possible ratio of surface area to volume, electrodes consisting of mercury films deposited on a planar platinum substrate have been used with improved sensitivity. In these films, which are typically 1-10 pm thick, the distance from the point innermost in the electrode is substantially reduced and the stripping curves are sharper. Mathematical evaluation of the diffusion equations show that sensitivity improves as the thickness of the film decreases (8, 18). The Hg-Pt OTE represents the extreme evolution in mercury film electrodes. Here the thickness of the mercury film is the minimum amount which is necessary to convert from “platinum character” which gives multiple stripping peaks (Figure 2 4 to “mercury character” which gives a well-defined stripping peak for lead (Figure 2B). This requires as little as 10 mC/cm2 of mercury-a film thickness of ca. 0.015 pm. The time interval for diffusion from the depth of the film to the mercury-solution interface is considerably reduced for this electrode compared to the thicker mercury film electrodes, and stripping curves are consequently sharper. A feature of the Hg-Pt OTE which warrants comment is the total recovery of PbZ+achieved during the stripping procedure at a normal scan rate. Comparison of the charge accumulated during reduction of Pb2+with the charge used for the anodic
stripping indicated complete recovery of lead from the mercury film in cyclic voltammetry with scan rates of up to 0.20 V/sec on 0.97mMPb*+-0.1MKN03. Initial experiments show that the Hg-Pt OTE does exhibit enhanced sensitivity for the stripping analysis of metal ions compared to the commonly used thicker mercury film electrodes. The electrode may prove useful for the analysis of trace amounts of Pb2+ and other metal ions which do not form intermetallic compounds with the platinum which is also dissolved in the mercury film. ACKNOWLEDGMENT
The authors appreciate the helpful suggestions of F.M. Hawkridge concerning the cell design. RECEIVED for review March 15, 1972. Accepted May 30, 1972. This paper was presented in part at the 164th National Meeting, American Chemical Society, New York, N.Y., August 1972. The authors gratefully acknowledge the financial support provided by the National Science Foundation Grants GP31236 (OSU) and GP9306 (CWRU) and the U S . Army Electronics Command, Contract DA AR07-68-C-0278. The authors also acknowledge the Department of Chemistry, Case Western Reserve University, Cleveland, Ohio, where this work was begun.
(18) W. T. de Vries, J. Elecrroanul. Cliem., 9,448 (1965).
Simultaneous Electrochemical and Photometric Monitoring of Intermediates Generated by Flash Photolysis J. I. H. Patterson’ and S. P. Perone Department of Chemistry, Purdue Uniaersity, Lufayette Ind. 47907 This paper describes instrumentation which allows the simultaneous electrochemical and photometric monitoring of transient intermediates generated by flash photolysis. An interface for an on-line digital computer is also described. This system was used to follow the second-order reaction of the ketyl-radical and ketyl-radical-ion generated by the flash photolysis of benzophenone. Because of the simultaneous measurements, it was possible to calculate the value of the molar absorptivity of the ketyl-radical-ion. It was found to be (9.5 & 1.3) x 103M-1 cm-l.
(1) S. P. Perone and J. R. Birk. ANAL.CHEM., 38, 1589 (1966). (2) J. R. Birk and S. P. Perone, ibid.,40, 496 (1968).
by flash photolysis (6). Both of these methods have distinct advantages and limitations. The electrochemical measurements have the disadvantage that they perturb the measured system as a result of electrolysis of electrochemically active species at the electrode. Because of theoretical limitations, it is possible to obtain meaningful kinetic data by continuous monitoring of electrolysis currents ( 2 ) only at times less than the half-life of the reaction if the rate of reaction is other than first-order. Using the time-delay method ( I ) , it is possible to obtain concentration measurements over a much larger time scale; but it is necessary to perform a separate experiment for each datum desired. Moreover, the shortest time at which a measurement can be made is determined by the nature of the solution under study. For highly conducting solutions, no severe limitation is imposed; however, as the conductivity decreases, the time at which the first meaningful measurement can be made increases because of the increased time necessary to charge the double layer. In the limiting case of non-conducting solutions no electrochemical measurement can be made. Electrochemical measurements, however, have the advantage of high and similar sensitivity for reactants and intermediates. The sensitivity is determined mainly by diffusion rates which can easily be deter-
(3) G. L. Kirschner and S. P. Perone, ibid.,44,443 (1972). (4) H . E. Stapelfeldt and S. P. Perone, ibid.,41, 628 (1969). (5) R. A. Jamieson and S . P. Pelone, J . Phys. Cliern., 76, 830 (1972).
