Preparation of Pure Fused Lithium Chloride-Potassium Chloride
Eutectic Solvent
H. A. Laitinen
Department of Chemistry and Chemical Engineering, University oJ Illinois, Urbana, Illinois
W. S. Ferguson
Research Department, The Ohio Oil Company, Littleton, Colorado
and
R. A. Osteryoung
Department o] Chemistry, Rensselaer Polytechnic Institute, Troy, N e w Y o r k
ABSTRACT
H y d r o l y t i c decomposition occurs d u r i n g the fusion of a eutectic m i x t u r e of
l i t h i u m chloride-potassium chloride containing traces of m o i s t u r e if the fusion
conditions are not controlled. The r e s u l t a n t c o n t a m i n a t i o n b y h y d r o x y l ion
g r e a t l y lowers the u t i l i t y of this m i x t u r e as a fused salt solvent. The effectiveness of various p r o c e d u r e s used for p r e p a r a t i o n of the fused salt solvent was
followed b y observation of the characteristic p o l a r o g r a p h i c r e s i d u a l c u r r e n t
using a p l a t i n u m microelectrode. A p r e p a r a t i v e m e t h o d is described which
involves d r y i n g the m i x t u r e u n d e r m o d e r a t e vacuum, fusion u n d e r a n h y d r o u s
h y d r o g e n chloride, and r e m o v a l of the h y d r o g e n chloride from the melt.
F o r t h e i n i t i a l w o r k of a l o n g - t e r m g e n e r a l i n v e s t i g a t i o n in f u s e d s a l t e l e c t r o c h e m i s t r y , a e u t e c t i c
m i x t u r e of LiC1-KC1 w a s s e l e c t e d as t h e f u s e d s a l t
solvent. A d v a n t a g e o u s c h a r a c t e r i s t i c s of this solv e n t a r e g o o d t h e r m a l s t a b i l i t y , w i d e s p a n of
e l e c t r o l y t i c d e c o m p o s i t i o n p o t e n t i a l , a b s e n c e of
s t r o n g l y a c i d i c or b a s i c p r o p e r t i e s , h i g h c o n d u c t i v i t y
a n d fluidity, a n d c o m m e r c i a l a v a i l a b i l i t y of its c o m p o n e n t s as t h e r e a g e n t g r a d e salts. F u r t h e r , t h e
m e l t i n g p o i n t is l o w e n o u g h a n d its c h e m i c a l r e a c t i v i t y is s u c h t h a t P y r e x glass c a n be u t i l i z e d as
a m a t e r i a l of c o n s t r u c t i o n . T h e p r o p e r t i e s of t h e
s o l v e n t t h u s p e r m i t , in t h e o r y , a s t u d y of t h e e l e c t r o c h e m i s t r y of a w i d e v a r i e t y of s o l u t e s w i t h a
m i n i m u m of i n t e r f e r e n c e f r o m s o l v e n t effects a n d
w i t h o n l y m o d e r a t e e x p e r i m e n t a l difficulty as h i g h
t e m p e r a t u r e w o r k goes.
I n p r a c t i c e , t h e p o t e n t i a l i t y of t h e s e d e s i r a b l e s o l vent properties, particularly the wide decomposit i o n p o t e n t i a l span, is l i m i t e d b y t h e p u r i t y of t h e
f u s e d s o l v e n t w h i c h c a n be p r e p a r e d . T h e m o s t
t r o u b l e s o m e a s p e c t of p r e p a r i n g a p u r e LiC1-KC1
eutectic fusion arises from the highly deliquescent
p r o p e r t y of t h e LiC1. U n l e s s s p e c i a l p r e c a u t i o n s a r e
t a k e n , h y d r o l y t i c d e c o m p o s i t i o n o c c u r s as t h e t e m p e r a t u r e is r a i s e d a n d HC1 is lost f r o m t h e s y s t e m .
Concurrently, the melt becomes contaminated with
alkaline products.
V a r i o u s m e t h o d s h a v e b e e n p r o p o s e d a n d u s e d to
prevent hydrolytic decomposition during the prep a r a t i o n of a n h y d r o u s c h l o r i d e s . T h e h e a t i n g of
h y d r a t e d r a r e e a r t h c h l o r i d e s w i t h gross a m o u n t s of
NH,C1 is o n e classic m e t h o d . F o r t h e p r e p a r a t i o n of
f u s e d LiC1-KC1 m i x t u r e s c o n t a i n i n g p o t a s s i u m
h e x a c h l o r o m o l y b d a t e , t h e m e t h o d of h e a t i n g t h e
s a l t m i x t u r e to f u s i o n in a s t r e a m of d r y , i n e r t gas
f o l l o w e d b y an e l e c t r o l y s i s of t h e m e l t f o r r e m o v a l
of o x y g e n as m o l y b d e n u m o x i d e s a t t h e c a t h o d e a n d
b y v o l a t i l i z a t i o n of m o l y b d e n u m o x y c h l o r i d e s h a s
b e e n u s e d (1). A n o t h e r p r o c e d u r e i n v o l v e s d r y i n g
t h e LiC1 a n d t h e KC1 s e p a r a t e l y b y first s l o w l y
h e a t i n g t h e s a l t s to b e t w e e n 480 ~ a n d 500~ u n d e r
h i g h v a c u u m (10 -~ to less t h a n 104 m m H g ) , t h e n
mixing the dried salts together under a controlled
a t m o s p h e r e in a d r y box, a n d f i n a l l y f u s i n g t h e d r y
m i x t u r e u n d e r h i g h v a c u u m (2, 3). T h e m e t h o d to
be described here involves drying the eutectic mixture under moderate vacuum, fusing the mixture
u n d e r a n h y d r o u s HC1 at a t m o s p h e r i c p r e s s u r e , a n d
t h e n r e m o v i n g t h e excess HC1 f r o m t h e f u s e d solvent.
Experimental Procedure
T h e a p p a r a t u s u s e d in t h i s w o r k i n c l u d e s t h e f o l lowing:
F u r n a c e : C e n c o - C o o l e y No. 13627, 115 V, 700 W,
C e n t r a l Scientific C o m p a n y , Chicago. A s e p a r a t e
h e a t i n g c i r c u i t e q u i v a l e n t to 25% of t h e o r i g i n a l
f u r n a c e h e a t i n g c a p a c i t y w a s i n s t a l l e d in t h e b o t t o m of t h e f u r n a c e c a v i t y . O n l y t h i s a u x i l i a r y
circuit was operated by the furnace controller.
Controller: Wheelco Indicating Controller Model
241-P, B a r b e r - C o l e m a n C o m p a n y , R o c k f o r d , I l linois.
Vacuum Pump: Cenco "Pressovac," Central Scientific C o m p a n y , Chicago.
C h e m i c a l s : T a n k gases of t h e M a t h e s o n C o m p a n y ,
516
Vol. 104, No. 8
PREPARATION
O F LiC1-KC1 E U T E C T I C
Joliet, Ill., w e r e used. T h e a r g o n w a s passed over
hot Cu to r e m o v e traces of o x y g e n . B o t h a r g o n
a n d HC1 w e r e d r i e d o v e r Mg (C10,)~. R e a g e n t g r a d e
LiC1 a n d KC1 w e r e u s e d to p r e p a r e t h e solvent.
Electrodes: A P t foil i n e q u i l i b r i u m w i t h a d i l u t e
s o l u t i o n of P t ( I I ) i n the f u s e d salt s o l v e n t w a s
used as t h e r e f e r e n c e electrode. This s y s t e m has
b e e n s h o w n to d e t e r m i n e a stable, n o n p o l a r i z a b l e
r e f e r e n c e p o t e n t i a l a n d has b e e n d e s c r i b e d in
v a r i o u s modifications ( 4 - 6 ) . T h e m i c r o e l e c t r o d e s
u s e d consisted of 26 g a u g e P t w i r e sealed into
C o r n i n g 0120 glass w i t h a b o u t 1 m m of t h e w i r e
p r o j e c t i n g ( t o t a l e l e c t r o d e a r e a of 1.4 m m ~) or 18
g a u g e t u n g s t e n w i r e sealed i n t o P y r e x glass w i t h
the w i r e g r o u n d flush to t h e glass ( t o t a l e l e c t r o d e
a r e a of 0.8 mm~).
Polarograph: The polarograms reported were obs e r v e d as a u t o m a t i c a l l y r e c o r d e d c u r v e s b y u s i n g
a n " E l e c t r o c h e m o g r a p h , " T y p e E, Leeds a n d
Northrup, Philadelphia.
F u s e d salt cell: T h e cell u s e d is d e s c r i b e d below.
T h e cell i n w h i c h the s o l v e n t p r e p a r a t i o n p r o c e d u r e is c o m p l e t e d has b e e n f o u n d v e r y u s e f u l i n
performing both chemical and electrochemical exp e r i m e n t s . As it is a g e n e r a l tool for f u s e d salt r e search (6), it is d e s c r i b e d i n g r e a t e r d e t a i l t h a n is
n e c e s s a r y m e r e l y for u s e as t h e s o l v e n t p r e p a r a t i o n
vessel.
