Vapor-Liquid Equilibriumof the Binary System HF-H20
Extending to ExtremelyAnhydrousHydrogen Fluoride
N. Miki, M. Maeno, and K. Maruhashi
Hashimoto Chemical, Limited, Kaizan-cho Sakai 590, Osaka, Japan
T. Ohmi*
Department ~ Electronics, Faculty ~ Engineering, Tohoku University, Sendai 980, Japan
ABSTRACT
The vapor-liquid equilibrium of the binary system HF-H20 was determined, especially in the range of low water concentrations. The purity of anhydrous hydrogen fluoride used in this work was 9N (99.9999999%) with a conductivity of 0.7
• 10 -6 S c m - ' (water concentration of 0.033 ppm). We elucidated that the vapor-liquid equilibrium of the binary system
HF-H20 obeyed Raoult's law below 1 x 10 3 tool dm -3 (18 ppm) water concentration. We determined the activity coefficients of H F and H20 at the boiling point of the binary system HF-H20.
Munter et al. (1, 2) reported the vapor-liquid equilibrium
of the binary system HF-H20 at atmospheric pressure over
a considerable range of composition. The boiling points
and vapor-liquid compositions of the binary system
HF-H20 determined by them are now utilized as the standard for hydrofluoric acid. Figure 1 shows their boiling
points and vapor-liquid compositions for the binary system HF-H20. They only measured the equilibrium up to
89% hydrogen fluoride (HF) concentration, and the equilibrium has not yet been measured in the range below 10%
water concentration. Vieweg (3) has reviewed and unified
the available data on vapor pressure over HF-H20 mixtures for the range of composition of 5~80% HF. It is important for promoting a technology of dehydration of HF that
the equilibrium of the binary system HF-H20 is determine d in the range of trace amounts of water. Here we
have determined the binary system equilibrium, especially in the range of low water concentrations, using the
conductivity method developed by ourselves (4, 5). We
have calculated the activity coefficient of HF and H20 at
the boiling point for the entire binary system HF-H20.
Experimental
Composition of the binary system HF-H20.--Figure 3
shows the relation between conductivities and water concentrations determined by known additions of water and
by the Karl Fischer method. The conductivity of anhydrous hydrogen fluoride in the closed system attained
the level of 0.7 • 10 -6 S c m - ' for continuous flowing anhydrous hydrogen fluoride. In the range of water concentrations below 1 • 10 -2 mol dm -3 (180 ppm), a definite
quantity of water was added to the anhydrous hydrogen
fluoride, and the conductivity was measured. In the range
of water concentrations over 1 • 10 -2 mol dm -3 (180 ppm),
the water concentrations were measured using the Karl
Fischer method. The calibration curve was obtained from
both results (4, 5). For the vapor-liquid equilibria, measured water contents less than 1 mol dm -3 (18,000 ppm) by
the conductivity method using the calibration curve and
greater than 1 mol dm -3 (18,000 ppm) using a neutralization titration method.
Measurement of Vapor-Liquid Equilibrium
Figure 4 shows the measurement apparatus for the vapor-liquid equilibrium. First, anhydrous hydrogen fluoride was introduced into the vessel A from the ultra puri-
Principle of measurement--Anhydrous hydrogen fluoride exhibits a hygroscopic nature, so measurements of the
equilibrium were carried out under the following two conditions: (i) an apparatus consisting of a closed circulating
system and (ii) measuring instruments set in the system.
Figure 2 shows the principle of the closed circulating
system. This system consists of a vaporization part and a
condensation part, with conductivity cells for measuring
water concentrations set in both parts. In this work, the
closed conductivity cells used were entirely made of
fluorocarbon polymer with two bright platinum electrodes.
Materials.--The purity of anhydrous hydrogen fluoride
used is 9N level (1 ppb m a x for every anion impurity and
0.1 ppb m ax for every cation impurity), and its conductivity is 0.7 x 10 -6 S cm -1 (at 0~ that is, the water concentration is below 1.85 • 10 -6 mol dm -3 (0.033 ppm). Anhydrous
hydrogen fluoride was charged into the apparatus from an
ultrapurifying process which is based on a new chemical
distillation principle (6). The new chemical distillation
principle consists of ultra dehydration and a fluorine oxidation process. The complexity of the chemical formulas
of the impurities converges to simple anhydrous structures due to the extreme dehydration of the hydrogen
fluoride, and the various valences of the impurities converge to the highest oxidation state by the fluorine oxidation. Consequently, all of the impurities having various
chemical formulas are simplified to structures removable
from hydrogen fluoride, which can be purified perfectly by
the distillation.
