ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
interaction is rather small resulting in much smaller retention and poor resolution. The results presented in this paper demonstrate the efficiency of the ligand exchange chromatography for separation of alkyl phenyl sulfides using Hg2+and Ag+ ion-loaded stationary phase and suggest analytical applications. Furthermore, the method may be appplicable to other types of sulfides as well. Resulting from our preliminary experiments, leaching of the metal salts from the stationary phase is a problem which should be solved to allow successful application of the method discussed in this paper in HPLC.
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(7) V. Horak, J. Pecka, and J. Jurjev, Collect. Czech. Chem. Commun., 32,3386 (1967). (8) V. Horak and J. Pedta, "Mechanisms of Reactions of Sulfv COmpouW', Vol. 4,Intra-Science Research Foundation, Santa Monica, Calif.. 1969, p 43. (9) V. N. Ipatieff, H. Pines, and S. M. Friedman, J . Chem. SOC.,80,2731
(1938).
(IO) S.Stahl, "Thin Layer Chromatography", Academic Press, New York, 1965,
p 494. (11) I.M. Kolthoff and E. E. Sandell, "A Textbook of Quantitative Inorganic Analysis", 3rd ed., Macmillan, New York, p 461. (12) K. A. Connors, "A Textbook of Pharmaceutical Analysis", Wiley, New York, 1967,p 49. (13) R. Aiger, H. Spitzy, and R. W . Frei, Anal. Chem., 48,3 (1976). (14) R. D. Dean and W. J. Dixon, Anal. Chem., 23,636 (1951). (15) A. T. Jams, A. J. P. Martin, and G. H. Smith, Biochem. J , 52,238 (1952). (16) I. E. Bush, "The Chromatography of Steroids", Pergamon Press, London, 1961,p 31. (17)R. W. Taft, Jr., "Steric Effects in Organic Chemistry", M. S. Newman, Ed., John Wiley & Sons, New York, 1956. (18) M. Charton, Pratt Institute, Brooklyn, New York.
LITERATURE CITED (1) L. R. Snyder, Anal. Chem.. 41, 314 (1969). (2) W. L. Orr, Anal. Chem., 38. 1558 (1966). (3) W. L. Orr, Anal. Chem., 39, 1163 (1967). (4) J. W. Vogh and J. E. Dooley, Anal. Chem., 47, 816 (1975). (5) V. Horak and J. Pedta, Collect. Czech.Chem.Commn., 32,3394(1967). (6) V. Horak and J. Pecka, Cokct. Czech. chem.Cornnun., 32,3055(1967).
RECEIVED for review May 2, 1979. Accepted August 7, 1979.
Observation of Electrochemical Concentration Profiles by Absorption Spectroelectrochemistry Richard Pruiksma and Richard L. McCreery' Department of Chemistry, Ohio State University, Columbus, Ohio 43210
A laser beam passing parallel to a working electrode surface was used to monitor an electrogenerated chromophore employing absorptlon spectrophotometry. By placing a 10-pm slit parallel to the electrode and intercepting the beam after passage by the electrode, a portion of the diffusion layer was sampled. Movement of the SUI relathre to the electrode allowed monitoring of concentration as a function of distance from the electrode surface. The resulting concentration profiles agree well with theory for both single- and double-step experiments for distances of 50-200 pm from the surface. Because of long optical path length (0.5 cm or greater), the present method Is much more sensitive than previous spectroelectrochemlcal methods. In addition, spatial resolution of the diffusion layer provides both fundamental information about mass transfer and addttlonal insight Into reactions accompanylng charge transfer.
