Anal. Chem. 1994,66,80&812
Adsorptive Square Wave Stripping Voltammetry for Determination of Azobenzene at Trace Levels Gang Xu, John J. O'Dea, Louise A. Mahoney, and Janet G. Osteryoung' Department of Chemistry, North Carolina State University, Raleigh, North Carolina 270958204
This paper describes the determination of azobenzene in trace amounts by adsorptive square wave stripping voltammetry (SWV). Tbe optimal SWV parameters found are step height A& = 5 mV, amplitude &W = 25 mV, and frequency f = 200 Hz. A detection limit of 3 X 1O-l2 M (0.5 pg mL-I) azobenzene was obtained after a 20-s preconcentration period at 0 V (vs SCE)at the static mercury drop electrode in 0.5 M acetate buffer (pH 4.7) with solution stirred. The correlation coefficients of the calibration curvesat concentration levels of 10-loM are greater than 0.999. The results of recovery tests for added azobenzene from 0.364 to 154.7 ng mL-l ranged from 93.3 to 101.2%,with relative standard deviations from 0.5 to 5.5%. The extremely low detectionlimit is due to reactant adsorption, discrimination against charging current, good signal-to-noise ratio provided by square wave stripping, and scope for optimization provided by the time and potential parameters of square wave voltammetry. The surface concentrationof azobenzeneat full electrode coverage is determined to be 1.2 X 10-lomol cm-*. This corresponds to an area of 1.38 nm2per molecule of azobenzene adsorbed on the mercury electrode, which agrees well with the calculated value of 1.35 nmz for a flat molecular orientation for azobenzene. Azobenzene was used as an effective insecticide and acaricide to control pests in fields and greenhouse~l-~ but discontinued for pesticide use in 1979.6 Azo compounds are widely used in industry as textile dyes, fur, leather, and polish colorings, and colorants in foods, drugs, and cosmetics. Many dyestuffs are azobenzene derivatives, such as Fast Yellow, Sudan Orange G, D&C Red No. 17 (Color Index No. 26 loo), and so on. As a result, azobenzene can become an environmental pollutant in the wastes discharged from relevant factories or impurities in colorants owing to use of impure starting materials or to incomplete reaction or side reactions during the dye manufacture. For instance, D&C Red No. 33 (Color Index No. 17200), which is permitted for use in ingested drugs, mouthwashes and dentifrices, and lipsticksin the United States,' is manufactured by coupling diazotized aniline with 8-amino- 1-naphthol-3,6-disulfonicacid in alkaline s ~ l u t i o n . ~ Azobenzene can arise either as a side reaction product of the oxidation of aniline or possibly from the decomposition of ( I ) Haring, R. C. J . Econ. Enromol. 1946, 39, 78. (2) Allen, W. W.; Nakakihara, H.; Schaefers, G . A. J. Econ. Enromol. 1957,50, 648. ( 3 ) Norman, P. A.; Spencer, H. Florida Enromol. 1952, 35, 19. (4) Budavari. S . , et al., Eds. The Merck Index, 1 lth ed.; Merck Rahway, NJ 1989; p 927. ( 5 ) Meister, R. T., et al., Eds. Farm Chemicals Handbook, Meister Publishing Co.: Wilioughby, OH, 1977; Vol. 63, p D25. (6) Reference 5. 1979; Vol. 65. p D25. (7) Marmion, D. M. Handbook of US.Colorants, 3rd ed.; Wiley: New York, 1991; pp 28, 112.