Fig. 1 is a n i l l u s t r a t i o n of t h e v a r i o u s cell c o m p o n e n t s . T h e cell as s h o w n is o n l y p a r t i a l l y ass e m b l e d a n d all of t h e v a r i o u s c o m p o n e n t s s h o w n
a r e n o t n e c e s s a r i l y to b e in use at one time. A n e x p l a n a t i o n of the f u n c t i o n of these c o m p o n e n t s serves
to i n d i c a t e w h i c h c o m b i n a t i o n is n e e d e d for a p a r t i c u l a r p u r p o s e . A l l of t h e glass p a r t s are P y r e x
( C o r n i n g 774) e x c e p t as o t h e r w i s e noted.
T h e side a r m t u b e A is the f u s e d salt c o n t a i n e r .
It is 33 c m l o n g b y 6 c m OD a n d i n d i c a t e s t h e scale
of the i l l u s t r a t i o n . I n use, the t u b e c o n t a i n s f u s e d
Fig. 1. Components of the fused salt cell
SOLVENT
517
salt to a d e p t h of a b o u t 41/z c m a n d p r o t r u d e s f r o m
t h e f u r n a c e a b o v e t h e l e v e l L - L w h i c h is 12 c m
f r o m t h e b o t t o m , b u t i n m o r e r e c e n t w o r k has b e e n
i n c r e a s e d to 15 cm. The m e l t i n s i d e t u b e A c a n b e
d i v i d e d into s e v e r a l p o r t i o n s b y the c o m p a r t m e n t s
B w h i c h a r e closed a t t h e b o t t o m b y m e d i u m p o rosity f r i t t e d glass disks. T h e s e p o r t i o n s of m e l t are
t h e n i n e l e c t r o l y t i c c o n t a c t w i t h one a n o t h e r b u t
c r o s s - c o n t a m i n a t i o n of the c o m p a r t m e n t c o n t e n t s is
p r e v e n t e d . A n u m b e r 13 r u b b e r s t o p p e r C serves
to close the t u b e A a n d to s u p p o r t w i t h i n t h e t u b e A
the i n e r t - g a s d e l i v e r y t u b e D, t h e t h e r m o c o u p l e
s h e a t h E, a n d v a r i o u s electrodes. T h o s e holes i n
s t o p p e r C w h i c h are n o t i n use are closed b y glass
p l u g s F.
T h e l a r g e holes i n s t o p p e r C p e r m i t easy m a n i p u l a t i v e access to t h e i n t e r i o r of t u b e A. D u r i n g t h e
e x p e r i m e n t , for e x a m p l e , i n d i v i d u a l c o m p a r t m e n t s
c a n be r e m o v e d f r o m t h e system, u s i n g h o o k H, or
k n o w n a m o u n t s of a s o l u t e c a n b e a d d e d to a n y one
of t h e c o m p a r t m e n t s w i t h i n t h e t u b e , u s i n g p l a t i n u m s p o o n G. T h e e n t r y of air i n t o t h e s y s t e m is
prevented by an argon atmosphere which blankets
the f u s e d salt.
T h e s e v e n holes i n the s t o p p e r m u s t h a v e p a r a l l e l
axes a n d m u s t fit t h e i r r e s p e c t i v e glass p a r t s (10
m m a n d 16 m m i n this case) v e r y s n u g l y b u t w i t h o u t b i n d i n g . It h a s b e e n f o u n d a d v a n t a g e o u s to
bore t h e holes w i t h a d r i l l press, u s i n g a r e g u l a r
t w i s t drill. T h e s t o p p e r is first f r o z e n i n a d r y ice"Cellosolve" mixture.
A p r i n c i p a l a d v a n t a g e of this cell is t h a t s e v e r a l
d i f f e r e n t s o l u t i o n c o m p o s i t i o n s c a n be m a i n t a i n e d
at one t i m e i n s e p a r a t e c o m p a r t m e n t s i n t h e t u b e .
Thus, a n e w b a t h of f u s e d salt s o l v e n t n e e d n o t b e
p r e p a r e d for each c h a n g e of e x p e r i m e n t a l c o n d i tion. T h e precise course of a n e x p e r i m e n t n e e d n o t
be p r e d e t e r m i n e d , since it is a s i m p l e m a t t e r to a l t e r
the c o m p o s i t i o n of a c o m p a r t m e n t at a n y t i m e w i t h out affecting t h e c o m p o s i t i o n of t h e rest of t h e c o m p a r t m e n t s . This is a n e x t r e m e l y v a l u a b l e a d v a n t a g e
i n f u s e d salt r e s e a r c h .
