V O L U M E 21, NO. 4, A P R I L
1949
497
ACKNOWLEDGMENT
The authors wish to thank George Calingaert, H. A. Beatty, and G. W. Thomson of the Ethyl Corporation, E. L. Baldeschwieler of the Standard Oil Development Company, H. H. Willard of the Univerjity of Michigan, and H. E. Tanis and J. K. Wolfe of t h k lsbopatory for helping in vapioua ways in connection with the present investigation.
Presented before the Diwsion of Andytied and Miero Chemiatwat the 114th Meeting of the A ~ R I C I CBEMICAL N SOCIETT, St. Louis. Mo., &Q part of s Symposium on New Electrical Methoda of I”aiy&. RECEIVED July 9, 1948.
LITERATURE CITED
(1) Aborn. R. H.. and Brown, 1,26 (1929).
(2) Liebhafsky, H. A.. Smith, H. M., Tania, H. E.. Winslow, Ji:. H., ANAL.CHEM., 19,861 (1947). (3) Michel, P. C.,and Rioh, T. .4.,Gem. Elec. Reo.. 50, 45 (February ,1947). (4) Sullivan, M. V.. and Friedman. H., IND. ENO.CHEM., ANAL. E ~ . . 18,304 (1946). (5) Winslow, E. H.9 Smith, H. M., Tanis, H. E., and Liebhslsky, H. A..AN.*L.CHKM..19. 886 (1947).
K. H., IND. ENR.CHEM.. AN*&. EO..
Multipurpose Electroanalytical Servo Instrument JAMES J. LINGANE Harwrrd Univemity, Cambridge 38, Mass. The instrument described provides a lariety of automatic services in the eleotruanalytical laboratory. Its chief oomponents are commercially available units and no special skill is required for assembly. Various applications are described, including methods based on electrolysis at controlled potential, constant total applied e.m.f., or constant current, and automatic potentiometric titrations.
T.
HE instrument described herein serves a number of purposes In the electroanalytical IahorrLt,ory. It functions as a potentiostat in automatically maintaining the potential of a working electrode constant during an electrolysis, as in eleetrogravimetrie determinations ai metals ( I , 4, 7), electrolytic separations of metal4 prior to h a 1 determinations by odher methods (@, coulometric analysis (6‘1, and electrolytic preparatinns by the controlled potential technique (4). Electrolysis a t constant total applied voltage, or n i t h constant current, may also he performed. ~~
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ment in automatic potentiomptric titrations (S) and in certaim other types of electrometric titrat,ions. As shown in Figure 1, the major components a i the instrument are commerciillly available units, and assembly requires very little special skill. The total cost of all parts is approximately $850. ELECTRICAL CIRCUIT
The electrical circuit is represented in Figure 2. The central component is a standard Brown Electronik precisian indicator (indicating potentiometer). which is eauimed with switches so
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operated with fuli amplifier sensitivity, in which condition the dead zone is less than 1scale division corresponding to a precision of =to.z%. The Brown potentiometer has a satisfactorily rapid responuej iull scale traverse requires only 12 seconds. As sunplied, the instrument normally has a capacitor connected across its innut terminals to suppress stray signals, and when a large resistanck is placed in the input circuit the time constant is increased so much t h a t t,hr msnonsc hroomes verv t,hia ~~.~.~.. ., slueeish. ~ ~ o o Thwefnre capaciwr was~di&nn&ed, and rapid rcsponso was maintained with as much a.6 100.000 ohms across the input t,erminals. I n ~
the negativk side of the input ci&t scpar&lv to onsurTstabl; operatban. The range of the potentiameter-controllcr may bo adjusted to a variety ai values hctween 10 mv. and 10 volts by means of the potential divider composed of the precision decade resistances, RI, Rz, and Ra, shown in thc upper right of the control panel in Figure 1. I n effect the instrument functions as a very high resistance voltmeter, and its range is given by 0.01 (R, Rn R a ) / ( R I+ , R , ) . Resistance RI comprises ten steps of 1000 ohms each, Rz nine 10,000-ohm steps, and Ra ten 100,000-ohm steps, and each is precise to = t O . l % (General Radio Co., Type 510). The total resistance of R, and RZ (100,000 ohms) is permanent1.y connected in the input circuit and by varying R8 the total input resistance may he adjusted from 0.1 to 1.1 megohms. This is amply large to prevent appreciable polarization of most of the types of electrode combinations employed in electroanalytical ‘ohemistry. For measurements with very high resistance electrodes, such as the glass electrode, an external vacuum tube amnlifier -~~~~~~ is internosed between the cell and the indicator-controller as descrihid below. By opening switch &, setting Rt to 10 (far left in Figure Z),
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ANALYTICAL CHEMISTRY
498
R? to 9 (far right), and R3 to zero, tht. iristruriit.rit r n i t j ~be used a s a
true potentiometer on the 0 to IO-mv. range. Potentiometer Ra (10-turn 1000-ohm Becknian Helipot) serves to adjust the zero point of the potentiometer-indicator to any desired position on the scale, and thus provides for input signals which change polarity during an experiment. This potentiometer is powered by a 1.5-volt dry cell, B in series with a 140,000-ohm fixed resistance, Rj, and thus has a range of 0 to 10 mv. By reversing the battery leads potentiometer Rq can also he u.wd to suppress or compensatca the signal frnm the snurc(1.
