A Coulometric Borane Monitor ROBERT S. BRAMAN,' DONALD D. DeFORD,2 THOMAS N. JOHNSTON, and LAURENCE J. KUHNS Callery Chemical Co., Callery, Pa.
b A continuous, automatic coulometric titration system was devised to monitor boranes in air. In this system, boranes scrubbed out of air into a sodium bicarbonate-potassium iodide electrolyte are titrated with coulometrically generated iodine. When compared to colorimetric procedures, this titrator was capable of determining diborane and decaborane in concentrations at least as low as 0.2 p.p.m. by volume. It is theoretically capable of detecting boranes in air at concentrations of 0.01 p.p.m. by volume. The gas sample flow, the magnitude of the coulometric generation current, the stoichiometry of the iodine reaction with the boron hydrides, and interferences were studied in relation to their effect on the system.
C
attention hab been given to the toxicological effects of boron hydrides. Because of their toxicity and volatility, boron hydrides in air are a safety hazard to personnel working in areas where they are ubed. 'rhe maximum allon able concentrations of diborane, pentaborane, and dec:tboiane have been set a t 0.1, "as w a r to zero as possible," and 0.5 p.p.m. [ volume), respectively, by the American ronference of Governmental Industrial Hygienists (1). Development of a continuous, automatic method for determining boranes i n air involves the following: a system for scrubbing boranes out of air, an appropriate physical or chemical method ior determining the scrubbed boranes, OxsinmmLL
1
Present addreah. .\rniour
Research
1 'oundation of Illinois Iristitute of Techtiology, Chicago, Ill. 2 Present address, Department of Chemi.try. Xorthwrrutwn rniversity, Evanston, Ill.
and instrumelitation to make the determination continuous and automatic. Several methods for determining boron hydrides in air or g:s mixt,ures have bcrw rcyorted in the literature. Some inwl\-c ckterminat'ion of boric acid by either microtibration or colorimet'ry aftcr the boranes have been scrublwd out of :i gas sample and hycirolyzod (5, 6. 1 4 ) . The scmitivity of tlie most wi(k1y uscol colorimetric intithod for boric acid is in the range of 2 pg., adequate for tlie deterriiinatio~~ of low concentrations of boranes in air if ling. saniplcs are taken. However, nonc of the‘ coicwinietric iwthods is sirnplc. anti :iccurwy depends upon terhniyur and the abatwe of int,erferencw. I 3 c ~ " m of this, it was decidcd thnt no conventional colorimetric method for boric acid could be used in :L rcasonal)lr Iiionitoring system. ~Iic,i,otit,ratioiirtyuircxs about ten times morv h r i c n c d for reasonably :rccuratc analyses than colorimetry and, therefor(,, is too illsensitive for determining lox conoentrations of boranes in air. Hill and Johnston uacrl ultraviolet spectrophotometry for det,ermination of decahorane in air (Y), but because the ultraviolet method does not detect pentahorane, alkyl pentaboranes, diborane. or alkyldiboranes, it is unsuitable for the general monitoring of boranes in air. Hill and Johnston also used the quinoline - decaborane complex in a colorimetric procedure for decaborane ( 9 ) , but this method has essentially the same disadvantages as the ultraviolet mvthod and is even lms suitable as ai1 analytical method hccuuse color development is slow. The reducing power of the boron hydritlrs has been usc~i for analysis.
Table I.