T h e f r i t t e d b a r r i e r s of t h e c o m p a r t m e n t s are v e r y
effective for p r e v e n t i n g diffusion of a s o l u t e out of
its c o m p a r t m e n t . E l e c t r i c a l l y , t h e b a r r i e r s are effic i e n t i n t h a t t h e r e s i s t a n c e is b u t 20 to 50 o h m s b e t w e e n t w o P t w i r e s i m m e r s e d i n the eutectic solv e n t of two d i f f e r e n t c o m p a r t m e n t s .
P r e p a r a t i o n of t h e eutectic s o l v e n t is c a r r i e d out
as follows. R e a g e n t g r a d e LiC1 a n d K C I a r e t a k e n
i n t h e p r o p o r t i o n 0.59 m o l e to 0.41 m o l e a n d m i x e d
together. T h e m i x t u r e is p l a c e d i n a s u i t a b l e vessel
a n d exposed to a v a c u u m (0.1 to 0.2 m m Hg) for 6
h r to r e m o v e excessive m o i s t u r e so t h a t t h e m a t e r i a l
w i l l n o t cake d u r i n g the n e x t step. A l l t r a n s f e r s of
t h e m a t e r i a l f r o m this p o i n t on m u s t be m a d e as
r a p i d l y as possible to a v o i d excessive p i c k u p of
atmospheric moisture. The vacuum-dried mixture
is b a l l - m i l l e d to a fine f r e e - f l o w i n g p o w d e r i n a
clean, t i g h t l y closed jar, u s i n g p o r c e l a i n b a l l s w h i c h
h a v e p r e v i o u s l y b e e n l e a c h e d w i t h 6N HC1, r i n s e d
with pure water, and oven-dried. The ground mixt u r e is stored i n 1-1b s c r e w - c a p j a r s u n t i l n e e d e d .
It has b e e n k e p t as long as 1 y e a r w i t h o u t c a k i n g ,
p r o v i d e d t h a t t h e g r o u n d m i x t u r e is v a c u u m - d r i e d
518
JOURNAL
OF T H E E L E C T R O C H E M I C A L
at r o o m t e m p e r a t u r e for a few h o u r s i m m e d i a t e l y
a f t e r b e i n g t r a n s f e r r e d into the 1-1b c o n t a i n e r s .
A l a r g e desiccator h a v i n g a n a p p r o p r i a t e l y d r i l l e d
L u c i t e p l a t e serves to sieve t h e b a l l s f r o m t h e m i x t u r e w i t h o u t u n d u e e x p o s u r e to a t m o s p h e r i c m o i s ture.
T h e s o l v e n t p r e p a r a t i o n for a n e x p e r i m e n t , w o r k ing f r o m t h e v a c u u m - d r i e d , g r o u n d m i x t u r e , is
c o m p l e t e d i n t u b e A, r e f e r r i n g to Fig. 1. T u b e A is
fitted w i t h a s t o p p e r C c a r r y i n g a gas d e l i v e r y t u b e
D a n d a t h e r m o c o u p l e s h e a t h E. A l l u n u s e d holes i n
t h e s t o p p e r are closed b y p l u g s F. The s p h e r i c a l
j o i n t of t u b e D is c o n n e c t e d to a stopcock at w h i c h
e i t h e r a r g o n or a n h y d r o u s HC1 is a v a i l a b l e at a
p r e s s u r e of 4-5 c m Hg i n excess of a t m o s p h e r i c
p r e s s u r e . T h e s i d e a r m of t u b e A is fitted w i t h its
p r o t e c t i v e d r y i n g t u b e I, w h i c h is i n t u r n c o n n e c t e d
to a v a c u u m m a n i f o l d . W i t h the stopcock of t h e gas
d e l i v e r y s y s t e m closed, t h e s y s t e m is e v a c u a t e d to
check for the p r e s e n c e of leaks. T h e s y s t e m is c o n sidered free of l e a k s if the p r e s s u r e r e a d i n g is 0.2
m m H g or lower. A i r is t h e n a d m i t t e d to a t m o s p h e r i c p r e s s u r e t h r o u g h t h e d r y i n g t u b e I.
T h e c h a r g e of g r o u n d eutectic m i x t u r e is t h e n
loaded into t h e cell t h r o u g h one of t h e l a r g e holes.
It s h o u l d be e m p h a s i z e d t h a t t u b e A does n o t c o n t a i n the c o m p a r t m e n t s B at this p o i n t a n d t h a t the
gas d e l i v e r y t u b e D is w i t h i n 1 c m of t h e b o t t o m of
t u b e A. A f t e r l o a d i n g t h e c h a r g e of e u t e c t i c m i x t u r e into t u b e A, the s y s t e m is m a i n t a i n e d e v a c u ated at r o o m t e m p e r a t u r e for a p e r i o d of 3 days.