aoa
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6, March 15, 1994
diazoaniline during the coupling reaction.8 Azobenzene may exist as an impurity in some pharmaceutical preparations also, for example, in Ketazone (Kebu~one)~ or phenylbutazone.1C-12 Azobenzene has been reported to be harmful to the liver and to be a chemical ~arcinogen.l3-~~ Therefore, there is a need for a fast, sensitive, and reliable trace analytical method for monitoring azobenzene and for the study of its carcinogenic biomechanism. High-performance liquid chromatography (HPLC) with a variety of detectors is the commonly used method for determination of a ~ o b e n z e n e . 8 J ~ JThe ~ J ~lowest ~ ~ detection limit reported for determination of azobenzene using HPLC is 0.3 ng in a 20-pL sample solution (15 ng mL-I, 8.2 X 10-8 M).19 The electrochemical behavior of azobenzene has been studied extensively.20-2s In protic media (water, methanol, ethanol, etc., or their mixture), azobenzene reduction occurs in an one-step, two-electron, two-proton reversible reaction.23-2s
AZOBENZENE
HYURAZOBENZENE
In strong acid hydrazobenzene undergoes the well-known benzidine rearrangement to yield benzidine and other minor products.26 In acetate buffer at pH 4.7, the conditions of the present study, the benzidine rearrangement does not proceed at an appreciable rate and thus does not contribute to the voltammetric response. There have been some publica(8) Bailey, J. E., Jr. J. Chromarogr. 1985, 321, 185. (9) Poctova, M.;Kakac, B. Cesk. Farm. 1981,30 (9,159 (in Czech); Anal. Absrr. 1982, 42, 1E46. (10) Fabre, H.; Hussam-Eddine, N.; Mandrou, B. J. Pharm. Sci. 1984,73, 1706. (1 I ) Matsui, F.; Lovering, E. G.;Curran, N. M.; Watson, J. R.J. Pharm. Sci. 1983, 72, 1223. (12) Maslowska, J.; Duda, J.; Janiak, J. Farm. Pol. 1988,44 (12), 719 (in Polish); Anal. Absrr. 1990, 52, 10E34. (13) Sax, N. I. Handbook of Dangerous Marerials; Reinhold: New York, 1951; p 36. (14) Weisburger, E. K. Basic LijeSci. 1983,24,23. (Organ Species Specif. Chem. Carcinog.). ( 15) Some Aromaric Azo Compounds. IARCMonogr. Eual. Carcinog. Risk Chem. Man 1975.8. 7 5 . (16) Riggin, R..M:; Howard, C. C.;Scott, D. R.; Hedgecoke, R. L. J. Chromarogr. Sci. 1983. 21. 321. (17) Marmion; D.'M. Handbook of Colorants, 3rd ed.; Wiley: New York, 1991; p 361. (18) Sternson, Larry A.; DeWitte,Wayne J. J. Chromarogr. 1977, 137, 305. (19) Burcinova, A.; Stulik, K.; Pacakova, V. J. Chromarogr. 1987, 389, 397. (20) Hillson, P. J.; Birnbaum, P. P. Trans. Faraday SOC.1952, 48,478. (21) Castor, C. R.; Saylor, J. H. J. Am. Chem. Soc. 1953, 75, 1427. (22) Wawzonek, S.; Fredrickson, J. D. J. Am. Chem. Soc. 1955, 77. 3985. (23) Markman, A. L.; Zinkova, E. V. J. Gen. Chem. USSR 1959, 29, 3058. (24) Laviron, E.; Mugnier, Y. J. Elecrroanal. Chem. 1980. 111, 337. (25) Compton, R. G.; Wellington, R. G.; Bethell, D.; Lederer, P.; O'Hare, D. M. J. Electroanab Chem. 1992, 322, 183. (26) Oglesby, D. M.; Johnson, J. D.; Reilley, C. N.Anal. Chem. 1966, 38, 385.
