ON T H E DEPOSITION O F ZINC F R O X ZINC CHLOR I D E DISSOLVED I N A C E T O N E BY HARRISON E A S T M A N P A T T E N
Introduction T h e electrolysis of zinc chloride in acetone solution possesses interest from several standpoints. First as to the physical state and purity of the zinc deposited ; second, concerning the current density and voltage requisite to deposit the zinc ; and finally as to the use of this solution for chlorination at the anode, especially when the anode itself is corroded.
Experimental Fused zinc chloride was dissolved iii acetonel to saturation and subjected to electrolysis between polished platinum electrodes 0.7 cm apart and 3 square centiiiieters in area.z Table I gives the current and current density corresponding to various pressures3 in Plate I. Current density is plotted as a function of the voltage, using amperes per square centimeter as abscissas and volts as ordinates. T h e points lie on a very regular curve ; especially considering the enlarged scale to which they are plotted. T h e residual current shows a current E. M. F. curve which approaches closely a straight line. T h a t it is not the true CR line of the solution is seen by comparison with the CR line of the upper part of the curve after passing the bend. This higher CR line gives a specific resistance of 33.42 ohins for the solution corresponding to a specific conductivity of 2-99 X IO-^ at 2 0 . 5 ~ ~ the temperature of the experiment. The acetone was dried over fused calcium chloride and distilled. A crit and description of the cell used may be found on page 318, Vol. 111, Trans. Am. Electrochemical Soc., 1903. For the method used see a former paper, I ‘ Decomposition Curves of Lithium Chloride and the Electrodeposition of Lithium,” Jour. Phys. Chem., March, 1904.
Harrison Eastma
484
Patten
TABLEI. Zinc Chloride in Acetone Platinum Electrodes Temperature 20.5' C Volts
I
0.30
o.ooo(?)
0.40
0.0001 0 0008 0.001 0.001
0.50 0.60 0~70 0.80 0. go I .oo 1.10 1.20
1.30 1.40 1.50 I .60 1.70 1.80
0.0014 0.0014 0.0017 0.0019 0.0019 0.0020 0.0022
2.30 2.40 2.50
0.0025 0.0026 0.0029 0.0029 0.0032 0.0035 0.0039 0.0046 0.006 0.007 j 0.0085
2.70
0.010
2.70 2.80 2.90 3.00 3.00 3.50 4.00
0.0114 0.013 0.0145 0.0159
1.90 2.00 2. I O 2.20
O.OIS2
5-50 6.00 7.00 8.00
0.0264 0.0337 0.0419 0.051 0.060 0.068 0.086 0.103
9.00
0. I 2 0
10.00
0.140 0. I80
4-50 5.00
12.00
Amperes Per sq. cm.
Amperes
I 0.00003 0.00026 0.00033 0.00033 0.00046 0.00046 0.00057 0.00063 0.00063 0.00067 0.00074 0.00084 0.00087 0.00097 0.00097 0.0010 0.001 I 0.0012
0.0015 0.002
0.0025
0.0028 0.0033 0.0038 0.0040 0.0048 0.0053 0.0060 0 0088 0.01 I 2
0.0139 0.017 0.020
0.0226 0.028 0.034 0.040 0.046 0.060
+
Zinc Chloride Dissolved in Acetone
485
By producing the CR line to the axis of volts a decomposition point of 1.90volts is obtained. Wiedeburg’s method of drawing the straight portions of the curve out to their intersection gives 2.14 volts. And the E. M. F. corresponding to the current density a t the bend of the curve is near 2 volts, the value given in the abstract of this article.’
,
3 I
2
I
0
001
002
003
004
005
Plate I. ZnC1, in acetone Platinum electrodes t = 20.5~C.
During the first stage” of electrolysis a discharge potential of 1.27 volts was obtained from a current density of 0.0006 ampere per square centimeter. I n this experiment no gas bubbles appeared a t either electrode. However in later experiments gas came off upon the cathode, though by no means copiously. Chloroform, chloracetone or other alkyl chlorides were formed a t the anode,’ 4 5 and a compact smooth coating of zinc was deposited upon the cathode. Zinc may be deposited steadily with a current density as low as 2.00 amperes per square foot, though the current efficiency is low. With a current density of 40 amperes per square foot, the zinc trees out from the edges of the Electrochemical Industry, August, 1903. primary electrolysis of the solute given in a former paper, “ Deconipositioii Curves of Lithium Chloride,” etc., 1. c. E. Schering. D. R. P., 29, 771, May 6, 1884. . Howe Abbott. Jour. Phys. Chem. 7, 84 (1903). J. E. Teeple. Jour. Am. Chem. SOC.26, 170 (1904).
* See the treatment of
Harrison Eastman Patten
486
cathode in long streamers. Use of a rotating cathode would tend to obviate this difficulty. Later experiments have shown that the zinc becomes spongy when the depth of coating exceeds about one millimeter, and before this pits appear. Analysis gave between 0.9 and 1.0percent of carbon in the deposit. On standing over night a zinc deposit redissolved in the solution from which it was obtained. From the above facts it appears that the deposition of zinc from acetone solution is not well adapted to procure chemically pure zinc or to give a thick coat for chemical purposes, and the preparation of zinc free from carbon is still before us. With the best so-called c. p. zinc electrodes as anode and cathode a current E. M. F. curve was determined in this same acetone solution of zinc chloride at 20' C, as shown in Table I1 and Plate 2. TABLE11. Zinc Chloride in Acetone Zinc Electrodes Temperature 20' C Volts
Amperes
0.037 0.043 0.048 0.065 0.074
0 .ooo I 97 0.000257 0.000352 0.000435 0.000526
0.091
~.000609
0.099 0.109 0.125 0.138 0.154 0.178
0.000701
0.212
0.00139 0.00174 0.00306 0.00350
0.260 0.443 0.502
Amperes Per sq. cm.
0.0000558 0.0000754 0.000I 03 0.000128 0.000154 0.000I 78 0.000203 0.000232 0.000256 0.000282 0.000307 0.000357 o.000407
0.000791 0.000875 0.000962 0.00 I 05 0.00122
'
0.000510 0 .oooig8
0.000103
For low voltage there is a slight indication of polarization,
Zilzc Chloride Dissolved in Acetone
487
but comparison with the points for higher voltages shows that these deviations are due to experimental error. T h e zinc anode was evenly corroded. Using a fresh saturated solution of zinc chloride in acetone a weighed zinc anode was corroded with an average current density of 0.0124 amperes per square centimeter until 4.6451 grams of zinc had dissolved away. T h e solution was then filtered from the spongy zinc and distilled, using a tower. T h e acetone was recovered intact, boiling at 54.7" to 55.0" C under a pressure of 738.8 nun.
Plate 11. ZnC1, in acetone Zinc electrodes
t = 200 c
I
This experiment is of general bearing as quantitative evidence that a solvent subject to attack by a n acid radicle may be used for continuous refinement where the anode is readily attacked. This confirms the lack of polarization shown in Plate All of the liberated chlorine is fixed by the zinc. A num2. ber of other metals similarly electrolyzed in acetone solution of their chlorides show no polarization. This work was done in the laboratories of physical chemistry and of applied electrochemistry, and the author takes pleasure in expressing his appreciation of the courtesy extended to him by both. University of Wisconsin, ] m e 22, 1904.
