ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, M A Y 1979
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Optical Monitoring of Electrogenerated Species via Specular Reflection at Glancing Incidence Richard L. McCreery, * Richard Pruiksma, and Robert Fagan Department of Chemistry, The Ohio State University, Columbus, Ohio 432 10
Light reflected from a planar electrode at small angles relative to the electrode surface was used to carry out conventional absorbance measurements on electrogenerated species. Compared to measurements involving optically transparent electrodes, the effective optical path length of the new technique is longer by factors of 100-200, leading to greatly Increased absorbance values. The absorbance is linear with the square root of time for times greater than 20 ms after the beginning of electrolysis, and its magnitude can be predicted from the geometry of the optical apparatus. The advantages of the new technique include the abillty to monitor weak or short-lived chromophores, the absence of a need for time averaging, and the lack of the requirement of an optically transparent electrode.
Numerous methods have evolved in recent years which make use of an optical probe to obtain UV-VIS spectral characteristics of solution components generated by an electrochemical reaction (1-3). These methods have been used to identify and characterize electrogenerated materials and to monitor the kinetics of decay of reactive species. The most common methods for examining electrogenerated materials in solution involve an optically transparent electrode (OTE), either a thin-film metal or metal oxide electrode on a transparent substrate or a “minigrid” electrode made from commercially available metal grids. While these spectroelectrochemical methods have proved extremely useful, they suffer from several drawbacks which hinder their use for examining weakly absorbing species or electrogenerated materials with short lifetimes. T h e main problem is a very short path length for both an optically transparent thin-film electrode or a minigrid, resulting in very small absorbance values for either short-lived or weakly absorbing species. The absorbance for an optically transparent electrode, where the optical beam is perpendicular to the electrode surface, is given by Equation I,
-
where Co> is the bulk concentration of a species undergoing Red, Do, is its a diffusion controlled reaction: Ox + nediffusion coefficient, t R is the molar absorptivity of the reduced form (assumed to be the only absorbing species a t the wavelength used), and t is the time after the initiation of generation of Red ( 4 , 5 ) . By analogy to the Beer-Lambert law, the term in brackets is the effective path length of the optical beam. This path length is very short for typical species (Do=r lo4 cm’/s), with values of 3.6 X cm a t 1 ms t o cm a t 1 s. Clearly, a strong chromophore is necessary to obtain measurable absorbance with the concentrations used in electrochemistry (ca. 1mM). With short-lived species, it is necessary to time average thousands of runs even for strong chromophores (6). 0003-2700/79/0351-0749$0 1.OO/O
A minigrid electrode does not improve these path-length problems and has the added problem of the lag time required for the absorbing species to “fill” the holes between the grid wires ( 7 ) . Internal reflection spectroscopy has also been used to monitor transient species, but the path length is limited to a few multiples of the length of the evanescent wave (ca. 2000 A) (8). A problem with both thin-film and minigrid electrodes is internal resistance which limits the magnitude of the current and creates uncompensated IR drops. This problem is severe with thin-film electrodes and results in poor transient response and uneven distribution o f applied potential. Changes in specular reflectance accompanying electrochemical events have long been used to characterize electrode surface changes and in a few cases to examine electrogenerated species in solution (9-11). Specular reflectance in a thin-layer cell was used to monitor stable solution species, with the beam reflected off the electrode a t a 40’ angle (12). F’hile this approach avoids the problem of electrode transparency aria resistance, the effective path length is only a factor of about two greater than that for an OTE. In addition, the magnitude of the incident beam angle is constrained by refraction from the walls of the thin-layer cell. Thus the sensitivity is only slightly improved, and the technique is still limited to strong chromophores of relatively stable species. A method based on multiple reflections within a reflective tube has been presented (13)and an attempt made to detect electrogenerated solvated electrons. Long effective path lengths were reported, but quantitation was difficult because of large variations in path length with beam geometry and position. In the technique reported here, light reflected from a planar electrode a t glancing incidence is used to make conventional absorption measurements. As the angle of incidence, measured relative to the electrode plane, is decreased, the optical path length through the diffusion layer increases greatly compared to a n OTE experiment. A large enhancement in absorbance, and therefore sensitivity, results, making spectroelectrochemical observation of weak chromophores and short-lived species possible.
THEORY For a light beam reflecting off a planar electrode at an angle a relative to the surface (Figure 11, the absorbance due to an electrogenerated species will be given by Equation 2.
