XANES Spectroelectrochemistry: A New Method for Determining

The formal potentials of the two redox couples determined using this method ... assumption that the formal potential of a redox couple in a real syste...
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Anal. Chem. 1999, 71, 4622-4628

XANES Spectroelectrochemistry: A New Method for Determining Formal Potentials L. Soderholm,*,† Mark R. Antonio,*,‡ Clayton Williams,*,§ and S. R. Wasserman*,|

Chemistry and Advanced Photon Source Divisions, Argonne National Laboratory, Argonne, Illinois 60439

Proof-of-concept experiments are presented outlining the use of X-ray absorption near-edge structure (XANES) in the determination of formal potentials. Two redox couples of Np in 1 M perchloric acid are used for the demonstration. XANES data obtained as a function of applied potential are used to quantitatively determine the relative concentrations of Np(VI)/Np(V) and Np(IV)/Np(III). Bulk electrolysis is done in the X-ray beam with the solution maintained under electrochemical control. Relative concentrations are extracted from the data using a principal component (factor) analysis. The formal potentials of the two redox couples determined using this method agree with those previously published. The results establish in situ XANES spectroelectrochemistry as a viable method for determining formal potentials. Examples, taken from environmental or heteropolyanion chemistry, are discussed for which this methodology will provide information currently unavailable from other established techniques. A detailed knowledge about the speciation of redox-active metal ions in complex environments is important in such diverse areas as catalysis, battery development, corrosion, and environmental sciences. There are a number of methods used to estimate the oxidation states of metal ions in complex samples, among the more simple of which are Eh/pH diagrams.1 These estimations rely on the underlying assumption that the formal potential of a redox couple in a real system can be extrapolated from simpler systems. Unfortunately, the redox behavior of a metal ion in solution can be dependent on a number of factors not easily estimated, including the influence of other complexing ions in solution.2 For example, the Ce(IV)/Ce(III) reduction potential is about 1.7 V less positive in 5.5 M K2CO3 than in a noncomplexing HClO4 solution.3 The reduction potential of an ion in solution can be determined experimentally by cyclic voltammetry, polarography, potentiometric titrations, or bulk electrolysis with optical detection or by otherwise measuring the change in concentration of the redox-active species as a function of applied potential. A plot of the relative concentrations of the oxidized and reduced species †

Chemistry; [email protected]. Chemistry; [email protected]. § Chemistry; [email protected]. | Advanced Photon Source Division; [email protected]. (1) Pourbaix, M. Atlas of electrochemical equilibria in aqueous solutions; Pergamon Press: Oxford, 1966. (2) Lindberg, R. D.; Runnells, D. D. Science 1984, 225, 925-927. (3) Hobart, D. E.; Samhoun, K.; Young, J. P.; Norvell, V. E.; Mamantov, G.; Peterson, J. R. Inorg. Nucl. Chem. Lett. 1980, 16, 321-328. ‡

