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8/10/2019 Anode Effect in Aqueous Electrolysis
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A n o d e E f f e c t i n A q u e o u s E l e c t r o l y s i s
HERBERT H. KELLOGG
Schoo l o f Mines Columbia Univers i t y Ne w York Ne w York
ABSTRACT
A phenomeno n which closely resembles anode effect in molten electrolysis can be
developed in the electrolysis of aqueous solutions at high current density. Normal
operation of the electrode ceases and a so-called tra nsit ion period begins when the
electrode temperat ure reaches the boiling point of the electrolyte. When the app lied
voltage is increased beyond a critical value the transition behavior suddenly changes
to the aqueous anode-effect. Duri ng this effect the surface tempe ratur e of the anode
rises far above the boiling point of the electrolyte. Evidence is provided which indicates
tha t the gaseous envelope surroun ding the anode duri ng the aqueous anode-effect is
main tain ed by the vapor ization of the electrolyte aga inst the hot anode surface. An
aqueous cathode-effect was also obtained. The relation between aqueous anode-effect
and anode effect in molten media is discussed.
INTRODUCTION
Since the earliest experiments with the electrolysis
of molten salts, the behavior called anode effect
has been reported and many investigators have at-
tempted explanations of this peculiar and trouble-
some phenomenon. The description of anode effect
by C. S. Taylor (1) is quoted below:
The anode, during t he normal course of electrol-
ysis, is surrounded with a large number of gas bub-
bles which are constantly escaping from it. These
small bubbles seem to form on the anode, and then
break away easily and escape from the electrolyte by
breaking through t he surface film. This smooth, even
evolution of gas around the anode is always a sign
of normal operation. The moment the anode effect
occurs, however, conditions are entirely different.
The anede appears to be entirely surrounded by a
film of gas, which, by covering the surface of the
anode, pushes the fused electrolyte away, and thus
produces the so-called 'non wetting' of the anode.
As the electrolyte is pushed away, small arcs form
between the electrolyte and the anode.
The lit erature on anode effect is rich in factual in-
formation on the occurrence and control of the
phenomenon in a variety of molten electrolyses (2, 3,
4). Noticeably lacking, however, is a satisfactory
hypothesis concerning the anode effect which will ex-
plain what forces are responsible for holding the
molten electrolyte away from the anode in the form
of a gaseous envelope despite the ever present hydro-
static forces which tend to collapse this envelope.
To the best of the author's knowledge, the idea that
this envelope is maintained by the rush of gases
evolved at the anode is the most widely held hypoth-
esis at the present time.
1 Manusc ript received August 2, 1949. This paper pre-
pared for delivery before the Cleveland Meeting, April 19
to
22
1950.
This paper shows that a phenomenon can be ob-
tained with electrodes which evolve gas in aqueous
electrolysis, which phenomenon closely fits the pre-
viously cited description of anode effect in molten
electrolysis. Furthermore, by analysis of pertinent
data on this aqueous anode-effect, an explanation
of the forces which form and maintain the gaseous
envelope has been arrived at. It seems very likely
that the explanations of aqueous anode-effect may
also be valid for the anode effect in molten electrol-
ysis, and it is contemplated to investigate this possi-
bility in a future paper.
133
EXPERIMENTAL
The experiments on aqueous anode-effect were
made with the following circuit: The cell was an 800-
ml Pyrex beaker in which were placed the anode to
be investigated (in most instances a platinum wire,
1.25 mm diameter, immersed to a depth of 8 mm)
and a platinum cathode (4 x 6 cm sheet). The elec-
trolyte (usually 1.0N H2S04) was added so as to
almost fill the cell. A glass stirring-rod, bent into a
loop at the bottom, was attached to a motor and
rotated at 450 rpm. A mercury thermometer was
used to record the temperature of the electrolyte.
Direct current was supplied to the electrodes by
means of a Voltage divider connected across a 115
volt source. An ammeter was placed in series with
the cell, and the total cell voltage was measured
across the electrodes with a voltmeter.
The measurements of electrode temperature were
made with an electrode constructed in the following
manner: A tube Of 25-20 chromium-nickel ste el,
closed atone end, 3.5 mm OD, 2.7 mm ID, and 50 mm
long comprised the electrode. A copper-constantan
thermocouple (30 gauge wire) was threaded through
a two-hoIe ceramic insulator and inserted into the
steel tube so that the junction was in contact with
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134 J O U R N A L O F T H E E L E C T R O C H E M I C A L S O C I E T Y April 1950
t h e c l o s e d e n d o f t h e t u b e . A n e f f i c i e n t t h e r m a l c o n -
t a c t b e t w e e n t h e t h e r m o c o u p l e j u n c t i o n a n d t h e t u b e
w a s m a d e b y p l a c i n g a b o u t 5 0 m g o f a l o w m e l t i n g
( 8 0 ~ a l l o y o f l e a d , t i n , a n d b i s m u t h in t h e b o t t o m
'
2
O 2
F L
o -o - - , ~
4 6 eO I o t2o
olts
FIG. 1. An ode effect with p latinu m-w ire anode (0.314 cm~
imm ersed area) in norm al H~SO4 at 66 ~ 4- 4 ~ C.
FIG. 2. Normal operation of platinum-wire anode. 1N
H2S04 at 40 ~ C, 27 vo lts, 5.4 ampe res, a bou t 8 X.
o f t u b e a n d t h e n g e n t l y h e a t in g t h e t u b e w i t h t h e
t h e r m o c o u p l e i n pl a ce . T h e r m M e m f w a s m e a s u r e d
w i t h a p o t e n t i o m e t e r .
