91 the effect of chloride ions on copper deposition l

7/30/2019 91 the Effect of Chloride Ions on Copper Deposition l'

http://slidepdf.com/reader/full/91-the-effect-of-chloride-ions-on-copper-deposition-l 1/7

T h e E f f e ct o f C h l o r i d e I o n s o n C o p p e r D e p o s i t i o n l'2

W . H . G A U V I N A ND C . A . W I N K L E R

McGill Universi ty , Montreal , Canada

A B S T R A C T

A d d i t i o n o f c h l o r i d e i o n s t o a n a c i d i f i e d c o p p e r s u l f a t e s o l u t i o n h a d n o e f f e c t o n t h e

e l e c t r o d e p r o c e s se s b e l o w a de f i n it e m i n i m u m c o n c e n t r a t i o n , c o r r e s p o n d i n g t o t h e p r e -

c i p i t a t i o n o f c u p ro u s c h l o r i d e . T h e l a t t e r w a s a d s o r b e d b y t h e d e p o s i t a n d a c t e d a s a

m i l d a d d i t i o n a g e n t , d e c r e a s i n g th e g r a i n s i z e w i t h a c o r r e s p o n d i n g i n c r e a s e in h a r d n e s s

a n d r e f l e c t i v i t y o f t h e d e p o s i t , a n d a s m a l l i n c r e a s e i n p o l a r i z a t i o n . I n c r e a s e d p o w d e r

f o r m a t i o n a n d , a t h i g h c h l o r id e c o n c e n t r a t i o n , f o r m a t i o n o f c u p r o u s c h l o r i d e w e r e n o t e d

a t t h e a n o d e . T h e g e n e r a l b e h a v i o r w a s n o t a f f e c t ed b y t h e a d d i t i o n o f b in d a r i n e t o t h e

e l e c t r o l y t e , b u t g e l a t i n c o n s i d e r a b l y i n c r e a s e d t h e p o w d e r f o r m a t i o n , t h e l a t t e r b e i n g

m a x i m a l a t g i v e n c h l o r i de a n d g e l a t i n c o n c e n t r a t io n s , a n d p r e v e n t e d t h e c h l o r i d e f r o m

e n t e r i n g t h e d e p o s i t , p r e s u m a b l y d u e t o t h e f o r m a t i o n o f a g e l a t in - c u p r o u s c h l o r i d e

c o m p l e x .

I N T R O D U C T I O N

Addition of chlorides (NaC1 or HC1) to acidified

copper sulfate electrolytes has been common prac-

tice in copper refineries for many years. Indeed,

their presence may be considered essential, since

it keeps the silver ion concentration very low, and

minimizes the codeposition of antimony and bis-

mut h with copper. The chloride concentration used

is usually from 10 to 50 mg/1 (1, 2).

Little information is available on the effect of

chloride ion on copper deposition. It has been re-

ported that the presence of chloride decreases thegrain size of the deposit (3), though this has been

contradicted (4); while an increase in chloride ion

concentration from 0 to 0.12 g/1 has been stated to

cause a slight decrease in cathode polarization, with-

out change in the anode polarization (4). It has

also been reported that addition of chloride ions to

electrolytes containing sulfite liquor as additio n agent

prevents brittleness of the copper deposit (5).

The most thorough investigation was published by

Yao in 1944 (6). Working with pure acidified electro-

lytes, in the absence of addition agents, he found th at

an increase of concentration at low chloride con-

centrations resulted in small increases in cathode

polarization and Vickers Hardness Number, and a

large decrease in the grain size. At concentrations

greater than about 15 rag/l, however, cathode polari-

zation and hardness decreased sharply with chloride

concentration, while an increase in grain size was

observed.

1 M a n u s c r i p t r e c e i v e d A p r i l 2 3, 1 95 1. T h i s p a p e r p r e p a r e d

f o r d e l i v e r y b e f o r e t h e D e t r o i t M e e t i n g , O c t o b e r 9 t o 12 ,

1951.2 C o n t r i b u t i o n f r o m t h e P h y s i c al C h e m i s t r y L a b o r a t o r y ,

M c G i l l U n i v e r s i t y , w i t h fi n a n c i a l a s s i s t a n c e f r o m t h e N a -

t i o n a l R e s e a r c h C o u n c i l o f C a n a d a .

71

The present study was made in view of the many

aspects of copper deposition in the presence of chlo-

ride that still require explanation, and in particular

to determine the effect of chloride in the presence

of addition agents, about which no information was

available in the literature.

E X P E R I M E N T A L

Reagent grade copper sulfate pentahydrate, sul-

furic acid, and sodium chloride were used through-

out. The composition of the electrolyte was the

same for all experiments, and consisted of 125 g/1

of copper sulfate pentahydrate and 150 g/1 of sul-

furic acid. Deposits were obtained on a cathode

between two anodes of high purity at a current

density of 2 am p/dm 2. Glass b atte ry jars were used,

and no agitation was provided. The same precau-

tions observed in previous studies (7-9) were again

followed for preparing and cleaning the electrodes,

and for drying the deposits. Chloride concentrations

were determined potentiometrically, using a silver-

silver chloride electrode as reference electrode. This

method was also used in the presence of addition

agents, since recently published results (10) con-

firm earlier conclusions (11) and indicate that theaccuracy of the method is little affected by proteins.

