have )vide application for the quantitative analysis of solids by infrared absorption.
W 0
z
POTASS IU M
2 c
ACKNOWLEDGMENT
T HI OCYANATE
One of the authors (S.E.W.) is indebted to the Board of Trustees Research Fund of Rensselaer Polytechnic Institute for a grant toward the purchase of the Model 21 Spectrometer, and to the Socony Mobil Oil Co., Inc., for partial support of this work.
il
.-ALUMINUM
SOAP
#
LITERATURE CITED
(1) Atkinson, J. V., Bakelite Co., private
communication to authors, Sept.
0
WAVELENGTH, M I C R O N S Figure 6. Spectra of potassium thiocyanate and an aluminum soap in potassium bromide
4, 1956. ( 2 ) Bakelite Co., technical data sheet on surface coating products, Oct. 19, 1953. ( 3 ) Barnes, R. B., Gore, R. C., TVilliams, E. F., Linsley, S. G., Peterson, E. I f . , IXD.ENG.CHEX., AXAL. ED.19, 620 (1947),. Browning, R. S., Riberley, S. E., Sachod, F. C., ANAL.CHEJI.27, 7 (1955). Childers, E., Struthers, G. IT.,Zbzd., 27, 737 (1955).
Harple,
Table II.
snnlplc
Designation VYChI
Results on Commercial Samples
7cPolv(vinl.1 hcctate)
Reported
VYDR
VYHH-1
9 4 13
Found-
84, 8 5 35,35 13 0 , 13 1
the absoibance ratio of the 4.7- and 5.8micron bands on 2-nig. samples of four aluminum soaps, the free fatty acids of which had been determined by chemical methods (6).
In view of the results obtained on these typical materials, this general method employing potassium thiocyaiiate as an internal standard should
\J7.
W.,Wiberlev S. E.,
Railer. W. H . . Zbzd.," >4, 635 (1952j. Hughes, H . K., associates, Ibid., 24, 1349 (1952). Kirkland, J. J., Ibid., 27,1537 (19%). Kuentzel, L. E., Ibid., 27, 301 (1955).
Miller, F. A , , Wilkins, C. H., Ibid., 24, 1253 (1952).
Kright, S . ,-4ppl. SDectroscopy 9, 105 (1955).
RECEIYED for review June 20, 1956. ilccepted November 23, 1956. Division of .4nalytical Chemistry, 130th Meeting, .4CS, A4tlantic City, S. J., September 1956.
Instrument for Controlled Potential Electrolysis and Precision Coulometric Integration GLENN L. BOOMAN Atomic Energy Division, Phillips Petroleum Co., Idaho Falls, Idaho
b Control of electrolysis potential within 3 mv. and response to changes occurring as fast as 10 psec. are attained with a 600-ma. capacity potentiostat circuit. A standard deviation of less than 0.05% for integration of electrolysis currents is possible over the range from 10 pa. up through 100 ma. The instrument is easily constructed and is adaptable to many different types of electroanalytical methods.
I
study of the coulometric determination of uranium (VI) at controlled potential, the N BEGINXIKG a
need arose for an instrument which could accurately control the mercury cathode potential and precisely integrate the resulting electrolysis current. Controlled potential methods of analysis have not been used t o the extent merited by their versatility and selectivity, mainly because of the complex electronic circuitry needed and the previous unavailability of suitable commercial instruments. For precision work with a rapidly stirred mercury clectrode, a servocontrol potentiostat does not have the necessary speed of response to compensate for the large random fluctua-
tions in current due to the continually changing electrode area. The servocontrol instruments such as those made by Fisher Scientific Co., Pittsburgh, Pa., and Analytical Instruments, Inc., Bristol, Conn., and the various servocircuits reviewed by Lingane (6) are necessary when large scale controlledpotential reductions are required, as in organic and inorganic synthesis by electrolysis. These are also suitable for many coulometric applications. For practically all analytical work a maximum available electrolysis current of 0.5 to 1 ampere is sufficient. This assumes the accurate pipetting of about VOL. 29, NO. 2 , FEBRUARY 1957
213
0.5 nil. of solution or the direct weighing of approximately 100-nig. samples. In this current range completely electronic control beconies econoniically feasible, giving the advantages of fast response time and elimination of mechanical systems. Also, by decreasing the sample size, a more efficient cell geometry can be attained, reducing the analysis time by a factor of 5 to 10 from the time required in large cells (9). The problem of integrating a varying electrolysis current has been attacked in many ways. Several types of chemical coulometers have been used, but they are characterized by the limitations and speed of chemical titrations or difficulties of exact gas-volume measurement ( 8 ) . Mechanical ball and disk integrators have been utilized in conjunction with strip-chart recorders (’7). Integrating relays and motor generators have been found useful for integration of fairly high cell currents (‘7, 9). Several circuits for electronic integration based on repetitive capacitor discharge time have been developed, but they lack accuracy and have difficulty with dead time ( 5 ) . For accurate integration of fast current changes, an electronic direct current system of suitably wide band response is needed. The usual type of resistance-capacitance integrator as used in analog computers seemed a logical choice for this application.
