AN APPARATUS FOR THE ELECTROLYTIC DETERMINATION O F METALS, USING A ROTATING CATHODE BY E. S. SHEPHERD
Gooch and Medway' have shown that by the use of a rotating cathode the time necessary for the quantitative precipitation of metals can be very much shortened. It has occurred to the writer that a much more convenient form of apparatus would be the one herein described. A sketch of the one now it1 use in this laboratory is here given.
I Instead of a platinum crucible, I have used the ordinary disk anode, shortening the stem to about 6 cm, and fastened it by a screw connector directly to the shah of the armature. The connection to the battery is made through the iron franie of the motor. The motor used is a toy motor, a very poor affair in its way, but sufficient for the purpose, and cheap enough to permit each cathode having its own motor. The use of belts as suggested by Gooch is very unsatisfactory, owing to the slipping, etc. It was found best to arrange a rheostat for each motor, Am. Jour. Sci. (4) 15,320 (1903).
Electrolytic Delermination of Metah
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since no two motors run on the same current, and it is also desirable to slacken the speed when removing the beaker andwashing the cathode. This rheostat consisted of one zero, two one-ohm and two two-ohm coils connecting through the switch (S), the other motor connection beiiig through the wire leading to M'. And a 110-volt circuit lamp may of course replace this form of rheostat. The cathode connection was made through four &volt 6C. P. lamps in multiple (I,) for storage-battery work, or these are replaced by the ordinary 110-volt lamp for dynamo circuit. The current was then regulated by loosening or tightening the lamps in their sockets. No difficulty was experienced in getting a good connection through the motor frame to the cathode. The beaker containing the electrolyte was supported by the wood support (C) on the brass posts (D). T h e screw for tightening the collar of (C) should be of such size as to allow manipulating this support with one hand, leaving the other free to manage the wash bottle, etc. The anode was a stiff platinum wire held in the usual electrode holder, connection being made through the brass posts (D). The distance from the motors to the base board is about 30 cm, and between motors I O cm. T h e disk electrode was used because we happened to have that form in stock. A more desirable form would consist of a disk of platinum gauze, thus allowing a stronger current to be used and shortening the time required. T h e brass connector which connects the cathode to the shaft is protected from corrosion by a rubber tube. A finger stall does very well. A number of determinations were made to see if the apparatus could not advantageously replace volumetric methods in commercial laboratories. A rather good chalcopyrite was treated with I O cc nitric acid, and after the reaction had subsided, with 7 cc strong sulphuric acid. It was then evaporated until white fumes of SO3 appeared ; taken up with water ; filtered, I gram NaCl added, and electrolyzed. T h e salt aided materially in preventing the deposit burning toward the close of the electrolysis. T h e time required was from 25 to 40 minutes, using 1.5 to 2
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Electrolytic Determination of Metals
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Chalcopyrite
Blende Time Minutes
Percent Cu
Percent Zn ____
~
a 6 C
22.57 22.75 22.88
d e
22.89 22.87
Time Minutes
25
30 30 35
35
.
A
67-25
30
B
67.84 67.80
40 40
C
I
Jour. Am. Chem. SOC. 24, I073 (1902).
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