation of vanadiumm in solution
799
9 G. L. Kelley and J. B. Conant, ibid., 1916, 8, 719. lo H. H. Willard and F. Fenwick, J. Amer. Chem. Sot., 1923,45, 84. I1 I. M. Kolthoff and E. B. Sandell, Znd. Erg. Chem., Analyt., 1930,2, 140. le H. H. Willard and R. C. Gibson, ibid., 1931,3, 88; G. F. Smith, Analyst, 1955, 80, 16. la H. H. Willard and L. L. Merritt, Jr., Znd. Eng. Chem., Anulyt., 1942,14,486. I4 G. A. Dean, Canad. J. Chem., 1961,39,1174. l5 G. L. Kelley and J. B. Conant, op. cit., ref. 9; H. H. Willard and P. Young, Znd. Eng. Chem., Analyt., 1932,4, 187. I6 G. A. Dean, Thesis, London University, 1959, p. 78 ff.
THE AIR-OXIDATION ALKALINE Chemistry Department,
OF VANADIUMIV IN SOLUTION
G. A. DEAN and J. F. HERRINGSHAW Imperial College of Science and Technology, London S.W.7 England
(Received 2 March 1963. Accepted 21 March 1963) Summary-The air-oxidation of vanadiumrv in alkaline solution proceeds via the reduction of the aerial oxygen to peroxide. The initial rate of oxidation is proportional to the concentration of sodium hydroxide; ironr*r catalyses the reaction and chromiumx11 inhibits it. At concentrations of vanadium not less than O.O02N, the rate of oxidation is controlled by the rate of diffusion of aerial oxygen. Under suitable conditions, quantitative recovery of vanadium as vanadiumV is obtained.
No air-oxidation of vanadiumrv occurs in strongly acid solution,r but in alkaline solution the standard redox potential of the vanadiumIV-vanadiumV system becomes sufficiently less positive (-0.74 and -0*85v in IN and 3N sodium hydroxide, respectively? for such oxidation to occur. Jackson3 found that below pH 2.45 oxidation at 14” was negligible over a period of 1 week: above this pH, the rate of oxidation rapidly increased. EXPERIMENTAL Regents Solutions of vanadiumrV in O*lN-sulphuric acid were prepared from GPR vanadium pentoxide by reduction with sulphur dioxide,& and those of vanadiumV in 1N sulphuric acid from AnalaR ammonium metavanadate by the method of Rae, Murty and Rao .I All other reagents were of AnalaR quality. Anumber of reagents are known6 to contain small amounts of impurities that can reduce vanadiumP Here the chief source of this error is the sodium hydroxide; the content of reducible impurity of several specimens of GPR and AnalaR grade was about0*2peq/g (compared with about0.005 peq/g for AnalaR sulphuric acid). The concentration of this impurity was reduced to less than 0.01 req/g in the following way: 40 g of sodium hydroxide pellets were swirled with 70-80 ml of water; when dissolution was complete, the solution was heated to boiling and 1 g of sodium peroxide was added, in small portions at a time, with swirling. The peroxide decomposed rapidly, but not violently, and no frothing occurred. The solution was boiled gently for 30 min more and then allowed to cool, and was diluted to 250 ml to give 4N sodium hydroxide. Method The reaction between vanadiumrv and oxygen in alkaline solution was studied by adding sodium hydroxide solution to a stirred standard solution of vanadium Iv. After the desired time had elapsed, the solution was aciditied. Amperometric (dead-stop) determinations were then made of either the residual vanadiumxv by means of permanganate or the vanadiumv produced by means of iron”. Tests showed that the distilled water and stock solutions were saturated with oxygen. In distilled water the concentration of dissolved oxygen at saturation is 0.0012N at 15”; in solutions of electrolytes it is leas, being about 0.0006N in 2N sodium hydroxide at 15’.’ In a typical reaction mixture of 5-6 ml of 1N sodium hydroxide and 10 ml of 0*005N vanadiumlv in 0.1 N sulphuric acid at 15”, the concentration of dissolved oxygen was found to be 0.0011N. MECHANISM (A)