The s y s t e m is n e x t g r a d u a l l y b r o u g h t to a t e m p e r a t u r e of 300~ d u r i n g a n 8 - h r p e r i o d b y a p p r o p r i a t e
a d j u s t m e n t of the f u r n a c e controls, v a c u u m b e i n g
m a i n t a i n e d all the while.
F u s i o n of the c h a r g e is effected u n d e r t h e f o l l o w i n g conditions. A n h y d r o u s HC1 is s l o w l y a d m i t t e d
to t h e e v a c u a t e d cell t h r o u g h gas d e l i v e r y t u b e D
u n t i l , w h e n the p r e s s u r e exceeds a t m o s p h e r i c , HC1
c a n be d i s c h a r g e d f r o m t h e cell t h r o u g h t h e s i d e a r m d r y i n g t u b e I into a disposal s u m p . A c o n v e n i e n t disposal s u m p for the HC1 is the i n t e r i o r of a
filter flask w h i c h is c o n n e c t e d to a r u n n i n g w a t e r
aspirator, the filter flask b e i n g also v e n t e d to t h e
a t m o s p h e r e . A slow flow of HC1 is m a i n t a i n e d
t h r o u g h t h e salt m i x t u r e w h i l e t h e t e m p e r a t u r e is
raised to 500~
f u s i o n o c c u r r i n g at 352~
A f t e r fusion, the m e l t is s a t u r a t e d w i t h HC1
w h i c h m u s t be r e m o v e d . Most of the HC1 c a n be
r e m o v e d b y e v a c u a t i n g t h e s y s t e m for a few m i n utes w i t h t h e w a t e r a s p i r a t o r a n d t h e n u s i n g a r g o n
to b r i n g t h e s y s t e m b a c k to a t m o s p h e r i c p r e s s u r e .
E v a c u a t i o n of the s y s t e m w i t h t h e w a t e r a s p i r a t o r
a n d t h e refilling w i t h a r g o n is r e p e a t e d once m o r e .
A t this p o i n t it is c o n v e n i e n t to add the c o m p a r t m e n t s B to the system, if t h e y a r e to be u s e d i n a n
e x p e r i m e n t . T h e i r p r e v i o u s t r e a t m e n t consists of
h a v i n g b e e n t h o r o u g h l y cleaned, r i n s e d , a n d v a c u u m - d r i e d at 400~
T h e y are a d d e d to the s y s t e m
one b y one, w h i l e hot, t h r o u g h t h e l a r g e holes i n
s t o p p e r C, u s i n g t h e h o o k H to m a k e the t r a n s f e r .
The c o m p a r t m e n t s fill b y g r a v i t y d u r i n g t h e final
step of the m e l t p r e p a r a t i o n .
As a final step, the s y s t e m is e v a c u a t e d on the
v a c u u m m a n i f o l d for 3 hr. T h e p u m p is p r o t e c t e d
August 1957
SOCIETY
IOO
I
A
g75
E
I
I
/
D/
.o 5 0 '
~ 25"
0
o
I
l
a
t/
i
0:5
!
l.b
U
!
?_0
2.5
50
- E in Volts vs. 0.I M Pffl]) Pt Reference
Fig. 2. PoJarograms for the eutectic solvent at 450~ A, solvent
still saturated with HCI after fusion; B, after evacuation for 2 hr,
most of HCI removed; C, after evacuation for 3 hr, HCI virtually
absent; D, typical curve for solvent fused without precaution to prevent hydrolysis.
f r o m HC1 v a p o r s b y a 24-in. c o l u m n filled w i t h a n
A s c a r i t e - M g ( C 1 0 , ) ~ m i x t u r e . T h e s y s t e m is b r o u g h t
b a c k to a t m o s p h e r i c p r e s s u r e w i t h a r g o n at t h e
c o m p l e t i o n of t h e e v a c u a t i o n p e r i o d a n d a n i n e r t
a t m o s p h e r e is m a i n t a i n e d o v e r the m e l t for t h e
b a l a n c e of a n e x p e r i m e n t b y a c o n t i n u o u s slow flow
of argon. A r g o n is d e l i v e r e d b y t u b e D to a l e v e l
j u s t a b o v e the s u r f a c e of t h e melt.
