The Electrochemical Reduction of Superoxide Ion and Oxidation of Hydroxide Ion in Dimethyl Sulfoxide Alvin D. Goolsby and Donald T. Sawyer Department of Chemistry, University of Culiforniu, Riverside, Culif. 92502 Ch ronopotentiometry, controlled potential coulometry, pH titrations, infrared spectrometry, and gas chromatography have been used to investigate the electrochemical reduction of superoxide ion and the oxidation of hydroxide ion in dimethyl sulfoxide. Reduction of superoxide ion at a gold electrode gives well defined, irreversible chronopotentiograms that are diffusion controlled and involve a one-electron process. I n the presence of tetraethylammonium perchlorate the reduction products are hydroxide ion, ethylene, triethylamine, and dimethyl sulfone. Oxidation of hydroxide ion gives well defined irreversible chronopotentiograms a t gold or platinum electrodes. The electrode reaction is a diffusion-controlled, one-electron process that gives water and oxygen as products. Electrochemical kinetic parameters and mechanisms consistent with the data are presented for the two systems.
SEVERALELECTROCHEMICAL STUDIES have been published recently concerning the oxygen-superoxide system in nonaqueous aprotic solvents (1-6). There is general agreement that oxygen is reduced in these solvents by a one-electron process at a potential of -0.7 t o -0.8 V cs. SCE with gold, platinum, or mercury electrodes. Diffusion coefficients for oxygen and superoxide ion as well as the electrochemical kinetic parameters for this system in dimethyl sulfoxide (DMSO) have been determined (5). In the same study the conclusion is made that superoxide ion in DMSO [using (C2Hs),NC1O4(TEAP) as the supporting electrolyte] is reduced by a one-electron process at a potential of -2.02 V cs. SCE with either gold or mercury electrodes. The suggested product is the peroxide dianion (Oz2-). Chronopotentiometric experiments indicate that a product of the superoxide ion reduction is oxidized on gold at an potential of $0.75 V cs. SCE; this was believed to be the peroxide dianion. Further work has established that the latter conclusion is incorrect. One of the products of the one-electron reduction of superoxide ion in D M S O is hydroxide ion, which is the species that is oxidized chronopotentiometrically at a n value of $0.75 V. The present paper summarizes the results of a detailed electrochemical investigation of the reduction of superoxide ion and of the oxidation of hydroxide ion in D M S O at gold, platinum, and mercury electrodes, EXPERIhlENTAL
A combination potentiostat-amperostat (7) with a Sargent Model SR strip-chart recorder was used for all of the electrochemical operations and measurements. Most of the chronoKolthoff and T. B. Reddy, J . Elecrrocl7em. Soc., 108, 980 (1961). (2) M. E. Peover and B. S . White, Cliem. Commun., 10, 183 (1965). (3) D. L. Maricle and W. G. Hodgson, ANAL. CHEM., 31, 1562 (1965). (4) E. L. Johnson, K . H. Pool, and R. E. Hamm, Ibid.,38, 183 (1 966). (5) D. T. Sawyer and J. L. Roberts, Jr., J . Electroaiiul. Chem., 12, 90 (1966). (6) M . E. Peover and B. S . White, Electrocl~iin.Acta, 11, 1061 (1 966). (7) A. D. Goolsby and D. T. Sawyer, ANAL.CHEM., 39,411 (1967). (1) I. M.
