Effect of Several Metal Ions upon Polarographic Reduction of Oxygen in Dimethylsulfoxide Edward L. Johnson, Karl H. Pool, and Randall E. Hamm
Department of Chemistry, Washington State University, Pullman, Wash. 99163 In solutions containing both oxygen and one of the metal ions Zn+2, Sr+2, TI+, Y+3, Cdfz, a new polarographic reduction wave was observed at potentials less cathodic than that required to reduce the most easily reducible species (either the metal ion or the oxygen) in the sample. The variation of the height of this new wave was investigated as a function of the concentration of oxygen and the various metal ions. The results of this study made possible a determination of the stoichiometry of the process responsible for the new wave. Controlled potential coulometry and controlled potential electrolysis were used to obtain information about the reduction process. The reduction products, prepared by electrochemical methods, were found to be the superoxides of zinc, strontium, and thallium; and the peroxides of cadmium and yttrium.
THE EFFECT of alkali and alkaline earth metal ions on the polarographic reduction of oxygen in dimethylsulfoxide (DMSO) solutions was reported (I). The reduction waves were markedly dependent upon the cation composition of the test solution. Peover ( 2 ) has reported similar type findings for the polarographic reduction of several quinones in dimethylformamide. Our earlier work had indicated that this shifting of the waves was quite complex and suggested that the metal ion was possibly participating in some type of catalytic reduction of the oxygen. Preliminary investigations with added zinc salts showed a somewhat similar shifting of the waves along with changes in the height of the shifted wave as a function of the zinc ion concentration. The purpose of this investigation was to examine the effect of added metal ions in more detail and, if possible, electrochemically produce and identify the reduction products. EXPERIMENTAL
Materials. The DMSO was obtained from the Matheson Company. This and the other chemicals were used as described previously (I). The nitrates of zinc, cadmium, strontium, thallium, and yttrium were of reagent grade and used without further purification. Stock solutions of the metal salts were prepared in DMSO. The zinc, cadmium, strontium, and yttrium solutions were standardized by complexometric titrations with ethylenediaminetetraacetic acid disodium salt. The thallous solution was standardized by amperometric titration with potassium iodide. Apparatus. A three-electrode polarograph of the ORNL type described by Kelley, Jones, and Fisher ( 3 ) was used to record all polarograms. The use of three-electrode polarography resulted in improved polarograms when compared to conventional two-electrode polarograms. The slope of the E us. log(id - i>ji plots for the first reduction wave of oxygen was found to be 61 mV. (1) E. L. Johnson, K. H. Pool, and R. E. Harnrn, ANAL.CHEM., 38, 183 (1966). (2) M. E. Peover and J. D. Davies, J. Elecfroanal. Chem., 6 , 46 ( 1963). (3) M. T. Kelley, H. C. Jones, and D. J. Fisher, ANAL.CHEM., 31, 1475 (1959).
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ANALYTICAL CHEMISTRY
I ’ -0.420
I
I
-0.583
I
I
-0.746 Potentiel, Voltr VI SCE
I
I
-0.909
Figure 1. The effect of added zinc ion to an oxygen-containing solution of DMSO Curve A . 25 ml of 0.1M KCIOain DMSO and 0.40mM in oxygen Curve B. Same solution but now 0.46rnMin zinc ion Curve C. Same solution but now 1.00mMin zinc ion; all curves start at -0.420 V. The base line has been offset to allow easy comparison of the curves
A conventional potentiostat utilizing operational amplifiers was used for both the controlled potential coulometry and the controlled potential electrolysis. A Honeywell “Electronik 19” recorder was used in the coulometric experiments to record current us. time data. All potentials were referred to aqueous saturated calomel electrodes and are so reported. Procedures. POLAROGRAMS. A 25-ml. sample of DMSO, which was 0.1M in tetraethylammonium perchlorate or potassium perchlorate, was placed in the test cell. After an appropriate oxygen concentration was obtained by bubbling oxygen and/or nitrogen gases through the test solution, the cell was closed off from the atmosphere. Addition of the metal salt solutions was accomplished by micropipets so that relatively concentrated solutions could be used to avoid dilution of the oxygen in the sample. Dried nitrogen gas was passed