T h e p h o t o g r a p h s w e r e m a d e w i t h a b e n c h c a m e r a
e q u i p p e d w i t h a 4 2 - m m M i c r o - S u m m a r l e n s . T h e
i l l u m i n a n t w a s a n I d e n t i f i c a t i o n F l a s h O u t f i t u s e d b y
t h e S i g n a l C o r p s , a n d c o n s i s t ed o f a p o w e r s u p p l y
a n d t w o g a s - d i s c h a r g e f l a s h - l a m p s . T h e f l a s h i n -
t e n s i t y i s r e p o r t e d a s e l e v e n t i m e s m o r e i n t e n s e t h a n
s u n l i g h t a n d t h e d u r a t i o n a b o u t 1 / 1 0 , 0 0 0 s e c o n d .
D I s c u s s i O N
Description of Aqueous A node Effect
T h e d i s c u s s io n o f t h e m a i n b o d y o f t h e d a t a a n d
t h e d e v e l o p m e n t o f a h y p o t h e s i s f o r t h e a q u e o u s
anode-e f fec t wi l l be fo l lowed wi th g rea te r ea se i f p re -
ceded by a desc r ip t io n o f a typ ica l e lec t ro lys i s which
resu l t s in the anode e f fec t .
F i g . 1 r e c o r d s in g r a p h i c a l f o r m t h e v o l t - a m p e r e
re la t ionsh ips fo r a cel l cons i s t ing o f a shee t - p la t inum
ca thode (4 x 4 e ra ) and a p la t inum -wire anode (0 . 31
e m 2 i m m e r s e d a r e a ) , w i t h a n e l e c t r o l y t e o f n o r m a l
H=S 04 . The fo l lowing desc r ip t ion app l ie s to the be -
h a v i o r o f th e w i r e a n o de , w h e n t h e b u l k t e m p e r a t u r e
i s m a i n t a i n e d a t 6 6 ~ 4 - 4 ~ W i t h a lo w v o l t a g e i m -
pres sed on the ce l l , bubb les o f oxygen a re evo lved
a t t h e a n o d e i n a p e r f e c t l y n o r m a l m a n n e r . T h e
v o l t - a m p e r e r e l a t i o n f o r t h e c e l l i s t h a t g i v e n b y t h e
reg ion A-B in F ig . 1 ; the cur ren t inc reases a lm os t
l inea r ly wi th inc reased vo l tage . F ig . 2 i s a pho to-
g r a p h o f t h e g a s e v o l u t i o n a t t h e w i r e a n o d e d u r i n g
t h i s n o r m a l o p e r a t i o n o f t h e c e ll ( c o m p a r e w i t h F i g .
3 w h i c h s h o w s t h e w i r e a n o d e w h e n n o c u r r e n t i s
f l o w i n g ) . T h e o x y g e n b u b b l e s f o r m r a p i d l y , b r e a k
a w a y f r o m t h e e l e c t r o d e , a n d r i s e q u i c k l y t o t h e s u r -
f a c e o f t h e e l e c t r o l y t e w h e r e t h e y b r e a k . T h e o p e r -
a t ion o f the ce l l i s qu ie t and s teady .
W h e n a c u r r e n t d e n s i t y r e p r e s e n t e d b y t h e p o i n t
B on F ig . l i s reached , a new behav ior beg ins . The
v o l t - a m p e r e r e l a t i o n o f t h e c e l l i s e r r a t i c . S p i t t i n g
a n d h i s s i n g n o i s e s a r i s e f r o m t h e a n o d e , a n d m a n y
o f t h e g a s b u b b l e s a r e p r o j e c t e d d o w n a n d a w a y f r o m
t h e a n o d e b y t h e s u d d e n s p i t s . T h e e l e c t r o l y t e
c l o s e t o t h e a n o d e i s h o t a s e v i d e n c e d b y t h e p r e s -
e n c e o f c o n d e n s e d w a t e r v a p o r i n t h e e v o l v e d g a s e s .
A f u r t h e r i n c r e a s e in i m p r e s s e d v o l t a g e c a u s e s n o i n -
c r e a s e ( o r e v e n a d e c r e a s e ) i n c u r r e n t t h r o u g h t h e
ce l l , a s shown by B-C on F ig . 1 . F ig . 4 i s a pho to-
g r a p h o f t h e w i r e a n o d e d u r i n g t h i s b e h a v i o r , w h i c h
t h e w r i t e r h a s c a l l ed t h e t r a n s i t i o n p e r i o d .
When the vo l t age i s ra i s ed to a c r i t i ca l va lue (nea r
C in F ig . 1 ) , an ins tan taneous change t akes p lace .
T h e v o l t a g e r i s e s s u d d e n l y a n d t h e c u r r e n t d r o p s
to a low va lue (po in t D in F ig . 1 ) . The loud sp i t t ing
a n d h i s s i n g , w h i c h a c c o m p a n i e d t h e p r e v i o u s s t a g e ,
c e a s e . T h e w i r e a n o d e i s c o m p l e t e l y s u r r o u n d e d b y a
g a s e o u s f i l m . O c c a s i o n a l t i n y s p a r k s c a n b e s e e n t o
f o r m i n t h e f i lm . F ig . 5 i s a p h o t o g r a p h o f t h e a n o d e
d u r i n g t h i s b e h a v i o r . T h i s is t h e p h e n o m e n o n w h i c h
t h e w r i t e r h a s c a l l ed a q u e o u s a n o d e - e f f e c t .
I n c r e a s e o f v o l t a g e t o p o i n t E o n F i g . 1 d o e s n o t
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Vol. 97 No. 4 ANODE EF FEC T IN AQUEOUS ELECTR OLYSI S 135
alter the behavior described for point D, except tha t
a dull red glow can be seen at the lower tip of the
anode. The writer has taken this glow to indicate
that the temperature of the electrode is high--proba-
bly 750~ Fur the r indication th at the electrode is
hot is that, if the circuit is suddenly opened while
the anode is operating anywhere in the E-F region
an elect rolyte of normal NaOH 2. Fig. 6, 7, and 8
summarize the volt-ampere relations and the anode
temperature for three different bulk-electrolyte tem-
peratures. The behavior for each electrolyte temper-
ature can again be conveniently divided into three
periods: (a) normal operation, A-B in Fig. 6, 7, and
8; (b) transition period, B-C; (c) anode effect, F-D-E.
