Electrochemistry of Niobium Pentachloride in N,N-Dimethylformamide Larry R. Sherman' and Vernon S. Archer2 Department of Chemistry, University of Wyoming, Laramie, W y o . 82070 Polarography, chronopotentiometry, and potential sweep chronoamperometry were used to study the reduction of NbCh in DMF. The NbCls was found to hydrolyze to give NbOCls and 2HCI due to residual water in the solvent. Two reduction steps were observed at the platinum disk electrode and three steps were observed at mercury electrodes. The second step observed at the mercury electrodes was due to HCI reduction. Electrode kinetic parameters were obtained for the niobium reductions at the mercury electrodes. Electron spin resonance spectrometry and vapor pressure osmometry were used to detect intermediates and determine the state of aggregation of the niobium species. Mechanisms are proposed for the niobium reductions.
THE FIRST ELECTROCHEMICAL reduction of niobium(V) to a lower oxidation state was performed by Stahler in 1914 ( I ) . He reduced the metal ion in aqueous H2S04and aqueous HC1 solutions and isolated (Nb)z(So& and NbCla .6Hz0 as products. This work was repeated by Kiehl and Hart in 1928 (2). Their work indicated a reversible reduction of NbCls to NbCla with a standard potential of -0.38 V us. SCE. This standard potential was dependent upon the pH of the solution. Zeltzer (3) performed the first polarographic reduction of NbC& in 1934. He observed a two-electron change at -0.83 V us. the normal calomel electrode in 0.1M nitric acid. In 1954, Cozzi and Vivarelli (4) showed that niobium(V) was reversibly reduced to niobium(1V) at -0.455 V us. SCE in concentrated aqueous hydrochloric acid. The niobium(1V) was found to disproportionate to niobium(II1) and niobium (V). They found that the niobium(II1) could be reoxidized to niobium(V) at -0.32 V us. SCE. By complexing the niobium (IV) with ethylene glycol, they were able to reduce the niobium(1V) to niobium(I1) before the former disproportionated. McCullough and Meites (5, 6) attempted to explain the mechanism for electrochemical reduction of niobium(V) in aqueous HCl solution. Their evidence indicated niobium(V) to be a dimer in high concentrations of hydrochloric acid. They felt that niobium(V) dimer was reduced to niobium(1V) dimer, which disproportionated into a niobium(V)-niobium (111) adduct. The latter was further reduced to a niobium (1V)-niobium(II1) adduct. The niobium dimers were presumed to be involved in dissociation equilibria with the respective monomers. It was presumed that monomeric nio1 Present address, Department of Chemistry, North Carolina Agricultural and Technical State University, Greensboro, N. C.
2741 1. 2 To whom correspondence and requests for reprints should be addressed.
(1) A. Stahler, Ber. Deut. Chem. Ges., 47,841 (1914). ( 2 ) S . J. Kiehl and D. Hart, J. Amer. Chem. SOC.,50, 2337 (1928). (3) S . Zeltzer, Coll. Czech. Chem. Commun., 4, 319 (1932). (4) D. Cozzi and S . Vivarelli, Ber. Bunsenges. Phys. Chem., 58, 177 (1954). ( 5 ) J. G. McCullough and L. Meites, J. Electroanal. Chem., 18, 123 (1967). (6) Zbid., 19, 111 (1968).