potentiometric experiments were made with a Leeds and Northrup coulometric cell (No. 7961). For the coulometric experiments a sealed gas-tight cell which could be filled completely was used to prevent the passage of gas in or out of the solution. A three-electrode system was used in each cell, and each had provision for the use of gold, platinum, or mercury electrodes. The area of the mercury electrode was determined geometrically; the areas of the platinum and gold electrodes were determined by chronopotentiometric reduction of ferricyanide ion. An aqueous Ag-AgCI reference electrode was used which has been described previously (5); its potential was 0.000 V cs. SCE. Infrared measurements were made with a Perkin-Elmer Model 137 NaCl spectrophotometer. The gas chromatographic analyses were performed using a Poropak-Q column and Carle Model 100 Micro Detector. Dimethyl sulfoxide (J. T. Baker analyzed reagent grade) was obtained in pint bottles t o minimize water contamination; the water content varied between 0.02 and 0.05%. Tetraethylammonium perchlorate (TEAP) was prepared by stoichiometric combination of distilled perchloric acid and reagent grade (C2HJ4NBr. The product was allowed to crystallize from the cooled solution and was recrystallized twice from cold water. Compressed oxygen (Liquid Carbonic Division of General Dynamics) was used as the precursor for the preparation of superoxide ion. The 25% aqueous solution of (C2H&NOH was obtained from Matheson Coleman & Bell. Known concentrations of superoxide ion to lo-”) were prepared by coulometric reduction of oxygen at - 1 .OO V cs. SCE a t a gold electrode (5). Solutions of hydroxide ion were prepared by syringe addition of 25% (C2HS),NOH to a known volume of DMSO containing 0.10F TEAP. This method gave solutions with reproducible hydroxide ion concentrations (deviation less than i 1 .O %) and minimized the addition of water. The 25% (C?H&NOH solution, which was standardized with potassium acid phthalate, was 1.72F in hydroxide ion. RESULTS AND DISCUSSION
Reduction of Superoxide Ion. As a preliminary part to detailed electrochemical investigations the long range stability of superoxide ion in DMSO has been studied. Solutions containing known concentrations of superoxide ion have been prepared and have been analyzed over a period of time by anodic chronopotentiometry at an amalgamated platinum inlay electrode after saturating the solution with nitrogen gas. A plot of ir1I2 (8) for superoxide ion oxidation U S . time indicates the rate of decomposition where i is the current and T is the transition time. This has provided the means for establishing the conditions for maximum solution stability. Increasing the water concentration in the D M SO increases the rate of decomposition of the superoxide ion. Also, if superoxide ion is generated by oxygen reduction at a mercury pool, there is rapid decomposition due to the presence of mercury. Even the presence of O2 gas in the (8) P. Delahay, “New Instrumental Methods in Electrochemistry,”
Interscience, New York, 1954, pp. 179-216. V O L 40, NO.
1 , JANUARY 1968
83
Table I. Electrochemical Kinetic Parameters for the Reduction of Superoxide Ion in DMSO containing 0.10F TEAP Electrode area: Hg, 0.212 cm*; Au, 0.644 cm2; r, 24 f 0.5" C O?- concn, Electrode moles/liter CYII, Log k ' 1 . h Hg
Au
7.55
x
10-4
6 . 1 2 x 10-4
0.44
-16.01
0.46
-17.86
in T with a given concentration and electrode. This establishes (8) that the reduction is a semi-infinite linear diffusion controlled process over a wide range of concentrations and currents. For irreversible chronopotentiometric systems expressions have been developed for evaluating the kinetic parameters for reduction processes (8)
+
nFAC3kof,h E = 2.3 RT log -anaF i ~
1
I
0
12
I
I
I
24 36 48 Time (sec)
I
L
60
7;
Figure 1. Chronopotentiometric reduction of superoxide ion in DMSO containing 0.10F TEAP
2.3 -~ RT log an, F
[
-2.3 RT nFACokob,h log (1 - a)naF i 2.3 RT log an, F
[
1-
(;)"'I
(1)
and for oxidation processes
E =
_ _
1-
(:>"'I
(2)
Gold electrode (0.64 crn2);current, 30 P A ; 1.0 X 10-3F 0 2 -