provided hy shunt resistor Rs and selecttd b y swit,eh S1,which also Jerves as an on-off switch. The precision decade resistance, R7, with ten 0.1-ohm steps (General Radio Co., Type 510, mount,ed direct1.y below the ammeter on the control panel) is used when it is desired to control the electrolysis current a t a constant value during an electrolysis. The Bron-n indicator-controller is connected to terminals 9 and 10, and thus controls the iR drop across R,. Resistance R7.is also useful !Then it is desired to record the change in current with time during an electrolysis, for iyhich purpose an auxiliary recording potentiometer is connected t o terminals 9 and 10. For maximum flexibility in the choice of circuit arrangements for different purposes the leads from the various components are connected to numbered plug jacks on the panel, as shown in Figure 1, and interconnection is made by flexible insulated wires equipped with banana-type plugs. High quality telephone type sivitchcb (General Control (20.1~ v c i owpcl for SI,S!, and SJ. APPLlC4TIO3 S
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i’ The cathode of the eleetrolvsis cell is connected to terminal
13 14 15
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Figure 2.
Controlled Potential Electrolysis. To perform an electrolysis with the potential of one of the electrodes of the cell automatically maint,ained at, a constant value, the folloxying connections are made.
Electrical Circuit
T ~ v o enclosed tip type single-pole >iIigle-thruw mercury snitches, Sc and Sa,are attached t o the drive mechanism of the potentiometer-indicator, so that the instrument functions as a sensitive controller. (The complete switch assembly is obtainable as a unit from the Brown Instrument Co.) By varying the angular displacement of these two snitches the net dead zone of the controller action can be changed from one or two scale divisions (0.2 to 0.4%) to any desired larger value, and the control point can be adjusted t o any desired scale position. The sivitch unit has provision for up to four mercury switches, and as these can be used either separately or in various combinations, a number of different switching actions are available. The lower part of the circuit in Figure 2 constitutes a potentiostat for electrolvtic determinations and coulometric analvsis. .General I Radio T’ariac Rectifier unlt (Type 1260.A) n i t h a maximum input of 135-volt alternating current and a maximum output of 4-ampere direct current a t 0 to 15 volts serves as a very convenient variable direct current source. To provide automatic control of the direct current output the Variac transformer in the rectifier unit is connected by reducing gears to a small reversible alternating current motor n hose action is controlled via leads 13, 14, and 15 by controller switches S , and Sj. The motor drives the T‘ariac transformer a t 0.33 r.p.m , which experience has shown is about the optimum value for most controlled potential electrolysis experiments. 1Iicrosnitches actuated by the drive mechanism serve as limit switches a t each end of the travel, and red and green pilot lights in the motor circuit (not shown in Figure 2) indicate when the motor is operating and its direction. The rectifier unit is powered by an auxiliary Variac transformer (General Radio Co., Type T’BlIT, 0 t o 135 volts, mounted on the panel below the rectifier unit in Figure 1) which serves as a manual control of the direct current output. This auxiliary transformer is also used t o adjust and match the total range of the rectifier unit to the load, so that the T’ariac transformer in the rectifier operates near the upper end of its range, and thus optimum controller action is obtained without hunting. The total applied e.ni.f. is indicated bv the voltmeter mounted on the left side of the panel of the rectifier unit. The electrolysis current is read on ammeter M, whose primarv range is 0 to 0.5 ampere. A second range of 0 to 5 aniperes is
‘jack 11 and the anode to jack’l2. Resistance R, is set t o zero. If the electrode whose potential is to be controlled is negative with respect to the reference electrode, the lead from the reference electrode is connected to jack 2, and a lead from the electrode is brought to jack 1. These connections are reverscld if the working electrode is positive with respect to the refcrencix electrode. The conncxction between the controllcr terminal jack and the working olectrodc should be made by a separate lpad dircctlv at thc ccll. rather than by a lead to termirials 11 or 12 on the panel, to avoid including in the apparent potential of t,hc norking electrode tht’ iR drop in the main leads betn.een terminals 11 and 12 and the cell. K i t h several amperes’ current, this iR drop can amount to 0.05 volt or even more. The motor-reversing leads, 13 and 15, are connected to the mercury s:n-itch