1356
bium(1V) was reduced to niobium(II), which reacted with monomeric niobium(1V) to produce monomeric niobium(II1) species as a final product. Gut (7) reported that niobium(V) gave two polarographic reduction waves at -0.52 V and -0.88 V us. a mercury pool electrode in acetonitrile and that niobium(IV) gave one polarographic wave at -0.89 V us. a mercury pool electrode in N,N-dimethylformamide (DMF). He assigned a total electron change of one to the first wave and, two to the second wave. Gutmann and Michlmayr (8) observed a single reduction wave for niobium(V) in D M F at -1.27 V vs. SCE. They assigned a two-electron change to the reduction. This paper includes polarographic and chronopotentiometric studies of the reduction of NbC16in DMF and attempts to explain certain aspects of the reduction mechanisms. EXPERIMENTAL
Apparatus. The electrochemical measurements were made with a Sargent Model XV Polarograph using a Sargent threeelectrode IR Compensator or with a Beckman Electroscan 30 using either the galvanostatic or potentiostatic mode. The vapor phase osmometric data were obtained on a Mechrolab Vapor Pressure Osmometer Model 301A. The electron paramagnetic resonance spectra were obtained on a Varian Electron Paramagnetic Resonance Spectrometer Model E-3. The visible spectra were obtained on a Beckman DK-2 Spectrophotometer. Most of the data involving linear relationships were processed by the method of least squares using a Philco 2000 Computer Model 211. A saturated cadmium chloride (in DMF)/cadmium amalgam reference electrode (SCdAE) was constructed similar to that described by Marple (9) except that anhydrous lithium perchlorate was used as the sole supporting electrolyte. The reference electrode had a potential of -0.567 V measured against an aqueous saturated calomel electrode (SCE) through a concentrated LiC104/DMF salt bridge. A dropping mercury electrode (DME) of conventional design was used for the polarographic work. The average drop time for the DME was 7.93 seconds at -0.075 V and 5.38 seconds at -1.08 V us. SCdAE with a mercury flow rate of 0.633 mg/ sec. The working electrodes used for chronopotentiometry and potential sweep chronoamperometry were a polished platinum disk electrode of surface area 0.36 cm2 and a mercury cup electrode of effective surface area 0.263 cm2. The mercury cup electrode was a very small mercury pool electrode at which conditions necessary to a linear diffusion field in the downward vertical direction were maintained. For controlled potential electrolysis and coulometry, either a mercury pool or a platinum gauze electrode of large surface area was used as the working electrode. The auxiliary electrode used in most cases was either a platinum spiral or a platinum gauze electrode. The auxiliary electrode was ordinarily isolated from the cathode by use of fritted glass connections. Beckman electrolysis vessels of standard design were used in all electrochemical work. (7) R. Gut, Helv. Chim. Acta., 43, 830 (1960). (8) V. Gutmann and M. Michlmayr, Monatsh. Chem., 99, 326 (1968). (9) L. W. Marple, ANAL.CHEM., 39, 844 (1967).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
Reagents. Technical grade D M F from Eastman Organic Chemicals was dried over molecular sieves, further treated with CaH2, and distilled at atmospheric pressure. The constant boiling fraction (143 “C at 595 mm) was used. The residual water concentration was monitored by Karl Fischer titration and was found to be in the range 10-25 ppm. The solvent gave a stable polarographic base line from f 0 . 3 to - 1.6 V us. SCdAE when tetraethylammonium perchlorate (TEAP) was used as supporting electrolyte. TEAP from Eastman Organic Chemicals was used as received, after drying in a vacuum oven at 80 “C for 24 hours. Niobium pentachloride (99.573 was used as received from Rocky Mountain Research Corporation, Denver, Colo. All other chemicals were reagent grade and were used without further purification. Sample Preparation. All solutions were prepared and transferred to electrochemical cells in a dry box. They ranged from 0.700 to 7.300mF in niobium pentachloride and were approximately 0.2F TEAP. After preparation a solution was transferred to an electrolysis vessel, thermostated at 24.9 0.1 “C,deoxygenated with dry argon gas, and electrolyzed. The auxiliary electrode compartment contained only 0.2F TEAP in DMF. RESULTS AND DISCUSSION Niobium Oxytrichloride. It is believed that this entire investigation was performed on NbOC13 rather than NbC15. Golub and Sych (10,11) found that NbC15is converted fairly rapidly to NbOC18in D M F due to reaction with residual water and with the solvent itself. The residual water in the D M F used in this study was approximately 1 m M and would hydrolyze most of the NbC15 in the less concentrated solutions according to the following equation: NbCls
