ELECTROCHEMISTRY O F BATHS O F FUSED ALUMISUM HALIDES.
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T H E ELECTROCHEMISTRY OF BATHS OF FUSED ALUPIIIKL7P\I HALIDES. I1
THEDEPOSITIOX POTENTIALS
O F CHROMIUM, ?vlOLYBDEKUhl, .4SD TUXGSTEX”
E. E. MARSHALL
AND
*
L. F. YKTEMA
Department of Chemistry, St. Louis University, St. Louis, Missouri Received August 18, 1041
The determination of the normal electrode potentials of molybdenum and tungsten is complicated by the tendency of these metals to become “pa~sive.”~ A number of workers have found that the pretreatment of a molybdenum electrode determines its potential. Muthmann and Fraunberger (3) showed that molybdenum, treated with concentrated chromic acid, gave an Ex value of +0.66 volt, while another piece of the same composition, activated by cathodic polarization in potassium hydroxide solution, gave an Ex valve of -0.74 volt. Russell and Rowell (6) placed molybdenum near mercury in the electromotive series and reported that in the active form it is about as active as active cobalt or nickel. Tungsten exhibits similar properties. Muthmann and Fraunberger (4) reported that the potential of passive tungsten is +0.88 volt and that of active tungsten -0.70 volt. Its position in the electromotive series is between bismuth and mercury, according to Russell and Rowell (5). It should be of interest therefore to determine deposition potentials of these metals in an alkali chloride-aluminum chloride bath and compare the potentials with those of elements previously studied ( 7 ) . Chromium, which also exhibits passivity, will be included in this report. , APPARATUS AND PROCEDURE
The apparatus and experimental procedures have been described previously
(7). The bath consisted of the following: aluminum chloride, 58.66 g., 66 mole per cent; sodium chloride, 7.80 g., 20 mole per cent; potassium chloride, 6.96 g., 14 mole per cent.
Reagents The sodium chloride and potassium chloride used in the bath were pulverized and then heated to 400°C. for several hours to drive off occluded water. The nickel and zinc chlorides mere dehydrated in the usual manner by heating in a stream of dry hydrogen chloride. The aluminum chloride and the chlorides 1 Presented a t the lOlst Meeting of the American Chemical Society, which was held in S t . Louis, Missouri, April, 1941. 2 This work constitutes a portion of a dissertation submitted by E. E. Marshall to the Faculty of the Graduate School of the University of Missouri in partial fulfillment of the requirements for the degree of Doctor of Philosophy. A discussion of this topic is to be found in Gmelin’s Handbuch der anorganischen Chemie, 8th edition, No. 53,pp. 67-68, and No. 54, p. 85.
354
E. E. MARSHALL AND L. F. YNTEMA
ment,ioned above were of “analytical reagent” quality. The chromous chloride was an anhydrous C.P. preparation. Molybdic anhydride was prepared by roasting ammonium molybdate, (NH&Mo~Oaa*4H20,of “analytical reagent” grade. Potassium hexachloromolybdate (111), &MoCla, was prepared by the method of Bucknall, Carter, and Wardlaw (1). Tungstic anhydride was prepared from tungstic acid, C.P. grade. It was twice dissolved in ammonium hydroxide, filtered, and precipitated with hydrochloric acid. The tungstic acid was converted to the anhydride by ignition. The potassium nonachloroditungstate (111), &WaClp, was prepared by the electrolytic reduction procedure of Collenberg and Backer (2).
