1910
Anal. Chem. 1985, 57, 1910-1912
Registry NO.7-HC (N),93-35-6;7-HC (C+),58048-06-9;7-HC (A-), 32942-70-4.
LITERATURE CITED
'C
A-
Flgure 6.
values, showing that it is possible to postulate the following equilibria (Figure 6). Attempts to assign a band to the tautomeric form (T)were unsuccessful due probably to the obscuring of that band, which should appear at 421 nm (AA = 65 nm) or 397 nm (Ah = 20 nm) according to eq 1with A*, = 325 nm and A*, = 478 nm, produced by the other bands. On the other hand, when AA = 153 nm (A*,, - A*, for the tautomer), eq 1predicts a band a t 478 nm, but as the neutral-anionic transition appears a t 471 nm and its quantum efficiency is greater than that of the neutral-tautomeric form transition (5). Again, it was not possible to observe that band. The experimental data for Aems are in very good agreement with the theoretical values from eq 1, and this fact could mean that the contributions of the different species to the main bands are negligible.
Wolfbeis, 0. S.; Fiirlinger, E . ; Kroneis, H.; Marsoner, H. Fresenlus' 2 Anal. Chem. 1983, 314, 119-124. Dienes, A.; Shank. C. V.; Kohn, R. L. IEEE J . Quantum Electron. 1973, OE-9, 833. Sherman, W. R.; Robins, E. Anal. Chem. 1988, 4 0 , 803-805. Fink, D. W.; Koehler, W. R. Anal. Chem. 1970, 42, 990-993. Moriya, T. Bull. Chem. Soc. Jpn. 1983, 56, 6-14. Yakatan, G. J.; Juneau, R. J.; Schulman, S. G. Anal. Chem. 1972, 44, 1044- 1046. Nakashima, M.; Sousa, J. A.; Clapp, R. C. Nature (London).M y s . Sci. 1972, 235, 16-18. Beddard, G. S.; Carlin, M. S.; Davidson, R. S. J . Chem. Soc., ferkln Trans 1977, 2 , 262-267. Miller, T. C.; Faulkner, L. R. Anal. Chem. 1978, 48, 2083-2088. Rechsteiner, C. E.; Gold, H. S.; Buck, R. P. Anal. Chlm. Acta 1977, 95, 51-58. Gold, H. S.; Rechsteiner, C. E.; Buck, R. P. Anal. Chlm. Acta 1978, 103, 167-173. Gold, H. S.; Rasmussen, G. T.; Mercer-Smith, J. A.; Whitten, D. G.; Buck, R. P. Anal. Chim. Acta 1980, 722, 171-178. Lloyd, J. B. F. Nature (London),Phys. Scl. 1971, 231, 64-65. Lloyd, J. E. F.; Evett, I. W. Anal. Chem. 1977, 49, 1710-1715. Vo-Dinh, T. Anal. Chem. 1978, 50, 396-404. Inman, E. L., Jr.; Winefordner, J. D. Anal. Chem. 1982, 54. 2018-2022. Lloyd. J. B. F. Analyst (London) 1975, 100, 82-95. Andre, J. C.; Bouchy, M.; Nlclause, M. Anal. Chlm. Acta 1977, 92, 369-378. Blackledge, R. D. J . Forenslc Sol. 1980, D5, 583-588. Lloyd, J. E. F. Analyst (London) 1980, 105, 97-109. Vo-Dinh, T.; Gammage, R. E.; Martinez, P. R. Anal. Chem. 1981, 53, 253-258. Paul, M. A.; Long, F. A. Chem. Rev. 1957, 57, 1-45. Drexhage, K. H. "Topics in Applied Physics"; Voi. 1, Dye Laser: Schiifer, F. P., Ed.; Springer: Berlin, 1973: Chapter 4. I
ACKNOWLEDGMENT Acknowledgments are made to A. B. Pomilio for the coumarins and to A. L. Peuriot for his assistance in literature searching.
RECEIVED for review October 22, 1984. Accepted April 12, 1985. We are indebted to CONICET, SUBCyT, and UBA for financial support.
Experimental Comparison of Relative Responses for Alternating Current and Square Wave Polarography with Irreversible and Nearly Irreversible Systems Ari U. Ivaska* Department of Analytical Chemistry, Abo Akademi, 20500 Turku 50, Finland
Donald E. Smith' Department of Chemistry, Northwestern University, Euanston, Illinois 60201
The relative responses of ac and square wave polarography were studied in four lrrevetslble or nearly ilreverslble systems. The in-phase component of the fundamental harmonic ac current was compared wlth the net current of the square wave technique. The two methods were found to give very slmilar responses in all lour systems when the same frequency and amplitude were used in the pertubation wave forms.
We have noted that direct comparisons of data obtained by ac and square wave polarographic or voltammetric methDeceased (January 1985). 0003-2700/85/0357-1910$01.50/0
ods, using comparable experimental conditions (same input wave form frequency and magnitude, as well as the same reactant and electrolyte concentration, etc.) are essentially nonexistent. We think that this situation deserves some form of empirical study to determine whether the two alternating potential techniques produce similar or divergent Faradaic results. It appeared to us that it would be most useful to compare data responses for irreversible or nearly irreversible systems. Such measurements are the most challenging as far as charging current compensation is concerned. Published theory (1-4),supporting experimental results (5-IO), and a review chapter (11) have given clear evidence that ac polarography is applicable and useful with irreversible systems. Publications considering irreversible systems for square wave polarography are infrequent (12),but this is of no consequence to the present investigation. 0 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
1911
Table I. Peak Currents (iJ,Peak Potentials (Ep),and Peak Full Width at Half Maxima ( WIl2) for the Reduction of Systems A, B, C, and D A' Bb ac ac digital analog SQW digital analog SQW
i, rA E,V &I,,
mV
2.83 -0.965 185
2.79 -0.950 188
2.40 -0.975 185
0.425 -0.740 190
0.411 -0.760 188
CC
Dd
ac
k$Wl,,,
0.390 -0.750 186
ac
digital
analog
SQW
digital
analog
SQW
1.24 -2.060 142
1.31 -2.040 138
1.03 -2.060 138
2.14 -1.615 190
2.29 -1.600 185
2.02 -1.610 185
mV M E U ( C ~ Oin~1.0 ) ~ M NaC104. C, 1.0 X lo-$M anthracene M Cr(C104)3in 1.0 M NaC104at pH 1(HC104). bB,1.0 X "A, 5.0 X M HC104 in 1.0 M LiC104. in ACN-O.10 M TBATFB electrolyte (second wave). dD, 1.0 X
We present below an experimental comparision of responws for square wave and ac polarography with irreversible or almost irreversible systems. Comparisions are made using the net current in square wave polarography and the in-phase component of the fundamental harmonic current in ac polarography. The former technique uses a time domain approach to effect minimization of the double-layer charging current while a frequency domain strategy is used in the ac technique. Four systems were selected for this comparison. They are (A) reduction of Cr3+ in aqueous 1.0 M NaC104, adjusted to pH 1using HC104, (B) reduction of Eu3+in aqueous 1.0 M NaC104, (C) reduction of the anthracene radical anion to its dianion in acetonitrile (ACN)-O.l M tetrabutylammonium tetrafluoroborate (TBATFB), and (D) reduction of solvated hydrogen ion in 1.0 X M HC104, 1.0 M LiC104. Systems A and E) are irreversible due to very slow heterogeneous charge transfer (7,8). System C is irreversible because of rapid protonation of the dianion (6). System D obviously consists of several steps (13). Nevertheless, the overall reaction is totally irreversible. EXPERIMENTAL SECTION
in the square wave experiment but the specified ac frequency was taken from an array of frequency domain responses. The electrochemical cell used the standard three-electrode configuration. The auxiliary electrode was a platinum wire in all cases. The reference electrode was a SCE in aqueous media and a Ag,AgI10.10 M tetrabutylammonium iodide-ACNI when acetonitrile was used. The working electrode was a dropping mercury electrode with a mechanically controlled drop life of 1 s and the flow rates were 1.17 mg s-l in the analog system, 1.30 mg s-l in the digital system for systems A and D, and 0.728 mg s? for systems B and C. All analog system data were normalized to digital system flow rates before presentation. Argon (Matheson Gas Products) was used to degas the cell solution. It was purified enroute to the cell by a column filled with molecular sieves followed by an oxygen scavenger (Ridox) column and a gas bubbler solvent saturation system. AI1 measurements were performed under a blanket of argon.