The effects of chloride were studied with gelatin

and with bindarine present in the electrolyte. Both

substances are commonly used in copper refining,

the former as glue, the lat ter as a by-product of the

sulfite pulp industry containing 50 to 60 per cent

calcium lignosulfonate as the active material.

R E S U L T S

in preliminary experiments, it was observed tha t

addition of chloride ion as sodium chloride to the

electrolyte had no obvious effect on deposition be-

Downloaded 27 Mar 2012 to 152.74.222.130. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp



72 JOURNAL OF THE ELEC TROC HEMI CAL SOCIETY February 1952

low a minimum concentration, which was quite re-

producible and amounted to 9 rag/1 at 25~ and

abou t 19 mg/1 at 50~ Above this minimum con-

centrati on, th e deposits became finer grained and

gradua lly changed to a salmon pink color. Increased

concentrations gave increased fineness to the struc-

ture.

Above the minimum concentration, t he anode proc-

esses were also affected. Corrosion was more ir-

regular and pitting and irregular depressions were

frequently observed. Differences in thickness showed

that solution occurred mainly from the upper part

of the anodes. A great deal of very fine copper powder

O O'0eI: k

L dC i

Z O04

h i

O

O 002_ J

TID

r1 -

Ota J

h A

C)

~9

Z

h J

b 9

OZ

- 0 . 1

- 0 . 2

f o w

yI , [

- 0 ' 3 , J t r

0 100 200 300 400 50 0

CHLORIDE ION, MG PER LITE R

F I G . ] Top: E f f e c t of c h l o r i d e c o n c e n t r a t i o n o n t h e

c h l o r i d e i n t h e d e p o s i t . Bottom: E f f e c t o f c h l o r i d e c o n c e n -

t r a t i o n o n t h e c a t h o d e e x c e s s w e i g h t ( A ) in t h e a b s e n c e o f

a d d i t i o n a g e n t ; ( B ) i n t h e p r e s e n c e o f g e l a t i n .

was found to accumulate d irectly under the" latt er

at the bottom of the cells, at both 25 ~ and 50~At still larger concentrations, less and less copper

powder was formed, b ut a grayish white precipitate

began to appear on the anodes, partially covering

them, and slowly accumulating in the cells in the

form of very thin flakes. Analysis showed that this

materi al was cuprous chloride. At sufficiently high

concentrations of chloride, the film completely sur-

rounded the anodes, and increased the anode polari-

zation severely. Analysis of the electrolyte showed

that the chloride ion concentration became depleted

at a fairly rapid rate especially at higher concentra-

tions. These effects were shown to be due to chloride

ion rather than sodium ion by the observation that

addition of sodium sulfate instead of sodium chloride

had no effect on the e lectrode processes, even afte r

prolonged electrolysis.

Presence of Chloride in the D eposits

Large deposits (avg wt 22.7 g) were obtained in

the presence of increasing amounts of chloride and

were stripped off the cathode which had been previ-

ously dipped in a solution of beeswax in carbon

tetrachloride. The deposits were dissolved ill 50 per

cent nitric acid, and their chloride content deter-

mined electrometrically with the results shown in Fig.

1, top.

Identical determinations when 50 mg of bindarine

was present in the electrolyte showed that, within

the experimental error, the amount of chloride en-

tering the deposit was not affected by this addition

agent. On the other hand, deposits obtained in thepresence of 10 and 50 mg/1 of gelatin contained no

chloride at the lower chloride concentrations, and

only traces could be detected at chloride concentra-

tions above 300 mg/1.

Increase in Weight o f Cathode Deposits

The excess weights of copper deposits obtained

in the presence of increasing amounts of chloride ions

were determined relative to the weight of a deposit

obtained in series under the same conditions, but

in the absence of these ions. The results, expressed

on the basis of the weight of the deposit in the coulom-eter in series, are shown in curve A, Fig. 1, bottom,

for a temperat ure of 25~ Addition of 50 mg/1 of

bindarine to the system did not in any way affect

these results, but addition of 50 mg/1 of gelatin

(curve B, Fig. 1) caused no increase in the excess

weight at tow concentration and gave a sharp de-

crease in cathode weight at concentrations above

100 rag/1.

Reflectivity and P hysical Appearance of Deposits

A previous study of the reflectivity of copper de-

posits (12) has shown that this property is verysensitive to minute changes in crystal size and orien-

tation. The change in the reflectivity of the deposit

caused by an increase in the chloride content of the

electrolyte was determined relative to a polished

silver standard, with the results shown in Fig. 2,

top. A definite increase in reflectivity was observed

up to a concentration of approximately 100 mg/1.

The slight decrease at higher concentrations is prob-

ably not to be ascribed to an increase in crystal

size, but to an increased roughening of the surface

and the appearance of irregularities on the deposits,

even after a short period of deposition.




