substances that react with iodine interfere chemically in the coulometric system. Methanol, methyl borate, boric acid, and benzene did not interfere when injected into the titration cell. Butene and hydrogen gases in air at high concentrations did not consume iodine, as indicated by the titrator. Hydrogen gas, when bubbled directly through the titration cell, caused the titrator to generate iodine. Ozone and nitrogen oxides should have a reverse effect on the instrument since they can oxidize the iodide-containing electrolyte to iodine.
LITERATURE CITED
(1) American Conference of Governmental
Industrial Hygienists, Threshold values for 1957, 19th Annual Meeting. ( 2 ) Austin, R. R., Am. Gas Assoc. Proc.
31,505 (1949).
( 3 ) DeFord, D. D., Braman. R. R.,Breese, R. F., 126th Meeting, ACS, S e w York, S . T., September 1954 ( 4 ) Eckfeldt, E. L., Proctor, I)-. E.,
Perley, G. .I.,50th Annivwsary Meeting, Electrochemical Society, I’hiladel-
phia, Pa., May 1952. (5) Feinsilver, L., et al., Chemical Corps Medical Laboratories Rcsearch Rept. 359, (May 1955). (6) Feinsilver, L., Bean, \T. C., I b z d , No. 170 (February 1953). ( 7 ) Hatcher, J. T., Wilcou, J, I-, A v a ~ . CHEII. 22, 567 (1950).
(8) Hill. \\ . H., et al., Iodometric Monitoring of Borane-Containing Atmospheres, CCC-1024-TR-129. (9) Hill, K.H., Johnston, M. S., ANAL. CHEM.2 7 , 1300 (1955). (10) llcKelvcy, J. hI., Hoelscher, H. E., Ibid., 29, 128 (1957).
(11) llessner, .-2. E., Ihid., 30, 547 (1958). (12) RamseJ-, W. J., Farrington, P. S., Swift, E. H., Ibid.3 22, 332 (1950). (13) Schaffer, P. rl., Jr., Briglio, A., Jr., Brockniati, J. A , , Jr., Ibid., 20, 1008 (1948). (14) Weatherby, J. H., Chemical Warfare Laboratories Spec. Publ. 2-5 (February 1958). RECEIVED for rcview February 18, 1960. Accepted June 2 7 , 1960. Pittsburgh Conference on .Inalytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1959.
Contro I led- Pote ntia I PoI a rogra phic PoIa rizing Unit with Electronic Scan and Linear Residual Current Cornpensation M. T. KELLEY, D. J. FISHER, and H. C. JONES Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, Tenn.
b A polarographic polarizing unit is described that has electronic scan, linear residual current compensation, and potential-control circuits. The performance of the electronic-scan circuit is superior to that of a conventional motor-driven multiturn potentiometer scanner. The use of this polarizing unit is particularly advantageous for high-sensitivity polarography, both regular and derivative
T
Oak Ridge Xational Laboratory Model Q-1988 controlled-potential and derivative polarograph has been described (4). I n this instrument, amplifiers continuously force the potential of the polarized electrode (which may be, for example, a dropping mercury electrode, D.M.E.) with respect to that of the reference electrode to equal the sum of the linearly increasing scan voltage and the fixed value of the initial voltage, independently of circuit and of cell resistances. Electrolysis current flows between the polarized electrode and a platinum working electrode, but no current flows between the polarized electrode and the reference electrode. iR drops in the bulk of the electrolyte cannot distort the form of the polarogram because no appreciable iR loss voltages are seen by the input of the potential control amplifier. This ORXL polarograph can be used for the analysis of very low concentrations of irHE
1262
ANALYTICAL CHEMISTRY
reversible and reversible species (3, 4). It is possible to record as a function of effective rather than applied voltage the instantaneous currents, the successive peak currents, the successive average currents, or the timc derivative of the polarographic w a v ~ . The voltage scan and linear residual current compensator circuits in the polarizing unit of the Modrl Q-1988 polarograph employ gauged iiiultiturn potentiometers that are motor driven through a magnetic clutch in order to obtain, respectively, a voltage and a compensating current that increase linearly with respect to time ( 4 ) . An improved polarographic polarizing unit has been devrlopcd that uses electronic operational amplifiers instead of these conventional electromechanical circuits. The principles of electronic scanning have been briefly described ( 3 ) . The electronic polarizing unit has three constituents: a potential-control system, an rlcctronicscan circuit, and a n electronic linear residual current compensator. This unit has been substituted for the rlcctromechariical polarizing unit of the Q-1988 polarograph without any modifications to the current amplifier or to the subsequent computing and rccording portions of the polarograph. Presumably this electronic mntrolledpotential polarizing unit ~ o u l dalso be used with the recording portions of other polarographic instrummts to
obtain polarograms that are recorded on an effective voltage scalp. PRINCIPLES OF OPERATION OF ELECTRONIC POTENTIAL-CONTROL SYSTEM
The electronic potential-control system functions as follows: The current amplifier, which follows the polarizing unit, through negative feedback maintains the polarized electrode at ground potential, so that the voltage drop across the current meawing resistor is not seen by the polarographic cell. The use of a current amplifier has the additional advantage of enabling the use of high valued current-nie‘t4uiiig resistors for inereaced s e n d \ - i t y and a better signal-to-noise ratio because this does not result in any iR voltage 10,s on the recorded polarogram. The inputs to the potential control amplifier, each n i t h respect to ground, are. the scan potential; arid the sum of the initial potential and the potential difference that exists betn een the polmzcd electrode and the reference electrode. Thii latter potential is the effectire t ~ l lpotential, nithout any appreciable iR losses, since no current flons through the circuit loop that include< the refcrence electrode. By mean< of current feedback “through” the cell, the potential control amplifier maintains the potential of its two inputs equal. nith rc5pect to ground (the sum of the offsLt and error voltages are negligible 11ith this amplifier). That i-. this amplifier causes the value of cell current to flon which chould flon f o r
each corresponding value of the sum of the initial and scan potentials. The &em causes this potential difference to be identical with the effective potential between the reference and microelectrodes, independently of the magnitude of the cell current, and independently of the values of circuit and cell resistances. By means of the potentialcontrol system, one obtains recorded polarograms that shorn cell current 2s. actual effective potential between the microelectrode and the reference electrode. The controlling electrochemical factors in this current feedback method of achieving controlledpotential electrolysis are those governing the actual current-voltage relation.hips. The optional computer section in the Type Q-1988 polarograph (4)that follows the potential-control system enables the choice of recording instantaneous, average, or maximum current L I S . effective potential, or the recording of the derivative of the wave 1's. effective potential, d i / d E us. E.
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Figure 1 . Circuit diagram of controlled-potential polarographic polarizing unit with electronic scanning and linear residual current compensation
DfSCRlPTlON OF POTENTIALCONTROL SYSTEM
The circuit diagram of the polarizing unit is shown in Figure 1. -4 current amplifier, m-hich has bern described ( 4 ) , holds the potential of the polarized electrode-cg., D.IJ1.E.-at ground potential. The initial potential source, which also has been described ( 4 ) , inserts a constant potential of selected polarity and magnitude in series with the reference electrode -e.g., f3.C.E.: I n t i one input (terminal 1) of the difference amplifier. Essentially zero current is drawn through the reference electrode. The potential control amplifier is a commercial operational amplifier (5) that is modified as shown in Figure 1 to function as a chopper-stabilized difference amplifier. The output of the difference amplifier is connected to a platinum-wire electrode that is immrrsed directly in the solution. The three-electrode cell that is required has been described (3, 4). Although the relative placement of the three electrodes in the cell is not critical, as a precaution, the reference and platinum electrodes arc platwl on opposite sides of the polarized electrode and the tip of the reference elertrode is placed close to the polarized rlectrode. Cell current is supplied by the potentialcontrol amplifier and flows through the current amplifier. The feedback loop through the cell, by delivering appropriate current to tlie platinum dectrode, forces the potentials a t the two inputs of the difference amplifier to be equal with respect to ground. The scan potential is applied to the R input terminal of the difference amplifier. The over-all action of the potential mntrol system is, therefore, to force the e.m.f. between the polarized elec-
M-5X
trode and the reference electrode to equal the sum of the potentials of the initial potential source and the scan voltage source independently of the value of the eel1 current. There is no distortion of the forms of regular waves or derivative peaks by the iR losses that affect the form of n-aves obtained with convcntional polarographic polarizing circuits. DESCRIPTION OF ELECTRONIC-SCAN CIRCUIT