AND
KINETICS
Dissolved Oxygen in Excess
Table I gives a qualitative summary of many consistent determinations made with aliquots of the same solutions of vanadiumv. The results in (d) were the same 193
G. A. DEAN and J. F. HERR~NGSHAW
794
whether the chromiumrrr was added before or after the air-oxidation, but before acidifying. The differences between (c) and (b) or (d) and (b) increased with the concentration of vanadiumrv present at the beginning of the air-oxidation. TABLE I.-TITRATIONS BY IRONII IN ACID SOLUTION Titre compared with the direct titration of the vanadium”
Treatment (a) Chromiumm
added
the same
(6) VanadiumV alone, or with chromiumrrr added, and air-oxidation in 1N sodium hydroxide (c) Vanadium reduced to vanadiumIv and airoxidation in 1N sodium hydroxide
the same slightly high
(d) As (c), but chromiumrr* added (e) Chromiumnl
alone and air-oxidation
high as in (6) and (c)
nil
The high results in (c) and (a) cannot be caused by impurities in the reagents because there is no corresponding error in (b). The high result in (a) cannot be due to air-oxidation of chromiumrn [c$ (e)] or to the induced air-oxidation of chromiumrn because the same result is obtained by adding the chromiumr* immediately before acidifying. It is postulated that the high results in (c) and (d) are caused by the formation of peroxide during the air-oxidation of vanadium Iv. The reaction can be formulated as follows : 0, + 2VO,*- + 2H,O + 2VO,- + 20H’ + H,O,
(1)
HsO, + 2VOS2- + 2VO,- + 20H’
(2)
followed by :
Reaction (1) is undoubtedly the result of several successive reactions; 0,’ and HO, are probably formed as intermediates. Direct tests with vanadiumrv and hydrogen peroxide in alkaline solution showed that the two overall reactions are comparable in rate, but that reaction (2) is somewhat slow at the concentrations prevailing during the air-oxidation. Thus, provided an excess of oxygen is initially present, peroxide should accumulate during the airoxidation and some peroxide should remain when all the vanadiumrv has been oxidised. This is in agreement with the experimental results. The higher results in (d> compared with (c) are caused by the oxidation of the chromiumrrr by the peroxide formed. In (c) the peroxide formed reacts only very slowly with the ironrr in the titration (see below). Under the optimum conditions for the production of peroxide, i.e., in the presence of a large excess of dissolved oxygen, the amount of free peroxide remaining corresponds to not more than one quarter of the maximum allowed by reaction (1). As the initial concentration of vanadiumrV is increased, this proportion of free peroxide finally present decreases, and, with an excess of vanadiumrv, the peroxide remaining becomes negligible.
Air oxidation of vanadiumlv in alkaline solution
A detailed examination equation of the type
795
of the kinetics was not attempted.