Vol. 99, No. 2 EFF ECT OF CHLOR IDE IONS ON Cu DEPOSITION 73

Gelatin increases the reflectivity of copper de-

posits to a considerable extent (12) and addition of

chlorides in concentrations up to 100 rag/1 did not

decrease it. At higher concentrations, however, Jr-

regularities and excrescences began to appear, which

decreased the refleetivity. Bindarine, on the other

hand, when present alone in an electrolyte (12) in-creases the reflectivity to an extent comparable with

that for chloride in Fig. 2. No appreciable changes

were observed when chloride was added, except th at

at high chloride concentrations, deep convection

current lines and irregularities were again observed

in the deposits.

2O>_-

h-

- 1 5I-L)kdJ

ks_w I 0rr

uJ>- 5F-<_1Wrr

OZ 80/

JL,J

75

OO 70r r

,n 65i, i

Zd3

a:: 60

:Z

I 1 I I

I I I I I I100 200 300

CHLORIDE ION, MG PER L ITER

F r o . 2 . Top: E f f e c t o f c h l o r i d e c o n c e n t r a t i o n o n t h e r e -

f l e c t i v i t y o f t h e d e p o s i t . Bottom: E f f e c t o f c h l o r i d e c o n c e r t -

t r a t i o n o n t h e h a r d n e s s o f th e d e p o s i t .

Hardness of Deposits

In Fig. 2, bottom, is plotted the relation between

chloride concentration and hardness of the deposit

as obtained with a Rockwell Hardness Tester, using

a ~-in. penetrator and 60-kg load, due precautions

being taken to minimize the effect of the base metal.

The maximum effect is comparable with that when

the electrolyte contained 50 rag/1 of bindarine alone

(82 Rockwell Number), but considerably smaller

than that when the electrolyte contained gelatin

(a Rockwell Number of 90.5 for 20 mg/1 of gelatin).

As with reflectivity, the hardness of deposits ob-

tained in the presence of gelatin or bindarine is not

affected when chlorides are added to the electrolyte.

Cathode Polarization

Measurements of the anode mid cathode polariza-

tions are most easily made in a Haring cell (13) with

good accuracy and reproducibility of results, pro-

riding the same initial surface is always used, and

sufficient time is allowed for steady state values to

be reached (7). Starting with an initial cathode

surface deposited at 25~ at an apparent current

density of 2 amp/dm', the rate of attainment of

steady state polarization values in the presence of

various amounts of chloride is shown by the typical

curves in Fig. 3. It should be noted that the values

observed after a few minutes of eleetrolysis--a pro-

cedure which was followed by Yao (6) are con-

siderably different from the final steady st ate values.

The effect of chloride ion on the steady state cathode

polarization is also shown in Fig. 3.

> 9 0B

8 5

! i 84

7s

0 1 O 2 0 3 0 4 0 5 0 6 0 2 0 4 0 6 0 8 0 1O O

T l i X / I E - M I N U T E S C H L O R I D E I O N , ~ ,/ IC ~ P E R L I T E R

F I G . 3 . Left: V a r i a t i o n o f c a th o d e p o l a r i z a t i o n w i t h

t i m e , i n t h e p r e s e n c e o f ( A ) 1 0 ra g , ( B ) 2 0 r a g , ( C ) 4 0 n ag , a n d

( D ) 1 0 0 i n g p e r l i t e r o f c h l o r i d e i o n . Right: V a r i a t i o n o f

s t e a d y s t a t e c a t h o d e p o l a r i z a t i o n w i t h c h l o r i d e i o n c o n c e n -

t r a t i o n .

No effect on the polarization measured in the

presence of 5 mg/1 of gelatin, or of 50 mg/1 bindarine,

was observed when 100 rag/1 of chloride ion was

added to the electrolyte.

Anode Solution, Copper Powder, and C~tprous

Chloride Formation

In an effort to explain some of these results, a

study was made of the effects of increased chloride

concentrations on the rate of solution of the anodes,

and on the formation of copper powder and cuprous

chloride. The extent of solution of the anodes was

found by direct weighing. At the end of the deposi-

tion, the electrolyte was filtered through a tared

sintered glass crucible, the powder and precipitate

washed with small portions of dilute sulfurous acid,

dried, and weighed. The total weight of copper

powder and cuprous chloride was thus obtained.

The cuprous chloride was then dissolved, the copper

powder weighed, and the weight of cuprous chloride




74 JOURNAL OF THE ELECT ROCHE MICA L SOCIETY February 1952

obtained by difference. The extent of anode solution

and of copper powder and cuprous chloride forma-

tion (the latter in terms of its copper content) were

calculated on the basis of the weight of the copper

deposited on the cathode in the coulometer in series.

The results are given in Table I. The peculiar be-

havior of anode corrosion in presence of chloride ions

has already been mentioned, but it was particularly

unusual at 2~ at which tempe ratu re the lower

part of the anode was covered with a pinkish zone,

very even and smooth, strongly reminiscent of the

appearance of an electrodeposit, and considerably

thicker than the rest of the electrode.