+ HzO
NbOCla
+ 2HC1
(1) When the solvent was added to NbC15, small quantities of white fumes, presumably due to HCl formation, were observed. When high concentrations of NbClb were used, the solutions were initially a deep yellow, fading to pale yellow, or colorless solutions in less than 20 minutes. Furthermore, polarograms of the niobium pentachloride solutions (Figure 1) gave three reduction waves; the second wave was enhanced by the addition of either hydrochloric acid or dry hydrogen chloride gas to the solution. In most freshly prepared solutions, the second wave was approximately twice the height of the first. If the HCl was due to the complete hydrolysis of NbC15 according to Equation 1, this wave would be expected to be proportional to the NbC15 concentration and to be approximately twice the height of the first wave, assuming the first wave involved a one-electron reduction step. Other electrochemical work supported this conclusion also. Neither Gutmann and Michlmayr (8) nor Gut (7) considered the hydrolytic production of HC1 as a factor in their polarographic investigations. It is believed that the first niobium reduction wave in acetonitrile, which Gut assigned to the reduction of Nb(V) to Nb(IV), corresponds to the first niobium reduction wave observed in this investigation. However, it is believed that the second reduction wave, which Gut assigned to the reduction of Nb(1V) to Nb(II), might have been due to the reduction of the hydrogen ion formed by the hydrolysis reaction. Since the NbCI4 should also hydrolyze to produce HCl, the single polarographic wave, observed by Gut for solutions of NbC14 in DMF, probably was due to hydrogen ion reduction. The KC1 used as supporting elec+
(10) A. M. Golub and A. M. Sych, Latc. PSR Zinat. Akad. Vestis, Kim. Ser., 1963,641. (11) A. M. Golub and A. M. Sych, Rum. J. Inorg. Chem., 9, 593
(1964).
E (V vs. SCdAE) Figure 1. Polarogram of a 6.58mF NbC15 solution in DMF Table I. Controlled Potential Coulometry A. Total Electronic Change at Platinum Cathode No. Mean
First reduction Second reduction Complete reduction
determinations 9 2 8
electronic change 2.96 2.03 5.06
Std dev 0.22 0.10
0.22
B. Total Electronic Change at Mercury Cathode First reduction 8 1.01 0.09 Second reduction 1 1.87 0.12 Third reduction 6 2.01 0.15 Complete reduction 6 4.90 0.17
trolyte probably was reduced at a potential too positive to allow resolution of the wave corresponding to reduction of Nb(IV). No correlation can be made between the work reported here and that reported by Gutman and Michlmayr (8). The only feasible explanation for the single polarographic wave, which they obtained for aged solutions of NbC15in DMF, seems to involve the possibility that this wave was a composite wave involving both reduction of Nb(V) to Nb(1V) and reduction of hydrogen ion. In freshly prepared solutions, they did observe a very small wave at - 1.86 V in addition to the wave at -1.27 V us. SCE. This wave might have been due to hydrogen ion reduction. From the results discussed below, it is felt that the following reduction sequence corresponds to the three polarographic waves observed in this investigation: First wave
+ le- +-2Hf + 2e- >
0.1v
Nb(V)
Nb(1V)
(2)
Hz(g)
(3)
-0.6V
Second wave Third wave
Nb(1V)
- 1.1v
+ 2e- +-
Nb(I1)
(4)
It should be mentioned that the third wave is probably due to reduction of a Nb(V)-Nb(II1) adduct rather than to reduction of Nb(1V). Electrochemistry at Platinum Electrodes. Potential sweep chronoamperograms of the NbC15 solutions, obtained at the platinum disk electrode using a scan speed of 50 mV/sec., showed two peaks having peak potentials at approximately +0.2 and -0.4 V us. SCdAE. The peak current of the first wave was considerably greater than that of the second wave. Controlled potential coulometry at the two peak potentials, using a platinum gauze cathode, indicated an electronic change of approximately “3” for the first reduction and of approximately “2” for the second reduction (Table Ia).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
1357
