of manganese(I1) and perchloric acid, and Headridge et al. (3) did the same for cadmium. The results are presented in Table V. In addition to the data listed, our half-wave potential for anhydrous manganese perchlorate is exactly the same as that of Headridge for the hexahydrate. In agreement with Headridge, we conclude that the effect of low concentrations of
water (below ca. 10-2M) on the half-wave potentials of the majority of metal ions in sulfolane is likely to be small. RECEIVED for review January 21, 1972. Accepted March 7, 1972. We thank the National Science Foundation for financial aid under grant numbers GP-6478X and GP-16342.
Voltammetric Behaviors at Platinum Electrodes and Decomposition Potentials of Alkali Tungstate and Polytungstate Melts Louis Meites,’ Ephraim Banks, and Charles W. Fleischmann2 Department of Chemistry, Polytechnic Institute of Brooklyn, Brooklyn, N . Y . 11201
The electrolytic preparation of tungsten bronzes from molten alkali polytungstates prompted a voltammetric study, at 750 “C, of the systems Li,W04-W03, NazW04W03, K2W04-W03,and some ternary melts. Platinum beads were used as indicator electrodes together with an Ag/Ag2W04reference electrode in a cell whose compartments were separated by a porcelain membrane. Reduction waves were obtained in Na2W04-WOaand Li,W04-WO,. The heights of the waves were proportional to the concentration of WO, up to about 2 mole %, above which.convective effects precluded the formation of plateaus. The main wave was always preceded by a small irreproducible prewave whose nature is not well understood. Bronzes were formed on the surfaces of electrodes maintained at potentials on the main WO1 wave in melts containing more than 5 mole % WO,. The anodic, cathodic, and full-cell decomposition potentials are reported for the polytungstates. Potassium tungstate containing low percentages of WO, is not molten at 750 “C, but potassium tungstate melts rich in WO, behaved like similar lithium and sodium tungstate melts.
A SYSTEMATIC STUDY of the electrochemistry of fused polytungstates is of particular interest because the electrolysis of a fused polytungstate is an excellent technique ( I ) for the preparation of large single crystals of tungsten bronzes, (metal)zWOs, which are recovered at the cathode. The electronic and catalytic properties of the bronzes are well known (2). The present work describes the voltammetric behaviors of the systems Li2W04-W03, Na2W04-W03, K2W04-W03, LiKW04-WOs, and NaKW04-W03. These systems do not appear to have been previously studied by voltammetric techniques, but the voltammetric properties of molten solutions of WOs in borax (3, 4 ) and Na20.Si02.B203 (5) have been reported. Present address, Department of Chemistry, Clarkson College ofTechnology, Potsdam, N.Y. 13676. Present address, NL Industries, Central Research Labs., P.O. Box 420. Hightstown, N.J. 08520. (1) E. Banks and A. Wold, in “Preoarative Inornanic Reactions.” Vol. 4, W. L. Jolly, Ed:, Wiley-Interscience, New York, N.Y., 1968, p 237. (2) P. G. Dickens and M. S. Whittingham, Quart. Rev. (London), 22. 30(1968). (3) Yu. K . Delimarskii, K. M. Bojko, and G. V. Shilina, Electrochim.Acta, 6,215 (1962). (4) G. V. Shilina and N. V. Ul’ko, Ukr. Khim. Zh., 28,172 (1962). ( 5 ) Yu. K . Delimarskii and S . S.Ognyanik, ibid., 29,932 (1963).