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versus the applied potential (a Nernst plot) yields the formal potential of the reaction.4 Whereas this methodology has broad applicability, there are cases in which it cannot be used, notably when there are interfering species in solution.5,6 For example, the reduction potential of rare-earth ions complexed to metal clusters, such as the heteropolyanions, is difficult to measure because the metal ion cluster is itself redox active7 and may be highly absorbing in its reduced state.8 This unfortunate combination renders the standard techniques ineffective for the determination of the formal reduction potential of a complexed metal ion. We have investigated the use of X-ray absorption near-edge structure (XANES) as a method of determining the relative concentrations of the redox components generated by in situ bulk electrolysis. Although there has been significant interest in X-ray absorption spectroscopy as a probe of ion speciation in electroactive systems, most of the work has centered on extended X-ray absorption fine structure (EXAFS) spectroelectrochemistry.9-12 The utility of XANES as a valence probe is well documented,13,14 although the underlying details of this spectroscopy are not well understood. In most cases, analyses are done qualitatively by comparing spectra from known standards with those from the samples of interest. We demonstrate herein that XANES can be used to quantitatively determine concentrations in a redox-active system that is under electrochemical control. We use the relative concentrations to obtain formal potentials for the redox reactions. Principal component (factor) analysis (PCA)15-17 is incorporated into our data treatment to optimize the quantitative extraction of (4) Bockris, J. O. M.; Reddy, A. K. N. Modern Electrochemistry; Plenum Press: New York, 1970. (5) Kristensen, E. W.; Igo, D. H.; Elder, R. C.; Heineman, W. R. J. Electroanal. Chem. 1991, 309, 61-72. (6) Xie, Y.; Dong, S. J. Electroanal. Chem. 1994, 365, 151-156. (7) Soderholm, L.; Liu, G. K.; Muntean, J.; Malinsky, J.; Antonio, M. R. J. Phys. Chem. 1995, 99, 9611-9616. (8) Papaconstantinou, E.; Pope, M. T. Inorg. Chem. 1970, 9, 667-669. (9) Dewald, H. D.; Watkins, J. W.; Elder, R. C.; Heinman, W. R. Anal. Chem. 1986, 58, 2968-2975. (10) Sharpe, L. R.; Heineman, W. R.; Elder, R. C. Chem. Rev. 1990, 90, 705722. (11) Smith, D. A.; Elder, R. C.; Heineman, W. Anal. Chem. 1985, 57, 23612365. (12) Igo, D. H.; Elder, R. C.; Heineman, W. R.; Dewald, H. D. Anal. Chem. 1991, 63, 2535-2539. (13) Bertram, S.; Kaindl, G.; Jove, J.; Pages, M.; Gal, J. Phys. Rev. Lett. 1989, 63, 2680-2683. (14) Bianconi, A. In X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS, and XANES; Koningsberger, D. C., Prins, R., Eds.; John Wiley and Sons: New York, 1988; pp 573-662. (15) Malinowski, E. R.; Howery, D. G. Factor analysis in chemistry; John Wiley and Sons: New York, 1980. (16) Wasserman, S. R. J. Phys. IV 1997, 7 (C2), 203-205. 10.1021/ac990080t CCC: $18.00

© 1999 American Chemical Society Published on Web 09/18/1999

relative concentrations. An advantage of this approach is that XANES is a single-ion probe that can interrogate one element in a complex mixture. This is in contrast to polarographic techniques, which cannot distinguish between different redox couples present in the same sample. Furthermore, the X-ray absorption by an ion is almost directly proportional to concentration;18 therefore, XANES provides information about both the oxidized and reduced species. This is in contrast to optical spectroscopy, in which there is often only one component of the redox couple that is optically active. Finally, XANES spectroscopy can be applied to any element in the periodic table. For our proof-of-concept experiments, we have chosen to look at the formal potentials of two neptunium (Np; Z ) 93) couples:

Np4+ + e- f Np3+

(1)

NpO22+ + e- f NpO2+

(2)

There are several reasons for this choice. The Np(III) and Np(IV) edges in XANES spectra are more separated than the Np(V) and Np(VI) spectra, which overlap enough to render their quantitative separation difficult. The electrolytic preparation of valence-specific Np ions in a 1 M perchloric acid solution is well documented.19-23 The pH of the HClO4 solution allows both redox couples to be accessed in the same solution with no observable hydrolysis or precipitation.1,24 These two different couples provide good examples of the use of XANES spectroelectrochemistry in the determination of formal reduction potentials. Previous work to determine Np formal potentials has relied on polarography or potentiometric titrations or optical spectroscopy to determine the valence-specific concentrations of Np. Recent interest in Np speciation itself centers on understanding its chemistry under environmentally relevant conditions.25-28 Under the standard conditions applied in our experiments, the higher-valent Np ions are in the form of hydrated neptunyl ions NpO22+ and NpO2+, whereas the lower-valent ions exist as simple hydrated ions Np(IV) and Np(III).29 Simple redox reactions involving either the (VI)/(V) or the (IV)/(III) couple, such as those shown in (1) or (2), proceed rapidly. Those reactions involving the creation or destruction of a [OdNpdO]+ moiety, such as the reaction that couples (1) and (2), are expected to proceed much more slowly in the absence of a large overpotential.30 Therefore, the kinetics involved in the formation or destruction of the dioxo ion can be used to isolate the two couples of interest. (17) Fernandez-Garcia, M.; Marquez-Alverez, C.; Haller, G. L. J. Phys. Chem. 1995, 99, 12565-12569. (18) Antonio, M. R.; Soderholm, L.; Song, I. J. Appl. Electrochem. 1997, 27, 784792. (19) Hindman, J. C.; Kritchevsky, E. S. J. Am. Chem. Soc. 1950, 72, 953-956. (20) Cohen, D.; Hindman, J. C. J. Am. Chem. Soc. 1952, 74, 4679-4682. (21) Sullivan, J. C.; Hindman, J. C.; Zielen, A. J. J. Am. Chem. Soc. 1961, 83, 3373-3378. (22) Li, