Chemistry
Physical & Theoretical Chemistry fields
Okayama University
Year 1988
Studies on the acid-base properties of the
AIBr3-NaBr melts
Hidetaka Hayashi
N Hayashi
Okayama University
Kyoto University
Z Takehara
A Katagiri
Kyoto University
Kyoto University
This paper is posted at eScholarship@OUDIR : Okayama University Digital Information
Repository.
http://escholarship.lib.okayama-u.ac.jp/physical and theoretical chemistry/22
2606
J. Electrochem. Soc., Vol. 136, No. 9, September 1989 9 The Electrochemical Society, Inc.
nealing. H i g h current density was o b t a i n e d from the P S Z
fuel cell at 1000~ The on-off cycle of discharge s u g g e s t e d
that La0.gSr0.,MnO3 c a t h o d e activity varies w i t h the discharge condition.
Acknowledgment
This w o r k was s u p p o r t e d by the P e t r o l e u m E n e r g y
Center.
M a n u s c r i p t s u b m i t t e d Nov. 30, 1988; revised m a n u s c r i p t
r e c e i v e d March 21, 1989.
REFERENCES
1. D. C. Fee, R. K. S t e u n e n b e r g , T. D. Claar, R. B. P o e p pel, and J . P . A c k e r m a n , F u e l Cell S e m i n a r Abstracts, p. 74 (1983).
2. J. N. Michaels, C. G. Vayenas, and L. L. H e g e d u s , This
Journal, 133, 522 (1986).
3. B. C. H. Steele, Sci. Ceram., 12, 697 (1984).
4. S. Ikeda, O. Sakurai, K. U e m a t s u , N. Mizutani, and
M. Katos, J. Mater. Sci., 2{}, 4593 (1985).
5. C. C. M c P h e e t e r s and T. D. Claars, F u e l Cell S e m i n a r
Abstracts, p. 64 (1986).
6. R. A. Miller, J. L. Smialek, and R. G. Garlick, Adv.
Ceram., 3, 241 (1981).
7. F. K., M o g h a d a m , T. Yamashita, and D. A. S t e v e n s o n ,
ibid, 3, 364 (1981).
8. J. J. Swab, in " P r o c e e d i n g s of 24th A u t o m o t i v e Techn o l o g y D e v e l o p m e n t Contract C o o r d i n a t i n g Meeting," p. 185 (1987).
9. K. T s u k u m a , Y. Kubota, and T. Tsukidate, Adv.
Ceram., 12, 382 (1984).
10. H. G. Scott, J. Mater. Sci., 10, 1527 (1975).
11. D. C. Fee, P. E. B a c k b u r n , D . E . Busch, T. D. Claar,
D. W. Dees, J. Dusek, T. E. Easler, W. A. Ellingson,
B . K . F l a n d e r m e y e r , R . J . Fousek, J . J . Heiberger,
T . E . Kraft, S. Majumdar, C.C. M c P h e e t e r s , F . C .
Mrazek, J . J . Picciolo, R . B . Poeppel, and S . A .
Zwick, F u e l Cell S e m i n a r Abstracts, p. 40 (1986).
12. J. E. Bauerle, J. Phys. Chem. Solids, 30, 2657 (1969).
13. A. J. B a r d and L. R. Faulkner, " E l e c t r o c h e m i c a l Methods," J o h n Wiley & Sons, Inc., N e w Y o r k (1980).
