ACKNOWLEDGMENT
fication of the toluene derivatives, which may complement the estimation from the ring proton region. The methyl proton shifts are listed in Table I. Roughly speaking, the behavior of the methyl proton shift in the 0-,m-, and p-monosubstituted toluenes is similar to that of the 0-,m-,and p protons in the monosubstituted benzenes, respectively (19). However, some studies of the methyl proton shift in the toluene derivatives have already been made (4, 30), and further discussion is not intended in this work.
The authors express their gratitude to Tokyo Kasei Kogyo Co., Ltd., by whom the majority of the samples were provided. They are also indebted to Hiroshi Tomita for the analysis and preparation of the samples by gas chromatography, to Masaru Yanagisawa, Tomoko Tanahashi, and Yukio Mochizuki for their help in the experimental work, and to Fumiko Taka for her data compilation.
(30) H. Yamada, Y. Tsuno, and Y. Yukawa, Bull. Chem. SOC.Jap., 43,1459 (1970).
RECEIVED for review February 18, 1972. Accepted April 11, 1972.
Rapid, Phase-Sensitive, Three-Electrode Alternating Current Polarography A. M. Bond Department of Inorganic Chemistry, University of Melbourne, Parkville, 3052, Victoria, Australia
D. R. Canterford Department of Physical Chemistry, University of Melbourne, Parkville, 3052, Victoria, Australia Previously, short controlled drop times have been employed with considerable advantage i n ac polarography to permit faster scan rates of potential and short recording times. I n this work, the possibility of using 3electrode phase-sensitive detection with this so-called “rapid” ac polarographic technique has been investigated. Results show that theoretical relationships derived for natural drop time ac polarography can be extended to the rapid ac method. Thus, with 3-electrode phase-sensitive instrumentation, excellent discrimination against the charging current is still obtained at short controlled drop times. I n fact, the degree of discrimination under rapid conditions was better than with natural drop time phase-sensitive ac polarography. This was particularly evident at high frequencies. With the rapid phase-sensifive ac technique, copper(l1) and cadmium(l1) could be detected down to the 5 x lO-’M to 1 x 10-6M level. The introduction of phase-sensitive readout to the rapid ac technique, therefore, provides a considerable improvement to results reported previously with non phase-sensitive instrumentation. Indeed, the technique appears to be highly attractive i n many aspects, having the advantage of permitting fast scan rates, and thus short analysis times, while maintaining excellent discrimination against the charging current. This i s i n contrast t o other polarographic techniques where the use of short drop times necessarily results i n a lowering of ability to discriminate against the charging current, and therefore a loss in sensitivity.
IN A RECENT ARTICLE (I), Bond pointed out the considerable advantages of using rapid alternating current (ac) and direct current (dc) polarographic techniques, with short controlled drop times, compared with the usual polarographic approach of using natural drop times of between about two and eight seconds. It was concluded in this article that the rapid techniques could be given much wider usage than presently accorded. In rapid polarography, the use of short controlled drop times permits fast scan rates of potential to be used, without (1) A. M. Bond,J. Electrochem. SOC.,118,1588(1971).