Results and Discussion
T h e p o l a r o g r a p h i c m e t h o d , u s i n g solid m i c r o cathodes, was f o u n d to be a n e x c e l l e n t w a y of d e t e c t i n g the p r e s e n c e of e l e e t r o a c t i v e i m p u r i t i e s i n
t h e s o l v e n t p r e p a r a t i o n s . C u r v e D of Fig. 2 is a
t y p i c a l p o l a r o g r a m of those o b s e r v e d for t h e s o l v e n t s
w h i c h w e r e p r e p a r e d by f u s i o n w i t h o u t p r e c a u t i o n
to p r e v e n t h y d r o l y t i c d e c o m p o s i t i o n or w h i c h w e r e
prepared by fusion under moderate vacuum. Evid e n c e of s o I v e n t c o n t a m i n a t i o n is s h o w n b y t h e
r a p i d l y i n c r e a s i n g c u r r e n t at --0.9 v. T h e c a t h o d e
process g i v i n g rise to this c u r r e n t is a c c o m p a n i e d
b y the f o r m a t i o n of v i s i b l e gas b u b b l e s at t h e m i c r o c a t h o d e surface, i n d i c a t i n g t h e process n o t to be
s o l v e n t decomposition. A n o t h e r e x p e r i m e n t a l fact
suggesting e o n t a m i n a t i o n is t h a t t h e P y r e x c o n t a i n e r of a s o l v e n t so p r e p a r e d is d e e p l y etched
w i t h i n 1 or 2 h r e x p o s u r e to t h e solvent.
The p r o c e d u r e of s o l v e n t p r e p a r a t i o n g i v e n h e r e
is designed, b y p r o v i s i o n of a n excess of HC1, to
shift t h e h y d r o l y t i c e q u i l i b r i u m
H ~ O -k CI- ~ OH-
-6 HCI
f
(I)
to the left. Under
these conditions the moisture
volatilizes from the system without the formation
of hydroxyl ion. Alkaline contamination is undesirable because its corrosive attack on the Pyrex cell
leads to further contamination
of the solvent and
because the electrode reaction
OH- -k e - ~ I/z H~ -k O:
(II)
i n t e r f e r e s w i t h a s t u d y of o t h e r electrode processes
w h i c h occur b e t w e e n a p p r o x i m a t e l y - - 1 a n d --2.5 v
vs. the P t - P t ( I I ) r e f e r e n c e system.
The effectiveness of t h e p r e p a r a t i v e p r o c e d u r e
g i v e n c a n be j u d g e d f r o m c u r v e C of Fig. 2, w h i c h
Vol. 104, No. 8
PREPARATION
OF LiC1-KC1 EUTECTIC
is a p o l a r o g r a m t y p i c a l of t h e e u t e c t i c s o l v e n t p r e p a r e d as specified. F o r c u r v e C t h e r e s i d u a l c u r r e n t p r e c e d i n g t h e t e r m i n a l p r o c e s s of l i t h i u m m e t a l
d e p o s i t i o n is less t h a n 3 tLa a t a m i c r o c a t h o d e of 0.8
m m '~ a r e a . T h e i n h e r e n t l y w i d e d e c o m p o s i t i o n p o t e n t i a l s p a n of t h e LiC1-KC1 e u t e c t i c m i x t u r e is
t h u s a c c e s s i b l e f o r t h e e l e c t r o c h e m i c a l s t u d y of s u c h
s o l u t e s as CrCI~, FeC12, CuC1, ZnCI~, a n d MgCI~.
N o n e of t h e s e s o l u t e s c a n b e s t u d i e d a t l o w s o l u t e
c o n c e n t r a t i o n s in t h e c o n t a m i n a t e d s o l v e n t b e c a u s e
r e a c t i o n ( I I ) is c o m p e t i t i v e w i t h m o s t of t h e e l e c t r o d e r e a c t i o n s of i n t e r e s t . F u r t h e r m o r e , t h e a l k a l i n e e n v i r o n m e n t of a c o n t a m i n a t e d s o l v e n t r e s u l t s
in p r e c i p i t a t i o n of s o m e of t h e s o l u t e s as i n s o l u b l e
species. T h e f r e e d o m f r o m a l k a l i n i t y of t h e s o l v e n t
w h e n p r e p a r e d as d e s c r i b e d is a t t e s t e d b y t h e f a c t
t h a t t h e a b o v e s o l u t e s r e m a i n in s o l u t i o n a n d t h e
f a c t t h a t t h e P y r e x g l a s s w a r e is o n l y s l i g h t l y e t c h e d
a f t e r d a y s of c o n t a c t w i t h t h e f u s e d s a l t solvent.
A l t h o u g h r e a c t i o n ( I ) is w r i t t e n as a n e q u i l i b r i u m , t h e r e a c t i o n is n o t s h i f t e d r e a d i l y to t h e l e f t
once it h a s p r o c e e d e d e x t e n s i v e l y to t h e r i g h t . F u sion of t h e e u t e c t i c m i x t u r e w i t h o u t p r e c a u t i o n to
prevent hydrolytic decomposition yields a solvent
t h e c o n d i t i o n of w h i c h c a n n o t b e m a d e s a t i s f a c t o r y
b y t h e p a s s a g e of HC1. T h e c o r r e c t i v e t r e a t m e n t
does n o t s e r v e in p l a c e of t h e p r e v e n t i v e t r e a t m e n t .
