Instrumental Methods of Derivative Polarography M. T. KELLEY and D. J. FISHER Analytical Chemistry Division, Oak Ridge Nafional laboratory, Oak Ridge, Tenn. ,The authors have studied several instrumental techniques for recording derivative polarograms with dropping mercury electrodes, because the derivative technique permits resolution of polarographic waves with closely spaced half-wave potentials. Their efforts were devoted largely to improving the use of simple resistancecapacity differentiating networks b y removing drop oscillations with minimum distortion. The parallel-T filter used to remove drop oscillations also permits effective application of the dual dropping electrode technique for recording derivative polarograms. The culmination of this work i s a derivative polarograph using a combination of a diode filter with the parallel-T filter. Although the waves recorded with this instrument still show some slight dissymmetry, they permit very good resolution of waves with half-wave potentials as close as 60 mv.
I
s MOST applications of polarography, the current is recorded a t a polarized microelectrode (such as the dropping mercury electrode) as a function of applied potential. The usual technique applies a linear rate of increase to the potcntial and the current is recorded a. a function of time (and hence of applied potential). The recorded polarographic r a v e s for reversible diffusion-controlled reductions a t the dropping mercury electrode conform to the Heyrovskf-IlkoriE equation:
Khen curves are recorded for systems involving more than one reduction reaction, the determination of the halfware potential and the diffusion current for the individual reactions becomes difficult n-lien the half-wave potentials arcs separated by less than about 0.2 volt. Experience of the authors and other investigators indicates that better reqolution of polarographic waves would be obtained if the derivatiye d i / d E n-ere recorded in place of the current itself. Differentiation of the Heyrovskj--1lkoviE equation gives the equation for the derivative curye R S follows:
The maximum value of the derivative
occurs a t E = El/* (the half-wave potential) and i = i d / 2 . At this point the derivative is:
di -=--
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(3)
Thus, the maximum value or peak height of a derivative wave is proportional to the diffusion current, and hence to the concentration of a reversibly reducible ion; the potential at which the peak occurs is the half-wave potential. I n practice it is difficult to determine d i / d E directly; however,
dE- di -dix - = dE
dt
(4)
dt
It is possible to record d i / d t directly using a constant rate of change of potential ( d E / d t ) . The values of d i / d t thus obtained are directly proportional to di/dE. A number of investigators have worked on the problem of recording
Figure 2. Circuit diagram of dual D.M.E. derivative polarograph
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voltage n-ith complex gating circuitry to eliminate this effect. Semerano and Riccoboni ( l e )and Heyrovskj. (6) have used two dropping mercury electrodes n i t h applied potentials differing by a few niillivolts to obtain differential polarograms. Because such polarograms suffered from the presence of
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Figure 1 . Derivative polarogram b y direct current tachometer method
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derivative polarograms. Leveque and Roth ( I O ) and Lingane and Williams (11) have used resistor-capacitor differentiating netIvorks, but the latter authors concluded that as the distortion of the waves was so severe the technique was not useful. Small alternating potentials superimposed on the main polarizing potential have been used by Breyer, Gutman, and Hacobian (S), Barker ( 2 ) ,and Hamm (6),and in the Cambridge Univector Voltaniograph (4). K h e n sine-wave potentials are superimposed, this method suffers from rather large condenser currents, especially a t low concentrations. Barker ( 2 ) and Hamm (5) used a square ware
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Figure 3. Dual polarogram
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VOL. 30, NO. 5, MAY 1958
929