Available Reducing Power of Some Boron Hydrides at Low Concentrations in Air Reducing MAC. hlAC, Boron Power, !3quiv. pg./'nis I'.P.lI. pequiv.,/Liter Hydride IGq./Mole Wt . of ;\ir (hy Vol.) of .lir 0,081 0.044 100 12 2.31 BzHs 0.017 0 037 20 2.67 1OU" &HI0 0 .0;36 0,0:32 20 3.16 100" B6H9 0 , 09:3 :300 0,055 40 :i. 06 ILuHic 1UW 0 018 0 033
These are not a c t w l masimiim allowable conc*rntr:itioii (11ACl) w l i w s , hiit arc) illrlided for comparison. 0
1258 *
ANALYTICAL CHEMISTRY
Feinsilver and Bean (6) used the rcaction of diborane with bromine in glacial acetic acid to measure 1 to 3 p.p.ni. of diborane with a precision of ztO.1 p.p.m. ( 5 ) . Only 1-liter air samples were required. Hill reported the use of iodine for analysis of dimethylamineborane in a similar procedure with comparable accuracy (8). Of the methods reviewed, those involving the oxidation of boranes with halogens held the greatest promise. The reaction stoichiometry of iodine with various boranes is very favorable. Table I presents data calculated from the maximum allon able concentrations from the reaction stoichiometries (11) of several boranm with iodine. Table I shows that a borane monitor operating on the basis of an iodine titration \ \ o d d have to produce and detect about 0.03 peq. of iodine if an air flow of I-liter per minute is used. Coulometric titration procedures are probably the best for tleterminmg small amounts of reducing materials v, ith iodine. For examplr. Ramsey. Farrington, aiid Swift reported titration of as little as 2 peq. of arseneous arsenic with coulometrically generated iodine, with rrlative errors of l ~ s sthan 0.2% ( I d ) . I n addition to allowing accurate analyses of small amounts of materials, coulometric titration can easily be adapted to continuous automatic analysis. Relatively feu investigators have described continuous, automatic coulometric analysis ( 2 , 4, I S ) . Shaffer, Briglio, and Brockman described an instrument for the determination of mustard gas in air and .lustin and his coworkers operated the Titrilog which employs electrolytically generated bromine. These instruments all operate with a variable coulometric current. The electrolysis current is automatically adjusted to a value just sufficient to titrate completely the reactive constituent in a sample stream, and the magnitude of the coulometric current IS a measure of the concentration of desired constituent in the sample stream. -1recorded plot of current 2's. time provides a continuous record of concentration as a function of time. If the zensitivity were adequate, this type of instrument n-ould undoubtedly be suit&le for monitoring boranes in air. DeFord, Branian, and Breese utilized :I constant generating current for con-
tinuous automatic coulometric analyses (3). The instrumentation however, was rathcr complex, and although it could be niodified to fit the needs of a continuous, automatic borane monitor, i t would not be superior to the titrator described here. .A nen, simplified, continuous, automatic coulometric titration system was developed, based upon the iodine reaction with boranes. The salient features of the system are the niethod of using the coulometric generation current and the amperometric detection system. These are operated alternately in a cycle by switching devices and a n appropriate chemical system. Boron hydrides in air are bubbled through an iodine solution in a reaction cell and titrated by the coulometrically generated iodine; the number of cycles registered or recorded is a measure of the amount of boranes present in the air samples. The applicability and sensitivity of the titration system were denionstrated with appropriate tests. Both batch saniples of boranes and gas sample containing horanc~swere titrated. APPARATUS
Design and Operation of Coulometric Titrator. Before designing t h e coulometric titrator, t h e boranc-indicating system had t o be investigated briefly. Since t h e iodine reaction vr-ith boranes was to be used in the coulometric titration, boranes could be detected indirectly as changes in iodine concentration in the titration cell or directly as reducing agents. T n o platinum foil electrodes (1 X 2 cm. and 1 X 1 cm. in area) with a n iinprrssed potential of about 0.2-volt direct current gave a signal of 25 pa. a t p H 5.5 with a pyridine-pyridinium buffer. Addition of 1.2 peq. of pyridine-borane to this solution reversed the indicating cwrrent by about 20 pa. Because excebs pyridine-borane gave less than half the signal of an equivalent amount of iodine, the indication of excess iodine was iised ratlirxr than excess borane to coiitiol the operation of the coulometric titrator. To use the amperonietric indication signal directly, a 0- to 100pa. metcr relay manufactured by Assembly ProduetF, Inc. (Model 35l-C), was employed. Amperometiic indication has been clificult to use in automatic coulometric titrators because of signal inducrd in the indicating electrodes b y the geneiating current. T o avoid this, the coulometric titiation system was designed t o operate the generating current and thc ampeiometiic indication circuit altri nately (Figure 1). This was done by a cycle timer (Industrial Timer Corp. Model &IC-1) which consisted of a series of microswitches activated by a seriw of cams rotating on the shaft of a synchronous motor. The microswitches were thus able to perform electrical functions a t time intervals set 1-y rotating rams.
I
I--
-r-
-47
I
R"
-
v
I_
- -"- J
C Y C L E TIMER
Figure 1
.
Wiring diagram for coulometric titrator
A.
Indication-system anode Main power switch S?. Cam-actuated microswitch for pulsing the register coil once per cycle Sg. Cam-actuated microswitch for keeping timer motor running during most of one cycle S1. Cam-actuated microswitch for controlling operation of indication system Sg. Cam-actuated microswitch for controlling generation time Sg. Manuol generation on-off switch Ry. Relay actuated by indication system for starting a cycle RI. 100-ohm Rz. 100,000-ohm RB. 10,000-ohm R d , Rg. 39,000-ohm 30,000-ohm, giving current of about 10 ma. R6. RI. 4700-ohm Power supply, conventional Sola transformer, 6 X 4 tube, and capacitor input fllter
SI.
Thr generating current of the titrator was supplied b y a simple, conventional, direct current power supply. A variety of generating currents could be set by varying a resistor in series with the generating electrodes. A generating current of 10.21 ma. fluctuated less than lYc undrr actual titration conditions. The titrator operates 111th an incremental, or "shot," type of action. Initially, the indicating circuit from the polarized platinum electrodes decreases to a value low enough to cause the meter relay t o contact a preset contact. il small curient through the contact then activates a heavier-duty relay which starts the motor of the cycle timer. The first microsnitch on the cycle tinier locks the cycle timer motor on for most of one cycle, independent of the meter relay. The second niicroswitch provides a short pulse of alternating curient to activate a register once for readout purposes. A third microsn itch turns off the indication circuit, and the fourth microsnitch then turns on the geneiating curient. Sfter the generating current is turned off by the microswitch on the cycle timer, the indication current is 511 itched on and the cycle timer is switched off. The cycle will reprat itself if the concentration of iodine in the reaction cell is sufficient to prevent the meter relay from switching. The percentage setting of the cams on the cycle timer and the switching functions performrd are given in Table 11. The generating period of the cycle timer was set a t 50% of the total cycle.
Since a n &second cycle was chosen, the total generation time was 4 seconds per cycle. The accuracy of the length of generation time depends upon the accuracy inherent in the cycle timer and upon the accuracy with which the rotary cams can he set to cover 50% of the total travel. Several checks of the generating period revealed that it was 4.00 seconds with no detectable deviation. This instrument was used at first. A slightly different model constructed by the Mine Safety Appliances Corp. was used in later experiments. This second instrument has a more sensitive meter relay; a -0.4 to +0.4 microammeter mas used in series with a zerosupression potentiometer. A stepping d a y differentiating circuit recorded the cycling rate as a function of time
Table II.
Cam Settings on Cycle Timer Per- Time
centage On, Switch Setting Sec. 1
5-95
7 2
2
6-10
03
3
65--5
4
10-60
Controla tinier motor Controls readout systmi
a
Controls indication system
40
Controls coulometric generation system
Off for most of cycle.
VOL. 32,
NO. 10, SEPTEMBER 1960
1259
sium iodide in 400 ml. of 0.531 sodium bicarbonate. This electrolyte gave a very low background when nitrogen was passed through the cell. The background was thought to be due entirely to volatilization of the slight excess of iodine present in the electrolyte during operation of the monitor. All experiments reported in this paper vere carried out with the bicarbonateiodide electrolyte, except for woik with pyridine-borane. OPERATIONAL PROCEDURES
Figure 2.
Record from borane monitor
a. Stepping switch output pattern b. Printout pattern output
on a strip-chart recorder. Differentiation was performed and recorded by a 20-step, stepping relay with a n automatic reset and a Varian Model G recorder. The stepping relay was actuated by pulses from the cycle timer. For each step about 0.5-mv. potential was applied t o the recorder. After 3 minutes the stepping relay reset. Figure 2,a, is an example of the recorded pattern. A third instrument was assembled by using the second borane monitor without the differentiating circuit and recorder. A Standard Instrument Corp. Tally Print recorder (Model TP-AR-4) was used as the readout device. This instrument recorded the number of cycles during each 5-minute period (Figure 2 , b ) . There is essentially no difference in the basic operation or current stability of these three instruments. Electrolysis Cell. T h e electrolysis cells used with both coulometric instruments were of essentially t h e same design. Figure 3,a. shon s t h e electrolysis cell used for all t h e reported esperiments, evcept t h e coulometiic titrations of pyridine-borane. One electrode served as a common anode for the generating and the indicating systemq. The magnetic stirrer did not disturb the indicating system of the titrator, but to avoid rapid loss of electrolyte volume through evaporation, it wai; necessary t o place a thin board or piece of asbestos between the magnetic stirrer and the titration cell. When in use for long periods, the magnetic stirier warmed the titration cell. Cell Electrolyte. Two different cell electrolytes were used in t h e evperiments. A pyridinium ion-pyridine buffer a t p H 5.5 in water consumed iodine at too high a rate to be useful for generating currents of 1.0 ma. An acceptable cell electrolyte was prepaled by dissolving 2 grams of potas-