over the surface while the cell was open for this addition of solution. All stirrings were done manually in order to keep the oxygen concentration approximately constant. The metal ion solutions were thoroughly de-aerated with dried nitrogen gas prior to use. The use of KCIOl as the supporting electrolyte was based on the fact that the first wave displays no shift in the presence of potassium ion ( I ) . No formation of KO2 was found in any electrolyses. CONTROLLED POTENTIAL COULOMETRY. A 25-ml sample of DMSO (0.1M in potassium perchlorate) containing a known concentration of oxygen and metal ion, was placed in the sample compartment of the cell. The counter electrode was a large piece of Pt gauze separated from the sample compartment by an agar salt bridge. The reference electrode was a saturated calomel electrode. The potential used for the electrolysis was selected from a polarogram that was run
prior to the electrolysis. A vigorous stream of nitrogen gas was kept over the surfrtce of the sample during the electrolysis. The mercury cathode and solution were stirred with a nichrome stirring rod connected to an overhead stirring motor. A 10-ohm resistor was placed in the mercury lead and the potential drop across it was recorded as a function of time. -~ 1 he xvzrage electrolysis required approximately 45 minutes when the oxygen cortcentration was 0.42mM. The maximum drift of the poi.entiostat during an electrolysis under these conditions was found to be approximately 12 mV. Experiments on oxygen solutions with only supporting electrolyte present galre an average n value of 1.03 for the first oxygen reduction wave. The cell arrangeCONTROLLED POTEN riAL ELECTROLYSIS. ment was the same as that used for the coulometry experiments. However, the sample solution was 25 ml of the stock 0.2M solutions of the metal ion. Oxygen gas was continually bubbled into the solution to keep it saturated. The average current through the cell under thesz conditions was. 4 niA. The electrolysis was continued for an average of about 12-14 hours. This was sufficient time to produce several hundred milligrams of the product. Two or three different 25-1111 samples were electrolyzed to check the reproducibility of the electrolysis in the case of each metal investigated. All polamgrams and electrolyses were thermostated at 25.0 i 0.1” (2. RESULTS AND DISCUSSION
Zinc. The half-wave potential (Eliz) of zinc was found to be approximately --0.95 V in a DMSO solution. There was some difficultyin making this estimate because there was an unsupprexsibie maximum similar to that observed by Kolthoff and Reddy lt4) for nickel and cobalt in DMSO. The diffusion coefficiertt of zinc was found to be 1.56 X 10-6 cm Y/sec. Adding zinc to an oxygen-containing DMSO solution produces a new wave at less cathodic potentials than is required for the usual reduction of oxygen as is shown by Figure 1. The height of the new wave increased linearly with the zinc ion concentration until a maximum height was obtained. With further additions of zinc, no further changes took place in the height of the wave. Using this situation a good amperometric titration of the oxygen in a DMSO solution could be made by adding a DMSO solution of zinc nitrate a s the titrant. Such amperometric titrations were performed on a number of solutions of different oxygen concentrations with a very sharp break in the titration curve in all cases. If the concentration of oxygen in the test sample was greater than cu. 0.42mM, the new wave was badly distorted, apparently because of some type of adsorption process. In order to determine if this process was an adsorption process, the temperature coefficient of the diffusion current for the reduction process was evaluated. This was accomplished by obtaining a low concentration of oxygen to prevent losses due to changes in solubility with temperature and then recording the polarogram at various temperatures in the range 20-30°C. Then an excess of zinc was added to the solution and again the polarogr,ims were recorded at various temperatures. The temperature coefficient of the oxygen reduction diffusion current was found to be 2.61 %/deg at 25°C. That of the new wave in the presence of zinc was found to be a negative -0.34z/deg at 25°C. According to Vlcek (3, _
_
~
( 3 ) 1. hl. Kolthoff and T. B. Reddy, J. Electrochem. Soc., 108, 980 (1 960). (5) A. A. Vlcek, ‘‘Progress in Inorganic Chemistry,” F. A. Cotton Ed., Vol. 5 , lnterscience, New York, 1963, p. 234.
