ANALYTICAL CHEMISTRY, VOL. 51, NO. 4, APRIL 1979
561
T hin-Layer Spectroe1ect rochemist ry for Monitoring Kinetic s of Electrogenerated Species Elmo A. Blubaugh, Alexander M. Yacynych,’ and William R. Heineman“ Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 4522 I
The applicability of thin-layer optically transparent electrodes for monitoring homogeneous chemical reactions of electrogenerated species has been demonstrated. The reactive species is coulometrically generated in the thin solution layer at a minigrid electrode, and the chemical reaction is optically monitored by light passing through the transparent electrode. Single- and double-potential step techniques are demonstrated using the benzidine rearrangement as a model system. Spectra in the 200-320 nm range were obtained with a rapid scanning spectrometer during electrogeneration and reaction. Absorbance vs. time data were then dlsplayed for analysis of the kinetics. Rate constants obtained for the acid-catalyzed s-’ in 0.05 F HCI benzidine rearrangement were 2.8 X and 3.0 X lo-* s-’ in 0.10 F HCI.
Thin-layer electrochemical cells restrict the diffusional path of the electroactive species by trapping a “thin slab” of solution between two barriers ( I , 2). Exhaustive electrolysis of electroactive species in the thin solution layer can be achieved in about 10-60 s with diffusion as the only mode of mass transport. Optically transparent thin-layer electrodes (OTTLE) enable spectroscopic observation of electrogenerated species via a light beam passing through t h e thin layer of solution and t h e electrode ( 3 , 4 ) . Therefore, the O T T L E is potentially useful for the optical monitoring of homogeneous chemical reactions t h a t an electrogenerated species may undergo. T h e O T T L E technique offers advantages which warrant its consideration as a method for kinetic studies. (a) Since the reactive species is trapped in the thin solution layer, very slow homogeneous chemical reactions should be observable. Such slow reactions are not always amenable to study by conventional electrochemical techniques in which diffusion of the reactive species away from the electrode is unrestricted. Perturbation of concentration gradients by convection limits many semi-infinite diffusion techniques to a time domain of less t h a n ca. 60 s. (b) Since electrogeneration of reactive species in the thin solution layer is quantitative, spectral observation of intermediates and products of the chemical reaction is not interfered with by starting material, unless a regenerative mechanism is involved. (c) T h e reactive species is homogeneously distributed across the thin solution layer after its quantitative generation from starting material. Such a homogeneous solution of reactive intermediate allows subsequent kinetic processes to be treated by conventional methods of data analysis for kinetics. This is less complicated t h a n non-thin-layer electrochemical and spectroelectrochemical techniques for which appropriate diffusion equations must be solved or simulated to extract information about kinetics (5-7). T h e measurement of reaction rate constants with an O T T L E was suggested during the early development of Present address: Department of Chemistry, Rutgers University.
New Brunswick, N.J. 08903.
0003-2700/79/0351-0561$01.00/0
thin-layer spectroelectrochemistry ( 2 , 3 )and electrogenerated intermediates were optically observed (8, 9). However, t h e quantitative measurement of rate constants has only recently been reported. Owens and Dryhurst measured t h e rate constant for the hydrolysis of a diimine electrogenerated by oxidation of 5,6-diaminouracil (10). McCreery reported t h e rate constant for the hydrolysis of electrogenerated p-quinone imine by a method in which the amount of reactive species was varied by potential control of the [O]/[R] ratio in the thin layer ( I I ) . This “non-quantitative generation” approach was effectively used to slow t h e reaction into a time frame observable with t h e thin-layer cell. Mark e t al. have used the O T T L E to spectrally observe the homogeneous reoxidation of electrogenerated cob(1)alamin (12-14). In this study, the utility of the OTTLE for the spectroscopic measurement of rate constants for homogeneous chemical reactions of electrogenerated species is evaluated for singleand double-potential step techniques. T h e benzidine rearrangement was selected as a model system. T h e reaction sequence involves the acid-catalyzed rearrangement of hydrazobenzene which is generated by reduction of azobenzene:
azobenzene
,I i-ifJ
rsolution e a c t i o n : =)-H
*
hydrazobenzene
hydrazobenzene
I
benzidine
“2
diphenyline Considerable d a t a on the benzidine rearrangement are available for comparison purposes since this reaction has been used as a model EC mechanism for evaluating numerous electrochemical techniques including polarography (Is), double potential step chronoamperometry ( I @ , double potential step chronocoulometry ( I 7 ) , twin-electrode thin-layer electrochemistry ( 2 8 , chronopotentiometry with step current reversal and reverse ramp current (19),thin-layer chronopotentiometry ( I 9 ) ,thin-layer potential-step chronocoulometry (19),cyclic voltammetry (20),and potential step generation with linear sweep reversal (21).