loss in precision of measurement ( I ) . Thus, considerable time saving, compared with conventional polarography, may be achieved in recording a polarogram, which makes the technique most attractive for routine analysis. Ac polarography has many advantages over dc polarography, particularly for fast electrode processes (2-4). These advantages are still maintained at short controlled drop times, and endeavors to improve the rapid ac polarographic technique even further would seem most desirable. In practice, the detection limit for a particular polarographic technique often occurs at a concentration of electroactive species, at which the charging or capacitive current masks or “swamps out” the faradaic current. Therefore, it is not only the absolute magnitude of the faradaic current but also the ratio of the measured faradaic current to the measured charging current which often determines the sensitivity of a technique. Many of the extensions to polarographic techniques developed in the past 20 years have aimed at providing discrimination against the charging current. For instance, in ac polarography, the employment of phase-sensitive detection provides considerable improvement in the detection limit and the precision of measurement at low concentrations ( 4 , 5). With dc polarography, techniques such as Tast (currentsampled) and pulse polarography, also provide discrimination against the charging current. In a recent comparison of a wide variety of polarographic techniques, (6), it was observed that with the dc techniques (2) B. Breyer and H. H. Bauer, “Chemical Analysis, Vol. XIII, Alternating Current Polarography and Tensammetry,” Interscience, New York/London, 1963. (3) H. Schmidt and H. von Stackelberg, “Modern Polarographic Methods,” Academic Press, New York/London, 1963. (4) A. M. Bond, ANAL.CHEW,44,315(1972). (5) D. E. Smith in “Electroanalytical Chemistry,” A. J. Bard, Ed., Marcel Dekker, New York, N.Y., 1966, Chap. 1, Vol. 1. (6) A. M. Bond and D. R. Canterford, ANAL. CHEM.,44, 721 (1972).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
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-.-
2
8
6
4
'
l
b
AE(mV)
Figure 1. Plots of ac peak current ( I D - ) us. applied ac voltage ( A E ) for rapid ac polarography Drop time = 0.32 sec. Phase-sensitive detection Solution is 10-3M copper(I1) in 1M NaNOa Frequency: (a) 10 Hz (c) 500 Hz (b) 100 Hz ( d ) 1000 Hz
[CUIX IO' M
Figure 3. Plots of ac peak current U p - ) us. copper(I1) concentration for natural drop time and rapid ac polarography Phase-sensitive detection. LIE = 10 mV, frequency
= 100
Hz Natural drop time Rapid drop time
= =
2.9 sec 0.32 sec
All previous work on rapid ac polarography appears to have been of the non phase-sensitive form. From our preliminary results, the introduction of phase-sensitive detection seemed likely to provide a considerable improvement to the rapid ac technique and give rise to a highly attractive polarographic method, in which one can use fast scan rates and still maintain considerable discrimination against the charging current. The present work, therefore, describes an investigation of rapid phase-sensitive polarography and considers its potential for use in routine analysis.
I; 0
I'
IO 20 [FREQUENCY~
~2"l
30
Figure 2. Plots of ac peak current (I,-> us. square root of frequency for natural drop time and rapid ac polarography Phase-sensitive detection. Solution is lO+M copper(II) in 1M NaN03. A E = 10 mV Natural drop time = 2.9 sec Rapid drop time = 0.32 sec
of Tast and pulse polarography, as theoretically expected, the use of short controlled drop times decreased the ability of these techniques to discriminate against the charging current. Similarly, with conventional dc polarography, the ratio of faradaic-to-charging current becomes less favorable as the drop time is decreased. It is apparent, therefore, that rapid dc techniques, while permitting fast scan rates and thus short analysis times, have the disadvantage of providing less favorable faradaic-to-charging current ratios. With ac polarography, however, our previous work indicated that phase-sensitive detection provided substantial discrimination against the charging current at all drop times. 1804
EXPERIMENTAL All chemicals used were of reagent-grade purity. A stock solution of copper(I1) was prepared by dissolving copper metal in concentrated nitric acid. Excess acid was boiled off and the solution was made up to volume with distilled water. Cadmium(I1) solutions were prepared by dissolving cadmium(I1) carbonate in 1M hydrochloric acid, which also played the role of supporting electrolyte. Sodium nitrate, l M , was used as supporting electrolyte for the copper(I1) solutions. All measurements were made at 25.0 0.1 "C and solutions were deaerated with oxygen-free nitrogen. Polarograms were recorded with the PAR Model 170 Electrochemistry System (Princeton Applied Research Corporation, Princeton, N.J.). Short controlled drop times of 0.16 or 0.32 second were obtained with Metrohm Polarographie Stand E354. All potentials reported in this work are relative to a silversilver chloride reference electrode (Agl AgC1, 5M NaCl), connected to the polarographic test solution by a salt bridge containing 1M NaN03 for the copper(I1) solutions or 1M NaCl for the cadmium(I1) solutions. Tungsten wire sealed in glass was used as the third or auxiliary electrode.