TABLE I. Copper powder and cuprous chloride format ion

C h l o r i d e i o n A n o d e C o p p e r C u C I I C o p p e r i nin solut ion solution powde r preci pitat e CuCI

mg/l % % [ % . %

Tempera ture: 25~ Current, density: 2 amp/din 2

0

10

30

50

100

250

500

100.7

102.6

102.6

102.4

101.5

102.3

103.1

0.22

1 . 9 3

1.96

1 . 7 5

0.68

0.22

0.16

0.4O5

3.07

3.78

I

0.26

1 . 9 7

2.43

Temperature

0

30

50

100

250

500

: 50~ Current densit y: 2 amp/din 2

0.28

1 . 4 3

1.52

1.22

0.34

0.21

102.1

103.3

103.5

103.2

104.0

104.6

i

0.11

2.263.47

m

0.07

1 . 4 1

2.23

Temperat ure: 2~ Current density: 1 amp/ dm ~

0

0

50

50

100

100

100.1

100.1

104.9

104.1

102.8

102.9

0.05

0.04

4.75

3.70

0.39

0.67

I

I

0.125

3.27

3.22

I

I

I

0.08

2.09

2.07

indicated that, within the experimental error, the

anode processes were not altered by the presence of

bindarine. In the presence of 50 mg of gelatin, Fig. 4

shows the same trends as in the absence of addition

agent, with the striking exception that the copper

powder formation was now considerably greater, re-

presenting 33.8 per cent of the coulometer deposit

weight for the optim um concent ratio n of 50 rag/1 of

chloride. Keeping the chloride at the latter con-

centration, an even greater yield amounting to 39.5

per cent of the coul ometer deposit, could be obtained

I' -

o

3,.D

LdI- "

~ o 2 0

9

o

L l 0o

0

Ld[Ln 0 I O 0 2 0 0 3 0 0

o CHLORI DE ION, MG PER LITER

FIG. 4. Effect of chloride concentration on the copper

powder and cuprous chloride formation in the presence of

gelatin. Insert: Copper powder formation in the presence

of 50 mg/l of chloride ion and varying amounts of gelatin.

9 C O P P E R P O W D E R 2 5 ~ 0 ~ " ~ ' o

o , , " ' 5 0 ~ ~ I

9 C O P P E R I N C u C I , 2 5 ~ I

TABLE II. Anode polariza tion in )resence of :hloride ions

C h l o r i de G e l a t i n B i n d a r i n e D e p o s i t i o n A n o d ei o n s p o l a r i z a t i o nr a g / I r ag ,. 1 m g / I h r m v

0

100

500

500

100

500

500

_ _ I

_ _ L

I

- - 5 0

- - 50

20

31.5

65.2

431.9

532.3

94.3

387.0

1050

Table I shows that, without addition of chloride

ion, the powder formation was quite small and in-

creased with temperatur e. In the presence of chloride,

the powder formation passed through a maximum

at a given temperature, then decreased sharply,

while the cuprous chloride formation increased. Max-

imum copper powder formation also seemed to de-

crease as the temperature increased. Finally, the

difference between the amount of anode dissolved

and the sum of copper going to powder and to

cuprous chloride remained approximately constant

at a given temperature.

Results obtained with increasing amounts of chlo-

ride ions and a fixed amount of bindarine (50 mg/1)

with 200 mg/1 of gelatin, as shown by the insert in

Fig. 4.Anode Polarization

Measurements of the anode polarization showed

tha t at 25~ and 2 amp /d m 2, chloride ion had no

effect whateve r until cuprous chloride formed, above

the minimum chloride concentration required. It

then rose slowly, with marked irregularities when

flakes of cuprous chloride became detach ed from the

anode. It was observed to increase more regularly

when a complete film of cuprous chloride surrounded

the anodes. Table II shows values of the anode

polarization at different times of deposition and for




Vo l . 9 9 , No . 2 E F F E C T O F C H L O R I D E I O N S O N C u D E P O S I T I O N 75

d i f f e r e n t e l e c t r o l y t e s . T h e f i r s t v a l u e , i n t h e a b s e n c e

o f c h lo r i d e io n , is a s t e a d y s t a t e v a l u e w h i c h r e m a i n s

e s s e n t ia l l y c o n s t a n t a f t e r t h e f i rs t h o u r o f d e p o s i t i o n .

T h e o t h e r v a l u e s s h o w e d m a r k e d f l u c t u a t i o n s a s

d e p o s i t i o n p r o g r e s s e d a n d a r e g i v e n m e r e l y t o i n d i-

c a t e t h e o r d e r o f m a g n i t u d e . I t s h o u l d b e m e n t i o n e d

t h a t t h e p a r t i M p a s s i v i t y i n d i c a t e d b y t h e h i g h

p o l a r i z a t i o n v a l u e s , e s p e c i a l ly i n t h e p r e s e n c e o f

g e l a t i n , m e r e l y r e f l e c t s t h e i n c r e a s e i n p o t e n t i a l

n e e d e d t o m a i n t a i n a g i v e n c u r r e n t d e n s i t y w i t h

i n c r e a s e d r e s i s t a n c e o f t h e c u p r o u s c h l o r i d e f i l m .

