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Electrochemical investigations in molten potassium disulphate at 430°C
J. ElectroanaL Chem., 127 (1981) 169-177
169
Elsevier Sequoia S.A., Lausanne--Printed in The Netherlands
ELECTROCHEMICAL INVESTIGATIONS IN MOLTEN POTASSIUM D I S U L P H A T E A T 430°C
P R O P E R T I E S OF V A N A D I U M SPECIES
ALAIN DURAND, GERARD PICARD and JACQUES VEDEL Laboratoire d'Electroehimie A nalytique" et Appliqube de I'E.N.S.C.P., (Laboratoire associb au C.N.R.S. No. 216), Ecole Nationale SupOrieure de Chimie de Paris, 11, rue Pierre et Marie Curie, 75005--Paris (France)
(Received 10th February 1981)
ABSTRACT Properties of vanadium compounds dissolved in molten potassium disulphate have been studied at 430°C as a function of solvent acidity. Potentiometric and voltammetric measurements, and electromigration experiments, allowed the determination of the formulae and stability of the species existing in the melt. The species, in acidic and basic medium are, respectively, VO2SO4 and V02(SO4)32 for V(V) and VOSO4 and VO(S04) 4- for V(IV). The corresponding acido-basic constants are, respectively, K v = 10 -t2.9_+o.t)and Kiv = 10 -0.3-+oA).The formal potential of the V(V)/V(IV) system is E ° =0.26___0.01 V [vs. Hg(I)/Hg(II) electrode] at pSO4=3.2. The oxidation of sulphur dioxide to sulphur trioxide is discussed by consideration of the species existing in the melt.
INTRODUCTION Industrial catalysts used to p r o m o t e the oxidation of sulphur dioxide usually contain v a n a d i u m pentoxide and potassium sulphate and are used at temperatures between 400 and 500°C. It is n o w assumed, f r o m Boreskov's works, that sulphur trioxide, reacting with potassium sulphate, forms a liquid film which dissolves v a n a d i u m pentoxide and serves as reaction m e d i u m [ 1 ]. Molten potassium disulphate is not an inert solvent. It undergoes a partial dissociation b y the equilibrium: $207a - (1) ~=z SO3(g ) + SO42- (1)
(1)
Therefore, molten potassium disulphate belongs to the class of solvents having an acidity system (solvoacidity), acids and bases being defined b y the exchange of SO4a ion, which takes part in equilibrium (1). I n previous work [2], it has been shown that acidity, defined b y pSO 4 = -log[SO 2-] where [SOaz - ] (mol kg - 1 ) is the concentration of free sulphate in the melt, could be measured f r o m the position of the voltamperometric reduction curve of the solvent, 0022-0728/81/0000-0000/$02.50 © 1981 Elsevier Sequoia S.A.
170
at a rotating gold micro-electrode. This allowed the determination of the dissociation constant Ki,p of equilibrium (1): Ki, p = P(SO3)[SO 2-] = 10 -3.2 atm mol kg -1
(2)
as well as the solubility S of potassium sulphate: S = 0.25 mol kg
1
Accordingly, acidity in molten potassium disulphate can vary between p S O 4 z 0.6 (basic medium) and p S O 4 = 3.2 (acidic medium), corresponding respectively to a saturated solution of potassium sulphate and to a sulphur trioxide saturated solution [P(SO3) = 1 atm]. In spite of its narrowness this acidity range allows a modification of chemical properties of solutes, as has been shown for dissolved species of P(V) [3]. The purpose of this work is to establish the nature of the vanadium species that can exist in this melt, the oxidation state and form of complexes, as a function of the acidity of the medium, and also to determine the acid-base and redox potential characterizing the possible exchange reactions. The formulae of the complexes allow an interpretation of observations dealing with the oxidation of sulphur dioxide to be proposed. EXPERIMENTAL
The electrochemical vessel is an airtight Pyrex cell which allows monitoring of the atmosphere above the melt. The cell is placed in a thermoregulated vertical furnace (t=430__+ I°C). Voltammograms were obtained by means of a Tacussel threeelectrode potentiostat, with linear sweep generator (scan rate 0.1 V min-~) and analogue recorder. The working electrode was a rotating gold micro-electrode, area 1 mm 2, rotation rate 500 rpm. The reference electrode was the m e r c u r i c / m e r c u r o u s sulphate/saturated potassium sulphate electrode described in our previous work. An electromigration cell has been used to determine the charge carried by certain vanadium complexes. It had three compartments, separated by fritted discs. The vanadium species to be observed was introduced in the central part and an electric potential difference was applied between two gold electrodes immersed in the side compartments. The cell was immersed in a thermoregulated potassium disulphate melt. The concentration of the V(IV) saturated solution was measured after dissolution in water of quenched samples. Titration was made by oxidation with Ce(IV) solution in the presence of ferricyanide [4]. Reagents were Merck p.a. hydrated vanadyl sulphate (VOSO 4, 3.5 H20), Merck p.a. vanadium pentoxide and Prolabo RP sodium metavanadate. RESULTS A N D DISCUSSION
Solute properties Vanadium(IV) solutions are green, and the V(V) solutions are dark red. Air bubbling in a V(IV) solution slowly changes its colour to dark red. Conversely,
171
sulphur dioxide reduces V(V) into V(IV), apparently faster than oxygen oxidizes V(IV). Vanadyl sulphate has a limited solubility in acidic medium. The solubility of VOSO 4 was determined by analysing saturated solutions. The experimental value for the solubility is S(VOSO4) = 0.12 mol kg 1 for pSO 4 = 3.2, and is > 1 mol kg 1 for pSO4 = 0.6. The solubility of V(V) compounds is > 1 mol kg - ~ for any value of the acidity.
