ANALYTICAL CHEMISTRY
1130
search Foundation. They are also grateful to Parke, Davis and Co. for financial help on the project. LITERATURE CITED
( I ) Eiclielberger, W. C.. La Mer, V. K., J . Am. Chem. SOC. 55, 3635 (1933).
(2) Fritz, J. S.,".%cid-Base Titrations in Nonaqueous Solvents," G. Frederick Smith Chemical Co., Columbus, Ohio, 1952. (3) Hall. X . F., Meyer, F., J . A m . Chem. SOC.49, 3047 (1927) (4) Higuclii, T., Concha, J., J . Am. Pharm. Assoc., Sei. Ed. 40, 174 (1951).
(5) Higuchi. T., Concha, J., Science 113, 210 (1951). (6) Higuchi, T., Danguilan, M. L., Cooper, A. D., J . Phys. Chem. 58, 1167 (1954). (7) Higuchi, T., Rehm, C. R., ANAL.CHEM.27, 408 (1955). (8) Pfeiffer, P., Ber. deut. chem.Ges. 48, 1796 (1915). (9) Spengler, H., Kaelin, A, Pharm.Acta H e h . 18, 542 (1943). (10) Van Lente, K., Pope. G., Trans. Illinois State Acad. Sci. 39, 77 (1946).
RECEIVED for review October I?, 1955. Accepted April 20, 1956. Based in part on thesis submitted b y Joseph A. Feldman t o the Graduate School, University of Wisconsin, August 19, 1955, in partial fulfillment of requirements for degree of doctor of philosophy.
Unique Polarographic Damping Circuit For Selective Elimination of Current Fluctuations Due to Dropping Mercury Electrode MYRON T. KELLEY and DALE J. FISHER O a k Ridge National Laboratory, Union Carbide Nuclear Co., O a k Ridge, Tenn.
The presence of current fluctuations due to the growth and fall of successive drops in dropping mercury electrode polarography causes some difficulty in measuring the height of polarographic waves for quantitative analytical purposes and is a severe limitation in all methods of derivative dropping mercury electrode polarography. A simple reliable filter circuit completely eliminates the drop oscillations from the recorded polarographic waYe with a negligible effect on the wave form, although the half-wave potential suffers some displacement, the magnitude and direction of which are dependent upon the rate and direction of scanning. One has merely to extend straight-line segments when measuring the wave height of a filtered wave. The filtered wave height is directly proportional to concentration. The filter is being used for established polarographic concentration determinations and for derivative polarography.
T
HE dropping mercury cathode is widely used in polarography
because i t uniquely exploits the advantages accruing from the properties of a high hydrogen overvoltage on mercury and of a freshly renewed electrode surface. I n addition t o the useful polarographic information present in the electrical dropping mercury electrode current, there are interfering components due to the charging of the double-layer capacity, to the giovth of diffusion current during the life of each drop, and to the severe transients caused by the fall of each growing drop. Though as applied t o established concentration determinations, the polarographic technique is relatively rapid, much tedious time is spent measuring "mid-points" of individual drop oscillations before the wave form corresponding t o the average current can be drawn nnd interpreted. An alternative method involves the nieasurement a t the peak of each oscillation, which requires a rigid control of recorder response and damping conditions, as it is almost impossible to build any sort of polarograph that is capable of recording the oscillations with no damping whatsoever. I n differential or in derivative polarography involving t r o dropping mercury electrodes ( I ) , it is necessary to synchronize the two drops and highly damp the resultant current. The fluctuation of the current of a single dropping mercury electrode is also a severe limitation in other methods of derivative polarography ( d ) , because the instantaneous values of the derivative take enormous excursions. This necessitates heavy resistancecapacity (RC) damping, which in turn distorts the polarographic
wave form and makes it impossible to record the true derivative of the polarographic wave. [In this paper the term R C damping is used to designate damping using the commonly used simple integrating filter consisting of a variable resistor in one leg of the recorder leads plus a capacitor directly across the input t o the recorder (8).The time constant-i.e., the product of value of the resistor in megohms times the value of the capacitor in microfarads-determines the magnitude of the R C damping a t any given frequency.] The result of compromise on this dilemma is to limit the field of application of derivative polarography, sincc with the required heavy RC damping, superior peak resolution is not realized between waves of close half-wave potential. At this laboratory, a filter circuit has been developed which completely eliminates the drop oscillations from the recorded polarographic wave n-ith a negligible effect on the form of the wave. Curves a and b of Figure 1 illustrate the inadequacy of ordinary RC damping. With a customary degree of R C damping (curve a ) , the frequencies corresponding to the average polarographic current are passed almost unattenuated but those corresponding to the di opping mercury electrode frequency and its rich harmonic content are only somewhat reduced. (The frequency selectivity of such a simple netiTork is relatively poor.) Heavy damping (curve b ) can eliminate the dropping mercury electrode components but not without reducing the magnitude of the lon-er frequencies, which must be passed unattenuated if
'A \
63
\
! b l R E J E C T l O N C U R V E 3 U E TO H rE A5V0Y SRE CC D A M ? I N G O
( C l R E J C C T I O N C J R V E 3 U E TO OUACRUPLE PARALLEL-T R C FILTER
i 1
1
i
, , 02. 04 .C5 .14 2 ,a . E ,2 2C 4.C FQEQJENCY
Figure
13.0 20.0
200
ZOSC
CPS]
1. Comparison between RC damping quadruple parallel-T RC filter damping
and
V O L U M E 28, NO. 7, J U L Y 1 9 5 6
Figure 2.