The decomposition voltages reported below are the minimum applied voltages needed to initiate bronze formation in the polytungstates. Such values do not appear to have been reported, except for the thallium system (6). EXPERIMENTAL
The molten solutions were prepared by fusing reagent-grade alkali tungstates or carbonates with tungsten(V1) oxide. The concentrations of WOa, based on the starting materials, are given in units of either mole percentage (m/o) or formula weights per liter (F). The latter were calculated using density data given in the literature (7);extrapolations were necessary for concentrations above 50 m/o at the working temperature, 750 “C. At this temperature, data could be obtained for L~zWO~-WOS and Na2W04-W03 containing zero to 55 m/o W03 because these are completely molten (8), but KzW04wo3 could be investigated only over the range from 33 to 55 m/o WOa. Unstable higher polytungstates have been reported (8, 9) but were not investigated in this work. The system LiKW04-WOa was examined over the range from 14 to 50 m/o W 0 3 . The ternary sodium-potassium system NaKWOa.W03 was also studied. The ternary compositions appeared to be completely molten, but phase diagrams were not available. Two different cell designs were employed. One, intended primarily for two-electrode voltammetry, was used to obtain most of the data given; the other was used principally for direct measurements of the decomposition potentials of melts. A voltammetric cell (10) was prepared from a 10-ml porcelain dish into which a 5-ml porcelain crucible was placed. The latter contained an Ag/Ag,WO4 reference electrode (IO), to which all potentials in this paper are referred; the porcelain walls served as a non-porous conductive septum between compartments. The other type of cell was prepared from high-purity recrystallized alumina. It consisted of a 5-ml flat-bottomed crucible separated into two sections by inserting a short vertical length of 8-mm 0.d. alumina tubing. Four 1 X 2-mm slits cut into the lower end of this tube provided ionic conduction between the compartments. The anode was placed within (6) M. J. Sienko, J. Amer. Chem. SOC.,81,5556 (1959). (7) K. B. Morris and P. L. Robinson, J. Chem. Eng. Data, 9, 444 (1964). (8) F. Hoermann, 2.Atforg.Allgem. Chem., 177,167 (1928-29). (9) V. Spitzin and A. Cherepneff, ibid., 198,276 (1931). (10) E. Banks, C. W. Fleischmann, and L. Meites, J. Electrochem. Soc., 117,652 (1970). ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
1133
0.4
0.2
Figure 1. Voltammogram of a platinum bead (0.03 cm2 area) in sodium tungstate at 750 “C
-0.5 -0.1
I
-0.3
-0.7 -0.9 1 -1.1 Potential vs. Ag/Ap2W04,VOItS
I
I
-0.4
1.4
/
1.2
1.0
0.8 U
‘.
e
0.6
3 0.4
new or reassembled cell components, fresh reagents, and two or more newly prepared electrodes. The indicator electrode was always a Pt bead, 1 mm in diameter, formed by fusing the end of an 0.6-mm diameter wire. The wire leading to the bead was shielded by threading it through alumina tubing. The exposed geometric area of such an electrode was 0.03 f 0.01 cm2; the reproducibility of this area includes that of forming and shielding the bead. The roughness factor of the beads was estimated to be 2.5-3.5 from data on the adsorption of hydrogen and oxygen onto similar beads sealed into glass, as measured (12) by galvanostatic charging in dilute sulfuric acid. For this purpose, glass was used in place of the ceramic shield so that the exposed area could be more clearly defined. Fresh electrodes were always used because of the report that platinum is attacked by molten tungstate (13), and because we found that the surface areas of Pt foils could be increased by prolonged anodic polarization (e.g., at 3 mA/cmZ for several hours) to oxygen evolution in molten tungstate. On the other hand, the surface roughness of a foil was found to decrease on standing at open circuit in the melt.