C,(x,t) is the concentration profile of the electrogenerated species as a function of distance from the electrode and dl is a path length increment. Since dx/dl equals sin a ,
2t R 2 A ( t ) = -Co) -Dox1f2t’l2 sin a
(3)
As before, this equation is based on the assumption that only C 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979 Liaht B e a n
Elec:rode
Specular reflection of a light beam from a planar electrode. See text for details
Figure 1.
Mask
Laser S'eerirg Mll,O,
Oii
I
I
0
01
0.2
03
0 5
04
T i m e (seconds)
Absorbance vs. time response for a potential step from -0.3 to -0.8 V vs. SCE; 0.59 mM methyl viologen in 0.3 M phosphate buffer, pH 7.0. Average of three runs Figure 3.
Electrode
Figure 2. Optical arrangement for absorbance measurements using reflected light. B = C = 67.0 cm; A = 0.3-3.0 cm. Beam and electrode aligned as described in text
the reduced form absorbs a t the chosen wavelength and that the charge transfer reaction is diffusion controlled. The factor (2/sin a ) represents the path length enhancement of the reflection measurement over that calculated for an OTE experiment with the beam perpendicular to the electrode. Except for this enhancement, the absorbance has the same behavior as that observed with an OTE, with a t1I2dependence of absorbance for a stable electrogenerated species.
EXPERIMENTAL A schematic diagram of the optical apparatus is shown in Figure 2. Distances B and C are equal, and have magnitudes of about 70 cm. The light source was a 0.5 mW, He/Ne laser (632.8 nm) whose beam was directed by two mirrors (beam steering device, Newport Research). The mirrors, cell, and photomultiplier were mounted on an optical rail (Newport Research). The working electrode was made by vapor deposition of 5000 A of gold film onto glass. The electrode was mounted on a three-dimensional micromanipulator and placed in a 2 x 1 X 1'/2 inch quartz cell made from '/,,-inch-thick quartz plate. The dimension of the working electrode along the optical axis was 37 mm. A platinum wire auxiliary electrode and an SCE were placed in the cell without special positioning. A 1P28 photomultiplier (PMT) with standard electronics served as a detector. The alignment of the working electrode with respect to the laser beam is of obvious importance and was accomplished as follows. The cell was lowered away from the working electrode, and the beam positioned to pass through a 3 mm2hole on the face of the PMT housing, without reflecting off the electrode. The distance from the beam to the electrode (A in Figure 2) was measured, with values ranging from 0.3 to 3 cm. Then the beam was directed onto the electrode using the steering device, and the electrode was rotated until the reflected spot impinged on the hole in the PMT housing. This adjustment ensures that the electrode plane is parallel to a line between the source and the PMT. Finally, the cell was repositioned without moving the working electrode and rotated to place the beam back on the PMT, to ensure the beam position is not altered by refraction within the cell. Provided the distances B and C of Figure 2 are equal, the apparent incidence angle p is sin-' (A/C). To determine the true incidence angle a (again relative to the electrode surface), refraction by the solution must be taken into account. From Snell's law, sin a = sin /3/1.331, where 1.331 is the refractive index of water at 632.8 mm. The a values reported herein have been so corrected. The index of refraction of the buffer used here was assumed to equal that of water to the required accuracy. Changes in index of refraction by dissolved salts are generally in the third or fourth decimal place (14). The reduction of methyl viologen to a blue cation radical was used as a test system in this work, since the radical absorbs well at the He/Ne laser wavelength and the system is well behaved electrochemically. An accurate concentration of methyl viologen is difficult to prepare from commercially available methyl viologen chloride, since the water content varies. To avoid this compli-
1
0 2
1
I
04
I
06
08
t% ( s e c k ,
Figure 4.
Absorbance vs. square root of time for data of Figure 3
cation, the glancing incidence technique was evaluated by comparing the absorbance of light incident at small values of a to the absorbance of light which is incident normal to the surface. The normally incident light will experience a path length twice that of an OTE experiment, and will have twice the absorbance given by Equation 1. The ratio of absorbance at any angle a to one half the absorbance at normal incidence is the absorbance enhancement of the present technique over an OTE experiment, and should have a value of 2/sin a . Using this approach, it is unnecessary to know Do, or t at 632.8 nm to compare the two methods, but literature values (15,16) under similar conditions cm2 s-' and c632,8 r 9700 M-' cm-'. are: Do, = 0.86 X Methyl viologen dichloride (K and K Laboratories) was dissolved in 0.3 M phosphate buffer with pH 7.0. The potential was controlled by a PARC model 173 potentiostat and was stepped from -0.3 to -0.8 V vs. SCE. The PMT output was monitored before and during the potential step with a minicomputer, and the absorbance vs. time curves were calculated by the computer. Three runs were averaged in all cases. Despite the negative potential of the step, hydrogen generation was too slow under these conditions to cause any apparent problems.
