Electrochemistry of Phenazine at a Platinum Electrode in Aprotic Solvents Donald T. Sawyer and Ralph Y. Komai
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Department
of Chemistry, University of California, Riverside, Calif.
The nonaqueous electrochemistry of phenazine has been studied as function of solvent and solution acidity in dimethylsulfoxide, dimethylformamide, and acetonitrile. Cyclic voltammetry, chronopotentiometry, and controlled potential electrolysis have been used to determine the stoichiometry, thermodynamics, and kinetics of the electron transfer processes. The results indicate that phenazine is reduced in neutral solutions by two one-electron steps with a stable radical anion produced by the first of these. Under acidic conditions, phenazine is reduced by a single two-electron process in DMSO and DMF, and by two reversible one-electron steps in acetonitrile. The interactions of oxygen and of hydrogen peroxide with phenazine and its reduction products have been determined. Reduction mechanisms are proposed which are consistent with the electrochemical data and the products. the electrochemistry of phenazine has been studied previously in aqueous solutions (l) and in acidic methanol (2), a systematic investigation in aprotic solvents has not been made. Two prevous studies at mercury electrodes, one in acetonitrile (3) and the other in dimethylformamide (4-6), included phenazine as one of a series of investigated compounds; detailed measurements were not reported. The difficulties with previous model compounds used for flavin electrochemical studies (7) have caused us to select phenazine(I) as a model for a detailed nonaqueous electrochemical investigation of the electron transfer reactions in dimethylsulfoxide (DMSO), dimethylformamide (DMF), and acetonitrile (CH3CN).
Although
i
Phenazine has been chosen as a model system because of its simplicity and its ring structure similar to the isoalloxazine ring system of flavin molecules. The latter have two nitrogens separated by two carbons in a highly conjugated system. Phenazine also has two nitrogens separated by two carbons in a system with the same order of conjugation as that found in flavins.
EXPERIMENTAL The electrochemical measurements were performed with a versatile instrument constructed from Philbrick operational amplifiers following the design of DeFord (8). Conventional (1) R. C. Kaye and . I. Stonehill, J. Chem. Soc. {London), 1952, 3240.
(2) D. N. Bailey, D. M. Hercules, and D. K. Roe, J. Electrochem. Soc., 116, 190(1969). (3) S. Millefiori, J. Heterocycl. Chem., 7, 145 (1970). (4) D. van der Meer and D. Feil, Reel. Trav. Chim. Pays-Bas, 87, 746 (1968). (5) D. van der Meer, ibid., 88, 1361 (1969). (6) Ibid., 89 51 (1970). (7) B. Janik and P. J. Elving, Chem. Rev., 68, 295 (1968). (8) D. D. DeFord, presented at the 133rd National Meeting, American Chemical Society, San Francisco, Calif., 1958.
92502
electrochemical techniques and equipment were used in conjunction with the DeFord instrument. For cyclic voltammetric and chronopotentiometric experiments, a Beckman platinum-inlay electrode was employed; a platinum gauze electrode was used for the coulometric and electrolysis experiments. The area of the inlay electrode was determined by chronopotentiometric reduction of ferricyanide ion. The reference electrode, which conisted of an aqueous Ag-AgCl electrode in 0.4F tetramethylammonium chloride solution (potential, 0.000 V vs. SCE), was connected by a cracked glass-bead salt bridge to a Luggin capillary. The shield tubes were filled with the supporting electrolyte used in the sample solution. For the coulometric experiments, a glass gas-tight cell was employed. Dimethylsulfoxide (DMSO) (J. T. Baker analyzed reagent grade) was obtained in pint bottles to minimize water contamination; the water content varied between 0.02 and 0.05 %. Tetraethylammonium perchlorate (TEAP) was prepared by stoichiometric combination of reagent grade perchloric acid and reagent grade tetraethylammonium bromide. The product was allowed to crystallize from the cooled solution and was recrystallized twice from cold water. The phenazine, a 25% aqueous solution of tetraethylammonium hydroxide (TEAOH), Spectroquality acetonitrile, and V,V-dimethylformamide were obtained from Matheson Coleman and Bell. A 0.0976F solution of TEAOH in DMSO was standardized against potassium acid phthalate prior to its use to standardize a solution of HCIO4 in DMSO. The latter solution was 0.1F in TEAP and 0.114F in HCIO4, and was prepared by first diluting concentrated perchloric acid to 1 OF with water prior to adding DMSO to it. Solutions of phenazine anion radical were prepared by controlled potential electrolysis at a platinum gauze electrode of phenazine in DMSO containing 0.1F TEAP. The UV spectra were recorded with a Cary Model 14 spectrophotometer using silica cells with solution path The solutions for lengths of 0.10, 1.0, 10, and 100 mm. spectrometric study were prepared from a stock solution of 1.0 X 10" 45678Fphenazine in acetonitrile containing 0.1FTEAP; dilutions were made with 0.1F TEAP in acetonitrile. Sample solutions were acidified by addition of microliter quantities of 4FHC104 from a Hamilton 50-µ1 syringe. The gas chromatographic instrumentation included a Varían Aerograph Model 1200 gas chromatograph equipped with a flame ionization detector, a Leeds & Northrup Speedomax H recorder and a 10% (by weight) sodium sulfatemodified Porasil C column operated at 65°C (9). A Hamilton 10-µ1 syringe was used to sample the coulometric cell which was fitted with serum caps. The system was calibrated with c.p. grade ethylene (Matheson Company). 123