00032700/94/030608088~.50/0 Q 1994 American Chemical Smbty
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tions12,27-29 on the electroanalytical determination of azobenzene; however, the sensitivity is insufficient for the analysis 0.0 of samples at trace levels of azobenzene. The adsorptive preconcentration of azobenzene has been used prefatorily in a polarographic determination scheme.30 A detection limit EIV M is reported in 50% methanol solution by linear of 5 X sweep voltammetry at the hanging mercury drop electrode ..........._. sw (HMDE). SCAN Adsorptive stripping voltammetry is widely recognized as - 0.7 a sensitive technique in electroanalytical c h e m i ~ t r y . ~ lIt v~~ 0 1 2 3 110 has been shown that adsorption of organic compounds which exhibit surface-active properties can be used as an effective time I s preconcentration step to lower detection limits in electroanFigure1. Potential-time waveform foradsorpthresquare wave strlpplng alytical determinations. The adsorptive behavior and the voltammetry. surface charge-transfer reaction of the azo-hydrazobenzene system on the mercury electrode have been studied by Laviron mulation time, ta, is equal to tc plus the time required, once and M ~ g n i e rand ~ ~ by O’Dea and Osteryoung.33 Both the scan starts, to reach the peak potential, E, azobenzene and hydrazobenzene are known to adsorb on the mercury surface. The inherent ability of pulse techniques to (2) t, = t, + IEp - Eil/fhE, decouple the Faradaic current from the non-Faradaic current is important for systems involving reactant a d ~ o r p t i o n . ~ ~ ? ~ ~ Among the pulse techniques, square wavevoltammetry (SWV) where Ei is the initial potential of a scan and f and AE, are plays a unique role since it offers the advantages of speed, the frequency and the step height of the square wave, sensitivity, and discrimination against background current. respectively. These advantages are due to high frequency, large pulse The square wave scanning potential waveform is shown in amplitude, and difference current measurement, respectively. the inset of Figure 1. The waveform consists of a symmetrical The rejection of background currents effects signal-to-noise square wave superimposed on a staircase with period 7 . The ratios which are large in comparison with those afforded by square wave amplitude is Esw,and the frequency f is equal to other commonly used voltammetric techniques. 1/2t, = 1 / ~ .The current is sampled at the end of both the Square wave voltammetry can be applied successfully for forward and reverse half cycles (at point 1 and point 2 in the detection of substances initially ads0rbed.363~ In parFigure 1). A difference (net) current is determined by ticular, Flores and Alvarez have indicated the advantage of subtracting the current measured at point 2 from that measured square wave voltammetry for determination of substances at point 1.38 For reversible reactions such as the reduction which undergo quasireversiblesurface reactions.39 The present of azobenzene, the net current is larger than either the forward work demonstrates that the combination of the adsorptive or reverse current. accumulation and square wave stripping is an extremely powerful analytical technique which can detect the analyte EXPER IMENTAL SECTION rapidly at bulk concentrations as low as subpicomolar. Azobenzene (Aldrich) was recrystallized three times from hot 90% ethanol. The remaining reagents were analytical In adsorptive square wave stripping analysis as applied grade. A stock solution of azobenzene was 1.4 mM in glacial here, the whole process is carried out on a single drop of a acetic acid. Working solutions were prepared daily from the stationary renewable mercury electrode of constant area. The stock solution and 0.5 M acetate buffer, pH 4.7. All solutions potential-time waveform is illustrated in Figure 1; AB is the were prepared in distilled deionized water (Milli-Q, Millipore). preconcentration step and BC is the stripping step. A new The stock solution and working solutions were kept in the drop is formed for each experiment. For the preconcentration dark immediately after the preparation. The solutions were step, the electrode is held at a potential favorable for deoxygenated with high-purity argon saturated with water. accumulation, for a conditioning time, tc. The total accuSquare wave experiments were performed by using a threeelectrode configuration with a static mercury drop electrode (27) Vanccsorn, Y.; Smyth, W. Franhin Anal. Chem. Symp. ser. 1980, 2, 299. (Electroanal. Hyg., Environ., Clin. Pharm. Chem.). (EG&G PARC Model 303A) of area 0.0108 cm2(small size), (28) Temerk, Y. M. Fresenius J . Anal. Chem. 1976, 282, 205. a saturated calomel electrode, and a platinum wire counter (29) Barek, J.; Berka, A.; Zima, J. Collect. Czech. Chem. Commun. 