Redox properties Redox properties were studied by means of voltammetric curves obtained with rotating gold micro-electrodes. Addition of VOSO 4 leads to a single oxidation wave, the limiting diffusion current of which is proportional to the concentration of the added vanadium (Fig. 1). Similarly, addition of V(V) leads to a single proportional reduction wave. Plotting the gold electrode potential vs. log(j--Jox)/(Jred--j) (wherejox, Jred a n d j are respectively the limiting current densities for V(V) and V(IV) and the current value at the considered potential), gives a straight line, with a slope equal to 0.135 +__0.005 V, in good agreement with the theoretical slope (0.140 V) derived for the reaction: V(V) + e ~=~V(IV) at 430°C. The same result is valid for any acidity value. Thus, in agreement with Tandy [5], V(IV) and V(V) are the only existing oxidation states of vanadium in molten potassium disulpahte. The proposition of Frazer and Kirkpatrick [6] who assumed a disproportionation of V(IV) into V(III)
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~ . . . . . .
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~oc
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fr 0,2 a_
_o//
I I r I I
_5C
/
.50
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pofenfiat/ V
b-
2
3
4
102 IV(]V)] / mot
5
kg "I
'Fig. 1. Oxidation of vanadium(V) in molten potassium disulphate (pSO4 =0.6), for various concentrations: (a) current-potential curves (gold micro-electrode); (b) variation vs. concentration of limiting current.
172 and V(V), has to be rejected. Such a reaction would lead to the formation of an oxidation wave (V(III) ~ V(V)) and of a reduction wave (V(V) ~ V(III)) the logarithmic plot of which has a slope of about 0.070 V.
Acido-basic properties of V(IV) For pSO 4 values ranging from 3.2 to 2.9, addition of VOSO 4 does not shift the reduction curve of the solvent. This compound dissolves, then, without solvolysis. In basic solution an excess of potassium sulphate (the strong base of the solvent) is dissolved by vanadyl sulphate. The dissolution reaction may be written: V O S O 4 _}_ nSO42_
--,
2nVO(SO4).+,
(3)
Equation (3) is an acidobasic reaction of the acid VOSO 4 and the strong base of the solvent SO42- . The n value was determined by acidometric titration in situ. The acidity of a basic solution (pSO 4 slightly greater than 0.6) was thoroughly determined. Then a known mass of K2SO 4 was added to the melt and back-titrated by vanadyl sulphate, until the initial pSO 4 value was reached again. Several determinations gave n = 2-+- 0.3 and consequently, the V(IV) species existing in the melt may be VOSO 4 in acidic medium and VO(SO4) 4 in basic medium.