terminals, 5 and 8, respectively, lead 14 is joined to citlier tclrniinal 6 or 7, and terminals 6 and 7 are connected together. Switches Sc and Sj are arranged in opposition to constitute a single-pole double-thron switch which controls the direction of rotation of the variable transformer in the rectifier, and they are set to the desired operating potential. The dead zone b e t m e n Sa and Sjshould be made large enough to prevent hunting. In many cases this dead zone may be as small as 1% of the indicator scale length, corresponding to a control precision of *0.005 volt when the controller range is 0 to 1 volt. When the potential of the working electrode tends t o fluctuate rapidly, a larger dead zone should be used, as otherwise hunting will occur. The auxiliary T-ariac transformer is set back to zero, and the rectifier transformer is run up t o near its upper limit by shorta is then circuiting motor lead 14 with either 13 or 15. Switch S closed and the set,ting of the ausiliary Variac transformer is increased by hand until the potential of the working electrode assumes the value of the cont,rol point. The apparatus may then be left to itself and the potential of the working electrode will be maintained automatieally a t the control point (plus or minus one half the dead zone) until the electrolysis is completed. The varied practical analytical applicationa of cont’rolled potential elcctrolysi.: are discussed in wveral recent publications (1, 4-7). Electrolysis with Constant Total Applied e.m.f. Leads from the cathode and anode of the cell are connect,ed to controller jacks 1 and 2, respectively. This control function has little value in analytical applications of electrolysis, but it is useful in other types of electrochemical experiments, such as electroplating. Constant Current Electrolysis. Resistance K7 is set t o the desired value and cont,roller terminals 1 and 2 are connected to terminals 9 and 10, respectively. The range of the indicator-controllel. and the value of RI used depend on the particular application. Usually R7 is set to 1 ohni and each millivolt potential rlrnp aero+ it then corre,*ponds t o 1 milliampere. By using tlw 0 to 0.01-volt range of the coniroller, currents of 0 to 10 niilliainprws may be measured or controlled tn ahout +0.02 millianiperr, and by employing the
499
V O L U M E 21, NO. 4, A P R I L 1 9 4 9 larger ranges of the indicator-controller, currents of several aniperea may be measured or controlled. The precision of current control depends on the dead zone between mercury switches S q and S6,and can be made as small as 0.5% or better. Still higher prrcision of current control can be achieved by employing an auxiliary potentiometer in series with the iIidicat,or-controller. If it is desired to control an electrolyzing current a t 1 ampere with a precision of =tO.O2% (+0.002 ampere), R7 would be set a t 1 ohm, the auxiliary potentiometer would be set at 0.995 volt, and connccted to terminals 1 and 10 in opposition to the if2 drop across R,, terminal 2 would be connected to terminal 9, and the 0 to 0.01-volt range of the indicator controller Iyould be uscd. The controller will then operate in the range 0.995 to 1.006 amperes, and for control a t 1.0000 * 0.0002 anipere mercury, switches S,and S5 are set a t the midpoint of the controller scale with the dead zone adjusted to 2% of the scale length. Manual Potentiometric Titrations. Leads from the titration cell are connected to controller jacks 1 and 2 , and the indicatorcontroller is set to the desired range. The change in potential is observed a s the titrant is added, and otherwise the classical potentiometric titration method is followed. Automatic Potentiometric Titrations. Previous papers ( 3 ) ibe the autotitrator unit shown on the desk in Figure 1 and tht. general methodology of automatic potentiometric titrations. If the titration is to be performed with only a singlr rate of tirlivery of the t,itrant, up to the equivalence point, onc of the i n t ” ~ n yswitches is connected as an on-off switch in series with thts motor which drives the syringe buret. The mercury switch is adjusted to the known equivalence point potential, or somewhat in advance, and it the0 functions to stop delivery of the titrant at the equivalence point’ (3). In order to achieve greater speed in automatic titrations, withIJlit, any sacrifice of accuracy, provision should be made for very rapid addition of the titrant until the end point is approached, For a slower delivery rate in the immediak vicinity of the end point, and finally for small incremental additions until the end point is exactly reached. Using the autotitrator and general technique previously described (S), this was achieved as follow: The syringe of the autotitrator was pon-ered by a small direct current motor of a type whose speed can be varied over a t least a fivefold range by