FIG. 3. Platinum-wire anode. No current flowing, im-
mersion 0.8 cm, about 87
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136 J O U R N A L O F T H E E L E C T R O C H E M I C A L S O C I E T Y Apr i l 1950
p a s s a g e o f c u r r e n t f r o m t h e b u l k o f t h e e l e c t r o l y t e
t o t h e e l e c t r o d e s u r f a c e a r e t h e b u b b l e w a l l s o f t h e
e v o l v i n g g a s . T h e c u r r e n t d e n s i t y i n t h e s e b u b b l e
w a l l s i s c o r r e s p o n d i n g l y h i g h , a n d t h e h e a t d i s s i p a t e d
b y I2R ' ' h e a t i n g i n t h e b u b b l e w a l l s i s h i g h e r t h a n
e l sewhere in the e lec t ro ly te .
T h e t e m p e r a t u r e o f t h e b u l k e l e c t r o l y te h a s o n l y
a m i n o r e f f e c t o n t h e v o l t - a m p e r e r e l a t i o n s i n t h i s
n o r m a l o p e r a t i o n p e r io d . F o r a g i v e n v o l t a g e , t h e
c u r r e n t i s s l i g h t l y h i g h e r f o r t h e h o t e l e c t r o l y t e t h a n
f o r t h e c o o l , a s w o u l d b e e x p e c t e d f r o m t h e h i g h e r
c o n d u c t i v i t y o f t h e h o t e l e c t r o l y t e .
e l e c t r o l y t e t e m p e r a t u r e . T h i s i s r e a d i l y e x p l a i n e d
s ince the co ld e lec t ro l y te wi l l o f fe r be t t e r cond i t ions
FIG. 5. Aque ous anode effect with platinum -wire
anode. IN H2SO4 at 40 ~ C, 70 volts, 1.[ am peres, abo ut 8X .
2. Tran sition Period]
F o r a l l t h r e e e l e c t r o l y t e t e m p e r a t u r e s t h e c h a n g e
f r o m t h e n o r m a l o p e r a t i o n t o t h e t r a n s i t i o n
p e r i o d i s c o i n c i d e n t w i t h t h e p o i n t a t w h i c h t h e
e l e c t r o d e t e m p e r a t u r e r e a c h e s 10 0 ~ 2 ~ T h e
r e g i o n c l o s e t o t h e e l e c t r o d e h a s b e c o m e s u f f i c i e n t l y
h o t t o v a p o r i z e t h e e l e c t r o l y t e , a n d t h e s p i t t i n g a n d
h i s s in g n o i s es w h i c h a c c o m p a n y t h e t r a n s i t i o n
p e r i o d a r e e v i d e n ce t h a t v a p o r i z a t i o n d o e s t a k e
p lace .
F i g . 6 , 7 , a n d 8 s h o w t h a t t h e c u r r e n t d e n s i t y a t
w h i c h t h e n o r m a l o p e r a t i o n e n d s a n d t h e t r a n s i -
t i o n p e r i o d b e g i n s is h i g h e r t h e l o w e r i s t h e b u l k -
FIG. a. Aqueous anode-effect with l)latinum-wire
anode. 1N ]|2SO4 at 90 ~ C, 74 volts, 0.12 ampe res, abou t 8X .
10
E
le ~
i
s
/ 50C
/
/
I
i
, j , . 4 0
. ~
Amp
~ ? 1
F D
2 o 4 0 6 0 8 0 /0 0 1 2 0
Vo l t s
FIG. 6. Anode effect with alloy~steel anod e (0.956 cm2
immersed area) in normal NaOH at 39~177~ C.
f o r h e a t t r a n s f e r a w a y f r o m t h e e l e c t r o d e , a n d , c o n se -
q u e n t l y , a h i g h e r c u r r e n t d e n s i t y w i l l b e r e q u i r e d t o
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Vol. 97, 25o. 4 A N O D E E F F E C T I N A Q U E O U S E L E C T R O L Y S I S 13 7
r a i s e t h e e l e c t r o d e t e m p e r a t u r e t o t h e b o i l i n g p o i n t
o f t h e e l e c t r o l y t e .
F i g . 6 , 7 , a n d 8 s h o w t h a t d u r i n g t h e t r a n s i t i o n
p e r i o d t h e c u r r e n t f a i ls t o i n c r e a s e a n d e v e n f a ll s
a s t h e v o l t a g e is f u r t h e r i n c r e a s e d . T h e e x p l a n a t i o n
o f t h i s b e h a v i o r i s f o u n d b y e x a m i n a t i o n o f t h e
e l e c t r o d e s u r f a c e d u r i n g t h e t r a n s i t i o n p e r i o d . F i g .
4 s h o w s t h a t t h e v a p o r i z a t i o n o f t h e b u b b l e w a l l s
d u r i ng t h e t r a n s i t i o n p e r i o d r e s u l ts i n t h e f o r m a -
t i o n o f a w i d e v a p o r f i l m w h i c h e n c l o s e s a l a r g e s e c -
t i o n o f t h e e l e c t r o d e . D u r i n g t h e t r a n s i t i o n p e r i o d
t h e s e f il m s h a v e a v e r y s h o r t l if e a n d a r e c o n s t a n t l y
I0
Amos
4OO
p
6 ~ 4 ' x 3 g
/ ,.
,,
9 1 0 (
. * - '
0 20 40 6 0 80 /oo Igo
Volts
Fro. 7 . Anode effect with alloy- steel a node (0.956 em 2
imme rsed area) in norm al N aOH at 66 ~ 4 - 4 ~ C .
4
2
L ,.:
g
2 0
40 go
V o l t s
~ ~
A m p ~
~o~
I0
80 I00 120
FIG. 8. Anode effe ct with alloy- steel anode (0.95g em 2
im m ersed a rea ) i n n o rm a l N aO H a t 8 9 ~ 2 ~ C .
f o r m i n g a n d b r e a k i n g - - a b e h a v i o r w h ic h i s r es p o n s i-
b l e f o r t h e u n s t e a d y r e a d i n g s o f v o l t a g e a n d c u r r e n t
d u r i n g t h i s p e r i o d . T h e p r e s e n c e o f t h e s e v a p o r f i l m s
w h i c h p a r t i a l l y e n c l o s e t h e a n o d e r e s u l t s i n a l a r g e
i n c r e a s e i n t h e r e s i s t a n c e o f t h e e l e c t r o l y t e p a t h n e a r
t h e a n o d e a n d , h e n c e , t h e c u r r e n t m a y d r o p e v e n
t h o u g h t h e v o l t a g e i n c r e a s e s .
D u r i n g t h e t r a n s i t i o n p e r i o d t h e e l e c t r o d e t e m -
p e r a t u r e r e m a i n s c o n s t a n t a t 1 00 ~ 4 - 2 ~ T h e r e
r e m a i n s s o m e d ir e c t c o n t a c t b e t w e e n t h e a n o d e a n d
t h e e l e c t r o l y te w i t h t h e r e s u l t t h a t t h e e l e c t r o d e
t e m p e r a t u r e i s p r e v e n t e d f r o m r i si n g a b o v e t h e b o i l -
i n g t e m p e r a t u r e o f t h e e l e c t r o l y te . T h e g a s e v o l v e d
b y t h e e l e c t r o l y s i s c o n t i n u e s t o e v o l v e as b u b b l e s ,
t h o u g h t h e f o r c e of t h e s u d d e n v a p o r i z a t i o n s o f t e n
p r o j e c t s t h e g a s b u b b l e s f a r a w a y f r o m t h e e l e c t ro d e .
3. Anode-EffectRegion
T h e c h a i n o f e v e n t s t h a t l e a d s t o t h e i n s t a n t a n e o u s
c h a n g e f r o m t h e t r a n s i t i o n p e r i o d t o t h e a n o d e
e f f e c t w i ll b e b e t t e r u n d e r s t o o d i f t h e c h a r a c t e r i s t i c s
o f t h e a l r e a d y f o r m e d a n o d e e f f e c t f i l m a r e f i r s t d i s -
c u s s e d .
T h e m o s t s t r i k i n g f a c t s , s h o w n i n F i g . 6 , 7 , a n d
8 , a r e t h e v e r y h i g h e l e c t r od e t e m p e r a t u r e s w h i c h
p r e v a i l w h e n t h e a n o d e - e f f e c t f il m i s p r e s e n t . T h e
l o w e s t a n o d e t e m p e r a t u r e r e c o r d e d u n d e r t h e s e c i r -
c u m s t a n c e s w a s 1 6 5 ~ ; t h e h i gh e s t w a s a b o u t 6 2 0 ~
T h e s e h i g h a n o d e t e m p e r a t u r e s , t o g e t h e r w i t h t h e
f a c t s o n t h e v a p o r i z a t i o n o f t h e e l e c t r o l y t e d u r i n g
t h e t r a n s i t i o n p e r i o d , s u g g e s t a s i m p l e a n d c o m p e l -
l in g e x p l a n a t i o n f o r t h e f o r c e s w h i c h m a i n t a i n t h e
g a s e o u s e n v e l o p e d u r i n g t h e a n o d e e f f e c t .
The anode-effect ilm is prima rily a water-vapor film
surrounding a hot wire. T h e e l e c t r o ly t e is p u s h e d b a c k
f r o m t h e e l e c t r o d e s u r f a c e b y t h e v a p o r p r e s s u r e o f
t h e e l e c t r o l y t e , w h i c h e x c e e d s o n e a t m o s p h e r e a s a
r e s u l t o f t h e h i g h e l e c t r o d e t e m p e r a t u r e . I f t h e f il m
a t t e m p t s t o c o l la p s e u n d e r t h e i n f lu e n c e of h y d r o -
s t a t ic f o r c e s w h e n t h e e l e c t r o l y t e s u rf a c e a p p r o a c h e s
c l os e t o t h e a n o d e , f u r t h e r v a p o r i z a t i o n w i l l t a k e
p l a c e a n d p u s h t h e e l e c t r o l y t e b a c k .
T h i s h y p o t h e s i s o f t h e f o r c e s w h i c h m a i n t a i n t h e
f i l m i s i n a c c o r d a n c e w i t h t h e v i s u a l o b s e r v a t i o n o f
t h e a n o d e - e f f e c t f i l m . T h e s u r f a c e o f t h e f i l m i s n o t
s t a t io n a r y , b u t v i b r a t e s r a p i d ly t o w a r d a n d a w a y
f r o m t h e e l e c t r o d e s u r f a c e ( s ee F i g . 5 ). S i n c e t h e
e l e c t ro d e c a n lo s e h e a t b y c o n d u c t i o n u p t h r o u g h i t s
l e n g th , t h e b o t t o m t i p o f t h e w i r e i s t h e h o t t e s t p a r t
( th i s w a s e s t a b l i s h e d e x p e r i m e n t a l l y b y t h e o b s e r v a -
t i o n o n t h e p l a t i n u m - w i r e a n o d e t h a t t h e b o t t o m
s e c t i o n c o u l d b e m a d e t o g l o w a t a r e d h e a t , w h i l e
t h e t o p g a v e n o v i s i b l e r a d i a t i o n ) .