RESULTS AND DISCUSSION 42
-
-0.7
-0.9
-1.1
-1.5
-1.3
Potential vs. Ag/Ag,W04
-1.7
,Volts
Figure 2. Voltammograms of a platinum bead (0.03 cm2 area) in a lithium tungstate melt at 750 “C In the presence of 1.00% free WOa (b) After adding 1 LizCOa to remove WOa
(a)
the tube. During electrolysis, the tube acted as a chimney for the evolved oxygen but, in contrast to the previous cell, the anolyte and catholyte were not completely separated. The electrolyses were carried out with the cell placed in a top-fed crucible furnace purged with dry argon and controlled to =kl “C. The argon was freed from oxygen, by passing it through chromous solutions ( I I), to decrease the concentration of oxygen dissolved in the melt. Voltammograms were obtained with a scan rate of 0.250 V/min, and were corrected for ohmic potential drop, using values of the cell resistances obtained from conventional ac measurements. The stated reproducibility of the data at each composition is based on measurements with completely (11) L. Meites, “Polarographic Techniques,” 2nd ed., Wiley-Interscience, New York, N.Y., 1965. 1134
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
~
~~
Behavior of the Alkali Tungstates Alone. Na2W04 and Li2W04are molten at 750 “C, but KzW04is not. Cathodic voltammograms for the lithium and sodium melts may be seen in Figure 1, in curve 6 of Figure 2, and in Figure 3 as the base curve ( O x WOs). It is well known that the ultimate reduction product in these systems is tungsten metal and that the anodic process releases oxygen (14). SODIUMTUNGSTATE.The voltammogram of NasW04 shown in Figure 1 was started at a potential sufficiently positive to evolve oxygen (Region I in the figure). The nearly constant cathodic current recorded between - 0.45 and - 1.O V (Region 11) is attributed to the reduction of dissolved oxygen previously formed at the surface of the indicator electrode. Cathodic sweeps beginning at potentials more negative than -0.6 V did not exhibit this current. As it is not certain that the melt became saturated with oxygen during the anodic portion of the scan, the current density for oxygen reduction, 1.7 mA/cm2, may be smaller than that corresponding to an oxygen-saturated molten tungstate. In Region 111 of Figure 1, cathodic processes were evident. The first of these began at a potential somewhat more negative (12) J. A. V. Butler, in “Electrical Phenomena at Interfaces,” J. A. V. Butler, Ed., Macmillan, New York, N. Y., 1951, p 204. (13) A. Magneli and B. Blomberg, Acta Chem. Scand., 5, 372
(1951). (14) K. C. Li and C. Y. Wang, “Tungsten.” American Chemical Society Monograph No. 94, 3rd ed., Reinhold, New York, N. Y., 1955.
2%
b
I
1.6
I 1.2
IA I .o
1.2 0.8
1.0
a
E
2
U E
0.6
C
c
0.8
E
a 0
12
II
V
N
i
0.4
0.6
0.2
0.4
0 -0.3
-0.9
-1.1
- 1.3
Potential vs. Ag/Ag2 W 0 4 ,Volts
-02 -1.0
-1.2
-1.4
-1.6
-1.8
-2.0 0.4
Potential vs. Ag/A$ W04 ,Volts
Figure 3. Voltammograms of platinum beads (0.03 cm* area) in sodium polytungstates at 750 “Cin the presence of 5.00,2.00, 1.00,0.50,0.25,0.10, and 0 mole WO,
0.6
than -0.9 V, and gave rise to currents that varied with different electrodes, presumably because of variations in surface roughness. Compare, for example, the curve for NaZWO4 in Figure 1 with the base curve of Figure 3. The inflection at about - 1.4sV in Figure 1 was interpreted to mean that a change in the cathodic process occurred at this potential, and is discussed further below. At still more negative potentials, a series of irreproducible inflections was observed before the final current rise. The potential at which this occurred, which is the cathodic decomposition potential of Na2W04,was also irreproducible but was in the vicinity of -2.6 V. These irreproducibilities at very negative potentials presumably reflect variations of the thicknesses or structures of solid products deposited onto the electrode. The potential of the onset of the cathodic process in Region I11 and the potential of the first inflection in Region IV were reproducible within hO.1 V in different melts and at different electrodes. Curves were also recorded with electrodes having unsheathed lead wires in order to ensure that the inflections were not artifacts caused by the sheathing. These reduction processes were not due to free W 0 3 as an impurity because the addition of up ot 0.5 m/o carbonate to the melt did not alter the voltammograms. It will be shown below that the wave due to the reduction of WO, is destroyed by carbonate, as expected according to Equation 1.