Studies on the Acid-Base Properties of the AIBr3-NaBr Melts
H. Hayashi*
Department of Applied Chemistry, Faculty of Engineering, Okayama University, Okayama 700, Japan
N. Hayashi and Z. Takehara*
Department of Industrial Chemistry, Faculty of Engineering, Kyoto University, Kyoto 606, Japan
A. Katagiri*
Department of Chemistry, College of Liberal Arts and Sciences, Kyoto University, Kyoto 606, Japan
Recently, a lot of w o r k has b e e n p e r f o r m e d on the acidbase properties of Lewis acidic m o l t e n salts. One of the
m o s t e x t e n s i v e l y investigated m e l t systems is t h e s o d i u m
c h l o r o a l u m i n a t e m e l t (175~176
in w h i c h the chloride
ion activity is d e p e n d e n t u p o n the c o m p o s i t i o n of the
melt. It is well k n o w n that u n u s u a l h i g h e r o x i d a t i o n state
ionic species, such as S(IV) (1, 2) or l o w e r o x i d a t i o n state
cluster c o m p o u n d s , such as W6C184§ (3), can be stable in the
c h l o r o a l u m i n a t e m e l t s b e c a u s e of the strong Lewis acidity
of t h e s e solvents (4-7). These m e l t s are n o w one of t h e potential materials for use in l o w - t e m p e r a t u r e m o l t e n salt
batteries (1, 2) or as solvents for organic synthesis (8).
T h e a l u m i n u m b r o m i d e - s o d i u m b r o m i d e melts also
s h o w t h e variable Lewis acidity. The m e l t i n g point of p u r e
a l u m i n u m b r o m i d e is lower than that of a l u m i n u m chloride (A1Br~: 97.5~ A1C13: 192.4~
w h e r e a s the boiling
p o i n t of the b r o m i d e is h i g h e r t h a n that of chloride (A1Br3:
255~ A1C13: 187~ (9). The liquidus t e m p e r a t u r e of the A1Br~-NaBr binary system is b e l o w 100~ in the 65-85 m o l e
p e r c e n t A1Br3 c o m p o s i t i o n ranges (10), while the liquidus
t e m p e r a t u r e of the A1C13-NaC1 melts is generally m o r e
t h a n 170~ At 200~ the v a p o r p r e s s u r e of the b r o m i d e
m e l t is a b o u t a tenth of that in the A1C13-NaC1 m e l t (9).
Therefore, from a practical p o i n t of view, the A1Br3-NaBr
m e l t s e e m s easier to h a n d l e t h a n the A1CI~-NaC1 m e l t for
m a n y c h e m i c a l processes. T h e s e observations are closely
related to the fact that the a l u m i n u m b r o m i d e forms a stable d i m e r m o l e c u l e (A12Br6) not only in t h e solid state but
also in the liquid state.
As d e s c r i b e d above, s o m e physical properties of the
h a l o a l u m i n a t e m e l t s are d e p e n d e n t on the halide species.
Therefore, the acide-base properties of the melts also m a y
v a r y w i t h the anion c o m p o n e n t , e v e n w i t h the s a m e
cationic constituent. F r o m t h e s e points of view, we investig a t e d t h e acid-base properties of t h e A1Br3-NaBr melts and
c o m p a r e d their characteristics w i t h those of the chloroal u m i n a t e melts.
* Electrochemical Society Active Member.
In b r o m i d e melts, "acid" is defined as a b r o m i d e ion acceptor, while the " b a s e " is the b r o m i d e ion donor.
Acid + B r - = Base
T h e basicity of t h e s e melts can be e x p r e s s e d as the p B r v a l u e (pBr- = - l o g a(Br-); a(Br ) m e a n s the b r o m i d e ion
activity). C o n c e r n i n g the acid-base e q u i l i b r i u m in the A1Br3-NaBr melts, T r e m i l l o n et al. (11) r e p o r t e d the equilibr i u m c o n s t a n t of the following c h e m i c a l reaction, w h i c h is
d o m i n a n t in the neutral region of the melt, by a c o u l o m e t ric titration m e t h o d
2A1Br4 = A12Brc + B r In this report, the over-all acid-base equilibria h a v e b e e n
t a k e n into a c c o u n t and individual e q u l i b r i u m c o n s t a n t
values e v a l u a t e d on the basis of the E M F m e a s u r e m e n t s of
t h e following A1/A1 c o n c e n t r a t i o n cell, especially in the
acid c o m p o s i t i o n ranges [up to 65 m o l e p e r c e n t (m/o)
A1Br3]
A1/A1Br3 - NaBr(sat.)//A1Br3 - NaBr(i)/A1
[1]
w h e r e " i " indicates a m e l t composition.
