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Thermodynamic diagrams for the vanadium-water system at 298·15K
THERMODYNAMIC VANADIUM-WATER
DIAGRAMS FOR THE SYSTEM AT 298.1X
K. POST and R. G. ROBINS School of Chemical Engineering, The University of New South Wales, Kensington, N.S.W. Australia
Abstract-Critically selected thermodynamic data have been used for the calculation of equilibrium conditions in the vanadium-water system at 24%1X. Potential-pH diagrams for concentrations of IO” and IO-%m atoms vanadium ner loo0 gm water and log concentration-pH diagrams for each of the stable oxidation states arc presented. -
INTRODUCTION Equilibrium conditions for the stable species in the vanadium-water system were recalculated using criti-
cally selected data and arc presented here in diagrammatic form. These diagrams show significant variation Table
1. Free energy data for selected vanadium at 298.15 K
V VZC vocryst v3 + vo+ VOH’+ v,o, tryst. voz+ VOOH + (VOOH): + v,o,“HV,O; V,04cryst. vo; vo; H2V1OOZ HV,,O;, v,,0:; HJVZO, HV,O:v,o:H,VO, HVO:vo: V,O,cryst. H,O,,
0.0 - 54.2 -96.6 - 60.08 ~ 107.99 - 112.79 - 272.3 - 106.7 - 157 -31x - 665.3 - 36065 -315-l - 140.3 ~ 187.3 -1646 -1841 ~ 1833.1 - 445.5 -428.4 -411 -244 -233 - 214.9 -3393 -56.7
species 6 :, I 1 I* 6 6 3,14 3.14 7 I 6 6
These to those published previously by PourbaixCl] differences are due to the selection of a number otdifferent stable species and to the use of more recently available free energy data. A summary of the details of the identification and characterization of the several solid and many ionic species considered here can be found in a number of recent reviews on the aqueous vanadium system[2S]. The present authors have further examined the incomplete and often contradictory literature on this system and it is from this analysis that the selection of both stable species and free energy data have been made. Thermodynamic diagrams as constructed in this work provide a rapid and reliable means for visual assessment ofequilibrium behaviour and reaction feasibility. The vanadium diagrams presented here have been useful to the authors in investigating the mechanism of oxidation for certain reactions in this system. The calculations required to obtain the potentialpH and log concentration-pH diagrams were based
P f 6 6 6 6 6 6 6 8
* Hill[3] misprints this species as VOH+, further his value of AGY for this species is miscalculated and should be - 110.6 kcal mole- 1 and not - 101 kcal inole-‘. Gnsequently Hills’ data for the species VO+ is incorrect and should be - 105.6 kcal mole-‘. However, in this paper Pourbaix’sCl] original data was used for the vanadium (II), and’(III) ionic species as it gave accurate link-up of all triple points in the diagrams and from this appears that the potential for the reaction V3* + HZ0 = VOzi + 2H+ + e-used by both Hill and Pourbaix is in error. 7 Data for these species is misprinted in NBS 270-5[6]. Free energy values used here were calculated from equilibrium constant data tabulated by Sillen[lS] and taken from refs.[9-131.
Fig. 1. E/pH diagram for the vanadium-water system at 298.15K. Lmv = 10” gm ats vanadium per 1000 g water.
402
K.