RESULTS
A neutral phenazine solution yields a well defined cyclic voltammogram in DMF, DMSO, or acetonitrile. A typical curve is illustrated in Figure la and indicates a reversible one-electron couple followed by an electrochemically irreversible one-electron reduction. The peaks for neutral solutions diminish upon addition of perchloric acid and a two-electron couple appears at more (9) A. F. Isbell and D. T. Sawyer, Anal. Chem., 41,1381 (1969).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
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Figure 2. Cyclic voltammograms for ImM phenazine in neutral and acidic acetonitrile at a platinum electrode Supporting
TEAP;
scan
0.1F electrolyte, rate, 0.10 V/sec
where all currents are absolute values, iap/iCp is the ratio of anodic to cathodic peak currents, (iCp)o the measured cathodic peak current, (iap)o the measured anodic peak current from the zero current line, and (isn)0 the measured cathodic current at the switching potential (see Table I). A plot of peak current vs. v11- yields a linear graph with a correlation coefficient of 0.987, which confirms the well behaved nature of this couple and the lack of kinetic complications. A similar conclusion for acetonitrile solvent is reasonable from the results of like
1. Cyclic voltammograms for ImM phenazine in neutral and acidic dimethylformamide at a plat-
Figure
inum electrode Supporting electrolyte, 0.1F TEAP; scan rate, 0.10 V/sec
tests.
positive potentials in DMSO and DMF (see Figure 16). The peak potentials are independent of acid concentration. In the case of acetonitrile, two new one-electron couples arise from the addition of acid (see Figure 26); the peak potentials of the more positive couple vary with acid concentration. The neutral solution peaks are completely replaced by the acidic peaks when the ratio of acid to phenazine is 2:1 for
all three solvents. The electrochemical data for phenazine in DMSO are similar to those in DMF for all conditions. With acetonitrile, the acidic data are at more positive potentials than for the other two solvents. The electrochemistry of neutral phenazine in DMSO is essentially reversible on the basis of the peak ratio test as calculated for scan rates, v, from 0.01 to 1.0 V/sec. The ratio of the anodic peak current to the cathodic peak current in cyclic voltammetry should be unity for a well behaved diffusion-controlled process involving no chemical processes in addition to the electron transfer. This is limited to cases which have switching potentials not less than 35jn mV beyond the cathodic peak (10). An empirical formula for determining this ratio of peak currents is given by lap
(lap)o
lev
(lcp)o
0.485(!sp)o (lep)
+ 0.086
·
,
y(nFirD0/RT)lliv'l*
,,,,
W
(D»IDTr/*
n is the number of electrons in the process, F the faraday, D0 and DT the diffusion coefficients of the oxidized and reduced species, R the gas constant, T the absolute temperature, v the scan rate, and a the transfer coefficient. The Dr. equation can be simplified by the approximation that D0 The results of a cyclic voltammetric study of phenazine in DMSO and acetonitrile are summarized in Table II. For neutral phenazine the·value of —log ks,n in DMSO is 2.1 for scan rates between 0.50 and 5.04 V/sec and 1.7 in acetonitrile between 0.12 and 7.46 V/sec. The values for the diffusion coefficients, D0, in the two solvents have been determined on the basis of chronopotentiometric measurements (12). As-
where
=
(1)
o
(10) R. S. Nicholson and I. Shain, Anal. Chem., 36, 706 (1964).
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The simple heterogeneous rate constant, kS}h (the rate constant at the formal potential of the rate determining couple for the electron transfer reaction), can be evaluated from the separation of the anodic and cathodic peak potentials as the scan rate is varied (11). Values for the analytical function, , are obtained from Nicholson’s working curve of peak separation vs. . The rate constant can then be determined from the relation
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
til) R. S. Nicholson, Anal. Chem., 37, 1351 (1965). (12) P. Delahay, "New Instrumental Methods in Electrochemistry,’’ Interscience, New York, N.Y., 1954, Chap. 8.