*
RESULTS AND DISCUSSION Theoretical, The theory for ac polarography with natural drop time has been well established ( 4 , 5) and documented
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
Table I. Variation of Peak Potential and Half-Width with Frequency for Natural Drop Time and Rapid AC Polarography. Natural drop time Rapid [Edelpeak, Half[EdclPeak, Halfvolt os. width, width, Frequency, volt~cs. AglAgCl AgjAgCl mV mV Hz loo0 800
600 500 200 100 10
0.083 0.082 0.081 0.080 0.078 0.076 0.075
71 68 67 63 56
53 48
0.080 0.079 0.077 0.075 0.073 0.070 0.069
a [Cu(II)] = 1 X lW3M. Applied ac voltage sensitive detection used in both cases.
=
65
60 58 57 54 52 47
10 mV. Phase-
Figure 4. Comparison of phase-sensitive and non phase-sensitive rapid ac polarography of 10e3M copper(I1) in 1MNaNO3
(5). With controlled drop times, the evidence available ( I ) indicates that the same theory, in a modified form, is applicable, In ac polarography the charging current is 90" out of phase with the applied ac voltage, whereas the faradaic current is usually 45" or less out of phase, depending on the nature of the electrode process. In principle, complete discrimination against the charging current should be possible with phase-sensitive readout if measurements are made either in phase with the applied ac voltage, or 180" out of phase. The theory for phase-sensitive detection assumes that no effects from the ohmic iR drop are present. However, because of resistance effects, complete separation of the faradaic and charging currents is not possible and some charging current is always measured in any ac experiment (4, 5). The use of 3-electrode instrumentation eliminates much of the resistance and enables the theoretical expectations to be approached. However, even with 3-electrode instrumentation, uncompensated resistance still exists which may or may not be important, depending, for example, on the ac frequency used, the magnitude of the resistance of the dropping mercury electrode (DME), and the nature of the solvent. If the theory for natural drop time ac polarography can be extended to short controlled drop times, then it would be expected that with phase-sensitive, 3-electrode instrumentation, a high but not complete discrimination against the charging current could still be expected with rapid ac polarography, as is the case with the natural drop time technique. The results obtained in this work with phase-sensitive, 3-electrode instrumentation strongly coniirm previous evidence ( I ) that the theory for natural drop time ac polarography can indeed be simply extended to controlled drop time ac polarography and that excellent discrimination against the charging current can be readily achieved. The results of a detailed study of the
electrode process are reported below to support these arguments. It has been shown elsewhere (6) that the reduction of copper(I1) in 1M NaNO, is reversible in the dc sense, but is quasi-reversible, with reversible dc charge transfer, on the ac time scale. Comparative results for the Cd(I1)
+ 2e + -
Cd(ama1gam)
electrode process, which is reversible in both the dc and ac sense in lMHC1, are included where necessary.
Drop time = 0.32 sec. AE = 10 mV. Frequency = 1OOOHz (a) Phase-sensitive detection (b) Non phase-sensitive detection
AC PEAKCURRENT cs. APPLIEDAC VOLTAGE.Figure 1 shows plots of peak current (Ip-) cs. applied ac voltage (AE), at various frequencies, for copper(I1) under rapid phasesensitive conditions. For all frequencies a linear relationship is evident, as was also found with natural drop times. AC PEAKCURRENT cs. FREQUENCY. Plots of peak current us. square root of frequency for natural drop time and rapid phase-sensitive ac polarography of copper(I1) are shown in Figure 2. Both plots approach linearity at low frequency and extrapolate to the origin. As the frequency is increased, considerable curvature occurs, till at 1000 Hz, IP- is becoming close to being independent of frequency. These results are as expected for a quasi-reversible ac electrode process. For the reversible cadmium(I1) reduction, linear plots of ZP- us. frequency were obtained up to 1000 Hz with both the natural drop time and rapid techniques. This is consistent with theory for this class of ac electrode process. DEPENDENCE OF AC PEAKCURRENT ON CONCENTRATION. Over the range lO-,M to 10-6M Cu(I1) a linear calibration curve of lp- us. concentration was obtained. Figure 3 shows the calibration curve at low concentrations. For comparison, the corresponding natural drop time calibration curve obtained under the same conditions is included. The much lower current per unit concentration recorded under rapid conditions is evident. As the faradaic current is directly proportional to the surface area of the electrode, this would be expected ( I ) . DEPENDENCE OF PEAK POTENTIAL ON CONCENTRATION. For copper(I1) in 1M N a N 0 3 the ac peak potential, [&]peak, under rapid conditions was independent of concentration and equal to 0.070 =t0.002 volt us. Ag AgCl. These results refer to an applied ac voltage of 10 mV with frequency 100 Hz. This value can be compared with the [Edelpeak value of 0.073 0.002 volt us. AglAgCl for natural drop time ac polarography at the same applied ac voltage and frequency. The rapid and conventional dc polarographic half-wave potentials were 0.069 and 0.073 volt us. Ag AgCl, respectively. VARIATION OF PEAKPOTENTIAL AND HALF-WIDTHWITH FREQUENCY. For copper(I1) in 1M NaNO,, [,!?&]peak was found to become more positive with increasing frequency
*
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VOLT
AglAgCl
vs
Figure 5. Effect of frequency on natural drop time phasesensitive ac polarography Solution is 2 X 10-4M cadmium@) in 1M HCI. Drop time = 2.9 AE=10mV Frequency: (a)100Hz; (b)300Hz; (c)lOOOHz sec.