temperature coefficient is indicative of an unstable product of the first step of the electrode process with adsorption as the rate-determining step. The ratio of the new wave height after it had attained its maximum height to that of the original oxygen wave was found to be 1.23. The normal reduction wave of zinc did not appear until the new wave had reached its maximum height. When additional zinc was added beyond that required to cause the new wave to attain its maximurn height, the zinc wave appeared but was long and drawn out. This distortion of the zinc wave is probably due to the adsorption process described above. At an oxygen concentration of ca. 0.40mM the shift in the of the new wave from that of the oxygen wave was found to be approximately -140 mV. However, it was found that the ElIzof the new wave was a logarithmic function of both the oxygen concentration and that of the zinc ion. Thus for a given oxygen concentration the E1,2 varied logarithmically with the zinc concentration until a concentration was reached which produced no further changes (when the new wave had attained its maximum height). For a fixed metal ion concentration the E,,* varied logarithmically as the concentration of oxygen was increased. The slope of the E I ; S . log(id - i ) / i plots for the new wave was found to be 61 mV. Controlled potential coulometry at a potential on the diffusion plateau of the new wave yielded an average n value of 1.03 (calculated on the basis of the oxygen concentration in the test solution). Controlled potential electrolysis yielded a grayish black precipitate. This precipitate was separated from the mercury pool and the DMSO solution by centrifugation. The precipitate was washed with acetone and air dried. During this process it was found that the precipitate was apparently composed of two components. One was jet black in color and the other was white. If the black precipitate was allowed to stand, it slowly decomposed into a white precipitate. The black precipitate dissolved in acidic solutions with the evolution of a gas. The white precipitate also dissolved in acidic solutions but no gas was evolved. If we assume the black precipitate to be zinc superoxide and the white precipitate to be zinc peroxide, the theoretical percentages of zinc and oxygen in the two compounds should be 50.6 and 49.4 in the superoxide and 67.2 and 32.8 in the peroxide. Superoxide disproportionates stoichiometrically in aqueous solutions forcing one to analyze for the resulting peroxide produced. When the black precipitate was analyzed, the percentage of zinc ranged from 52.3 to 56.5 % and that of oxygen (as superoxide) ranged from 46.1 to 50.3% with averages of 54.6% Zn and 48.0% oxygen. The zinc was determined by EDTA titration and the oxygen was determined by permanganate titration. In a similar manner the white precipitate was analyzed and found to contain from 66.5 to 67.8% Zn with 67.2x Zn as the average, and from 32.5 to 33.8 oxygen (as peroxide) with 33.3 % as an average. The white precipitate obtained from the decomposed black precipitate gave the same analysis. The mercury pool was analyzed for possible zinc content but none was found. Since the new wave is limited by the zinc and oxygen concentrations, it should be possible to obtain the stoichiometry of the reduction process by considering the flux ratio. When the wave has obtained its maximum height the flux ratio can be expressed by: x =
CZn
D’” zn/Coz D1‘202
(1)
where x is given by: VOL. 39, NO. 8, JULY 1967
889
x Zn+2
+ Oz + y e-
= products
(2)
Equation 1 is the usual expression for the end point condition of an amperometric titration. The actual flux ratio for a series of oxygen concentrations ranging from 0.1 1 to 1.06mM was found to be 0.49. Using this value along with the evidence presented above we may write the reduction process as: Zn+2
+ 2 Oz + 2e-
= Zn(Oz)2(adsorbed)
(3)
The fact that the ratio of the new wave height to that of the original oxygen wave is somewhat greater than 1 may be due to the reaction Zn(Oz)2(adsorbed) = Zn02
+ Oz
(4)
