ANALYTICAL CHEMISTRY
178 (12) Sartori, G., Gart. chim. itaZ., 64, 3 (1934), through Kolthoff, I. M., and Lingane, J. J., “Polarography,” p. 270, New York, Interscience Publishers, 1941. (13) Semerano, G., and Capitano, V., Mikrochemie ver. Mikrochim. Acta, 30, 71 (1942). (14) Spalenka, hl., Z . anal. Chem., 128, 42 (1947). (16) Toropova, V. F., J . Applied Chem. (U.S.S.R.),18, 177 (1945).
(16) Tyler, W. P., and Brown, W. E., 15,520 (1943).
IND. ENG.CHEM.,ANAL.ED.,
RECEIVED May 14, 1948. Taken from a thesis submitted by C. A. Reynolds to the Department of Chemistry and the Committee on Graduate Study of Stanford University in partial fulfillment of the requirement for the degree of doctor of philosophy, July 1947
Apparatus for Electrolysis at Controlled Potential C. J. PENTHER
AND D.
J. POMPEO, Shell Development Company, Emeryuille, Calif.
A unitized controlled potential apparatus is described which contains a source of accuraie reference potential with n range of 0 to 4.41 volts, a sensitive unbalance detector and electrode potential correcting unit, and a source of line-supplied direct current which is independent of line voltage variations. Using a mercury cathode, and with initial current as much as 100 times final current, the seositivitp to changes in electrode potential is about 1 mv. and the instantaneous deviation from the control point not more than 1 1 0 mv.
T
H E apparatus described was designed t o be fully automatic and as nearly foolproof as possible, for use a8 a routine tool in an analytical laboratory. I t can be used for the preparation of samples in the systematic polarographic analysis of inorganic or organic mixtures, as xell as in organic synthesis ( d ) , and has been proposed as a means of direct analysis of alkali metals by separation using a rotating silver anode and mercury cathode by the method of Smith ( 5 ) . Earlier apparatus designed to accomplish work of this nature has been described by Hickling ( d ) , Caldwell, Parker, and Diehl (I), and more recently by Lingane (3). The requirements of a routine laboratory apparatus were met by combining the best features of the above-mentioned equipment-namely, complete line operation (except for the battery in the reference potential potentiometer), wide range of voltage and current, high sensitivity, and freedom from need of operator attention and maintenance.
Mounted directly above the potentiometer are two meters with corresponding range switches used to measure electrode potential and current. The one on the left measures current on four ranges of 0.1, 0.5, 1.0, and 5.0 amperes. The one on the right (ranges changed after photographing) measures cathode-anode potential in three ranges of 3.0, 15, and 30 volts. The upper large knob to the right of the potentiometer is the Helipot voltage control. A friction clutch between the Brown motor and Helipot shaft permits setting the electrode voltage manually when starting an electrolysis. The motor is rated a t 27.5 r.p.m. and a 5 to 1 gear reduction is used to drive the potentiometer. As the Helipot has ten turns, the effective maximum 27 5 speed of the slider is -= 0.55 r.p.m. 5 x 10 The lower knob is the amplifier gain control, which enables the operator to adjust the amplification to the optimum value of maximum sensitivity without hunting. The potentiometer gain control mounted in the Electronik amplifier is disconnected and shielded leads are extended to the panel control. The change in cell voltage is indicated by two pilot lamps which are mounted above the upper knob and are identified as increasing
METHOD
A block diagram of the apparatus is shown in Figure 1. A
f7?
b e d s & Northrup potentiometer, Type 7665-5 with a special range of 4.44 volts, is used as a reference potential. The difference voltage between this reference voltage and that developed between the mercury or platinum cathode and a saturated c a b me1 half-cell is detected by a Brown Electronik amplifier (Brown Instrument Company, 4482 Wayne Ave., Philadelphia 44,Pa.). The output of this amplifier controls the direction and speed, corresponding to the sign and magnitude of input unbalance, of a two-phase motor which drives a Helipot potentiometer (Helipot Corporation, 1011 Mission St., South Pasadena, Calif.). A voltage picked off the sliding arm of this potentiometer varies the grid voltage of a pair of vacuum-type rectifier tubes, the plate currents of which serve to vary the output of a saturable reactor. The reactor maintains the primary voltage of a step-down transformer supplying the low voltage rectifier a t a value that provides a constant cathode potential independent of electrode current and line voltage.