D a t a w h i c h i l l u s t r a t e t h e s l o w r e l e a s e of m o i s t u r e
b y t h e g r o u n d e u t e c t i c m i x t u r e a r e s h o w n in T a b l e
I. T h e s e d a t a w e r e o b t a i n e d b y d e t e r m i n i n g t h e
v o l a t i l e m a t e r i a l c o l l e c t e d in a t r a p , c o o l e d w i t h a
d r y i c e - " C e l l o s o l v e " m i x t u r e , w h i c h e s c a p e d a 195.4
g s a m p l e of t h e g r o u n d e u t e c t i c m i x t u r e u n d e r t h e
c o n d i t i o n s i n d i c a t e d . I t w a s c o n c l u d e d t h a t at r o o m
t e m p e r a t u r e a n d 0.1-0.2 m m H g p r e s s u r e t h e m a j o r
p o r t i o n of t h e m o i s t u r e is r e m o v e d f r o m t h e g r o u n d
e u t e c t i c m i x t u r e in 3 d a y s . T h e n u m e r i c a l v a l u e s
of G a r d n e r , B r o w n , a n d J a n z (2, 3), r e p o r t e d on a n
e x t e n d e d s t u d y of w e i g h t c h a n g e s as a f u n c t i o n of
t i m e a n d t e m p e r a t u r e of a l k a l i c h l o r i d e s m a i n t a i n e d
at 10-' to 10 -~ m m Hg, c a n n o t b e c o m p a r e d to t h e
d a t a of T a b l e I b e c a u s e of t h e d i f f e r e n c e s in t e m p e r a t u r e a n d p r e s s u r e c o n d i t i o n s . B o t h sets of d a t a
y i e l d t h e s a m e g e n e r a l conclusion, n a m e l y , t h a t
m o i s t u r e is t e n a c i o u s l y h e l d b y LiC1, e v e n a t l o w
pressures.
A s u m m a r y of o b s e r v a t i o n s , b a s e d on t h e r e s u l t s
of a l t e r i n g t h e e x p e r i m e n t a l c o n d i t i o n s g i v e n a b o v e
for t h e p r e p a r a t i o n of t h e f u s e d s a l t solvent, i n d i c a t e s t h e c o n d i t i o n s p r e s c r i b e d to b e o p t i m a l for
t h e p r o d u c t i o n of a h i g h q u a l i t y s o l v e n t w i t h a
Table I. Moisture collected from 195.4 g of eutectic mixture
Evacuation*
period, hr
0-24
24-60
60-74
74-84
84-94
Temperature
condition
Room t e m p
Room t e m p
Room t e m p
Room t e m p
S l o w l y incr.
to 245~
Moisture
collected
in trap, g
0.65
1.00
0.10
0.00___0.05
0.15
* P r e s s u r e m a i n t a i n e d a t 0.1-0.2 m m Hg.
Cumulative
moisture
collection, g
0.65
1.65
1.75
1.75
1.90
SOLVENT
519
m i n i m u m of p r e p a r a t i v e effort for t h e a m o u n t s of
s o l v e n t r e q u i r e d in e l e c t r o c h e m i c a l studies.
T h e q u a l i t y of t h e s o l v e n t p r e p a r a t i o n , as m e a s u r e d b y t h e p o l a r o g r a p h i c c u r v e , is l o w e r e d if t h e
t i m e of t h e " r o o m t e m p e r a t u r e " e v a c u a t i o n of t h e
g r o u n d e u t e c t i c m i x t u r e is less t h a n 2 d a y s . D u r i n g
t h i s p e r i o d t h e p r e p a r a t i v e o p e r a t i o n r e q u i r e s no
o p e r a t o r a t t e n t i o n a n d l i t t l e in t h e m a n n e r of e q u i p m e n t . S h o r t e n i n g t h e p e r i o d is t h e r e f o r e n o t p a r t i c u l a r l y to b e d e s i r e d . S h o r t e n i n g t h e p e r i o d d u r i n g
w h i c h t h e m i x t u r e is h e a t e d , u n d e r v a c u u m , f r o m
r o o m t e m p e r a t u r e to 300~ is d e l e t e r i o u s to t h e solv e n t q u a l i t y . S u b s t i t u t i o n of t h e v a c u u m c o n d i t i o n d u r i n g t h i s s t e p b y 1 a r m of HC1 p r e s s u r e is
also d e l e t e r i o u s . E x p e r i e n c e s h o w s t h a t t h e f u s i o n
s t e p m u s t b e p e r f o r m e d u n d e r HC1 i n o r d e r to p r e vent excessive hydrolysis because the vacuum cond i t i o n s p r e s c r i b e d a r e insufficient for c o m p l e t e r e m o v a l of t h e m o i s t u r e p r i o r to fusion. F u s i o n of t h e
m i x t u r e at l o w p r e s s u r e of HC1 is w i t h o u t b e n e ficial effect on t h e s o l v e n t q u a l i t y . T h e t i m e r e q u i r e d f o r r e m o v a l of t h e HC1 f r o m t h e m e l t is n o t
appreciably shortened by interrupting the evacuat i o n w i t h f r e q u e n t flushes of t h e s y s t e m w i t h a r g o n .
