E
F G H
I K
+ +
M / I O ZnSO, M/5 H,SO,. M / I O ZnSO, M / s o H,SO, M/IO resorcinol. Saturated solution of ZnO,H, in 3 M NaOH. Add 60g solid NaOH (3M/2) per liter of solution G. Add Iig resorcinol (M/Io) per liter of solution G. Dilute solution G to one-fourth with 3 M NaOH.
+-
burners. In this bath a given temperature could be kept to within 2'-3' of the temperature desired. This bath would accommodate four solutions at one time and so the runs were made in sets of four each. The solutions were contained in 500 cc beakers, each cell having two zinc anodes with a rotating cathode placed between them. The cathodes were zinc tubes, one centimeter in diameter and dipped full five centimeters into the solution, making a total cathode surface of a trifle over sixteen square centimeters. Each cathode was turned by an individual motor, making 200-300 revolutions per minute. The current and voltages were all taken on the specially brightened surface of the connector which held the rotating cathode fast to the shaft of the motor. In each particular case the readings were quite constant but that of course does not mean that they were free from error. An imperfect contact between the voltmeter lead and the rotating cathode, an imperfect contact between the rotating connector and the rotating cathode, or a slight fluctuation in the temperature would all introduce quite serious errors. While it is hoped and believed that errors due to these causes were small, the measurement of the voltages was a secondary matter and it would not be safe to base conclusions on minor fluctuations in the voltages. The polarization voltages were not determined in any case. By means of a sliding resistance, current densities were varied in steps of 0.5 ampere usually up to j amp/dm2. After that the current density was increased rapidly up to the values a t which the deposits became hopeless. In Tables 11-X are given all the data except the limiting current densities, these last being put all together in Table XI. In the runs with solutions A, B, C and D a t 20' the immersed portion of the cathode was twice what it was in the later runs and the motor gave a lower speed. The limiting values in Table X I are not comparable therefore with the corresponding values at 40' and 70'. To show that this difference is due to rate of stirring, a couple of runs were made
371
Electrolytic Precz2itntion of Zinc
TABLE I1 Temperature
1
20'
___ ~
"mp Volts dm2
Sol.
. -~ -
-
Volts
Kemarks
0.91 0.41 0.55 1.23
A l l deposits granular but spreading
1.34 0.86
Same as preceding but C smoother
-~ - I _ -
A
A l l deposits granular a n d spiral
1.0 0.55 1.0 0.32 1.0 10.41 1.0 I 0.84
B C D -__
~
A
2.0 2.0 2.0 2.0
B C D
0.97 0.54 0.73 1.90
1
Deposits granular but bec'o m i n g smoother
A
-
A I B ' C D ,
~~
A
3.0
3.0 3.0 3.0
2.5 2.5 2.5 2.5
B C D
0.97 1.55 3.10
B
snioother
c
D
I. I O
2.82
~
3.5 3.5 3.5 3.5
1.70 I. I O
1.65 3.20
Deposits still granular but very metallic looking
~
A B
I .40
C D
1.90 3.60
1.90
A B
5.5 5.5 5.5 5.5
c
D
A B
C D
1
1
2.15 1.60
All deposits stronger and smoother
No change in 1.50 deposits
2.00
2. I O
4.00
A, B a n d C good; D poor
2.00
4.00
2.65 1.70 2.40 4.50
Same as in preceding run
3.00
All fair
~
6.5 6.5 6.5 6.5
2.90 2.15
A , B and C good; D bad
B
2.50 4-30 ~
A B C
7.5 7.5 7.5
C
8.5 13-50 Only fair
3.20 2.20
A1 1 deposits fair
IC
deposits
7.0 7.0
2.20
8.0
3-30 All treeing
2.65
~~
A
!I B
2.80
8.0 8.0
2.50
3-30 ~
I! I
;I
c
9.0
3-50 Poor and nonadherent
372
TABLE I11 Temperature Sol.