1985, 50, 1819. electrode. An EG&G PARC 273 potentiostat was used as (30) Barek, J.; Hrncir, R. Collect. Czech. Chem. Commun. 1986, 51, 25. the source of applied potential and as a measuring device, (31) Wang, J. Voltammetry Following Non-electrolytic Preconcentration. In Electroanalyfical Chemistry; Bard, A. J., Ed.;Marcel Dekker: New York, which was controlled by a PDP8/e minicomputer (Digital 1989; Vol. 16, pp 1-88. (32) Kalvoda, R.; Kopanica, M. Pure Appl. Chem. 1989, 61, 97. Equipment Corp.). Software developed in our laboratory (33) ODea, J. J.; Osteryoung, J. G. Anal. Chem. 1993, 65, 3090. provides precise control and timing of the operation. The (34) Delahay, P. J . Phys. Chem. 1966, 70,2373. (35) Holub, K.; Tessari, G.; Delahay P. J . Phys. Chem. 1967, 71, 2612. data were analyzed off-line on a 486-based PC (33 MHz) (36) Wcbbcr, A.; Shah, M.; Osteryoung, J. G. Anal. Chim. Acta 1983, 154, 105; computer by the data analysis program.33 1984, 157, 1, 17. For all experiments the accumulation was carried out at (37) Ribcs, A. J.; Osteryoung, J. G. Anal. Chem. 1990, 62, 2632. (38) Osteryoung,J. G.;O’Dca, J. J. Square-wavcVoltammetry. In ElecrrwMlyficol 0 V where no current flowed during the conditioning time, tc. Chemistry;Bard, A. J., Ed.;Marcel Dekker: New York, 1986; Vol. 14,p 209. (39) Rodriguez Flores, J.; Fernandez Alvarez, J. M. Elecfroanalysis 1992, I ,347. At the end of the conditioning time, the waveform was applied AnalytlcalChemistry, Vol. 66, No. 0, March 15, 1994
808
3 0)
A
t
7,o-
4
-
5.0-
\
3.0-
V -0.1
-0.2 E/V
.o-
-1
.ot
L
-2.01 0.0
1
-0.3
-0.4
-0.5
vs.SCE
Fmrr 2. Square wave voltammogram of 1 pM azobenzene In 0.5 M acetate buffer pH 4.7; A€, = 5 mV, E,* = 25 mV, f = 200 Hz; 4 = 5 s; (a) forward current, (b) reverse current, (c) net current.
with selected values of AE,,E,,, and$ The final potential of the scan was -0.7 V. In general, each experiment was repeated three times under the same conditions at an airconditioned room temperature of 22 O C . For solutions at nano- or subnanomolar levels of concentration, an E G t G PAR Model 305 magnetic stirrer, operated under computer control, was employed for convective mass transport. For higher concentrations the accumulation was carried out in quiescent solution by diffusion. For square wave voltammetry, in which the experimental result is a set of discrete points, the peak height is defined as the maximal value of the parabola defined by the largest current value and the two adjacent values. RESULTS AND DISCUSSION Optimization of Conditions for Adsorptive %ware Wave Voltammetry. The peak current was optimized by changing AE,,E,,, andfthrough the range 2-20 mV, 10-100 mV, and 50-300 Hz, respectively, using Orthogonal Design. The optimal square wave parameters were identified to be AE, = 5 mV, E,, = 25 mV, a n d f = 200 Hz, at which the effective scan rate is 1 V s-1. Figure 2 illustrates a typical square wave voltammogram of 1 pM azobenzene under these conditions. Unless otherwise specified, all experiments were performed with these SWV parameters. The voltammogram in Figure 2 displays that the contributions of the forward and reverse currents to the net current are approximately equal and the difference between the two peak potentials is only 0.008 V. This suggests that reduction of azobenzene to hydrazobenzene is reversible. As shown below, the maximal amount adsorbed is a monolayer. This layer neither is desorbed nor rearranges on the voltammetric time scale and the product layer can be reoxidized at will. The kinetics of this surface process have been ~haracterized.~~ To determine the optimal accumulation potential, the potential range from 0 to 4 . 4 V was examined with 5 X lo-' and 5 X M azobenzene solutions. Accumulation potentials ranging from 0 to -0.1 V yielded similar peak heights. The peak heights decreased and the baseline deteriorated as the accumulation potential was made more negative. The peak shape obtained by accumulation at 0 V is the most well-defined. The effect of acetate buffer (electrolyte) concentration on the peak current was tested over the range from 0.001 to 1 010 Analytical Chemlstty, Vol. 66, No. 6, March 15, 1994
0.0
-0.1
-0.2 E/V
-0.3
-0.4
-0.5
vs.SCE
F@n 3. Square wave voltemmograms of azobenzene with different concentrations of Triton X-100: 5 pM azobenzene In 0.5 M acetate buffer, pH 4.7, r, = 5 8; concentration of Trlton X-100 (v/v, %): (a) 0, (b) 0.0001, (c) 0.0003, (d) 0.0006.
