electrode history, while the soluble product oxidations run were essentially independent of electrode history, a t least as long as the electrode had been previously treated in standard manner by ethyl acetate pretreatment and no electrode fouling compound had been electrolyzed The limiting current reproducibil ty is very good as illustrated in Tables I and 11. The electroie has been applied to continuous measurement of monamine oxidase enzyme activity where it has been shown
to yield highly reproducible results over relatively long periods of time (12).
RECEIVED for review September 30, 1970. Accepted January 29, 1970. William D. Mason Was a fellow Of the American Foundation for H-mmaceutical E h ~ a t i o n . (12) W. D. M~~~~ and Carter L. 0lson, A ( 1970).
~ cHEL1,, ~ 42, ~ 488,
Optical Ring-Disk Electrode System James E. McClure Research Seroices Department, American Cyanamid Co., Stamford, Conn.
THEROTATED DISK electrode and rotated ring-disk electrode systems and their applications are well known ( I , 2). This paper describes a n extension of the above two systems. The new system is a n optical ring-disk electrode (ORDE) in which a n optically transparent ring surrounds the disk electrode. This arrangement allows recording of optical spectra of unstable materials generated at the disk electrode. The system has been tested by recording the spectra of a stable species (IrC16*-) and a n unstable species (monocation of triphenylamine) which were generated electrochemically.
n MOTOR
LIGHT DETECTOR WORKING ELECTRODE CONTACT BEARINGS
EXPERIMENTAL
Apparatus. The arrangement used for the O R D E studies is shown in Figure 1. The shaft of the rotating electrode assembly was machined from a length of 6i8-in. o.d., 3/16-in. i d . , stainless steel tubing (final 0.d. was 9116 in.). The bearing assembly and contacts for electrical connection to the working electrode were duplicated from a rotated disk electrode system which was designed by Stonehart (3). The shaft was driven through a pulley and belt arrangement with a n Electrocraft Model E500-M electric motor and speed control system. The Electrocraft E500-M is capable of speeds u p to 5000 rpm. A 3 to 1 pulley (PIC Design Corp.) ratio was used in the connection between the drive motor and rotating shaft. The optical ring-disk electrode was constructed by positioning several hundred light wires (American Optical Company) around a platinum rod (l/*-in. diameter) in a hole -5/32 in. drilled into a trifluorochloroethylene cylinder and then sealing with epoxy resin (see Figure lb). After the resin was cured, the face of the O R D E was sanded and then polished t o a mirror finish by conventional resinographic techniques (0.1 p alumina was used for the final polish). The thickness of the optically transparent ring, which is shown in the crosshatched area of Figure lb, is -0.5 mm. The optical ring is immediately adjacent t o the platinum disk electrode. Eccentricity of the trifluorochloroethylene holder was ~t0.0005in. A similar value was determined for the eccentricity of the platinum disk. The light wires surrounding the platinum electrode extend up through the hollow stainless steel shaft and terminate -’/* in. from a photomultiplier tube. Light from a Heath EU-700 monochromator was focused on the O R D E through a window in the bottom of the electrolysis cell with a l/s-in. 0.d. light guide. A Pacific Photometric (1) R. N. Adams, “Electrochemistry at Solid Electrodes,” Marcel Dekker, Inc., New York, 1969, p 67. (2) A. C . Riddiford in “Advances in Electrochemistry and Electro-
chemical Engineering,” P. Delahay, ed., Vol. 4, Interscience, New York, 1965, p 47. (3) P. Stonehart, Anal. Chirn. A m . , 37, 350 (1967).
WORKING ELECTRODE
\ CHROMATOR
WIRES
(ai
(b)
Figure 1. Arrangement for optical ring-disk electrode studies Model 15 Recording Photometer with shielded 1P21 photomultiplier tube was used for light detection. A Wavetek Model 114 signal generator and Model 61RS Wenking potentiostat were used to provide a square-wave excitation signal t o the working electrode. A Princeton Applied Research Model HR-8 lock-in amplifier was used for signal detection in the light measuring circuit. The cell (3l/*-in. diameter) was blackened to exclude light. A platinum counter electrode was isolated from th6 working electrode compartment with a fine porosity frit. Reagents. Na*IrCls and NaJrCI, were obtained from A. D. Mackay and K & K Laboratories, respectively. Eastman triphenylamine and Mallinckrodt nanograde acetonitrile were used without further purification. Procedure. The O R D E system was tested with the IrC163IrC16*- e- system. IrC162- was generated from 4 m M IrC163- (in 0.2M HC104) a t the disk electrode with a square-wave excitation signal. The light attenuation due t o the periodic appearance of IrCI6*- in the light path at the optical ring was measured with a photometer and lock-in amplifier. The spectrum of the monocation of triphenylamine
+
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
551
0.0002
c
I
I
I
I
400
425
450
, A
47s Inn)
I
I
500
125
Figure 2. Spectrum of IrCW (0.2M HClO4) obtained with A, the ORDE system and B, a Cary Model 14 spectrophotometer Spectrum A was obtained by generating IrC16*- from a 4mM solution of IrClK with a 2 Hz square-wave excitation signal. The potential was stepped between + O S and +1.0 V us SCE. Spectrum B was obtained from a solution of IrC162(TPA) was obtained in a similar manner. were carried out at room temperature.