RESULTS The absorbance vs. time curve for reduction of a 0.6-mM viologen solution with an a angle of 1.48' is shown in Figure 3. The small noise level of the curve, which is typical, results from the large absorbance values compared to a n OTE experiment. A plot of A vs. t1I2 for the same experiment is shown in Figure 4,and demonstrates a t1f2dependence of absorbance a t times longer than 20 ms. Good A vs. tl/' linearity was observed for angles greater than 0.4'; deviations a t smaller angles are discussed below. Table I lists data from experiments a t different incident angles a t times from 25 to 500 ms after the beginning of the potential pulse. The theoretical enhancement is the value 2/sin a , determined from the experimental geometry and Equation 3. The observed enhancement is the observed average A / t ' f 2divided by one half the A / t 1 I 2value for a normal incidence (a = 87') experiment. This enhancement
ANALYTICAL CHEMISTRY, VOL.
Table I. Predicted and Observed Absorbance Enhancements as a Function of a: predicted observed mean enhanceenhancea ("1 A/t"Z ment ment 87. 1.99 1.48 1.22 0.96 0.71 0.45 0.19
0.0200 0.546 0.759 0.966 1.126 1.336 1.890 2.187
___
2.00 57.5 77.5 93.9 118 162 254 594
54.6 75.9 96.6 113 134 189 219
Table 11. Observed Absorbance Enhancement as a Function of Time after Initiation of Electrolysis pre-
seconds a (")
1.99 1.48 1.22 0.96 0.71 0.45 0.19
0.05 56.7 75.1 80.5 98.0 120 123 81.5
0.025 0.013 0.005 54.6 74.7 66.7 82.4 81.9 67.4 39.3
50 61 44 45 47 34 18
dicted enhance0 . 0 0 1 ment
30 11 28 3.4 9.5 -0.1 9 . 8 -0.1 15 0.3 11 -1.2 1.0 -11
57.5 77.5 96.6 118 189 254 594
is the increase in absorbance for the glancing incidence experiment over an experiment using an OTE with the beam perpendicular to the electrode surface. Table I1 lists the observed enhancement as a function of time for points a t 50 ms and less after the beginning of the potential pulse. At times longer than 50 ms, the enhancement is constant to within 5 % , and equal to the observed values from Table I. However, a t times shorter than 50 ms, the enhancement decreases both with decreasing time and with decreasing angle. As mentioned earlier, the absorbance is not linear in t 1 / 2in this time region. For a light beam at normal incidence to the electrode surface, an A vs. t1/2plot was linear after 21 ms. At times below 21 ms, the A vs. t1/2 curve for a normal incidence experiment had a shape similar to that in Figure 4. Finally, no significant change in reflectance of the gold electrode was observed when a background experiment was conducted with buffer containing no viologen.
DISCUSSION T h e agreement between the predicted and observed absorbance enhancement for incident angles of 0.96" and above (Table I) confirms Equation 3. The agreement with theory allows the calculation of an effective path length for a given experimental configuration and pulse time, and absorbance may be related to concentration in the same fashion as an experiment based on an OTE. The theory for kinetic systems developed for OTE experiments (1)also applies to the glancing incidence approach, with the latter technique having much greater sensitivity. When it is necessary to know the absorbance enhancement accurately, enhancements of a t least a factor of 100 over conventional OTE experiments may be achieved, when all other parameters are equal. For angles a t 0.71' and 0.45', the absorbance was linear with t'/', but the enhancement was 18-2570 lower than expected. While a n effective path length cannot be calculated accurately a t these small angles, the A vs. t1l2linearity allows the experiments to be used for many purposes, such as spectral characterization and kinetics. The enhancements of up to a factor of 200 over OTE experiments will make such experiments valuable even when absolute quantitation is difficult. For the smallest angle used here (0.19O)the absorbance was
51, NO. 6, MAY 1979
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not linear with t 1 / 2until late in a 500-ms run. Data acquired a t this angle may be useful for detecting short-lived species, but the absorbance vs. time behavior is too unpredictable for quantitation. The decrease in the enhancement at times below 50 ms cannot arise from the optical geometry, so it must be caused either by electrode surface roughness or by slow response of the potentiostat/cell combination. At times below 10 ms, the diffusion layer thickness approaches the magnitude of surface irregularities of the electrode. A beam reflected a t glancing incidence may not interact with the electrogenerated species within the microscopic depressions of the electrode surface and, therefore, will not have the