Amp
Remarks
din2
Sol. -
.~ ~
A B
0.50 0.15
No precipitate
C
0.35
granular. Not very adherent
D
A B C D
0.30
0.54 0.36
0.57 0.53
1.0
1
0.96
B
0.60
C D
0.87 1.46
C D
A
A l l deposits finely crystallineandadherent. D very hard and fine
B C D
~ _ _ _
A
“4mp Volts dm‘ I .o 1.0 I .o
2.0 2 .o 2.0 2.0
..
0.39 0.30 0.46 0.36
Becoming finely granular and more adherent
0.77 0.41 0.78
All good and a d heren t somewhat miral. A poorest, D best
-~
A l l deposits good, especially A which was
Remarks ____
-
A
)
__
All fine
0.93 0.36 0.98 I .60
B C
deposits
bright ~-
~
A B C D
1.20
0.35 1.30 1.80
0.40
-
~~~~
A B
C D -
A B C D
_
1-30 A l l
0.60 I .40
__
deposits
fine
1
2. IO
1.50 1 A, C, D fiue. B slightly spotted b u t 2.50 adherent
A B C D
1.60
0.go 1.60
6.0 6.0 6.0
1.00
spotted
2.60 4-70 6.50
A l l deposits l i A good
1
6.0 6.0 6.0 6.0
A, C, D good, B hard and spotted
1.90 1.10 2.00 3.10
6.0
2.90
4.00
.- - .
2.10
I D
___
1.90
_ -. -.
A 11 deposits fine
1.25
adherent
~-
A 115.0 B ‘15.0 C 15.0 D ~15.0
-- ..-
1.00
~~
~
4.I O 2.80
4-90 7-40
~~
_~~
_ _ _-
-
A, €3, C good, D poor
TABLE IV Temperature 70'
~~
Sol.
'
.4mp Volts dm2
1
Remarks -
A B C D
Remarks
__
~
0.5 10.30 0.5 10.15
0.5 , 0.30 0.5 , 0.20
separate crystals
0.39
A l l deposits crystalline and ad heren t
'0.51
Same as preceding
I. IO 0.30
0.20
0.45 0.44
~
_-
0.28
1.0 ~
-
~
-
0.25
~
A l l deposits crystalline and adherent. D best ~-
__. -~
Same as preceding
0.55 0.60 ~
0.72 0.26
0.65 0.70
3.5 3.5 3.5 3.5
1
0.69 0.79
. - ..
-.
A B C D
1-32 0.35 0.78
~
1
1.20
0.44 0.86 1.30
1.10
____~_
- -~
~
Remarks
Sol.
lG%< :I
0.3 5 0.40
Crystal line a n d non-adherent
B
t.0
F G
H
1.0 I .o 1.0
~
- .______
~~
0.40 0.68 0.18
All good, E and F the better
0.33 ~
E F G
i ~
2.5
0.52
2.5
1.05
All good, G and H smooth and adherent
~~
E F G H
4.0 4.0 4.0 4.0
~
-
0.78 1.80
0.44 0.47
F, G, H good,
E fair
Volts
Remarks
E, G, H good and adherent: F crystalline 0.28 a n d non-ad0.32 ~-herent
0.37 0.55
~
E
0.52
G H
0.90 0.38 0.35
F
-_ E I; G
H
H ~~
A-11 deposits adherent, C most so. All crystalline, D least so
TABLE V Temperature zoo
Volts
0.I O 0.18
~
Same as preceding
All good
__
0.56 1.32 0.36 0.42
E fair, Fcrystalline, G and H good
374 TABLE VI Temperature 40' --
~~
Sol.