S e v e r a l a t t e m p t s u s i n g o t h e r m e a n s to r e m o v e
m o i s t u r e a n d h y d r o x y l i o n f r o m t h e m e l t p r o v e d to
b e u n s u c c e s s f u l . T h e c h e m i c a l r e a c t i o n of e i t h e r
SOCI~ or SiCI~, i n t r o d u c e d as t h e v a p o r c a r r i e d b y a
s t r e a m of d r y N, c o n s t i t u t e d t h e m o s t n e a r l y s u c cessful e l i m i n a t i o n of m o i s t u r e a n d h y d r o x y l ion.
The hydrolytic reactions were slow and incomplete
u n d e r t h e c o n d i t i o n s of t h e e x p e r i m e n t s . Use of
acidic metal chlorides in the same capacity was
r u l e d o u t b e c a u s e of t h e c a t i o n c o n t a m i n a t i o n
t h e r e b y i n t r o d u c e d into t h e solvent. P r o l o n g e d e l e c t r o l y s i s of t h e m e l t b e t w e e n g r a p h i t e e l e c t r o d e s
f a i l e d to p r o d u c e a p u r e s o l v e n t , as j u d g e d b y t h e
polarographic criterion, probably because this
m e t h o d does n o t e l i m i n a t e o x i d e c o n t a m i n a t i o n . No
n o t i c e a b l e i m p r o v e m e n t in t h e r e s i d u a l c u r r e n t
characteristic was produced by the scavenging effect of c a t h o d i c a l l y d e p o s i t e d Li. M e l t s p r e p a r e d in
contact with Vycor and Alundum containers had
t h e s a m e r e s i d u a l c u r r e n t c h a r a c t e r i s t i c as t h o s e
p r e p a r e d in P y r e x , i n d i c a t i n g t h e c o n t a i n e r m a t e r i a l to b e of l i t t l e a p p a r e n t significance.
Acknowledgment
T h e a u t h o r s t a k e t h i s o p p o r t u n i t y to e x p r e s s t h e i r
a p p r e c i a t i o n to t h e D i a m o n d O r d n a n c e F u z e L a b o r a t o r y , D e p a r t m e n t of D e f e n s e , f o r f i n a n c i a l s p o n s o r s h i p of a s s i s t a n t s h i p s h e l d b y t w o of t h e m
(R.A.O. a n d W . S . F . ) d u r i n g e x e c u t i o n of t h e r e search.
M a n u s c r i p t r e c e i v e d Sept. 28, 1956. This p a p e r was
p r e p a r e d for d e l i v e r y before the Cincinnati Meeting,
May 1-5, 1955.
A n y discussion of this p a p e r w i l l a p p e a r in a Discussion Section to be p u b l i s h e d in the J u n e 1958
JOURNAL.
REFERENCES
1. S. Senderoff and A. Brenner, This Journal, 101, 16
(1954).
2. H. J. G a r d n e r , C. J. Brown, and G. J. Janz, "The
P r e p a r a t i o n of D r y A l k a l i Chlorides f o r Conduc-
520
JOURNAL
OF
THE
ELECTROCHEMICAL
tance Studies in Aqueous Media and Molten Salts,"
Technical Status Report on Contract No. Nonr591(06), supported by the U. S. Office of Naval
Research.
3. C. J. Brown, J. J. Gardner, and G. J. Janz, "Electrochemistry of Molten Alkali and Alkaline E a r t h
Halides C o n t a i n i n g Halides of T i t a n i u m , " A n n u a l
SOCIETY
August 1957
Report, March, 1956, on Contract No. Nonr-591 (06).
4. R. A. Osteryoung, Ph.D. thesis, U n i v e r s i t y of Illinois,
1954.
5. W. S. Ferguson, Ph.D. thesis, U n i v e r s i t y of Illinois,
1956.
6. H. A. L a i t i n e n and W. S. Ferguson, Anal. Chem., 29,
4 (1957).