solution contributes slightly to the instability of superoxide ion. Thus, solutions of superoxide ion in DMSO are most stable when the water content is minimized, the product is generated at gold electrodes, and the solution is degassed with nitrogen a t the end of the superoxide generation period. The slowest rate of decomposition that has been obtained for these conditions is 2.9 per hour. Quantitative coulometric reoxidation of the superoxide ion to oxygen indicates that the rate of decomposition is higher than 2.9 per hour during the superoxide ion generation period. This probably is due t o the scavenging of low concentration impurities and the relatively high oxygen concentration (2.1 x 1 0 - 3 ~ ) . Using concentration corrections based on the results from the coulometric reoxidation of the superoxide ion and anodic chronopotentiometry, the difin DMSO confusion coefficient for superoxide ion, DO?-, c m 2 sec-' taining 0.10F TEAP, has a value of 0.73 X at 25" C. This value is approximately 2 5 z lower than an earlier value (5)which was based on both anodic and cathodic chronopotentiograms. Superoxide ion is reduced by a one-electron process on gold or mercury in DMSO containing 0.10F TEAP. This has been established by chronopotentiometry and by controlled potential coulometry (5). The chronopotentiometric reduction of superoxide ion at a gold electrode ( E l l 4 ,-2.05 V cs. SCE) is illustrated by Figure 1, which also includes a reverse chronopotentiogram showing a short anodic wave for the oxidation of residual superoxide ion (Eo.22, -0.75 Vcs. SCE) and a major anodic wave (Eo.22, +0.70 V cs. SCE) due t o a product of the superoxide ion reduction. For the superoxide ion reduction wave the value of ir1'2/ACo varies less than + 3 for a fourfold range in concentration; the value of i ~ 1 ' 2 varies less than +4z for a sixfold range
z
z
84
ANALYTICAL CHEMISTRY
where E is the potential of the working electrode us. normal a the transfer coefficient for the hydrogen electrode ("E), reduction process, n, the number of electrons involved in the rate-determining charge transfer step, A the electrode area, F the faraday, R the gas constant, T the absolute temperature, C" the bulk concentration of electroactive species, T the transition time, t the time of electrolysis, and k o f , hand k o o , h the heterogeneous rate constants for the reduction and oxidation processes, respectively. The irreversibility of the superoxide ion reduction proThe cess has been tested by plotting E cs. log [l - ( f / ~ 1'2], ) reduction at gold gives a straight line, whereas reduction at mercury gives only an approximate straight line. These data have been used with Equation 1 to evaluate the electrochemical kinetic parameters for the reduction process, which are summarized in Table I . The value of iT1'? for the reduction process is always slightly greater than it is for the oxidation process with a given superoxide ion concentration and electrode. The added magnitude for the reduction process probably is due to minor intermediate decomposition to a reducible species such as oxygen. The reverse chronopotentiometric wave which is obtained following superoxide ion reduction (Figure 1) has an Eo.22 value of +0.70 V cs. SCE. This value is almost identical to the E l , 4value for hydroxide ion oxidation which is discussed in the next section. Furthermore, plots of Et cs. log [l - ( t / ~ ) I / 2 ]for the reverse wave and the hydroxide ion oxidation wave are linear and have essentially the same slopes and intercepts. This behavior indicates that the two waves are due to the same process--i.e., hydroxide ion oxidation. For convenience, and t o avoid superoxide ion decomposition during the coulometric experiments, oxygen has been reduced directly by two electrons to obtain the one-electron reduction products of superoxide ion. The same processes are assumed t o occur to give the same products that result