cos2-+ wo3 ----f Con + w04*-
(1)
It was further demonstrated that these reduction processes were not due to water in the melt, for essentially identical curves were obtained with melts prepared from crystals of Na2WO4.2H20, from anhydrous Na2W04,and by remelting
Figure 4. Voltammograms of platinum beads (0.03 cmz area) in Na2W04containing AgzW04at 750 “C (1) and (2) 1.00% Ag(I) at two different Pt beads, (3) 0.40% Ag(I), and (4) 0.40 Ag(I) with the & d o n of polarization reversed at -0.85 V, just before reaching the cathodic peak
a melt previously studied. This last result may also be taken as evidence that interfering impurities were not being leached from the porcelain cell components. A voltammogram recorded in an all-alumina cell also had the same shape after being corrected for the polarization of the large Pt gauze employed as a counter electrode in place of the reference compartment. Voltammograms of silver(1) were also obtained so that any leakage from the reference compartment would be recognized, and are shown in Figure 4. If the potential was maintained at a value within Region 111, the cathodic current decreased to a value on the extrapolated base line within a few minutes. At a potential such as - 1.7 V, following the inflection in Region IV, the current remained steady for three hours but no deposit accumulated on the electrode. Nevertheless, after one such polarization a few milligrams of a tan product was recovered by cooling the melt and dissolving it in water. This material was insoluble in water, as is WO,, but unlike wo3 it dissolved in concentrated mineral acids to give pale yellow solutions. A positive tungsten blue test (15) was obtained, showing that the (15) F. Feigl, “Qualitative Analysis by Spot Tests,” 3rd ed.,Elsevier, New York, N.Y., 1946, pp 94,306.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, J U N E 1972
1135
10 . U
Figure 5. Forward and reverse voltammograms of a platinum bead (0.03 em2 area) in Na2W04at 750 "C. Arrows indicate sweep direction
E
i
3
a5
0
-1.3
-1.5
-1.7
-a5
product could be reduced to W 0 2 . ~(16), ~ and permitting its tentative identification as a tungsten oxide having a composition between wo2.75and W03. When the cathodic sweep was reversed at any potential more negative than -1.0 V, anodic currents were obtained on scanning toward more positive potentials, as shown in Figure 5. When the electrode potential was stepped from -1.7 V (where a current of $0.6 mA was sustained) to -0.9 V, the anodic current at this potential decreased with time to a steady value of -0.05 mA. When a pause at open circuit was interposed between the polarizations at -1.7 and -0.9 V, both the magnitude of the anodic current and the time required for it to fall to zero decreased as the length of the pause increased; the data are shown in Table I. The fact that the decay time was unaffected by stirring (compare entries 3 and 4) showed that the oxidizable material was not dissolved in the melt at the electrode surface but was deposited on the electrode, and that it decomposed or dissolved away with time. In summary, three reduction processes were observed in Na2W04. One process began at - 1.O V and did not lead to a sustained cathodic current; a second began at potentials more negative than about - 1.5 V and led to a sustained reduction with deposition or adsorption of some of the reduced material on the electrode; a third, at very negative potentials, involved the final reduction to tungsten. The natures of the first two reduction processes may be similar to those which have been reported for sulfate melts (17). LITHIUM TUNGSTATE.In Li2W04,cathodic processes similar to those observed in Na2W04began at about -1.0 V, but cathodic decomposition occurred sharply at -1.7 V, as shown in Figure 2, curve b. This final cathodic decomposition potential is 0.9 V more positive than that for Na2W04. The cathodic processes in both melts may be the same and the difference between the measured values may reflect a difference between the rates of reduction in the two melts. Voltammetry of W 0 3 in Alkali Polytungstates. THE REDUCTION WAVEFOR WOa. Adding W 0 3to sodium or lithium tungstate at 750 "C gave rise to a new reduction wave. The potassium melts of comparable W 0 3 content are not molten at this temperature. Voltammograms for Na2W04containing from zero to 5 m/o W 0 3 may be seen in Figure 3. A typical reduction wave in Li2W04is shown as curve a of Figure 2. 'This figure also shows that the W 0 3 reduction wave was eliminated by adding an equimolar quantity of carbonate to the melt, as expected according to Equation 1 above. (16) N. V. Sidgwick, "The Chemical Elements and Their Compounds," Vol. 11, Clarendon Press, Oxford, 1950, pp 1047,1049. (17) K. E. Johnson and H. A. Laitinen, J. Electrochem. SOC.,110, 314 (1963).
