Anal. Ch8m. 1984, 56,301-302
30 1
Metal Substrate Effects on pH Response of Tetracyanoquinodimethane Modified Electrodes Sir: Recent publications have reported on the use of polymer modified electrodes as potentiometric pH sensors (1, 2). Cheek et al. ( I ) argued that in the case of two polymercoated electrodes, namely, poly(l,2-diaminobenzene)and oxidized phenol polymer, the observed pH responses were due to the platinum and carbon surfaces rather than the polymer films. This conclusion is in contradiction with that of Heinemann et al. (2)who suggested that the Nernstian pH response of poly(l,2-diaminobenzene)coated platinum electrodes was due to the protonation of the amine linkages in the polymer. In recent work we have described the voltammetric and spectral response of tetracyanoquinodimethane (TCNQ) modified electrodes (3-5). The effect of pH on the cyclic voltammetry of these modified electrodes can be understood on the basis of the classical 3 X 3 nine-membered square scheme characteristic of quinone-hydroquinone couples (6-9). As for other compounds with quinoid structures, TCNQ exhibits quasi-reversible behavior in protic solvents with an apparent direct 2e-, 2H+ exchange in acidic p H region (IO). This scheme is complicated further by dimer formation (4, 11),disproportionation of the anion radical in strongly acidic solutions (IO, 12, 13) 2TCNQ-M'
+ 2H'
F=
TCNQHz
+ TCNQ + 2M+
(1)
and the apparent reduction of neutral TCNQ by hydroxide ions in very basic media (pH >12) TCNQ
+ Mf + OH- + TCNQ-Mf + OH.
(2)
or 2TCNQ
+ 2M+ + 20H- + 2TCNQ-M+ + HzO + ' / 2 0 2 (3)
In basic solution the formation of TCNQ- is clearly signaled by its electron spin resonance and visible absorption spectra. Similar reactions have been observed by Melby et al. (IO) for interaction of TCNQ with the basic groups of glass and by others for a variety of quinone species (7, 14). While quinone-hydroquinone couples do exhibit subtle electrochemical behavior, they are the basis for a widely used and reliable redox pH electrode. Thus an attempt to utilize TCNQ modified electrodes as a pH sensor seemed to be a logical step based on the kinship between TCNQ and benzoquinone. However, in agreement with Cheek et al. ( I ) , the results support the idea that the electrode substrate plays an important role in determining the response to the pH of the bulk solution.
EXPERIMENTAL SECTION The TCNQ polymer used in this study was synthesized by treatment of 2,5-bis(oxyethanol)TCNQ monomer (15) with stoichiometric amounts of adipoyl chloride in N,N-dimethylacetamide (3). Analytical grade reagents after recrystallization were used for the buffer solutions (16). The film preparation and the electrochemicalapparatus used are described elsewhere (3-5). The potential measurements were carried out with a Fairchild 7050 multimeter and a saturated calomel electrode, SCE. The potential values measured with the help of the multimeter were checked with a Leeds & Northrup 8691 potentiometer and the deviation was within 10.2 mV. The solutions were purged with nitrogen or argon before each measurement. The platinum electrode substrates were either transparent platinum on quartz or Beckman Pt-disk electrodes. The gold electrode substrate was a transparent gold on quartz disk, while
carbon paste electrodes were made in the usual way (17, 18). No special pretreatment of the platinum electrode was applied, but both the platinum and the Pt/TCNQ electrodes were cycled several times between +0.3 and -0.3 vs. SCE and were held at +0.3 V for 5 min before the open-circuit measurements, except for investigation of the virgin TCNQ films. The gold and the carbon paste coated and uncoated electrodes were studied in the same way. All measurements were carried out at room temperature.