SENSITIVITY
APPARATUS
OYETER
The entire a paratus is housed in a steel case occupying a table space 50 cm. 8 0 inches) long by 45 cm. (18 inches) deep. The rear 7.5 inches are occupied by the alternating current power supply and covered with a cane-pattern sheet metal cover which provides adequate ventilation as well as protection. The front section of the cabinet is made of 18-gag6 steel sheet and supports the Duralumin control panel on a slope of 30” running from a height of 6 inches a t the front edge to 12 inches a t the rear. The Brown amplifier is secured as a unit to the inside rear wall a t the bottom. As shown in Figure 2, a photograph of the complete apparatus, the Leeds & Northrup potentiometer has been removed from its case and mounted on the panel.
I
I
Figure 1. Block Diagram of Controlled Potential Electrolysis Unit
V O L U M E 21, NO.
1, J A N U A R Y 1 9 4 9
179 action furnished by the clicking of the switch blade on the gear teeth, although in this case there is no indication of direction. Amplifier and power supply fuses and the power switch and pilot light are located in the UDDW rieht corner a i the omel. Terminals for the electrode;, bin&ns posts for t6e &kine
power supply can bepurcbased as a unit,-contro'lied d.c. power supply, Type 5253 (Eleetro Engineering Works, 6021 College Ave.. Oakland. Calif.). The 75-watt lam" RCPORI t h p ~. n ~ i r n 9 rnf ~ . . step-down transformer E-5255 provides an auxiliary load on the reactor, so as to retain relatively h o a r control when the electrode ~~
Figure 2.
Controlled Potential Electrolysis U n i t
and decreasing. They are actuated by a leaf-spring, single- ole, doublethrow switoh which has the tip of its center arm light& in contact with the teeth of a small gear an the motor shaft. When the motor turns slowly, an indication of a slight unbalance between the reference and cell potenti&, one or the other of the Pilot lamns blinks slowlv on and off. Ii the unbahnee is ereater k d correction is more &pid, the frequency of flashing increases. An added operator convenience is the audible signal of corrective ~~
~~
~
0
~~~~
~
~.~
The "ring" shunt shown for the ammeter has the advantage, over individual shunts, of not including the switch contact resistance in the measuring circuit. The meter is also less susceptible to overload damage, as accidental opening of the switch contact opens the load circuit rather than the shunt circuit. Resistance values indicated are calculated and must be adjusted at amembly to tho meter used. The terminal boards shown are provided to permit individually removing the panel, amplifier, and power su ply from the assembly. In addition to the seven leads of the Erectronik amplifier BS furnished by the manufacturer, two additional leads and shield are brought out of the case for the gain controllocated on themain panel. A small transformer is necessary to isolate the voltage applied to the leaf-spring switch and neon pilot lamps from ground and the power circuit. The 1003-mid. condenser across the reference electrode and cathode is part of an antihunt filter. When a calomel reference electrode is used, its internal resistance makes up the resistive element. When controlling the electrode potential directly, a 1000-ohm resistor is connected between the reference electrode pin jaok and the anode binding post. OPERATION
Operation is vcry simple. Having once started an electrolysis, the operator may leave the apparatus unattended until the electrolysis is completed. The following steps are followed: Insert power plug in service outlet, standardize o. tentionieter, connect working and reference electro$es to their respective terminals, turn on power, manually adjust electrode potential to starting value by panel knob and voltmeter, and set potentiometer to eontrol potential and lock in galvanometer key.
Figure 3. Wiring Diagram of Controlled Potential Electrolysis Unit
The circuit will immedistely start correcting, and if the sensitivity is not too high, will quiek?y settle down to where the lights w e blinking dowly, perhaps first indicating increasing voltage and then decreasing, but finally indicating decreasing voltage only as the input potential decreases with decreasing load. If there is "hunting," the sensitivity should be decreased. Couversely, closest control is obtained at high sensitivity and the gain control should be advanced as far &s possible without hunting. The galvmometer on the potentiometer should he clamped, except when stsndardizing, because the e.m.f. developed a c r o s ~its coil as i t swings in the magnetic field is such as to increase the tendency to hunt. Sensitivity may be determined by noting the numher of millivolts change in Potentiometer setting required to actuate the motor as indicated by the pilot lights. This can be done only when the eleetrolysis has proceeded to 8 stage where there is little change in electrode current.