-64
-0s
-04
-07
-0a
VOLT vS&Aga
Figure 7. Rapid ac polarogram showing interference from low frequency beats Drop time = 0.32 sec. AE = 10 mV. Frequency = 100 Hz. Phase-sensitive detection. Solution is 2 X lO+M cadmium(II) in lMHCl
-04
-04
-05
VOLT vs.
-07
As/!
Figure 6. Rapid phase-sensitive ac polarogram of 2 X lO-4M cadmium(I1) in 1M HCl recorded at a frequency of 1000 Hz Drop time 10 mV
=
0.16 sec. AE
=
for both natural drop time and rapid phase-sensitive ac polarography, as shown in Table I. From this table, the halfwidth can be seen to approach, at low frequencies, the theoretical value for a reversible diffusion controlled ac electrode process [ i e . , 90/n = 45 mV for Cu(II)] for both techniques. For cadmium(I1) reduction, [Edclpeak and the half-width were independent of frequency for both techniques, as expected for a reversible electrode process. These theoretical-experimental correlations demonstrate conclusively that theoretical predictions made for natural drop time ac polarography can be extended to controlled drop time ac polarography, with a high degree of precision. Phase-Sensitive, Three-Electrode Detection and Uncompensated Resistance Effects with Rapid AC Polarography. In the theoretical section, discussion was limited to a consideration of faradaic currents. However, the total current in ac 1806
polarography consists of the vectorial addition of the faradaic and charging currents. Therefore, the usefulness of phasesensitive detection with rapid ac polarography will depend on the ability or the technique to discriminate against the charging current at short drop times. Figure 4 shows a comparison of phase-sensitive and non phase-sensitive rapid ac polarography for copper(I1) at an applied ac voltage of 10 mV and frequency of 1000 Hz. The enormous discrimination against the charging current with phase-sensitive detection is obvious. Because of the presence of ohmic iR drop effects, whether or not 3-electrode instrumentation is used, complete discrimination cannot be obtained. The degree of discrimination will depend on several variables, as follows. For ac electrode processes involving charge transfer, both the faradaic and charging currents increase with frequency. However, the charging current increases at a greater rate than the faradaic current; so the higher the frequency, the less favorable the ratio of faradaic to charging current (4). Furthermore, at higher frequencies, larger currents per unit concentration are produced, so larger ohmic iR drops are present, Therefore, the degree of discrimination is generally greater, the lower the frequency. The second major limitation placed on the degree of discrimination is the effective resistance present, which for the case of 2-electrode polarography is the total cell resistance. A 3-electrode system eliminates much of the cell resistance and is therefore considerably superior to a 2-electrode system. Even with a 3-electrode system, however, uncompensated resistance, particularly from the DME, is still present and ideality is not reached ( 4 , 5 ) . However, the magnitude of the uncompensated iR drop actually decreases with decreasing controlled drop time (drop size) in comparison with the same electrode at natural drop time. This can be shown as follows.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
-. 02
0.1
0
VOLT vs
-01
-02
w4c1
Figure 8. Rapid ac polarogram recorded at a frequency of 49.5 Hz Drop time = 0.32 sec. AE = 10 mV. Phase-sensitive detection. Solution is lO+M copper(I1) in 1M NaN03
The fundamental harmonic alternating current is directly proportional to the electrode area (5) and, therefore, to tZ‘’3 (7), whereas the resistance of the growing mercury drop is directly proportional t o t-’Ia (8). Thus, in ac measurements at a DME, the iR drop is proportional to t 1 I 3and therefore decreases, the shorter the controlled drop time. In other words, the effect of uncompensated resistance is less serious for rapid ac polarography than for natural drop time ac polarography. The above conclusions were readily confirmed, particularly at high frequency, as shown in Figures 5 and 6. Figure 5 shows how, with natural drop time phase-sensitive