Acido-basic properties of V(V) In acidic medium, additions of sodium metavanadate involve a decrease in pSO 4. Conversely, vanadium pentoxide does not change the acidity in acidic medium but does change it in a basic medium. Titration of a (SO 3 + V205) mixture by potassium sulphate allowed the determination of the reaction stoichiometry, after deduction of the quantity of K2SO 4 needed to neutralize the excess of sulphur trioxide. The reaction m a y be written: V205 + 2SO42- ~ 2VO3 + S~O72-
(4)
Characterization of the dissolved species The species dissolved in the melt may be written either in a "sulphate form", as we have done for eqn. (3), or in an "oxide form" as for eqn. (4). More generally, TABLE 1 Various ways of writing vanadium species dissolved in molten potassium disulphate Form
Sulphate form
~=~
Oxide form+ solvent
V(IV)
Acidic Basic
2VOSO4 2VO(SO,)4-
~=~ ~=~
V2032+ + $207z V202- + 3S2O2
V(V)
Acidic
2VO2SO4-
~=~
V205 -~- $2 O 2 -
Basic
VO2(SO4)32
~
g o 3 + $2072-
173 each constituent of the two acid-base couples may be written either in a "sulphate" or in an "oxide" form, as indicated in Table 1. The real nature of each species was determined either by electromigration or by potentiometry. The migration is easily observed owing to the colours of the solutions. For V(V) in acidic medium (where the existing compound is the acid form of the acid-base couple), V205 was introduced in the central tube of the electromigradon cell. After electrolysis, the anodic tube was coloured red and the cathodic tube remained colourless. Consequently, the solvated form of V205 in acidic medium is anionic. For vanadyl sulphate in acidic medium, the green colour remained in the central tube, indicating that the dissolved species is the neutral one. In both circumstances, the sulphate form corresponds to the observed properties. Potentiometric characterization" was carried out for V(IV) in basic medium (pSO4 = 0.6). In the sulphate form the V(IV) species is monomeric and alternatively dimeric in the oxide form. Vanadium(V) is always monomeric. The redox equilibria are, respectively: V(V) + e = V(IV)
(sulphate form)
2V(V) + 2e=[V(IV)] 2 (oxide form) to which correspond the equilibrium potentials:
Esf = Cte + (2.3RT/F) log([V(V)]/[V(IV)])
(5)
Eof =Cte+ (2.3RT/2F) Iog([V(V)]2/[(V(IV)2)])
(6)
o2o
~-~0, lc4,o 0,~c
C),05 0,0 _o,o5
_Z [og([V(~]/mot kg- 1
Fig. 2. Variationof the equilibriumpotentialof a solutionof V(IV) and V(V)vs. the V(IV) concentration ( p S O 4 = 0.6).
174 Or, in a simplified form: Esf = Cte -
0.140 log[V(IV)]
Eof = C t e --
0.070 log[V(IV)]
Known concentrations [V(IV)] of vanadyl sulphate were added to a V(V) solution (VOSO 4 was neutralized by the excess of K2SO4). Variation of E vs. log[V(IV)] is plotted in Fig. 2. The slope of the straight line (0.140 ± 0.005 V) shows that the V(IV) species in basic medium is VO(SO4) 4- , i.e. the monomeric form. Characterization of the basic V(V) species is more difficult because neither technique allows one to distinguish between the two forms. However, the quasi-equality of the diffusion coefficients of V(IV) and V(V)-obtained by chronopotentiometry [7]--allow one to' assume a similar structure of the compounds and to conclude in favour of the complex VO2(SO4) ~ . The diffusion coefficient values are respectively: D[V(IV)] = (5.6 ± 0.7) )< 10 -7 cm 2 s - ' D[V(V)]----(5±I.3)×10
7cm2s I
A c i d i t y constants. P o t e n t i a l a c i d i t y d i a g r a m
The two acid base equilibria are then: V02(S04)32
~:~ V02SO 4- -~- SO2-
VO(S04) ~- ~ VOSO 4 + 2S042
(7) (8)
with the two constants: Kv = [VO2SO4] [SO 2 ]/[VO2(S04)32 ]
(9)
and
KIV ~-[VOSO4][SO42-]2/[go(so4)~ ]
(10)
In acidic medium, the redox reaction is VOSO4 + $2O2- = VO2SO 4 + 2 SO 3 + e
(11)
and the equilibrium potential is obtained by applying the Nernst relation: E = e °' + ( 2 . 3 R T / F )
log([V02SO4] Vs2o3/[VOSO4] )
(12)
The V(IV) and V(V) balances are respectively: IV(IV)] = [VOSO4I + [VO(SO4) ~- ]
(13)
[V(V)] = [VO2SO4] + [VO2(SO4)32-]
(14)
where [V(IV)I and [V(V)] are the total concentrations of V(IV) and (V).