* Electrochemical Society Active Member.
120
2
"0.,
100
o
\
80
4-'-
C~
\
60
\
E 40
IX
20
20
40
HF
60
80
I00
(Wt%)
Fig. 1. Boiling points and vapor-liquid composition of the binary system HF-H20. 1, Liquid phase; 2, vapor phase.
J. Electrochem. Soc., Vol. 137, No. 3, March 1990 9 The Electrochemical Society, Inc.
787
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788
I
Vaporization part
HF gas
T .
K --
.
.
.~
.
"
.
0
f
Cooler
T : Thermocouple
K : Cell
0
HI:" liq
T2
Condensation p a r t
<
J
"5
G
Fig. 2. Schematic diagram of the closed-circulating system for vaporliquid equilibrium.
t y i n g p r o c e s s , after w h i c h a definite q u a n t i t y o f w a t e r w a s
a d d e d f r o m i n j e c t i o n hole I. A t this t i m e , t h e w a t e r c o n c e n t r a t i o n was m e a s u r e d u s i n g c o n d u c t i v i t y cell K1. T h e hyd r o g e n fluoride w a s t h e n h e a t e d to i n c r e a s e its v a p o r p r e s s u r e a n d e x h a u s t e d f r o m t h e p o i n t F in Fig. 4 b y r e d u c e d
p r e s s u r e to e l i m i n a t e t h e air in t h e s y s t e m . S u b s e q u e n t l y ,
h y d r o g e n fluoride w a s statically v a p o r i z e d , a n d t h e n t h e
v a p o r o f h y d r o g e n fluoride was c o n d e n s e d b y c o n d e n s e r G
to r e t u r n to t h e v e s s e l A. T h e k e y p o i n t for this e x p e r i m e n t
is t h e static v a p o r i z a t i o n m e t h o d , b e c a u s e v i g o r o u s boiling
disturbs the vapor-liquid equilibrium. Circulation was
c o n t i n u e d until t e m p e r a t u r e s TI, T2, a n d T~ a n d c o n d u c t i v ities kl a n d k2 r e a c h e d c o n s t a n t values.
II
AHF outlet
Fig. 4. Measurement apparatus for vapor-liquid equilibrium: A, vessel; B, tower; C, condensed hydrogen fluoride, D, capillary; E, control
vessel for pressure; F, vacuum system; G, condenser; H, heater; I, injection hole with fluoro-polymer rubber; T1, T2, T3, thermocouple; K=, K2,
cell.
Results and Discussion
The vapor-liquid equilibrium of the binary system
HF-H20 h a s b e e n d e t e r m i n e d at a t m o s p h e r i c p r e s s u r e , esp e c i a l l y in t h e r a n g e of low w a t e r c o n c e n t r a t i o n s . Table I
a n d T a b l e II p r e s e n t t h e l i q u i d a n d v a p o r c o m p o s i t i o n s
a n d r e l a t e d boiling p o i n t s d e t e r m i n e d for t h e b i n a r y syst e m HF-H20.