beats between the dropping frequencies of the two electrodes, Airey and Smales (1) used a device to synchronize the drop times of the two electrodes. In the earliest viork done at this laboratory, a study of the Leveque and Roth (10) technique confirmed the conclusions of Lingane and Williams (11): Because of the loss of signal in a passive resistor-capacitor differentiating network, it was necessary to use longer time constants in this network than were most desirable. Because the differentiating network tended to pass the drop oscillations almost unattenuated while strongly attenuating the desired signal, it was necessary to follow the differentiating network by a very heavy integrating network. Both of these factors tend to distort the derivative wave badly. Little success was achieved until a filter was designed to attenuate drastically the current oscillations due t o the growth and fall of mercury drops without introducing the distortion in the polarographic wave characteristic of simple integrating filters. A quadruple parallel-T filter, which has been described in detail (7), was used for this purpose. It is a low-pass filter with a n attenuation of at least 30 db. for all frequencies higher than 0.2 cycle per second (drop times less than 5 seconds). A direct current tachometer was the first device tried for differentiating a polarographic wave. The tachometer was driven through suitable gears by the balance (pen) motor of a Brown recorder in a high-sensitivity polarograph (9), so that it showed its maximum output when the current showed the greatest rate of change. The tachometer output was fed through an integrating filter (to remove commutator ripple) and a voltage divider to a second recorder. The polarogram obtained (Figure 1) is well defined and symmetrical and shows deficiency in only one respect: The base lines where the rate of change of current was small show irregularities due to the discrete commutator segments of the tachometer. Probably an alternating current tachometer (perhaps a 400-cycle unit) and rectifier would remedy this situation, but the stepwise nature of the Brown recorder balancing would still introduce some of this effect. The dual dropping electrode technique was tried using the circuit shown in Figure 2. The use of the quadruple parallel-T filter unit completely eliminated the beats between the drop frequencies of the two electrodes which were reported by previous workers (6, 12). The filter enabled smooth derivative curves to be obtained without resorting to the drop synchronixation technique of Airey and Smales (1). The diffusion currents of the two electrodes were balanced by varying the
930
ANALYTICAL CHEMISTRY
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Figure 5. Unamplified RC derivative polarogram with parallel-T filter
Figure 6. Block diagram of electromechanical amplifier method of derivative polarography
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mercury head on one electrode, which changed the drop time as ne11 as the diffusion current. This balancing is accomplished more conveniently by making the current-measuring resistor for one of the electrodes variable. A derivative curve is shorn in Figure 3. The wave is well defined and has excellent symmetry. However, because the signal input to the recorder is the difference between the outputs of the electrodes, the sensitivity of the recorder must be high. Therefore the recorded curve s h o w considerable random fluctuation due to noise originating in the recorder amplifier. Other dis-
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APPLIED P O T E N T I A L
advantages are the necessity of using two dropping electrodes and two reference electrodes and of matching the diffusion currents of the two electrodes by varying the load resistors or the mercury head. The authors directed most of their effort in derivative polarography toward improving the Leveque and Roth technique. Figure 4 shows the results for
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Figure 12. Polarogram for solution of lead and indium (with diode filter)
this technique with the recording polarograph of the authors (note the large drop oscillations and the unsymmetrical wa,ve distortion). Figure 5 shows the results of introducing the quadruple parallel-T filter. Because it was still necessary to use a differentiating network of 20-second time constant to obtain sufficient signal, the peak of the derivative curve is broadened and the trailing edge of the peak is lengthened. The authors concluded, as had Lingane and Williams (II), that in order to use a differentiating network of more suitable time constant (1 to 3 seconds), the signal must be amplified to compensate for the loss occurring in such a network. An extra spiral slide-wire was installed on the Brown recorder of the conventional polarograph of the authors. Sufficient voltage was applied to this slide-wire so that a potential could be taken which was proportional to the input to the recorder, but larger. This arrangement and a typical derivative polarogram are shown in Figures 6 and 7, respectively. The method suffers from much the same difficulty as the tachometer technique: The discontinuous nature of the Brown recorder balancing leads to irregularities in the derivative curve, particularly where the rate of current change is small. Several other forms of voltage amplification n-ere tried using direct current amplifiers of the chopper type. The general form of the circuit for this
