Wide-Range Instrument for Controlled Potential Electrolysis R . W. LAJIPEIERE Oak Ridge .Vational Laboratory, Oak Ridge, T e n n .
C
OSTKOI,I,ED pit,ential electrolysis is a promising nirttiod for analysis, or quantitative separat,ion of elrnirnts from solutions. .4 nuniber of instrument,s havr bern designcd for this purpose and are considered in an e:trlier paper (9). In addition, an article by Ashley ( I ) gives an excelleiit survcxy of the field arid literature to date.
Frequently occasioiis arise where it, is desirable to supply a current of several amperrs or a potential up to 50 or 100 volte a t the anode of an electrolytic cell-for example, the conccntratioii of reducible ion usually encountered in macro work may involve initial currenk of a few amperes. In the case of R side reaction such as gas cvolution, much rurrent, and also a high
Figure 1. Circuit Diagram R-l. 5000, 4-watt wire-wound potentiometer R$.R6, R7, R8, RQ, R10, E l l . R12, R13, R14. 2:00, I/* watt, 5 % 0 to 50 pa. direct current meter, 1200 ohms internal resistance M2, .M3. 0- t o 1-pa. direct current meter, 55 ohms internal resistance Red. 1. Selenium rectifiers, 75 @ a ,full wave Red 2 . Selenium rectifier. Fansteel V32OM PL1. 120-volt pilot li h t P L Z . 120-volt neon p5ot liqlit Brown converter Brown 27-r.p.m. Servo m o t o r J 1 . &pin chassis connector socket J 2 , J 3 . I-pin chassis connector so(bket Gear box, 100/1 reduction R l 5 . 62.5 ohms, 1% prec/s/on RIB. 27.8 ohms, lYO prec!s!on R17. 5.10 ohms, lYO precision R18. 2.520 ohms, lYo precision Rl9. 0.500 ohms, 1 % prec/sjon R20. 0.250 ohms, 1YCprecisiqn R21. 0.0500 ohms, 1% pfeasion R22. 195 ohms, 1% precision S S S , 59B. Cersrnic wafer switch SSC. Microswitoh noruially open
T1. 120 to 120, 40-volt ampere isolating transformer T2. Unit Transformer Co. 0 to 6 transformer T3. Sola 120-volt, 500-volt ampere constant voltage transformer T4. 230-volt Pi/t-ampere Variac T5. 115-volt, 5-ampere Variac L1. 8-henry, 40-ma. choke L2,L3,L4,L5. 0.16-henry 5-ampere choke F 1 F 2 F3. 10-ampere fuse Sl: Double- ole single-throw 12-ampere switch S 2 S 7 58. bauble-pole double-throw 3-ampere switch S3' 54: Microlimit switch (mounted on T4) S5: Normally open push button, spring return S6. Isolantite wafer switch, 1 pole 11 positive C1, C4. 1 mfd., 1000 volts C2 0 5 mfd. 600 volts C3: 20 mfd.,'250 volts C5. 0 1 mfd 500 volts C6. 600 mfd:, 125 volts C7 C8. 1000 mfd 150 volta R1: 5000, 25-wati'wire-wound potentiometer R2. 2500, 5 watts R2. 2500,5 watts R3. 3000, 5 watts
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V O L U M E 23, NO. 2, F E B R U A R Y 1 9 5 1
Electrodeposition is a proinihing method of separati n g one element at a time from a solution containing
several elements. The method is easily carried out bj remote control; hence i t is particularly suitable for use with radioacti\e solutions. 4 closely controlled solution-cathode potential is desirable in order to pre\ent more than one element from depositing on the cathode. Se\eral amperes of current capacitj are useful where concentrations of the desired element are high; also, in some cases, to orerride the effect of hydrogen etolution. Anode poten-
nncidt~ voltage are required to maintain the solution-cathode
potc,iiti:il. For nonaqueous studies the current required niay be IOM. \)ut the voltage high owing to the large resistance of the solutiori. For these reasons an iristrument wis built which can supI J I ~up to 5 amperes, 100 volt,s, or 250 volt-amperes. The latter liiiiitntion simply meaiis the product of currpnt times voltage cxrinot exceed 2 5 G f o r example, a t 80 volts the current should iiot vrceed 250/80 or 3.12 amperes. POWER SUPPLY