Figure 1 shows a typical polarogram. The limiting diffusion current of the second wave was increased by addition of HC1 gas. In addition it was possible to remove the second wave completely by the addition of solid Li2CO3 to the solution. This treatment was not used routinely, as the added Li2COa reduced slightly the diffusion currents of the two remaining waves and shifted their half-wave potentials to more negative values. It became apparent that the first reduction wave observed at the platinum disk electrode had been due to the reduction of two different species and had been resolved into two waves at the DME. As expected, the solvated proton had a considerably larger cathodic overpotential at a mercury electrode than at a platinum electrode. The first and third waves were found to be due to stepwise niobium reduction. Controlled potential coulometry at the mercury pool cathode, using potentials on the plateau regions of the three polarographic waves, indicated that the electronic changes were approximately “1 ,’) “2,” and “2” for the first, second, and third reduction waves, respectively (Table Ib). Polarograms of solutions, which had been “half-reduced” by controlled potential electrolysis on the plateau region of the first wave, were of the same form as those of the original solutions, except that the limiting diffusion current of the first wave was proportionately less in each case. No anodic waves for these solutions were observed within the working limits of the DME. The diffusion current constant I d is defined from the Ilkovic equation by the expression:
0.4
0.2
’ W
Q
U
9
0.C
>
v
u
- 0.2 - 0.4
TIME ( s e d Figure 2. Chronopotentiogram of a 0.693mF NbC15 solution in DMF, using a current of 100 pA at the platinum disk electrode As expected chronopotentiograms of the NbC15 solutions at the platinum disk electrode showed two waves having quarter-wave-potentials of approximately $ 0 . 3 and -0.3 V os. SCdAE (Figure 2). For the solutions of lower concentration, it was found that the ratio of transition times, r 2 / r 1for , the two chronopotentiometric waves was approximately the theoretical value to be expected for nl = 3 and nz = 2. (For a 0.693mFNbC15 solution, the ratio was 1.81, compared with the theoretical value of 1.78.) Based on the other work reported here, it is thought that the mechanism is more complex than a simple two-step reduction and that this agreement is therefore meaningless. Addition of dry HC1 gas was found to increase the peak current for the first potential sweep chronoamperometric wave and the transition times for both chronopotentiometric waves. Cyclic potential sweep chronoamperometry and cyclic chronopotentiometry indicated the two reduction steps to be completely irreversible. The single anodic wave observed during reverse sweeps was quite small and was presumed to be due to oxidation of adsorbed hydrogen gas. Further investigation at platinum electrodes was abandoned at this point, as it was obvious that proton reduction was a major contributor to the first reduction step. Polarography. Polarograms of the NbClj solutions showed three irreversible reduction waves having half-wave potentials of approximately -0.1, -0.6, and -1.1 V us. SCdAE.
Technique used Polarography Chronopotentiometry
1358
where id is the maximum limiting diffusion current in microamperes, t the drop time in seconds, m the rate of mercury flow in mg/sec, D the diffusion coefficient in cm2/sec, n the total electron change involved in the reduction, and c the bulk concentration of electroactive species in millimoles/liter. Id was calculated for the first and third reduction waves. The mean values of Id were 1.76 and 3.83 p A 1. sec1’2 rng-213 mmole-1 for the first and third waves, respectively. From the polarographic data, the diffusion coefficients of the electroactive species responsible for the first and third waves (corresponding to the first and second niobium reductions) were determined. The diffusion coefficient values are summarized in Table 11. Electrode kinetic parameters for the two niobium reductions were obtained by making use of the following equation for irreversible polarographic waves derived by Oldham and Parry (12): -
E
= RT In
anaF
1.35 k,’d-!-
D
-
[
In 2i(3id - i) Sid(id - i)
]
(6)
where E is the potential of the working electrode us. the reference electrode used, k,” the heterogeneous rate constant at (12) K. B. Oldham and E. P. Parry, ANAL.CHEM., 40,65 (1968),
Table 11. Diffusion Coefficients D(cm2/sec) First niobium reduction Std dev Second niobium reduction 6.15 X 6.62 x
cm,F
2.55 x 1.21 x 10-7
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
7.29 x 2.31 X loe6
Std dev 2.83 1.58
x x
10-7
Technique used
fflle