POST
Table 2. Standard free energy change at 298.1X
AND
R. G. ROIXNS
for equilibria between vanadium (2 # 0)
species of different oxidation
state
AGykcal mole- ’ 0 = II
VI+ + 2e- = V VO + 2H+ + 2e-
:
II = III 3 4 5 III = IV 6 7 8 9 10 IV = v 11 12 13 14 15 16 17 18 19 20 21 22 23 24 2.5 26 27
+ 54.2 + 39.9
= V + H,O
Vlf + e- = vz+ V,O, + hH+ + 2r- = 2V2+ + 3H,O V,O, + 2H+ + Ze- = ZVO + H,O
f
5.9
-6.2 + 22,4
VO’+ + 2H+ + e- = V3+ + Ha0
- 10.1
VzO4 + 2H* + 2e- = V,Os + V,O;+ 6H+ + 4e- = 2V,Os 2V0”+ + 2e- + H,O = V,Oa HV,O; + 3H+ + 2e- = V,03
- 13.9 - 49.4
Hz0 + 3H,O + 2H+ + 2H,O
- 2.2 -25.0 - 126.0 - 44.2 -32-5 - 182.9 - 187.9 - 195-8 - 56.8 -92.0 - 126.8 - 202-2 -23-l -51,4 - 39.7 - 28% - 45.7 - 64-8 -101-O
2V0:+ lOH+ + 4e- = V,O, + 6H,O V,O, + 6H+ + Ze- = 2V02” + 3H,O V,O, + 2H+ + 2e- = V204 + HZ0
H2V,oO:a + 14H+ + lOe- = N,O., + 8H,O HV,,O:, + 15H+ f lOe- = 5V,O, + BH,O I’,,028 + 16H+ f lOe- = 5V,04 + 8H20
HV,O:2HV,O:2Vz044VO:VO; + HaV,O, H,V,O; H,V,OT HV,O:-. 2HVO:2VO:-
+ 5H+ + 2e- = V,O., + 3H,O + XH+ + 4e- = V,O;+ 5Hz0 + IOH+ + 4e- = V,O;+ 5H,O + 14H+ + 4e- = V,O$- + 7H,O 2H+ + e- = V02+ + H,O + 7H+ + 2~ = 2VO’+ + 5H,O + 3H+ + Ze- = V,O, + 3H,O + 2H+ + 2a- = HV,O; + 2H,O + 4H’ + 2e- = HV,O; + 2H,O + 5H+ + Ze- = HV,O; + 3H,O + 7H+ + 2e- = HV,O; + 3H,O
upon the general equilibrium equation for an aqueous system containing vanadium pA, + xH+ + ze- = ~$3, + cH,O Where A, and B, are the particular vanadium species and the subscripts 4 and p define the number of vanadium atoms associated with each species.
Fig. 2. E/pH diagram for the vanadium-water system at 29%15K. xmv = 1O-2 gm ats vanadium per 1000 g water.
The half cell reduction potential for this equation can be written E
=
EO
RT ZF
In
ai(,x ‘ko
a& x a”,+
and if the temperature is taken as 298.15 K, the activity of water assumed to be unity, the term log ~ln+
Fig. 3. Log Zmv/pH diagram for the vanadium (II+vater system at 29%1X.
The vanadium-water system
403
Table 3. Standard free energy change at 29%15K for equilibria between vanadium species of the same oxidation state (2 = 0)
_
II
AGY kcal mole-’
29
VO + ZH+ = V’+ + Hz0
-14.3
III 31 32 33 34 35
VOH’+ + H+ = V3+ + H,O VO+ + H+ = VOHZ+ V,O, + 6H+ = 2VSf + 3&O VzO3 + 4H+ = 2VOH2+ + Hz0 V,O, + 2H+ = 2VO+ + Hz0
-4.0 -4.5 - 18.0 -10.0 -0.4
2HV,O; = V,O$- + Hz0 HV,O, + 5H+ = 2V02+ + 3H,O V20., + 4H+ = 2V02+ + 2H20 HV,O; + H+ = V,O, + Hz0 V,O;- + 2H+ = 2V204 + Hz0 (VOOH): + + 2H+ = 2VOz+ + 2Hz0 V,O,‘- + 6H+ = Z(VOOH):+ + HZ0
-0.7 -22.8 -11.7 -11.1 -21.6 -8.8 -27-4 -14.0
IV 36 37 38 39 40 41 42 43
HV,O;
V 44 45 46 47 48 49 50 51 52 53 54 55 56 57
HSVZO; HV,oO:, HI’&-
+ 3H+
= (VOOH):’
+ 3H+ = 2V0,’ + 3Hz0 + H+ = H2VIOO:i + 2H+ = H,V,O;
ZH,VO, + H+ = H3V20; V,,O& + H+ = HV,,O& SHsVzO;
+ Hz0
= HV,,O&
+ Hz0
+ 7HzO
5HV,O:- + 9H+ = Vi.05; V@:+ H = HVIO:2HVO:- + H+ = HV,O:-
+ 7H,O
+ Hz0 2VO9+ 2H+ = VzO:+ Hz0 iwo:+ H+ = Ffzvop VO:+ H+ = HVO:5H,V,O; + H+ = HaVl,,Oi; + 7H20 HlV,r,O;; + 14H+ = lOV0; + 8&O V,,O:; + H+ + 7H,O = 5H,V,O;
:: 60
HVlO+- + H+ = 2VO3 + Hz0
2HVO:= V,O$+ H,O V,O5 + ZH+ = 2VO: + Hz0 H,V,O, + H+ = V20, + 2H,O H,V,,O;, + 4H+ = 5Vz0, + 3Hz0
2: 63
defined as pH and with AGo = -zFE” zE --=_,_ 0.059 1
-AGo 1364
it follows that
-xpH-lo+
I
Since many of the vanadium ions are polymeric it is convenient to express concentrations in terms of the number of g atoms of vanadium per 1000 g of solvent, so that if the activities of the species A and B are equal to their molal concentrations rnAand mB it follows that my, = qm, and+,, and - AGo 2-E ---=-_xpxpH 0.059 1 1364