The current was then reset in the same manner. -4bout six to ten current steps are necessary for each determination, before background current is reached. An electrolysis time of 7 to 10 minutes was found to be sufficient.
The number of coulombs corresponding to the material reduced or oxidized at the working electrode could then be obtained by integrating the area under the recorded current-time curve. This integration mas accomplished by weigh-
Figure 1. Circuit arrangement for “potential-limit’’ electrolysis in constant-current steps Manual control
COULOMETRY BY “POTENTIAL LIMIT” METHOD
For some preliminary coulometric investigations a simple manual control circuit was built. This unique electrical circuit applies a constant current to the cell which can be changed in discrete increments to keep the working electrode potential from exceeding the control point. A series-pentode constant-current source was used, consisting of a 6SJ7 tube with manually controllable grid potential as shown in Figure 1. As the cutoff charscteristics of the 6557 approach a logarithmic curve, a smooth linear control of the logarithmic decrease in electrolysis current was obtained. The cathode potential was monitored with a Beckman Instruments amplifier control Unit as used on the Model K automatic titrator. Because of the high inpiit impedance of this amplifier, a high resistance reference electrode could be used. The current floxving through the cell was recorded on a Brown, millivolt, recording potentiometer by measuring the voltage drop across a resistor in series with the cell. To do a coulometric determination, the current was set a t a value rhich would allow electrolysis to proceed for about 1 minute before the working electrode reached the control point. 214
ANALYTICAL CHEMISTRY
40
W m LL r
a
5
30
[i
0
I i w c C 0 3
$
20
3 [i
k W W
10
0
;I-’0
I
2 3 ELECTROLYSIS T I M E , M I N U T E S
Figure 2. Current-time curve for controlled-potential reduction of 10 y of uranium(V1). Citrate electrolyte
7
ing the amount of paper under the stepshaped curve, making proper allowance for background current. The density of the paper was periodically checked and found to be sufficiently uniform in each roll to be within other experimental error. Figure 2 shows a typical current-time curve. When this potential-limit method is used, fast changes in working electrode potential do not affect the analytical results, since the average control potential is well below the chosen control point. The simple current control method shown in Figure 1 is not sufficiently precise to allow integration by summing the current-time products for the current steps. A rather simple automatic instrument rould be made which would furnish about ten accurately known current steps to the electrolysis cell and record the time each step was used. The further development of this instrument %as dropped in favor of continually controlling the electrode potential.