F i g u r e 5 s h o w s t h e relation b e t w e e n t h e b o i l i n g p o i n t s
a n d v a p o r - l i q u i d c o m p o s i t i o n for t h e low w a t e r c o n c e n t r a t i o n s in t h e b i n a r y s y s t e m HF-H20, u s i n g t h e d a t a in Table
II. T h e b i n a r y s y s t e m in t h e r e g i o n o f low w a t e r c o n c e n t r a t i o n s has b e e n a c c u r a t e l y d e t e r m i n e d in t h i s w o r k . T h e
c o n v e n t i o n a l boiling p o i n t s h a v e b e e n d e m o n s t r a t e d to coi n c i d e well w i t h o u r d a t a for h y d r o g e n fluoride c o n c e n t r a t i o n s u p to 89%. O n t h e o t h e r h a n d , in t h e r e g i o n o f v e r y
l o w w a t e r c o n c e n t r a t i o n s , t h e v a p o r c o m p o s i t i o n (line 2) is
c o m p l e t e l y d i f f e r e n t f r o m t h e p r e v i o u s l y r e p o r t e d data
(1, 2). It is c o n s i d e r e d t h a t this d i f f e r e n c e is c a u s e d b y t h e
analytical m e t h o d . T h e p r e v i o u s w o r k e r s s a m p l e d t h e cond e n s a t e , d i l u t e d w i t h ice, a n d a n a l y z e d for h y d r o g e n fluoride b y titration. It is c o n s i d e r e d likely t h a t t h e s a m p l e s ab-
CMzo (ppm)
1.8x10"3 1.8xlO
"2 1.8xlO
-I I.SxlO~ 1.8xlO= 1.8xlO2 1.8xlO~ IBxl04
10-1
_A
10-2
tf
'E
r
Table I. Composition in liquid and vapor phase at boiling point of the
system HF-H20
T1
(~
(•
108.5
113.7
113.2
101.5
76.0
57.1
35.5
29.8
20.3
19.8
19.5
19.5
19.5
Liquid
kl (0~
1
--------46800
25500
9041
947
502
CH2O
(ppm)
T2
(~
Vapor
7'3
k2 (0~
(~ (•
6Scm-1)
754000 a
580000"
563000a
454000"
340000"
259000 a
125000a
84000a
5570
2390
600
44
20
108.3
113.6
113.0
101.4
75.8
57.0
35,4
29,8
20,2
20,0
19,8
19.5
19.5
15.5
15.6
15.6
15.4
15.4
15.3
15.3
15.2
14.8
15.1
15.1
15.2
15.0
----41000
28000
13500
8700
2022
1081
350
20
10
CH2o
(ppm)
932800a
481000 a
38,9500a
52900"
4600
2700
1000
570
89
46
12
0.64
0.45
"Neutralization titration.
I0"~
"90 10-4
.>_
I0 "5
o
s o r b e d m o i s t u r e in t h e c o u r s e of t h e analysis p r o c e s s ,
w h i c h r e s u l t e d in h i g h e r c o n c e n t r a t i o n s o f water. T h e conv e n t i o n a l m e t h o d is n o t able to m e a s u r e a c c u r a t e l y a t r a c e
a m o u n t o f w a t e r in H F b e c a u s e o f its o p e n s y s t e m configuration and the accompanying absorption of water from
t h e e n v i r o n m e n t d u e to e x t e r n a l l e a k a g e o f t h e m e a s u r i n g
s y s t e m a n d b a c k d i f f u s i o n of w a t e r f r o m t h e o p e n e n d of
t h e s y s t e m . In this w o r k , t h e m e a s u r i n g s y s t e m w a s
/
iO-e
7
/
Table II. Activity coefficients in the binary system HF-H20
HF
(weight Percent)
/
iO-V
10-7
10-6
Water
10-5
10-4
Concentration
I0-3
10-2
i0-I
10o
CHao ( tool dm -a)
Fig. 3. The relation of water concentration and conductivity of hydrogen fluoride. (A, - - 9
(O~ this work; (B, - - 0 - - ) (O~ this work
(Karl Fiscber's method); (C, ----) (15~ Ref. (11); (D, - - - - ) (15~
Ref. (7).
Liquid
Vapor
24.6
42.0
43.7
54.6
66.0
74.1
87.5
91.6
99.443
99.761
99.940
99.9956
99.9980
6.72
51.90
61.05
94.71
99.54
99.73
99.90
99.943
99.9911
99.9954
99.9988
99.999916
99.999955
Vapor
pressure
Activity
Boiling
point
(mmHg)
coefficient
(~
Po (HF) Po (H20) 71 (HF) 72 (H20)
108.5
113.7
113.2
101.5
76.0
57.1
35.5
29_8
20,3
19.8
19.5
19.5
19.5
9614
10810
10690
8175
4321
2554
1311
1090
787
768
760
760
760
1021.5
1215.2
1195.3
801.6
301.4
129.8
43.4
31.5
17.9
17.3
17.0
17.0
17.0
0.02
0.09
0.10
0.18
0.27
0.41
0.67
0.77
9.97
1.00
1.00
1.00
1.00
0.90
0.52
0.45
0.12
0.04
0.06
0.14
0.17
0.68
0.86
0.90
1,00
1.00
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I}
Table III. Activity coefficients in the binary system HF-H20 from the
data of Munter and Vieweg
2
I
I/
30
~
20
80
90
99
Fig. 5. Boiling points and vapor-liquid composition for low H20 concentrations in the binary system HF-H20 1, liquid phase; 2, vapor
phase.