When
-5.2 -5.0 -17.1 - 14.2 - 79 - 10.4 - 88.0 - 17-4 - 19.1 -37-9 -11a -18-I -15.4 -1@6 +25 -29-O - 1.7 + 2.0 -7.2 -206
= pm,
the equation defines a boundary on a potential-pH diagram of total vanadium concentration Cm, about which the available vanadium is equally distributed between species A and species B. Diagrams 1 and 2 are baaed on this relationship and in its reduced form (where z = 0), it forms the basis of diagrams 34. FREE
ENERGY
DATA
AND
THE
DIAGRAMS
The free energy data shown in Table 1 have been used in the calculations required for the construction of the potential-pH and log concentra tion--pH diagrams. For each species listed, these data have been critically selected from the most recent and reliable sources available. Reactions between all possible combinations of these species were considered, and in Tables 2 and 3, only those which represent stable equilibria are shown. In the potential-pH diagrams, Figs. 1 and 2, constructed at 10’ and IO-’ g at of vanadium per
404
K.
POST
AND
R G. ROBINS
lo3 g. of water respectively, the non vertical boundaries (those between species of different oxidation state (z # 0) are defined by the relations contained in Table 2. All other boundaries in Figs. 1-6 (where z = 0) are defined by the relations contained in Table 3. In all diagrams broken lines are used to represent boundaries at which the available vanadium is equally distributed between the two ionic species which predominate in the adjacent regions. The unbroken lines define conditions at which the enclosed solid species exists in equilibrium with the adjacent ion at the particular specified concentration. In the log concentration-pH diagrams, Figs. 3-6, boundaries between adjacent species for which 4 # p appear as non vertical lines. This implies that the position of these equilibria on a potential-pH diagram is a function of concentration and it should be noted that in this respect the diagrams presented here differ in construction from those of Pourbaix. This presentation, which requires that the potential-pH diagrams be read in conjunction with the log
PH
Fig. 4. Log km,/pH
i L-4
B
PH
Fig. 6. Log Lm,/pH
diagram for the vanadium (Vjwater system at 298.15K.
concentration-pH diagrams (Figs. 3-6) avoids the diagrammatic complexities which can complicate interpretation of the equilibrium behaviour such as exists between vanadium (IV) and vanadium (V) in Pourbaix’s diagrams in the pH region of 2-5-7-5. In all instances, differences between the diagrams presented here and those originally published by Pourbaix arise from the use of more recent information concerning the existence and free energies of the various vanadium ionic and solid species. They are presented here, in this form, as they provide a concise and up to date summary of the complex behaviour of the vanadium-water system at equilibrium. The diagrams were constructed as a guide to experiments on the oxidation of vanadium (IV) to vanadium (V) using molecular oxygen and in this region have been found to relate accurately to changes observed in this study.
diagram for the vanadium (IIIjwater
system at 2%+15K.
REFERENCES
-1. M. J. N. Pourbaix, 2. 3. 4. 5. 6.
7. 8. 9. Fig. 5.
Log >,m,/pH diagram for the vanadium (IV jwater system at 298.15K.
10.