1260
ANALYTICAL CHEMISTRY
Coulometric Titration. T o test t h e operation of t h e first recorder, a fenbatch and continuous titrations of various reducing materials were run. A titration cell was assembled (Figure 3,b), consisting of a generating anode, an indicating circuit anode: a cathode common to both systems, and a n air bubbler for st>irring. The cell was filled with 100 ml. of buffered electrolyte. T o operate the instrument, the meter relay was set and the titrator was turned on. The titrator immediately started to cycle and generated iodine. When enough iodine was present to yield an amperometric signal greater than the set signal, the titrat’or stopped cycling. Samples were added to the titration cell from a buret for continuous titration and from a pipet for batch titration. The procedure followed for the analysis of air with the second instrument was relatively simple; the sample was drawn through the sintered-glass tube of the electrolysis cell (Figure 3,a) by a pump. This method of air sampling was considered superior to pushing the air through the titration cell, because it avoided possible absorption of boranes onto the walls of the connective tubing and the pump diaphragm. Synthetic gas samples \\-ere prepared by passing dry nitrogen over a volatile boron hydride in a diffusion apparatus (Figure 4) similar to one described by RIcKelvey and Hoelscher (IO). Dir
Figure 3.
Electrolysis cells
a. In IO-cm. long, 6-cm. diameter bottle containing 1 5 0 ml. of solution A. Gas diffusion tube 6. Magnetic stirrer C. Platinum screen electrode (others were platinum foil) b. In 250-ml. three-necked flask A. Gas diffusiontube 6. Common cathode C. Platinum foil electrode (others were coiled platinum wire)
borane was dissolved in mineral oil; decaborane TTas used undiluted in the diffusion apparatus. Gas samples of constant composition mere obtained. Flow rates of sample gases were determined with a calibrated rotameter. Various flow rates nere used in comparing analysis procedures. il flow rate of about I liter per minute was acceptable for sampling air in routine monitoring. The results with the borane monitor and those with reference methods mere compaied by switching the sample gas stream from the monitoring cell to the scrubbers used in the reference method. At no time n-as any change in sample gas floiv rate indicated by a calibrated flowmeter after sn-itching the gas stream. Colorimetric Method for Decaborane. A method was based upon
1
Figure 4. System for comparison of methods for determining boranes in air A.
50-ml. flask containing borane used as diffusion apparatus B. Titration cell of the coulometric analyzer C. 3-way stopcock for diverting air sample
D.
Diffusion tube
the formation of a p-naphthoquinoline(benzo [f]quinoline)-decaborane comples for determining decaborane in air. Air was bubbled through a solution of 2.5% p-naphthoquinoline in xylene. The transmittance at 490 mp mas read after the xylene solution had been heated in a boiling mater bath for 5 minutes. Analytical results were obtained from a standard curve prepared fi,om pure decaborane. The deterniination range was 0 to 20 pug. of decaborane pel' milliliter. This method was superior to one based upon the quinolinedecaborane complex ( 5 ) . Colorimetric Method for Diborane. T h e procedure for t h e determination of dihorane in air was based upon the colorimetric method of Hatcher and \Tileox for boric acid ( 7 ) . Air cont'ainiiig dihorane was bubbled through 13 nil. of water in a Mine Safety Appliawes microimpinger. Boric acid producrd by the hydrolysis of bhe diborane was converted to a carmine complex by atlding 5 ml. of Concentrated sulfuric aciti and 5 ml. of carmine reagent to 1 nil. of scrubbing solution. The s o h tion resulting after 45 miiiut,es was compared to a reagent blank a t 585 nip in a Beckiiian DU sprctrophotonieter. T l i ~reagent was prepared b y dissolving 12.5 mg. of carminic acid (National Aniline Division, Allied Chemical Corp.) i n 500 ml. of concentrated sulfuric acid. A standard curye n.as prepared from 1)oi.ic acid.
Table 111.