DISCUSSION

C a r e f u l c o n s i d e r a t i o n o f t h e m e c h a n i s m o f c o p p e r

d e p o s i t i o n f r o m s u l f a t e s o l u t i o n s i s r e q u i r e d b e f o r e

a s a t i s f a c t o r y e x p l a n a t i o n f o r s o m e o f t h e r e s u l t s

p r e s e n t e d c a n b e o f f e r e d .

I f a s h e e t o f c o p p e r is i m m e r s e d i n a c o p p e r s u l f a t e

s o l u t io n , i n t h e a b s e n c e o f a n y c h l o r id e s o r a d d i t i o na g e n t s , t h e f o l l o w i n g r e a c t i o n w i l l o c c u r u n t i l e q u i l i b -

r i u m p r e v a i l s a t t h e m e t a l - s o l u t i o n i n t e r f a c e :

Cu ++ + Cu +~- 2C u +. ( I )

F r o m t h e e q u i l i b r iu m c o n s t a n t fo r t h i s r e a c t i o n

( 1 4 - 1 6 ), i t c a n b e c a l c u l a t e d t h a t t h e e q u i l i b r i u m

c o n c e n t r a t i o n o f c u p r o u s i o n s i n t h e b o d y o f a 0 .5

m o l a r c o p p e r s u l f a t e s o l u t i o n , s u c h a s t h a t u s e d i n

t h i s i n v e s t i g a t i o n , w o u l d b e c l os e t o 0 . 8 7 . 1 0 - 3 g r a m

m o l e / 1 a t 2 5 ~ I n t h e a b s e n c e o f t h e m e t a l o r o f a

s u i t a b l e p r o c e s s t o m a i n t a i n t h e c u p r o u s i o n c o n -

c e n t r a t i o n , t h e l a t t e r w i ll c o n t i n u o u s l y d e c r e a se ,

o w i n g t o o x i d a t i o n b y d i s s o l v e d a t m o s p h e r i c o x y g e n

i n a c i d i c s o l u t i o n s , o r t h r o u g h h y d r o l y s i s ( 1 7 ) . S i m -

i l a r l y , i t i s p o s s i b l e t o e s t i m a t e ( 1 8 ) t h e e q u i l i b r i u m

c o n c e n t r a t i o n o f c u p r o u s i o n s i n t h e a n o d e f i l m t o

b e a p p r o x i m a t e l y 1 . 1 0 - 3 g r a m m o l e /1 a n d t h a t i n

t h e c a t h o d e f il m t o b e a p p r o x i m a t e l y 0 . 6 5 . 1 0 -~

g r a m m o l e / 1 u n d e r t h e o p e r a t i n g c o n d i t i o n s u se d .

T h e f a c t t h a t t h e c u p r o u s i o n s a r e li k e l y t o b e p r e s e n t

i n t h e f o r m o f t h e c o m p l e x ( C u S O 4 ) - d o e s n o t i n

a n y w a y a l t e r t h i s d i s c u s s i o n .

I f c h l o r id e io n s a r e n o w a d d e d t o a n e q u i l i b r i u m

s o l u t i o n o f c u p r i c a n d c u p r o u s i o n s s t i l l w i t h o u t

e l e c t r o l y s i s , f o r m a t i o n o f t h e l i t t l e s o l u b l e C u C 1p r o c e e d s a c c o r d i n g t o t h e e q u a t i o n

C u + + C 1 - ~ C u C I . ( I I )

U s i n g 2 . 2 . 1 0 - 7 as t h e v a l u e o f t h e s o l u b i l it y p r o d u c t

c o n s t a n t a t 2 5 ~ ( 1 4 ), i t c a n b e c a l c u l a t e d t h a t f o r

a 0 . 5 m o l a r c o p p e r s u l f a t e s o l u t i o n , p r e c i p i t a t i o n o f

C u C 1 w i ll c o m m e n c e w h e n t h e c o n c e n t r a t i o n o f c h lo -

r i d e i o n i s g r e a t e r t h a n 2 . 5 . 1 0 - 4 g r a m m o l e / l , o r