predicted path length. An additional surface effect which may be important in some cases is the change in reflectance of the electrode caused by changes in potential or the electrolysis of solution components. Changes in reflectance of the gold electrode were negligible in background runs with viologen absent, ruling out significant error in absorbance from an interaction between the buffer and the electrode. The linearity of observed absorbance with t1/2for viologen solutions is unlikely if the reflectance of the gold surface were changed by electrolysis; thus electrode reflectance changes can be assumed negligible for the gold/ viologen system. No attempts were made to optimize the transient response of the potentiostat/cell combination, and at least some of the slow response of the absorbance was caused by this deficiency. The fact that the deviations from linearity for A vs. t l i z plots at short times are similar for both glancing and normal incidence implies that cell response is a significant source of deviation below 25 ms. Clearly, the cell response could be improved when experiments require measurements in this short time frame. The lack of agreement between theory and experiment at angles of 0.7" and below is caused at least in part by scattered light from the incident beam which is not reflected directly off the electrode. A t small incident angles, deflection of the incident beam by diffraction, dust in the solution or air, or imperfections in the glass optics and cell will create stray light which will decrease the observed absorbance. A laser source for this work is a convenience, but not a necessity. The only quality of the source which is important is collimation, and sufficiently parallel light can be achieved easily without a laser. An experiment using a continuum source and a monochromator would be conducted in the same fashion as one using a laser source, provided the light is collimated before entering the electrochemical cell. Work at small angles would require a narrow beam which could be easily generated by a suitable mask. The vapor-deposited gold electrode used to examine the glancing incidence technique was not difficult to fabricate and is useful for a variety of systems. However, any electrode having sufficient reflectance and planarity can be used with success. Platinum foil (0.005 inch thick) and a mercury film on platinum were tested, and yielded results comparable to the gold film electrode. When sufficiently polished, glassy carbon would also be a suitable electrode. As pointed out by others ( I 2 ) , reflectivity changes with wavelength can be significant with metals in the UV. The thick gold film or any bulk metal electrode will have much lower resistance than thin films used for OTE experiments, so uncompensated IR effects will usually be negligible. The effective path length increases of factors of 100-200 allowed by this reflection technique have four important implications. First, electrogenerated species with lifetimes in the region of 10 ms and greater create much larger absorbances than those from the corresponding OTE experiment, resulting in improved sensitivity and signal-to-noise ratio.
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
While the present experiments did not yield satisfactory results below 10 ms, t h e problems in this time region are technical and amenable t o improvement. Second, the relatively long path lengths of the reflection mode permit weak chromophores to be monitored, in contrast to most O T E experiments. Third, extensive time averaging is not usually necessary, again in contrast to OTE methods. Many reactions of electrogenerated species are irreversible, and a long period of time must elapse between runs to re-establish initial conditions. In many cases this is impractical, so time averaging is commonly applied to mechanisms involving reversible coupled reactions, such as redox reactions between an electrogenerated component and a solution species. The present technique will be useful where time averaging is not permissible or is very time-consuming. Fourth, a wider range of concentrations may be used with the present method compared to OTE experiments. Lower concentrations are usable because of the longer optical path length, and higher concentrations are possible because of the lower resistance of the working electrode relative to thin-film electrodes. T h e concept of using reflected light for absorbance measurements on electrogenerated species is not a new one. However, when the reflection angle is very small with respect to the electrode surface, significant enhancements in path
length are observed and important experimental advantages result.