a series resistor. The variable resistance, Rs (radio potentiometer, mounted on the extreme left of the control panel), serves this purpose, and its resistance depends on the particular motor used. The motor leads from the autotitrator are connected t o jacks 3 and 6, and jacks 4 and 5 are connected together, so that Rs is in series with the motor. Switch S,is set to open at or slightly in advance of the equivalence point potential as in titrations a t a single delivery rate. Jacks 7 and 8 are connected to jacks 3 and 4, respectively, and slyitch S j i s set to change from closed to open at a value ne11 in advance of Si. Switch SS shorts out’ resistance Rs and thus permits high speed delivery of the titrant until the potential corresponding to its aet point is reached. The opening of 85 puts Rs in the motor circuit and the delivery rate slows down for the remainder of the titration. The action in the vicinity of the equivalence point is csactly the same as originally described ( 3 ) . Usually a high speed delivery rate of 5 to 6 nil. per minute is employed, and RS is adjusted so that the slow delivery rate is 0 . j to 1 rnl. per minute. The proper setting of Sa and Sj depends on the characteristics of the titration curve; ordinarily R5 is set to open a t 1 t o 5% before the equivalence point. The optimum setting is easily established by trial in any particular titration. Cnder these conditions a titration requiring 25 ml. is completed in 5 to 6 minutes. Because of the slon speed delivery nrar the equivalence point t,he incremental additions at the end are very small, and usually amount, t o on1.v 0.005 t o 0.01 nil. The precision and accuracy of the titrations depend, of course, on the characteristics of the titration curve and on the proper control of the ot’herfactors (5). Under average good conditions a 25-ml. titration may be performed with a precision of +0.05%, nr even better in some cases. .A 10-ml. titration requires only 3 minutes, and a precision of *0.17c is ordinary. For automatic titrations lyith thc glass electrode an elcc-
tronic amplifier must be interposed b e t w e n the titration cell and the controller. Thc ubiquitous Model G Beckman pH meter (shown on the left of the autotitrator in Figure 11 qerves ideally aithout any major alteration.. I t is only necessary to attach t v o binding po from the internal metal shield) to the outer case of the instrument and connect internal leads from these to the terminals of the null point indicating meter. Leads, which ordinarily do not need to be shielded, are then connected between these binding posts and controller terminals 1 and 2, so that the potentiometer controller functions as a null point indicator in exact mimicry of the meter in the Beckman instrument. The Beckman pH meter is calibrated and operated exactly as in ordinary use, and the usual titration tvpe electrodes are employed as shonn in Figure 1. The zero point of the indicator-controller is adjusted to eoincide with the operating point of the mercury snitches. The pH meter is set to the appropriate equivalence point pH, and the switch button is locked in the closed position to start the titration. The titration then proceeds automatically until the restoration of balance stops the autotitrator. The signal in the output circuit of the Beckman instrument, indicated by the meter and the indicator controller, varies from zero to about 340 millivolts for extreme off-balanee from the low to high side. This output when plotted R S ordinate against offbalance pH on the abscissa yields an S-shaped rurve, and the null point of the meter corresponds to a point on the steepest section, so that the sensitivity in terms of p H is maximal at the balance point and falls off rapidly with increasing unbalance in either direction. This is highly advantageous because the range of the indicator-controller may be set t o a relatively small value-e.g., 0 to 0.5 volt-and a very small change in p1-I a t the balance point produces a largr displacement and consequentlv very precise operation. Other Applications. The instrument is convenient for manual amperometric titrations n i t h either the dropping mercury electrode or a platinum nijcroelectrode. The nccessarv polarizing e.m.f. is supplied b\ the rectifier unit, and the currcnt is nicRsured by connecting the potmtionirJter-indicator across a precisely known resistance-e g., 100 or 1000 ohms-in series n i t h the titration cell. The poat.ibilitv of prrforming automatic amprrometric titrations is aLo being investigated. Automatic titrations using polarized electrodei, according to the dead stop end-point technique, may al-o be peiformed. X suitable polaiizing c.m.f. of a fern- millivolts is applied to the two platinum electrodes and the current is measured by the potentiometer-contioller in terms of the 2R drop across either Rr or an auxiliary reiistance in