.\.- shown in Figure 1, power is drawn from the l2O-volt, 60cyrle line and goes through the various circuit components in thr following order: Sola constant voltage transformer 2'3, regu1:iting Variac T4, range control Variac T5, selenium fullw:ivv rwtifier Red. 2, syninietrical LC filter, anode-current meter 5 1 2 , uiiode voltmeter M3, polarity-reversing switch 88, anodeourrelit fuse F3, and finally to the electrolytic cell. Tile Sola tranafwmer prevents ordinary changes in line voltage from :iffr,c.tingthe apparatus and also srrves to isolate the equipment c~lectricallyfrom the line. In niouiiting a Sola near sensitive :iniplilirrs it Ehuuld Ijr /)orlie in mind that i t is a strong source of magnetic leakage flux. This radiation is emitted much lesv morigly in some directioiis than in others: thus, with a little care it is posible to locate the unit in the same cabinet (Figures 3 arid 1)with the control circuit in such a manner that i t has no effect on control. Tlie regulating Variac, T4,serves to vary the anode potential (all ~ ~ o t r n t i a lare s reierred to cathode as ground or zero) as dict:tt.id by the control circuit, thereby holding a constant solutioii-c.:ithode potential in the cell. I t is driven by a Brown motor (part 76750-3, Broivn Instrument Co., Philadelphia, Pa.) through a 100 to 1 gear reduction arid friction clutch. The clutc~hprrmits a quick initial setting to be made by hand a t the start o f an electrolysis. \Vhen seeking halance the Srariac turns a t :i s p w d that takes 5 minutes to cover its range, This holds the rate o f change of anode voltage to 20 volts per minute or less. This rate of change is fast enough for any foreseen plating operation, yet slow enough to permit very close control without hunting. h 24Gvolt Variac is used to gain the smoother control that rerjult,s from the greater number of turns (600 turns coinparetl t o 300 for a similar 120-volt Variac.). The brush has some smootliing effect, presumably because it comes in contact with more than one turn a t a time, with the result that the jumps in direct current anode voltage are similar to those from a Variac with 800 turns. If we let AVp rqu:iI thc step in probe voltage, R the set'ting of the range control \-ariar in volts, and V , the prolw voltage in volts, AT.', = 0.01 1 KI', millivolt. R is roughly equal to the anode potential; t h u j f'cir the worst conceivable condition, R = 100 and VT,= 5 volts. Then ATTP = 5.5 inv. A more usual set of conditioris would be li = 20, and V , = 2, which gives AT.'= = 0.44 niv. In each case regulation could riot be closer than these limits.
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tials of the order of 50 to 100 bolts are sometimes desirable, as in the case of nonaqueous solutions of high internal resistance. In such cases the current requirements are usuallj low.. 4 simple rugged instrument was built using standard components which will deliver up to 100 volts or 250 volt-amperes, will regulate to 2 mv. over a 12-hour period, and will operate equally well at either polarity. Such an instrument will fulfill practically all requirements for trace separations and w i l l be useful where sizable quantities are to he removed from solution.
The range control Variac, 7'5, is u3ed to set the upper liniit of direct current voltage obtainable from the regulator. Its setting is roughly equal to the direct current anode potential. It is usually advisable to set this only a little higher than the maximum anode potential expected during the electrolysis in order to reduce the voltage per turn on T4. This control also is used to vary the direct current output by hand when it is desired to use the machine as a variable voltage power supply. I n this case T4 is turned to its maximum position and the regulating system is shut off.
RI5
ANODE
AMPERES
5 AMP. POSITION
Figure 2.