First Niobium Reduction Polarography Chronopotentiometry Potential sweep chronoamperometry
0.596 0.618
Second Niobium Reduction Polarography Chronopotentiometry Potential sweep chronoamperometry
0.245 0.232
Table 111. Electrode Kinetic Parameters Std dev of ana k f " cm/sec
Std dev of
kj"
0.054 0.080
3.56 1.40
x x
10-4 10-4
3.61 6.52
x x
10-4 10-6
0.060 0.120
9.38 2.14
x x
10-7
9.52 1.16
x x
10-8 10-7
0.53
0.24
zero volts os. the reference electrode in cm/sec, a the cathodic transfer coefficient, n, the number of electrons involved in the rate determining charge transfer step, i the instantaneous current at the time of release of the mercury drop. The other quantities of interest are defined in relation to Equation 5 . The values of k f " and an, are summarized in Table 111. It was impossible to calculate standard rate constants as the standard electrode potentials could not be determined. Each individual polarographic wave conformed very closely to Equation 6. The mean values given for the parameters were the averages obtained from the analysis of 30 to 40 different polarograms. The large standard deviation of k," was due to limitations in the measuring process and the fact that relatively small deviations in potential measurements can cause rather large deviations in k f o due to the logarithmic relationship involved. Potential Sweep Chronoamperometry. Potential sweep chronoamperometry of the NbCls solutions at the mercury cup electrode tended to support the polarographic results. A typical potential sweep chronoamperogram, obtained using a scan speed of 50 mV/sec., is shown in Figure 3. Three peaks, corresponding to the three polarographic waves, are present, with peak potentials of approximately -0.33, -0.80, and - 1.45 V US. SCdAE. Cyclic potential sweep chronoamperograms of the NbCls solutions failed to show any anodic peaks within the working range of the mercury cup electrode. One solution was electrolyzed at the first peak potential until approximately half of the niobium had been reduced. A potential sweep chronoamperogram of this solution showed the same cathodic waves, except that the first wave was proportionately smaller, but no anodic wave corresponding to reoxidation of the niobium was present. Values of an, were determined from potential sweep chronoamperometric data by using a method of Reinmuth and Rogers (I.?), depending on the shift in half-peak potential Ep/2with voltage sweep rate v according to the following equation (Table 111):
(7) Chronopotentiometry. Chronopotentiograms of the NbCls solutions at the mercury cup electrode showed three waves corresponding to the three polarographic waves observed. Chronopotentiometric data obtained from the first and third waves were used to calculate diffusion coefficient values by making use of the Sand equation: (13) W. H. Reinmuth and L. B. Rogers, J. Amer. Chem. SOC., 82, 802 (1959).
240
3 160
.-
v
BO
0 E ( V VS. SCdAE)
Figure 3. Potential sweep chronoamperogram of a 2.5mF NbC& solution in DMF, using a scan speed of 50 mV/sec at the mercury cup electrode
where 7 is the transition time in seconds, i the constant electrolysis current, n the total electron change involved in the reduction process, F the faraday, A the electrode area, D the diffusion coefficient, and c the concentration of electroactive species. The diffusion coefficient values obtained by chronopotentiometry and polarography are compared in Table 11. The large discrepancy between the two values of the diffusion coefficient, determined from the second niobium reduction step, could be due to differences in sample treatment. The data used in obtaining the polarographic values were obtained by running polarograms on the solutions as prepared. The data used in obtaining the second chronopotentiometric value were obtained by running chronopotentiograms on solutions which had been exhaustively electrolyzed at potentials sufficient to completely reduce the Nb(V) through the first step and to remove the hydrogen ions from the solution. This procedure could have altered the nature of the niobium species being reduced. Electrode kinetic parameters for the two niobium reductions were obtained by making use of the following equation for an irreversible chronopotentiometric wave (14):
where t is the time of electrolysis. All other quantities of interest were defined in relation to Equations 5, 6 , and 8. Values of k," and ana obtained by the several different techniques are compared in Table 111. (14) P. Delahay and T. Berzins, J . Amer. Chem. SOC.,~~, 2486 (1953).