1136
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
Table I. Magnitude and Rate of Decay of Anodic Current at -0.9 V after Prepolarization at -1.7 V in Na2W04' Time at open Initial current at Decay time, No. circuit, min -0.9 V, mA min 1 2 3 4 5
5 1 0.5
0.Y 0.17
-0.02 -0.05 -0.1 -0.1 -0.16
0.75 0.9 1.4 1.5 3.6
a The indicator electrode was polarized at - 1.7 V for 3 min, then allowed to stand at open circuit for the time recorded in the second column. It was then brought to -0.9 V, where the initial current is given in the third column; the last column gives the time required for that current to fall to zero. b Stirring during time at open circuit.
The reduction waves obtained for W03 had plateaus reminiscent of polarographic waves. Waves of this shape have been reported by other workers (18, 19) for reductions at stationary electrodes in melts, and we have observed similar waves (IO)for the reduction of Cd(I1) from sodium tungstate using the present cell design. Peaks, rather than waves, are expected in stationary-electrode voltammetry (20, 21) but were rarely observed. This is presumably because convection in the melt prevents the build-up of a stable diffusion layer around the electrode at the sweep rate employed. Waves having pronounced maxima were, however, obtained for the reduction of Ag(1) from Na2W04,as shown in Figure 4. The current due to W 0 3 reduction was clearly transportlimited since the height of this wave could be increased by stirring. At concentrations above 2 m/o W03, however, a plateau could not be detected, presumably due to an increase in melt convection. INTERPRETATION OF THE WAVE. Table I1 gives the relationship between the WOI concentration and the wave height in lithium and sodium polytungstate melts, as well as values of the potentials, which were previously presented graphically (22). Though the relative uncertainty of the ratio of average wave height to the concentration of W 0 3is large at the lowest concentrations because the wave heights were reproducible to (18) E. Black and T. DeVries, ANAL.CHEM., 27,906 (1955). (19) H. A. Laitinen, C. H. Liu, and W. S. Ferguson, ibid., 30, 1266 (1958). (20) J. E. B. Randles, Trans. Furnday S O C . , 327 ~ , (1948;. (21) A. SevEik, Collect. Czech. Chem. Commun., 13,349 (1948). (22) E. Banks, C. W. Fleischmann, and L. Meites, J. Solid State Chem., 1,372 (1970).