Coulometric Titration of Pyridine-Borane Samples Sample
T ~ p of e Run
Batch (sample added in 2-ml. increments from 2-nil. pipet)
1 2 3
4 5
6
Continuous ($ampleadded dropvise from buret jl
7 8 9 10
Total Vol., 111. 8 10 10 14 20 20 21 18
16.40 12,Oi
Kumbere of Cycles
Borane, Peq. Presenta Found
34 41
62 8i 84 104
78 61 56
Av. 0
Calculations based on iodometric titration of stock solution of pyridine-borane.
Oxide. Several samples of arsenous oxide standard solutions were titrated with the second titrator a t 300 ua. (Table IV). Coulometric Titration of Decaborane. S a m d e s of recrvstallized decaborane in hexane 1vere"titrated batchwise with the second instrument set a t a generating current of 300 pa. (Table IV). Bicarbonate buffer was the titration medium. Determination of Decaborane and Diborane in Air. An air stream containing decaborane was analyzed by the borane monitor and colorimetry. Decaborane was diffused into the air stream in the apparatus described in Figure 4. The results of the four tests (Table lr)a t decaborane concentrations of about 0.3 p.p.m. show satisfactory agreement between the two methods. To help maintain good scmhhing efficiency, an excess of iodine was always maintained in titrations of the air sample streams. Results ivith the borane monitor and the carniinic acid colorimetric procedure in analyzing an ail, stream containing diborane n-ere also compared (Table Y). Agreement was acceptable.
EXPERIMENTAL RESULTS
I h t c h coulometric titrations of vari0.001.1- pyridine-borane w r o run 11)- pipetting samples directly into the titration cell (Figure 3,b). The generating current of the titrator n-as set a t 10.21 ma. and the meter relay a t 20 pa. The calculated results: (Tahle 111) were based upon titration of the pyridine-borane stock sample with standard iodinr. Samples in runs 1 to 6 were addetl in 2-nil. increments from a 2-ml. pipet. Continuous r u m were made by adding sample to the titration cell drop\>-ise from a buret. The titrator followed the buret flon- within one or two cycles. Coulometric Titration of Arsenous
011samounts of
%
14.4 -2.i 18.6 0.5 19.1 3.2 26.3 1.2 36.9 -0.3 35.6 -3.8 -0.9 44.0 -0.9 33.0 25.8 4.0 23.7 5.8 0 . 6 1 i 3 . 0 std. dev
14.8 18.5 18.5 26.0 37.0 37.0 44.4 33.3 24.8 22.4
45
Difference,
DISCUSSION
Laboratory tests revealed that the borane monitor is capable of determining boranes in concentrations near the maximum allowable concentration,
Table
IV.
Batchwise Titrations
Sumbers
Coulometric
peq.
of Cycles Present
Found Difference,yc
ilrsenous Oxide 11 50 52 48 52 54
0 141 0.617 0.61i 0.617 0.617
45 46 42 46
0,538 0.538 0.538 0.538 1.076
is a
0,137 0.622 0 646
0.597 0.646
-2 8
0,s 4. I -3.2
4.i
8.8 0 . 6 1 i 0.617 -4v. 2.16 + 4 . 6 std. dev.
Uecaboranen 0.5GO
4.2 6.5 -2.8 0.573 6.5 0.970 -9.i Av. 0 . 9 i 7 . 1 atd. dev. 0.573 0.523
40 equiv./mole present.
but it does have some disadvantages. All boranes react in the same way with the iodine, and therefore no differentiation among boranes is obtained. However, the maximum allon-able concentrations for all boranes are very low and any borane concentration indicated by the monitor is significant. Acetone and peroxides and any other
Analysis of Air by Borane Monitor and Colorimetry Borane >lonitor Colorimetry KO. Wt.; P.p.m. Wt., P.p.m. of cycles pg, (by vol.) Absorbance fig. (by vol.) Decaborane
Table V. .4ir Collection Run Liter/Min. Time, 1 h . 1 2 3
,t
0.487
1.?I 20
0,420 0.4'20
"0
0.420
47
88 85 93 12s
11.1 10.7 11.6 15.4
0.28 0.24 0.26 0.15
0,045 0.060
0,064 0,074
Diborane (0.420 liter/min.)
a
5
9.3 12,4 13.2 16.9
Difference5
0.25 0.29 0.30 0.16
Av.
-0.03 0.05 0.04 0.01 0.02 i 0.036 SD
Corrected for background of 0.02 pg./'min. P p.m. by vol., colorimetry minus borane monitor,
VOL. 32, NO. 10, SEPTEMBER 1960
1261
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 l potential, 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