Circuit diagram of single-section parallel-T RC filter
1131
ceived many hundieds of hours of usage with no malfunction. experienced. The high impedance of the filter restricts its immediate application to some commercial polarographs. The current-measuring recorders of these polarographs can be readily modified to use the circuitry described by Kelley and LIiller ( g ) , but such a modification might also require other wiring changes, since the original design of many polarographs did not consider the problem of alternating current pickup, which ran be disastrous to the operation of a high impedance recorder. I t is impractical to design a low impedance parallel-T filter because oil-filled capacitors of the capacity required are prohibitive in size and cost. Electrolytic capacitors are unsuitable, because their capacity is somewhat variable, depending on their previous history, and their leakage resistance is undesirably low and also variable. PERFORMANCE O F FILTER
the resulting wave form is to be undistorted. A frequency-selective filter is needed, for i t is the value of the average currents during the life of consecutive drops that is of interest. For the frequencies corresponding to the fundamental and several harmonic values of the drop times ordinarily employed in polarography, inductance-capacity filters are impractical because the very large but high Q inductors needed cannot be obtained. The parallel-T R C filter (6, 6) is frequency-selective and, fortunately, requires only inexpensive resistors and oil-filled capacitors. The circuit of a single-section parallel-T R C filter is s h o m in Figure 2. This filter very strongly rejects a frequency, fi given 1 by = where the symbols are as indicated in Figure 2.
mC,
The network should operate from a source whose impedance is small compared with R and into a load which is large compared with R.
The design null frequency of the quadruple parallel-T R C filter shown in Figure 3 is 0.2 cycle per second (a drop time of 5 seconds). The filter removes over 99% of the otherwise recorded excursions caused by the growth and fall of each drop of the dropping mercury electrode. The measured wave height is not identical n-ith that obtained without the filter. However, without the filter, mid-points of recorded oscillations are often arbitrarily taken as indicative of average current ( 3 ) . The output of this filter may indicate more nearly a true average of the current flon-ing a t any time than do the mid-points of recorded oscillations without this filter. The recorded half-wave potential is retarded about 40 mv. (for a scanning rate of 0.1 volt per minute), but this known retardation is not a disadvantage for concentration determinations and may be taken into consideration for identification purposes. The precision with which a single filtered wave or with which replicate filtered waves may be measured is a t least as good as that for the unfiltered waves. One has merely to extend recorded straight-line segments when measuring the wave height of a filtered wave. The relationship between wave height and concentration is linear, n-ith no degradation observed due to the presence of the filter. The relative effects of ordinary R C filters and of composite quadruple parallel-T RC filters terminated Kith a small amount
DESCRIPTION OF PRACTIC4L FILTER
A practical composite filter for polarographic use consists of four parallel-T R C filters Rhich reject in order the tenth harmonic, fourth harmonic, second harmonic, and fundamental frequency of the dropping mercury electrode, followed by a simple R C section having a short time-constant. If the design null frequency of the fundamental parallel-T filter corresponds to the lowest drop time to be used, the filter 1%-illbe equally effective for faster dropping rates as well, because the composite filter is essentially a low-pass network n-ith an attenuation of a t least 30 db. for all frequencies above the fundamental design frequency. The quadruple parallel-T R C filter, curve c of Figure 1, has a much sharper cutoff than does a simple R C damping filter. The combined filter is frequent--selective, rejecting the diopping mercury frequency and its harmonics, but passing virtually unattenuated the polarographic n-ave frequency. The use of a snitch is suggested, so that the filter may be removed from the circuit n-henever desired. I t is sometimes convenient to record the oscillations-for example, n-hen diagnosing troubles due to diop irregularities. The circuit of a practical quadruple parallel-T RC filter is shonm in Figure 3. It is inserted in series (1-4, 1B) with the high impedance lead going to the simple R C filter network of the polarograph, and point 2 is returned to the ground bus. The impedance of the filter is high, but the introduction of it causw no difficulty in a polarograph using a high impedance recorder such as that described by Kelley and Miller ( 2 ) . I t has been installed in all polarographs u s ~ dat this laboratory, and has re-
