Electrochemistry of Osmium in Sodium Hydroxide Solutions +8, +6, and + 4 States James G . Connery and Richard E. Cover Departnient of’ Chemistry, St. John’s Unicersity, Jamaica, N . Y . 11432 Constant-current coulometry and voltammetry were used to elucidate the nature of the various species present in sodium hydroxide solutions of osmium(lV) and osmiurn(V1). The existence of three osmium(V1) species, three osmium(lV) species, and an electroactive solid has been demonstrated and their chemical natures deduced. The conclusions are consistent with all data available on osmium behavior in these media.
POLAROGRAPHY of osmium in the +8, +6, and +4 states in sodium hydroxide solutions has been investigated by Meites ( I ) , Cover and Meites ( 2 ) , and Cover (3). These investigators found a single polarographic wave for the reduction of either osmium(VII1) or osmiuni(V1) to the + 4 state in hydroxide concentrations above 0.25M. Similarly, in hydroxide concentrations below 3.8M, a single anodic wave was found for osmium(1V) corresponding t o its oxidation to the + 6 state. Complications were observed, however, in hydroxide concentrations outside these ranges. Both osmium(VII1) and osmium(V1) give a n additional cathodic wave in 0.05 to 0.25M hydroxide; an additional anodic wave is observed for osmium(1V) in 3.8 to 9.4.44 hydroxide. These additional waves led Cover and Meites t o postulate a mechanism in which osmium(V) was produced a t the electrode subsequently disproportionating t o give osmium(V1) and osmium(1V). The picture was further complicated by their observation of the transient formation of a precipitate during controlled-potential electrolyses under certain conditions. The work reported here was undertaken to jnvestigate the validity of the disproportionation mechanism and t o elucidate the chemistry involved. EXPERIhlENTAL
Apparatus. The cell used throughout this work was a double-diaphragm, cylindrical, controlled-potential electrolysis cell ( 4 ) . The diameter of the main compartment was 6 cm with a total volume of about 380 ml. In operation, four electrodes were positioned in the cell’s main compartment. A 1-cm layer of triple-distilled mercury was used as a pool electrode for controlled-potential and constant-current electrolyses. A modified vibrating dropping mercury electrode ( 5 ) (VDME) was inserted through the top and used t o follow the various osmium species amperometrically or to run voltammograms and polarograms. The remaining two electrodes were reference electrodes of the silver-silver chloride, saturated sodium chloride type (6). (1) L. Meites, J . Am. Cliem. Soc., 79,4631 (1957). (2) R. E. Cover and L. Meites, Ibid., 83,4706 (1961). (3) R. E. Cover, M. S. thesis, Polytechnic Institute of Brooklyn, 1960. (4) L. Meites, AXAL.CHEM., 27, 1116 (1955). (5) D. A. Berrnan, P. R . Saunders, and R. J. Winzler, Ibid., 23,1040 (1951). (6) L. Meites and S . Moros, Ibid., 31,23 (1959).