technique is shown as the voltage amplifier connection in Figure 8 and a typical derivative curve is shown in Figure 9. The chief fault of these derivatives is the presence of excessive noise in the output of the amplifier. This was characteristic of all amplifiers of this type if they possessed the required input impedance (about 1 megohm) and gain. This was remedied by altering the feed-back circuit of the amplifier so that the major part of the voltage drop in the measuring resistor was cancelled out by a potential of opposite sign generated in the feed-back loop of the amplifier. This current amplifier arrangement (shown as alternative connection in Figure 8) permitted the use of much higher valued measuring resistors and hence increased the magnitude of the signal to be amplified. The arrangement was a substantial improvement over any of the schemes tried as shown in Figure 10. Instruments employing this scheme have been in active use for several years. The symmetry of derivative polarograms can be improved if the damping system is modified still further. Because the current in most polarograms increases as the appliedvoltage increases, the output of the current amplifier is amplified to a value of several volts (25 volts gives full scale deflection of the recorder) and is then passed through a thermionic diode, The output of the diode charges a capacitor of appreciable size (20 microfarads). The discharge rate of the capacitor is made very slow so that the charge on the capacitor essentially follows the peaks of the drop oscillations. Following the diode comes the differentiating network, the parallel-T filter (usually reduced to two sections with the diode), a simple resistance-capacity filter (the capacity is reduced from that required without the diode), and finally the usual high impedance recorder. This arrangement has been the most satisfactory thus far. This instrument (shown in Figure 8 as an alternative connection) has been described (8). Figure 11 shows try0 derivative curves for thallium which are typical of the performance of this instrument. When only two sections of the parallel-T filter are used (dual-T curve), the wave height is slightly greater and the symmetry of the derivative wave is somewhat better. The slight residue of drop oscillations in this wave is hardly enough to make the wave height difficult to measure. The symmetry of the waves is poorer when multiple electron reductions are involved. I n Figure 12 the separation of the waves for lead and indium where the half-waves differ by 160 mv. is essentially complete. Symmetry of the waves could be improved by using a slower rate of scanning with, VOL. 30, NO. 5 , MAY 1958
931
however, a corresponding sacrifice in wave height. Figure 13 demonstrates the maximum resolution capabilities of the instrument. The two waves are those of thallium and lead with halfwave potentials differing by only 40 mv. The normal polarograms show no indication of an inflection point between the two waves, while the derivative curve clearly shows the composite nature of the wave. For quantitative determination, it would be necessary to consider the effect of each wave on the height of the other, but qualitatively the resolution is very definite. This instrument has been used for many derivative polarograms, including several which are irreversible (nickel in potassium chloride and iodate are examples of the latter). The derivatives of these irreversible waves are normal in shape but are somen-hat broader than would be expected. This is a distinguishing feature of this method of derivative polarography as compared to alternating current methods which show no waves a t all or greatly reduced sensitivity on irreversible waves. The instrument has found its greatest application in rather dilute solutions (10-6 to l O - 4 M ) in which maxima are seldom a problem.
Figure 13. Regular and derivative polarograms of solution of lead and thallium (with diode filter)
The dual dropping-electrode technique without synchronization gives well-defined derivatives, but will probably have limited application because the sensitivity is limited and the use of t\To electrode systems is awkward. The use of a tachometer as a differentiating device falls short of satisfactory performance because of mechanical defects of the available equipment. The use of resistor-capacitor differentiating networks, proposed by Leveque and Roth (IO),can be made to give very satisfactory polarographic n-aves. By adding parallel-T and diode filters and using a suitable current amplifier, this technique is satisfactory for analytical use, even though the shape of the waves still deviates from the theoretical equation.
SUMMARY
LITERATURE CITED
The use of the parallel-?' filter has made possible several techniques of recording derivatire polarograms.
(1) hirey, L., Smales, A. A,, Analyst 75,
287 (1950). (2) Barker, J. C., Brit. Patent 709,826 (June 2, 1954).