I n o r d e r t o p r o v e t h a t a h o t w i r e i s c a p a b l e o f s u p -
p o r t i n g a fi l m , s u c h a s i s f o r m e d d u r i n g t h e a n o d e
e f f e c t , t h e f o l l o w i n g e x p e r i m e n t w a s p e r f o r m e d : A
n i e h r o m e w i r e , 0 . 1 0 c m d i a m e t e r a n d 1 2 c m l o n g ,
w a s f o r m e d i n to a l o o p a n d c o n n e c t e d t o a d i r e c t -
c u r r e n t s o u r c e ; a c u r r e n t o f 3 9 a m p e r e s w a s p a s s e d
t h r o u g h t h e w i r e . A s s o o n a s t h e w i r e w a s r e d h o t i t
w a s p l u n g e d i n t o a b e a k e r o f w a t e r , w i t h o u t d i s-
c o n n e c t i n g t h e c u r r e n t s o u rc e . A v a p o r f i lm w a s s e e n
t o s u r r o u n d t h e e n t i r e l e n g t h o f t h e w i r e . F i g . 9 is a
p h o t o g r a p h o f t h a t p a r t o f t h e w i r e c lo s e t o t h e s u r -
f a c e o f t h e w a t e r .
a By use o f a n ickel wire, 0 .81 ma t d iam eter , as an anode
the au thor was ab le to ob tain s ' ach a h igh elect rode tem-
pera tu re the n ickel mel ted ( rap - - 1452 C) .
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138 J O U R N A L O F T H E E L E C T R O C H E M I C A L S O C I E T Y April 195
I n t h i s e x p e r i m e n t t h e r e i s n o q u e s t i on b u t t h a t
t h e g a s e o u s e n v e l o p e a r o u n d t h e h o t w i r e i s a w a t e r
v a p o r f i l m . T h e r e i s n o e l e c t r o l y s i s d u r i n g t h i s e x -
p e r i m e n t a n d n o o t h e r p o s s i b l e s o u r c e o f l a r g e q u a n -
t i t i e s o f g a s o t h e r t h a n w a t e r v a p o r . T h e s t r i k i n g
s i m i l a r i t y b e t w e e n t h e h o t - w i r e f i l m ( F i g . 9 ) a n d
the aqu eous anode -e f fec t fi lm (F ig . 5 ) i s s t ron g ev i -
d e n c e t h a t t h e a n o d e - e f f e c t f i l m i s a l s o m a i n t a i n e d
b y t h e v a p o r i z a t i o n o f w a t e r .
FIG. 9. Vapor fihn surrounding hot wire (about 8X). A
nichrome wire (0.10 cm diam) was heate d to red heat b y
passing 29 amperes through it; the red hot wire was then
plunged into distilled w ater at 72 ~ C, witho ut disconnecting
the current source. Wire passes through the surface of the
water at the top of the picture.
A n o t h e r p o i n t o f si m i l a r it y b e t w e e n t h e a n o d e -
e f f e c t f i l m a n d t h e h o t - w i r e f i l m i s t h a t t h e t h e r m a l
e n e r g y w h i c h m u s t b e d i s s i p at e d i n o r d e r t o m a i n -
t a i n t h e s e f i lm s is o f t h e s a m e o r d e r o f m a g n i t u d e
f o r b o t h f i l m s. W i t h w a t e r a t 8 7 ~ t h e c r it i c al cu r -
r e n t r e q u i r e d t o m a i n t a i n t h e h o t - w i r e f i lm i s a b o u t
2 9 a m p e r e s . T h i s c o r r e s p o n d s t o a h e a t d i s s i p a t i o n
o f a b o u t 4 0 w a t t s p e r s q u a r e c e n t i m e t e r o f s u r f a c e
a r e a o f t h e w i re . T o j u s t m a i n t a i n t h e a n o d e e f f e ct ,
w i t h a n e l e c t r o l y t e t e m p e r a t u r e of 8 7 ~ r e q u i r e s
a b o u t 4 5 v o l t s a n d 0 . 3 5 a m p e r e s , w h e n t h e p l a t i n u m -
wire anode , i m m ers ed to a d ep t h o f 0 . 8 cm , is used .
N o t a l l o f t h i s v o l t a g e d r o p o c c u r s a t t h e a n o d e f i l m ,
h o w e v e r . T h e a p p r o x i m a t e v o l t a g e d r o p a t t h e f i l m
c a n b e o b t a i n e d b y s u b t r a c t i n g f r o m t h e t o t a l v o l t -
a g e d r o p , t h e v o l t a g e d r o p o f th e c e ll w h e n i t i s
o p e r a t i n g w i t h o u t a n o d e e f f e c t a t t h e s a m e c u r r e n t .
Thus , wi th norm a l H2S O4 a t 87~
T o t a l v o l t a g e d r o p t o j u s t m a i n t a i n a n o d e e f f e c t
= 45 vo l t s .
V o l t a g e d r o p f o r n o r m a l o p e r a t i o n a t 0 .3 5 a m -
pe res = 3 . 5 vo l t s .
Vol ta ge d rop a t f i lm = 45 - 3 . 5 = 41 . 5 vo l t s .
I m m e r s e d a r e a o f e l e c t r o d e = 0 . 31 4 c m 2 ( P t w i r e
i m m e r s e d 0 . 8 c m ) .
41.5 X 0.35
He a t d i s s ipa te d by f i lm = - = 46
0.314
w a t t s / c m 2.
T h e c l o s e a g r e e m e n t o f t h e s e t w o f i g u r e s f o r t h e
p o w e r r e q u i r e d t o m a i n t a i n t h e t w o k i n d s o f f i l m
a l so s u p p o r t s t h e w a t e r - v a p o r t h e o r y o f t h e a n o d e -
effect f i lm.
A p r o p e r t y o f t h e a n o d e - e f f e c t f i l m w h i c h i s o f
c o n s i d e r a b l e in t e r e s t a n d i m p o r t a n c e i s i t s a b i l i t y t o
c o n d u c t e l e ct r i c c u r r e n t. T h e a n o d e e f f e ct w o u l d b e
im poss ib le i f the f ilm were a non cond uc tor , s ince i t i s
t h e h e a t d i s s ip a t e d b y t h e c u r r e n t t h a t h e a t s t h e
e l e c t r o d e s u r f a c e a n d m a i n t a i n s t h e v a p o r f i l m . T h e
m e c h a n i s m o f c o n d u c t i o n t h r o u g h t h e v a p o r f i lm i s
b y n o m e a n s c le ar , b u t t h e f o l l o w i ng o b s e r v a t i o n s
and d i s cus s ion th row som e l igh t on i t .