Table II. Data for WOI Wave Concentration of WOa Ratio of average Mole E I I ZV, US. Average wave wave height to Solvent per cent Formal Ag/Ag,WOi‘ height, mAb formality of WO, Na2W04 0.10 0.013 ... 0.06 5 f 4 0.25 0.033 -1.74 0.15 5*2 0.50 0.066 -1.65 0.31 4.7 j=0.8 1 .oo 0.131 -1.60 0.63 4.8 z!= 0 . 4 2.00 0.264 -1.56 1.33 5 . 0 f 0.2 Li2W04 0.70 0.115 -1.40 0.37 3.2 Z!= 0.4 1.oo 0.164 -1.34 0.55 3.3 f 0 . 3 Each value is the average obtained in 5 replicate measurements, reproducibility: Z!=O.Ol volt. * These values are based on data obtained with at least two different platinum beads and at least two independent melts of each composition; their overall reproducibility is *0.05 mA. 5
only f0.05 mA, the ratio appears to be independent of concentration. It is common (23) in molten-salt voltammetry to compare the current-potential data with the predictions of well-known polarographic equations ( I I ) , including the HeyrovskjrIlkovii. equation
for reversible reduction to a product that diffuses away from the e!ectrode surface in either direction, and the KolthoffLingane equations E = const
+ 2.3RT log(il - i) nF
(3)
and Eli2 = const
+ 2.3RT -log c nF
- 0.1
-0.3
-0.5
-ID
-0.7
-12
-14
(4)
for reversible reduction to a product deposited onto the electrode surface. In these equations, n is the number of electrons transferred, C is the concentration of the electroactive species, i is the current at the potential E , and i l is the limiting current of the wave. The data in Table I1 and Figure 3 show that the half-wave potential ITliz shifts with the concentration, and the magnitude of the shift agrees with that predicted by Equation 4 for a reversible one-electron process. It would be expected that the reduction would obey this equation because tungsten bronzes are always found to deposit on the electrode and not throughout the catholyte ; that is, the reduced species does not travel away from the electrode but immediately undergoes the necessary chemical reactions (22) to precipitate the bronze. Moreover, a plot of E against log (il - i)/i was not linear, but one of E against log(i8 - i) was linear over at least a decade in i. The slope of such a plot varied between 0.15 V and 0.23 V, compared to a theoretical value of 0.203 V at 750 “C for a reversible one-electron transfer. Laitinen and Osteryoung have critically reviewed (24) the interpretation of such data for solid microelectrodes in molten salts and have cautioned against too rigorous an interpretation of the slopes obtained. The dependence of EIIZ on the concentration of W 0 8 over a wide range of concentrations has been presented (22) as supporting the idea that the simple molecular species WOa (23) Yu. K. Delimarskii and B. F. Markov, “Electrochemistry of Fused Salts,” The Sigma Press, Washington, D.C., 1961. (24) H. A. Laitinen and R. A. Osteryoung, in “Fused Salts,” B. R. Sundheim, Ed., McGraw-Hill, New York, N.Y., 1964, p 255.
-0.4
Potential
M.
Ag/Aq,WO4
,Volts
Figure 6. Forward and reverse voltammograms for a platinum bead (0.03 cm2 area) in 20 % W03-80% Na2W04at 750°C. Sweeps 1, 1’; 2, 2’; and 3, 3’ reversed at points 1*, 2*, and 3*, respectively