closed, as s h o w n in Fig. 4 and, m o r e o v e r , t h e s y s t e m w a s
c o m p l e t e l y c o v e r e d b y a n i t r o g e n e n v i r o n m e n t i n o r d e r to
s u p p r e s s t h e i n c u r s i o n of w a t e r t h r o u g h e x t e r n a l leakage.
Figure 6 shows the relation between the water content in
t h e v a p o r a n d l i q u i d p h a s e s i n t h e b i n a r y s y s t e m HF-H20.
T h e c o n c e n t r a t i o n o f w a t e r in t h e v a p o r p h a s e i n t h i s w o r k
is d i f f e r e n t f r o m t h e v a l u e o b t a i n e d b y M u n t e r a n d V i e w e g
o v e r t h e r a n g e of 60-90% H F c o n c e n t r a t i o n i n t h e l i q u i d
phase.
E v e r y p a r t i a l p r e s s u r e i n t h e v a p o r - l i q u i d e q u i l i b r i u m of
t h e b i n a r y s y s t e m H F - H 2 0 g e n e r a l l y o b e y s H e n r y ' s l a w [1]
p~
P%
~,
X~
=
=
=
=
at t~
[1]
p a r t i a l p r e s s u r e of n t h c o m p o n e n t
v a p o r p r e s s u r e of n t h c o m p o n e n t at t~
a c t i v i t y coefficient of n t h c o m p o n e n t
m o l e f r a c t i o n of n t h c o m p o n e n t i n l i q u i d
T h e d a s h e d l i n e i n Fig. 6 i n d i c a t e s t h e r e l a t i o n b e t w e e n
the water concentrations in the vapor and liquid phases
o b t a i n e d b y c a l c u l a t i o n u s i n g ~/H2o= 1. I t is r e c o g n i z e d t h a t
i0-5
I0 -4
102
i
lO-Z
I
i
!
i
o This work
A
i0 I
9 MuntePs data
8433
9183
10046
9120
3250
1769
1224
836
950
1087
939
192
71
38
0.02
0.03
0.05
0.14
0.34
0.54
0.70
0.98
0.91
0.80
0.33
0.12
0.40
0.82
Vieweg (3)
10
2.01
20
6.78
30
19.26
40
43.52
50
74.25
60
90.99
70
98.73
80
99.91
102.8
106.6
110.3
111.7
106.2
87.0
65,5
47,6
8433
9184
10046
10337
9120
5743
3250
1923
836
950
1087
1136
939
469
192
82
0.02
0.03
0.05
0.08
0.13
0.21
0.34
0.50
0.98
0.91
0.80
0.63
0.43
0.38
0.17
0.04
the experimental data coincide with the theoretical line
b e l o w 1 x 10 -~ m o l d m -3 0 8 p p m ) o f water. T h e s l o p e of
the relation between the water concentrations in liquid
a n d i n v a p o r b e c a m e 1 i n t h e r a n g e b e l o w 1 • 10 -z m o l
d m -3 (18 p p m ) w a t e r in h y d r o g e n fluoride. T h i s r e s u l t led
to t h e c o n c l u s i o n t h a t t h e v a p o r - l i q u i d e q u i l i b r i u m of t h e
b i n a r y s y s t e m H F - H 2 0 o b e y s R a o u l t ' s l a w b e l o w 1 x 10 -3
m o l d m -z (18 p p m ) w a t e r c o n c e n t r a t i o n .
T h e a c t i v i t y coefficient of H20(~/s2o) i n H F h a s n o t b e e n
r e p o r t e d p r e v i o u s l y . A c t i v i t y coefficients of H F a n d H20
h a v e b e e n d e t e r m i n e d o v e r a w i d e r a n g e of c o m p o s i t i o n at
t h e b o i l i n g p o i n t s at a t m o s p h e r i c p r e s s u r e in t h e b i n a r y
s y s t e m HF-H20, as c a l c u l a t e d f r o m t h e f o l l o w i n g e q u a t i o n
[2] (8, 9). T a b l e III s h o w s t h e a c t i v i t y coefficients c a l c u l a t e d
f r o m t h e d a t a of M u n t e r e t a L a n d V i e w e g (2, 3).