(6) G. Porter, “Rates and Mechanisms of Reactions,” S. L. Friess. E. S. Lewis, and A. Weissberger, Ed., 2nd ed., Wiley, New York, N.Y., 1963, Chapter 19.
POTENTIOSTATIC CHRONOAMPEROMETRY has been employed as a method for monitoring intermediates generated by flash photolysis (I-j), and has been applied successfully to the determination of several photochemical mechanisms ( 4 , 5 ) . More commonly, photometric methods introduced by Norrish and Porter have been used to monitor reactions generated Present address. The Milton S. Hershey Medical Center, The Pennsylvania State University. Department of Biological Chemistry. Hershey, Pa. 17033.
1978
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
mined or estimated for any electrochemically active species. As opposed to electrochemical monitoring, photometric monitoring has the advantage that it does not perturb the measured system. Thus, a continuous record of absorbance us. time can be related to homogeneous kinetic equations for any period of time desired. However, if the absorptivities of the intermediates are unknown, there is no means of calculating their concentrations. In many cases, there is no easy method for determining the absorptivities of the intermediates; therefore, many rate constants for photochemical processes are reported relative to the absorptivity. The purpose of this work was to develop a system which will perform simultaneous electrochemical and photometric monitoring of transient intermediates generated by flash photolysis. This system combines the advantages of both monitoring techniques and supplies complementary data on the system under investigation. Moreover, these complementary data can be obtained simultaneously to minimize any ambiguities which might arise if separate experiments were required. In addition, although either monitoring technique might be insensitive to some intermediate or product, it is much less likely that both will be. The simultaneous data acquired by this instrumentation will help deal with complex measurement problems such as mixed electrolytic currents or overlapping absorption bands. The simultaneous independent monitoring of two different reaction intermediates can aid in the elucidation of complex photochemical mechanisms. In addition, the system provides data which allow the straightforward calculation of the absorptivities of transient intermediates. Results are presented here showing the calculation of the absorptivity of the benzophenone radical-ion generated by the flash photolysis of benzophenone. EXPERIMENTAL Flash Radiation. The photochemical flash system is essentially as previously reported (3) with optics as shown in Figure 1. The main difference between this and previous work is that photometric monitoring optics are included. Also, the light entering the cell is not focused on the working electrode, but rather is focused such that photolysis will be as uniform as possible at the bottom of the cell along the optical path of the photometer. Electrochemical Instrumentation. The potentiostat used in this instrument employs positive feedback IR compensation to overcome the problem of high uncompensated resistance found in 80% ethanol solutions used in this work. The potentiostat is similar to that of Brown and Smith (7, 8). In this work, a high speed amplifier (120A, Analog Devices, Norwood, Mass.) is used for the current-to-voltage converter. Analog Devices 149 amplifiers are used as reference electrode follower and the controlling amplifier. The cell potential can be controlled two different ways: 1) A battery supply is used to set the initial potential and a function generator (Model F23OA, Data Royal Corp., San Diego, Calif.) applies a potential step. 2) The computer can control the cell potential through two digital-to-analog converters (DAC); one 10-bit DAC (DAC-491OB, Datel Systems, Canton, Mass.) sets the initial potential, and a fast-setting 10-bit DAC (DAC-VlOB, Datel Systems) applies a potential step. In experiments which do not require the high speed capabilities of the potentiostat, the band-width of the output is limited by passing the signal from the current-to-voltage ~~
~
~~~
(7) E. R. Brown, T. G. McCord, D. E. Smith, and D. D. De Ford, ANAL.CHEM., 38, 1119 (1966). (8) E. R. Brown, D. E. Smith, and G . L. Booman, ibid., 40, 1411 (1968).