The circuit of the electronic scan portion of the polarizing unit is shown in Figure 1 and is outlined by a dashed line. In the electronic-scan method, the scan potential, which increases linearly with respect to time, is derived from the. output of an integrator that is supplied with a constant potential of suitable magnitudr. That is, the electronicscan circuit generates a linear voltage, scan by rharging a high quality tapacitor with a constant current of selected sign so that E,,,,
=
Q/C
=
f ldt/C'
= kt
This property of a capacitor is enforced through negative feedback a t high gain by an operational amplifier which has certain necessary characteristics that enable its use for this application ( 5 ) . The operation of this circuit as well as that of other closely related sawtooth generators, including bootstrap cathodefollon-er, single-tube Miller integrator, phantastron, sanatron, and sanaphant cirrnits, may be regarded ( 1 ) as depending upon an effect described by Miller-that is, this negative fredback
circuit is a variety of the Miller integrator class of sweep generators. I n effect, conditions are provided such that all of the current to be integrated flows without impediment into the capacitor and is stored there and such that the voltage that is developed across the capacitor may be utilized as a polarographic scan source 11 ithout appreciable drain of charge from the capacitor. The scan rate is adjusted by selecting, with the scan-rate switch, the value of a resistor that is connected in series with the constant voltage and the input of the integrator. Although not indicated in Figure 1, it is possible to insert a reversing switch between the output of the constant potential source and the electronic-scan integrator if it is desired to scan in either direction of polarization. Also, additional positions on the scan rat(, swit(Z1i can be used to provide other \tan rates by means of added resistors; an open position on this switch can be usrd t o cause the scan potential to remain at a value reached when tlie scan rate sn-itch is turned to the open position. The alternating current line operated constant potential source is a silicon diode rectified, Zener diode regulated circuit eimilar to that of the constant potential source used for the initial potential. A commercial operational amplifier (5), provided with capacitance feedback and resistance input, is used for the integrator. A bias adjustment is provided to balance out the zero offset of the integrating amplifier so that closing the scan switch will erase the output of the amplifier to zero volts. Opening the scan switch inimediatrly starts another scan cycle. VOL. 32, NO. 10, SEPTEMBER 1960
1263
The output of the integrator varies linearly from 0 to +lo0 volts in a length of time determined by the values of the constant potential, resistor, and capacitor, because the amplifier, through feedback a t high gain, enforces the impedance function of the capacitor. The scan voltage varies linearly from 0 to $2 volts in the chosen time interval. Scan rates of 100 mv. per minute and of about 18 volts per minute are obtained with the resistor values shown in Figure
1. DESCRIPTION OF ELECTRONIC LINEAR RESIDUAL CURRENT COMPENSATOR
At current sensitivities of the order of 1 pa. full scale and below, over reasonable span intervals, the residual current appears to increase linearly with EDME. To make available most of the recorder scale for the diffusion current, the counter current compensation method of Ilkovii: and Semerano ( 2 ) is applied in the linear compensator. A current of opposite sign, increasing at a selected constant rate, is applied to the input terminal of the current amplifier. The circuit diagram of the electronic linear compensator is outlined in Figure 1 by a dashed line. The slope potentiometer selects a fraction of the output of the integrator. A commercial operational amplifier ( 5 ) , provided with a resistive feedback network, functions as a n inverter-Le., i t multiplies the selected portion of the voltage output of the integrator by -1. By a switch, this negative compensating potential is applied through a 4megohm resistor t o the input of the current amplifier and in this way is passively transformed to a current that linearly compensates for the residual current. The bias potentiometer is adjusted to balance out the zero offset of the amplifier so that i t delivers zero compensating current \+-bile the scan switch is in the off position. During the running of a preliminary nonderivative polarogram, the slope potentiometer is adjusted to obtain a compensated limiting current that is parallel to the volage axis on the recorder. The linear compensator is not needed for derivative polarograms. The recorder zero-set can be more conveniently used to suppress the portion of the output of the derivative computer that is due to the slope of the residual current.