-d[vanadi~IV]~d~
As expected, an
= ~~oxyge~][vanadiumrv]
does not describe the results satisfactorily. Effect of hydrogen peroxide
on the titration by iron”’
The peroxide formed in the alkaline oxidation is of less consequence because at the low concen~ations encountered here it reacts only slowly with iron*1 in acid solution. The results given in Table II were obtained for titrations of 20 ml of 0*~08~ vanadiumv by appro~mately 0=02N iron= at 20°C in a solution 1N to suiphuric acid and 0*5M to orthophosphoric acid. TABLEII
~nditions No addition of peroxide
Solution lo-‘N to peroxide Solution 10-sN to peroxide
Titre,
Titre calculated if the peroxide reacted quanti~tively with ironu,
mi
ml
0.880 0.883 O-9
0.995 2.05
The observed increases in the titres correspond to only a few per cent of the peroxide present. The presence of hydrogen peroxide at the end-point causes a drift in the values of the amperometric current because of the slow oxidation of irorP. Such a drift was detected when, in the experiments summarised in Table I, relatively large concentrations of vanadiumXv had been air-oxidised and this is additional evidence for the fo~ation of peroxide. The peroxide formed is usually too dilute to be detected by the colour of pervanadium. In the last titration mentioned above, the vanadium was initially present as pervanadium and the solution was distinctly red. The end-point coincided with the discharge of this colour. Eflect of some vor~ab~~~ The rate of oxidation by dissolved oxygen is greatly increased as the concentration of sodium hydroxide is increased, the initialrate being proportional to the concentration of sodium hydroxide. The results in Table III are for solutions initially 0.0008N to vanadiumIV at 15”. TABLEIII.-EFFECX OFCONFLATION OFSODIUM HYDROXIDE Normality of sodium hydroxide
OGO6
Percentage of vanadiumXv oxidised after 1 set after 10 set
’
0.056
0.18
o-37
0.92
1.9
3.8
-
-
12
18
30
65
90
26
48
76
95
100
100
100
Traces of iron catalyse the reaction, whilst chromium~ inhibits it (Table IV). The air-oxidation of chromiumur IS . very slow, e.g., 0401 equivalents- % per day for 0.05&f chromiumrrr in 1N sodium hydroxide. s
G. A. DEAN and J. F. HERRINGSHAW
796
TABLE IV.--EFFecr OF IRON AND CHROMIUM.
(All solutions initially OGOOSNto vanadium3 Percentage of vanadiumxv oxidised after 1 set in 0*3N-sodium hydroxide at 18” Added ironm, M Chromium absent Solution 0.031 M to chromiumru
nil
10-S
104
10-a
20
34
85
*
3
4
10
17
* When the concentration of ironrn is greater than 10-aM, precipitation occurs and oxidation of the vanadium is incomplete. In the presence of chromiumrrr, this precipitation is inhibited.
Between 15 and 50”, a rise of 10” increased the rate of reaction in 0*3N sodium hydroxide by a factor of l-4. (B) D@usion-controlled Conditions If the vanadiumvr is in excess of the dissolved oxygen, initially the vanadiumrv reacts rapidly with the oxygen in solution, and this is followed by a slower oxidation of the remaining vanadium Iv at a rate equal to the rate of diffusion of atmospheric oxygen into the solution. Experiments were made with solutions stirred without perturbation of the surface, in which the concentration of vanadiumrv was varied between O-003 and 0.008N, the liquid-air interface between 7.5 and 70 cm*, and the time between 3 and 30 min; consistent results were obtained for the rate of oxidation of vanadiumrv per cm8 of interface. The presence of iron rrr did not increase the rate of this oxidation. Between 15 and 50”, a rise of 10” increased the rate of oxidation by a factor of about l-15. The rate of air-oxidation of vanadiumrr in 2N sulphuric acid was the same as that of vanadiumIv in 0.3N sodium hydroxide. At higher concentrations of sodium hydroxide, the rate of oxidation was decreased as shown in Table V. TABLEV