EXPERIMENTAL Optically transparent thin-layer cells were constructed by sandwiching gold minigrid (500 Ipi) between two quartz plates separated by two 2-mil Teflon tape spacers ( 4 ) . Cell thicknesses were ca. 0.23 mm. The mercury-coated gold minigrid electrode was prepared by a previously reported procedure (22). The C 1979 American Chemical Society
562
ANALYTICAL CHEMISTRY, VOL. 51, NO. 4 , APRIL 1979 A
t ,2
t---+----t----l - 3 . i X -3.6;: - c . e z
I3.L - 3 . 2 c ; o
-1
,>, ,.
~
ti
"'5,
sc.5
Figure 1. Cyclic voltammogram of azobenzene on Hg-Au minigrid OTTLE. 1 m M azobenzene, 0.050 FHCI, 0.200 FKCI, 4 4 % ethanol. Scan rate, 10 m V s-'
O ? n E was masked with black vinyl tape so that the optical beam passed through a 2 X 2 mm area in the center of the minigrid. Reference and auxiliary electrodes were placed in a small solution cup into which the OTTLE was dipped. OTTLE cells were subjected to 10 min of radio-frequency plasma discharge prior to use for removal of any organic film. A potentiostat of conventional operational amplifier design was used to provide potential control. All potentials were measured vs. an SCE using a Fluke 8008A digital voltmeter. All chemicals were of reagent grade quality. Azobenzene (Eastman Kodak Co.) was recrystallized from ethanol until its absorption spectrum remained constant (23). A 44% ethanol-56% water solvent mixture was prepared by combination of equal volumes of 95% ethanol and deionized, doubly distilled water. Solutions were 0.050 F o r 0.100 F HC1 with a concentration of 1mM azobenzene and a sufficient amount of KC1 to give an ionic strength of 0.25. Solutions were deoxygenated by nitrogen bubbling before use and were kept under nitrogen and in the dark to avoid photooxidation of the azobenzene. Single-potential step experiments were performed on mercury-coated gold minigrid OTTLEs. The electrode assembly was positioned in the deoxygenated sample compartment of a Harrick Rapid Scan Spectrometer, RSS-B. External triggering and data acquisition were accomplished through a data linkup with a Raytheon 704 computer. Spectra in the range of 220 to 320 nm were obtained at a rate of 10 spectra per second. The absorbanceetime curves at fixed wavelengths of 293 and 300 nm were then plotted via data reduction procedures previously reported (24). New solution was drawn into the OTTLE from the reservoir cup before each measurement. Double-potential step experiments were performed on a gold minigrid OTTLE. The electrode assembly was placed in the deoxygenated sample compartment of a Cary 14 spectrophotometer. Deoxygenated solution was added t o the solution cup, and new solution was drawn into the cell before each measurement a t different reaction times, t ~ External . triggering of the potentiostat was provided by a PAR 175 Universal Programmer. Absorbance-time curves were obtained at 325 nm for different reaction times, t R .