E
F G H
E
F G H
Remarks
0.5 0.5 0.5 0.5
1.5 1.5 1.5 1.5
0.30 A l l deposits 0.41 fair and crys0.04 talline
E
0.07
H
I .o I .o I .o I .o
0.60 A l l .deposits 0.80 good 0.06
F G
2.0 2.0
I -04 0.07
H
2.0
0.12
E F
3.0 0.go A l l deposits 3.0 I .70 good 3.0 0.I I 3.0 0.19
0.09
F G
H
2.5 2.5 2.5 2.5
F G
H
3.5 3.5 3.5 3.5
0.42
F and G good,
0.56
E and H fair
0.06 0.09
0.63
0.76 A l l deposits 1-33 good 0.09
_
G H
0.14
-~
E
F G
_
~
E
Volts
. _ _ _ - ~
A1 1 deposits good ~
__
E
1.00 A l l deposits 1.90 good 0.12
F G
0.26
H
4.0 4.0 4.0 4.0
1.12 2 -00
A l l deposits good
0.13 0.28
TABLE VI1 Temperature 70° Sol.
, Amp
Remarks
;i;;;a Volts
Sol.
i
A & :
E
F G
0.13 E a n d H good, F and G nonadherent 0.02
0.20
0.30 0.03 0.03
0.23
H
0.02
E
0.29
H
0.04 0.04
Volts
Remarks
~
~
Same as preceding
~~
F G
Same as pre0.53 ceding
H
F G
0.43 A l l deposits 0.92 good 0.06
H
0.05
E
E F G
2.0 2.0 2.0 2.0
0.36 0.75
Same as preceding
0.05
0.04 0.53 1.13
A l l deposits good
0.06 ~-
0.65 1.26
0.07
0.06
A l l deposits good
E
F G H
4.0 4.0 1.55 good, D poor 4.0 0.08
4.0
0.07
Electrolytic P?-ecz.itntzo?z of Zzkc
_.
S P Volt dmL
Sol.
1
Remarks
~1
375
.__...____
~
Sol.
j 3 Volt
Remarks
~
I K
0.5 0.5 0.5
L
0.05
0.05 0.08 0.05
I
0.08
K
0.08
L
I, K and L good, M fair
0.08
I
0.IO 0.11 0.10
0.06 0.07
1.0
I .o
0.07 0.07
I K
2.0 2.0 2.0 2.0
0.09 0.09 0.09 0.og
A l l deposits good
0.12 0.12 0.12 0.12
A l l deposits good
M ~
K
A l l deposits good
I
K
L
0.IO
M
0.14 A l l deposits 0.14 good
I
0.14
L
~
Sol.
'
As _ ~
_
Volt
~
~~
L
M
'
1
I good, K very 4.0 0.15 fair, L fine, 4.0 0.16 s m o o t h and 4.0 0.16 adherent. M 4.0 0.15 only fair
~
Volt '
I K
1.0
0.04
L
1.0
0.05 0.05 0.05
I .o
0.06
I
2.0
0.08
K
2
L
2.0 2.0
M
A l l deposits good
M
0.07
'
__
Remarks
0.06
1 K
3.0 3.0
0.04 I, K and L 0.04 good, M fair
0.06
1
3.0
Remarks
~
0.07
K
~
3.0
TABLE IX Temperature __-
0.05
I
K M
0.14
I, K and L good, M fair
M
L
0.08
M
I .o 1.0
2.5 2.5
3.5 3.5 3.5 3.5
0.09 0.09 0.09 0.09
__
A l l deposits good
A l l deposits 0.13 good
0.12
0.12 0.11
I K
L
I .o
.o 0.08
0.IO 0.IO 0.I I 0.10
I
0.13 0.14 0.13 0.14
L
M
A l l deposits good
0.08 0.08
M K
A l l deposits good
A l l deposits good
I, K and L good, M fair. M good zinc but thin
376 - -_.
