Methods for constant potential control

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METHODS FOR CONSTANT POTENTIAL CONTROL E. B. THOMAS and R. 1. NOOK John Carroll University, Cleveland, Ohio

SEVEEAL methods of maintaining a constant potential of cathodes with respect to a solution during reduction and deposition a t definite, predetermined potentials have been described.' Each method employs a mechanism actuated by small potential differences between the cell (consistingof the cathode on which deposition is progressing and a reference electrode) and some "bucking potential" from a fixed battery. The fixed battery may take the form of a student type potentiometer (0- to 2.20-volt nominal range) in the conven- operated . tional manner for potentials no greater than 2.20 volts, or if desired, this range may be extended to 4.40 volts bv the simule ewedient of driving twice as much current throuih the'potentiometer. his may be accomplished by standardizing the potentiometer against the Weston saturated standard cell with the potentiometer reading 0.5091 volt instead of 1.0183. fi is necessary that the battery furnishing current for the potentiometer be increased porportionately. After such standardization, of course, the r e d i g of the potentiometer must be doubled toobtain the true potential. The galvanometer used to indicate balance-in the usual arrangement should be "shorted out" of the circuit after calibration has been completed. Three different methods of cathode potential control have been used here, each having in common the controlled rectifier circuit of Figure 1. A five-ampere capacity Variac, its variable arm controlled by a geareddown, reversible motor, feeds the primary of the transformer TP. The output of this transformer is rectified by the full-wave-bridge selenium rectifier and filtered by the condenser and choke combination indicated. Theinductances used are the secondaries of 110:20 volt, 250 volt-ampere transformers which have d. c. resistances of 1.3 and 1.2 ohms, respectively. These showed impedances of 510 ohms each with 60-cycle a. c. Since the circuit employed is a full-wave bridge, the ripple frequency will he 120 cycles per second and the effective impedance of each will be about 1000 ohms a t low currents. Two of the methods employed used a galvanometer and photocell arrangement, the g a h m m e t e r requiring a current of 0.8 microampere to deflect the beam of light from its zero position to a point of incidence on the ( a ) C A L U ~ LC. L ,W., R. C. PARKER,AND H. DIEHL,Ind. Eng. Chern., Anal. Ed., 16, 147 (1944). ( b ) DIEKL, H., "Electrochemical Analysis with Graded Cathode Potential Control," G. Frederick Smith Chemical Co., Columbus, Ohio, 1948, 56 pp. (c) HICKLING, H., Trans. Faraday Sac., 38, 27 (1942). ( d ) J. J.. Ind. Eng. Chern., A d . Ed., 17, 332 (1945). ( e ) LINGANE, Anal. Chem., 21,178 (1949). PENTHER,C. J., AND D. J. POMPEO,

photosensitive surface of the cell. The resistance of the galvanometer coil was 300 ohms. A relay circuit similar to that shown in the R.C.A. brochure on phototubes2 was used, two such circuits being mounted on the same chassis in order that correction might be made for applied voltages which were too large or too small. A reversible (series) motor, geared down to 1 r. p. m. drove the variable arm of the Variac of Figure 1. Di-

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ficulty was experienced in the form of overcorrecting (hunting) with this setup, the solution to which was found to he reduction of the motor speed, so that the arm of the Variac turned no faster than r. p. m. In the second method, using galvanometer and photocells, the relays were replaced by a two-phase induction motora as shown in Figure 2. This circuit shows considerable promise for recording variable currents and/or voltages by mounting the photocells and recording pen on a carriage actuated by a two-phase reversing motor of the proper speed. In operation the miniature thyratrons (Figure 2) are biased to cutoff by the rectifier network. m e n a beam of light falls on P C I , the negative bias on the grid of tube A is reduced, allowing tube A to "fire," and sending a rectified pulse through winding LI of the motor. Since the d a t e of tuhe A (andof tube B for that matter) is feci by a. c., the grid of tube A regains control of the valve action of the tuhe a t the end of a half cvcln. I~~f PC I i.s. still ~" - . . ..... illn...minated a t the beciinnine of a new cvcle. " , the above ~

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"Phototuhes," Form PT-20 R1, p. 7. R d i o Corporation of America, Hasrisou, New Jersey, 1940.16 pp. The motors indicated in Figures 2, 3, 4, and operated a t 22 volts 60-cycle a. c., are War Surplus "Waste Gate Motors for Electronic Turbo Supercharger Regulator," 400-cycle, geareddown, induction motors, manufactured by Minneapolis-Honeywell Regulator Co., and are presently available m the surplus market at a cost of about three dollars each.