. .
Breyer, B., Gutman, F.. Hacobian S.,Australian J . Sei. Research 1 3 558 (1950). Cambridge Instrument Co., Ltd., 13 Grosvenor Place, London, S W, 1. Endand. Tech. Bull., Sheet 313. Hanim,-R. E , , Asn~.CHEJI. 30, 350 (1958). Heyrovskg, J., Chetii. / i 3 f y 40, 222 (1946); dyalust 72. 22cl (1947). Kellev. M. r.."FlshPr. D. J.. . ~ A L .
CH&. 28, 1,130 ( l n & j , ' (8) Ilelley, hl. T., Fisher, D. J., Southm-ide Chemical Conference, l l e m phis, Tenn., Ilecemher 1956: I S A Journul, to be ptibli.shed. (9) Kelley, 11. T., AIiller, H. H., .%XAL. CHEJI. 24, 1895 (1952). (10) Leveque, 3f. P., Roth, F., J . chitn. phys. 46, 480 (1949). (11) Lingane, J. J., Williams, R., J . A m . C'hem. SOC.74, 790-6 (1952). (12) Semerano, G., Riccoboni, L., Gazz. c h h . ital. 72, 297 (1942). RECEIVED for review September 2 j 2 1957. ilccepted December 26, 1957. Division of Analytical Chemistry, Beckman Award Symposium Honoring Ralph H. lluller, 131st Meeting, ACS, Miami, Fla., rlpril 1 0 5 i .
Analysis of Explosives by Nonaqueous Titration ROY D. SARSON Explosives Division, Olin Mathieson Chemical Corp., Mount Braddock Works, Mount Braddock, Pa.
,The value of nonaqueous titrations as applied to explosives and to the use of automatic differential titration with a view to reducing analysis time was determined. Trinitrotoluene, dinitrotoluene, pentaerythritol tetranitrate, and hexogen are titrated as acids in methyl isobutyl ketone. Nitroglycerin, nitrocellulose, mononitrotoluene, and ammonium nitrate are titrated as acids in dimethylformarnide. Inorganic nitrates are titrated as bases in glacial acetic acid. Trinitrotoluene, dinitrotoluene, and mononitrotoluene are differentially resolved by titrations in methyl isobutyl ketone and dirnethylformamide. Extraction of explosive mixtures with methyl isobutyl ketone separates nitrobodies from inorganic nitrates. Nonaqueous titration of explosives with indicators and the
932
ANALYTICAL CHEMISTRY
automatic differential titrator results in an accurate and time-saving means of quantitative analysis.
titrations with particular respect to explosives and their ingredients have been investigated. Ammonium nitrate and sodium nitrate are differentially titrated as bases in acetic acid-chloroform solvent. ii'itroglycerin, nitrocellulose, mononitrotoluene (MITT), and ammonium nitrate are titrated as acids in dimethylforniamide and ethylenediamine. Dinitrotoluene (DST), trinitrotoluene (TNT), hexogen ( R D X ; hexahydro 1,3,5-trinitro-s-triazine), and pentaerythritol tetranitrate (PETN) are titrated as acids in dimethylformamide and methyl isobutyl ketone (4 methyl-2-pentanone). J I N T , DNT, and T N T are OXAQUEOUS
differentially resolved n ithin certain limits. The insolubility of inorganic nitrates in methyl isobutjl ketone provides a means of separation and subsequent direct titration of some organic nitrates. Through the use of both indicators and the automatic titrator, routine analyses can be carried out with considerable saving of time and nith accuracy equal to present methods. The nitrometer has been the standard device available to the explosive cheniist for the determination of nitrate explosives. Although capable of excellent precision, this device iq somewhat cumbersome and requires maintenance. The titanous chloride method has largely supplanted the nitrometer for the differential analysis of explosive mixtures. This method is capable of good precision but, because of the neces-