! TABLE X Temperature 70' ~
~
~~~
Volt
1
1.0
0.03
1
1.0
0.04 0.04 0.05
1
~
Remarks ~~
I
0.03 0.03
K
L
0.03
M
~-~ ~-
I
0.04 0.05 0.05
K
L
I thin and fair,
I
Kgood. Lfair, M good
K
I K L
0.05 0.07 0.06 0.07
Same as preceding
0.07 0.09 0.07 0.08
Same as preceding
K
L
M
L
I
1.0
M
1.0
I
2.0
-
~
0.06
I
I
1
~. ._
Same as preceding
M
M
Remarks
~
0.04
~
-_
~
K
~
L
I
M I
2.0
0.05 0.05
2.0 2.0
0.06
IC, L and M good, I very poor ~
_
_
-
-
~
Same as preceding
0.05
3.0
0.06 0.08
Same as preceding
0.07
0.08
0.08 0. IO
Same as preceding
0.09 0. I O
TABLE XI Current densities a t which deposits go bad -~
-__-___--~~
___
Temperature 7oQ Volts ____
46.5
12.0
1
12.0
1
59.0 55.8 65.0 15.2
7.2 4.5 6.0 9.4 3.9 4.6 0.8
0.07
0.03
9.3 12.4 1
Run a t slower speed than the others.
* Run a t higher speed than the others.
0.30
0.30 0.15 0.35
6
on February 16, 1907, with solution A and another motor. By varying the speed of the motor, the current density at which the deposit became bad could be varied a t will. With the highest speed, about 1 2 0 0 K. P. M., treeing began a t the lower edges of the cathode when the current density was 60-65 amp/dm2. Even at these values there were no signs of spongy zinc and the deposit was excellent over the bulk of the cathode surface. The limiting value for solution G at 40" and at 70" were obtained on a special run made in 1906. These enormous fluctuations, which were entirely unforeseen when the work was planned, make the actual figures for the current densities of practically no value so far as comparisons are concerned. They are valuable in showing what high current densities may be used both in acid and alkaline solutions provided precautions are taken t o prevent impoverishment at the surface of the cathode. In fact there is really no evidence that there would be any upper limit in most cases if one could prevent all impoverishment a t the cathode surface. On the other hand it must be remembered that the danger from treeing increases rapidly with increasing fall of potential through the electrolyte. In general, the alkaline solutions begin to give bad results at a lower current density than the acid solutions. Against this we must set the facts that the deposits from the alkaline solutions are smoother than those from the acid solutions, that the current efficiency is apparently higher, and that the voltage across the terminals is very low. This last is because zinc will precipitate readily from a strongly alkaline solution while it does not come out so readily from strongly acid solutions. The presence of resorcinol seems to have very little effect one way or the other. A few runs on current efficiency were made. The solutions selected were C at 70°, G at 70°, I at 40" and a plating solution at 20". The plating solution had the composition: ZnS04.7H,0, 82.7 grams; (NH,),SO,, 22.6 grams; water, 500 cc. Stationary cathodes were used, consisting of thin copper plates carefully cleaned. The solutions were run in series
with a copper coulometer. With a current density of 1.5 amp/dm2, the deposits from solutions G and I soon became so bad as to be unweighable. The following results were obtained with solution C at 70" and with the plating solution at 20° in a run lasting sixty-four minutes. TABLE XI1 -- ~
Solution
C at
70'
~~~
i
~
~~~~
Curretit density
1.5 amp/dmZ
February, 1907.