Figure 1. Overall optics Cell Interference filter = Flash lamp = Monochromator = First surface mirror = Linear parabolic mirror = Photomultiplier housing S = Light source W = Working electrode C
=
F L M MI M2 P
=
converter through an active low pass filter (9). This decreases the high frequency noise associated with the electrochemical signal, thereby increasing the sensitivity of the measurement. When the hanging mercury drop working electrode (HMDE) is switched out of the potentiostat circuit for a timedelay measurement ( I ) , it is necessary to switch an auxiliary working electrode (Pt wire) into the circuit to maintain system stability (3). This switching is accomplished using a “doublepole single-throw Field Effect Transistor (FET)” (CAG-7, Crystalonics, Cambridge, Mass.) which has a maximum “on-resistance” of 6 ohms. The switching of this FET is controlled by the output of a count-down register composed of six adjustable-count up-down counters (SN74192N, Texas Instruments, Dallas, Texas) with the output of the last counter setting a flip-flop (SN7476N, Texas Instruments). Before a time-delay experiment is started, the count-down register is set by an on-line digital computer (described below), and the flip-flop is cleared, switching the working electrode out of the potentiostat circuit. When the experiment starts, a 10-KHz pulse train is applied to the input of the countdown register. At the end of the period set in the count-down register, the final flip-flop is set, switching the working electrode into the circuit, and the electrochemical experiment begins. This system allows any delay from 100 psec to 99.9999 sec in increments of 100 psec to be set with an uncertainty of ~ t 0 . 2psec. This logic does away with the necessity of setting a monostable for each delay as was previously necessary ( I , 3,5). Photometric Instrumentation. The photometric instrumentation is similar to that normally used for kinetic spectroscopy (IO) with optics as shown in Figure 1. The light from a halogen lamp, S, (No. 64610, Osram, Berlin, Germany) passes through an interference filter, F, (Monopass Filter, Optics Technology Inc., Palo Alto, Calif.) with a band-pass of approximately 50 nm for the UV ranges and 20 nm for the visible. The light is then focused so that it will pass through the reaction cell, C, as closely as possible to the center and the bottom of the cell. The optical path length through the cell is 2.5 cm; the maximum dimensions of the light beam are 2 X 4 mm. The light then passes through a high intensity UV-Visible monochromator, M, (No. 33-86-07 UV-Visible (9) Actioe RC Networks, Burr Brown, 1966, p 68. (10) J. W. Boag, Photochem. Phorobiol., 8, 565 (1968).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
1979
Figure 2. Computer interface Al, Al, A3, A4, = Analog Devices 149 amplifiers R1, = 1KQ1% Metal film resistors Rz, = lOKQ 10% Metal film resistors R8, = 49.9 KQ 1% Metal resistors
I
EC. Ottrmt
R4
Grating in a No. 33-86-26 Monochromator Housing, Bausch & Lomb, Rochester, N.Y.) to a UV-Visible photomultiplier, P, (No. 9781B, EMI, Plainview, N.Y.). The monochromator is equipped with variable slits which for this work were set with the entrance slit at 1.0 mm and the exit slit at 0.5 mm. The output of the photomultiplier is connected to a currentto-voltage converter which is mounted on the back of the photomultiplier housing. The output of this current-tovoltage converter then passes through two additional stages of amplification so that no one stage has excessive gain which would decrease the system band-pass. The latter amplification stage has additional inputs for a battery source and a DAC (DAC-4910B, Datel Systems), allowing the addition of a static offset potential. Computerization. The basic components of the computerized data acquisition system are shown in Figure 2. The system is built around a Hewlett-Packard 2116B computer (Hewlett-Packard, Palo Alto, Calif.) equipped with a Teletype, high speed paper tape reader and punch, 16K words of core memory, and direct memory access. In addition, the computer is equipped with a data acquisition system (11) which includes a unipolar (0 to +10.23 volts) analog-todigital converter (ADC) with a 1-psec conversion time (ADC-F, Analog Devices), and a 5-MHz clock (MCO-50, Wyle Systems, El Seguendo, Calif.) with count-down logic which will give frequencies from 1 MHz to 0.01 Hz in 1-2-5 steps. The signal from the photometer is amplified at A1 to bring the signal to the full range of the ADC. The signal is then routed to a track-and-hold amplifier (SMH-1, Datel Systems) which allows sampling of the signal at a precise time as determined by the timing logic. The output of this track-andhold is then multiplexed to the ADC using a multiplexer composed of a set of “single-pole double-throw FET” switches (CAG-20, Crystalonics, “ON” resistance = 50 ohms) at the summing point of amplifier A4. The electrochemical signal can be inverted at A2 and then amplified by A3 or go directly to amplifier A3. This allows the signal for either oxidation or reduction current to result in a positive voltage for input to the ADC. A voltage from a DAC (DAC 4910B, Datel Systems) is also added to the input signal at A3 to ensure that the input to the ADC is slightly positive (approximately f0.1 V) to ensure a valid base line and allow measurement of small negative deflections of the input signal. From amplifier A3, the signal passes to a track-and-hold amplifier, then to the second channel of the multiplexer, and finally to the ADC. The timing of data acquisition is controlled by the computer in conjunction with external timing logic. The timing (11) E. D. Schmidlin and G. L. Kirschner, Purdue University, Lafayette, Ind., unpublished work, 1970. 1980