175 1,0
m
0,~
.~ Iv°# S
7 0,2 _ ~
__
Y 0
pso4 Fig. 3. Potential-acidity diagram: experimental points and calculated curve for [V(V)]=2× 10 2 mol kg-1 and [V(IV)]= 10-2 mol kg -1
The combination of eqs. (2), (9), (10), (12)-(14) gives
E= E ° + (2.3RT/F) + (2.3RT/F)log(1
log([V(V)]/[V(IV)]) + [SO42- ] 2 / K i v ) ~
(2.3RT/F)log(1
+ [SO42- ]/K
+ (4.6RT/F)pSO 4
v) (15)
with E ° = E °'+ 2 logK i E °, Kiv and K v were determined from potential vs. acidity measurements for a solution having known [V(V)] and [V(IV)] values. The best set of values was obtained by a least-squares method, using the pit-mapping technique described by Sillen [8]. TABLE 2 Comparison between acido-basicproperties of P(V) and V(V) Formulae pK
Phosphorus(V)
Vanadium(V)
P O 3S O 3 / / p o
VO 2SO 4//VO2(SO 4 ) 2-
1.4
3-
2.9
176 The following values were obtained: E ° = --0.260-+- 0.007 V
[vs. H g ( I ) / H g ( I I ) electrode]
PKiv = - l o g Kiv = 5.3 ± 0.1 PKv = - l o g K v = 2 . 9 ± 0 . 1 The calculated curve is compared with experimental results on the potential-acidity diagram (Fig. 3). A good agreement is observed. A comparison can be established between acidobasic properties of V(V) and those of P(V) [3]. These properties are assembled in Table 2. In both cases only one acid-base system is observed and both acidic forms have the same basic formula. However, the base corresponding to P(V) is stronger than that corresponding to V(V), and their formulae are different. The first corresponds to the so-called oxide form and the latter to the sulphate form. The difference of behaviour may be related to the difference between the ionic radius of P(V) (35 pm) and V(V) (59 pm). Both ions have a rare gas configuration and form complexes with oxide ions, which are increasingly stable with decreasing size of the central ions. Thus, P(V) ion forms very stable bonds with three oxide ions, while V(V) ions link only two oxide ions, leaving enough room for two SO 2 ions. Finally, the difference between the formulae of the complexes allows one to suggest an explanation for certain observations dealing with sulphur dioxide oxidation. This concerns, first, the inhibiting influence of sulphur trioxide on its production rate [9] and secondly, the lack of SO 3 formation during the bubbling of SO 2 in a molten K2S207q-V205mixture [10]. The p K values determined above show that for pSO 4 > 3 the acidic forms of the complexes are more concentrated in the solution than the basic forms. These high acidity values are presumably encountered during SO 3 formation and in K2S20 7 + V205 mixtures because of vanadium pentoxide acidity. In such acidic media, oxidation of SO 2 into S03 may be written as: 2VO2SO 4- + SO2 + SO 3 --~ 2VOSO 4 + $2O2 and
(16)
2 V O S O 4 + 1 0 2 ~- S2072
(17)
~ 2VO2SO 4 + 2SO 3
where it can be seen that the reduction of V(V) into V(IV) does not give SO3; sulphur trioxide formation appears.during the reoxidation of V(IV) by oxygen (eqn. 17). On the other hand, a slightly, soluble species, VOSO 4, is involved in eqn. (17), a possible cause of slowing down. In basic media, the reactions are 2VO2(804)32- q- SO 2 if- $2 O2
~ 2 V O ( S O 4 ) ~ - if- SO 3
2VO(SO4) ~- + 1 0 2 -----)2VO2(804)32 - q-- S2 O2
(18) (19)
This time, SO 3 production occurs during reduction of V(V). Indeed, if a sulphur dioxide stream is bubbled in a KzSO 4 saturated solution of V(V), a dissolution of potassium sulphate is observed, caused by the reaction: KaSO,(s) + SO s ~ 2K + + $2072-
(20)
177
A change in the complex structure presumably corresponds to the change in the formula, allowing a better electron transfer in the vicinity of the metal. ACKNOWLEDGEMENTS
The authors wish to express their thanks to Professor B. Tremillon for his helpful suggestions and encouragement. REFERENCES 1 2 3 4 5 6 7 8 9 10
G. Boreskov, Ann. G6n. Chim., Privat, Toulouse, 3 (1967) 211. A. Durand, G. Picard and J. Vedel, J. Electroanal. Chem., 70 (1976) 55. A. Durand, G. Picard and J. Vedel, J. Electroanal. Chem., 70 (1976) 65. G. Chariot, Chimie Analytique Quantitative, Vol. II, Mbthodes sblectionnbes d'analyse chimique des 616ments, Masson, Paris, 1974, p. 562. G.H. Tandy, J. Appl. Chem., 6 (1956) 68. J.H. Frazer and W.J. Kirkpatrick, J. Am. Chem. Soc., 62 (1940) 1659. A. Durand, G. Picard and J. Vedel, to be published. L.G. Sillen, Acta Chem. Scan&, 16 (1962) 159. P. Mars and J.G.H. Maessen, Proc. 3rd Int. Cong. Catal., Amsterdam 1 (1964) 226; J. Catal., 10 (1968) 1. A.R. Glueck and C.N. Kenney, Chem. Eng. Sci., 23 (1968) 1257.