LITERATURE CITED Winograd, N.; Kuwana, T. " S p e c t r o e W o c b m i s ~at OpScaliy Transparent Electrodes", in "Electroanalytical Chemistry", Vol. 7, Bard, A. J., Ed.; Marcel Dekker: New York, 1974. Kuwana, T. Ber. Bunsen, Phys. Chem. 1973, 7 7 , 858. Heineman, W. R . Anal. Chem. 1978, 50,390A. Strojek, J.; Kuwana, T. J . Nectroanal. Chem. 1968, 76, 471. Winograd, N.; Blount, H. N.; Kuwana, T. J. Phys. Chem. 1969, 73, 3456. Winograd, N.; Kuwana, T. J. Am. Chem. SOC. 1971, 93. 4343. Petek. M.; Neal, T. E.; Murray, R. W. Anal. Chem. 1971, 43, 1069. Winograd, N.; Kuwana, T. J . Electroanal. Chem. 1969, 23, 333. McIntyre, J. D. E. "Specular Reflection Spectroscopy of the Electrode Solution Interphase", in "Advances in E W o c h e m i i W and EWochemical Engineerlng", Vol. 9, Muller, R., Ed.; Wiley-Interscience: New Ywk, 1973. Bewick, A.; Tuxford, A. M. Symp. Faraday SOC. 1970, 4 , 116. Aylmer-Kelly, A. W. 6.;Bewick, A,; Cantrill, P.; Tuxford, A. M. Faraday Diss. Ch8m. SOC. 1973, 56, 96. Kissinger, P. 1.;Reilley, C. N, Anal. Chem. 1970, 42, 12. Walker, D. C. Anal. Chem. 1967, 3 9 , 896. O'Brien. R . J. Chem. Eng. Data 1973, 78, 142. Steckhan, E.; Kuwana, T. Ber. Bunsen. Phys. Chem. 1974, 78, 253. Landrum, H. L.; Salmon, R. T.; Hawkridge, F. J. Am. Chem. SOC. 1977, 99, 3154.
RECEIVED for review December 6, 1978. Accepted January 25, 1979. This work was supported in part by grant 28412
from the National Institute of Mental Health.
Electrochemical Study of the Degradation of Vitamin K, and Vitamin K, Bisulfite J. C. Vire, G. J. Patriarche," and G. D. Christian' Intstitut de Pharmacie, Universit6 Libre de Bruxelles 2051 1, Boulevard du Triomphe, 1050 Bruxelles, Belgium
Polarography Is used to follow the degradation and the products of vitamin K, bisulfite ( I ) and vitamin K, (11). The degradation scheme is outlined in Figure 2. The predominant degradation of I in alkaline solution is to 11. In neutral solution, gradual isomerization to the naphthoquinone sulfonate (VIII) becomes significant. I1 is degraded primarily via rearrangment to the epoxynaphthohydroquinone ( V I ) in the absence of oxygen, except In very alkaline medium (pH 12) where formation of the naphthohydroquinone(VII) becomes significant. Small amounts of the epoxnaphthoquinone (111) occur by reaction of I1 with O2 or H202formed with VII. V I and I V degrade to the phthiocol ( I V ) and the dinaphthaienetetrone dimer (V), the former becoming more dominant at higher pH. I is more stable than I1 at pH 1 7. The relative rates of degradation are reported.
K vitamins are present in vegetables where they seem to play an active role in photosynthetic mechanisms (1). They also play a role in cellular respiration as electron transporters (2) and in oxidative phosphorylation ( 3 ) . These properties result from the reversible character of this quinone/hydroquinone redox system. In the human body, they are synthesized by microorganisms in the intestine to provide required physiological quantities Permanent Address: Department of Chemistry, University of Washington, Seattle, Wash. 98195. 0003-2700/79/0351-0752$01 .OO/O
and are implicated in synthesis of four blood coagulation factors in the liver ( 4 , 5 ) . Deficiencies of vitamin K in the body are infrequent, but may occur if obstruction of t h e bile or intestine occurs and in cases of liver disease such as cirrhosis ( 4 , 5 ) . Deficiencies are generally accompanied by a decrease in the prothrombin level with an increase in blood coagulation time. At advanced stages, the capillaries may become fragile and hemorrhage. Symptoms are reversed by incorporation of K vitamins in the diet, except in cases of some liver diseases. In recent spectrophotometric studies of naphthoquinones of the important K group vitamins, we have observed several properties related to complex formation with titanium(1V) (6). We have also studied the electrochemistry of vitamin K3 (menadione) and its bisulfite derivative (7-10). We were interested in the stability of the active forms of these compounds in aqueous solution. The present work was undertaken t o determine both the rates of degradation and the degradation products of vitamin K3 and vitamin K3 bisulfite in neutral and alkaline media.
EXPERIMENTAL Direct current (dc), alternating current (ac), and differential pulse (dp) polarographies were used to identify and monitor the compounds resulting from the degradation of these two vitamins. Polarograms were obtained with solutions of pH 6-12 at regular intervals during periods ranging up to between 20 and 50 days. These periods correspond to stabilization of the developed chemical degradation processes. The solutions were stored in the dark. They were deaerated with nitrogen and stored in Teflon-stoppered volumetric flasks to exclude oxygen. Control experiments with electrolyte solutions demonstrated no po0 1979 American Chemlcal Society