Atlas of ElectrochemicalEquilibria i/t Aqueous Solution. Pcrgamon, Oxford (1966). A. A. Ivakin, Dokl. Akad. Nauk SSSR-Uralskii filial institut Khimii. Trudy. No 18. 3-17. (1968). J. 0. Hill, I. G. Worsiey and L. G. H&pler,‘Chem. Reo. 71(l), 127, (1971). M. T. Popk and B. W. Dale, Q. Rev. them. Sot. 22, 527 (1968). Gm&ns handbuch der anorganischen chemie System Number 48, Vanadium, Tiel, A.B., Lieferung 1, 2 Verlag Chemie GMBH. Weinheim/Bergstr. (1967). D. D. Wagman, W. H. Evans, V. B. Parker, 1. Harlow, S. M. Bailey, R. H. Schumm and K. L. Churney, Selected Vulues of Thermodynamic Properties. Nat. Bur. Stand. (U.S.) Tech. Note 27c-5 (1971). H. T. Evans and R. M. Garrels, Geochim. cosmochim. Acta 15, 131 (1958). W. M. Latimer, Thp Oxidation states of the elements and their potentials in aqueous solutions Prentice Hall, N.Y. (1952). F. J. C. Rossotti and H. S. Rossotti, Acta them. scand. 10, 957 (1956). L. Newman and K. P. Quinlan, J. Am. chum. Sot. 81, 547 ( 1956).
The vanadium-water Il. V. Varoqui and J. Brenet, C.r. hehd. Se’anc. Acad. Sci., Paris. 252. 3033 11961).
12. Yu. 1: Sainikov, Zh. Neorg.
ir. L.’Zolotavin
and I. ya. Bezrukov,
Khim. 8, 923 (1963). 13. A. A. Jvakin, Zh. prikl. Khim. 39, 277 (1966).
system
405
14. 6. Lutz and H. Wendt, Ber. Burzscr~ys. phys. Chem. 74(41. . ,_ 372 (19701. 15. L. G. Sill& (Editor) Stability constnnls of metal-tin
complexes. Spec. Pub. No. 17, The Chemical London (1964) and supplement Spec. Pub. (1971).
Society, No. 25
DIAGRAMS FOR THE SYSTEM AT 298.1X
K. POST and R. G. ROBINS School of Chemical Engineering, The University of New South Wales, Kensington, N.S.W. Australia
Abstract-Critically selected thermodynamic data have been used for the calculation of equilibrium conditions in the vanadium-water system at 24%1X. Potential-pH diagrams for concentrations of IO” and IO-%m atoms vanadium ner loo0 gm water and log concentration-pH diagrams for each of the stable oxidation states arc presented. -
INTRODUCTION Equilibrium conditions for the stable species in the vanadium-water system were recalculated using criti-
cally selected data and arc presented here in diagrammatic form. These diagrams show significant variation Table
1. Free energy data for selected vanadium at 298.15 K
V VZC vocryst v3 + vo+ VOH’+ v,o, tryst. voz+ VOOH + (VOOH): + v,o,“HV,O; V,04cryst. vo; vo; H2V1OOZ HV,,O;, v,,0:; HJVZO, HV,O:v,o:H,VO, HVO:vo: V,O,cryst. H,O,,
0.0 - 54.2 -96.6 - 60.08 ~ 107.99 - 112.79 - 272.3 - 106.7 - 157 -31x - 665.3 - 36065 -315-l - 140.3 ~ 187.3 -1646 -1841 ~ 1833.1 - 445.5 -428.4 -411 -244 -233 - 214.9 -3393 -56.7
species 6 :, I 1 I* 6 6 3,14 3.14 7 I 6 6
These to those published previously by PourbaixCl] differences are due to the selection of a number otdifferent stable species and to the use of more recently available free energy data. A summary of the details of the identification and characterization of the several solid and many ionic species considered here can be found in a number of recent reviews on the aqueous vanadium system[2S]. The present authors have further examined the incomplete and often contradictory literature on this system and it is from this analysis that the selection of both stable species and free energy data have been made. Thermodynamic diagrams as constructed in this work provide a rapid and reliable means for visual assessment ofequilibrium behaviour and reaction feasibility. The vanadium diagrams presented here have been useful to the authors in investigating the mechanism of oxidation for certain reactions in this system. The calculations required to obtain the potentialpH and log concentration-pH diagrams were based
P f 6 6 6 6 6 6 6 8
* Hill[3] misprints this species as VOH+, further his value of AGY for this species is miscalculated and should be - 110.6 kcal mole- 1 and not - 101 kcal inole-‘. Gnsequently Hills’ data for the species VO+ is incorrect and should be - 105.6 kcal mole-‘. However, in this paper Pourbaix’sCl] original data was used for the vanadium (II), and’(III) ionic species as it gave accurate link-up of all triple points in the diagrams and from this appears that the potential for the reaction V3* + HZ0 = VOzi + 2H+ + e-used by both Hill and Pourbaix is in error. 7 Data for these species is misprinted in NBS 270-5[6]. Free energy values used here were calculated from equilibrium constant data tabulated by Sillen[lS] and taken from refs.[9-131.