M with 1 X 1 P M azobenzene. The peak height increases with increasing concentration of acetate buffer. However, for acetate concentration larger than 0.5 M, the peak current decreases with concentration of electrolyte, presumably because increasing the ionic strength of the solution changes the activity of the adsorbate. The maximum peak current was obtained with acetate concentration of 0.5 M. Adsorptive Behavior and Basis of Quantitative Analysis. Figure 3 shows square wave voltammograms obtained for 5 pM azobenzene without and with Triton X-100 present at various concentrations. It is clear that even a small amount of the surfactant, 0.0001%, caused the peakcurrent to decrease by almost 50%, due to competitive adsorption of azobenzene and Triton X-100 on the electrode surface. Most adsorption is diffusion-controlled. For diffusioncontrolled adsorption at a spherical electrode, the relationship between surface concentration of adsorbate,,'I and time is@ = 1.128C*(Dta)'/2+ C*DtJr
(3)
where c* is the bulkconcentration of reactant, D the diffusion coefficient, r the static mercury drop electrode radius, and ta the accumulation time. For the experimental conditions employed here, the spherical correction of eq 3, C*Dta/r,is negligible.@ Therefore,
r = 1.128C*(Dt,)'/2
(4)
The Faradaic current is proportional to this value of I';33*41 that is, the contribution of diffusing material to the signal during the course of the voltammogram is negligible. Under these conditions, the peak current, Zp is given by I, = [nFAr/t,]\k,
(5)
where A is the area of the electrode, t, the time at which current is sampled, and @, the maximal value of the dimensionless current, which depends on the square wave (40) Ribes. A. J.; Ostcryoung, J. 0. J. Elccirounal. Chem. 1990, 287, 125. (41) Mah0ncy.L. A. Flow 1njtctionAnalysiswithEl~trochcmicalDctcction.Ph.D. Dissertation, SUNYAB,1993; p 135.
C'td
'2/
u Ms"'
f/Hz
Flguro4. Net current vs C*tl'*: 1-9 pM azobenreneIn 0.5 M acetate buffer, pH 4.7.
Figure 5. Dependence of net current on frequency: 1 1Marobenzene in 0.5 M acetate buffer, pH 4.7, & = 5 8.
parameters and on the model for the process; n and F have the usual meanings. Equations 4 and 5 show that, for diffusioncontrolled adsorption, the peak current is proportional to the product of solution concentration C and square root of accumulation time ta, and as long as ta is kept constant, Zp increases linearly with increasing C*. This is the basis of quantitative analysis by adsorptive square wave stripping voltammetry. Figure 4 illustrates that a linear relationship between Zp and Cta112for azobenzene, as expected from eq 4 for diffusioncontrolled adsorption, is obtained up to 24 pM s1/2. At larger values of this product, deviations from linearity occur, and finally Zpreaches a limiting value, independent of C and ta, which corresponds to complete coverage of the electrode by a monolayer. It follows that adsorptive square wave stripping voltammetry should be employed under conditions of coverage where peak current increases linearly with the concentration of the analyte. When this dependence deviates from linearity, the experimental conditions should be modified, for instance, by decreasing t,, diluting of the solution, etc. To achieve good reproducibility of analytical results, it is essential that the electrode surface area, rate of solution stirring, and preconcentration time should be as consistent as possible. A blank assay should be performed in parallel with each determination to detect impurities that may not contribute to the response without accumulation. The linear range extends up to about 23.5 pA or 2.3 mA cm-2. Thus for the optimal