The potential of these electrodes was -0.051 i 0.002 V cs. the SCE at 25.0” C. These reference electrodes made contact with the solution in the cell by a Y-shaped salt bridge filled with a saturated solution of sodium chloride. The auxiliary electrode in the third cell compartment consisted of a spiral of platinum wire. All potentials in this work were referred to the saturated calomel electrode. In addition t o the electrodes, a glass stirrer and two gas inlet tubes were positioned in the cell’s main compartment. The glass stirrer was rotated at 600 rpm by a Sargent constant speed synchronous rotator (S-76485). Prepurified nitrogen scrubbed with chromous chloride was continually passed through the cell. A sintered borosilicate gas dispersion cylinder was used to remove all dissolved oxygen from the solution. The second gas inlet, a 6-mm borosilicate tube, was used to pass nitrogen over the solution after deaeration. The cell’s main compartment was fitted with a polyethylene cap with provisions for insertion of the electrodes, the stirrer, and the gas tubes. All controlled-potential electrolyses were performed with a modified Jones-Lingane (7) potentiostat. The circuitry was changed slightly to permit oxidations at negative potentials. A Model 7960 Leeds and Northrup coulometric analyzer was used as the constant-current generator. This instrument 0.004 mA was used was calibrated and a current of 6.430 for all coulometric experiments. All voltammograms, polarograms, and current-time curves were recorded with a Sargent Model (XV) polarograph. When polarograms were used for detailed analyses of the polarographic waves, the voltage axis was carefully calibrated to k0.5 mV with a Leeds and Northrup Model 7655 potentiometer, and iR drop corrections were made. During coulometric experiments when both osmium(V1) and osmium (IV) concentrations were followed, a switching circuit similar to that of Meites and Schlossel (8) was used alternately t o fix the potential of the VDME on the plateau of the osmium (VI) cathodic wave a t - 1.05 V or on the plateau of the osmium (IV) anodic wave a t -0.33 V. For the coulometric oxidation studies o n osmium(IV), 100% current efficiency was obtained at the mercury pool electrode if the potential of the pool was less than -0.35 V. For all of the coulometric experiments, the potential of the pool was monitored continuously with a Sargent Model SR recorder. Solutions of osmium(V1) and osmium(1V) were prepared by controlled-potential electrolysis by the methods of Cover and Meites (2). All experiments were performed at 25.00 0.05 O C. Reagents. All reagents used were Mallinckrodt or Baker ACS reagent grade. The 5.00M sodium hydroxide solution contained 5.00M sodium perchlorate. The sodium perchlorate was prepared as solid NaC104. H 2 0 by the neutralization of Baker reagent 70% perchloric acid with 50% sodium hydroxide. When the neutralization was complete, the p H
*
(7) J. J. Lingane and J. L. Jones, Ibid., 22, 1169 (1950). (8) L. Meites and R. H. Schlossel, J . Phys. Cliem., 67, 2397 (1963). VOL 40, NO. 1, JANUARY 1968
87
Table I. Polarographic Data for Osmium Oxidation m state NaOH, M Wave“ E ‘12, V (mlnP) nP Calcd Theory VI11 0.05-0.20 c1 -0,628 dz 0.003 -0.28 1.03 z!= 0.08 -0.29 0.0 VI11 0.05-0.20 c2 -0.92 to -0.81 3.17 0.64 f 0.03 2.0 2.0 VI 0.1-9.4 Cl -0.596 dz 0.009 0.17 0.84 It 0.12 0.14 0.0 VI 0.1-0.25 c2 -0.84 to -0.78 3.03 0.65 zt 0.05 2.0 2.0 VP 10.0 c3 -0.85 ... ... ... ... IV 0.1-9.4 A1 (-0.41 to -0.59) -1.54 -1.18 5 0.13 1.8 2.0 IV 4.0-9.4 A2 (-0.44 to -0.50) -2.49 -1.46 dz 0.21 3.6 4.0 IVC 0.25 A3 (-0.390 dz 0.006) ... -0.78 f 0.06 ... ... IV-VIbJ 10.0 A4 (-0.310 dz 0.004) ... ... ... ..* a C indicates a cathodic wave; A indicates an anodic wave. Data obtained at the VDME only. c Average of all data obtained from polarograms of products of controlled-potential electrolytes on the plateaus of waves C1 of osmium (VIII) and osmium(V1). Wave due to electroactive precipitate.
was adjusted to 8.0 and the solution filtered through fine porosity sintered borosilicate. The salt was isolated and dried at 180” C for 24 hours. Stock solutions of osmium(VII1) were prepared by dissolving Mallinckrodt reagent grade osmium tetroxide in 0.05M sodium hydroxide. These osmium solutions were assayed using the potentiometric titration procedure described by Cover (9).