Experimental
B e c a u s e of the e x t r e m e m o i s t u r e sensitivity of alumin u m bromide, all e x p e r i m e n t s w e r e d o n e in a g l o v e box. In
o r d e r to k e e p the m o i s t u r e level and t h e o x y g e n c o n t e n t to
a m i n i m u m , argon gas refining and circulating equipm e n t w e r e a t t a c h e d to the glove box. The o x y g e n c o n t e n t
inside the b o x was m a i n t a i n e d b e l o w 0.1 v o l u m e p e r c e n t
and the d e w point was b e l o w -70~ H i g h purity alumin u m b r o m i d e (Mitsuwa Chemical) was u s e d as received.
S o d i u m b r o m i d e (Wako Chemical) had b e e n dried at 160~
in v a c u o for 2 days. The electrodes of the c o n c e n t r a t i o n
cell w e r e m a d e of a l u m i n u m wire (99.99%, rare metallic),
w h i c h w e r e electropolished in 20% a q u e o u s N a O H or
cleaned in a 6:1 m i x t u r e of HNO3 and H3PO4 at 80~ for 2-3
rain. T h e e l e c t r o c h e m i c a l cell used was m a d e of P y r e x
J. Electrochem. Soc., Vol. 136, No. 9, September 1989 9 The Electrochemical Society, Inc.
glass. T h e r e f e r e n c e e l e c t r o d e c o m p a r t m e n t , w h i c h w a s always saturated with NaBr, was separated from the bulk
m e l t e i t h e r b y a b e t a - a l u m i n a t u b e ( N G K s p a r k plug, 15~))
o r b y a s o d i u m i o n s e l e c t i v e glass t u b e (HORIBA). I n several e x p e r i m e n t s , a h a n d m a d e P y r e x t h i n film m e m b r a n e
also s e r v e d as a d i a p h r a g m in t h e c o n c e n t r a t i o n cell. T h e
c o m p o s i t i o n of t h e m e a s u r i n g c o m p a r t m e n t w a s c h a n g e d
b y a d d i n g k n o w n a m o u n t s of e i t h e r A1Br3 or N a B r , a n d t h e
E M F v a l u e s of t h e cell w e r e m e a s u r e d a n d r e c o r d e d w i t h a
h i g h i n p u t i m p e d a n c e digital v o l t m e t e r ( T A K E D A , TR6855). A p r o p o r t i o n a l t h e r m o e l e c t r o n i c c o n t r o l l e r w a s
u s e d to c o n t r o l t h e m e l t t e m p e r a t u r e at 210 ~ -+ 2~ T h e
temperature of the melt was measured by the use of a
Chromel-Alumel thermocouple.
Results and Discussion
T h e f o l l o w i n g c h e m i c a l s p e c i e s h a v e b e e n r e p o r t e d to
e x i s t in t h e A1Br3-NaBr melts, o n t h e b a s i s of R a m a n spect r o s c o p y (12)
N a § AIBr3, A12Br6, A1Br4 , A12Br7 , B r
w h e r e N a § acts o n l y as a c u r r e n t - c a r r y i n g s p e c i e s a n d is ind e p e n d e n t of t h e a c i d - b a s e e q u i l i b r i u m i n t h i s melt. T h u s ,
t h e f o l l o w i n g t h r e e c h e m i c a l e q u i l i b r i a c a n b e set u p
among the above species
2A1Br3 = A12Br6
K0
[2]
A1Br4- + A1Br3 = A12Brc
K1
[3]
2A1Br4- = A12Brc + B r -
K2
[4]
S i n c e n o t h e r m o d y n a m i c q u a n t i t y c o n c e r n i n g [2] is
available, t h e f o l l o w i n g c h e m i c a l e q u i l i b r i u m w a s cons i d e r e d i n s t e a d of [2] a n d [3]
A1Br4 + 1/2 A12Brs = A12BrT-
K'I
[5]
w h e r e t h e e q u i l i b r i u m c o n s t a n t is d e f i n e d as K',.