Table I.
x
Peak Current Ratios from Cyclic Voltammetry of the First Neutral Reduction of Solution of Phenazine in DMSO (0.1F TEAP) at a Platinum Electrode
(V/sec)
(hp)o>
µ
(tap) »
µ
µ
(/ ) >
10 ~3M
a 1.01
(tap)
(tap)
(icp)
(hp)
x1'2 (V/sec)1'1
µ
(icp),
0.01
12.2 12.2 12.3
6.9 6.8 7.2
7.0 6.9 6.7
0.931 0.922 0.912
0.922
0.10
12.2
0.02
17.2 17.3 17.4
9.8
10.0 10.0
0.14
17.3
9.5
10.1
0.946 0.937 0.934
0,939
10.0
0.05
26.2 25.8 25.5
14.0 14.0 14.0
15.8 15.3 15.4
0.948 0.942 0.956
0.949
0.22
25.8
0.1
37.3 37.5 37.2
18.8 19.0 19.8
23.5 23.3 23.3
0.960 0.953 0.970
0.961
0.32
37.4
0.2
57.2 56.6 57.3
27.8 27.0 27.1
36.1 36.3 36.9
0.953 0.959 0.959
0.957
0.45
57.0
0.5
99.2 99.0 99.3
40.0 40.0 40.0
67.5 69.2 68.0
0.962
0.970
0.71
99.2
115.0 113.5 112.0
47.5 47.0 49.0
83.5 85.5 81.5
1.012 1.040 1.026
1.026
1.00
113.5
1.0
0.981
0.966
Average 0.961
Table II. x
Variation of Peak Potentials with Scan Rates and the Calculated Heterogeneous Rate Constants Phenazine in Neutral DMSO
(V/sec)
Ecp
V
vs.
V
SCE
vs.
SCE
5.04 2.44 2.38 1.22 0.498
-1.280 -1.240 -1.240 -1.210 -1.190
-1.040 -1.060 -1.080 -1.080 -1.080
2.50 2.52 1.22 0.48
-0.400 -0.420 -0.396 -0.392
+0.056 +0.016 +0.060 -0.040
7.46 5.04 2.52 1.24 0.248 0.123
-1.290 -1.248 -1.240 -1.220 -1.200 -1.204
,
mV
240 180 160 130 110
+ X 102 7.7 15.0 19.5 30.0 44.8
-log ks,h 2.3 2.2 2.1
2.0 2.1 Average 2.1
Phenazine in Acidic DMSO 456 436 456 352
1.05 1.26 1.05 2.75
3.2 3.0 3.4 3.0 Average 3.2
7.42 12.2 21.7 36.0
1.9 1.8 1.7 1.6 1.4 1.5 Average 1.7
Phenazine in Neutral Acetonitrile
-1.048 -1.052 -1.088 -1.100 -1.120 -1.128
242 196 152 120 80 76
suming that the neutral and acidic species have the same diffusion coefficients, the —log ks,h value for the two-electron process in acidic DMSO is 3.2 for scan rates between 0.48 and 2,50 V/sec. In the case of acidic acetonitrile, the two one-electron processes have —log ks,h values of 1.8 and 1.4, with the more positive process listed first, for scan rates between 0.24 and 5.04 V/sec. Values of ««„ may be calculated from cyclic voltammetric data relating the difference between peak potentials for a given reduction to the logarithms of the scan rates (10) according to the realation (Ep)2
—
(Ep)
=
=
(RT/anaF) In (xi/x2)1/2 (0.0591 /2ana) log (v,¡v2)
(3)
120 150
where (Ep)i and (Ep}> are the peak potentials for scan rates of xi and x2, and na the number of electrons in the rate-determining step. The data have been treated as points for a line rather than using the equation to make a two-point determination. Least squares fits of Ep vs. log (x) yield (from their slopes) «« -values of 0.33 (phenazine-DMSO), 1.14 (acidic phenazine-DMSO), 0.71 (phenazine-CH3CN), and 0.23 [acidic phenazine-CH3CN (peak 2)]. These results indicate one-electron processes for phenazine in both solvents for neutral conditions and for acidic acetonitrile. The ratedetermining step in acidic DMSO appears to be a two-electron process with an a value of 0.57. Controlled potential coulometry has been used to determine the number of electrons in the total process for each of the ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
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oxidation-reduction reactions in DMSO.