COMPLETELY ELECTRONIC POTENTIOSTAT AND INTEGRATOR
K i t h the design considerations of fast response and current capacity of a t least 500 ma. in mind, a completely electronic potentiostat and an integrator were constructed. Three direct-coupled amplifiers were used, connected as shown in Figure 3. Amplifier 1, the potentiostat anode supply circuit, furnishes sufficient potential to the anode to keep the reference electrode above ground potential by the desired control volt-
age. Amplifier 2, the cell load-compensating circuit, furnishes sufficient voltage to resistor, Rz, to keep the mercury pool cathode a t ground potential. The use of two amplifiers to control the working electrode potential is preferable to a floating power supply or a differential scheme. The variable resistor, R1, can be used to limit the initial current to any desired value, in cases where high initial current is obtained. By the use of this resistor, potential control is maintained and the potential automatically approaches the control point as the cell current decreases. Amplifier 3 operates as a resistance-capacitance integrator. The output voltage of amplifier 3 is given by:
Output voltage =
This output voltage will remain a t the output terminal until the integrating capacitor is discharged, allowing the final integrated current value to be read when desired. By proper choice of Rr, Ra, and C, any range of electrolysis currents can be integrated within the output current limitations of the potentiostat. The complete coulometer schematic is shown in Figure 4. The potentiostat amplifiers, A-1 and A-2, are capable of furnishing up to 600 ma. to the electrolysis cell. This high value of current was obtained by using a total of six 6AS7-G control tubes with the direct-current voltages furnished by a half-wave, unregulated, selenium rectifier supply. An unregulated supply
CATHODE Figure 3. Interconnection of amplifier circuits for controlledpotential reduction and electronic integration
could be used in this stage, as a very large direct-current gain is driving the output stage which has a gain less than unity. The three direct-current amplifier units are identical each consisting of a 60-cycle chopper amplifier with a n open loop gain of 1000 and a seriesparallel connected direct current amplifier having an open loop gain of 30,000 (IO). Thus each amplifier has a n open loop direct-current gain of 30,000,000. With 1 0 0 ~ ofeedback, the amplifier response is flat from direct current to 100 kc. The output voltage range is k100 volts. .4ny drift in the direct-current amplifier is reduced by a factor of 1000, the gain of the chopper amplifier channel. Zero grid current is drawn by the amplifier, since capacitive coupling to the direct-current unit is used. Both of the amplifier units are commercially available analog computer components. The plug-in units used in this circuit were the K2-X and the K2-P amplifiers manufactured by the George A. Philbrick Go. (IO). A Philbrick Type R-100 direct-current pon-er supply was used to supply the regulated plus and minus 300-volt potentials t o the amplifier units. Regulation of this unit is given as 0.1% us. input and 0.02% us. output. Stabilization against large line changes was effected with a Sorenson Model 1001 alternating current regulator. For most n-ork this alternating current regulator would not be necessary. For accurate integration, the integrating capacitor must have lorn leakage and low dielectric absorption. Mylar dielectric is suitable and considerably less expensive than polystyrene on Teflon dielectric units. 4 30-mf. capacitor was chosen as a compromise between cost and reliability of integration. With a small capacitor, the input resistance to the integrator can be made large, giving very little leakage from the capacitor due to the offset voltage appearing a t the amplifier input. However, small values of capacity result in a low feedback factor for amplifier 3, which produces increased noise and instability. An input resistance of 1.33 megohms could be used with the 30-mf. capacitor, resulting in only a small reduction of input impedance. Also with this choice of values, the output voltage could be read directly as milligrams of uranium in the sample. The precision of integration and potential control is well demonstrated by the data in Table I. The slightly low readings on the high current ranges are due to uncompensated lead resistances. The calibration circuit arrangement is shown in Figure 5. The calibration reference voltage, el, was furnished by a laboratory potentiometer or by a Weston standard cell. The resistances, Rz, used around amplifier 2 to effect range changes were VOL. 29, NO. 2 , FEBRUARY 1957
215
+
-
ln6i
ovt-2
?I17 V
AC Line
Mercury Pool Cathode
t Do Millivolt Range Recorder Or Manual Potentiometer
KZP Chopper Amplifier
ZDRr
U
50 KR
Figure
4.
Wiring schematic for electronic coulometer
R2
General Radio 0.05% plug-in units. T o determine the number of milligrams or micrograms of material reduced in the cell, exactly one hundredth of the output voltage was measured, using a precision voltage divider and a laboratory potentiometer or a J. -4.Fluke Model 800 differential voltmeter directly without the voltage divider. The use of the differential voltmeter allowed a resolution of 10 mv. or nearly 0.017, of the voltage in the range usually used. The potentiostat control, as measured with a Tektronix Type 532 oscilloscope, was within 1 mv. without stirring and within 3 mv. with stirring. Control of the reduction potential rvas best when the reference electrode was inch from the mercury surface. to This circuit can be simplified for the determination of small amounts of material. For example, with less than 4 peq. of material being reduced, the 6AS7-G output stages are not needed. If the analysis is concerned with less than 0.04 geq. of material being reduced, amplifier 2 and the integrator input resistor can also be eliminated, allowing all of the electrolysis current to flow directly into the integrating capacitor. 216
0
mKRTen Turn
Ten
ANALYTICAL CHEMISTRY
133 Mag.
DFigure
5.