which produces more reducible oxygen. That this reaction possibly occurs at the electrode surface is shown by the d e composition of the black precipitate to zinc peroxide upon standing. The apparent stabilization of superoxide anion by zinc and the adsorption of this product account for the anodic shift of the reduction potential. Strontium. The data here are quite similar to those of the zinc case. The diffusion coefficient was found to be 1.55 X 10-6 cmZ/sec. At an oxygen concentration of 0.40mM the shift in the half-wave potential was found to be -65 mV (compared to -140 mV for the analogous zinc case). The ElIzof the new wave also showed a logarithmic dependence upon both the oxygen and strontium concentrations. The temperature coefficient of the wave height was found to be -0.50%/deg at 25°C. Plots of E us. log(& - i)/i for the new wave gave a slope of 58 mV. The coulometric experiments produced an n value of 0.99 based on the oxygen concentration in the test solution. The ratio of the new wave height (when it had attained its maximum height) to the original oxygen wave was found to be 1.03. The controlled potential electrolysis produced a jet black precipitate that appeared to be relatively stable with time. This product was washed with acetone and air dried. The precipitate dissolved in acidic solutions with the evolution of a gas. Analysis of the precipitate found it to be 59.7% strontium and 43.0% oxygen (as superoxide). The ranges were 58.9 to 61.3% Sr and 41.2 to 4 3 . 7 z oxygen. The theoretical values for Sr(O& are 57.8 % strontium and 42.2 % oxygen. As in the zinc case the flux ratio may be obtained from amperometric titration curves. The value of the flux ratio was found to be 0.50. With the above evidence we can represent the reduction process as: Sr+2
+ 2 O2 + 2e-
= Sr(Oz)z(adsorbed)
(5)
The ratio of wave heights suggests that the disproportionation reaction is probably very slow. Again, as in the case of the zinc, it appears that stabilization of the superoxide anion by the Sr+* ion and adsorption of this product account for the anodic shift of the oxygen reduction wave. Thallium. Although the thallium(1) ion reduces approximately 260 mV less cathodic than the first reduction wave of oxygen, it was found that additions of thallium(1) ions to oxygen-containing DMSO solutions also produced a new wave which occurred at less cathodic potentials than that required for the thallium(1) reduction. The diffusion coefficient of thallous ion was found to be 3.56 X 10-6 cmZ/sec. It was found that the oxygen could be amperometrically titrated with thallous solutions and a typical titration curve is shown in Figure 2. At an oxygen concentration of 0.39mM, the halfwave potential of the new wave was found to be 150 mV less cathodic than the thallous reduction. The new wave
890
ANALYTICAL CHEMISTRY
Figure 2. Amperometric titration of 0.38mM oxygen with 0.206 M thallium(1)nitrate also displayed a logarithmic dependence upon both the oxygen and thallium(1) ion concentrations. The temperature coefficient of the wave height was found to be 1.44 %/deg at 25°C. An average value of 57 mV was obtained for the slope of the E us. log(id - i)/i plots. The ratio of the new wave height to the original height of the oxygen wave was found to be 1.02. The coulometric experiments yielded an average n value of 0.98 (based on the oxygen concentration). Controlled potential electrolysis produced a jet black precipitate which analyzed for 83.0% thallium and 17.0% oxygen (as superoxide). The ranges were 81.5 to 83Sz thallium and 14.5 to 20.0% oxygen. The theoretical values for TlOz are 8 6 S x thallium and 13.5% oxygen. The black precipitate dissolved in acidic solutions with the evolution of a gas. The flux ratio for the process was found to be 1.02. This may be used along with the evidence above to write the process as:
+
T1+
+ + e0 2
= T102
(6)
Apparently in this case the stabilization of the superoxide anion by thallous ion is the most important factor in the anodic shift of the reduction of the oxygen. This is supported by the positive temperature coefficient and by the observation that high concentrations of oxygen did not result in distorted polarograms as in the zinc and strontium cases. Cadmium. Cadmium ion was found to reduce approximately 60 mV before the first oxygen reduction wave. The diffusion coefficient was found to be 1.46 X 10+ cm2/sec. As in the thallous case, solutions containing both cadmium and oxygen produced a new wave at less cathodic potentials than that required for the cadmium reduction. The halfwave potential of the new wave was approximately 60 mV less cathodic than the half-wave potential of the cadmium wave when the oxygen concentration was 0.40mM. A logarithmic dependence upon the concentration of cadmium and oxygen was observed as in all the previous cases. The temperature coefficient of the diffusion current was found to be +3.70 %/deg at 25 O C . This is approximately 40 % greater