RESULTS AND DISCUSSION A freshly prepared virgin TCNQ film electrode in contact with aqueous buffers usually showed a potential ca. +0.220 f 0.020 V vs. SCE. This potential difference was independent of the solution pH in the range pH 2-12. After a long period of time (2 h), there was no change in the optical spectrum characteristic of the neutral TCNQ film (Amm = 412,432 nm). In contrast to this behavior, when the platinum surface was only partially coated, the originally yellow-orange color of the film turned green-blue a t pH > 7.5-8 and the bands at A, = 455, 655, 728, 830, and 1055 nm characteristic of the -1 oxidation state of the TCNQ (5) appeared in the visiblenear-infrared spectrum. In the ESR spectrum the TCNQ-. signal appeared, as well. This color change was reversible: when the electrode was immersed in acidic solution, the neutral TCNQ was regenerated. More interestingly, after cycling the film between +0.3 and -0.3 V vs. SCE and holding a t +0.3 V for 3 min to reoxidize fully the film, the open-circuit pH response obtained was the same as in the case of the partially coated electrode. The potential of the coated electrode was somewhat more negative than that of the bare platinum at certain pH values, which is indicative of a mixed potential, but the slope of the potential/pH curve was essentially the same (0.055 f 0.005) V/pH for both the coated and the bare electrode. Figure 1shows the potential variation as a function of pH for bare Pt, virgin Pt/TCNQ, and electrochemically cycled Pt/TNCQ electrodes. These data were obtained for the same transparent electrode. It should be noted that the measured potential varied with different platinum electrodes by as much as k0.020 V. A similar phenomenon has been observed in previous papers for platinum electrodes when a correlation with the surface pretreatment of the electrodes was found ( I , 19). The preparation of a clean, well-defined platinum surface, the adsorption of different species in contact with aqueous solutions, and the formation of surface oxide films have been subjects of controversies (19) and these problems have not been resolved yet. Even for the Pd/PdO electrode, which was recommended as pH sensor, similar voltage variations at a single pH were found, although the surface pretreatment was identical for all 34 electrodes studied (20). Despite the scattering of the data it can be concluded that, in the case of TCNQ modified platinum electrodes, the pH response is mainly due to the platinum substrate. While the gold and the carbon paste electrodes have not been extensively studied, similar effects could be observed for both electrodes; only the potential/pH slopes were somewhat smaller for these electrodes. The cyclic voltammograms of the TCNQ modified gold and carbon paste electrodes were identical with those observed on platinum substrates. The standard potential of the TCNQOI- couple in the TCNQ-modified electrodes is +0.020 V vs. SCE ( 4 ) . As can be seen in Figure 1, the reaction of the platinum substrate with TCNQ can become spontaneous in basic media. This
0003-2700/84/0356-0301$01.50/00 1984 American Chemical Soclety
Anal. Chem. 1004. 56.302-304
302
w
5). Water is undoubtably incorporated into the film as well during this process which renders the TCNQ film fully electroactive, thus permitting the redox equilibrium, via eq 7 or 8, to be established. These results also suggest the absence of “pinholes” in the virgin films for the transport of water to the platinum substrate. Registry No. Pt, 7440-06-4; Au, 7440-57-5; C, 7440-44-0; TCNQ, 1518-16-7; TCNQ-., 34507-61-4; 2,5-bis(oxyethanol)TCNQ-adipoyl chloride copolymer, 83462-96-8; 2,5-bis(oxyethano1)TCNQ-adipoyl chloride SRU,83462-97-9.
I
-0.2
2
4
6
8
IO
12
LITERATURE CITED
14
PH
Figure 1. pH response of (1) bare platinum electrode, (2)platinum electrode coated with a virgin TCNQ polymer film, and (3)platinum electrode coated with a TCNQ polymer film and cycled between +0.3 and -0.3 V vs. SCE. can be envisioned with the formation of surface platinum oxides via the following reactions:
+ H 2 0 + PtOH + H+ + ePtOH + PtO + H+ + eT C N Q + e- * TCNQPt + H2O + T C N Q * PtOH + TCNQ- + H+
(5)
+ H2O + 2TCNQ * PtO + 2TCNQ- + 2H+
(8)
Pt
(4)
(6)
(7)
or
Pt
(1) Cheek, G.; Wales, C. P.; Nowak, R. J. Anal. Chem. 1983, 55, 380. (2) Heineman, W. R.; Yacynych, A. M. Anal. Chem. 1980, 52, 345. ’ Day, R. W.; Inzeit, G.; Klnstie, J. F.; Chambers, J. Q. J. Am. Chem. SOC. 1982, 104, 6804. 1 Inzelt, G.; Day, R. W.; Kinstle, J. F.; Chambers, J. Q. J. Phys. Chem. 1983, 8 7 , 4592. Inzelt, G.; Day, R. W.; Kinstle, J. F.; Chambers, J. Q . J. Electroanel. Chem ., in press. Jacq J. Electrochim. Acta 1987, 12, 1345. Chambers, J. Q. I n “The Chemistry of Quinoid Compounds”; Patai, S., Ed.; Wlley: New York, 1974; Chapter 14. Laviron, E. J. Nectroanal. Chem. 1983, 146, 15. Inzelt, G.; Chambers, J. Q.; Day, R. W.; Klnstle, J. F., submitted for publication to J . Am. Chem . SOC Melby, L. R.; Harder, R. J.; Hertler, W. R.; Mahler, W.; Benson, R. E.; Mochel, W. E. J. Am. Chem. SOC. 1962, 8 4 , 3374. Boyd, R. H.; Phillips, W. D. J. Chem. Phys. 1965, 4 3 , 2927. Yamaglshi, A.; Sakamoto, M. Bull. Chem. SOC.Jpn. 1974, 47, 2152. Yamaglshl, A. Bull. Chem. SOC.Jpn. 1978, 4 9 , 1417. Lazorov, St.; Trlfonov, A,; Popov, Tz. 2.Phys. Chem, (Lelpzig) 1968, 238, 145. Hertler, W. R. J . Org. Chem. 1976, 41, 1412. Robinson, A. R.; Stokes, R. M. “Electrolyte Solutions”, 2nd ed.; Butterworths: London, 1959. Taken from “CRC Handbook of Chemistry and Physics”, 49th ed.; Weast, R. C., Ed.; The Chemical Rubber Co.: Cleveland, OH, 1968; pp D-78-80. Adams, R. N. “Electrochemistry at Solid Electrodes”; Marcel Dekker: New York, 1965. Chambers, C. A.; Chambers, J. Q. J. Am. Chem. SOC. 1966, 8 8 , 2922. Oilman, S . I n “Electroanalytlcal Chemistry”; Bard, A. J., Ed.; Marcel Dekker: New York, 1968; Vol. 2. Grubb, W. T.; King, L. M. AnalChem. 1980, 5 2 , 270.
.
Precedents for eq 4 and 5 are discussed by Gilman (19). The charge consumed by a typical TCNQ film electrode as determined by cyclic voltammetry or chronocoulometry is 0.3-3 mol cm-2. Since a platinum mC cm-2; Le., (3.1-31) X surface contains 1.3 X 1015atoms/cm2 or 2.2 X lo4 mol/cm2, there is sufficient reactant to reduce a reasonably thick TCNQ film assuming a surface roughness factor in the range 1-10. Gyorgy Inzelt I n this view the TCNQ polymer film acts as a n indicator of Department of Physical Chemistry and Radiology the platinum surface redox state. The large molar absorpL. Eotvos University tivities of TCNQ and TCNQ-. (10) and the facile charge Budapest, Hungary transport process operative for these electrodes (3-5) render James Q. Chambers* the TCNQ polymer ideal for this purpose. James F. Kinstle This explanation implies the penetration of water molecules Roger W. Day to the platinum substrate surface in the cycled and not in the Mark A. Lange virgin TCNQ film electrodes. Penetration of water is reaDepartment of Chemistry sonable since cyclic voltammetric response at cycled TCNQ University of Tennessee film electrodes is found for ions like Fe(CN)63-/4-,Br-/Br3-, Knoxville, Tennessee 37996-1600 and Fe(EDTA)-I2-. The Fe(CN)63-/4-couple appears ca. 0.2 V positive of the TCNQ wave and is attenuated in magnitude for review August 1, 1983. Accepted November 1, somewhat relative to the bare platinum response ( D F ~ ( c N ~ - RECEIVED 1983. This research was supported by the U.S. Army Research N 1 x lo4 cm2 s-l). Office (Project No. P-17715-c) and The University of TenThere is also compelling evidence that counterions are innessee. corporated into the film during the “breaking-in” process (3,
Laser Microprobe Mass Analysis of Refinery Source Emissions and Ambient Samples Sir: Particulate emissions from industrial plants, automobiles, and other sources have a significant impact on the quality of the environment. A typical ambient sample is made up of a collection of particles of differing morphologies from various sources. For both regulatory and environmental
reasons, it would be useful to have a procedure that can determine and quantitate the extent of contributions of various sources to an ambient sample. The major analytical problem involved in such an effort is to contend with the heterogeneity of the particulates that make up an ambient sample. A me-
0003-2700/84/0356-0302$01.50/00 1984 American Chemical Society