square wave parameters, the signal for the unknown must be less than 2.3 mA cm-2 in order to remain in the linear range. The combination of eqs 4 and 5 predicts that the slope in the proportional region should have the value 1.01 pA (pM sl/2)-1, whereas the experimental value obtained is 0.97.This excellent agreement supports quantitatively this description of the process as reaction of adsorbate accumulated under diffusion control. The limiting concentration-independent value of current is 33.64PA, which corresponds to a surface concentration of 1.2 X 1 0 - ' O mol cm-2 or area per molecule of 1.38nm2. Azobenzene has two geometrical isomers, trans and cis. However, the trans is the ordinary form and the commercial product is nearly pure transe42 The calculated molecular area of adsorbed trans-azobenzene for the flat
orientation, based upon the method proposed by Soriaga et a1.43*44and data on bond lengths and angles from X-ray diffraction,45is 1.35 nm2. This excellent agreement suggests that the molecular orientation of azobenzene adsorbed on mercury from 0.5 M acetate buffer, pH 4.7,is flat. Equation 5 also predicts that peak current, Z,, should increase linearly with increasing frequency, assuming constant qP.Figure 5 displays this linear relationship for azobenzene up to 550 Hz. The deviation from linearity above 550 Hz may be due to kinetic complications or to the effect of capacity on background currents. The optimal frequency used (200 Hz) falls within the linear region. Determination at Trace Levels. Most neutral organic molecules are adsorbed on the electrode surface from aqueous solution primarily because of hydrophobic forces. In general, the less soluble an organic molecule is in aqueous media, the stronger the adsorption. Unlike many previous w0rkers,2&~3 we used a pure aqueous medium of 0.5 M acetate buffer, pH 4.7, with no methanol or ethanol. This lowers the solubility of azobenzene below the values obtaining when even a trace of lower alcohol is present and produces a lower detection limit. We employed standard addition to determine the detection limit. Figure 6A shows the square wave voltammogram of 10mL of 0.5M acetate buffer, tc= 20 s with stirring. Stirring is used at the lowest levels of concentration to decrease the time required to accumulate a detectable amount. Figure 6B is the voltammogram which was obtained after adding 10 pL of lo-* M azobenzene to the blank solution of Figure 6A, tc = 20 s, with stirring. The final concentration of the solution in the cell was 1 X 10-11 M. A well-defined net peak current is observed on the voltammogram and a detection limit of 3 pM (0.5 pg mL-l) is estimated from it for a signal-to-noise ratio of 2. The beautiful flat baselines obtained are due to the effective discrimination against charging current and background provided by square wave voltammetry. The detection limit can be made far lower if necessary by extending the accumulation time. That is, the signal depends, for given conditions, only on the product A r (eq 5). In extensive
(42) Hartlcy, 0. S.J. Chrm. Soc. 1938, I, 633.
(43) Soriaga, M.P.;Hubbard, A. T. 1. Am. Chrm. Soc. 1982, 104, 2735. (44) Soriaga, M. P.; Wilson, P. H.; Hubbard, A. T.J . ElcctroawI. Ckcm. 1982, 142. 317. (45) J. J: dclangc; J. Monteath Robertson; I. Woodward. Proc. R.Soc. London 1939, ~ 1 7 1 , 3 9 8 .
Analyticel Chemistry, Vol. 66,No.
6,M r c h 15, 1994
011
Tablo 1. ROCOVH~T I l k
B
'
0.5-
L
- I 0.0 0
-0 2
-0 4
-0 6
amt (ng/mL) added found0
1-0- 0 0
-02
-04
-06
E/V vs SCE E/V vs SCE Flgurr 6. Voltammograms of azobenzene, 4 = 20 8, stirred: (A) 10 mL 0.5 M acetate buffer: (B) 1 X 1O-l1M azobenzene in 0.5 M acetate buffer.