8 .9 r a g/ 1 . T h e e x p e r i m e n t a l o b s e r v a t i o n t h a t d e p -

o s i ti o n b e g i n s t o b e a f f e c te d b y t h e p r e s e n c e o f

c h l o r id e io n a t a p p r o x i m a t e l y t h i s c o n c e n t r a t i o n

g i v e s s t r o n g r e a s o n t o a s s u m e t h a t C u C 1 , s t i l l i n

c o l l o i d a l f o r m s i n c e i t h a s h a d n o t i m e t o g r o w i n t o

a v i s i b l e p r e c i p i t a t e , i s r e s p o n s i b l e f o r t h e a d d i t i o n

a g e n t e f f e ct o f c h l o ri d e s. T h i s i s f u r t h e r s u p p o r t e d

b y t h e a d s o r p t i o n t y p e o f c u r v e f o r t h e c h l o r i d e c o n -

t e n t o f t h e d e p o s i t ( F i g . 1 ). T h e s o l u b i l it y p r o d u c t

c o n s t a n t a t 50 ~ c o u ld n o t b e o b t a i n e d f ro m p u b -

l is h e d d a t a , b u t c a n b e e s t i m a t e d t o b e 1 . 2 . 1 0 - 6

f r o m t h e m i n i m u m c o n c e n t r a t i o n t h a t w i l l a f f e c t

d e p o s i t i o n a t t h i s t e m p e r a t u r e . I t m u s t b e p a r -

t i c u l a r l y n o t e d t h a t s o l u t i o n o f C u C 1 t o f o r m t h e

c o m p l e x i o n ( C u CI _~ )- a c c o r d i n g t o t h e e q u a t i o n

C u C 1 + C I - ~ - ( C u C1 2 )- ( I I I )

c a n o c c u r t o a p p r e c i a b l e e x t e n t o n l y in t h e p r e s e n c e

o f c o n s i d e r a b l y l a r g e r c o n c e n t r a t i o n o f c h l o ri d e i o ns

t h a n w a s u s e d i n t h i s i n v e s t i g a t i o n ( 1 7 , 1 9 - 2 1 ) .

T h e A n o d e P r o c es s es

S o f a r , o n l y e q u i l i b r i u m c o n d i t i o n s i n t h e a b s e n c eo f e l e c t r o l y s i s h a v e b e e n c o n s i d e r e d . I f t h e p r o c e s s e s

o c c u r r i n g a t t h e a n o d e d u r i n g e l e c t r o l y s is in t h e

a b s e n c e o f c h l o r i d e o r a d d i t i o n a g e n t s a r e n o w e x -

a m i n e d , t h e f o l lo w i n g t h r e e r e a c t i o n s a r e p o s s i b l e :

Cu ~ Cu +~ + 2e , ( IV)

Cu - -~ Cu + + e , (V)

Cu+ - -~ Cu++ + e . (V I)

E q u a t i o n ( I V ) p r e d o m i n a t e s d u r i n g a n o d i c s o l u ti o n .

S i n c e t h e a n o d e f i l m c o n t a i n s c u p r o u s i o n s i n c o n -

c e n t r a t i o n h i g h e r t h a n i n t h e s o l u t i o n , t h e s e i o n sp a s s b y m i g r a t i o n a n d d i f f u s i o n a c r o s s t h e a n o d e

f il m . U p o n r e a c h i n g t h e s o l u t i o n s i d e o f t h i s l a y e r ,

a r e g i o n w h e r e t h e e q u i l i b r i u m c o n c e n t r a t i o n o f

c u p r o u s i o n s i s l e s s , r e a c t i o n ( I ) g o e s t o t h e l e f t ,

a n d m e t a l l i c c o p p e r i s p r e c i p i t a t e d a s a v e r y f i n e l y

d i v i d e d p o w d e r . T a b l e I s h o w s t h a t , i n t h e a b s e n c e

o f c h l o r i d e s , t h i s p o w d e r f o r m a t i o n i s r a t h e r s m a l l ,

s i nc e m o s t o f t h e c u p r o u s i o n s d i f f u si n g f r o m t h e

a n o d e f il m m e r e l y s e r v e t o r e - e s t a b li s h t h e c u p r o u s

i o n c o n c e n t r a t i o n i n t h e s o l u t io n , w h i c h i s c o n s t a n t l y

b e i n g d e p l e t e d b y o x i d a t i o n a n d h y d r o l y s i s . I t i s

i n t e r e s t i n g t o n o t e t h a t , u p o n f o r m a t i o n o f m e t a l l i c

c o p p e r a c c o r d i n g t o r e a c t i o n ( I ), a n e q u i v a l e n t

a m o u n t o f c u p r i c i o n s i s a l s o f o r m e d , r e s u l t i n g i n a

s t e a d y i n c r e a s e o f t h e c u p r i c io n c o n t e n t o f t h e s o l u -

t i o n a s e l e c t r o l y s i s p r o c e e d s . S i m i l a r l y , t h e a c i d i t y

w i l l s l o w l y d e c r e a s e w i t h t i m e , o w i n g t o t h e o x i d a -

t i o n o f c u p r o u s i o n s i n t h e s o l u t i o n . S in c e t h e c u p r o u s

i o n s c o n t i n u o u s l y l e a v i n g t h e a n o d e f il m m u s t b e

r e p l a c e d t o m a i n t a i n t h e e q u i l i b r i u m v a l u e , r e a c t i o n

( V ) m u s t a l s o o c c u r c o n c u r r e n t l y w i t h ( I V ) , a n d i t

i s s e e n a t o n c e t h a t c o p p e r , e v e n u n d e r i d e a l c o n d i-

t i o n s , m u s t d i s s o l v e a n o d i c a l l y w i t h a t l e a s t s l i g h t l y

g r e a t e r t h a n 1 0 0 p e r c e n t e f f i c i e n c y .




76 JOURNAL OF THE ELEC TROC HEMI CAL SOCIETY February 1952

If now the Mdition of chloride ions to the solu-

tion is considered, the free chloride migrating to the

anode film will cause further prec ipitation of cuprous

chloride. The amount of cuprous chloride formed in

the film will be considerably larger than in the body

of the solution and cuprous chloride will diffuse

across the anode layer. Two possibilities must now

be considered in turn. If the original addition of

chloride to the solution was small, say up to about

50 rag/l, the concentr ation of Cu + ions in the solu-

tion will not be appreciably decreased and will soon

be restored to the equilibrium value. It will be possi-

ble for the molecules of CuC1 which have diffused

away from the anode layer to dissociate back into

chloride and cuprous ions according to equa tion (II),

and the latter will, in turn, precipitate metallic

copper since the equilibrium value for the cuprous

ions is now exceeded. Powder formation in the pres-

ence of chlorides will therefore follow the overallreaction:

2CuC1 ~ Cu ++ + 2C1- + Cu. (VII)

However, if large additions of chloride have been

made to the solution, the concentration of free cup-

rous ions in the solution will be drastically reduced,

its free chloride ion concentration will be corre-

spondingly increased, and although the formation of

cuprous chloride in the anode film will be higher, less

and less of the CuC1 diffusing across the anode layer

will dissociate according to the above equation as the

chloride conten t increases. Table I clearly shows that

as the yield of copper powder decreases after pass-ing through a maximum, that of cuprous chloride

increases. Anode convection currents will at first

wash away the undissoeiated salt, and may even

carry it to the cathode, where its presence may be

responsible for the deep vertical convection lines

which have been observed. At still higher concen-

trations of chloride, the cuprous chloride forms a

slowly growing film on the anode.

The effect of chloride ions on the anode processes

does not seem to be altered in any way when

bindarine is added to the system, but it is strikingly

changed when gelatin and chloride are both present.

Whereas a maximum powder formation amounting

to about 2 per cent of the weight of the deposit in

the coulometer was observed at 25~ in the presence

of chloride alone, opt imum concentra tions of chloride

and gelatin increased the powder formation to 39.5

per cent, indicating that the anode now dissolved

largely in the cuprous form. This phenomenon can be

explained in the light of the proposed mechanism if

it is assumed that the effective concentration of

cuprous chloride in the solution is radically decreased,

allowing the dissociation of the salt formed in the

anode film to proceed unhampered upon diffusion to

the solution side of the anode layer. Formation of a

complex between cuprous chloride and gelatin ap-

pears to be a reasonable possibility, as it has been

shown to occur at least in basic media not only with

gelatin but with other proteins as well (22-2t). The

large increase in powder formation can be explained

by accelerated diffusion under the influence of a

much larger effective concentra tion gradient.

The Cathode Processes

In the absence of chlorides or addition agent, the

three following cathode reactions are possible:

Cu++ + 2e ~ Cu, (VIII)

Cu + -t- e --* Cu, (IX)

Cu ++ + e+ Cu +. (X)

Under steady state conditions in the cathode film,

reaction (VIII) preponderates, since (IX) cannot

occur until the potential requirements for the dep-

osition of cuprous ions are met. If the cuprous ion

concentration demanded by equation (I) were at all

times maintained, reaction (VIII) would be the only

one to occur, and two faradays of electricity would

convert one gram ion of cupric ions to metallic

copper. The efficiency of deposition in this case

would be 100 per cent. In practice, however, de-

pletion of the cuprous ions does occur, and to restore

the equilibrium, reaction (X) must take place to

some extent, resulting in less than 100 per centefficiency of deposition. It can be predicted, there-

fore, that all conditions leading to depletion of the

cuprous ions by favoring its oxidation and hydrolysis,

such as dissolved oxygen, low acidity, high temper-

ature, low apparent current density (or, alterna-

tively, current densities near the critical value), will

decrease the cathode efficiency. Ample experimental

evidence has been presented to support these views.

As an example of an extreme case, it is possible to

electrolyze a strong, hot copper sulfate solution at

low current density with no copper whatever de-

positing at the cathode, the whole of the current

being used for the conversion of cupric to cuprous

ions (25, 26).

The cathode behavior will not be greatly affected

in the presence of small additions of chloride, since

no appreciable depletion of the cuprous ions in the

film occurs, and deposition will proceed normally

except for the addition agent effect resulting from

adsorption of CuC1, with a corresponding increase in

weight of the deposits, as shown by the first part of

curve A, Fig. 1, bottom. But when the chloride ion

concentration is high, the cuprous ions concentration

will be depleted to a much greater extent, and the




Vo l . 99 , No . 2 E F F E C T O F C H L O R I D E I O N S O N C u D E P O S I T I O N 7 7

reduction of Cu ++ to Cu+ [reaction (X)] will proceed

to an increasing extent, resulting in decreased dep-

osition efficiency. The second part of curve A, Fig.