180
ANALYTICAL CHEMISTRY acid in approximately 75-ml. volume. The cathode area was approximately 10 sq. cm. and the electrolytemercury interface was stirred b small propeller-type glass stirrer proximately 14 X 7 mm.) which was half in the mercury and half in the electrolyte. The stirrer revolved a t approximately 1100 r.p.m. The anode consisted of four turns of Xo. 16 (B.&S.) platinum wire wrapped around the stirrer about 2 em. from the surface of the mercury. The copper was first deposited a t a potential of 0.300 volt vs. saturated calomel electrode and, although not shown on this curve, the cadmium was subsequently deposited a t a potential of 0.800 volt us. saturated calomel electrode. The voltage varied from a peak of 9.5 volts 5 minutes after starting to slightly less than 2 volts in 50 minutes. In the same time the current reached a peak of 1.1 amperes and leveled off a t 0 011 ampere, a ratio of 100 to 1. During this time, control was maintained within j=!J.OlO volt. Near the end of the runs, the control potential was shifted 5 millivolts to 0.305 volt. I t can be seen from the recorder trace that a new control point was immediately established and maintained.
&E
TIME IN MINUTES
Control Characteristics of Controlled Potential Apparatus
Figure 4.
Deposition of copper on a platinum cathode is shown in Figure 5.
5
$ I
6
----___
1.7
Depsltlon of Copper on Plirtimrn
d U
1.b Cathode: 5 x 2.5 cm. 45 mah p t . A ~ d e r 5 I 1.25 cm. 45 msh p t .
0
10
Figure 5.
PO
30
40 50 TIME IN MINUTES
50
70
The electrolyte contained approximately 100 mg. of copper and cadmium and 2 ml. of sulfuric acid in approximately 150-ml. volume. The electrodes 1% ere 45-mesh platinum gauze; the anode dimensions were 5 x 1.3 em. and the cathode dimensions 5 x 2.5 em. The anode was rotated at approximately 600 r.p.m. The copper was first deposited a t a potential of 0.100 volt vs. the saturated calomel electrode. After completion of the copper deposition, 5 grams of sodium hydroxide were added to the electrolyte and then enough acetic acid just to dissolve the precipitate. In this experiment also the cadmium was later deposited a t a potential of 1.OOO volt is. the saturated calomel electrode The temperature of the electrolyte during the cadmium deposition was held a t 65’ C. There is markedly less fluctuation using a platinum cathode and the maximum electrode deviation from the control point is 2 mv.
80
Control Characteristics of Controlled Potential Apparatus ACKNOWLEDGMENT EXPERIMENTAL RESULTS
Tests were made using both a mercury pool and platinum gauze :as a cathode. An auxiliary potentiometer, set to the control potential, and a Leeds & Northrup Speedomax Model G recorder were connected in series across the cathode and saturated calomel half-cell so as to record the actual deviation from the control point during an electrolysis. Figure 4 is a plot of cell voltage and current and deviation of cathode potential from the control value against time for the deposition of copper into a mercury cathode. The electrolyte contained approximately 100 mg. each of copper and cadmium, 8.14 mg. of zinc, and less than 2 mg. of sulfuric
The authors are indebted to Louis Lykken and T. D. Parks for suggesting operating requirements and to V. H. Gunther for experimental test data* LITERATURE CITED
c. M.’., P a r k e r , c.8 and Dlehl, H., I S D . ExG. CHEM., ANAL.ED.,16, 147 (1944). Hickling, A., Trans. Faraday SOC.,38, 27 (1942). jyngane, J. J., lxD.EKG. CHEW., AXAL. ED.,17, 5 (1945). Lingane, J. J., Swain, C. G., a n d Fields, bl., J . Am. Chern. S O C , 65, 1348 (1943). S m i t h , E.F., “Electro-Analyses,” p. 292, Phlladelphla, Blackston’s Sons, 1911.
(1) Caldwell, (2) (3) (4) (5)
RECEIVED May 28,
1948.