W h e n t h e p l a t i n u m - w i r e a n o d e i s u s e d i n a n e l ec -
t ro ly te o f su l fu r ic ac id , the on ly v i s ib le s igns o f
c u r r e n t c o n d u c t i o n a r e r a t h e r i n f r e q u e n t a n d t i n y
s p a r k s a c r o s s t h e a n o d e f i l m . O n t h e o t h e r h a n d , i f
a l i t t l e sod ium su l fa te i s added to the e lec t ro ly te (o r
i f s o d i u m h y r o x i d e i s t h e e l e c t r o l y t e ) , t h e a n o d e f i lm
is s een to em i t a ye l low g low, cha rac te r i s t i c o f so -
d i u m e m i s s i o n . W i t h a c e l l v o l t a g e o f 7 0 v o l t s a n d
a n e l e c t r o l y t e t e m p e r a t u r e o f 4 0 ~ t h e a n o d e s u r -
f a c e i s c o v e r e d w i t h a g r e a t m a n y ( p e r h a p s 1 0 0 )
br igh t ye l low spo t s , bu t the re i s no gene ra l g low.
W h e n t h e v o l t a g e i s r a i s e d t o 1 10 v o l t s a y e l l o w
g l o w p e r v a d e s m o s t o f t h e a n o d e s u r f a c e . T h e i n -
t e n s i t y o f t h e g l o w v a r i e s o n a n y o n e p o s i t i o n o f t h e
s u r f a c e w i t h a p e r i o d i c i t y t h a t i s s i m i l a r t o t h e v i -
b r a t i o n o f t h e v a p o r f i lm d e s c r i b e d e a r li e r . T h e g l o w
does no t ex i s t ou t in the wide por t ions o f the f i lm ;
i t i s conf ined c lose to the anode sur face .
B a s e d u p o n t h e a b o v e o b s e r v a t i o n s , a n d w i t h o u t
r i g o r o u s p r o o f , t h e f o l l o w i n g h y p o t h e s i s r e g a r d i n g
c u r r e n t c o n d u c t i o n a c r o s s t h e f i l m i s o f f e r e d :
V e r y l i t t l e c o n d u c t i o n c a n t a k e p l a c e a c r o s s t h e
wide por t ions (0 .2 to 2. 0 m m th ick) o f the f i lm , a s
e v i d e n c e d b y t h e l a c k o f s o d i u m g l o w i n t h e w i d e
p o r t i o n o f t h e f il m . I n t h o s e p o r t i o n s o f t h e f i lm
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Vol. 97 No. 4 ANODE EF FE CT IN AQUEOUS ELECT ROLYS IS 139
which are sufficiently thin (perhaps 0.2 mm judging
from the photograph in Fig. 5), the potential gradient
is sufficiently high to cause ionization of the gases
within the film and c urren t conducti on will take place
by migrat ion of the gaseous ions. This ionization may
be sufficiently intense to cause a visible discharge in
the gas, as evidenced by the sodium glow noted
above. Since any one position on the film is periodi-
cally approaching closer to the electrode and then
receding, the conductio n across the film at th at point
will also var y and the intens ity of th e glow will vary,
thus, the flickering aspect of the glow is described.
Since appreciable current still flows during the
anode effect, there must be some electrolytic reaction
taking place at the anode. On the other hand, visual
observation of the anode during the anode effect
shows no formation of gas bubbles. The seat of the
electrolytic reaction must now be the electrolyte-gas
interface. This type of electrolysis, where the metal
electrode is separated from the electrolyte by a
gaseous region, has been described as long ago as
1887 by Gubkin (5) and studied at len gth by Klemenc
(6). However, these studies involved low pressures
(5-15 mm of Hg) in the gas phase and real glow-
discharge conduction was obtained.
To prov e that during the anode effect oxygen gas
is evolved into the water-vapor envelope and exits
through the neck of this envolope to the atmosphere,
the following experiment was performed: The upper
portion of the platinum-wire anode was sheathed
with a close-fitting ceramic insulator, so that a tip
about 0.5 cm long of platinum was exposed. The up-
per end of the sheath was cemented to the wire to
prevent gas leakage. The electrode was immersed so
that the 0.5 cm of bare platinum and about 0.2 cm
of the sheath were below the electrolyte surface.
When the anode effect was developed with this elec-
trode a large bubble of noneondensable gas formed
at the top of the bare platinum and broke off from
time to time when it grew too large. Tests proved
the gas to be oxygen. Fig. 10 shows the anode effect
under these conditions.
Fig. 6, 7, and 8 show that the el ectrolyt e temper -
ature has a marked effect on both the current and
the electrode temperat ure during the anode effect.
A low electrolyte temperature gives rise to a high
current and a high electrode tempera ture during the
anode effect. The explanation of this peculiar re-
lationship is to be found in the thickness of the vapor
films formed at different electrolyte temperatures.
Fig. 5 and Fig. 5a show the anode film at electrolyte
tempe ratu res of 40~ and 90~ respectively. With
the high electrol yte temp erat ure the film is quite uni-
form and it vibrates only slightly. With the low elec-
trolyte temperature the film vibrates violently and
is very wide (1-2 mm) in some places and extremely
thin in others. It is reasonable to expect that the
cooler the electrolyte, the more closely the film can
approach the electrode before it is heated sufficiently
by the hot surface to cause vaporization. As dis-
cussed previously, the main conduction through the
film occurs at the very thin sections; thus, the cold
electrolyte makes possible a larger current because
the film approaches closer to the electrode surface.
As would be expected, the intensity of agitation
of the electrolyte also affects the current during the
anode effect. At a given electrolyte temperature, in-
FIG. 10. Aqueous anode-effect with par tly insulated
platinum wire. The top part of the electrode is sheathed
with a ceramic insulator. The large bubble breaks off
periodically when it grows too large. 1N H2SO4 at 67~ C,
77 volts, 0.15 amperes, about 8X.
creased agitation causes an increased current to flow.