RESULTS AND DISCUSSION The digital ac and square wave polarograms of system D are illustrated in Figure 1. Results of all four systems are summarized in Table I where the peak currents, peak potentials, and the peak full width at half maximum are given. Admittance data were converted to ac currents using
The reagents HC104 (Mallinckrodt, Inc.), NaC104 and LiC104 (G. F.Smith Chemicals), and Cr(C104)3.6H20and E U ( C ~ O ~ ) ~ . H ~ O The most important conclusion to be reached from Figure (Alfa Chemicals) were used without futher purification. An1 and Table I is that the peak characteristics are very similar thracene (Aldrich Chemical Co.) was recrystallized from methanol in ac and square wave polarograms for aJl four systems studied. before use. Acetonitrile ("distilled in glass", Burdick and Jackson Ac responses seem to be slightly larger than the observed Laboratories, Inc.) was dried by passing through an alumina column immediately before use (14). Tetrabutylammonium square wave responses, but this is of little consequence because tetrafluoroborate ("electrometric grade", Southwestern Analytical the differences are within the realm of small variations in the Chemicals, Inc.) was used without further purification. Highly technique application. purified H20was obtained by pwing departmental distilled water The ac and square wave responses of the chromium system through a Millipore Corp. "Milli-Q" water purification system. (system A) also were studied as a function of concentration. An analog phase-selective fundamental harmonic ac polaroIn both cases linear responses were obtained to concentrations graph with positive feedback iR compensation (15) and a comas low as 5 X M and detection limits ( S I N = 2) were puterized array processor enhanced Fourier transform Faradaic estimated to be 1 X 10" M for both methods, again showing admittance measurement device (FT-FAM) (16)were used for close similarity between the two methods. Other observable5 ac measurements. Square wave measurements were done with illustrating the similarities are the following. First, with rethe computerized measurement system. In the latter case the square wave exitation system was computer generated so that it versible systems one predicts and observes that the current had the same amplitude and frequency as the sinusoidal ac signal magnitudes are proportional to the square root of frequency used in the ac experiments. One cycle of a symmetric square wave for both techniques. With irreversible systems the rigorous was superimposed on a staircase dc voltage at the end of each ac polarographic theory (3)predicts a very slight dependence period. A new mercury drop was used for every staircase dc of the peak magnitude and peak potential on frequency. This voltage. The current was digitally sampled with 20-pus sampling was observed in these studies for both techniques, again intervals and averaged from the last 5% of the points for each demonstrating the similarity of ac and square wave observahalf-cycle. T h e differencecurrent magnitude was then calculated. bles. In light of this empirical evidence it is obvious that both In both techniques the exitation signal had an amplitude of 40 phase selective ac and square wave techniques are as powerful mV peak to peak, or AE = 20 mV, and a frequency of 97.7 Hz. In the FT-FAM case the pertubation was done the same way as in studying irreversible or nearly irreversible systems. The
1912
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
Registry No. TBATFB, 42942-5; Cr, 7440-47-3;Eu, 7440-53-1;
H+,12408-02-5;NaC104,7601-89-0; LiClO,, 7791-03-9 anthracene radical anion. 34509-92-7.
LITERATURE CITED
POTENTIAL, V
Z
w
a 1.0 a 3
O0.5
0.D
-1.3
-1.4
-1.5
-1.6
-1.7
-1.8
-1.9
Delmastro, J. R.; Smith, D. E. J. Elecfrmnal. Chem. 1967, 14, 261-288. fimmer, B.; Siuyters-Rehbach, M,; Siuyters, J. H. J. Necfroanal. Chem. 1967, 74, 189-180. Smith, D. E.; McCord, T. 0. Anal. Chem. 1988,4 0 , 474-481. Ruzic, I.; Smith, D. E.; Feldberg, S. W. J. Necfroenal. Chem. 1974, 52, 157-192. Tirnmer, B.; Siuyters-Rehbach, M.; Sluyters, J. H. J. Elecfroanal. Chem. 1967, 14, 181-191. MoCord, T. G.; Smith, D. E. Anal. Chem. 1970,42, 2-8. Schwall, R. J.; Ruzic. I.; Smith, D. E. J. €/echoanal. Chem. 1975,6 0 , 117-123. Matusinovic, T.; Smith, D. E. J . Elecfroanal. Chem. 1979, 9 8 , 133-139. Matusinovic, T.; Smith, D. E. Inorg. Chem. 1981, 20,3121-3122. Bond, A. M. Anal. Chem. 1972, 4 4 , 315-335. Smith, D. E. CRC Crit. Rev. Anal. Chem. 1971,2 , 247-343. O’Dea, J. J.; Osteryoung, J.; Osteryoung, R. A. Anal. Chem. 1981, 53, 695-701. Heyrovsky, J.; Kuta, J. “Principles of Polarography”; Academic Press: New York, 1966; p 235. Schaar, J. C.; Smith, D. E. Anal. Chem. 1982, 5 4 , 1589-1594. Brown, E. R.; McCord, T. G.; Smith, D. E.; DeFord, D. D. Anal. Chem. lQ66,38, 1130-1 136. Schwall, R. J.; Bond, A. M.; Loyd. R. J.; Larsen, J. G.; Smith, D. E. Anal. Chem. 1977,49, 1797-1805.
POTENTIAL. V Flgure 1. Irreversible reduction of solvated H+ in 1.0 X M HCIO,, 1.0 M LiCiO,: (A) digital ac polarographic response: (B) digital square
wave polarographic response.
RECEIVED for review February 14, 1985. Accepted April 10,
techniques differ oely in the methods of measurement and data interpretation. It should also be remembered that the ideal square wave is composed of an array of odd-harmonic sine waves of increasing frequencies, and decreasing magnitudes.
1985. The authors thank National Science Foundation Grants (CHE77-15462 and CHE82-10831) for support of this work. Personal grants from the Orion Corporation Research Foundation (Finland) and Academy of Finland to A.U.I. also are acknowledged.