/
0,oCl 0.0
, 0.2
I
0.4
C
,
,
0.6
0.8
/ nM
, l o 1.0
2 0
4 0
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6.0
8.0
/ nn
Fburr 7. Azobenzene calibration curves: (A) 10-loM level concentratlons, 4 = 40 8 , stirred, slope 0.13 FA nM-', y-intercept 0.11 PA, correlation coefflcient 0.999:(B) lo-@M level concentrations, 4 = 30 8 , stlrred, slope 0.082 pA nW1, y-Intercept 0.03 FA, correlation coefflcient 0.9994.
experiments we have found that the factor which controls the detectionlimit is not sensitivity or background noise, but rather our ability to maintain a clean environment and thus to avoid contamination of the sample solution. Calibration curves at concentration levels of 1Wo, l e 9 , 1 P , and lo-' M were obtained with correlation coefficients of 0.999, 0.9994, 0.9996, and 0.9999, respectively. The calibration curves of 10-10 and 10-9 M azobenzene are given in Figure 7. From the calibration curve in the 10-loM range wecalculatethevalue 3S1/m,whereSI is thestandard deviation of the y-intercept and m the slope. This quantity gives an estimate of 50 pM for the detection limit. This method of estimating is much more conservative than the conventional method based on the results of Figure 6. Thus it is not surprising that the value obtained is 10 times larger. The recovery test is based on the additionof a known amount of azobenzene to the blank solution and determination of this amount by calibration curves carried out in parallel with the determinations. The results of recovery tests with addition of amounts from 0.364 to 154.7 ng mL-1 are listed in Table 1. Each entry in the table represents the mean of six experiments, and each experiment involved three potential scans. Recoveriesrangingfrom93.3 to 101.2%wereobtained. The reproducibility of the results in terms of the relative standard deviation ranged from 0.5 to 5.5%. 812
Analytlcel Chemlsby, Vol. 88, No. 8, h4arch 15, 1994
0.364 0.910 1.274 1.638 3.640 9.100 12.74 16.38 27.30 81.90 118.3 154.7
0.358 0.896 1.289 1.611 3.400 9.130 12.64 16.32 27.60 82.30 119.0 154.6
RSD
rec
(7%)
(%)
5.6 3.3 1.6 1.2 0.9 1.1 1.2 1.2 1.3 0.8 2.1 0.5
98.30 98.50 101.2 98.30 93.30 100.3 99.20 99-60 101.2 100.6 100.6 99.96
a Each entry represented the mean of six experiments and each experiment involved three potential acana.
CONCLUSION In summary, adsorptivesquare wave strippingvoltammetry is a highly sensitive, rapid, precise, and accurate analytical technique for trace determination of azobenzene and, by extension, of other adsorbed organic compounds. This approach is also attractive due to the relatively low cost of instrumentation and short time required for the analysis. We have not addressed here the problem presented by complex sample types. These problems will need to be addressed case by case, as the specific nature of the problem depends on the type of sample. In principle, problems of interferences can be addressed by means of separation techniques. In this work the times are rather long, at least 1 s. However, the intrinsic speed of the square wave stripping technique is compatible with the time resolution required of detectors in conventional HPLC. The requirement for significant enhancement of the signal due to accumulation is only that the period at zero current be large in comparison with the pulse width. Under these chemical conditions, the standard rate constant (k,) for the charge-transfer reaction (eq 1) of adsorbed azobenzene is ks= 160 s-1.33 The dimensionlessrate constant at 200 Hz is k,?, = 0.4. For values of k*t, near unity, there is substantial thermodynamic control of the reaction, which reduces the effect of uncompensated resistance in comparison with that found with totally irreversible reactions. Reversibility also provides an enhanced signal (as shown in Figure 2). Furthermore, the results of Figure 4 show clearly that there are no problems arising from multiple surface states or multiple peaks over a wide range of surface concentration. Thus, under well-controlled conditions of mass transport, calibration curves have the same slope over many orders of magnitude change in concentration. All of thesc features make adsorptive square wave stripping voltammetry an attractive analytical approach for this class of compounds. ACKNOWLEDQMENT This work was supported in part by the U.S.National Science Foundation under Grant CHE9208987. Received for review August 20, 1993. Accepted De@" 15,
1993." Abstract published in Aduancc ACS Absrructs. February 1, 1994.