Fig. 1. E/pH diagram for the vanadium-water system at 298.15K. Lmv = 10” gm ats vanadium per 1000 g water.
402
K.
POST
Table 2. Standard free energy change at 298.1X
AND
R. G. ROIXNS
for equilibria between vanadium (2 # 0)
species of different oxidation
state
AGykcal mole- ’ 0 = II
VI+ + 2e- = V VO + 2H+ + 2e-
:
II = III 3 4 5 III = IV 6 7 8 9 10 IV = v 11 12 13 14 15 16 17 18 19 20 21 22 23 24 2.5 26 27
+ 54.2 + 39.9
= V + H,O
Vlf + e- = vz+ V,O, + hH+ + 2r- = 2V2+ + 3H,O V,O, + 2H+ + Ze- = ZVO + H,O
f
5.9
-6.2 + 22,4
VO’+ + 2H+ + e- = V3+ + Ha0
- 10.1
VzO4 + 2H* + 2e- = V,Os + V,O;+ 6H+ + 4e- = 2V,Os 2V0”+ + 2e- + H,O = V,Oa HV,O; + 3H+ + 2e- = V,03
- 13.9 - 49.4
Hz0 + 3H,O + 2H+ + 2H,O
- 2.2 -25.0 - 126.0 - 44.2 -32-5 - 182.9 - 187.9 - 195-8 - 56.8 -92.0 - 126.8 - 202-2 -23-l -51,4 - 39.7 - 28% - 45.7 - 64-8 -101-O
2V0:+ lOH+ + 4e- = V,O, + 6H,O V,O, + 6H+ + Ze- = 2V02” + 3H,O V,O, + 2H+ + 2e- = V204 + HZ0
H2V,oO:a + 14H+ + lOe- = N,O., + 8H,O HV,,O:, + 15H+ f lOe- = 5V,O, + BH,O I’,,028 + 16H+ f lOe- = 5V,04 + 8H20
HV,O:2HV,O:2Vz044VO:VO; + HaV,O, H,V,O; H,V,OT HV,O:-. 2HVO:2VO:-
+ 5H+ + 2e- = V,O., + 3H,O + XH+ + 4e- = V,O;+ 5Hz0 + IOH+ + 4e- = V,O;+ 5H,O + 14H+ + 4e- = V,O$- + 7H,O 2H+ + e- = V02+ + H,O + 7H+ + 2~ = 2VO’+ + 5H,O + 3H+ + Ze- = V,O, + 3H,O + 2H+ + 2a- = HV,O; + 2H,O + 4H’ + 2e- = HV,O; + 2H,O + 5H+ + Ze- = HV,O; + 3H,O + 7H+ + 2e- = HV,O; + 3H,O
upon the general equilibrium equation for an aqueous system containing vanadium pA, + xH+ + ze- = ~$3, + cH,O Where A, and B, are the particular vanadium species and the subscripts 4 and p define the number of vanadium atoms associated with each species.
Fig. 2. E/pH diagram for the vanadium-water system at 29%15K. xmv = 1O-2 gm ats vanadium per 1000 g water.