Experimental procedure Since it was found that both molybdic anhydride and tungstic anhydride are rather insoluble in the alkali chloride-aluminum chloride bath a t 156”C., the temperature at which the previous work was done, naphthalene (boiling point, 218OC.) was used in the vapor thermostat. It was therefore considered necessary to determine the deposition potentials of typical elements a t this higher temperature, in order to establish whether there might be a marked change in the values. The data found for nickel, zinc, and aluminum are given in table 1. I n the case of these three metals the temperature effect is negligible, if existent. It was therefore assumed that the elements of the chromium group might be placed in their positions in the series, using data obtained at 218%. instead of 156OC. Chromium: The first experiments were performed with chromic chloride. It was found to be insoluble in the bath. The data obtained were characteristic of those in which no “foreign” metal was added. No chromium metal was deposited on the cathode. If potassium dichromate was used, a small amount, two or three drops, of chromyl chloride distilled over, chlorine was evolved, and chromic chloride precipitated. It was found that chromous chloride is soluble in the bath and that chromium metal is discharged by electrolysis. A bath was prepared containing 0.82 g. of chromous chloride (Le., a 1 mole per cent solution). Gray, fairly smooth deposits of chromium were formed a t a deposition potential of +0.16 volt and a decomposition potential of 1.95 volts. Figures 1 and 2 show typical graphs of I versus E relationships for deposition potentials as measured against the aluminum reference electrode and for decomposition potentials as measured from anode to cathode. Data are collected in table 2. MoZybdenum: The molybdenum deposition potentials were determined both with baths containing hexavalent molybdenum and with baths containing trivalent molybdenum. The former type contained 0.96 g. of molybdic anhydride. The deposition potential of spongy metal was $0.30 volt, and the decomposition potential was 1.86 volts. The large potential difference between reference electrode and cathode shown on the graph in figure 1,-about $1.7 volts at less than 1 ampere per dm?,-is probably the measurement of a reduction potential. Baths of the second type were prepared by adding 2.84 g. of potassium hexachloromolybdate to a melt containing l .50 g. less potassium chloride
TABLE 1 Deposition potentials and decomposition potentials 1 mole per cent solutions; temperature, 218°C. 11
I
DEPOSITION po?wTu
Aluminum..
.. .. . . . . . . . . . . . . .
DECOYPOSITTON P O T E ” .
-0.04 -0.04 -0.03
2.10 2.09 2.08 2.08
2.08 Mean.. . . . . . . . . . . . . . . . , . . . . Zinc. . . . . . . , . .
-
2.08 2.08 (2.09a t 156°C.)
-0.03 (-0.02 a t 156°C.)
. . . . . . .. .. . . . . .
1.78 1.76 1.80 1.82 1.84 1.82 1.84
$0.24 0.22 0.26
0.26 0.25
Mean. . . . . . . . . . . . . . , , . . . . . ,
-
-
+0.25 ($0.24 a t 156°C.)
1.81 (1.82a t 156OC.)
+0.75
1.25 1.20 1.20 1.23 1.23 1.23
Nickel. . . . . . . .. . . . . . . . . . . . . . . .
0.77
0.83 0.82
Mean. . . . . . . .
... . .
,
i
. . . .. . . .
-
-
$0.79 (+0.80 a t 156’C.)
(1.25 a t lSe’C.)
1.22
mo a mo 00 @om o oom mo
o2
2 a I5
05t
0
0
0
-
0.
0oo MOOS 0 0
0
8% 8
0
-
o omm
a
0 K3MoC&
VOL
.
: CrC$ I
rs
FIG.1. Deposition potentials of chromium and molybdenum. I uerswI E (measured from aluminum reference electrode t o cathode). Temperature, 218OC. 355
356
E. E. MARSHALL AND L. F. YNTEMA .iO
Xrode -Colhode
G
-0s'
-oa
-,z
1 -20
-/6
-ZJ -22 VOLTS
-32
-36
-40
-44
-48
FIG.2. Decomposition potentials of chromium and molybdenum compounds. I versus
E (measured from anode to cathode). Temperature, 218°C. TABLE 2 Deposition potentials and decomposition potentials at 218°C.
-_
.-
11 DECO16pOSITION POTENIIALS
1
Chromium:
CrC12, 1 mole per cent . . . . . , . Mean
...
.
-
i
$0.16
1.95
1
+O .28 0.32 0.30 0.34 0.30 0.32
1.87 1.84 1.86 1.88 1.87 1.78
Moos, 1 mole per cent. . . . , . . . . . KsMoCls, 1 mole per cent . . . . . . .
1.94 1.99 1.93
__
. .. , . . . .... , . . .. ..
Molybdenum :
+0.16 0.16 0.16
ii
I Mean.. . . . . . . . . . . . . . . . . . . . . . . , . .I
-
-
$0. 31
1.87
+O .43 0.40
1.64 1.68
0.38 0.36 0.40
1.73 1.68
I
Tungsten:
WOJ, 2 mole per c e n t . . . . . . . . . , . . . ICJW2Clo,2 mole per cent.. . . . . . Mean.. . . . , . . . . . , . , , , , , , , , ,
. . . .. .