When the poten-
tial is controlled at —1.30 V vs. SCE, the results indicate that 1.00 faraday per mole of phenazine is required for reduction; reoxidation requires 0.96 faraday per mole of electroactive material when this process is reversed and the potential is controlled at —0.50 V vs. SCE. Reduction of phenazine at —1.90 V vs. SCE indicates that 1.83 faradays per mole are required for the total reduction at the second peak. Cyclic voltammetry after such electrolysis indicates the occurrence of a post-electrochemical reaction during the time-scale of coulometry to give a species that is oxidized by a two-electron process with a peak potential of —1.1 V vs. SCE. The post-electrochemical reaction is not a reaction with oxygen because the cyclic voltammetry and electrolysis have been carried out in an oxygen-free atmosphere. The one-electron product from the first neutral reduction is a purple radical that gives an ESR spectrum in agreement with previous work (13). The product of the second oneelectron process is orange; the solution passes through a red color during the course of reduction. Controlled potential electrolysis of phenazine at —1.30 V vs. SCE yields a solution of the radical species that can be examined by cyclic voltammetry in the absence of starting material. The oxidation-reduction peaks of the radical species coincide with those observed in cyclic voltammograms of phenazine (Figure 1). Controlled potential coulometry indicates that the reduction of phenazine in acidic DMSO is a two-electron process. The reduction requires 1.87 ± 0.07 faradays per mole of phenazine when the concentration of perchloric acid is at least twice that of the phenazine and the potential is set at —0.41 V vs. SCE. Reoxidation of the reduction product requires 91.8% of the number of coulombs for the reduction and yields the starting material. The initial color is pale yellow. Upon reduction, the color changes first to green and finally to golden yellow. Controlled potential coulometry of phenazine in acetonitrile with an excess of perchloric acid (greater than a 2:1 acid-phenazine mole ratio) requires 1.00 faraday per mole when the potential is set at +0.19 V vs. SCE. When less than a 2:1 ratio of acid to phenazine is present, the number of faradays required is equal to one-half the number of moles of acid. The potential for reoxidation of the product is dependent on acid concentration. The reduction is reversible and coulometry indicates a 95 % recovery of starting material. Reduction of the first one-electron product requires another 0.88 faraday per mole. This too is a reversible reaction on a coulometric basis. The initial solution is yellow-chartreuse in color and becomes green during reduction. The final product of the second reduction is colorless. The chronopotentiometric data for the first neutral phenazine wave in DMSO, between currents of 30 and 80 µ , indicate a constant value for ir112 of 45.7 X 10~6 A sec1/2 for a 1.02 mM solution. The diffusion coefficient for phenazine in DMSO, D, can be calculated from the Sand equation (12)
irll2/C
=
nvll2FAD'l2/2
(4)
where i is the current, r the transition time, A the electrode area, C the bulk concentration, and F the faraday. For phenazine at a platinum electrode (A, 0.233 cm2), D has a value of 6.26 X 10~6 cm2 sec-1. For a neutral solution of 1.00 X 10~3F phenazine in acetonitrile (0.1F TEAP), between currents of 20 and 70 µ , (13) E. W. Stone and A. H. Maki, J. Chem. Phys., 39, 1635 (1963).