T h e e l e c t r o d e r e a c t i o n at a n a l u m i n u m e l e c t r o d e i n t h e s e
m e l t s c a n b e d e s c r i b e d as
A1Br4 +3e
=AI+4Br-
[6]
T h e E M F of t h e c o n c e n t r a t i o n cell is g i v e n b y
0.5
I
I
I
oO
oOI
2607
Table I. Calculated mole fraction values for Br-, AIBr4 , AI2Br7in the AIBr3-NaBr melt
X(A1Br3)
0.497
0.51
0.55
X(Br-)
X(A1Br4 )
X(A12Br7- )
0.01
0.49
--
-0.48
0.02
0.39
0.11
E1 =
4RT
a(Br )ref
in -
3F
a(Br-)i
RT
a(A1Br(),
+ - - in
3F
a(AlBr4-)ref
-
-
[7]
w h e r e r e f c o r r e s p o n d s to t h e N a B r s a t u r a t e d melt.
Figure 1 shows the relationship between the measured
EMF values and melt compositions. Near the equimolar
c o m p o s i t i o n , t h e r e is a n a b r u p t c h a n g e o f t h e EMF. T a b l e I
i n d i c a t e s m o l e f r a c t i o n s of t h e v a r i o u s s p e c i e s w h e n K2 is
a s s u m e d to b e zero. G o i n g f r o m 49.7 to 51.0 m/o A1Br3, t h e
m o l e f r a c t i o n v a l u e of A1Br4 is a l m o s t c o n s t a n t , w h i l e t h a t
o f B r - s h a r p l y d e c r e a s e s . ( A b o v e 50 m/o A1Br3, t h e brom i d e ion c o n c e n t r a t i o n is zero i n t h i s e x t r e m e case.) So,
t h e a b r u p t c h a n g e of t h e E M F v a l u e s n e a r t h e e q u i m o l a r
p o i n t i n Fig. 1 is a t t r i b u t e d to t h e a c t i v i t y c h a n g e s o f B r .
T h e r e f o r e , in t h e 49.7-51.0 m/o A1Br3 region, t h e s e c o n d
t e r m in Eq. [7] c a n b e n e g l e c t e d a n d Eq. [7] is r e p l a c e d b y
Eq. [8].
E2 =
4RT
3F
a ( B r )r~f 4 R T
X(Br-)~f
In - - -In - a(Br-)i
3F
X(Br-)i
[8]
I n t h e last e x p r e s s i o n , t h e a c t i v i t y t e r m a is s u b s t i t u t e d for
b y t h e m o l e fraction, X, s i n c e t h e a c t i v i t y v a l u e s are q u i t e
s m a l l a n d a c t i v i t y coefficients are e x p e c t e d to b e close to
unity.
T h e K2 v a l u e c a n b e o b t a i n e d b y a c u r v e fitting p r o c e d u r e b e t w e e n t h e o b s e r v e d E M F v a l u e s a n d Eq. [8] b y
u s i n g t h e d a t a i n t h e n e u t r a l region.
I n o r d e r to o b t a i n a K'I value, t h e d a t a s e t in t h e 51.0-65.0
m/o AIBr~ r e g i o n w a s used. I n t h i s region, t h e A1Br4- conc e n t r a t i o n is r e l a t i v e l y h i g h (>0.1 m o l e fraction), so t h e act i v i t y coefficient of A1Br4-, w h i c h is e x p r e s s e d b y Eq. [9],
w a s t a k e n i n t o a c c o u n t in t h e c a l c u l a t i o n p r o c e d u r e (7)
R T In (YN) = WT(1 -- X) 2
[9]
w h e r e YN is t h e a c t i v i t y coefficient, a n d WT is t h e w o r k constant.
cl o
0.4
0.6
0.5
0.3
0.4
O.2
[]
>
0.3
El
"'
0.2
I.IJ
0.1
I
0
@
50
0.1
J
55
I
60
AIBr 3
mlo
i
65
Fig. 1. Relationship between the measured EMF values and melt
compositions, 9 beta-alumina, 9 Pyrex glass-a, [] Pyrex glass-b, gg
Na + ion selective glass.