Table 11. Infrared Determination of Dimethyl Sulfone Yield from Oxygen Reduction with a Gold Electrode a t -2.05 V us. SCE in DMSO Containing 0.10F TEAP Volume of solution, 40 ml; IR cell path length, 0.025 mm Moles of OI Absorption peak, Moles of Coulombs consumed T (at 8.82 microns) [DMSOJ, F DMSOI produced 5.7 0.0114 f 0.0004 4 . 6 x lo-' 100 5 . 2 x 10-4 195 10.0 x 10-4 11.0 0.022 f 0,001 8 8 x
when superoxide ion is reduced by one electron. This has been confirmed by several step-wise experiments. An acid-base titration of the hydroxide ion produced by the quantitative two-electron reduction of a known amount of oxygen has established that 1.OO i 0.05 mole of hydroxide ion is formed per mole of O2reduced (or per mole of 02-reduced by one electron). The infrared spectrum of a solution obtained by the large scale reduction of oxygen at -2.05 V has a n absorption peak at 8.82 microns which is characteristic of dimethyl sulfone (DMSO?). The resulting solution also has the strong odor of a n amine. When the solution is warmed and the volatile gases that are evolved from it are collected in a small volume of chilled DMSO, strong infrared lines are obtained at 5.87 microns and 8.33 microns for the resulting solution. These lines are characteristic of (C2Hs)3N. The yield of DMSO2 from the two-electron reduction of O2(at - 2.05 V cs. SCE) has been determined by comparison of the height of the absorption peak a t 8.82 microns to that for solutions containing known amounts of DMSO2 in DMSO. The results are summarized in Table I1 and indicate that 1.00 =t0.10 mole of DMSO2 is produced per mole of oxygen reduced. The presence of ethylene above the DMSO solution (0.10F TEAP) after the two-electron reduction of oxygen has been established by gas chromatography. Oxidation of Hydroxide Ion. Chronopotentiometric measurements have established that hydroxide ion is oxidized o n gold o r platinum electrodes in DMSO containing 0.10F TEAP at a n El!,value of f0.75 V cs. SCE. Figure 2 illustrates the oxidation wave for hydroxide ion as well as the reverse wave obtained when the current is reversed. Chronopotentiometric studies of this process for a number of different hydroxide ion concentrations indicate that the value of the chronopotentiometric constant, i r ' / ? / A C , varies less than * 4 z for a sixfold range in concentration. Also, values of i T 1 ' 2 for a given concentration and electrode vary less than k 2 . 5 z for a sixfold range of r values. The diffusion coefficient for hydroxide ion, DorI-, in DMSO containing 0.10F TEAP has been determined from the chronopotentiometric data (8) to have a value of 0.47 X 10-5 c m ? sec- I . A major problem in obtaining quantitative data has been the rapid decomposition of hydroxide ion in DMSO containing 0.10F TEAP. The rate of decomposition in a deoxygenated solution is approximately 13 per hour on the basis of chronopotentionietric measurements and acid-base titrations. The products of the decomposition are ethylene, triethylamine, and water on the basis of gas chromatographic and infrared analyses. They undoubtedly are produced by this reaction (9). OH-
_-
+ (CpHs).iN+
-+
H20
+ (C2Hj)3N + CH:!=CH:!
(3)
(9) R. T. Morrison and R. N. Boyd, "Organic Chemistry." Allyn and Bacon, 1962, p. 533.
Table 111. Electrochemical Kinetic Parameters for the Oxidation of Hydroxide Ion in D M S O containing 0.10F TEAP Electrode area: Pt, 2.14 cm2; Au, 0.644 cm2; t , 24" f 0.5" C OH- concn, Electrode moles/liter (1 - cY)!ltL lOgk"b,h A. Chronopotentiometry Pt 0 37 -8.9 1.72 X 1.46 x lo-: -8.7 0.35 Au B. Galvanostatic method Pt 3.44 x Pt 7.74 x
t1.60
i
10-7
-10.9 -9.5
0.48 0.37
10-1
I
I
1
2 3 4 Time (sec)
I
1
1
I
L
+0.80
w
ooot-
0'
I
I I I I
>
-2.401 0
5
6
Figure 2. Chronopotentiometric oxidation of hydroxide ion in D M S O containing 0.10F TEAP Platinum electrode (2.14 cm?);current, 400 PA; 4.30 X lO-3F OH-
Plots of E us. log [l - (f/r)1'2] for the oxidation of hydroxide ion at platinum or gold give straight lines and thereby indicate that the oxidation process is irreversible (8). Kinetic parameters for the electrode reaction have been evaluated from these plots using Equation 2 and are summarized in Section A of Table 111. VOL. 40, NO. 1, JANUARY 1968
85
Table IV.