jl
0.8 0.6
a 1.0
Q
0.4 0.2
-L
2800
500 600 700 800 9001000 X (nm) Figure 4. Visible spectra of the blue Nb(1V) intermediate measured against the unelectrolyzed NbC16 solution. Curve 1 is the initial spectrum. Curve 2 was run 4 minutes later. Curve 3 was run approximately 10 minutes later. Curve 4 is the base line Mechanism. The electrode kinetic parameters give slight insight into the mechanism for the electrochemical reduction. In addition controlled potential coulometry gives a certain amount of useful information. Several additional techniques were used for obtaining data with regard to intermediates, state of aggregation, and final reduction products. It had been observed that, during exhaustive electrolysis under controlled potential conditions, a transient blue species formed in the vicinity of the cathode. Gut (7) had previously reported that NbC14formed blue solutions in DMF. Figure 4 shows visible spectra of the blue intermediate obtained as a function of time. The successive spectra of the blue intermediate showed absorption maxima between 750 and 800 nm. The half-life for this species was approximately 4 minutes. Theoretically, niobium(1V) species should be paramagnetic, giving a 10-line electron paramagnetic resonance spectrum. A spectrum of this type has previously been reported for N b (IV) in concentrated alcoholic HC1 solutions by Lardon and Gunthard (15). By rapid removal of a sample of the blue solution from the vicinity of the cathode and freezing it in liquid nitrogen, it was possible to obtain the expected 10-line ESR spectrum, which is shown as a line spectrum in Figure 5. As the solution warmed up, the blue color faded and the ESR signal decreased. This paramagnetic species appears to have a short lifetime above 175 OK. One possible explanation for the disappearance of the paramagnetic species involves the disproportionation of the paramagnetic Nb(1V) to diamagnetic Nb(V) and Nb(II1) species. In order to determine the state of aggregation in the original NbCls solutions and in the solutions produced by exhaustive electrolysis, a vapor pressure osmometric study was made. This work indicated that the DMF solutions contained four moles of particles for every formula weight of NbCls added. When the solution was exhaustively electrolyzed, using nonisolated platinum working electrodes, at a cathodic potential sufficient to remove the solvated protons and to reduce the Nb(V) through the first reduction step, three of the four particles were removed from the solution. When the solutions were exhaustively electrolyzed at the mercury pool cathode under conditions such that only Nb(V) could be reduced through the first reduction step, approximately one to one and one-half of the four particles were removed from the solution. These controlled cathode potential electrolyses were performed without supporting electrolyte and were carried only (15) M. Lardon and H.H. Gunthard, J . Chem. Phys., 44, 2010
(1966). 1360
L
3100
FIELD (Gauss) Figure 5. Electron paramagnetic resonance line spectrum of the blue Nb(1V) intermediate. No corrections for line width variations were made. ( T = -120 "1
to approximately 95 2 completion due to the increase in solution resistance. The data obtained were good to approximately =t= ' 1 2 particle. From the above data, the following two mechanisms are postulated as possibilities for the first reduction of Nb(V) at a mercury electrode in DMF: [I]
-0.1v + le- +
[NbVOCl2]+
Pale yellow or colorless species [Nb'VOClZ]
[Nb1vOCl2]
(10)
Blue, paramagnetic species
k +
Blue, paramagnetic species
+
+
'12
Cl-
(11)
or -0.1v + le- +
Pale yellow or colorless species
Blue, paramagnetic species
Blue, paramagnetic species (13) Colorless species Golub (10) has indicated that niobium(V) exists in cationic form in DMF. The recent work of McCullough and Meites
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
(5, 6) indicates that niobium(V) exists principally as the dimer
