response will become quite small if an interferant is present. It is interesting to consider the kinetic behavior of an electrode which does not meet our arbitrary definition of an acceptable steady-state response, i.e., a 50.55. The transient response of an electrode with a = 1.0 is shown in Figure 3, curve d. It is evident that this electrode is for practical purposes, not significantly slower than a perfect (diffusion controlled) electrode. With a = 1.0 we find that the potential will be within 0.1 and 1.0 mV of the final reading when Dt/L2is about 2.77 and 1.79, respectively. Thus an increase in a from zero to unity causes only a minor increase in the steady-state response time of an immobilized enzyme electrode. We may conclude that when the magnitude of the steady-state response is acceptable ([P]z=o/[S]o > 0.5; a C 0.55), the transient response will be dictated by diffusion within the immobilized fluid and not by the amount of enzyme. Under these circumstances there is no point in adding more enzyme solely to improve the response time. The results for the zero-order reaction are not analytically important because the product concentration is independent of the bulk substrate concentration, i.e., from Equation 18 at steady-state ( t = a,z = 0) we find that:
1 VL2
[PI,=”= -2 -(steady-state; 0
[SI 9 KM )
(22)
Neither the amount of enzyme nor the Michaelis constant influence the transient behavior of the electrode in the zero-order kinetic region; it is purely diffusional as seen in Equations 18 and 20. The electrode potential will be within 0.1 mV and 1 mV of the final value when Dt/L2 is greater than 1.98 and 1.06, respectively. Montalvo and Guilbault (12) found that an uncoated univalent ion electrode required 23 s to respond to added ammonium chloride (0 to 10 mM increase) to within 98% of steady-state but the response time increased to 42 s when a 350-pm enzyme layer was placed over the electrode. A 65-pm layer had almost no discernible effect, Le., the intrinsically slow response of the electrode was the dominant factor. Very
few of the papers on enzyme electrodes present sufficient data for a detailed comparison but there is definite qualitative agreement with transient response curves presented in the literature (1,3,12-14), particularly with respect to the extreme dependence of response time on the thickness of the enzyme layer. Long response times are expected for species whose diffusion coefficients are small or when the enzyme membrane system acts to decrease diffusion coefficients. The model described here is not applicable to electrodes whose sensing surface is inherently slow to respond to the product. Based on the above estimates of Dt/L2required to achieve steady-state, it is evident that any significant difference in the transient behavior of the electrode when it is operated under zero-order and first-order conditions indicates that the amount of enzyme on the electrode is too low for fast response. The most appropriate action to take would be to increase the enzyme concentration within the trapped layer but not increase the thickness of the layer.
LITERATURE CITED (1) G. G. Guilbault and J. G. Montaivo, Jr., J . Am. Chem. SOC.,92, 2533 (1970). (2) W. J. Blaedel, T. R. Kissei, and R. C. Boguslaski, Anal. Chem., 44, 2030 (1972). (3) C. Tran-Minh and G. Broun, Anal. Cbern., 47, 1359 (1975). (4) P. Racine and W. Mindt, Experlentia Suppl., 18, 525 (1971). (5) L. D. Meil and J. T. Maioy. Anal. Chern., 47, 299 (1975). (6) L. D. Mell and J. T Maloy, Anal. Chem., 48, 1597 (1976). (7) S. W. Feidberg in “Electroanalytical Chemistry”, Vol. 3, A. J. Bard, Ed.. Marcel Dekker, New York, N.Y., 1969, Chapter 4. (8) F. R. Shu and G. S. Wilson, Anal. Chem., 48, 1679 (1976). (9) M. Mascini and A. Liberti, Anal. Chim. Acta, 88, 177 (1974). (10) L. D. Bowers and P. W. Carr, Anal. Chem., 48, 544A (1976). (11) D. L. Powers, “Boundary Value Problems”, Academic Press, New York, N.Y., 1972. (12) J. G. Montaivo, Jr., and G. G. Guilbault, Anal. Chem., 41, 1897 (1969). (13) G. G. Guiibault and F. R. Shu, Anal. Chim. Acta, 56, 333 (1971). (14) G. G. Guilbault and E. Hrabankova, Anal. Chem., 42, 1779 (1970).
RECEIVED for review October 8,1976. Accepted January 21, 1977. This work was supported by a grant from the National Institutes of Health (GM 17913).
Voltammetry and Potentiometry of Tetrathiafulvalene Halides J. Q. Chambers,*’ D. C. Green, F. B. Kaufman, E. M. Engler, B. A. Scott, and R. R. Schumaker’ I.B.M.T. J .
Watson Research Laboratory, P . O . Box 218, Yorktown Heights, New York 10598
The electrochemistry of tetrathlafulvaiene (lTF) and related donor molecules has been studied in the presence of bromlde Ions In acetonitrile. Evldence for radlcal catlon bromide salt formation is found in the cyclic voltammograms of tetraselenafulvalene and benzotetrathlafulvalene. Coulometric generation of the radical cations followed by addltlon of stoichiometric amounts of halide salts affords a convenient preparation of the donor halide salts. The soiublllty product of TTFBr, ( x 3: 0.8) In acetonltrlle has been estlmated from the potentlometrlc titration curve of TTF wlth tetraalkylammonlum trlbromlde. ‘Present address, Department o f Chemistry, University of Tennessee, Knoxville, Tenn. 37916. Present address, I.B.M. Research Laboratories, San Jose, Calif.