Agitation makes possible a more rapid rate of heat
transfer away from the electrolyte-gas interface, with
the result that this interface remains cooler, can ap-
proach more closely to the electrode surface, and can
allow a larger current to flow.
The high electrode temperature obtained with the
cold electrolyte is a secondary effect. Since the cold
electrolyte allows a large current to flow, and the
large current will dissipate more heat than the small
one, the electrode will become hotter. In brief, the
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140 J O U R N A L O F T H E E L E C T R O C H E M I C A L S O C I E T Y April 1950
e l e c t r o d e t e m p e r a t u r e i s a f u n c t i o n o f, a m o n g o t h e r
fac tors , t he r a t e of hea t d i s s ipa tion .
4. Change rom Transition Period to Anode-Effect
With the foregoing d i s cus s ion of the anode-e f fec t
f i lm after i t i s formed, i t i s now poss ible to discuss
the c r i t i ca l f ac tors which re su l t i n the change f rom
t h e t r a n s i t i o n p e r i o d t o t h e a n o d e e f fe c t.
I t i s e v i d e n t t h a t t h e a q u e o u s a n o d e - e f f e c t b e g i n s
a t a c r i t i ca l vo l t age ra the r tha n a t a c r i t i ca l cur re n t
dens i ty . F ig . 1 , 6 , 7 , and 8 show tha t fo r the evolu-
t i o n o f o x y g e n t h e c r i t i ca l v o l t a g e i s a b o u t 4 5 - 5 0
v o l t s. I f h y d r o c h l o r i c a c i d i s t h e e l e c t r o l y t e a n d
chlor ine i s evolved , t he c r i t i ca l vo l t age for anode
e f fec t i s abo ut 30-35 vol t s . App aren t ly , t he re fore , t he
n a t u r e o f t h e g a s e v o l v e d a t t h e a n o d e h a s s o m e
bear ing on the c r i t i ca l vo l t age . These fac t s sugges t
the fo l lowing hypothes i s for the ch a in of event s which
l e a d f r o m th e t r a n s i t i o n p e r i o d t o t h e a n o d e e f fe c t.
D u r i n g t h e t r a n s i t i o n p e r i o d t h e r e i s s u ff i ci e nt
h e a t d i s s i p a t e d t o v a p o r i z e m u c h o f t h e e l e c t r o l y t e ;
howev er , an anode-e f fec t f i lm canno t form unless the
20 40 ~ 0 ~0 /00 f20
olts
FIG. II. C ~thode effect w ith I)l~tinu m-wire catho de
(0.314 cm 2 im me rs ed are ~) in no rm fl H,,SO4 at 66 ~ -4- 3 ~ C.
a n o d e t e m p e r a t u r e r i s e s a p p r e c i a b l y a b o v e t h e b o i l -
i n g p o i n t o f t h e e l e c t r o l y t e . T h e t y p e o f c o n d u c t i o n
d u r i n g t h e t r a n s i t i o n p e r i o d t h a t r es u l t s i n t h e
vapo r i za t ion of the bu bble wa l l s can neve r g ive a
t e m p e r a t u r e a b o v e t h e b o i l i n g p o i n t o f t h e e l e c t r o -
ly t e , s ince a s soon as a pa r t i cu la r bubble wa l l i s
v a p o r i z e d c o m p l e t e l y th e c i r c ui t is b r o k e n a t t h a t
p o i n t a n d t h e c u r r e n t c e as e s f o r t h a t p a r t i c u l a r p a t h .
However , a s the vo l t age i s p rogres s ive ly inc reased ,
a po in t i s r eached where the re i s suf f i c i en t po ten t i a l
drop ac ros s some th in s ec t ion of one of the t rans i en t
f i lms to cause conduc t ion th rough the gas f i lm. As
soon as th i s occurs , t he hea t d i s s ipa ted by th i s cur -
r e n t c a n r e s u l t i n a l o c a l e l e c t r o d e - t e m p e r a t u r e i n
exces s of 100~ Th e hea t can be cond uc ted a long
t h e m e t a l e l e c t r o d e a n d c a u s e t h e v a p o r f il m t o s p r e ad
and be s t ab le a s a comple te enve lope . In shor t , i t i s
s u g g e s te d t h a t f o r a q u e o u s a n o d e - e f fe c t u n d e r t h e
condi t ions c i t ed , t he c r i t i ca l f ac tor for the onse t o f
anode e f fec t i s a suf fi c i en t po te n t i a l d rop ac ros s a
t r a n s i e n t v a p o r f il m t o c a u s e a n a p p r e c i a b l e c o n -
d u c t i o n t h r o u g h t h e g a s p h a s e .
Aqueous Cathode-effect
In a l l o f t he exp e r iment s and h ypoth eses d i s cus sed
above the re i s no fac tor which i s pecul i a r t o anodes
a lone . I f t he hypotheses a re cor rec t , t hen one could
p r e d i c t t h a t a c a t h o d e w h i c h e v o l v e d g a s, a n d w h i c h
i s ope ra t ed a t a h igh cur ren t dens i ty should show a
s imi l a r behavior and deve lop a vapor f i lm under
proper condi t ions . Such i s found to be the case . I f
the p l a t inum-wi re e l ec t rode i s made a ca thode in a
1N su l fur i c -ac id e l ec t ro ly te , i t fo l lows a behavior
exac t ly s imi l a r t o th a t o f t he anod e . F ig . 11 records
FIG. 12. Aqueous cathode- effec t with platinum- wire
cath ode. 1N H2SO4 at 49 ~ C, 70 volts, 1.0 ampe res, about 8X.
t h e v o l t - a m p e r e c h a r a c t e r i s t i c s f o r t h e p l a t i n u m
c a t h o d e a n d F i g . 1 2 i s a p h o t o g r a p h o f a q u e o u s
c a t h o d e - e f f e c t . T h e c a t h o d e e m i t s a b r i g h t b l u e
glow, cha rac te r i s t i c o f hyd roge n emis s ion , if t he
vol t age i s ra i s ed to 110 vol t s .