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
ir112 has a value of 96.2 X 10-6 A sec1/2 for the first wave; this indicates a value of 2.87 X 10-6 cm2 sec-1 for the diffusion coefficient. A plot of log [(r1/2 r1,2)/(z1/2)] vs. potential yields a straight line in agreement with the equation for a reversible chronopotentiogram (12) -
E
=
ET/4
+ (RT/nF) In
(
-
—-/t
J
(5)
The slope of this plot indicates a one-electron process. Chronopotentiometry of acidic phenazine in acetonitrile results in decreasing values of ir1,2 as i is increased. A plot of ir112 vs. i has a negative first derivative; this indicates a preceding chemical step of the type (14)
Y^=±0;0—>R
(6)
Ultraviolet spectra have been recorded of acidic phenazine in acetonitrile (µ quantities of 4F HC104). For phenazine concentrations from 10~6 to 10" , identical spectra are obtained when absorbance is divided by path length and concentration. The wavelength of the absorption maximum is 2567 A with a molar absorptivity of 1.00 X 105 l./mole-cm. The UV spectrum confirms that an equimolar amount of proton per phenazine protonates the phenazine completely; the spectrum is the same as that obtained with a large excess of acid. A comparison of visible spectra for 10-3 and 10-4F solutions of acidic phenazine indicates that the spectra are similar. No spectrometric evidence has been obtained for a dimeric species. The lack of an oxidation peak for the second reduction of phenazine implies that a chemical reaction occurs following the electrochemical reaction. Previous work has established that tetraethylammonium ion can act as a proton source (75). Coulometry at —1.75 V of a 1.0 X 10~3M phenazine solution in DMSO with 0.1F TEAP in a closed cell (degassed with nitrogen) yields gas samples that give an ethylene peak by gas chromatographic analysis. The area of the peak increases as the electrolysis proceeds. Interactions between Phenazine and Oxygen. The electrochemistry of oxygen in nonaqueous solvents has been studied previously (15, 16). A well defined reversible one-electron couple, present at —0.75 V vs. SCE, and an irreversible reduction at 2.05 V vs. SCE are observed. The product of the reversible reduction is the superoxide ion, 02~. The product of the second reduction is the dianion, 022-, which abstracts a proton from the tetraethylammonium ion present as supporting electrolyte. The hydroperoxide ion reacts with DMSO to yield dimethylsulfone and hydroxide. Neutral solutions of phenazine and oxygen behave the same in both DMSO and acetonitrile. The separate reversible couples of both oxygen and phenazine are observed when a solution of phenazine in DMSO is oxygenated. This behavior indicates the lack of interaction of these starting materials as well as the lack of a phenazine-superoxide interaction. The only discrepancy between the behavior of the mixed solution and a cyclic voltammogram, which is the sum of the voltammograms of the individual species, is the behavior of the irreversible phenazine reduction (second reduction). The potential for this reduction is considerably different from that of the irreversible oxygen reduction. The observed currents, however, indicate that there is a relation between this phenazine reduction and the concentration of the oxygen, especially —
(14) W. H. Reinmuth, Anal. Chem., 33, 322 (1961). (15) A. D. Goolsby and D. T. Sawyer, ibid., 40, 83 (1968). (16) D. T. Sawyer and J. L. Roberts, Jr., J. Electroanal. Chem., 12, 90 (1966).
when the oxygen is in great excess such that the current of the second phenazine reduction becomes much greater than that of the first. The combination of phenazine and oxygen in acidic media in both DMSO and acetonitrile gives normal acidic phenazine couples which remain unchanged with successive cyclics. The acidic phenazine couples are present as long as the cathodic scan does not sweep as negative as the oxygen reduction. Once this is done, acidic phenazine is not present in the region of the electrode and the neutral phenazine couple is observed in DMSO. The mixed system of oxygen and phenazine in acidic media differs only in degree from DMSO to acetonitrile. In the latter, there is a reduction of peak currents. Some neutral phenazine species is formed which corresponds with the loss of acidic species. The addition of oxygen gas into a solution of dihydrophenazine, formed by the electrolysis of an acidic phenazine solution in DMSO at —0.65 V vs. SCE, causes the immediate loss of dihydrophenazine. The resulting solution is the same as a neutral solution of phenazine and oxygen, without a trace of protons or of the acidic form of phenazine. The electrolysis of phenazine in acidic acetonitrile has been carried out in steps. The one-electron product is formed at +0.25 V vs. SCE with cyclic voltammetry used to verify the electrolysis. When oxygen is bubbled into the system, there is no loss of the acidic species as determined by a series of cyclic voltammograms. If the cyclic scan is as negative as the oxygen couple, then the acidic couples are reduced in size. Elimination of oxygen from the system followed by continued electrolysis at —0.25 V vs. SCE yields the fully reduced species. Addition of oxygen, as in DMSO, eliminates the acidic reduced species to give unprotonated oxidized phenazine and oxygen. The reaction of oxygen and dihydrophenazine is faster than can be measured by cyclic voltammetry to give oxidized neutral phenazine and hydrogen peroxide. With an acidified solution of oxidized phenazine and oxygen, there does not appear to be a significant reaction of electrolytically formed dihydrophenazine with oxygen. An oxidation wave is present and successive cyclics show negligible change in the currents of the couple. This indicates that the oxygen reaction with dihydrophenazine does not occur during a cyclic voltammetric time scale. Within the diffusion layer for respective concentrations of dihydrophenazine and oxygen of 1 mM and 3 mM, there is no noticeable reaction over a seven-second period during the voltage scan. The reaction of superoxide ion with dihydrophenazine