169
Elsevier Sequoia S.A., Lausanne--Printed in The Netherlands
ELECTROCHEMICAL INVESTIGATIONS IN MOLTEN POTASSIUM D I S U L P H A T E A T 430°C
P R O P E R T I E S OF V A N A D I U M SPECIES
ALAIN DURAND, GERARD PICARD and JACQUES VEDEL Laboratoire d'Electroehimie A nalytique" et Appliqube de I'E.N.S.C.P., (Laboratoire associb au C.N.R.S. No. 216), Ecole Nationale SupOrieure de Chimie de Paris, 11, rue Pierre et Marie Curie, 75005--Paris (France)
(Received 10th February 1981)
ABSTRACT Properties of vanadium compounds dissolved in molten potassium disulphate have been studied at 430°C as a function of solvent acidity. Potentiometric and voltammetric measurements, and electromigration experiments, allowed the determination of the formulae and stability of the species existing in the melt. The species, in acidic and basic medium are, respectively, VO2SO4 and V02(SO4)32 for V(V) and VOSO4 and VO(S04) 4- for V(IV). The corresponding acido-basic constants are, respectively, K v = 10 -t2.9_+o.t)and Kiv = 10 -0.3-+oA).The formal potential of the V(V)/V(IV) system is E ° =0.26___0.01 V [vs. Hg(I)/Hg(II) electrode] at pSO4=3.2. The oxidation of sulphur dioxide to sulphur trioxide is discussed by consideration of the species existing in the melt.
INTRODUCTION Industrial catalysts used to p r o m o t e the oxidation of sulphur dioxide usually contain v a n a d i u m pentoxide and potassium sulphate and are used at temperatures between 400 and 500°C. It is n o w assumed, f r o m Boreskov's works, that sulphur trioxide, reacting with potassium sulphate, forms a liquid film which dissolves v a n a d i u m pentoxide and serves as reaction m e d i u m [ 1 ]. Molten potassium disulphate is not an inert solvent. It undergoes a partial dissociation b y the equilibrium: $207a - (1) ~=z SO3(g ) + SO42- (1)
(1)
Therefore, molten potassium disulphate belongs to the class of solvents having an acidity system (solvoacidity), acids and bases being defined b y the exchange of SO4a ion, which takes part in equilibrium (1). I n previous work [2], it has been shown that acidity, defined b y pSO 4 = -log[SO 2-] where [SOaz - ] (mol kg - 1 ) is the concentration of free sulphate in the melt, could be measured f r o m the position of the voltamperometric reduction curve of the solvent, 0022-0728/81/0000-0000/$02.50 © 1981 Elsevier Sequoia S.A.
170
at a rotating gold micro-electrode. This allowed the determination of the dissociation constant Ki,p of equilibrium (1): Ki, p = P(SO3)[SO 2-] = 10 -3.2 atm mol kg -1
(2)
as well as the solubility S of potassium sulphate: S = 0.25 mol kg
1
Accordingly, acidity in molten potassium disulphate can vary between p S O 4 z 0.6 (basic medium) and p S O 4 = 3.2 (acidic medium), corresponding respectively to a saturated solution of potassium sulphate and to a sulphur trioxide saturated solution [P(SO3) = 1 atm]. In spite of its narrowness this acidity range allows a modification of chemical properties of solutes, as has been shown for dissolved species of P(V) [3]. The purpose of this work is to establish the nature of the vanadium species that can exist in this melt, the oxidation state and form of complexes, as a function of the acidity of the medium, and also to determine the acid-base and redox potential characterizing the possible exchange reactions. The formulae of the complexes allow an interpretation of observations dealing with the oxidation of sulphur dioxide to be proposed. EXPERIMENTAL
The electrochemical vessel is an airtight Pyrex cell which allows monitoring of the atmosphere above the melt. The cell is placed in a thermoregulated vertical furnace (t=430__+ I°C). Voltammograms were obtained by means of a Tacussel threeelectrode potentiostat, with linear sweep generator (scan rate 0.1 V min-~) and analogue recorder. The working electrode was a rotating gold micro-electrode, area 1 mm 2, rotation rate 500 rpm. The reference electrode was the m e r c u r i c / m e r c u r o u s sulphate/saturated potassium sulphate electrode described in our previous work. An electromigration cell has been used to determine the charge carried by certain vanadium complexes. It had three compartments, separated by fritted discs. The vanadium species to be observed was introduced in the central part and an electric potential difference was applied between two gold electrodes immersed in the side compartments. The cell was immersed in a thermoregulated potassium disulphate melt. The concentration of the V(IV) saturated solution was measured after dissolution in water of quenched samples. Titration was made by oxidation with Ce(IV) solution in the presence of ferricyanide [4]. Reagents were Merck p.a. hydrated vanadyl sulphate (VOSO 4, 3.5 H20), Merck p.a. vanadium pentoxide and Prolabo RP sodium metavanadate. RESULTS A N D DISCUSSION
Solute properties Vanadium(IV) solutions are green, and the V(V) solutions are dark red. Air bubbling in a V(IV) solution slowly changes its colour to dark red. Conversely,
171
sulphur dioxide reduces V(V) into V(IV), apparently faster than oxygen oxidizes V(IV). Vanadyl sulphate has a limited solubility in acidic medium. The solubility of VOSO 4 was determined by analysing saturated solutions. The experimental value for the solubility is S(VOSO4) = 0.12 mol kg 1 for pSO 4 = 3.2, and is > 1 mol kg 1 for pSO4 = 0.6. The solubility of V(V) compounds is > 1 mol kg - ~ for any value of the acidity.