II
il
11I ,,
~
-
-
$0.39
1.68
than the usual amount. Black spongy deposits were formed at f0.31 volt. The decomposition potential was 1.85 volts (see figures 1 and 2 and table 2).
ELECTROCHEMISTRY OF BATES OF FUSED ALUMINUM HALIDES.
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The formation of a red condensate on the walls of the electrolysis cell indicates the loss of a small amount of molybdenum from the bath by the formation of & volatile compound.
. I e
a
1 +?4
+I2
109
+I0
+OK
+04
+02
I 0
I
-02
-04
VOL T5
FIG.3. Deposition potential of tungsten. I Versus E (measured from aluminum reference electrode to cathode). Temperature, 218°C.
1
os
0
0
-08
~o m
-12
, -16
-20
-24 JOL
-28
rs
,
,
,
-32
-36
-40
I
-44
FIG.4. Decomposition potentials of tungsten compounds. I v e r s u ~E (measured from anode t o cathode). Temperature, 218OC.
Tungsten: When 1 mole per cent solutions of tungstic anhydride were used, they soon became depleted because of the formation of a volatile tungsten compound that collected &s red needle-like crystals in the cooler part of the electrolytic cell. Accordingly, measurements were made on 2 mole per cent solutions
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E. E. MARSHALL AND L. F. YNTEMA
prepared by adding 3.10 g. of tungstic anhydride to the bath. The deposition potential was found to be $0.40 volt and the decomposition potential 1.66 volts. In another series of experiments baths containing 2 mole per cent of the trivalent tungsten compound, potassium nonachloroditungstate, were used. 5.36 g. of this salt was added to a bath containing 1.50 g. less potassium chloride than the usual amount. The deposition potential was found to be +0.38 volt and the decomposition potential 1.70 volts. There appears to be an electrolytic reduction characterized by a potential of about 1.3 volts (see figures 3 and 4 and table 2). DISCUSSION
The fact that, in the experiments on both molybdenum and tungsten, similar values are obtained for deposition potentials from solutions containing the metals in the trivalent and in the hexavalent states indicates the probability that there is a preliminary reduction of the higher valence form and that the last step in the reaction, the reduction to free metal, is the same in each case. These data place the metals of the chromium group among those characterized by a high discharge potential. The order found, as compared to aluminum as reference, is as follows: iron, $0.47; tungsten, +0.39; cadmium, $0.36; molybdenum, $0.31; zinc, +0.24; chromium, +0.16. The value for hydrogen, referred to aluminum as standard, is +1.05 volts (7). If the series, as given, is converted to the hydrogen standard, it becomes: iron, -0.58; tungsten, -0.66; cadmium, -0.69; molybdenum, -0.74; zinc, -0.81; chromium, -0.89. It is interesting, if not significant, that these values for molybdenum and tungsten are close to the corresponding EX values that Muthmann and Fraunberger (3, 4) found for the active elements in an aqueous system. SUMMARY
1. Deposition potentials of nickel, zinc, and aluminum from alkali chloridealuminum chloride baths are practically the same a t 156OC. and 218°C. 2. If the deposition potentiala of the metals of the chromium group are compared with those previously reported, the following series, of increasing activity, is obtained: iron, tungsten, cadmium, molybdenum, zinc, chromium. The authors wish to acknowledge the assistance of Brother Anthony Gerhardt,
S.M., who prepared the trivalent tungsten compound. REFERENCES (I) BUCKNALL, CARTER, AND WARDLAW: J. Chem. Soc. 1927,513. (2) COLLENBERG AND BACKER: Z. Elektrochem. SO, 233 (1924). (3) MUTAMANN AND FRAUNBERGER: Ber. Bayr. Akad. 84,201 (1904). (4) MUTHMANN AND FRAUNBERGER: Ber. Bayr. Akad. 34,204 (1904).
(5) RUSSELL AND ROWELL: J. Chem. Soc. 1926, 1881. (6) RUSSELL AND ROWELL: J. Chem. SOC.1926, 1889. (7) VERDIECK AND YNTEMA: J. Phys. Chem. 46,344 (1942).