INSTALLATION OF ELECTRONIC POLARIZING UNIT
The electronic polarizing unit that is shown in Figure 1 has been installed in the prototype of a Node1 Q-1988 Oak Ridge h’ational Laboratory controlledpotential and derivative polarograph, which has been described (4). 1264
ANALYTICAL CHEMISTRY
In Figure I, the circuit is shown as it is when the function switch on the ORNL Q-1988 polarograph is in the operate position. The wiring of the initial potential source and of the current amplifier is identical with that described previously (4). The wiring a t the function switch is the same as before ( 4 ) except that the section originally in series with G+ is not used. The values of the resistors a t the scan rate switch are given approximately in Figure 1 because a t the time of fabrication of the electronic polarizing unit, they are trimmed to produce the desired scan rate values: trimming is necessary because of production tolerances on the values of the Zener diode voltage and the capacitance of the integrating capacitor. The Philbrick R-100B power supply (5) that is used to operate other electronic portions of the Q-1988 polarograph also delivers regulated and -300 volts to the electronic polarizing unit. The heater voltages for the amplifiers in the unit are obtained from the polver transformer in the Q-1988 polarograph. The Philbrick Type USA-3 amplifier ( 5 ) was modified to convert it to a chopperstabilized difference amplifier as shown in Figure 1. Circuit diagrams, specifications, and maintenance information for the Philbrick K2-X and USA-3 amplifiers and the R-100B power supply are available from the manufacturer ( 5 ) . The USA-3 amplifier is mounted in a USA-3-1\13 modular package (5) so that i t can be plugged into the chassis of the electronic polarizing unit. The error voltage indicator attached to the difference amplifier will be illuminated if the amplifier is unable to balance its inputs. It is possible to substitute for the error voltage indicator shown in Figure 1 the voltage limit type of error indicator that !vas used before (4). The Type 11-500 silicon diode is manufactured by Sarkes Tarzian, Inc., Rectifier Division, Bloomington, Ind. The Type 652CO Zener diode is manufactured by Texas Instruments, Inc., Dallas, Tex. If this polarizing unit is substituted for the polarizing unit in an existing polarographic instrument, the converted apparatus will record current us. effective voltage between the polarized and the reference electrodes. A three-electrode cell will be needed, similar to that described before (3, 4). The recorder of the polarographic instrument may be retained. It will be necessary to use the circuit of the polarizing unit described in this paper together with those of the current amplifier, voltage divider, and the zero set (4). A Philbrick R-100B power supply will be required. If desired, any or all of the computer sections that were described before (4) could also be added to the converted
+
instrument. These computer sections would provide for the recording of any or all of the follon-ing: average current polarograms, maximum current polarograms, derivative polarograms-all at controlled-potential.
PERFORMANCE OF ELECTRONIC CONTROLLEDPOTENTIAL POLARIZING UNIT
The performance of the Philbrick amplifiers and power supply has been satisfactory. These units have given good service The amount of electrical noise in the scan voltage is much less than that from an electromechanical circuit. I n the scan voltage obtained from a motor driven potentiometer, there are appreciable one cycle per turn variations when a niultiturn potentiometer is used that has a 50.1% linearity tolerance (3). This noise is sufficient to cause trouble n-hen polarograms are recordd a t very high sensitivity of solutions of very low species concentration. Slight mechaniral misalignment of the components of an electromechanical scanning unit will cause a n increase in the amount of this type of variation. For this reason, it is much easier to fabricate and to maintain the electronic polarizing unit; also, the cost of components is about the same in the two methods. I n addition, it is more convenient to operate the electronic scan switch than the magnetic clutch that was needed in the electromechanical system (4). The stability of the Zener diode regulated potential sources compares very favorably with that of batteries. However, good filtering of the initial potential source is required to avoid superimposing an alternating voltage on the voltage of the polarized electrode. It has not been found necessary to use a chopper-stabilized operational amplifier for the scanning integrator. The scan rate of the integrator is stable and reproducible over a period of months. The bias adjustment is not critical. The electronic scanning circuit performs very well for both regular and derivative polarography a t very high sensitivity. The electronic linear residual current compensator also uses an unstabilized amplifier. It is sufficiently stable for satisfactory use in compensating regular polarograms, even a t very high current sensitivity. Probably because of fluctuations in zero offset, the output is not stable enough for derivative polarography a t high sensitivity. As derivative polarography inhermtly compensates for linear regions of residual current, there is no need for the linear compensator in derivative polarography; therefore the extra cost of a chopperstabilized linear-compensator amplifier is not justified.