is rapid because neutral phenazine is present immediately after the oxygen reduction, or, considering the voltage scan rate, within three seconds after superoxide ion begins to form. The reaction is complete, and no acidic oxidation peak is detected. Further experimentation is necessary to determine the mechanistic behavior of this system. A definite solvent effect is present because the acetonitrile data show a decrease in, but not the elimination of, the oxidation peak for dihydrophenazine. Interactions between Hydrogen Peroxide and Phenazine. Addition of hydrogen peroxide to neutral phenazine yields electrochemical behavior that is the same as for the two substances separately. However, the anion radical, formed by electrolysis at —1.25 V vs. SCE, reacts rapidly with two equivalents of hydrogen peroxide per radical. A cyclic voltammogram in the anodic direction, one-half minute after the addition, indicates that no phenazine radical remains, but some superoxide ion is present. Electrolysis of the same solution to obtain the radical, followed by the addition of one
Figure 3. First-order kinetic data for the reaction of dihydrophenazine with hydrogen peroxide in DMSO. i represents the anodic peak current of dihydrophenazine Initial concentrations: lmM dihydrophenazine and O, 1.11 m H202; , 1.48mMH202; and ·, Z.OOmM h2o2
equivalent of hydrogen peroxide per radical, indicates most but not all of the phenazine is converted from the radical to the oxidized state in a rapid reaction. A superoxide oxidation peak is observed again. As a comparison, a similar solution has been electrolyzed to the radical and allowed to decay without the addition of hydrogen peroxide. The decay has been monitored by measuring the peak current for the oxidation of the phenazine radical. The decay of the radical is zero order with a rate constant of —8.5 X 10“7 mole/l.-sec. This radical decay occurs at a rate considerably slower than that observed for the mixed system. Dihydrophenazine reacts at a measurable rate with hydrogen peroxide. This has led to a series of kinetic experiments for mixtures of dihydrophenazine and hydrogen peroxide. Qualitatively, the reaction takes place much slower than the comparable reaction with oxygen. The first-order plots of t vs. In (iaP) for the oxidation peak of the dihydrophenazine are presented in Figure 3. The variation of the rate constant with the concentration of hydrogen peroxide indicates that the peroxide dependence is approximately one-half order. The three values for the first-order rate constant are 1.7 X 10~4, 4.4 X 10~4, and 5.0 X 10“4 sec-1 for respective initial concentrations for hydrogen peroxide of 1.11 X 10~3, 1.48 X 10-3, and 2.00 X 10”3M. DISCUSSION AND CONCLUSIONS
In an earlier study (/), the polarographic reduction of phenazine in aqueous buffers was investigated over a pH range from pH 0 to 13.0 with a variation of half-wave potential from +0.1 V to —0.8 V. At low pH values (below pH 2) two one-electron steps were observed; these coalesced to a single two-electron step as the pH was increased. At pH ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
·
719
Table III.
Oxidation-Reduction Mechanisms for Phenazine in Aprotic Solvents Ep, V vs. SCE (v, 0.1 V/sec)
cath.
anod.
-1.17
-1.11
A. Neutral solutions Ph
+
e-^±Ph-
EtiN
+
HPh" Ph~ + e~ —Ph2"->HPh" —> Ph + H+ + 2e~ B. Acidic DMF or DMSO HPh+ + 2e~ —> HPh" HPh- + HPh+ —> H,Ph + Ph H2Ph —>- HPh+ + H+ + 2e~ C. Acidic CH„CN HPh+ + H+ + e" H,Pht H2Pht + e~ HjPh
-1.85
-1.11
-0.29
+0.03 +0.33 +0.06
+0.43 +0.12
7.38, the half-wave potential was —0.45 V. From the polarographic data, the pKa values for protonated phenazine and the diprotonated radical were determined to be 3.2 and 3.8, respectively. These aqueous data are in strong contrast with the present data for nonaqueous solvents. The aqueous system involves two one-electron reductions at low pH and one two-electron reduction at higher pH, whereas the nonaqueous data indicate two one-electron reductions for aprotic conditions and one two-electron reduction under more
acidic conditions. A related nonaqueous study has been made of the electrochemical oxidation of 4,10-dihydro-5,10-dimethylphenazine (17). The dimethylated phenazine gives two reversible one-electron couples in acetonitrile with anodic halfpeak potentials of +0.11 V us. SCE and +0.83 V vs. SCE. The difference between this simple system and the phenazine system must be attributed to the two methyl groups. These inhibit the oxidation relative to hydrogen atoms and the methyl groups remain associated with the phenazine after oxidation; the protons are easily exchanged in the phenazine system. There is a similarity to the acidic couples which are observed in acetonitrile at positive potentials although the difference between potentials of the successive reactions is much less in the phenazine case. Cyclic voltammetry has been used to study a series of azines at a mercury electrode in acetonitrile (3). Phenazine was an atypical member of the series because of its stable dianion and attendant reversible second wave at a scan rate of 0.4 V/sec. This is in contrast to the present results where an irreversible second wave is observed at a platinum electrode with scan rates up to2V/sec. Phenazine has been studied under acidic conditions in methanol at a platinum electrode (2). The behavior is similar, but not identical, to that observed for acidic acetonitrile. The two one-electron waves in methanol coalesce to a single wave as the acidity is increased. A series of polarographic studies of reduction mechanisms in DMF included phenazine as one of the investigated compounds (4-6). The general conclusion was that the anion products abstract protons from water in the solvent. However, in the present study no evidence for a chemical reaction following the first neutral reduction is observed. The peakcurrent ratios at a platinum electrode in DMSO indicate an essentially reversible reaction without protonation of the first anion product. (17) R. F. Nelson, D. W. Leedy, E. T. Seo, and R. N. Adams, Z. Anal. Chem., 224, 184 (1967).