Redox properties Redox properties were studied by means of voltammetric curves obtained with rotating gold micro-electrodes. Addition of VOSO 4 leads to a single oxidation wave, the limiting diffusion current of which is proportional to the concentration of the added vanadium (Fig. 1). Similarly, addition of V(V) leads to a single proportional reduction wave. Plotting the gold electrode potential vs. log(j--Jox)/(Jred--j) (wherejox, Jred a n d j are respectively the limiting current densities for V(V) and V(IV) and the current value at the considered potential), gives a straight line, with a slope equal to 0.135 +__0.005 V, in good agreement with the theoretical slope (0.140 V) derived for the reaction: V(V) + e ~=~V(IV) at 430°C. The same result is valid for any acidity value. Thus, in agreement with Tandy [5], V(IV) and V(V) are the only existing oxidation states of vanadium in molten potassium disulpahte. The proposition of Frazer and Kirkpatrick [6] who assumed a disproportionation of V(IV) into V(III)
e,J 15[ 'E E ,,,¢
~5C. . . . . . .
_.
.
.
.
~ . . . . . .
.
.
]O .C
~oc
.
.
/
~'
/
2
50---
fr 0,2 a_
_o//
I I r I I
_5C
/
.50
1
0,L,
pofenfiat/ V
b-
2
3
4
102 IV(]V)] / mot
5
kg "I
'Fig. 1. Oxidation of vanadium(V) in molten potassium disulphate (pSO4 =0.6), for various concentrations: (a) current-potential curves (gold micro-electrode); (b) variation vs. concentration of limiting current.
172 and V(V), has to be rejected. Such a reaction would lead to the formation of an oxidation wave (V(III) ~ V(V)) and of a reduction wave (V(V) ~ V(III)) the logarithmic plot of which has a slope of about 0.070 V.
Acido-basic properties of V(IV) For pSO 4 values ranging from 3.2 to 2.9, addition of VOSO 4 does not shift the reduction curve of the solvent. This compound dissolves, then, without solvolysis. In basic solution an excess of potassium sulphate (the strong base of the solvent) is dissolved by vanadyl sulphate. The dissolution reaction may be written: V O S O 4 _}_ nSO42_
--,
2nVO(SO4).+,
(3)
Equation (3) is an acidobasic reaction of the acid VOSO 4 and the strong base of the solvent SO42- . The n value was determined by acidometric titration in situ. The acidity of a basic solution (pSO 4 slightly greater than 0.6) was thoroughly determined. Then a known mass of K2SO 4 was added to the melt and back-titrated by vanadyl sulphate, until the initial pSO 4 value was reached again. Several determinations gave n = 2-+- 0.3 and consequently, the V(IV) species existing in the melt may be VOSO 4 in acidic medium and VO(SO4) 4 in basic medium.