(9) R. E. Cover, Ph.D. thesis, Polytechnic Institute of Brooklyn,
Controlled-potential reductions (12) of osmium(VII1) and osmium(V1) solutions in 0.25M NaOH present evidence vital t o understanding this phenomenon. For both of these oxidation states, when the electrolyses are performed a t a potential on the plateau of cathodic wave, C1, transient precipitation is observed and plots of log i us. time are decidedly nonlinear. When electrolyses are performed at a potential on the plateau of the second wave, C2, precipitation is not observed and the log i 6s. time plots are linear. Furthermore, when polarograms are run on the products of the electrolyses where transient precipitation occurs, a n anodic wave is found which has not been observed under any other conditions. This wave, labeled A3 in Table I, has a half-wave potential of -0.390 i.0.006 V. The amperometric response of the VDME t o osmium(V1) concentration a t - 1.05 V was linear in the range of 0 to 9.0 m M in 10.OM NaOH. The maximum relative error in estimating osmium(V1) concentrations was & 0.75 %. Similar calibrations for osmium(1V) in this medium showed definite nonlinearity above the 1.OmM level. This was taken t o be caused by partial precipitation of osmium(1V) during its preparation. A permanent black precipitate was observed in these solutions. To follow osmium(1V) in precipitating solutions, it was assumed, then, that VDME response was linearly related t o the true concentration of osmium(1V). A proportionality constant relating osmium(1V) and osmium(V1) limiting currents was computed from data in the concentration interval where both calibration plots were linear. The ratio of +4 t o +6 limiting currents when corrected for residual currents was -0.896 f 0.019. This datum was then used in determining osmium(1V) concentrations. Coulometric oxidation studies were made on 4.0mM osmium(1V) in 10.OM NaOH. The plots (Figure 1) of the potential of the mercury pool us. time are revealing. In nonprecipitating osmium(1V) solutions, curve b, a single potential break was observed at about -0.28 V corresponding t o the loss of 100% current efficiency. O n the other hand, two distinct potential breaks were observed in precipitating solutions, curve a. The second potential break at about -0.21 V corresponds t o the single potential break observed in nonprecipitating solutions and likewise indicates loss of current efficiency.
1962. (10) L. Meites, “Polarographic Techniques,” second ed., Interscience, New York, 1965, pp. 246-7. (11) L. Meites and Y . Israel, J . Am. Clwm. Soc., 83,4903 (1961).
(12) R. E. Cover, Polytechnic Institute of Brooklyn, unpublished work, 1960.
RESULTS AND DISCUSSION
PolarographicData. It can be shown (IO)that for the ratedetermining step of a n irreversible electrode process Ox
+ ne + rn OH-
+
red
(1)
that when the drop time does not vary significantly, a plot of half-wave potential cs. log OH- should have a slope of (0.05915 V )(rn/np) where p is the transfer coefficient and n is the number of electrons involved in the rate-determining step. The half-wave potential data of Cover and Meites were so analyzed and the (m/np) values calculated are tabulated in Table I. Meites and Israel ( I I ) demonstrated that for a n irreversible electrode process, where the drop time does not vary significantly, a plot of E cs. log [i/(id-i)], measuring maximum currents, should have a slope of (-0.0542 Vjnp). The various polarographic waves were so analyzed, and the (np) values were calculated and are tabulated in Table I. The (m/n/3)and (np) values in Table I were obtained by the unweighted least-squares technique. The (np) values were obtained by averaging data from three or more polarograms run throughout the pertinent hydroxide concentration range. Behavior in Precipitating Solutions. The controlled potential preparation and coulometric oxidation of osmium(1V) in 10.OM NaOH is complicated by precipitation when the total osmium concentration is greater than l.OmM. The transient formation of a black solid is observed during the electroreduction of osmium(V1) or the electrooxidation of osmium(1V) solutions. In both cases, the initial and final solutions are clear and homogeneous. During the reactions, however, the solutions become turbid and almost opaque.