Normality of sodium hydroxide
0.06
0.31
0.88
3.7
6.6
Pate of oxidation at 15”, peg. mir+. cm-p
0.30
0.31
0.31
0.16
0.05
Presumably the lower solubility of oxygen in the more concentrated accompanied by a reduced rate of diffusion. ANALYTICAL
solutions is
APPLICATION
As the oxidation of vanadium rv by oxygen in alkaline solution has been shown to be rapid and complete, and as the oxidant is inactivated on acidification, the reaction may be of use in the determination of vanadium. The oxidation of vanadiumrv to vanadiumv before titrating with ironI sulphate is a common operation in vanadium determinations and requires not only quantitative oxidation but also easy and complete removal of the excess of oxidising agent. The problem is unexpectedly difficult, as is evidenced by the continuous attention that it has received, and various reagents, namely nitric acid,s permanganate, persulphate,B perborate,lO bromate,ll perchloric
191
Air oxidation of vanadiumrv in alkaline solution
acid12 and ozone,13 have been proposed; all of these leave something to be desired, particularly when the concentration of vanadium is small. The recovery of vanadiumv after air-oxidation of vanadiumrv was determined as follows. For 40-mg quantities of vanadium, lO-ml aliquots of 0~073N vanadiumv solution were treated with sulphur dioxide in the presence of IN sulphuric acid, this treatment being followed by expulsion of the excess of sulphur dioxide in a stream of TABLEVI.-RECOVERY OF 40 mg OF VANADIUM*
0*02N iron” sulphate, ml Method Direct titre, ml
Titre after applyingmethod, ml
Air-oxidation
34.79 f 0*003 (3) 34.80 f 0.003 (3)
34.77 + 0.02 (3) 34.82 f 0.014 (4)
-0.06 f 0.07 +0.06 3 0.05
Permanganate-azide
33.76 f 0.003 (4)
33.67 f 0.016 (4)
-0.27
Silver-persulphate
3364 f 0.006 (4)
33.77 f 0.09 (6)
$0.39 f 0.28
Difference
* The f values are standard deviations from the mean ; the figures in parentheses of determinations.
%
f 0.05
are the number
carbon dioxide. The solution was made 0.5N to sodium hydroxide, which gave a total volume of about 35 ml. As the vanadiumrv IS . in excess of the dissolved oxygen, the solution was aerated until oxidation was complete. The solution was then made 1N to sulphuric acid and 05 M to phosphoric acid, and was titrated amperometrically by 0~02N ferrous sulphate (Table VI). For 2*4-mg quantities of vanadium, 10 ml of TABLE VII.-RECOVERY OF 2.4 mg OF VANADIUM
Direct titre
4.142 4.741
4.141 -
4.812 4.813
4.755 4.155
4.142 -
4.149 4.750
4.814 -
4.757 4.756
nil
+0.05
+0.03
$0.03
0.01 N ironn sulphate, ml
Titre after applying air-oxidation method, ml Difference,
%
0.004753N vanadiumv solution were diluted with 70 ml of 0.2N sulphuric acid. An excess (15 ml) of 20-vol hydrogen peroxide was added, and the solution was boiled until the decomposition of the pervanadium was complete and for a further 5 min. (This procedure converts vanadium, whatever its initial valency, into a mixture of -98 o/oof vanadiumv and ~2 o/oof vanadium Iv; all traces of peroxide are decomposed.l* The air-oxidation of the remaining vanadiumrv produces insufficient peroxide to affect the titration). The solution was allowed to cool to 20”, and 8 ml of 4N sodium hydroxide were added with swirling. After 5 min, the solution was acidified and titrated as above by O*OlNiron” sulphate (Table VII). The titres thus obtained were compared with the “direct titre” of an untreated aliquot of vanadiumv solution. Air-oxidation of the vanadiumv solution showed that less than 0.02% of the vanadium was present as vanadiumIv. In Table VI, the recovery of vanadiumv by two well-known method@ is presented for comparison.