than that of the ordinary oxygen reduction process. The slope of the E us. log(id - i)/i plots was found to be 59 mV. The ratio of the new wave height (after it had attained its maximum height) to ihat of the original oxygen wave was found to be 2.04. The coulometric experiments failed since the new wave was not sufficiently separated from the cadmium reduction wave to preclude any interference from this wave. However, controlled potential electrolysis was used to produce the product of the reduction. These experiments yielded a paleyellow precipitate. The precipitate dissolved easily in acidic solutions with no apparent gas evolution. Attempts to analyze the product were not successful since it was found to be very impure. This might have been due to "DMSO" of solvation since the precipitate was very gelatinous. Analysis of the mercury pool for cadmium resulted in none being found. The flux ratio of the process was found to be 1.01. Using the above data we may represent the overall process as: Cd+Zf
0 2
+ 2e-
=
CdOz
(7)
However, the slope of 59 mV suggests that a better representation might be: Cd+2 + O2
2 Cd02+
= fast
+ e-
Cd+'
=
CdOz
+ + CdOn 0 2
trium ion. It was assumed that the diffusion coefficient of the yttrium ion would not be greatly different from that of the strontium ion and to a first approximation we could use the same value. With this assumption we obtained an average flux ratio of 0.67. This value is consistent with the above data and allows us to write the overall process as:
+ 3 On + 6e-
2 Y+3
Y2(0J3
(10)
Another possible representation of the process is suggested by the slope value of 73 mV and the rather large temperature coefficient. This implies a one-electron reaction followed by a kinetic disproportionation. This may be represented by the polarographic reduction to yttrium superoxide with subsequent disproportionation of the superoxide product. This accounts for the large temperature coefficient since an increase in the temperature would increase the rate of disproportionation and produce more oxygen at the electrode surface to enter into the reduction process. Reduction of several metal ions in dimethylformamide in the presence of oxygen has shown that peroxides are the products of the wave which is less cathodic than the reduction wave of the metal or that of oxygen. In the DMSO solutions of zinc and strontium one may try to calculate the formation constant of the superoxide. One could represent the process as
(9)
Equation 8 is consistent with the slope data and with the flux ratio as well. Again the apparent stabilization of the superoxide anion by the cadmium accounts for the anodic shift of the oxygen process. Yttrium, Yttrium was selected as example of a multivalent cation which is reducible only at very negative potentials. As was expected, addition of yttrium ions to the oxygen-containing solul.ion also produced a new wave at less cathodic potentials than that required for the oxygen reduction. At an oxygen concentration of 0.39 mM the half-wave potential of the new wave was approximately 140 mV less cathodic than the normid oxygen reduction. The logarithmic dependence upon oxygen and yttrium concentrations was also observed. The temperature coefficient of the wave height was found to be +2.90z/deg at 25°C. This value is somewhat larger than 1 hat of the normal oxygen reduction. The slope obtained from the E us. log(id - i)/i plots had a value of 73 mV. The ratio of the new wave height (after it had attained maximum height) to that of the original oxygen wave was found to be 1.85. The coulometric experiments yielded an n value of 2.39 based on the oxygen concentration. Controlled potential electrolysis yielded a grayish black precipitate which analyzed for 64.4 yttrium and 34.0 % oxygen (as peroxide). The theoretical values for Y 2 ( 0 &are 65.0% yttrium and 35.0% oxygen. The ranges were 63.8 to 65.0% yttrium and 33.5 to 34.3% oxygen. The sample dissolved easily in acidic solutions with no apparent gas evolution, Using the amperomei.ric titrations to obtain the flux ratio requires a knowledge of the diffusion coefficient of the yt-
=
0 2
Mn+
+ nOz-
+e +
M(Oz),(adsorbed)
-
02-
M(O,),(adsorbed)
-+
M(OJ,(precipitate)
(11) (12) (13)
The overall formation constant of M(02), is then given by (14) where
This can be related to the shift in E112by Eiiz = E i i z x ~ ~? )Eii202 ~ =
0.059 l~g[OzlK'[M]""
assuming that the activity of M(Oz),(solid) is unity. For the case of zinc we find that K' = 6.4 X 108 and for the case of strontium K' = 3.5 X 107. Because of the assumptions made, it is doubtful that these constants have any real meaning. To attempt to calculate similar formation constants for the peroxide compounds would involve further assumptions about the disproportionation reactions and is therefore thought to be worthless. RECEIVED for review March 21,1966. Resubmitted March 6, 1967. Accepted May 1, 1967. Work supported by the Research Committee of Washington State University.
VOL 39, NO. 8, JULY 1967
891