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
The cyclic voltammograms presented in Figures 1 and 2 indicate the marked effect of solution acidity on the phenazine system. On the basis of the electrochemical data, oxidationreduction mechanisms are proposed for phenazine in aprotic solvents for neutral and acidic conditions; these are summarized in Table III. In neutral solutions, the electrochemical reduction of phenazine proceeds by a reversible one-electron diffusion-controlled reaction followed by an irreversible one-electron reaction. The dianion is sufficiently basic to abstract a proton from tetraethylammonium ion, yielding triethylamine and ethylene (or it may react with H20 to abstract a proton and leave OH- which reacts with TEA+); the monoprotonated species is oxidized by a two-electron process at the potential for the radical anion oxidation to starting material. Previous phenazine studies (2-6), although under different conditions and with other mechanisms, all agree that proton abstraction is of prime importance as a post-chemical step.
In acidic solutions of DMF or DMSO, phenazine is protonated and is reduced by a two-electron process. The reduced product adds a second proton; if excess free protons are not present, it abstracts a proton from protonated phenazine.
In acidic acetonitrile, the potential of the first couple for phenazine is acid-dependent while the second couple is not. Phenazine is protonated when an equimolar amount of protons is present in acetonitrile just as is the case in DMF or
DMSO. Phenazine Interactions with Oxygen and Hydrogen Peroxide. Neutral phenazine does not interact with molecular oxygen or with superoxide ion. These conclusions are based upon the cyclic voltammetric behavior of a solution containing both oxygen and phenazine. The reduction peaks for oxygen and phenazine are each unaffected by the presence of the other in solution. However, the current for the second reduction peak of phenazine is proportional to the oxygen concentration. A tentative explanation is that the final product of the reduction, HPh-, reacts with the oxygen reduction product, Or. The product of the HPh~-02- reaction must be the phenazine radical which may once again be reduced at the same potential, thus yielding a larger current than observed for the first reduction.
HPh- + Or
Ph- + HOr
—>
(7)
A previous electrochemical study of superoxide in DMSO (16) indicates that the hydroperoxide ion reacts with DMSO
HOr + DMSO
DMSOí + OH-
—>-
(8)
The electrochemical data include an anodic peak at +0.80 V vs. SCE, which may be attributable to hydroxide ion. Dihydrophenazine reacts rapidly with oxygen to give neutral phenazine.
H,Ph + 02
Ph
—3»
+ H202
(9)
The kinetic experiments for dihydrophenazine with varying amounts of hydrogen peroxide indicate a first-order dependence for dihydrophenazine, and probably a half-order dependence for hydrogen peroxide. A possible reaction mechanism for such a dependence is H202
^
OH + H2Ph
2
(10)
H20 + HPh
(11)
The HPh is not a stable species in DMSO and tionation reaction is probable. 2
HPh'
—*
H2Ph
+
a
dispropor-
Ph
mechanisms. (12)
The concentration of HPh' should
be small and can be disof the reaction. in the early portion regarded, especially
~dLtÍ2Ph-]
MHPh P
=
fci[H2Ph][ OH]
=
kJC1'2 345678[H2Ph][H202]112
df
-
-
Further experimentation is required to verify the postulated
HHPhI2
(13)
Received for review October 6, 1971. Accepted December 14, 1971. This work was supported by the National Science Foundation under Grant No. GP-16114. We are grateful to the Public Health Service for an Environmental Sciences Traineeship for R. Y. K., PHS Grant No. 5 TOl ES 00084-03.