W i t h c a t h o d es , h o w e v e r , t h e r e i s y e t a n o t h e r p h e -
n o m e n o n t h a t t a k e s p l a c e . I f t h e s o l u t i o n c o n t a i n s
sodium su l fa t e , o r i f sod ium hydroxide i s used a s the
e l e c t r o ly t e , t h e c a t h o d e ef f e c t i s n o t o b t a i ne d , i ~ -
s t ead , t he ca tho de sur face i s cove red wi th a mul t i -
r u d e o f w h a t a p p e a r t o b e s p a r k d i s c h a r ge s ( b lu e
color ) , no wide vap or f i lm deve lops , and the sur -
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Vol . 97 No . 4
ANODE EF FEC T IN AQUEOUS ELECTROL YSIS 141
face of the catho de remains at 100~ or slightly
lower. Appare ntly some kind of film does form around
the cathode because the meniscus at the electrolyte
surface dips down, just as during the "cat hode effect"
or anode effect. However, the wide vapor-envelope
which can be clearly seen during the true "cathode
effect" is absent in this case. The explanation of
this phenomenon lies out of the scope of this paper,
but the writer feels that a different mechanism of
electric conduction between electrolyte and the elec-
trode surface is probably responsible for this phe-
nomenon.
Rela t i on Between Anode Ef f ec t i n Aqueous
and Mo l ten Electrolysis
The writer does not claim that the explanations
found valid for aqueous anode-effect are necessarily
valid for anode effect in molten media. I n p articular,
the chain of events which leads to molten anode-
effect is very likely different. In molten media with
a graphite anode, the e lectrol yte usually has a high
contact-angle against graphite (7, 8). On the other
hand, platinum and the alloy steel used in this paper
are completely wet (have 0 ~ conta ct angle) by the
electrolyte used. The non-wettability of the graphite
anode by the molten media may contribute to the
incidence of anode effect by making it possible for
gas bubbles to adhere strongly to the anode. More-
over, the high temperatures found in molten elec-
trolysis undoubtably have some effect upon the ease
with which the gas film can ionize and thus con-
duet current.
The writer does feel, however, that the concept of
the gaseous envelope stabilized by the vaporization
of the electrolyte close to the surface of a hot anode
should be closely investigated for molten anode-
effect. The explanations for the stability of the
gaseous envelope to be found in the literature all
center about a
gaseous f i lm- -one
stabilized by a
rush of noneo ndensable gases (02, CO, CO2) (9).
There are at least two considerations that make such
an explanation untenable. In the first place, in a
laboratory cell, when the anode effect starts, the
current will usually drop to ~ or ~ao of its previous
value. This means that during anode effect only 89
or ~o as much gas is being evolved as before anode
effect. Thus, one is forced to explain how a small
amount of gas will cause a gaseous envelope to form,
where five or ten times that amount of gas was un-
able to do so. Second, it is not hard to show by
means of hydrodynamics th at the velocity of gas re-
quired to hold back the electrolyte from the anode,
at any reasonable depth in the electrolyte, is very
large and far more than one could obtain from molten
electrolysis.
The writer is planning a future paper which will
attempt to apply the vapor-film theory of aqueous
anode-effect to anode effect in molten electrolysis.
At this time, however, it can be pointed out that it
is entirely possible that the electrolyte in the Hall
aluminum cell could vaporize if in contact with
a hot anode. Waddington and Pearson (10) have
recently pointed out that the current carriers in
cryolite are Na + and possibly AIF~. The tra nsfer-
ence of these ions will result in an anode layer of
electro lyte which is impoverish ed in NaF and rich
in A1Fa. A1Fa is a relativ ely unstabl e compou nd a nd
volatilizes with decomposition ar ound 1000 to 1100 ~
C (11). Thus, the anode may be surrounded with an
electrolyte which can volatilize at a temperature
about 100~ higher than the operating tempera ture
of the cell (1000~ and it is possible tha t a vapor
film could be formed if the anode became over-
heated.
CONCLUSIONS
1. A phenomenon which occurs when electrodes
which evolve gas in aqueous media are operated at
high current densities has been described and named
"aqueous anode-effect" because of its similarity to
anode effect in molten electrolysis.
2. Normal operation of the anode was found to
cease when the electrode temperature reached the
boiling point of the electrolyte. Under these condi-
tions, the anode enters a so-called "transition period"
where vaporization of the bubble walls leads to an
increased electrical resistance at the anode and the
current through the cell falls as the voltage is in-
creased.
3. The "transition period" behavior instantane-
ously changes to the "aqeueous anode-effect" when
the voltage is raised to a critical value that will per-
mit conduction through the gas phase.
4. The gaseous envelope which encloses the anode
during "aqueous anode-effect" is maintained by the
vaporization of the electrolyte close to the surface of
tile anode, which surface was found to be at a tem-
perature far above the boiling point of the electro-
lyte.
5. It was also shown that an "aqueous cathode-
effect," entirely similar to the anode effect, could be
obtained.
6. The relation between "aqueous anode-effect"
and anode effect in molten media was discussed and
it was pointed out that there is a possibility that the
anode-effect film in molten media is also stabilized
by vaporization of the electrolyte.
CKNOWLEDGEMENTS
The writer is indebted to Professors M. D. Hass-
ialis, T. A. Read, and A. F. Taggart for helpful
criticisms and suggestions. Mr. M. A. Kaei, Asso-
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42
J O U R N A L OF T H E E L E C T R O C H E M I C A L S O C I E T Y
A p r i l 1 9 5 0
ciate in Metal lurgy, aided with much of the experi-
menta l work .
Any discussion of this paper will appear in a Discussion
Section, to be published in the December 1950 issue of the
JOURNAL.
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