1
-
~~
Deposit
Percent efficiency
1.6708
89.3
379
EZectrolytic Prec+itation of Zinc TABLE XI11 Solution
Volt
G at 20' G a t zoo
0.6
Current density
0.13' 1
3.2 amp/dm2 amp/dm2
20.0
Time deposit
~
1
~
Percent efficiency
40 min 0.3948 15 min 0.g30g
99.49 100.70
The zinc obtained with a current density of 3.2 amp/dm2 was most excellent in quality, there being only a slight roughening on the cathode a t the surface of the electrolyte. The zinc deposited at the higher current density appeared as rounded granules of the metal adhering slightly t o a very finely crystalline under deposit. These runs also show the extremely low voltage at which zinc is precipitated from its alkaline solutions. In the experiments previously described, the deposits were not saved as it was necessary to use the cathodes over and over again. A few subsequent runs were made for the special purpose of studying the quality of the deposit under the microscope. The cathodes were copper plates 5 x 15 cm, immersed to a depth of I O cm in the solutions. These plates received a thin coating of zinc from a cyanide plating solution so that there might be no evolution of hydrogen due to local action between the zinc and the copper. This plating solution was made up as follows:. $ Dissolve I gram neutral ZnS0,.7H20 t o 20-30 cc water. Add enough potassium cyanide solution to redissolve the precipitated zinc cyanide and dilute to 150 cc solution per gram of hydrated zinc sulphate originally taken. Electrolyze at any temperature with a current density of 5.0 amp/dm '. To test the effect of varying conditions, the following runs were made : Effect of temperature Solution A , I O amp/dmZ,at 20°, 40°, 70'. I
Solution A,
I
amp/dm2, a t 40°, 70'.
Effect of current density Solution A,
1.0 amp/dm2 and I O
amp/dm2, a t 70'.
Solution A , 1.0amp/dmz and I O amp/dm2, a t 40'. Solution A 2 0 cc 40 percent formaldehyde, 1.0 amp/dm2 and IO amp/dni2at 40'.
+
Effect of concentration Solutions A and D, 1.0 anip/dm', at 40'. Solutions A and D, IO atnp/dmZ,a t 40'. Solutions G and K , 3.0 ainp/dm2, a t 40'. Solutions G 2 0 cc 40 percent formaldehyde and K percent fornialdehyde, 3.0 anip/dm', a t 40'.
+
+
20
cc 40
Effect of reducing agent* Solutions A and A at 40'. Solutiolis A and A at 40'. Solutions G and G at 40'. Solutions Kand K a t 40'.
,
+ 20cc40 percent formaldehyde, anip/dni*, + zocc 4opercent formaldehyde, amp/dni', + 20cc 40 percent fornialdehyde, 3.0 amp/dm2, + 20cc 40 percent formaldehyde, 3.0 amp/dmL, 1.0 IO
The solutions were all stirred thoroughly to prevent the collection of bubbles on the surface of the cathode, the acid solutions by means of a swiftly revolving test-tube brush and the alkaline ones with a revolving zinc plate. Both methods were effective. One anode and one cathode were used and the solutions were contained in, 500 cc beakers. Zinc anodes and the prepared copper (practically zinc) cathodes were always used. From these runs it could in general be said that an increase in concentration or in temperature causes an increase in crystal size, while an increase in current density has the opposite effect and decreases the size of the crystals. Formaldehyde makes the deposit nearly amorphous. From sodium zincate solutions we get a deposit which is very finely crystalline. The general results of this paper may be summed up: ~I Formaldehyde was used instead of resorcinol in these rims because there was less likeliliood of any formation of a coniplex salt and because the decomposition products are cleaner.
I . Good deposits of zinc may be obtained from acid or alkaline solutions even at such high current densities as 60 amp/dm2. 2 . The rate of rotation of the cathode has an enormous effect on the upper limit of the current density. 3 . Since zinc will precipitate readily from strongly alkaline solutions, the resistance of these solutions and consecluently the voltage across the terminals can be made very low for any given current density. 4. A high current efficiency may be obtained in alkaline solutions. 5. More finely crystalline deposits are obtained from alkaline than from acid solutions. 6. Increasing temperature or concentration increases coarseness of crystals. 7. Presence of formaldehyde decreases the size of the crystals. 8. Increasing the current density decreases the size of the crystals. This investigation was suggested by Professor Bancroft and carried out under his supervision.
Coriiell Zhiiversiiy
\