ac polarography for 2 X 10-4MCd(II), the measured charging current and the charging-to-faradaic current ratio both increase substantially with frequency. The higher the frequency, at constant drop time, the larger the alternating current and therefore the larger the iR drop. Thus the ability of phasesensitive detection to discriminate against the charging current decreases with increasing frequency as the uncompensated resistance term becomes more important. Figure 6 shows that for the corresponding rapid technique, very little charging current is measured even at a frequency as high as 1000 Hz (compare Figures 5c and 6). These results are in contrast to those expected with rapid dc polarography, since with this technique the direct current is proportional to l l f 6 (9), and therefore the iR drop is pro(7) J. Heyrovsky and J. Kuta, “Principles of Polarography,” Academic Press, New York/London, 1966, p 40. (8) I. M. Kolthoff, J. C. Marshall, and S. L. Gupta,J. Electroanal. Chem., 3,209 (1962). (9) J. Heyrovsky and J. Kuta, “Principles of Polarography,” .Academic Press, New York/London, 1966, pp 77-83.
portional to t-1/6. Hence uncompensated resistance effects become more serious in the case of rapid dc polarography in comparison with natural drop time dc polarography. Analytical Implications. From this work it was found that the application of phase-sensitive detection to rapid ac polarography provides considerable improvement over the corresponding non phase-sensitive form, and indeed, some improvement over phase-sensitive ac polarography with natural drop time. This feature, coupled with the fast scan rates possible, makes the rapid phase-sensitive technique very attractive for routine analytical work. For the copper(I1) and cadmium(I1) systems studied in this work, detection limits between 5 X IO-’M and 1 X 10-BM were found. Scan rates of up to 50 mV/sec could be used without any loss of precision. In ac polarography, the choice of a suitable frequency is often quite critical. Very high frequencies are not used because of unfavorable faradaic-to-charging current ratios and low frequencies are not used because of noise problems. For natural drop time ac polarography, a frequency of between 50 and 100 Hz provides a useful compromise ( 4 , 6), but with rapid phase-sensitive ac polarography the choice is not so critical and we have found that higher frequencies can be tolerated, providing a high faradaic current per unit concentration, with little noise, while maintaining satisfactory discrimination against the charging current. With the instrumentation used in this work, interference was encountered from low frequency beats as shown in Figure 7. This interference can be attributed to incomplete shielding and filtration of the mains frequency of 50 Hz. This was confirmed by recording a series of polarograms at I-Hz intervals around 50 Hz, showing that the interference was most severe at 49-50 Hz (see Figure 8). If adequate shielding and filtration are provided as is the case when the rapid drop timer is connected to a Metrohm Polarocord E261 via the Metrohm AC Modulator E393, then the rapid ac technique can be used without any interference from low frequency beats. CONCLUSION
From this work we have found that phase-sensitive detection in ac polarography with short controlled drop time can be used with considerable advantage. Theoretical relationships in ac polarography appear to be essentially independent of drop time. However, because of the lower iR drop associated with the short controlled drop time technique, better discrimination against the charging current is more readily obtained with rapid phase-sensitive detection than with natural drop time phase-sensitive ac polarography. This feature, combined with the much faster scan rates possible, permits us to conclude that rapid phase-sensitive ac polarography is considerably superior to conventional ac techniques. The high sensitivity and precision and the short analysis times possible make this technique extremely attractive for routine analysis. RECEIVED for review November 8, 1971. Accepted April 25, 1972.
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