The use of spectroscopic probes to monitor electrochemical events has become widespread and spectroelectrochemistry has been used for a variety of purposes (1-3). The majority of applications of spectroelectrochemistry involves light absorption, with the beam transmitted through the electrode or reflected off its surface. For the techniques used to examine solution species in the vicinity of the electrode, two major objectives have been realized, spectral characterization of electrogenerated materials and kinetic monitoring of reactive species. The most common techniques use optically transparent electrodes, with the spectrophotometric beam being perpendicular to the electrode plane. This approach yields an integrated absorbance throughout the entire diffusion layer, and the time course of this absorbance has been used to diagnose reaction mechanisms for electrogenerated species and to spectrally characterize reactive intermediates. Internal reflection spectroscopy (IRS) has been used for the same purposes, with the region of the diffusion layer within one wavelength of the electrode surface being sampled by the evanescent wave ( 4 ) . 0003-2700/79/0351-2253$01.00/0
These previous spectroelectrochemical methods based on absorption suffer from two major drawbacks when applied to the monitoring of transient electrogenerated species. First, the optical path length is very short, being limited to the thickness of the diffusion layer or the length of the evanescent wave. Thus these techniques have been applied primarily to strong chromophores or species with relatively long lifetimes, allowing measurement of absorbance value vs. time transients with acceptable signal-to-noise ratios. Second, these techniques cannot supply information about the concentrations of electroactive species (or their reaction products) as a function of distance from the electrode surface. IRS can provide surface concentrations, and a transmission or reflection experiment can provide total concentration in the diffusion layer, but neither allows spatial resolution of the diffusion layer. Reflection spectroscopy at a glancing incidence angle (5) can greatly extend the bptical pathlength but still does not resolve the concentration gradients of electroactive species. Despite the fundamental importance of concentration vs. distance profiles to electrochemistry, they have not been observed using absorption techniques. Several experiments have been reported which make use of the refractive index gradient of a diffusion layer to construct a concentration vs. distance profile (6-10). While these interferometric techniques have succeeded in some cases, they lack both sensitivity and selectivity because they are based on refractive index changes accompanying electrochemical events. It is very difficult to monitor more than one solution component using changes in refractive index, and fairly large refractive index gradients are required to be measured interferometrically. Hence refractive index techniques have allowed observation of diffusion profiles only for fairly concentrated solutions (ca. 0.1 M) of single components (e.g., CuS04). Application of such techniques to reactive systems a t millimolar levels would be extremely difficult. The objective of the present work is to construct concentration vs. distance profiles for an electrogenerated species using absorption spectrophotometry. The light beam is or@ 1979 American Chemical Society
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. .
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Figure 2. Method for aligning electrode plane parallel to fiitered laser beam. P,, pentaprism; P,, Pgr cornercube reflectors; D = 1.3 m the electrode flatness was not disturbed by the gold coating process. After removal of PI the laser beam will pass parallel to the electrode plane. The cell was constructed from 1.6-mm quartz plate and had dimensions of 60 X 40 X 30 mm with the 30-mm dimension being along the optical axis. To ensure that its presence did not adversely affect the alignment of electrode and beam, it was positioned so that the reflection of the beam off its face was directed back onto the incoming beam. This process aligned the cell face perpendicular to the beam and assured that the cell and solution did not deflect the beam. The 10-pm slit was moved vertically (along a dimension perpendicular to the electrode) with a high resolution micrometer (Lansing Research, Ithaca, N.Y.) having resolution and readability of 0.13 pm. The light leaving the slit was collected by a small lens (focal length = 50 mm) and directed onto the active surface of a 1P28 photomultiplier tube. As much of the light transmitted by the slit as possible was used for absorbance measurements. Diffraction by the slit and any scattering by the cell wall on the exit side are unimportant, since the light has already passed the electrode and the slit has already selected a particular region of the diffusion layer. A commercial potentiostat (Princeton Applied Research Model 173) driven by a laboratory computer (Hewlett-Packard 1000) controlled the potential using a Pt auxiliary electrode and SCE reference electrode. No particular care was taken with auxiliary and reference electrode placement, except to avoid the laser beam. The computer monitored the PMT output before and during a single or double potential step, and calculated the absorbance vs. time transients. Since the initial light intensity varied with slit movement, the PMT high voltage supply was adjusted to bring the PMT output voltage into the optimum range of the analog-to-digital converter. The test system used for this work was the oxidation of N,N,”,”-tetramethylparaphenylenediamine (TMPD) to its cation