0
l
50
I
60
I
70
,I
80
AIBr3 mlo
Fig. 2. Comparison of experimental and calculated aluminum electrode potentials in the AIBr3-NaBr melt as a function of melt composition. Solid line: the calculated curve; circles: the observed EMF values.
J. Electrochem. Soc., Vol. 136, No. 9, September 1989 9 The Electrochemical Society, Inc.
2608
0
I
I
I_
4
A
x
m.
0
..J
I
8I
I
I
I
5O
60
70
80
AIBr 3
mlo
Fig. 3. Activity profiles of the species in the AIBr3-NaBr melt. Curve
a: AIBr4-, curve b: AI2BrT-, curve c: AI~Br6, curve d: Br-.
The mathematical procedure developed by Osteryoung
(7) is applied to the calculation of K2, K'I and WT.
At first, the initial value of K2 was determined by using
the data set in the neutral region. Then the K'~ value was
obtained by using the data set in the acidic region. These
iterative procedures were repeated until the least squares
fit of Eq. [7] to the experimental data was given. Finally,
the following values are obtained
K'I = 3.5, K2 = 4.0 • 10-6, W T
:
1.0 x 103J moP]
The solid line in Fig. 2 shows the calculated curve of the
aluminum electrode potential by using the above constant,
while the circles indicate the observed EMF values.
As described above, Tremillon et al. determined the
equilibrium constant of the auto-dissociation of A1Br((11)
K = [A12Brc] [Br-]; - log K = 4.40 +_ 0.05
(The unit of K is tool 2 kg-2.)
This value corresponds to the K2 value of 5.0 x 10 -~ and
agrees well with that obtained in this work. K2 was determined to be 2.23 x 10 -7 at 200~ (7) for the A1C13-NaC1
melts, which is slightly smaller than that of the bromoaluminate melts. This implies -that for the same cationic
constituent the amount of "free" halide is less in the
chloroaluminate system than in the bromoaluminate melt.
Figure 3 shows the activity profiles of each chemical species, derived from the K'I and K2 values. The activity
values are indicated in units of mole fraction. It is clear
from the illustration that the dominant anion species is
A1Br4 below 60 m/o A1Br3. As a whole, the activity profiles
are similar to those of the chloroaluminates.
Summary
The acid-base properties of the A1Br3-NaBr melts were
examined on the basis of the EMF measurements of an
aluminum -aluminum concentration cell and the following
information was obtained:
1. The A1Br3-NaBr melts show a composition-dependent Lewis acidity that is similar to that found in the chloroaluminate melts.
2. The equilibrium constants of the two chemical equilibria (Eq. [4] and [5]) that characterize these melts have
been determined: K'I = 3.5, K2 = 4.0 x 10 -6.
3. For the same cationic constituent, the amount of
"free" halide in the chloroaluminate melt is less than that
in the bromoaluminate melt. This implies that the molecular dimer (A12Br6) is very stable and behaves as a strong
Lewis acid in the bromoaluminate melts.
Manuscript submitted Feb. 1, 1988; revised manuscript
received Feb. 6, 1989.
Kyoto University assisted in meeting the publication
costs of this article.
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2. K. Tanemoto, A. Katagiri, and G. Mamantov, ibid., 130,
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3. A. G. Cavinato, G. Mamantov, and X. B. Cox III, ibid.,
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120, 223 (1973).
8. H. L. Jones and R. A. Osteryoung, in "Advances in
Molten Salt Chemistry," Vol. 3, J. Braunstein, G.
Mamantov, and G. P. Smith, Editors, Chap. 3, p. 121,
P l e n u m Press, New York (1975).
9. C. R. Boston, in ibid., Vol. 1, Chap. 3, p. 121, (1971).
10. J. Kendall, E. D. Crittenden, and H. K. Miller, J. Am.
Chem. Soc., 45, 963 (1923).
11. B. Tremillon and J. P. Duchange, J. Electroanal.
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