798
G. A. DEANand J. F. HERRINGSHAW
Small differences between groups of direct titres are due to small differences in the concentrations of the iron” solutions used. The recovery of vanadiumv by air-oxidation is evidently quantitative under the conditions used. The reaction is thus a useful one where rapid and complete oxidation of vanadium in pure solution is required to be followed by a titration with a reducing agent. It is specially recommended for very dilute standard vanadiumv solutions which, under acid conditions, are often slightly reduced because of the presence of dust or organic impurities. The method has proved to be as satisfactory at vanadium concentrations of 3 x 10-6N. It cannot be used in the presence of ions that are precipitated by alkali. Small amounts of iron (i.e., insufficient to be precipitated by the alkali, p. 796) do not interfere. ChromiumIn interferes by being partially oxidised to chromiumvr. Thus, in the determination of 2.4 mg of vanadium, initially present as vanadiumrv, the presence of 2.4 mg of chromiumrrr caused a positive error of 2.5 ‘A; pre-treatment with hydrogen peroxide in dilute acid reduced the error to f l-1.5 %,* most of which is due to oxidation of chromium rrr during the pre-treatment rather than during air-oxidation. For smaller amounts of chromiumrrr, the error decreases approximately proportionally, but the exact value is somewhat dependent upon the conditions. With 2.4 mg of vanadium and O-1 mg of chromium, no error was discernible. Acknowledgement-We thank Imperial Chemical Industries Ltd. (Billingham Division) for awarding a bursary to one of us (G. A. D.), during the tenure of which this work was carried out. Zusammenfasstmg-Bei der Luftoxydation von Vanadin(IV) in alkalischer Lasung wird Luftsauerstoff zu Peroxyd reduziert. Die Anfangsgeschwindigkeit der Oxydation ist der NaOH-Konzentration proportional; Eisen(II1) beschleunigt die Reaktion, Chrom(II1) hemmt sie. Bei Vanadinkonzentrationen urn 0,002N ist die Oxydationsgeschwindigkeit durch die Andiffusion von Luftsauerstoff bestimmt. Unter geeigneten Bedingungen verliiuft die Oxydation zu Vanadin(V) quantitativ. R&urn&-L’oxydation a Pair du vanadiumn en solution alcaline s’effectue par la conversion de l’oxygene de Pair en peroxyde. Le taux initial d’oxydation est proportionnel a la concentration de la soude; le fern1 catalyse cette &action tandis que le chromern l’inhibe. Pour une concentration de vanadium de O,OO2N, le taux d’oxydation est soumis au taux de diffusion de l’oxygene.de pair. En op6rant dans des conditions convenables on peut recup&er dune man&e quantitative du vanadium sous forme de vanadiumv. REFERENCES 1 N. H. Furman, J. Amer. Chem. Sot., 1928,50,1675. * H. H. Willard and G. D. Manalo, Analyt. Chem., 1947,19,462. s 0. E. A. Jackson, Thesis, London University, 1934, p. 83. d F. P. Treadwell and W. T. Hall, Analytical Chemistry, Ninth English Edition. John Wiley and Sons, Inc., New York, 1942. Vol. II, p. 567. 5 V. P. Rao, B. V. S. R. Murty and G. G. Rao, 2. analyt. Chem., 1955,147,161. BL. Meites, Analyt. Chem., 1952,2&l, 1058; G. A. Dean, Canad. J. Chem., 1962,40,545. TInternational Critical Tables, Vol. III, p. 271. * G. L. Kelley, J. A. Wiley, R. T. Bohn and W. C. Wright, Znd. Erg. Chem., 1919, 11, 632. * We have foundlB that this error can be eliminated by keeping the solution hot for 10 min at pH 7 At this pH, the residual vanadiumIv after the peroxide treatment and before the air-oxidation. reduces any chromium v1 formed during the pre-treatment with hydrogen peroxide.
Air oxidation of vanadiumm in solution
799
9 G. L. Kelley and J. B. Conant, ibid., 1916, 8, 719. lo H. H. Willard and F. Fenwick, J. Amer. Chem. Sot., 1923,45, 84. I1 I. M. Kolthoff and E. B. Sandell, Znd. Erg. Chem., Analyt., 1930,2, 140. le H. H. Willard and R. C. Gibson, ibid., 1931,3, 88; G. F. Smith, Analyst, 1955, 80, 16. la H. H. Willard and L. L. Merritt, Jr., Znd. Eng. Chem., Anulyt., 1942,14,486. I4 G. A. Dean, Canad. J. Chem., 1961,39,1174. l5 G. L. Kelley and J. B. Conant, op. cit., ref. 9; H. H. Willard and P. Young, Znd. Eng. Chem., Analyt., 1932,4, 187. I6 G. A. Dean, Thesis, London University, 1959, p. 78 ff.