1, bottom, occurring beyond a critical chloride ion

concentrati on of between 50 and 75 mg/1, is really a

composite curve, representing a combination of the

increase in weight due to CuC1 adsorption, and the

decrease due to lowered deposition efficiency.

Addition of bindarine to an electrolyte containing"

chloride ions does not in any way alter the various

effects on the cathode process observed in the pres-

ence of chloride alone. Cuprous chloride is still ad-

sorbed by the deposit, and to very nearly the same

extent. Gelatin, however, appears to prevent the ad-

sorption of CuC1 by the deposit, as shown by the

analyti cal tests, and also by the first part of curve B,

Fig. 1, bottom, which does not exhibit the regular

increase due to adsorption in presence of chloride

alone. However, the second par t of this curve closelyfollows the behavior of curve A, indicating that the

depletion of cuprous ions in the cathode film must

again operate. These observations, of course, support

the assumption previously made that the effective

concentrat ion of CuC1 in the solution is considerably

reduced by the formation of a gelatin-cuprous

chloride complex.

In an effort to explain the depression observed in

the polarization-chloride ion concentration curve

(Fig. 3), fur ther expe riments were made a t low halide

concentrations in the presence of gelatin. Entirely

new phenomena were encountered, discussion of

which will be presented in a subsequent paper.

A C K N O W L E D G M E N T

Grateful acknowledgment is made to the Com-

mittee on Research, McGill University, for financial

assistance in the preparation of this manuscript.

A n y d i s c u s s i o n o f t h i s p a p e r w i l l a p p e a r i n a D i s c u s s i o n

S e c t i o n , t o b e p u b l i s h e d i n t h e D e c e m b e r 1 95 2 i s s ue o f t h e

JOURNAL.

R E F E R E N C E S

l . Y U -LIN YAO, Trans . E lec trocbem. Sou . , 8 6 ,3 6 5 (1 9 4 4 ) .

2 . ANON., C a n . M i n i n g J . , 58, 685 (1937).

3 . K . A R ND T , " T e c h n i s c h e E l e k t r o c h e m i e , " p . 29 7, V e r l a g

F . E n k e , S t u t t g a r t ( 19 29 ).

4 . E . W . R o u s e A ND F . K . A U BE b , Trans . E lec trochem. Sou . ,

52, 189 (1927).

5 . A . I . M O TO R ~ N , Tsve lnge Meta l . , 4, 54 (1940).

6 . YU-LIN YAO, Trans_ Eleetrochem. Sou. , 86,371 (1944).

7 . W . H . G A U V lN A ND C . A . WIN K LER , C a n . J . R e s e a r c h ,

A21, 37 (1943).

8 . W . H . G A U V IN A ND C . A . WIN K LER , Can. J . Research ,

B21, 81 (1943).

9 . W . H . GA ~'VIN AND C. A. WINKLER, Can. J . Research ,

B21, 125 (1943).

1 0 . Y U -LIN Y A O , Trans . E lec trochem. Sou . , 85,213 (1944).

11 . I . M . KOLTHOFF AND O. TOM ICEK, Chem. Weekb lad . , 21 ,

106 (1924).

1 2 . W . H . G A U V IN A ND C . A . WIN K LER , U n p u b l i sh e d re su l t s .

1 3 . H . E . H A m N G , Trans . E lec trochem. Sou . , 49,417 (1926).

14 . R. LUTHER, Z . p h y s i k . C h e m . , 34 , 488 (1900).

15 . G. Bi iDLANDER, Z . E l e k t f v c h e m . , 7, 159 (1907).

1 6 . E . H E I N ~ R T n , Z . E l e k t r o c h e m . , 37, 61 (1931).

1 7 . F . F6 R STER A N D G . COFFE'rTI,Z . E l e k t r o c h e m . , 10, 736

(1904).

18 . A. BRENNER, Proc . An t . E lec trop la ters ' Soc . , 28, 95

(1940); 29, 28 (1941).

19 . G. BbDLANDER AND O. STORB ECK, Z. anorg. Chem., 31, 7

(1902).

20 . R. LUTHER, Z . p b g s i k . C h e m . , 3 6 ,3 8 6 (1 9 0 1 ) .

2 1 . A . A . N o Y E s A ND M IN G C H O W, J . A n t . C h e m . S e c . , 40 ,

739 (1918).

2 2 . H . D . B A ER N STEIN , J . B i o l . C h e m . , 7B, 481 (1928).

23 . W . EHRENBERG AND P. WULFF, K o l l o i d - B e i h e f t e , 42, 1

(1935).

24 . W. D . TREAI)WEbL, S. JANETT AND M . BLUMENTHAL,

Heb~. CAirn. Aura, 6 , 513 (1923).

25. F. FOERSTER AND O. SEIDEL, Z. anorg. Chem., 14, 106

(1897).

26 . F. J . SCHWAB AND J. BAUM, J . P h y s . C h e m . , 7,493 (1903).

91 the effect of chloride ions on copper deposition l

Documents