The half cell reduction potential for this equation can be written E
=
EO
RT ZF
In
ai(,x ‘ko
a& x a”,+
and if the temperature is taken as 298.15 K, the activity of water assumed to be unity, the term log ~ln+
Fig. 3. Log Zmv/pH diagram for the vanadium (II+vater system at 29%1X.
The vanadium-water system
403
Table 3. Standard free energy change at 29%15K for equilibria between vanadium species of the same oxidation state (2 = 0)
_
II
AGY kcal mole-’
29
VO + ZH+ = V’+ + Hz0
-14.3
III 31 32 33 34 35
VOH’+ + H+ = V3+ + H,O VO+ + H+ = VOHZ+ V,O, + 6H+ = 2VSf + 3&O VzO3 + 4H+ = 2VOH2+ + Hz0 V,O, + 2H+ = 2VO+ + Hz0
-4.0 -4.5 - 18.0 -10.0 -0.4
2HV,O; = V,O$- + Hz0 HV,O, + 5H+ = 2V02+ + 3H,O V20., + 4H+ = 2V02+ + 2H20 HV,O; + H+ = V,O, + Hz0 V,O;- + 2H+ = 2V204 + Hz0 (VOOH): + + 2H+ = 2VOz+ + 2Hz0 V,O,‘- + 6H+ = Z(VOOH):+ + HZ0
-0.7 -22.8 -11.7 -11.1 -21.6 -8.8 -27-4 -14.0
IV 36 37 38 39 40 41 42 43
HV,O;
V 44 45 46 47 48 49 50 51 52 53 54 55 56 57
HSVZO; HV,oO:, HI’&-
+ 3H+
= (VOOH):’
+ 3H+ = 2V0,’ + 3Hz0 + H+ = H2VIOO:i + 2H+ = H,V,O;
ZH,VO, + H+ = H3V20; V,,O& + H+ = HV,,O& SHsVzO;
+ Hz0
= HV,,O&
+ Hz0
+ 7HzO
5HV,O:- + 9H+ = Vi.05; V@:+ H = HVIO:2HVO:- + H+ = HV,O:-
+ 7H,O
+ Hz0 2VO9+ 2H+ = VzO:+ Hz0 iwo:+ H+ = Ffzvop VO:+ H+ = HVO:5H,V,O; + H+ = HaVl,,Oi; + 7H20 HlV,r,O;; + 14H+ = lOV0; + 8&O V,,O:; + H+ + 7H,O = 5H,V,O;
:: 60
HVlO+- + H+ = 2VO3 + Hz0
2HVO:= V,O$+ H,O V,O5 + ZH+ = 2VO: + Hz0 H,V,O, + H+ = V20, + 2H,O H,V,,O;, + 4H+ = 5Vz0, + 3Hz0
2: 63
defined as pH and with AGo = -zFE” zE --=_,_ 0.059 1
-AGo 1364
it follows that
-xpH-lo+
I
Since many of the vanadium ions are polymeric it is convenient to express concentrations in terms of the number of g atoms of vanadium per 1000 g of solvent, so that if the activities of the species A and B are equal to their molal concentrations rnAand mB it follows that my, = qm, and+,, and - AGo 2-E ---=-_xpxpH 0.059 1 1364
When
-5.2 -5.0 -17.1 - 14.2 - 79 - 10.4 - 88.0 - 17-4 - 19.1 -37-9 -11a -18-I -15.4 -1@6 +25 -29-O - 1.7 + 2.0 -7.2 -206
= pm,
the equation defines a boundary on a potential-pH diagram of total vanadium concentration Cm, about which the available vanadium is equally distributed between species A and species B. Diagrams 1 and 2 are baaed on this relationship and in its reduced form (where z = 0), it forms the basis of diagrams 34. FREE
ENERGY
DATA
AND
THE
DIAGRAMS
The free energy data shown in Table 1 have been used in the calculations required for the construction of the potential-pH and log concentra tion--pH diagrams. For each species listed, these data have been critically selected from the most recent and reliable sources available. Reactions between all possible combinations of these species were considered, and in Tables 2 and 3, only those which represent stable equilibria are shown. In the potential-pH diagrams, Figs. 1 and 2, constructed at 10’ and IO-’ g at of vanadium per
404
K.
