4019
ELECTROCHEMICAL REDUCTION OF OXYGEN
tions from other mechanisms acting simultaneously with the proton-transfer processes. Acknowledgments. This work was supported by Grants GRt-12299 from the National Institute of General Rfedical Sciences and HE-01253 from the National Heart and Lung Institute.
culated data agree qualitatively with experimental results in these ranges. Likepvise this mechanism is not expected to be appreciably applicable between these p H ranges. However, these calculations need to be refined before they can be used subtractively t o determine contribu-
Proton Transfer in the Two-step Electrochemical Reduction of Oxygen in N,N-Dimethylformanide by Helen J. James and Robert F. Broman* Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68608
(Received November 19, 1970)
Publication costs borne completely by T h e Journal of Physical Chemistry
Second-order rate constants for the chemical reaction in the ECE reduction of oxygen iii N,N-dimethylformamide (DMF) with six different proton donors are determined by polarography. The mechanism of reaction during the lifetime of the polarographic mercury drop is shown to involve only a protonation of the superoxide ion and not of the hydroperoxide ion. The values of the second-order rate constants depend both on the acidity of the proton donor and on the proximity of a phenyl ring to the acidic proton within the proton-donor molecule. The phenyl ring is not a necessary requirement for the reaction to occur although it does increase the rate of reaction, Water is shown not to be a major contaminant since water strongly associates with DMF and is not available as a proton donor, even at moderately high water concentrations.
Introduction The reduction of oxygen in protic and aqueous solutions has been postulated to proceed by an ECE reduction through an unstable superoxide intermediate.lJ Superoxide ion, Oa-, has been shown to be a stable intermediate in the reduction of oxygen in solvents with a low availability of protons.3r4 These solvents include dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile, and pyridine. The reduction of oxygen in such aprotic solvents in the presence of proton donors can be studied as an ECE mechanism in which the chemical step is the superoxide protonation. An ECE mechanism, that is, a chemical reaction coupled between two electron transfers, has been investigated for a number of organic compound reduct i o n ~ . I~n ~these ~ studies the chemical reaction is the protonation of the organic anion of such compounds as anthraquinone, naphthalene, and benzophenone, and the reactions were studied in DRIF solutions. The mathematical model for the ECE mechanism has been solved for chronopotentiometry,’ potentiostatic electrolysis,* cyclic ~ o l t a m m e t r y ,and ~ polarographY.’0-’2 Polarography iS often the most Con-
venient method to use since the data can be evaluated easily and the assumptions used in the theoretical ca~culationsare the least limiting; therefore, pOlarOgraPhY 1 ~ used ~ sextensively in this ~ O r k . A Proposed mechanism for oxygen tion on in DMF might be of the form
O2
+
+ e-
-+0 2 .
-
(1)
+ x-
0 2 . - HX b_ H O ~ .
(1) J. P. Hoare, “The Electrochemistry of Oxygen,” Interscience, New York, N. Y., 1965. (2) F. Rallo and L. Rampazzo, J.Electroanal. Chem., 16, 61 (1968). (3) D. L. Maricle and W. G. Hodgson, A n a l . Chem., 37, 1562 (1965). (4) M . E. Peover and B. S. White, Chem. Commun., 183 (1965). (5) P. H. Given and M. E. Peover, J . Chem. Soc., 385 (1960). ( 6 ) H. B. Mark, Jr., Rec. Chem. Progr., 29,217 (1968). (7) A. C. Testa and W. H. Reinmuth, A n a l . Chem., 33,1320 (1961). (8) G. S. Alberts and I. Shain, ibid.,35, 1859 (1963). (9) R. S. Nicholson and I. Shain, ibid., 37, 178 (1965). (10) I. Tachi and M. Senda in “Advances in Polarography,” I. Longmuir, Ed., Paragon Press, New York, N . Y . , 1960, p 454. (11) R. S. Nicholson, J. M . Wilson, and M. L. Olmstead, A n a l . Chem.., 38.542 (1966). . . , (12) J. Janata and H. B. Mark, Jr., J.P h y s . Chem., 72,3616 (1968). T h e Journal of Physical Chemistry, Vol. 76, N o . 26, 1071
HELENJ. JAMES AND ROBERT I?, BROMAN
4020
the expression becomes
HOz-
+ HX “I,HzOz+ X-
The competition of superoxide ion and hydrogen peroxide anion for the proton donor will determine whether the last step of this mechanism occurs or not during the drop lifetime; that is, if k’ is considerably greater than k , the reaction will occur, and if k’ is less than IC, the reaction will not occur. Hydrogen peroxide has an acidity comparable to the phenols used in aqueous solutions and is less acidic than the carboxylic acids used in this study. Therefore, the second protonation step in the mechanism should not necessarily be disregarded. Janata and nfark’s development of the ECE mechanism12 used a system similar to that above in which it was assumed that a second protonation did occur. An ECE mechanism shows a liinetic- as well as a diffusion-controlled portion of the limiting current. The limiting current can be given as iiim =
fiF(dNo,/dt)di,,
+ %F(d”o,./dt)kin
(5)
where n is the number of electrons transferred in each reduction step, F is the Faraday, N is the number of moles of reducible species at the electrode surface, and t is time in seconds, assuming that the chemical step of the mechanism is both fast and reversible. Nicholson, Wilson, and Olmsteadll analytically solved an E C E mechanism under pseudo-first-order conditions, and Janata and developed their model using a reaction-layer concept. The following disoussion is based on Janata and Xark’s work as applied to the E C E mechanism in the reduction of oxygen in DMP. The reduction of oxygen to superoxide ion in eq 1 is the diffusion-controlled portion of the current and the combined eq 2 and 3 are the kinetic-controlled portion. Using the reaction-layer theory and the assump tion that the diffusion coefficients of oxygen and superoxide are approximately equal, Janata and derived the equation ilim
=
+
nFqDo,C0,/(3aDozt/7)”~
21 = AIC’o,.