RESULTS AND DISCUSSION Cyclic Voltammetry. Cyclic voltammograms of azobenzene were used t o determine appropriate potentials for the potential step experiments. A typical voltammogram obtained on a mercury-coated gold minigrid (Hg-Au) OTTLE is shown in Figure 1. The wave for reduction of azobenzene t o hydrazobenzene is clearly defined although somewhat distorted because of resistance in the OTTLE. The oxidation wave for hydrazobenzene t o azobenzene is obscured by oxidation of the mercury film to form mercurous chloride. The Hg-Au O T T L E was used for the single-potential step experiments to facilitate comparison with previous measurements made on mercury electrodes (19) and to extend the negative potential range. T h e latter aspect enabled the potential to be stepped well beyond the peak potential so that resistance effects on the rate of electrolysis were minimized. A voltammogram obtained on a gold OTTLE is shown in Figure 2. In this case, both reduction and oxidation waves
ii
i
-
--
rrl
--
Figure 2. Cyclic voltammogram of azobenzene on Au minigrid OTTLE 1 m M azobenzene, 0 050 F HCI, 0 200 F KCI, 4 4 % ethanol. Scan rate, 10 m V s-'
x
r-
Figure 3. Spectra recorded on Hg-Au OTTLE during single-potential step spectroelectrochemistry. Potential step 0.000 V to -0.600 V vs. SCE. 1 m M azobenzene, 0.05 FHCI, 0.20 FKCI, 4 4 % ethanol. Each spectrum represents 100 signal averaged spectra
are reasonably well defined, although merging of the reduction wave with the onset of hydrogen evolution restricts the accessible negative potential range. The gold OTTLE was used for the double-potential step experiments so that the potential could be stepped beyond the oxidation wave for the conversion of hydrazobenzene back to azobenzene. Cyclic voltammograms performed on a solution of hydrazobenzene after a 30-min reaction time t o generate benzidine and diphenyline gave no waves between +0.2 and -0.8 V, substantiating electroinactivity of the reaction products in this potential range as previously reported (19). Spectra. Spectra were recorded during a single-potential step experiment to enable selection of wavelengths for optically monitoring the benzidine rearrangement. Figure 3 shows spectra that were recorded during a single-potential step experiment in a Hg-Au OTTLE. T h e potential was stepped from 0.0 to -0.600 V vs. SCE and spectra were recorded a t the rate of 10 spectra per second with the Rapid Scanning Spectrometer. Signal averaging was used t o improve the signal-to-noise ratio. Each spectrum represents an average of the 100 spectra recorded during k5 s of the indicated time. T h e spectra show azobenzene (spectrum a t open circuit), which was then reduced t o hydrazobenzene (5-45 s) with subsequent rearrangement to products (750 s). Severe overlap of the azobenzene peak a t 233 nm, the hydrazobenzene peak at 245 nm, and the benzidine peak a t 255 nm is apparent. This is quite different from previously reported spectra that exhibit less overlap of the hydrazobenzene and product spectra (23). The peaks shown in Figure 3 are shifted to shorter wavelengths than those previously reported because of protonation in the acidic media.
Single-Potential Step Thin-Layer Chronoabsorptometry. The single potential step approach to the measurement of kinetics in the O T T L E involves quantitative electrochemical reduction of azobenzene to hydrazobenzene in the
ANALYTICAL CHEMISTRY, VOL. 51. NO. 4, APRIL 1979
Table I.
563
Rate Constants for Benzidine Rearrangement Determined by Thin-Layer Spectroelectrochemistrya technique
supporting electrolyte
single-potential step
0.05 F HC1, 0 . 2 F KC1, 44% EtOH
h,
0.05 F HC1, 0 . 2 F KCI, 44% EtOH 0.10 F HCI, 0.15 F KC1, 44% EtOH 0.10 F HCI, 0.15 F KC1, 44% EtOH
' Potential step:
a Temperature, 24.5-25.5 "C. -0.600 to t0.300 V VS. SCE.
l o 3 k , s-'
l o 3 s t d . dev., SK'
2.7 2.8 30 30 8.7'
0.36 (Ar = 3 ) 0 . 3 (N= 3 ) 3.2 ( N = 3 ) 3.9 ( N = 3 ) 0.24 ( N = 2 )
11c
1 . 2 ( N = 2) 1 . 5 (N= 2 )
293 300 293 300 325 325 325
0.10 F HC1, 0.15 F KC1, 44% EtOH
double-potential step
nm
0.00 t o -0.600 to + 0 . 3 0 0 V vs. SCE.