+
88
ANALYTICAL CHEMISTRY
1.5
0
I
I
I
I
50
100
I 50
200
150
- __
so
0
x [Os(Wl Coulometric oxidation of 3.9m.M osmium(1V) in 10.OM NaoH
To clarify the cause of the additional potential break, a series of voltammograms were run with the VDME during the coulometric oxidation of 4.0mM osmium(1V) in 10.OM NaOH. In addition t o the voltammetric waves characteristic of the + 4 and +6 states in this medium, two unexpected waves were observed. A small cathodic wave, C3, was found having a half-wave potential of -0.85 V. In addition, a well defined anodic wave, A4, was observed with a half-wave potential of -0.310 f 0.004 V. This anodic wave was best resolved at the time of the first mercury-pool potential break. When polarograms were run in unagitated solutions this anodic wave could not be observed. An electroactive precipitate can most suitably explain this behavior. When voltammograms are run with the V D M E in agitated solutions, convection is sufficient t o transport the solid to the electrode surface where it is oxidized. At the D M E , on the other hand, convection is minimized and transport t o the electrode is not sufficient t o produce an observable polarographic wave. Cover (3) had previously found controlled-potential electrolytic evidence for a n electroactive precipitate under similar conditions. Because of the transient nature of the precipitation, the possibility that the solid contained multiple oxidation states of osmium was considered. The precipitate could conceivably contain the f 4 , + 5 and $6 states. Because no experimental evidence has been found for the existence of osmium(V) in solution, the involvement of this oxidation state seems unlikely. For a precipitate containing +4 and +6 osmium species, one may write: (2)
with a solubility product of K.,
=
[Os(VI)]” [Os(IV)]‘
100
[OS(VI)I
u. Oxidation of 4.07mM osmium(1V) b. Oxidation of 0.814 mM osmium(1V)
+ 9 Os(1V) $ Os(V1)p Os(Iv)q (solid)
150
Figure 2. Concentration-product time curve
Figure 1. Potential of mercury pool cs. time in 10.OM N a O H
p Os(V1)
100
l i m e , minutes
Time, m i n u t e s
(3)
If there exists a time interval during coulometric oxidation during which the precipitate and its aqueous species are in equilibrium, then the K., relationship will be satisfied. Of the sets of p and q tested, (0, l), (1, 0), (1, 1>,(2, l ) , a n d (1, 2), only the set ( p = 1, q = 1) gave a plateau in the producttime curve (Figure 2). The limiting product was (1.28 0.02) x 10-6MZ.
Osmium(V). The coulometric oxidation studies of osmium (IV) in 10.OM N a O H indicate that osmium(V) does not play a significant part in the chemistry of the element in hydroxide media. The voltammograms run during coulometric oxidation in both precipitating and nonprecipitating 10.OM N a O H solutions indicate that the osmium(1V) species causing waves A1 and A2 are in equilibrium because the ratio of wave heights (A2:Al) is essentially constant a t 1.1 i 0.1 for all these experiments. This equilibrium then permits monitoring the total osmium(1V) concentration by measurement of total anodic wave height. O n the basis of the coulometric current, in 0.81mM osmium(1V) solutions, 9 8 z of the coulometric current was used directly to oxidize the + 4 species causing wave A1 . The disproportionation mechanism for coulometric titrations discussed by Cover and Meites (13) is clearly not supported by the experimental evidence. The osmium(1V) and osmium(V1) concentration-time curves d o not possess the type of curvature required for moderately rapid disproprotionation. In addition, no direct evidence was obtained for the existence of osmium(V) either from voltammetric observations or mass-balance computations. For a direct two-electron oxidation under these conditions, the +6 and $4 species concentration-time curves should have slopes of identical magnitude but opposite sign-namely, i 1.599 X 10-5M minute-’. The observed slopes were 1.602 (+ 0.006) X 10-5 and -1.598 (i0.008) X for osmium(V1) and osmium(IV), respectively. Because the direct two-electron oxidation and disproportionation mechanisms are homeomorphic for large values of the disproportionation rate constant, disproportionation could only be valid here if the steady-state osmium(V) concentration were 2 X 10-5Mand if the disproportionation rate constant were 2 4 x 104M-I minute-’. Voltammetric Evidence. The voltammetric characteristics of osmium(1V) and osmium(V1) strongly indicate that multiple species of both oxidation states exist in solution. Cover (3)studied the effects of mercury pressure on polarographic wave heights with these results. In 3.8M NaOH, the osmium(V1) C1 wave is diffusion-controlled while in 0.25M NaOH, the C1 wave is partially kinetically controlled. The