radical (Wurster’s blue). The one-electron character of this oxidation is well established (1I), but additional experiments were carried out to verify its suitability for the present work. At pH 7, TMPD exhibits a reversible voltammetric wave centered at +0.04 V vs. SCE. Potential step experiments from -0.2 to +0.2 V at a graphite paste electrode produced current transients which were linear with t-’lz for at least 15 s. The diffusion coefficient determined from chronoamperometry at pH 7 is 8.3 X lo4 cmz/s. Using reflectance spectroelectrochemistry with a normal incident angle ( 5 ) ,absorbance was linear with t l i z for a period from 0.2 to 15 s indicating a stable chromophore for this period. The radical was not sufficiently stable (i.e. a half-life of minutes) to determine its molar absorptivity by normal means, so the absorptivity was calculated from the reflection data to be 4200 M-‘ cm-I at 632.8 nm. It was assumed that the diffusion coefficients for the reduced and radical forms are equal. Thus for the time frame of the present work, TMPD is an appropriate test system, exhibiting an uncomplicated oxidation to a blue chromophore. The sensitivity of TMPD to air oxidation was controlled by careful degassing with argon which had been passed through a Cr2+ solution. Theoretical concentration vs. time curves at various distances from the electrode were determined from standard digital simulation techniques (12). The effect of the 10-pm slit was
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Figure 3. Fraction of bulk concentration vs. time at various distances from the electrode for a single-step experiment. Solid lines are theoretical curves (listed from top) at 25, 38, 70, 102, 133, 197, and 260 p m from the electrode. Points are experimental results: ( 0 )25 pm, (B) 38 p m , (A)70 pm, (0)102 pm, ( 0 )133 pm, (0) 197 pm, (A) 260 pm
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2 .1
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Figure 5. Fractional concentration vs. time at various distances for
a double-potential step experiment. Numbers indicate distances from electrode. Solid lines are theoretical, points are experimental. Step from -0.20 to +0.20 V vs. SCE at t = 0.0 s. Step from +0.20 to -0.20 V vs. SCE at t = 5.0 s
C T _ \ J Z
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--_r .
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Figure 4. Fractional concentration as a function of distance for various
times after a single-potential step. Solid lines are theoretical curves (listed from top) for 10, 8, 6, 4, 2, and 1 s. Points are experimental results: ( 0 )10 s, (m) 8 s, (A)6 s, (0)4 s, (0) 2 s, (A) 1 s incorporated by averaging the simulation “boxes” corresponding to this dimension.
RESULTS Experimental absorbance vs. time curves were converted to concentration vs. time curves using the molar absorptivity for the radical cation and the geometric pathlength of the electrode (0.45 cm). Because of the long pathlength, absorbance values were large, ranging from 0.05 to 2.0 units. Concentrations were then divided by the bulk concentration of TMPD (1.0 mM) to obtain fractional concentration of radical vs. time. Theoretical curves of fractional concentration vs. time as a function of distance were determined from the simulated results and the values of the diffusion coefficient for TMPD. Experimental and theoretical plots of fractional concentration vs. time are shown in Figure 3, for a single step experiment to a potential on the diffusion limit for production of radical (0.2 V vs. SCE). As expected, the curves rise sharply at distances close to the electrode, since electrogenerated material reaches these distances rapidly. The concentration vs. time plots at various distance may easily be converted to concentration vs. distance plots a t various times, commonly referred to as diffusion profiles. These profiles for various times are shown in Figure 4. The solid lines represent theoretical diffusion profiles, and the points represent the experimental results. Fractional concentration vs. time profiles for double-step experiments are shown in Figure 5 . After 5 s of generation of radical at the diffusion controlled rate, the potential was returned to a value required for diffusion controlled reduction back to TMPD (-0.2 V vs. SCE). Note that the peak in the electrogenerated radical concentration occurs later at greater distances from the electrode. The concentration vs. time plots
Flgure 6. Concentration vs. distance for five times during forward step of double-potential step experiment. Theoretical curves from the top: 5, 4, 3, 2, 1 s. Experimental points: (A) 5 s, ( 0 )4 s, (0)3 s, (A)2
s,
(e) 1 s
were converted to concentration vs. distance plots, shown in Figures 6 and 7 . Figure 6 includes five profiles before the potential switch while Figure 7 shows five profiles after the switch. Figure 8 compares theoretical and observed concentration vs. time profiles at a distance very close to the electrode (13 pm). Not only is the correspondence poor, but the experimental results are quantitatively irreproducible.
DISCUSSION While the quantitative agreement between theory and experiment for the single step shown in Figure 3 is not outstanding, the important qualitative features are apparent. As the distance increases, the concentration rises more slowly, as expected for electrogenerated material diffusing away from the electrode. In fact at 260 pm, more than 5 s elapse before any radical is observable. The concentration profiles of Figure 4 have the expected shape, and quantitative agreement again is good, although not outstanding. In both Figures 3 and 4, it is apparent that the quantitative disagreement is poorest at short (