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process is repeated, resulting in another pulse through wind'mg LI. The phase relationship between the currents in L1and Lzdetermines the direction of rotation of the motor. In the case a t hand the current through L2 was 0.32 ampere, and that through LI during the period that one or the other of the thyratrons was firing was about 30 milliamperes. It was found practical to mount the control unit (galvanometer and lamp included) in a light-tight box and to prevent "overshooting" of the galvanometer with brass "arrests" which l i t the swing of the galvanometer coil and mirror to the extent that light will fall on one or the other of the photocells a t all times that the galvanometer is deflected from its zero position. The galvanometer is slightly under-damped and its sensitivity is controlled by an Ayrton shunt. It should be noted that one side of the 110-volt line is connected directly to the chassis in Figure 2 and in Figure 3. Unless the l i e cord plug is inserted in the 110-volt line socket in such manner that the "hot" side of the 110-volt line is connected to the lead containing the switch of these two figures, the chassis will be

"hot," and serious shock may be experienced by one inadvertently touching the chassis. The entire difficulty may be avoided by operating the circuits of Figures 2 and 3 through an isolation transformer. Suitable transformers are the Freed 118-505 and the Stancor P6160. However, should one wish to operate without these transformers, one may test for this "hot" condition of the chassis by connecting a 110-volt lamp between the chassis and some well-grounded point, such as a water line. If the lamp glows when the line cord is plugged into the 110-volt outlet, the chassis is "hot" and dangerous. Reversal of the line cord plug will eliminate this danger. We have found it convenient to mark all 110-volt outlets with a spot of red paint on the "hot" side, and to mark all power line bayonet plugs with which a definite polarity must be maintained in a similar manner. The control unit4of Figure 3 has given the mcst satisfactory performance of all those tested to date. The d. c. "error voltage" between the fixed cell (student type potentiometer) and the controlled cell (cathode of electrodeposition cell and reference electrode) is impressed between the vibrating reed of an auto radio

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vibrator and the center tap of a lme-to-grid, or a centertapped input, double button microphone transformer. The reed is vibrated a t 60 cycles per second and any "error voltage" is amplified by the input transformer and the three stages of resistance-coupled voltage amplification following. Originally four stages of voltage amplification were employed; elimination of one stage gave a more stable instrument of sufEcient sensitiveness for the problem a t hand. At full sensitivity (maximum gain) an unbalance between the two oppo&g voltages of less than one-half of one millivolt will cause rotation of the motor. The output pulse of the voltage amplifier is fed to the paralleled grids of the two 6SN7's, which act as phase discriminating power amplifiers since their plates are connected directly to thecenter-tappedpower transformer secondary. The phase relationship between the amplified pulse and the voltage of this secondary produces a rectified pulse in the motor winding L1 of Figure 3. The phase relationship between this pulse and its counterpart through the second winding (IJ2)determines the direction of rotation of the motor. Figure 4 shows the block diagram of the complete assembly as ordinarily employed. The student type potentiometer is standardized by means of the Weston

' It will be noted that the principle upon which this unit operates is the same as that of the Brown Electronik Amplifier, though the Brown instrument is of much higher sensitivity.

APRIL, 1950

standard cell, the galvanometer shorted out of the circuit and thee. m. f. posts of the potentiometer connected to oppose the e. m. f . developed by the cathode and the reference half-cell, in this case a calomel electrode. The algebraic sum of the desired potential drop between the cathode and solution and the potential of the reference electrode is set on the potentiometer. The stirring motor, rectifier, and amplifierare turned on, and deposition allowed to proceed, using first the high-range ammeter to indicate deposition current, and later the lower ranges. The proper potential drop a t the cathode includes the cathode overvoltage, and must he determined experimentally.' It may be only approximated from the reversible standard electrode potential and the limiting concentration to which one wishes to deposit the ion from solution. The choice of the vibrator (in this case an ATR 610) was purely arbitrary, the only requirements being that the a. c. used to ~roducethe 60-cycle oscillation be isolated from the vibrating reed and from the two contacts, and that the reed be able to be "floated" above ground potential. In this vibrator the screw contact which shorted the reed to the vibrator frame a t the completion of each cycle was removed, and the four insulated spacing screws adjusted "by ear" until the characteristic 60-cycle hum was detected. I t was found that when the vibrator was adjusted in this manner, reversal of the polarity of the input test signal resulted in a 180" phase shift in the pulse applied to the grids of the power amplifiers as observed on a cathode ray oscilloscope screen. 'Seep. 23 of footnote lb.

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The inclusion of a small Alnico horseshoe magnet in the position indicated in Figure 3 will "polarize" the vibrator so that the direction of the response of the motor will always be the same for a definite polarity of the input voltage. This magnet may be eliminated if the operator will take the trouble to test the direction of motor response a t the beginning of a determination and reverse the polarity of the input if necessary. In other experiments on this problem, d. c. amplifiers followed by the so-called trigger circuits have been employed, as well as various input modulating circuits6 but none gave the sensitivity or the stability of the vibrating reed input device. Tests are under way using the Western Electric special mercury relay (D168479) as a 60-cycle "chopper" in the input circuit, but optimum conditions for polarizing this relay have not yet been determined. All the components and materials used in these experiments were obtained in the surplus market a t a small fraction of their original cost so that no reliable estimate of the expenses involved may be made. Summary. Several methods for the continuous, automatic control of the potential drop between an electrode and the solution from which a metal is being plated have been described. Such potential control allows the quantitative separation of many metals without the undue fatigue of the operator experienced when manual potential control is used. GREENWOOD, I. A., J. V. HOLDAM, AND D. MACME,"Electronic Instruments," McGraw-Hill Book Co., New York, 1948, 721 pp.