diagram is shown in Figure 3. During an experiment, the computer counts the pulses of a 20-KHz (50-psec) clock which starts coincident with the beginning of the experiment (with an uncertainty of *0.2 psec) in order to determine the elapsed time. When a clock pulse occurs, a data acquisition cycle is started and the software checks ahead to determine if electrochemical or photometric data are to be acquired coincident with the next 50-psec clock pulse. If data are to be acquired, the appropriate bits of an output register are set to enable a data acquisition cycle (with the timing in Figure 3) to be started by the next 50-psec clock pulse. Depending on the control bits output, either or both track-and-hold amplifiers switch to the hold mode at a known time, thus allowing assignment of the exact time (accurate to 1 0 . 2 psec) to each datum. If only an electrochemical or photometric datum is desired, the timing sequence for that datum is identical to that shown in Figure 3, with the appropriate half of the system inactive. Once the datum has been digitized, it is transferred directly to core memory using a direct memory access channel, such that the software does not have to handle data as they are input. (Complete circuit diagrams and program listings are available from the authors on request.) Cell and Electrode Placement. The cell and electrodes used in this work are the same as those used in previous work (3). However, in this work, the working electrode placement was considerably different. It is necessaiy for this work that the working electrode be placed slightly off the center line of the cell such that it does not block the photometric light path. In addition, it is necessary that the working electrode be placed such that it and the photometric beam sample the same average concentration of intermediate within the concentration gradient. This concentration gradient is caused by the absorption of light from the flash as it passes through the solution, resulting in less photolysis as the depth increases (2). Reagents. All reagents are as reported in (3). RESULTS AND DISCUSSION
Benzophenone in 80% ethanol and 0.1M NaOH was used as the photochemical system to evaluate the instrumentation. The mechanism for the photochemical reduction of benzophenone in alcoholic solvents is basically ( I , 12) as follows: 0
0 !I
(12) A. Beckett and G. Porter, Trans. Faradcry SOC., 59, 2038 (1963).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
El Figure 3. Timing diagram for timing logic 1. Convert an electrochemical datum 2. Convert a photometric datum
OH
0
,
[CsHs-c-csH~]*
IO
I
I1
----f C ~ H ~ - C - C F 5~ protic solvent (11)
OH
0-
I
I
Figure 4. Electrochemical monitoring of the reaction of the benzophenone radical-ion Monitored at -800 mV cs. SCE Slope = 4.8 X lo7 A-l s ~ c - ~ ’ ? Intercept = 8.2 X lo6 A-l sec-1,2
meanings ( 4 ) . Thus, a plot of l/itl’* us. time has a slope proportional to k2 and an intercept related to Co. The integrated rate equation for a second-order process occurring in homogeneous solution is given in Equation 6 .
To express this in terms of absorbance, the Beer’s law relationship is substituted, resulting in: CsH5-C-c-csHs
1
(IV) Here, the benzophenone (I) is excited to the triplet state which then reacts with an alcoholic solvent abstracting a hydrogen atom, producing a benzophenone free radical (11) and a free radical of the alcohol. At high pH, the ketyl free radical (11) is in equilibrium with the ketyl radical anion (111) which will predominate at the pH of this study (0.1MNaOH). The final step of the mechanism suggested by Beckett and Porter (12) is the reaction of the ketyl radical (11) with the ketyl radical-ion (111) to form benzopinacol-ion (IV). In this study, the second-order reaction of the ketyl-radicalion was monitored electrochemically and photometrically. An equation which expresses the current-time behavior for a potentiostatic chronoamperometric measurement of a photolytic species involved in a second-order chemical reaction was derived ( 2 , 4 ): Tl/2
kza’’2 +-nFAD1i2 t
nFAD1l2Co
(5)
where kz is the second-order rate constant, Co is the concentration of the intermediate at the beginning of the experiment, and the other symbols have the usual electrochemical
1
A,- eb’z
1
CsH5 CsH5
1 -
-1_ - kif
(4)
(7)
where A , is the absorbance measured at time t , e is the molar absorptivity of the reacting species, b is the cell path length, and A0 is the absorbance at the beginning of the reaction. Thus, a plot of l / A t us. time will have a slope of kz/eb and an intercept of 1/AO. Using the value of k2 calculated from the electrochemical data, it is possible to calculate the molar absorptivity from the slope of the plot of Equation 7 . Finally, using the intercept and the molar absorptivity, it is possible to calculate the concentration of the intermediate at the beginning of the experiment. This concentration can be compared with the value calculated from the electrochemical data in order to determine if the photometric beam and the working electrode are monitoring the same average concentration. a1!2
A value of ~- the electrochemical constant, for benzoFA 0112’