P Yi
~h at t~
[2]
Pi xi
~
p
pi
x~
y~
=
=
=
=
=
a c t i v i t y coefficient of i t h c o m p o n e n t
t o t a l p r e s s u r e of s y s t e m
v a p o r p r e s s u r e o f i t h c o m p o n e n t at t~
m o l e f r a c t i o n of i t h c o m p o n e n t in l i q u i d
m o l e f r a c t i o n of i t h c o m p o n e n t in v a p o r
T h e r e s u l t s are s h o w n i n T a b l e II a n d Fig. 7. T h e v a l u e s
o f ~H~o c a l c u l a t e d f r o m t h e r e p o r t e d d a t a (1-3) are d i f f e r e n t
H2OinLiquid Phase
I00
1.0
80
.
60
.
.
40
.
(wt %)
20
0
.
E
)
a Vieweg*s data
---Raoult's law
,1--
~ , I0 ~
102.8
106.6
I10.3
106.2
65.5
44.9
33.3
I0 t (mol dm-a)
I0 ~
lO-J
i
Munter (1)
10
2.0
20
7.1
30
19.4
50
80.0
70
98.8
81
99.3
89
99.5
I00
HF ( w t % )
p, = ~nP~
Vapor
pressure
Activity
Boiling
(mmHg)
coefficient
point
(~
Po (HF) Po (H20) ~t (HF) ~2 (H20)
(weightHF
percent)
Liquid Vapor
40
2
789
:~ o.8
i1-.
j
//
o
0
(
0.6
0
r" I0 -I
//
Dt'~ 10-2
r
/~r
.>_
0
<
sp 9
0.4
o.,
sS
~.0
10-3
o
-r~ 10-4
lO "ll
./' I
10-4
10-3
I
I0 -z
I
10-I
H20 of Liquid Phase
X"j
._~.~-~.
0
I
I0~
I
I01
I0 z
(wt %)
Fig. 6. Vapor-liquid equilibrium of the binary system HF-H20
20
, .---- ,~
40
60
80
O0
HFin Liquid Phase
(wt%)
This work o ?'~ (HF) Munter's data ,, 1', (HF)
9 I'2 (H20)
9 1'2 (H20)
Vieweg'sdata u 1', (HF)
9 ~'2 (H20)
Fig. 7. Activity coefficients of the binary system HF-H20
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790
J. Electrochem. Soc., Vol. 137, No. 3, March 1990 9 The Electrochemical Society, Inc.
from those calculated from our data, particularly at low
water concentrations in HF, though the reported values of
~]H~ coincide well with our data in the binary system
HF-H20 with decrease of water. The activity coefficient for
HF (~HF) monotonically increases and approaches 1 with a
decrease of water concentration, while the activity coefficient for H20 (~H2o)exhibits a m i n i m u m value at about 70%
HF in the binary system HF-H20. The existence of the
m i n i m u m value of "m2o is explained as follows. The liquid
structure of the HF-H20 system has already been demonstrated to exhibit a m i n i m u m 'H chemical shift at about
70% H F by NMR measurement, due to a m a x i m u m formation of H30§
and, moreover, the density of HF
solutions is well known to exhibit a m a x i m u m value at
about 70% HF because of the strong hydrogen bond (10). In
the extremely anhydrous region of the binary system
HF-H20, the value of ~H2oapproaches 1, where water molecules ideally dissociate.
Conclusion
The vapor-liquid equilibrium of the binary system
HF-H20 has been determined, especially in the range of
low water concentrations.
It has been demonstrated experimentally that the vaporliquid equilibrium of the binary system HF-H20 obeys
Raoult's law below 1 • 10-3 mol dm -a (18 ppm) water concentration. We have determined the activity coefficients of
H F and H20 at the boiling point of the binary system
HF-H20. It has been shown that the activity coefficient of
H20 has a m i n i m u m value at a concentration of about 70%
H F and approaches 1 with a decrease of water concentration.