16'
Potential step: + 0 . 3 0 0 t o -
Table 11. Previously Reported Rate Constants for Benzidine Rearrangement technique
supporting electrolyte 0.100 F HC10, , 0.15 F NaCIO,, 35.5% EtOH 0.063 F HCIO,, 0.15 F NaC10,. 35.5% EtOH 0.075 F HClO;, 0.15 F NaClO;, 44.0% EtOH 0.040 F HCIO,, 0.15 F NaCIO,, 44.0% EtOH 0.0997 F HC10, , 0.15 F NaCIO,, 38.5% EtOH 0.0629 F HC10, , 0.187 F NaC10, , 38.5% EtOH 0.0997 F HClO,, 0.15 F NaC10, , 38.5% EtOH 0.0629 F HClO,, 0.187 F NaClO,, 38.5% EtOH 0.050 F HC1, 0.0200 F NH,C1, 44% EtOH
twin-electrode thin-layer
thin-layer current-reversal chronopotentiometry thin-layer double-potential step reaction quenching with spectrophotometry
103 k ,
s-1
ref
22.3
(18)
4.23 2.16 31.0 13.9 20.0
(19)
9.0
8.6
1.32
(19)
(25)
+
2 3 3 -r
~~
~
_-
~
---
- -,I-
L
-
, . -, .
t T E , i
+
40c
323
---c
Figure 4. Absorbance-time curve at 293 nm during potential step from 0.000 to -0.600 V vs. SCE. 1 mM azobenzene, 0.050 FHCI, 0.200 F KCI, 4 4 % ethanol
-.as
1
t
Figure 5. Kinetic plot of single-potential step absorbance-time data
at 293 nm. 1 mM azobenzene, 0.050 FHCI, 0.200 FKCl solution, 44%
ethanol thin solution layer by a potential step from 0.00 t o -0.60 V vs. SCE. T h e electrolysis and subsequent rearrangement of hydrazobenzene are monitored spectrally. Severe overlap a t the lower wavelengths necessitated optical monitoring of' the reaction at >250 nm. T h e wavelengths selected for the single-potential step measurements were 293 a n d 300 nm where t h e absorbance change due to t h e benzidine rearrangement was greatest (difference between 45- and 750-s curves) a n d t h e spectrometer signal-to-noise ratio was best. A typical absorbance-time curve a t 293 nm is shown in Figure 4. T h e rapid decrease in absorbance during the first 30 s corresponds to the reduction of azobenzene to hydrazobenzene; t h e slow subsequent decrease is due to rearrangement of hydrazobenzene. T h e rearrangement of hydrazobenzene is an acid-catalyzed pseudo-first-order reaction (15-21). Consequently, plots of absorbance-time data according to Equation 1 should be linear with a slope of -k a n d an intercept of In ( A , - A m ) . In (A, - A J = -ht + In ( A , - A _ ) (1) where k = pseudo-first-order rate constant, s-'; i10 = initial absorbance = btX,"AB [HAB],; A , = final absorbance = ~ ( E ~ , ~ ~ [+D €x,~z[Bz],); P], A, = absorbance a t time t = ~(~x,HAB[HA +Beh,Dp[DPIt ], + tA,sz[BZ]t);in which HAB = hydrazobenzene, DP = diphenyline, BZ = benzidine; b = pathlength of thin-layer cell, t = molar absorptivity. A typical plot is shown in Figure 5 . In all cases, t h e plots were linear
with the correlation coefficient being greater t h a n 0.99, Second-order plots were nonlinear, substantiating the existence of a first-order reaction. A summary of rate constants obtained with 0.05 F HC1 and 0.10 F HCl at two monitored wavelengths is shown in Table I. The pseudo-first-order rate constant is larger for the greater acid concentration as would be expected for the acid-catalyzed rearrangement. The rate constants are in good agreement with those obtained by electrochemical techniques under similar solution conditions shown in Table 11. T h e rate constant of 2.7 x s reported here for 0.050 F hydrochloric acid is intermediate between the twin-electrode thin-layer values of 5 - l for perchloric acid con2.16 X lo-" s-' and 4.23 X centrations of 0.040 and 0.075 F,respectively (18). It also compares favorably with t h e 1.32 x s-' value obtained by a nonelectrochemical technique in 0.050 F HCI, 0.200 F KH,Cl, 44% ethanol (25). The rate constant of 3.0 X lo-*s-' obtained for 0.10 F HC1 is comparable to the value of 2.23 X 10 * i 0.06 X 10 5-l reported for 0.10 F HClO,, 35.5% ethanol obtained by the twin-electrode method (181 and 3.10 X IO-* s - ' by thin-layer chronopotentiometry and 2.0 x lo-' s-' by thin-layer potential step in 0.0997 M HC104, 0.15 M KaClO,, 38.5% ethanol (19). These results were obtained under non-ideal spectroscopic conditions since the total absorbance change resulting from the benzidine rearrangement was less t h a n 0.05 au. This
'
564
ANALYTICAL CHEMISTRY, VOL. 51, NO. 4, APRIL 1979
-
2.00
+
P i
c
--
? I+ d
.
-
1.00
-c ~--+~
125
i c
3 : -
+...
-.
~
~
4:s ilP'
5
*
Flgure 6. Double-potential step absorbance--time curves at 325 nm. Potential step from +O.OOO t o -0.600 to 4-0.300V vs. SCE. 1 m M azobenzene, 0.100 FHCI, 0.150 FKCI, 4 4 % ethanol. (1) t , = 50 s, (2) f R = 100 S, (3) tR = 150 S, (4) t , = 200 s, and (5) t , = 300 s
indicates that good results can be obtained with rather small optical changes. T h e precision of repetitive measurements was within 10% for all of the runs. The upper limit of rate constants obtainable by this method is determined by the time required for quantitative generation of reactive species in the thin solution layer. The electrolysis time for the present study was found to be about 30 s. Kinetic plots were severely curved in the region 0-30 s, so A--t data a t times greater than 30 s were used in the analysis. 'The electrolysis time can be shortened by cell designs which minimize iR drop or decrease the diffusional path length. The magnitude of lower rate constants obtainable is ultimately limited by the problem of edge effects (18) and convection. T h e effect of edge-diffusion into the thin-layer cell is minimized by focusing t h e optical beam in the center of the minigrid region ( 4 ) . The minigrids above and below the beam then serve as a buffer zone between the volume being optically monitored and the edges of the minigrid. OTTLEs in which the minigrid covers the entire cell can be used to eliminate the edge effect for extremely slow reactions (13).
Double-Potential Step Thin-Layer Chronoabsorptometry. T h e double-potential step thin-layer spectroelectrochemical approach involves first the complete reduction of azobenzene to hydrazobenzene by a potential step from 0.300 to -0.600 V vs. SCE.T h e potential is maintained at 0.600 V for a time interval, t R , during which the rearrangement reaction proceeds. At the end of t R , the remaining hydrazobenzene is electrolyzed back to azobenzene by returning the potential to 0.300 V. T h e double-potential step experiments were performed a t a wavelength of 325 nm where the absorbance change is attributable only to azobenzene, eliminating spectral interference from hydrasobenzene and reaction products. Typical optical responses for varying lengths of reaction time tR are shown in Figure 6. This double-potential step technique is the optical analogue of the thin-layer potential step approach with charge monitoring previously described by Oglesby, Johnson, and Reilley (19). A pseudo-first-order rate constant can be calculated from the data in Figure 6 by Equation 2. ~
LAf, the change in absorbance accompanying the first potential step, is a measure of the amount of hydrazobenzene generated Le., azobenzene electrolyzed). Mb,the absorbance change accompanying the second potential step, is a measure of the amount of hydrazobenzene remaining after reaction time t ~ The . reaction time. t H , was taken as the time interval between the two intersections of the extrapolated dotted lines for the forward and reverse potential steps as shown for curve 2. A typical plot of In (A&/ L 4 b ) vs. t H is shown in Figure 7 . T h e plot is linear as expected with the slope equal to the pseudo-first-order rate constant h. Rate constants o htained
/
/"
i -&T-z--z--x-70* __
0.00
-