Calibration circuit for coulometer
Table 1.
Calcd Current, ?ria. 0 01013 0 1013
1 013 10 13 101 81
Coulometer Calibration Data
(Six calibration determinations at each current level) Calcd. Av output,
Coulombs (10-hlin ) 0 006078 0 06078 0 6078 6 078
61 09
Volts/ Coulomb 12320 1233 123 3 12 26 1 223
% Standard Deviation 0 0 0 0 0
05
03 04 04
02
This coulometer can be used for controlled potential oxidations by merely interch anging amplifiers 1 and 2. The only maintenance work found necessa ry in a year of continuous op-
eration has been replacement of faulty capacitors and noisy choppers in two of the K2-P stabilizing amplifiers, occasional tube replacements, and replacement of the 1.6volt dry cells in
the potential setting circuit about twice a month. The operation of the instrument, as shown in Figure 4, for a controlled potential reduction with simultaneous integration of the electrolysis current is very simple. The balance of the amplifiers is checked. With the desired prereduction potential set on the ten-turn potentiometer, the potentiostat (amplifier 1) output switch is turned on. The output switch of amplifiers 2 and 3 are left on a t all times, During the prereduction step the capacitor in the integrating circuit is shorted by inserting a double plug into the capacitor terminals on t h e coulometer front panel. When t h e prerednction step is completed, as indicated by a constant background current or a sufficient amount of time, t h e potentiostat output switch is turned off. The desired reduction potential is then set on the ten-turn potentiometer and the shorting plug is removed from the integrator. Turning on the potentiostat output switch starts the reduction and the total integrated current can be read a t the output of amplifier 3. The coulometric determination is complete after enough time has been allowed either for background current to be attained or for no significant change to be observed in the integrated current reading. When the electrolysis is completed, the potentiostat output switch is turned off and the integrated current value read a t the output of amplifier 3. CELL DESIGN
Figure 6.
Figure
Electrolysis cell
7. Electronic coulometer
In order to obtain essentially complete reduction in a reasonable length of time, the ratio of electrode area to solution volume must be kept large (9). The electrolysis cell used in this study, a 50-ml. borosilicate glass test tube with a U-shaped length of 6-mm. glass tubing connected to the bottom for electrical connection to the mercury, is shown in Figure 6. Five milliliters of electrolyte were usually used. This cell could he slipped into a holder which contained the silver-silver chloride reference electrode, the platinum wire anode compartment, the nitrogen inlet tube, and the stirrer. The anode compartment is separated from the sample by a fritted glass disk. For the electrolysis of samples requiring more than 50 ma., the fritted glass anode separator proved unsuitable, because resistive heating in the frit resulted in gas formation and subsequent interruption of current flow. The use of Rohm & Haas Amberplex Type C-1, cation exchange membrane glued with Duco cement to the end of an 8mm. glass tube, proved very suitable for high current work. Sulfuric acid, lM, was usually used as the anolyte. An 1800 r.p.m. synchronous motor was used to drive a flabbladed glass stirrer VOL. 29, NO. 2, FEBRUARY 1957
217
which was about one half immersed in the mercury. Indentations were made in the cell walls to increase stirring efficiency and to prevent breaking of electrical connection to the mercury due to stirring of solution in the 6-mm. connecting tube. Figure 7 is a photograph of the cell, electrode assembly, and the plug-in panel of the coulometer.
POSSIBLE CIRCUIT IMPROVEMENTS
The dry-cell batteries may be eliminated by changing the amplifier bias arrangement (11). The battery used in the control-potential adjustment is replaceable by a simple silicon diode regulator. To obtain higher output currents, Type 6336 tubes can be substituted for the Type 6AS7-G tubes. These higher-current capacity tubes are rated a t 400 ma. per envelope, but are otherwise similar to the 6AS7-G. The performance of the cathode-follower output stage can be visually checked if the 47-ohm plate resistors are replaced with a low voltage pilot light such as Type 47. Any unbalance in triode load can then be easily detected. A circuit for automatic electronic balancing could be built along the design of Cederbaum and Balaban (3).
introduce the proper voltage to make the amplifiers balanced whenever the operate switches are returned to the “run” position.