POST
AND
R G. ROBINS
lo3 g. of water respectively, the non vertical boundaries (those between species of different oxidation state (z # 0) are defined by the relations contained in Table 2. All other boundaries in Figs. 1-6 (where z = 0) are defined by the relations contained in Table 3. In all diagrams broken lines are used to represent boundaries at which the available vanadium is equally distributed between the two ionic species which predominate in the adjacent regions. The unbroken lines define conditions at which the enclosed solid species exists in equilibrium with the adjacent ion at the particular specified concentration. In the log concentration-pH diagrams, Figs. 3-6, boundaries between adjacent species for which 4 # p appear as non vertical lines. This implies that the position of these equilibria on a potential-pH diagram is a function of concentration and it should be noted that in this respect the diagrams presented here differ in construction from those of Pourbaix. This presentation, which requires that the potential-pH diagrams be read in conjunction with the log
PH
Fig. 4. Log km,/pH
i L-4
B
PH
Fig. 6. Log Lm,/pH
diagram for the vanadium (Vjwater system at 298.15K.
concentration-pH diagrams (Figs. 3-6) avoids the diagrammatic complexities which can complicate interpretation of the equilibrium behaviour such as exists between vanadium (IV) and vanadium (V) in Pourbaix’s diagrams in the pH region of 2-5-7-5. In all instances, differences between the diagrams presented here and those originally published by Pourbaix arise from the use of more recent information concerning the existence and free energies of the various vanadium ionic and solid species. They are presented here, in this form, as they provide a concise and up to date summary of the complex behaviour of the vanadium-water system at equilibrium. The diagrams were constructed as a guide to experiments on the oxidation of vanadium (IV) to vanadium (V) using molecular oxygen and in this region have been found to relate accurately to changes observed in this study.
diagram for the vanadium (IIIjwater
system at 2%+15K.
REFERENCES
-1. M. J. N. Pourbaix, 2. 3. 4. 5. 6.
7. 8. 9. Fig. 5.
Log >,m,/pH diagram for the vanadium (IV jwater system at 298.15K.
10.
Atlas of ElectrochemicalEquilibria i/t Aqueous Solution. Pcrgamon, Oxford (1966). A. A. Ivakin, Dokl. Akad. Nauk SSSR-Uralskii filial institut Khimii. Trudy. No 18. 3-17. (1968). J. 0. Hill, I. G. Worsiey and L. G. H&pler,‘Chem. Reo. 71(l), 127, (1971). M. T. Popk and B. W. Dale, Q. Rev. them. Sot. 22, 527 (1968). Gm&ns handbuch der anorganischen chemie System Number 48, Vanadium, Tiel, A.B., Lieferung 1, 2 Verlag Chemie GMBH. Weinheim/Bergstr. (1967). D. D. Wagman, W. H. Evans, V. B. Parker, 1. Harlow, S. M. Bailey, R. H. Schumm and K. L. Churney, Selected Vulues of Thermodynamic Properties. Nat. Bur. Stand. (U.S.) Tech. Note 27c-5 (1971). H. T. Evans and R. M. Garrels, Geochim. cosmochim. Acta 15, 131 (1958). W. M. Latimer, Thp Oxidation states of the elements and their potentials in aqueous solutions Prentice Hall, N.Y. (1952). F. J. C. Rossotti and H. S. Rossotti, Acta them. scand. 10, 957 (1956). L. Newman and K. P. Quinlan, J. Am. chum. Sot. 81, 547 ( 1956).
The vanadium-water Il. V. Varoqui and J. Brenet, C.r. hehd. Se’anc. Acad. Sci., Paris. 252. 3033 11961).
12. Yu. 1: Sainikov, Zh. Neorg.
ir. L.’Zolotavin
and I. ya. Bezrukov,
Khim. 8, 923 (1963). 13. A. A. Jvakin, Zh. prikl. Khim. 39, 277 (1966).
system
405
14. 6. Lutz and H. Wendt, Ber. Burzscr~ys. phys. Chem. 74(41. . ,_ 372 (19701. 15. L. G. Sill& (Editor) Stability constnnls of metal-tin
complexes. Spec. Pub. No. 17, The Chemical London (1964) and supplement Spec. Pub. (1971).
Society, No. 25