is reduced at lower concentrations but that polymers of W 0 3 are reduced in the higher polytungstates. The assumption of a one-electron transfer is supported by a comparison of the limiting currents of WO, with those of Cd(1I) and Ag(1): these limiting currents were found to be 0.8, 0.5, and 0.3 mA for 0.4 m/o Cd(II), Ag(I), and WOa, respectively. Further support for a one-electron transfer is that W O Bhas been reported to be reduced through W(V) in other melts (3-5). DEPOSITIONOF TUNGSTEN BRONZES.Maintaining a potential on the WOs reduction wave resulted in the deposition of a tungsten bronze on the electrode. It should be noted that bronzes were obtained even though the design of the voltammetric cell permitted complete isolation of anolyte and catholyte. This result would appear to preclude the suggestion (25) that primary anode products (such as WOZ) are necessary to reduce the melt. (25) M. J. Sienko and B. R. Mazumder, J. Amer. Chem. SOC.,82, 3508 ( I 960). ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, J U N E 1972
1137
~~
Table 111. Decomposition Potentials of Alkali Polytungstates at 750 “C Measured half-cell decomposition Full-cell decomposition potentials Concentration of WOa potentials, volts“ Calculated from System d o Formal Cathodic Anodic half-cell data Measured LizWO~-WOa 0.00 0.00 -1.7oa -0.176 1.53b 1.50~ 0.70 0.12 -1.36 ... ... 1.OO 0.164 -1.29 ... ... ... 4.00 0.663 -1.22 0.03 1.25 5.80 0.968 -1.18 0.04 1.22 ... 0.04 1.19 ... -1.15 10.0 1.68 0.04 1.12 -1.08 15.0 2.59 0.09 1.13 ... -1.04 20.7 3.66 0.13 1.15 ... -1.02 24.0 4.31 -0.98 0.11 1.09 ... 30.0 5.57 0.12 1.05 ... -0.93 40.0 7.80 0.17 1.03 -0.86 45.0 9.01 0.19 0.84 -0.65 50.0 10.3 0.20 0.63 -0.43 55.0 11.5 NazW04-W03 0.00 0.00 ... -0.37 ... 2.25-4 0.25 0.033 -1.72 -0.40 1.35 ... 0.50 0.066 -1.61 -0.32 1.29 1 .OO 0.131 -1.54 -0.28 1.26 1.15 0.157 ... ... ... 1.28 2.00 0.264 -1.52 -0.28 1.24 ... 3.00 0.403 -1.51 ... ... ... 5.00 0.671 -1.46 -0.25 1.21 ... 8.05 1.10 ... ... ... 1.23 10.0 1.38 -1.42 -0.22 1.20 ... 15.0 2.14 -1.38 -0.19 1.19 ... 20.0 2.95 -1.33 -0.14 1.19 24.8 3.82 ... ... 1.150 30.0 4.69 -1.19 -0.10 1.09 ... 40.0 6.59 -1.06 -0.04 1.02 1.05 50.0 8.97 -0.95 -0.01 0.94 0.94 55.0 10.1 -0.84 0.08 0.92 ... 57.3 10.7 ... ... ... 0.81 KzWOd-WOa 35.0 4.61 -1.26 -0.09 1.17 ... 40.0 5.49 -1.19 -0.06 1.13 ... 46.0 6.63 -1.12 0.01 1.13 ... 50.0 7.46 -0.99 0.10 1.09 55.0 8.57 -0.76 0.17 0.93 ... -0.10 0.87 ... e -0.97 NaKWOd-WOa 50.0 1.20 ... e -1.22 -0.02 LiKWO4-WO3 14.2 0.00 1.15 ... e -1.15 19.8 1.14 ... e -1.13 0.01 25.0 1.14 ... e -1.08 0.06 30.0 1.15 ... e -1.09 0.06 32.0 0.02 1.13 ... 33.0 e -1.11 0.04 1.14 35.0 e -1.10 0.09 1.01 40.0 e -0.92 -0.78 0.15 0.93 ... 46.0 e -0.69 0.16 0.85 50.0 e a All potentials are referred to the Ag/Ag2W04 electrode. Average deviations are 0.02 V for half-cell values and 10.04V for calculated full-cell values below 5 m/o W 0 3 , kO.01 V for half-cell values and 3~0.02V for calculated full-cell values above 5 m/o WO,. Average deviation 10.05V. Average deviation f0.025 V for this value and those below it in this column. Not calculated because density values not available. I
I
In contrast to previous reports (14) that bronzes are recovered only from melts containing at least 5 m/o WOa, a few small crystals were prepared from Na2W04containing 1.15 m/o WOS. Nevertheless, it was generally found that bronzes were not obtained from melts containing less than 5 m/o WOa. In such melts, the electrode discolored after a few minutes of electrolysis, but a product did not build up. In melts containing low concentrations of WO, it is presumed that the reaction represented by Equation 5 goes to the left. 3xMzW04
+ (6 - 4X)woa + XW = 6MSW03
.
.
.
tungsten bronzes, and its reversibility has been demonstrated (26). It may be observed from the THE CATHODIC PREWAVE. figures that the W 0 8reduction wave was preceded by a smaller wave or peak. The magnitude, shape, and potential at the onset of this prewave varied with the particular electrode employed but were independent of the concentration of WOS. During polarization at potentials on the prewave, the current fell within a few minutes to the extrapolated base-line value.