Comparative Study of a Wide Variety of Polarographic Techniques with Multifunctional Instrumentation A. M. Bond Department
of Inorganic Chemistry, University of Melbourne, Parkville,
3052, Victoria, Australia
D. R. Canterford Department of Physical Chemistry, University of Melbourne, Parkville, 3052, Victoria, Australia
Recent advances in electronics have enabled the pos-
sibility of building extremely versatile multifunctional
Such instrumentapolarographic instrumentation. tion provides opportunity for realistic experimental evaluation and comparison of a large number of polarographic techniques. Using the PAR Model 170 Electrochemistry System, a comparative study of the determination of copper in 1M NaNO by conventional and rapid dc; Tastdc; derivative Tastdc; conventional and rapid ac (both phase-sensitive and nonphase-sensitive); Tast ac; pulse; derivative pulse; differential pulse and inverse or anodic stripping dc, ac, and pulse polarography has been undertaken. The results are reported critically, with emphasis on limits of detection and suitability for routine analysis. The development of the highly promising technique of rapid phasesensitive ac polarography is reported. The electrode parameters for the copper(ll) reduction are obtained from ac measurements. The past two decades have seen the development of a considerable number of new polarographic (voltammetric) techniques (see References 1-8 for example) to supplement the conventional form of dc polarography, which has been in existence for almost 50 years. Pulse polarography, phase(1) B. Breyer and .
H. Bauer, “Chemical Analysis, Vol. XIII, Alternating Current Polarography and Tensammetry,” Interscience, New York/London, 1963. (2) H. Schmidt and M. von Stackelberg, “Modern Polarographic Methods,” Academic Press, New York-London, 1963. (3) D. E. Smith, in “Electroanalytical Chemistry,” A. J. Bard, Ed., Marcel Dekker, New York, N.Y., 1966, Vol. 1, Chap. 1. (4) J. J. Lingane, “Electroanalytical Chemistry,” second ed., Interscience, New York, N.Y., 1958. (5) L. Meites, “Polarographic Techniques,” second ed., Interscience, New York, N.Y., 1965. (6) G. Chariot, Ed., “Modern Electroanalytical Methods, Proceedings of the International Symposium on Modern Electrochemical Methods of Analysis, Paris, 1957,” Elsevier, Amsterdam, 1958. (7) J. K. Taylor, E. J. Maienthal, and G. Marinenko, in “Trace Analysis, Electrochemical Methods,” G. H. Morrison, Ed., Interscience, New York, N.Y., 1965. (8) G. P. Rao and S. K. Rangarajan, Trans. Soc. Adran. Electrochem. Sci. Technol., 4, 116 (1969).
sensitive ac polarography, second haromonic ac polarography, square wave polarography, derivative polarography, and differential polarography may be cited as examples in this respect. As well as being important in fundamental studies, such as detailed examination of electrode processes and their mechanisms, these new techniques have been widely used in trace analysis. Polarographic techniques for trace analysis (7, 8) have infiltrated almost every field in which analytical chemistry is used. The scope of these applications is illustrated by the following examples: pollution, with particular reference to water analysis (9,10); pharmaceuticals (77); rocks and minerals, (72-74); pesticides (15); and petroleum products (16). The extraordinary variety of methodology possible in polarography, which has become available in such a short period of time, has created a problem. A new worker in the field, and indeed even the expert, is now confronted with an extremely difficult task in choosing which method to use for the trace determination of a particular species. This almost overwhelming choice of methodology, within the discipline of polarography, is in contrast to most other analytical disciplines, such as atomic absorption spectrometry, neutron activation, spectrophotometry, and X-ray spectrography. Each of the polarographic methods, in general, requires unique features of instrumentation and, at least in the past, each method has required the purchase of separate polarographic equipment. That is, if one wished to use (9) E. J. Maienthal and J. K. Taylor, Adran. Chem. Ser., No. 73, 1968.
(10) E. B. Buchanan, Jr., T. D. Schroeder, and B. Novosel, Anal. Chem., 42, 370 (1970). (11) R. Kalvoda, Cesk. Farm., 4, 501 (1955). (12) D. Cozzi, in “Progress in Polarography,” P. Zuman and I. M. Kolthoff. Ed., Vol. 2. Interscience, New York, N.Y., 1962, pp 703-711. (13) S. L. Tackett and P. T. Ong, Anal. Lett., 3, 169 (1970). (14) A. M. Bond, T. A. O’Donnell, A. B. Waugh, and R. J. W. McLaughlin, Anal. Chem., 42, 1168 (1970). (15) J. G. Koen, J. F. K. Huber, H. Poppe, and G. den Boef, J. Chromatogr. Sci., 8, 192 (1970). (16) T. Ishii and S. Musha, Rev. Polarogr. (Japan), 16, 61 (1969).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
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