LITERATURE CITED
( I ) Chance, B., Hughes, V., MacSichol E. F., Sayre, D., Killiams, F. C., eds., ‘VaveformS.” R.I.I.T. Radiation Laboratory Series,’’ Vol. 19, 1st. ed., pp. 257-9, 278-88, and 664-5, LIcGraw Hill, Sew York, 1949. ( 2 ) IlkoviE, D., Semerano, G., Collection Czechoslov. Chem. Communs. 4, 176 (1932). (3) Kelley, M. T., Fisher, D. J., Cooke,
IT. D., Jones, H. C., “ControlledPotential and Derivative Polarography,” presented at, and to be published by Pergamon Press Ltd. in the Proceedings of the Second International Congress of Polarography, Cambridge, England, Aug. 24-29, 1959, pp. 158-
Inc., Boston, Mass., “GAP/R Electronic Analog Computers,” “-4pplications Manual for Philbrick Octal PlugIn Computing Amplifiers,” and catalog data sheets. RECEIVED for review January 2,5, 1960. Accepted July 1, 1960. Work performed under Contract No. W-7405-eng-26 at Oak Ridge National Laboratory, operated by Union Carbide Nuclear Co. for the U. S.Atomic Energy Commission.
182.
(4)Kelley, 11. T., Jones, H. C., Fisher, D. J., A N A L . CHEM.31, 1475 (1959). (5) Philbrick, George A., Researches
Automatic Coulometric Titrator Application to the Determination of Sulfur in Petroleum by High Frequency Combustion JOHN R. GLASS and EDWARD J. MOORE Research Department, Socony Mobil Oil Co., Inc., Paulsboro, N. 1.
In the determination of sulfur by combustion to sulfur dioxide, the sulfur dioxide must b e titrated a t the rate at which it is formed. An automatic coulometric titrator has been developed which relieves the analyst of titrating continually as the combustion proceeds. Titrant (iodine) is generated using from 0.2- to 5-second increments of constant current times to within 0.001 second. The generation of the titrant is controlled by amplified amperometric signals from a pair of detecting electrodes.
coulometric titrators have been described and the instruments have been reviewed by DeFord (3-5). When sulfur is determined by combustion to sulfur dioxide (1, 6),the sulfur dioxide must be titrated at the rate a t which i t is formed. An automatic coulometric titrator for this purpose is shown schematically in Figure 1. ARIOUS
+haft of the stirrer. The c..amber at the middle of the cell has a larger diameter to provide space for the ends of the four electrodes and the four b1atic.s of the stirrer. The chamber a t the upper part is baffled to permit the gases to leave the cell without loss of electrolyte. The combustion gases enter through the small inlet a t the left and make thorough contact with the electrolyte in the loner part of the cell. At the p w k of t h r combustion the SO? may lxl evolved momentarily at a rate fa-ter than iodine can be generated. The gradual mixing of the electrolyte iii the lower section with that in the middle section enables the titrator to keep pace with the SO2 evolution. The
the constant current supply. This cycle of operation ib repeated many times during the combustion uiitil no more sulfur dioxide is liberated. The clock totals the time during whirh the constant current supply is connected and generating iodine. The product of the current and time (coulombs) is a measure of the amount of titrant added, hence the amount of sulfur in t h sample. TITRATION CELL
The titration cell is shown in Figure 2 . The chamber at the lower part of the cell is a long narrow annular space between the cell wall and the enlarged
si ELECTROLYTE
I
CLOCK
GENERAL OPERATION
The sample burns in an oxygen atmosphere. The combustion products, JThich include sulfur dioxide, are swept into the titration cell by a stream of nitrogen. The sulfur dioxide is absorbed and continuously titrated with coulometrically generated iodine. The detecting electrodes respond to the iodine concentration. When the iodine concentration is below a preset level, the detecting electrodes cause the amplifier and relay to connect the constant current supply to the pair of generating electrodes, thereby coulometrically generating free iodine. K h e n the iodine concentration rises above the preset level, the detecting electrodes cause the amplifier and relay to disconnect
L
CONSTANT CURRENT SUPPLY
AMPLIFIER
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Figure 1 .
Schematic diagram of apparatus VOL. 32, NO. 10, SEPTEMBER 1960
1265