Coulombs 4.006 8.006 12.006 14.53
Controlled Potential Coulometric Oxidation of Hydroxide Ion with a Platinum Electrode a t +0.90 V us. SCE in DMSO Volume of solution, 45.7 ml; t , 24" f 0.5" C; electrode area, 2.10 cm2 OH- consumed, ir1'2 x 106, pnoles (for 0 2 A ) sec1/2 [O,],X lo4, F O2formed, jmoles Ratio OH-/02 41.5 83.0 124.3 150.7
126 400 652 845
The kinetics for the oxidation of hydroxide ion also have been studied by the galvanostatic method; the resulting kinetic parameters are summarized in Section B of Table 111. The galvanostatic method involves extrapolation of the chronopotentiometric potential-time curve to zero time for a number of chronopotentiograms (using various currents) and plotting E l = , , us. log i. The slope of the resulting line is proportional to the value of (1 - a)n, and the data points are related t o the value of k O b ,h by Equation 2 (8). When controlled potential coulometry is performed on a solution with a known hydroxide ion concentration, the oxidation reaction produces 0.82 electron per hydroxide ion at +0.80 V us. SCE. This value apparently is low due to the decomposition of hydroxide ion and indicates that one electron per hydroxide ion actually is released on oxidation. The reverse chronopotentiometric wave from hydroxide ion oxidation is illustrated by Figure 2. This wave indicates that O2 is produced by the oxidation reaction [02 is reduced at -0.75 V us. SCE ( 5 ) ] . Controlled potential coulometric oxidation of hydroxide ion with a platinum electrode a t $0.90 V us. SCE has been used to establish the stoichiometry for the oxidation products. The results are summarized in Table IV. The buildup of O2 as a product has been monitored by cathodic chronopotentiometry ; its concentration has been calculated by using a value of 3.23 X 10-5 cm2 sec-1 for the diffusion coefficient of oxygen (5). During the early part of the electrolysis, approximately one oxygen molecule is produced per eight hydroxide ions consumed. However, as the electrolysis nears completion the yield approaches one O2 per four OH- ions and the chronopot.ntiometric oxidation wave due t o hydroxide ion disappears. Infrared measurements have established that the water content of the electrolysis solution increases during the course of hydroxide ion oxidation. Also, during the electrolysis the solution turns a deep bronze color. Attempts t o analyze the colored material have not been successful, but it represents less than 1 of the known analyzed products.
1.21 3.84 6.28 8.14
5.53 17.6 28.7 37.2
7.51 4.73 4.33 4.05
0.10F TEAP. Gas chromatography, infrared spectrometry, and electrochemical methods have confirmed that the products of the reduction are hydroxide ion, ethylene, triethylamine, and dimethyl sulfone. These conclusions can be summarized by a set of mechanisms which are consistent with the kinetic data and the observed products. O2
+ e-
+
02-
(-0.75 V)
(4)
This sequence must occur rapidly because hydroxide ion is observed by reverse chronopotentiometry following the superoxide ion reduction. The chronopotentiometric (1 - a)n, values (Table 111) and the controlled potential coulometry results also establish that hydroxide ion is oxidized irreversibly by a one-electron process at f0.75 V 5s. SCE on platinum or gold. Electrochemical methods and infrared spectrometry have confirmed that the products of the oxidation are water and oxygen. These conclusions can be summarized by two possible sets of mechanisms consistent with the kinetic data and product analysis. A. OH-+ 20H.
OH. +
+ e-
H20z
(f0.75 V)
(8) (9)
CONCLUSIONS
The first set of mechanisms is consistent with the observed decomposition of H z 0 2in DMSO.
The chronopotentiometric una values (Table I) and the controlled potential coulometry results establish that superoxide ion is reduced irreversibly by a one-electron process at -2.05 V us. SCE on gold or mercury in D M S O containing
RECEIVED for review August 4, 1967. Accepted September 29, 1967. Work supported by the National Science Foundation under Grant No. GP 7201.
86
0
ANALYTICAL CHEMISTRY