phenone was obtained by performing potential step experiments on benzophenone in acid solution where n is known to be one ( 2 ) . In order to obtain the value of the electrochemical constant for the experiments involving the ketyl radical-ion, it was assumed that the diffusion coefficients of the radical-ion and benzophenone are similar ( 2 ) and that the effective area of the working electrode in the flash photolysis experiments was 7 5 z of that in the potential step ex-
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972 *
1981
0
100
200
300
TINE I N S E C )
400
2
Figure 5. Photometric monitoring of the reaction of benzophenone radical-ion Monitored at 610 nm Slope = 7.4 sec-1 Intercept = 1.4
periments. The reason for this latter assumption and the errors inherent in it are discussed below. To obtain the data necessary for the calculation of the absorptivity of the ketyl radical-ion, a solution of benzophenone in 0.1M sodium hydroxide and 80% ethyl alcohol was subjected to a 10-psec flash of approximately 200 joules. The reaction of the radical-ion was monitored simultaneously using potentiostatic chronoamperometry at - 800 mV us. SCE and photometry at 610 nm. A typical set of data is plotted in Figures 4 and 5. Using the slope of the plot of Equation 5 for the electrochemical data along with the electrochemical constant derived for the radical-ion, it was possible to calculate the second-order rate constant. The average value obtained from four data sets (each of at least three individual experiments) was (1.6 f 0.3) X 105M-' sec-I. Using the value of the second-order rate constant obtained from the electrochemical data and the slope of the plot of Equation 7 for four sets of photometric data, an average value of (9.5 i 1.3) X 10aM-' cm-I was calculated for the molar absorptivity of the benzophenone radical-ion. The concentration of the radical-ion at the beginning of the experiment was also calculated for each set of data. As an example, the intercept of the plot of Equation 5 in Figure 4 results in a value of 3.2 x lO-SM for the concentration of the radicalion at time zero while the intercept of the plot of Equation 7 in Figure 5 results in a value of 2.9 X 10-3M. The value of the second-order rate constant derived here agrees well with previously reported values (2, 12), and the
1982
agreement of Co derived from the two sets of data indicate that the two measurement systems are monitoring very nearly the same average concentration. However, the value of the molar absorptivity of the radical-ion derived in this study is approximately a factor of two larger than that previously reported by Beckett and Porter (12). The reason for this latter discrepancy may lie in the assumption made by Beckett and Porter that for every molecule of benzophenone which is destroyed, a benzophenone radical is formed. While this may be true for the overall reaction, there is reason to question the time scale on which all radicals are formed. Several mechanisms have been suggested which would result in the formation of radicals long after the initial hydrogen abstraction step. Filipescu and Minn (13) have suggested a mechanism in which half of the radicals are produced in a slow second-order reaction (kz = 2.75 x 10-aM-' sec-I). The assumption made by Beckett and Porter will result in a value for the absorptivity which is the minimum value possible. In their procedure, the presence of any reaction producing radicals after the initial hydrogen abstraction step should have been taken into account, and would result in an estimated value for the absorptivity which is larger than that reported. The largest experimental error in the work reported here is due to the need to estimate the effective area of the working electrode for chronoamperometric monitoring after a flash. The effective electrode area is less than the total electrode area because the irradiation of the solution adjacent to the electrode is not 100% efficient. Because of the spherical shape of the electrode, the area of the electrode away from the flash is in a shadow cast by the front of the electrode. The value used in this work for the effective electrode area is based on previous work (2, 14) which indicates that a value of 75 % of the total electrode area is a reasonable estimate. Another potential source of error is due to the gross inhomogeneity of the photolyzed solution seen as a concentration gradient. However, the inhomogeneous nature of the solution after a flash should not be reflected to any great extent in the calculation of the absorptivity of the radical ion because the absorbance values which were used in the calculation represent an average value over the width of the optical beam. The instrumentation described here appears potentially very useful for the study of complex photochemical systems. A study using this instrumentation to investigate the mechanism of the photochemical reduction of iron(lI1) oxalate is currently in progress. RECEIVED for review February 25, 1972. Accepted June 28, 1972. This work was supported by a Public Health Service Grant No. CA-07773 from the National Cancer Institute. J. I. H. Patterson received a Graduate Traineeship from the National Science Foundation. (13) N. Filipescu and F. L. Minn. J . Amer. Chem. SOC.,90, 1544 (1968). (14) R . A. Jamieson, Ph.D. Thesis, Purdue University, Lafayette, Ind., 1971.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