95193.
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ANALYTICAL CHEMISTRY, VOL. 49, NO. 6, MAY 1977
Studies on the solid state conductivity and crystal structure of tetrathiafulvalene (TTF) halide salts have revealed the existence of new materials containing segregated stack phases of the type found in conducting TTF-TCNQ crystals (1-8). These structures, which have been identified for the mixed-valence nonstoichiometric TTF-halide salts, TTFX, where X = C1, Br, or I and 0 < a C 1, often have variable compositions and are difficult to purify by recrystallization. On the other hand, the known stoichiometric TTFX or TTFXz salts are insulators. For example, the TTFBr0.7-0.8 salt is 13 orders of magnitude more conducting than the stoichiometric TTFBr salt (3, 9). These salts have been prepared typically by halogen oxidation of the neutral donor or by metathetical reactions of TTF salts in appropriate solvents (10). These methods do not permit the control over the stoichiometry of the TTFX,
salts which can be realized by electrosynthesis via generation of the donor radical cations followed by metathetical reaction with halide salts (11). The salts can also be prepared by photooxidation of the donors in CX4 solutions (8). The voltammetric behavior of TTF, its selenium analogue, and some TTF derivatives in the presence of bromide is presented below. These experiments along with potentiometric measurements in tetraalkylammonium tribromideTTF solutions provide further insight into the formation and properties of these highly interesting radical cation salts.
EXPERIMENTAL Chemicals. Spectrograde acetonitrile (Burdick & Jackson) was distilled under Nz from P205prior to use and was stored in a dry argon atmosphere. Supporting electrolytes (Southwestern Analytical Chemicals, Inc.) tetraethylammonium perchlorate, TEAP, and tetraethylammonium chloride, TEACl, were dried at 80 “C under vacuum and tetraethylammonium bromide, TBABr, was used without further purification, as was tetramethylammonium tribromide (Eastman). TTF (12,13),BzTTF (14), TMTTF (15),and TSeF (16) were prepared according to previously published methods. Electrodes. The working electrode for the cyclic voltammetric experiments was a Beckman platinum disk electrode (area = 0.228 cm2)which was polished to a mirror finish with Buehler 0.03-pm lapping compound. A large platinum screen electrode (3.5 X 5 cm) was used for the coulometric generation of the donor radical cations. The reference electrode was a Ag wire immersed in 0.01 M AgN08, 0.1 M TEAP in acetonitrile. The indicating electrode for the potentiometric measurements was a 1-cm2platinum foil which had been lightly platinized in a 3% chloroplatinic acid solution containing a trace of Pb(I1). It was found that this electrode gave more stable potential readings than a polished platinum electrode in acetonitrile. Instrumentation. A conventional three-electrode potentiostat (Tacussel Model PRT 100-1X)was used for both the voltammetric and coulometric studies. It was driven by a Tacussel Model GSTP signal generator for the cyclic voltammetric experiments. The potentiometric measurements were made with a Data Precision Model 1450 digital voltmeter. All experiments were conducted in a dry, oxygen-free argon filled dry box (Vacuum Atmosphere Go., Model HE243 Dry Lab equipped with a Model 40-1 Dri-Train gas purifying unit). Powder x-ray diffraction data were used to identify the donor dication, radical cation, and halide deficient salts which were isolated from electrolysis experiments (3, 1I)..
RESULTS AND DISCUSSION Cyclic Voltammetry. The effect of bromide ion on the voltammetric behavior of TTF, tetraselenafulvalene (TSeF), and benzotetrathiafulvalene (BzTTF) has been determined. These donor molecules which become more difficult to oxidize
TTF TSeF BzTTF in the order, TTF (Ell2= 0.03 V, Ag/O.Ol M AgN03) TSeF (Ell2= 0.16 V), BzTTF (Ell2= 0.30 V), exhibit two reversible one-electron waves in their voltammetric behavior (14,17,18). These half-wave potentials, in combination with the formal potential for the Br3-/Br- couple in acetonitrile (0.15 V), establish that the reaction of the radical cation with bromide (Equation 1) is most favored for BzTTF. This reaction represents a means by which a mixed-valence salt may be obtained 2R‘ t 3Br- = 2R t Br; (1) with partially reduced stacks of donor radical cations. The electrochemical behavior is in accord with this view in that cyclic voltammograms of both TSeF and B z n F exhibit waves which are related to bromide-radical cation salts, while voltammograms of TTF exhibit only waves due to TTF and the Br3-/Br-, Br2/Br3- couples in acetonitrile (19-22).
Table I. Voltammetric Parameters of 6.44 x TSeF in 0.1 M TEAP, CH,CN [ Br- 1
Epmod
EpCBth
OM 0.78 x l o - , M 3.5 x 10-3 M 7.8 x 10-3 M
0.195 Vu 0.16c 0.13~ 0.11~
0.130 Vu 0.12c
... ...
M iPlv1
164b 198 191 224
a Volts vs. Ag/O.Ol M AgNO,, 0.1 M TEAP; sweep rate: 0.381 V s-’. Current function in pA/VIIZs-”’. Sweep rate: 0.342 V s-l.
Cyclic voltammograms of TSeF in the presence of bromide ions (added as tetra-n-butylammonium bromide, TBABr) are shown in Figure 1 and data are given in Table I. Addition of TBABr results in the sharpening of both the first and second TSeF oxidation wave on the initial positive-going potential sweep and a shift of the waves to more negative potentials. For the first TSeF wave (0x1 in Figure 1) a t a 1:l molar ratio of Br- to TSeF, the peak width decreases from 60 mV to ca. 40 mV and the current fuction, ip,$lz, increases A large current maximum which from 160 to 190 pAV4.5 appears in the vicinity of the second TSeF oxidation wave in these voltammograms is greatly diminished in subsequent cycles, and at the higher bromide concentrations only the bromide waves (19-22) are evident in multicycle, steady-state voltammograms. Increased amounts of TBABr shift the first oxidation wave to negative potential, 50 mV for A log C = 1, but do not significantly alter the shape of the peak voltammogram. This is consistent with an electrode reaction in which a solid bromide salt is formed on the electrode surface. These results suggest the participation of bromide ions in the electrode reaction. The solid TSeFBro.8,which precipitates from solution when TSeF’. is treated with TBABr TSeF t 0.8Br- -+.TSeFBr,.,(s) + 0.8e(2) is the suspected electrode product in the voltammetric wave 0x1, and may be adsorbed on the electrode surface. Evidently an electrode product passivates the platinum electrode for further TSeF oxidation, but not for Br- oxidation on subsequent cycles. For BzTTF the one-electron BzTTF oxidation wave and the Br-/Br< oxidation wave are nearly coincident at the M level. Cyclic voltammograms of BzTTF, TBABr mixtures (Figure 2) exhibit a reduction “postwave” which is not characteristic of either BzTTF’. or Br3- reduction. I t is likely that this wave, marked by “R’ in Figure 2, is due to reduction of BzTTFBr,. On repeated cycling the platinum electrode passivates for reduction of BzTTF oxidation products. Coulometry. Treatment of electrogenerated solutions of TTF’. and TSeF’. with TBABr in acetonitrile produces TTFBr, and TSeF,, 0.7 < x < 0.8. When the TTF solution is slightly overoxidized and the mixing is rapid, the insulating TTFBr salt can be precipitated (11). Similar results were obtained for BzTTF. Coulometric oxidation of a 1.6 X M BzTTF, 0.1 M TEAP, CH&N solution consumed 1 F/mol of electricity and resulted in precipitation of a perchlorate salt. After dilution to dissolve the precipitate, BzTTFC1, and BzTTFBr, salts were precipitated by addition of the respective tetraalkylammonium halides. Solubilities of the salts were determined by spectrophotometry using either the 440 nm band (log E = 3.40) or the 625 nm band (log e = 3.26) of BzTTF’.. The bromide salt was about five times more insoluble than the chloride salt; the latter had a solubility in acetonitrile of 5 X mol L-’ at room temperature. Rough conductivity measurements on the solids with a volt-ohm meter indicated that the BzTTFC1, was nonconducting (a < 10-GQ-lcm-l)and that the BzTTFBr, ANALYTICAL CHEMISTRY, VOL. 49, NO. 6, MAY 1977
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Figure 1. Cyclic voltammograms of 6.44 X M TSeF, TEABr mixtures in CH,CN, 0.1 M TEAP. Starting potential: -0.6 V; switching potential: 0.98 V vs. Ag/O.Ol M AgN03, 0.1 M TEAP. (A) [Br-] = 0, sweep rate = 0.381 V/s, 0.05 mA per division; (B) [Br-] = 7.8 X M, sweep rate = 0.342 VIS, 0.05 mA per division; (C) [Br-] = 3.5 X M, sweep rate 0.342 V/s, 0.10 mA per division; (D) [Br-] = 7.8 X M, sweep rate = 0.342 V/s, 0.20 mA per division salt was conducting (u > 10-2Q-'cm-'). This result is in accord with the view that a positive or not too negative E" for Equation 1 favors formation of the mixed valence compound (11).
Potentiometric Titration of TTF. Although cyclic voltammograms of TTF-TBABr mixtures gave no indication of TTFBr,, potentiometric studies using a platinized platinum electrode permitted the formation of this salt to be followed. Solutions of tribromide which were prepared either by constant potential coulometric oxidation of tetrabutylammonium bromide solutions or from reagent grade tetramethylammonium tribromide were used as the oxidant. In acetonitrile the tribromide is stable with respect to disproportionation to bromide ion and bromine and the E" of the Br