All experiments
RESULTS AND DISCUSSION The iridium system was chosen because its relevant electrochemical behavior, solution properties, and optical characteristics are known (4). The spectrum of IrCp- obtained with the O R D E matched a spectrum of IrClC2- obtained with a Cary Model 14 Spectrophotometer (Figure 2). This agreement indicated that the O R D E system was performing satisfactorily. During generation of IrC16-2 from 4 m M IrCls3-, absorbance values as low as 0.0004 were readily recorded. A squarewave excitation voltage and lock-in amplifier detection system were used t o obtain the spectrum with the ORDE. This approach was required because a signal fluctuation was present in the photometer output. This fluctuation or noise was predominantly due to the rotation of the light wires at the face of the photomultiplier tube. The signal attenuation due t o the small amount of light absorbing material reaching the optical ring was small relative t o this fluctuation in the detector circuit and could not be recorded by conventional dc techniques. The square-wave excitation signal caused the light absorbing material t o appear a t the optical ring periodically. The lock-in amplifier was used t o detect the resulting periodic attenuation signal from the photometer output. With this arrangement, the small signal due t o the light absorbing material was readily recorded. A spectrum of oxidized triphenylamine (TPA) in acetoni-
trile was obtained with the O R D E system at 3600 rpm. [The electrochemical oxidation of TPA has been studied by Seo and Adams (5).] The oxidation was carried out under conditions for which the half-life of the monocation (at the electrode surface) is about 60 milliseconds. This value was calculated from the known rate constant for T P A . + decomposition (6). A peak and shoulder were observed at 648 nm and 575 nm, respectively. These values are in reasonable agreement with values of 656 nm and 570 nm for a peak and shoulder, respectively, reported by Lewis and Lipkin (7) for TPA.+ in a rigidmedium. Seo and Adams (5) reported a peak at 640 nm for TPA.+ in acetonitrile. A shoulder observed a t 680 nm is presumably due t o the dication of tetraphenylbenzidine (TPB). We have observed a peak at 684 nm for the final dark blue solution resulting from a two-electron bulk oxidation of TPA at f1.3 V cs. SCE. A blue two-electron oxidation product has also been reported by Dvorak (8). TPB2+ is present because some TPA.+ dimerizes to form TPB before the T P A , + can be swept away from the disk electrode. TPB can then undergo further oxidation at the potential at which TPA, +is produced. The absorbance at 680 nm becomes more evident at lower rotation rates, because more time is available for TPB t o form and undergo oxidation at the disk electrode. The variability of rotation rate appears t o be a useful feature of the system. Thus, if a spectrum with two or more peaks is observed, the peak due t o a primary product can be established by increasing the rotation rate and observing which peak increases relative to the others. (The current cs. rpm1’2 relationship is obeyed with the ORDE system. This behavior was demonstrated with ferrocyanide in 1M KCI.) The spectrum of T P A . + could have been obtained with less interference from the higher oxidation state of TPB by carrying out the measurement at lower bulk concentration of TPA. The high concentration (7 m M ) was used to demonstrate that the system could be used to distinguish the primary product under conditions for which the product is fairly unstable. The ORDE system should find use in the study of electrode reaction mechanisms by providing additional information (spectral) about such systems. Emphasis can also be placed o n use of the system to obtain optical spectra of unstable materials. Such species could be generated either directly or indirectly. Additional experiments are planned in order to establish the limitations of the system in terms of application to studies of short-lived species. The use of transparent thin film rotated electrodes is also being considered. The results will be reported in a future publication. RECEIVED for review October 29, 1969. Accepted January 7, 1970. (5) E. T. Seo, R. N. Adams, et nl., J . Amev. Chem. SOC.,88, 3498
(1966).
(4) I. A. Poulsen and
2032 (1962).
552
c. S .
Garner, J . Amer. Chem. Soc., 84,
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
(6) R. F. Nelson and R. N. Adams, ibid., 90, 3925 (1968). (7) G. N. Lewis and D. Lipkin, ibid., 64,2801 (1942). ( 8 ) V. Dvorak, Microckem. J . , 12,99-116 (1967).