(4)
+ AoCV~,.-(Cp~~)’/*
(7)
For either case, that is whether the reaction represented by eq 4 occurs or not, the number of moles of oxygen reduced must equal the number of molcs of unprotonated superoxide that diffuse away from the electrode. Thus
+ AOC’O~.-(C’HX)’/~
AlCo, = AIC’o,.-
(8)
If the reaction in equation 4 does occur, there are two moles of the proton donor consumed per mole of superoxide protonated and A~(CHX- C’HX)
=
2AOC’Op.-(C’HX)1/z
(9)
where Az = (7DHX/37d)L’2, However, if the reaction in eq 4 does not occur, only 1 mol of proton donor is consumed per mole of superoxide protonated arid Az(Cax - C’HX)
=
AoCI*o,.-(CBH~)1/2 (IO)
By solving eq 8 and either 9 or 10 for CPo2.-and setting them equal, an equation that is cubic with respect to (C’HX)’/’ is obtained. If the reaction in eq 4 occurs, the result is
+ A~C’HX/A~+
(C”HX)’/’
(2-41Co,/Az - CHX)(C’HX)’/~ - A1Cwx/Ao
0 (11)
The equation is similar if the reaction in eq 4 does not occur but the coefficient of 2 is eliminated from the third term in the equation. By use of a computer, these equations can be solved using a Newton iterative methad for various values of k and concentrations of proton donor and oxygen. From these solutions, values for Co,.- and the limiting currents can be calculated. The calculated limiting currents can be compared to observed currents in order to approximate the rate constant, k , and to determine whether the reaction in eq 4 occurs during the lifetime of a drop.
Experimental Section All chemicals were reagent grade. Solid reagents
were dried over phosphorus pentoxide in a vacuum nFqC”0, - ( 1 0 - 3 k D ~ , C P ~ ~ ~(6) ) 1 / * desiccator. Liquids were used as received. Stock solutions of 0.1 F tetraethylammonium perchlorate where q (=0,85(mt)2/8) is the surface of dme in cm2, DO,is the diffusion coefficient of oxygen in cm2/sec, (TEAP) in DMF were made and used for all reagent solutions. Co, is the bulk concentration of oxygen in mM units, A multipurpose electroanalytical instrument as dek is the second-order rate constant in M-’ sec-I, scribed by Goolsby and Sawyer13 was used for all elecCI*02.-is the reaction volume concentration of supertrochemical experiments, along with a Clevite 1\Iark oxide, CHxP is the reaction volume concentration of 250 Brush recorder. A low-temperature bath mainHX, and t is the drop time corresponding to maximum tained reaction solutions at 5’ (*0.1’) where kinetic current of an undamped polarogram. By defining experiments were performed. The electrochemical A. = (10-3kDo,)1/2 cell had three compartments separated by mediumporosity sintered-glass frits. The compartments conAI = (7D0,/3&)~/‘
I = i11,/2nFq The Journal of Physical Chemistry, Vol. ‘76,N o . $6, 1071
(13) A . D. Goolsby and D. T. Sawyer, Anal. Chem., 39,411 (1967).
ELECTROCREVICAL REDUCTION OF OXYGEN
4021
tained a dropping mcrcury electrode (dmc), platinum clcctrode, and a saturatcd calomrl clectrodr! (SCP), as the working, auxiliary, and rcfcrrnco clcctrode, rcspcctivcly. Oxygcn concrntrations wcrc dvtermincd by a modificd Winlclcr method. l4 A rcviscd sampling technique was ncccssary a t 5' and consistcd of placing 0.5-1.0 ml of dcacratcd DIIB' coolcd in a n ice bath into the sampling syringe before addition of the DAIF sample to be analyzrd. This prevented outgassing of the sample as it warmed to ambient temperature.
Results Oxygen reduction in 0.1 F TEAP solutions of DMF in the absence of proton donors was performed to vcrify the formation of superoxide ion and to characterize its reduction p a r a m ~ t c r s . ~ At 5' the polarographic El/?was -0.54 V us. sce and the plot of log ((id - i)/i) us. E had a slope of 0.065. These parameters agree well with a rrported E,,, of -0.57 V us. sce in 0.1 F tetrabutylamnionium perchlorate solutions of DAW4 and with a slope of 0.070 reported for the reduction in 0.1 F TBAP solutions of DNS0.3 The doubling of thr limiting current with an added excess of proton donor also verifies the superoxide formation. The wave in the absence of proton donors can be classified as quasireversible and was found to have a transfer coefficient, a , equal to 0.75 and an electron-exchange rate constant, ICO, of 2.9 X 10-3 cm/sec, using the graphic method of Vavricka and I