Circuit for Measuring Anode Current
The filter is made symmetrical, since either side of the output niay be grounded. Chokes are connected in such a way as to minimize leakage flux (by making mutual inductance of adjacent chokes additive). Otherwise the sensitive balancing circuit might be disturbed. The capacitances are held to the valurs shown, and a bleeder resistance of 3000 ohms is connected across the output to provide a small enough resistance times capacity constant under all conditions to prevent hunting. (If the resistance times capacity constant is too large the output voltage can only be decreased slowly if cell resistance is high. This causes T4 to overshoot and resuks in sustained oscillations.) The reduction factor of the filter is sufficient to limit ripple in anode potential to 2 mv. a t 100 volts; hence there is essentially no ripple in solution-cathode potential. The meters Jf2 and M 3 read anode current and voltage, respectively. A range switch with multiplier resistors (not shofi-n) provides ranges of 5, 10, 50, and 100 volts for M 3 . The current, ranges are 1, 5, 10, 50, 100, 500, 1000, and 5OOO ma.; the circuit used for switching in the appropriate meter shunt,e is shown in Figure 2. A rather complex arrangement is used which avoids
ANALYTICAL CHEMISTRY
2M)
errors from current times resistance drop in the switch contacts and still permits the use of commonly available components REGULATING SYSTEM
anode voltage expected during the electrolysis. The machine is turned on and the proper reference potential is established. Subsequent operation is completely automatic. Operating characteristics me the same for either polarity of anode or probe.
The heart of the regulating system is a standard Brown amplifier except for special high impedance input transformer T2, and a Brown converter (parts 76020-1 and 75829-1, respectively). Voltage is ususlly supplied to the probe lead from a reference half-cell and a salt bridge connecting the reference electrode with the solution in the electrolytic cell. This is balanced against a reference voltage set up on potentiometers R4 and S6. M1 gives a coarse voltage reading and has full soale ranges of 0.1, 0.5, 1.0, 5.0, and 10.0 volt8. It is common procedure to connect a potentiometer between the reference electrode and oathode in order to read the voltage accurately. If the probe potential differs from the reference voltage, a small current How8 through the converter and the input transformer. This current is amplified and causes T4 to rotate in a direct.ion that corrects the error. The circuit responds to a *l-mv. difference, and the probe current is then le88 than 0.1 #a,, a value small enough to prevent polarization of the reference eleotrode. A separate lead, that carries only t,he signal current, connects the cathode to the regulator chassis and hmcc the midpoint of the input transformer.
Figure 4.
Figure 3. Front View of Instrument
This extra lead to the cathode is clearly necessary to prevent errors which result from current times resistance drop. Additional velocity damping has been added to the Brawn motor by feeding direct current from Red. Pinto the line phase of the Brown motor. The antiresonant circuit preterits 60-cycle alternating current from feeding back through the rectifier. O r d i n d y R1 is left a t zero, as this gives maximum damping and does not affect regulator sensitivity. To avoid hunting, permissible amplifier gain is limited by response time divided by filter re sistance times capacity constant. With the components shown the full gain of the amplifier may he safely used.
Rear View with .iccess Door Removed
One such regulator has been in operation a t the Oak Ridge National Laboratory for 2 years; others have been in ~u8eat Massachusetts Institute of Technology for a shorter period. Experience with these has shown that drift over a 12-hour period is less than 1 mv. Drift is independent of anode current or changes in anode current. Instantaneous departures from reference potential during the course of an electrolysis are within * 5 mv. However, much greater Huctuations may occur where periodic cathode phenomena are encountered. This is discussed in some detail in an earlier paper (8). These departurea cannot he observed unless a fast response instrument such .a B cathode-ray oscilloscope is used to observe probe potential. The machine has been used 8s a two-terminal regulator to hold constant mode potential by oonneoting the probe lead to the movable arm of a variable resistor whose ends are connected to anode and cathode. It can also he used as constant current source by connecting the probe to .s smell resistance in seriea with the cathode lead. ACKNOWLEDGMENT
The author wishes to thank L. B. Rogers and J. C. Griess for many helpful suggestions and for supplying data obtained during electrolyses. This work was performed a t the Oak Ridge Nationill Laboratory under contract 7405 eng. 26, between the Atomic Energy Commission and the Oak Ridge National Laboratory. LITERATURE C I T E 0
OPERATING PROCEDURE
The electrolytic cell is connected to the instrument in the usual way, and the range control is set a little higher than the maximum
(1) Ashley, 6. E.9.. ANAL.CHEM.,21,70 (1949). (2) LsmDhere. R. W., and Ropers. L.R., Ihid.. 22,463 (1950). R ~ c e i v e nMay 8, 1950