Manuscript submitted Nov. 10, 1988; revised manuscript
received Aug. 13, 1989.
Hashimoto Chemical, Limited, assisted in meeting the
publication costs of this article.
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1504 (1949).
3. R. Vieweg, Chem. Tech. (Berlin), 15, 734 (1963).
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(1980).
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(1990).
6. N. Miki, U. S. Pat. 4,668,497 (1987).
7. R. Ukazi and I. Kageyama, Jpn. Anal., 9, 604 (1960).
8. R. L. Jewry, and W. Davis, Jr., J. Phys. Chem., 57, 600
(1953).
9. Chemical Handbook, Japan Chemical Association
(1984).
10. M. F. A. Dove and A. F. Clifford, "Chemistry in Nonaqueous Ionizing Solvents," Vol. II, Part I. pp. 212218, Pergamon Press (1971).
11. Draft, International Standard ISOfDIS 3700-2 (1978).
Conductivity and Dissociation Equilibrium of Extremely
Anhydrous Hydrogen Fluoride
N. Miki and M. Maeno
Hashimoto Chemical, Limited, Kaizan-cho Sakai 590, Osaka, Japan
T. Ohmi*
Department of Electronics, Faculty of Engineering, Tohoku University, Sendai 980, Japan
ABSTRACT
We have studied the conductivity of extremely anhydrous hydrogen fluoride (AHF) and obtained a m i n i m u m conductivity of 0.7 • 10 -6 S cm -1 at 0~ We have recognized that water in A H F at concentrations below 1 • 10 -3 tool dm -3 shows
an ideal dissociation and that the limiting equivalent conductance of AHF is 436 S cm 2 mol-1 at 0~ The ideal relationship
between the conductivity and water concentration obtained in this work was extrapolated to the ultra-micro water concentration region. The conductivity of 0.7 x 10-6 S cm -1 corresponds to a water concentration of 1.85 • 10 -6 tool dm -3
(0.033 ppm) in the above relationship; however, this conductivity is not due to the dissociation of water but to that of hydrogen fluoride. We recognized that the relationship between the conductivity and water concentration of A H F coincides
completely with that of ultra-pure water in the ultra-micro conductivity region (10 -8 S cm-1).
The relationship between the dissociation of water and
the conductivity in anhydrous hydrogen fluoride is important for the electrochemical theory, the analytical technology of water, and the dehydration technology of hydrogen
fluoride.
Frendenhagen et al. studied conductivities over the
range from 0.013 to 0.5N water in hydrogen fluoride and
obtained a conductivity of 1.4 x 1O-5 S cm -1 for anhydrous
hydrogen fluoride at 15~ (1-4).
Kilpatrick et al. improved the methods for the purification of anhydrous hydrogen fluoride, which yielded acid
with m u c h lower conductivity, 1.6 • 10-6 S cm ~1 at 0~
than reported previously (5-7).
Rogers examined the dehydration of hydrogen fluoride
by CoF2 and Hyman tried the fractional distillation of hydrogen fluoride, but their conductivities for anhydrous hydrogen fluoride were only at the 10 -4 S cm -1 level (8, 9).
Ukazi et al. analytically studied conductivities over the
range from 0.1 to 5% water in anhydrous hydrofluoric acid
* Electrochemical Society Active Member.
with a conductivity of 10 2 S cm i, which was distilled
after heating a mixture of HF and F~ to 200~176
(10).
Netzer et al. described a modified fractionation system
from which they obtained hydrogen fluoride with a conductivity of about 1 x l0 -6 S cm -1 at 25~ (11).
The International Organization for Standardization
(ISO) has presented a conductometric method for determination of water concentrations in industrial anhydrous hydrogen.fluoride over the range of 0.01-0.4% water (12).
We have reported previously the conductivities below
0.01% water concentration in anhydrous hydrogen fluroide
(13). We now report that we have measured the conductivity of anhydrous hydrogen fluoride with lower water concentrations and studied theoretically the dissociation of
water in extremely anhydrous hydrogen fluoride.
Experimental
Materials.--Hydrogen fluoride was purified by an ultra
purifying process to reach a very low level of impurities as
shown in Table I. The ultra purifying process, which is
based on a new chemical distillation principle, consists of
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