CONCLUSION
Simple potentiostat and integrator circuits have been designed for use in controlled potential oxidations or reductions and simultaneous integration of electrolysis currents. The application of the instrument to the coulometric determination of uranium a t controlled-potential has been shown ( 2 ) to give excellent precision. By using the analog computer amplifiers as differentiating circuits (4, l a ) , the first and second derivative curves of a potentiometric titration could be easily obtained. The use of the potentiostat amplifiers as a constant current source for coulometry a t constant current would involve merely changing the input leads. K i t h a high precision integrator available as in this coulometer, careful control of current in a constantcurrent titration would become unnecessary and a coulometric titration with electrochemical generation of reagent could be accomplished with the
is in fact what can be obtained automatically making use of a series resistance, such as R1 in Figure 3, along with internal reagent generation with one instrument. This type of coulometry could well be called a “potential-limit” or controlled-time method, being intermediate to controlled-voltage and controlled-current coulometry and possessing the advantage of being able to choose the electrolysis time, independent of cell geometry, by adjusting the resistance, R1. High resistance glass electrodes, and polarized electrode systems may be used with the potentiostat, because the current drawn from the electrode system will be less than 10-10 ampere, considering the average amplifier unbalance of 50 pv. Application of this type of resistancecapacitance integrator to polarographic measurements was shown in another study ( 1 ) . The total cost of parts for the coulometer was less than $600.
ACKNOWLEDGMENT
The author wishes to thank the Instrument Department of the Idaho Chemical Processing Plant for help in the construction of the coulometer, Wayne B. Holbrook for obtaining many of the data, and James E. Rein and Ralph C. Shank for encouraging the development of the instrnment.
LITERATURE CITED
11
-
(1) Booman, G. L., Ph.D. thesis, University of Washington, 1954. (2) Booman, G. L., Holbrook, W. B.,
5800
I 10.5,~f.
POLYSTYRENE
WLANCE 100
150
RUN
1
14K,IOW
t ~ l - v # + o
RUN
+300
-
Rein, J. E., U. S. Atomic Energy Commission, IDO-14369(1956). ( 3 ) Cederbaum, I.,Balaban, P., Rev. Sci. Instr. 2 6 , 7 4 5 (1955). (4) Korn, G. A., Korn, T. M., “Elec-
tronic Analog Computers,” McGraa-Hill, New York, 1952. (5) Lewis, I. A. D., Collinge, B., Rev.
1
24 Meg.
b + 300 Figure 8.
Automatic balancing circuit
A method for using such a circuit with this coulometer is shown in Figure 8. Some other low grid current tube might be used in place of the Type 5800. Connecting the output of the K2-P unit during balancing either to ground or to the input would decrease the effect of alternating current pickup on the balance voltage. The use of this circuit would 218
ANALYTICAL CHEMISTRY
operator decreasing the rate of addition of “titrant” as the end point was approached. This technique would allow even better manual titration control than is possible with normal volumetric procedures, as the usual stopcock to control liquid flow would be replaced by a potentiometer giving accurately known control with a much finer delivery. The semiconstant current technique
Sci. I n s t r . 24, 1113 (1953). (6) Lingane, J. J., “Electroanalytical Chemistry,” pp. 202-50, Interscience, New York, 1953. ( 7 ) Ibid., pp. 246-50. ( 8 ) Ibid., pp. 349-53. ( 9 ) Meites, L., ANAL. CHEY. 27, 1116 (1955). (10) George -4. Philbrick Researches, Inc.,
Boston, “Applications Xanual, GAP/R-K2 Series,” p. 4, 1956. (11) Ibid., p. 8. (12) Ibid., p. 12.
RECEIVED for revie\v July 9, 1956. Accepted November 12, 1956. Conference on .4nalytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1956. The Idaho Chemical Processing Plant is operated by Phillips Petroleum Co. for the U. S. Atomic Energy Commission under Contract No. AT( 10-1)-205.