Acido-basic properties of V(V) In acidic medium, additions of sodium metavanadate involve a decrease in pSO 4. Conversely, vanadium pentoxide does not change the acidity in acidic medium but does change it in a basic medium. Titration of a (SO 3 + V205) mixture by potassium sulphate allowed the determination of the reaction stoichiometry, after deduction of the quantity of K2SO 4 needed to neutralize the excess of sulphur trioxide. The reaction m a y be written: V205 + 2SO42- ~ 2VO3 + S~O72-
(4)
Characterization of the dissolved species The species dissolved in the melt may be written either in a "sulphate form", as we have done for eqn. (3), or in an "oxide form" as for eqn. (4). More generally, TABLE 1 Various ways of writing vanadium species dissolved in molten potassium disulphate Form
Sulphate form
~=~
Oxide form+ solvent
V(IV)
Acidic Basic
2VOSO4 2VO(SO,)4-
~=~ ~=~
V2032+ + $207z V202- + 3S2O2
V(V)
Acidic
2VO2SO4-
~=~
V205 -~- $2 O 2 -
Basic
VO2(SO4)32
~
g o 3 + $2072-
173 each constituent of the two acid-base couples may be written either in a "sulphate" or in an "oxide" form, as indicated in Table 1. The real nature of each species was determined either by electromigration or by potentiometry. The migration is easily observed owing to the colours of the solutions. For V(V) in acidic medium (where the existing compound is the acid form of the acid-base couple), V205 was introduced in the central tube of the electromigradon cell. After electrolysis, the anodic tube was coloured red and the cathodic tube remained colourless. Consequently, the solvated form of V205 in acidic medium is anionic. For vanadyl sulphate in acidic medium, the green colour remained in the central tube, indicating that the dissolved species is the neutral one. In both circumstances, the sulphate form corresponds to the observed properties. Potentiometric characterization" was carried out for V(IV) in basic medium (pSO4 = 0.6). In the sulphate form the V(IV) species is monomeric and alternatively dimeric in the oxide form. Vanadium(V) is always monomeric. The redox equilibria are, respectively: V(V) + e = V(IV)
(sulphate form)
2V(V) + 2e=[V(IV)] 2 (oxide form) to which correspond the equilibrium potentials:
Esf = Cte + (2.3RT/F) log([V(V)]/[V(IV)])
(5)
Eof =Cte+ (2.3RT/2F) Iog([V(V)]2/[(V(IV)2)])
(6)
o2o
~-~0, lc4,o 0,~c
C),05 0,0 _o,o5
_Z [og([V(~]/mot kg- 1
Fig. 2. Variationof the equilibriumpotentialof a solutionof V(IV) and V(V)vs. the V(IV) concentration ( p S O 4 = 0.6).
174 Or, in a simplified form: Esf = Cte -
0.140 log[V(IV)]
Eof = C t e --
0.070 log[V(IV)]
Known concentrations [V(IV)] of vanadyl sulphate were added to a V(V) solution (VOSO 4 was neutralized by the excess of K2SO4). Variation of E vs. log[V(IV)] is plotted in Fig. 2. The slope of the straight line (0.140 ± 0.005 V) shows that the V(IV) species in basic medium is VO(SO4) 4- , i.e. the monomeric form. Characterization of the basic V(V) species is more difficult because neither technique allows one to distinguish between the two forms. However, the quasi-equality of the diffusion coefficients of V(IV) and V(V)-obtained by chronopotentiometry [7]--allow one to' assume a similar structure of the compounds and to conclude in favour of the complex VO2(SO4) ~ . The diffusion coefficient values are respectively: D[V(IV)] = (5.6 ± 0.7) )< 10 -7 cm 2 s - ' D[V(V)]----(5±I.3)×10
7cm2s I
A c i d i t y constants. P o t e n t i a l a c i d i t y d i a g r a m
The two acid base equilibria are then: V02(S04)32
~:~ V02SO 4- -~- SO2-
VO(S04) ~- ~ VOSO 4 + 2S042
(7) (8)
with the two constants: Kv = [VO2SO4] [SO 2 ]/[VO2(S04)32 ]
(9)
and
KIV ~-[VOSO4][SO42-]2/[go(so4)~ ]
(10)
In acidic medium, the redox reaction is VOSO4 + $2O2- = VO2SO 4 + 2 SO 3 + e
(11)
and the equilibrium potential is obtained by applying the Nernst relation: E = e °' + ( 2 . 3 R T / F )
log([V02SO4] Vs2o3/[VOSO4] )
(12)
The V(IV) and V(V) balances are respectively: IV(IV)] = [VOSO4I + [VO(SO4) ~- ]
(13)
[V(V)] = [VO2SO4] + [VO2(SO4)32-]
(14)
where [V(IV)I and [V(V)] are the total concentrations of V(IV) and (V).