(5)
This reaction has employed (1) for the thermal synthesis of 1138
.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
(26) 0. Brunner, “Beitrage zur Kenntnis der Wolframbronzen,” Dissertation, Zurich, 1903.
N o product was accumulated, nor did such polarization alter the characteristics of the W 0 3 wave. When the sweep was reversed at a potential following (more negative than) the prewave, an anodic peak was observed. Forward and reverse sweeps obtained for a melt containing 20% WOr80X Na2W04are shown in Figure 6. The scan was begun at a potential sufficiently positive to evolve oxygen, and oxygen reduction contributes to the background current during the rest of the forward sweep. When the potential was reversed at the point on the prewave marked 1* in the figure, an anodic peak appeared at -0.8 V. Since reversal at a potential more positive than -0.9 V did not give an anodic peak, this peak is attributed to oxidation of some materia@) formed in the process responsible for the prewave. The discontinuities that appear in the curve are the result of correcting the original curves for iR drop. They reflect the rapid decrease of current that occurred when the material being oxidized was completely depleted and are, in fact, evidence that deposited material was stripped from the electrode. The sharp rise in current at - 1.33 V corresponds to the onset of the formation of tungsten bronze. The forward and reverse sweep (3 - 3* - 3') illustrates that sweep reversal at - 1.33 V resulted in an anodic peak due to the oxidation of the bronze. This peak was followed by the anodic peak attributed to oxidation of the product formed on the prewave. As pointed out above, the bronze is formed after the reduced species equilibrates with the melt, and therefore the electrooxidation of the bronze is not the reverse of electroreduction Of
was.
DECOMPOSITION POTENTIALS. The cathodic decomposition potential of each melt is defined as the potential corresponding to the onset of formation of bronze. At concentrations above 5 m/o WOO,this process did not become current limited and these decomposition potentials were, for all practical purposes, also the cathodic decomposition potentials of the melt. For the pure melts, the cathodic decomposition potential was, as normally defined, that of the final cathodic process for the melt. The anodic decomposition potentials were obtained from anodic voltammograms. The following reaction is presumed to occur.
Xwo~+ W042---f WOz + O2 + Xwo3 + 2e
(6)
Table IV. Decomposition Potentials for Sodium Polytungstates at 645 "C
Concentration of WOa mio Formal 18.0 20.0 24.0 s,b
2.71 3.03 3.75
Measured full-cell decomMeasured half-cell decom- position position potentials, voltsn potentials, Cathodic Anodic V -1.28 -1.25 -1.19
-0.16* -0.15 -0.11
1.10 1.10 1.08
as for Table 111.
In several melts, the voltammograms showed current oscillations whose frequency increased as the potential became more positive. These were attributed to the evolution of oxygen at the electrode. Full-cell decomposition potentials of the polytungstates are of particular interest for the preparation of tungsten bronzes. These values were calculated from the average half-cell values, and are presented in Table 111. Three values obtained in NazW04-W03 at a lower temperature, 645 "C,are given in Table IV. For the system Na2W04-WOI, full-cell decomposition potentials were also determined directly using the alumina cell described above. Agreement with values calculated from half-cell data was good. As would be expected in view of the demonstration above that the reduction of W03 to bronze obeys reversible equations, the decomposition potential was found to be very small (typically 0.05 V) when a bronze previously deposited from the melt was used as the anode. In a melt containing 4 0 x W 0 3 and 60z Na2W04 a small decomposition potential (0.06 V) was also obtained with a tungsten anode. RECEIVED for review December 9, 1971. Accepted February 22, 1972. Special thanks are due the National Aeronautics and Space Administration for support given under the NASA Predoctoral Traineeship Program. This paper is abstracted in part from a dissertation submitted by one of us (C.W.F.) to the Polytechnic Institute of Brooklyn in partial fulfillment of the requirements for the degree of Ph.D. in Chemistry.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
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