P L 4 D R UP iE ?A R ALL EL-? R C F -TER S A M P ' N G A N D R C D A h ' ? l h G Of C ' 5:; 75 I / m l . T l 0 lMChOj
C - POLAROGRAM W I T H HEAVYR C D A M " ! N G 0' 53 S;;
-
7 5 y / n I TI' C,Md\3)
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, iB L--__ S.6 i' 03
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35
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Figure 4. Effect of heavy RC damping and quadruple parallel-T RC filter damping on log i / ( i d - i) TS. Ed.e relation
1132
ANALYTICAL CHEMISTRY
of R C damping are shown from different viewpoints in Figures 4 and 5 . The data for both figures were obtained from polarograms of a solution containing 7 5 y per ml. of thallium(1) in 0.1M potassium nitrate. The observed relationship bet\veen log z/(id - i) and E d . o . is plotted in Figure 4. Here it is seen that the distortion introduced by heavy R C filter damping is much worse than that due to the parallel-T R C filter. The polarograms from n-hich the dat,a of Figure 4 were taken are shoFn in Figure 5 , which shows the relative effect upon the appearance of the recorded waves of heavy R C damping and of quadruple parallel-T R C damping. Again, one may readily see that the effects of heavy R C damping are much worse than those of parallel-T R C damping and that very little distortion is introduced by use of this filter. The retardation of observed half-wave potential due to the time lag of the parallel-T R C filter is seen clearly in the plot in Figure 4. For reversible reactions a t the dropping mercury electrode one may make two polarograms x i t h the filter in succession in first the forward and then the reverse direction of polarization. It is seen in Figure 6 that the arithmetic average of the two values of half-wave potential thus obtained is identical JTith that obtained in the normal direction of scanning with ordinary R C damping. This procedure has been used a t this laboratory as an indication of whether a particular reaction is reversible.
7 ( C
P.
N O R M A L F O L A R O G R A M RC D A M P I N G OF 0 1 SEC
8 POLAROGRAM WITH QUADRUPLE P 4 R A L L E L - T R C FILTER D A M P I N G A N D R C D A M P I N G OF 0 1 SEC
0 SP0.
C
P O L A R O G R A M WITH H E A V Y RC D A M P I N G
OF 50 SEC
i L!
-0.1
-0.2
-0.3
-0.4
-0.5
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E d ? VOLT5
VI.
-a7
q.8
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I
lllw
l
-ch.*7k*-.8
Figure 5.
I -0.2
-0.4 EdrlOLTS
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l
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I
-0.3
-0.4
-0.5
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E d e V O L T 5 vs.5 C
I -0.7
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) -0.9
E
Effect of heavy RC damping and quadruple parallel-T RC damping on wave form 75 r/m). TI 0.1 M h h O
+
APPLICATIOSS OF FILTER
Two principal advantages have been realized from the general use of the filter. There is a great reduction of time and labor in the analysis for concentration by established polarographic procedures, and the use of this filter drastically simplifies the attainment of a derivative polarographic wave. I t has been successfully used as an aid for obtaining derivative waves by each of several techniques, the description of which is beyond the scope of this paper.
b hOEMAL P O L I E O G I I M
8 PCLAROGRAM WITH FlLTER I N , FORWARD OlRECTiON OF P O L b R l Z l T l O N C FOLAROGRAU W I T H F I L T E R I N , fiEVERSE DIRECT O N OF POLbRlZATiON
ACKNOWLEDGXIENT
The authors wish to acknowledge ’the able assistance of Hugh H. Miller in obtaining the data presented in Figures 4, 5 , and 6. LITERATURE CITED (1)
1
0 5 Ip o .
Airey, L., Smales, -1.A., Analyst 75,
1
287-304 (1950).
( 2 ) Iielleg, AI. T., lliller, H. H., ANAL. C H E M . 24, 1895-9 (1952). (3) Kolthoff, I. M., Lingane, J. J., “Polarography,”
P.
\
39, Interscience, Kew
York, 1946. (4)
Lingane, J. J., Williams, R., J . Am.
Chem. soc. 74, 790-6 (1952). (5) Stanton, L., Proc. I.R.E.. Wates and Electrons 34, 447-56 (1946). (6) Terman, F. E.. “Radio Engineers’ Handbook,” PP. 918-20, AIcGraw-Hill, New York, 1943. RECEIVED f o r review December 12, 1965. Arcepted l f a r c h 24, 1956.
Figure 6.
Effect of quadruple parallel-T RC filter upon half-wave potential (for reversible reactions) 100 y/ml. TI + 0.1M KNO,