175 1,0
m
0,~
.~ Iv°# S
7 0,2 _ ~
__
Y 0
pso4 Fig. 3. Potential-acidity diagram: experimental points and calculated curve for [V(V)]=2× 10 2 mol kg-1 and [V(IV)]= 10-2 mol kg -1
The combination of eqs. (2), (9), (10), (12)-(14) gives
E= E ° + (2.3RT/F) + (2.3RT/F)log(1
log([V(V)]/[V(IV)]) + [SO42- ] 2 / K i v ) ~
(2.3RT/F)log(1
+ [SO42- ]/K
+ (4.6RT/F)pSO 4
v) (15)
with E ° = E °'+ 2 logK i E °, Kiv and K v were determined from potential vs. acidity measurements for a solution having known [V(V)] and [V(IV)] values. The best set of values was obtained by a least-squares method, using the pit-mapping technique described by Sillen [8]. TABLE 2 Comparison between acido-basicproperties of P(V) and V(V) Formulae pK
Phosphorus(V)
Vanadium(V)
P O 3S O 3 / / p o
VO 2SO 4//VO2(SO 4 ) 2-
1.4
3-
2.9
176 The following values were obtained: E ° = --0.260-+- 0.007 V
[vs. H g ( I ) / H g ( I I ) electrode]
PKiv = - l o g Kiv = 5.3 ± 0.1 PKv = - l o g K v = 2 . 9 ± 0 . 1 The calculated curve is compared with experimental results on the potential-acidity diagram (Fig. 3). A good agreement is observed. A comparison can be established between acidobasic properties of V(V) and those of P(V) [3]. These properties are assembled in Table 2. In both cases only one acid-base system is observed and both acidic forms have the same basic formula. However, the base corresponding to P(V) is stronger than that corresponding to V(V), and their formulae are different. The first corresponds to the so-called oxide form and the latter to the sulphate form. The difference of behaviour may be related to the difference between the ionic radius of P(V) (35 pm) and V(V) (59 pm). Both ions have a rare gas configuration and form complexes with oxide ions, which are increasingly stable with decreasing size of the central ions. Thus, P(V) ion forms very stable bonds with three oxide ions, while V(V) ions link only two oxide ions, leaving enough room for two SO 2 ions. Finally, the difference between the formulae of the complexes allows one to suggest an explanation for certain observations dealing with sulphur dioxide oxidation. This concerns, first, the inhibiting influence of sulphur trioxide on its production rate [9] and secondly, the lack of SO 3 formation during the bubbling of SO 2 in a molten K2S207q-V205mixture [10]. The p K values determined above show that for pSO 4 > 3 the acidic forms of the complexes are more concentrated in the solution than the basic forms. These high acidity values are presumably encountered during SO 3 formation and in K2S20 7 + V205 mixtures because of vanadium pentoxide acidity. In such acidic media, oxidation of SO 2 into S03 may be written as: 2VO2SO 4- + SO2 + SO 3 --~ 2VOSO 4 + $2O2 and
(16)
2 V O S O 4 + 1 0 2 ~- S2072
(17)
~ 2VO2SO 4 + 2SO 3
where it can be seen that the reduction of V(V) into V(IV) does not give SO3; sulphur trioxide formation appears.during the reoxidation of V(IV) by oxygen (eqn. 17). On the other hand, a slightly, soluble species, VOSO 4, is involved in eqn. (17), a possible cause of slowing down. In basic media, the reactions are 2VO2(804)32- q- SO 2 if- $2 O2
~ 2 V O ( S O 4 ) ~ - if- SO 3
2VO(SO4) ~- + 1 0 2 -----)2VO2(804)32 - q-- S2 O2
(18) (19)
This time, SO 3 production occurs during reduction of V(V). Indeed, if a sulphur dioxide stream is bubbled in a KzSO 4 saturated solution of V(V), a dissolution of potassium sulphate is observed, caused by the reaction: KaSO,(s) + SO s ~ 2K + + $2072-
(20)
177
A change in the complex structure presumably corresponds to the change in the formula, allowing a better electron transfer in the vicinity of the metal. ACKNOWLEDGEMENTS
The authors wish to express their thanks to Professor B. Tremillon for his helpful suggestions and encouragement. REFERENCES 1 2 3 4 5 6 7 8 9 10
G. Boreskov, Ann. G6n. Chim., Privat, Toulouse, 3 (1967) 211. A. Durand, G. Picard and J. Vedel, J. Electroanal. Chem., 70 (1976) 55. A. Durand, G. Picard and J. Vedel, J. Electroanal. Chem., 70 (1976) 65. G. Chariot, Chimie Analytique Quantitative, Vol. II, Mbthodes sblectionnbes d'analyse chimique des 616ments, Masson, Paris, 1974, p. 562. G.H. Tandy, J. Appl. Chem., 6 (1956) 68. J.H. Frazer and W.J. Kirkpatrick, J. Am. Chem. Soc., 62 (1940) 1659. A. Durand, G. Picard and J. Vedel, to be published. L.G. Sillen, Acta Chem. Scan&, 16 (1962) 159. P. Mars and J.G.H. Maessen, Proc. 3rd Int. Cong. Catal., Amsterdam 1 (1964) 226; J. Catal., 10 (1968) 1. A.R. Glueck and C.N. Kenney, Chem. Eng. Sci., 23 (1968) 1257.