in aqueous HC1 solutions. The vapor phase osmometric data would admit the possibility of the Nb(V) being either monomeric or dimeric in DMF. The four particles obtained for each NbCls dissolved could be NbOC12+, C1-, and two HCl. [It has been reported that HC1 is a weak electrolyte in D M F (16).] The four particles could also be two HC1, ll/zC1- and [Nb2O2Cl3I3f. The fact that three particles were removed upon electrolysis at a platinum electrode and that 11/2 particles were removed at the mercury electrode is subject to ambiguous interpretation also. Either a monomeric or a dimeric Nb(IV) species might be expected to give the observed ESR spectrum. Both Cozzi and Vivarelli ( 4 ) and McCullough and Meites (5,6) indicate that Nb(1V) disproportionates to Nb(V) and Nb(II1) in aqueous HCl solution. Therefore, it would seem reasonable to account for the disappearance of the paramagnetic Nb(1V) species by a similar reaction. The fact that the original niobium reduction wave did not reappear, after electrolysis at the mercury cathode using potentials on the first polarographic wave, would indicate that the Nb(V), generated in the disproportionation reaction, is tied up in a form which is much harder to reduce than the original Nb(V) species. This gives added support to the Nb(V)-Nb(II1) adduct as a final product of the first reduction process. The known chemistry of niobium compounds would tend to favor dimeric structures throughout. If the third polarographic wave is examined carefully (Figure l), it appears to be two closely overlapping waves, each corresponding to a one-electron change. Exhaustive electrolysis at potentials on the third polarographic wave produced a white insoluble precipitate. Electrogravimetric analyses indicated this final reduction product to be NbO. (16) D. S. Reid and C. A. Vincent, J . Electroanal. Chem., 18, 427 (1968).
With these considerations in mind, the following mechanism is reasonable for the second niobium reduction:
NbO(S)
+ 11/2 C1-
(15)
This mechanism is, by no means, the only possibility. As the disproportionation reaction, following the first reduction step, has a half-life of approximately four minutes, it is entirely conceivable that the unstable paramagnetic species could be reduced simultaneously with the Nb(V)-Nb(II1) adduct to give NbO(S). The mechanisms proposed are consistent with the experimental data. More involved studies of the species present in solution at various stages in the electrolysis might give more definitive data. RECEIVED for review May 15, 1970. Accepted July 22, 1970. This paper is based on a thesis submitted by Larry R. Sherman to the Graduate School of the University of Wyoming in partial fulfillment of the requirements for the Ph.D. degree. L. Sherman wishes to acknowledge and thank the Analytical Division of the ACS for a 1967 Summer Fellowship, the National Aeronautics and Space Administration for a Research Traineeship, and the Colorado-Wyoming Academy of Science for a 1968 Research Grant-in-Aid.
Basic and Practical Considerations for Sampling and Digitizing Interferograms Generated by a Fourier Transform Spectrometer Gary Horlick' and Howard V. Malmstadt Department of Chemistry and Chemical Engineering, University of Illinois, Urbana, 111. 61801
A number of problems associated with the sampling and digitizing of interferograms and their effect on the resulting spectra are investigated and described. These include studies of the sampling rate, the accuracy of the frequency axis, the effects of missed, extra, and bad points, and the resolution required for the analog-to-digital converter. These problems are all studied using computer simulation on real and synthetic interferograms, and spectra are calculated to demonstrate the erroneous effects that can result. Criteria are established for selecting the sampling interval and a number of examples are given to show how the interval and the spectral region under investigation determine the final labeling of the frequency axis. It is shown that the occurrence of a missed, 1 Present address, Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada. To whom requests for reprints should be sent.
extra, or bad point during the sampling and digitizing steps can distort the final spectrum. Thus these types of errors must be avoided if accurate spectra are to be calculated. Finally, it is shown that a lack of resolution in the analog-to-digital conversion step results in a loss of spectral resolution.
WITH THE AVAILABILITY of commercial instrumentation Fourier transform spectrometers are becoming more common. The main distinguishing feature of this spectrometric technique is that the spectrum is obtained by taking the Fourier transformation of an interferogram which is generated by a twobeam interferometer ( I , 2 ) . Considerable data handling is necessary with Fourier transform spectrometry and many (1) Gary Horlick, Appl. Spectry., 22,617 (1968). ( 2 ) H. A. Gebbie, Appl. Opt., 8 , 501 (1969).
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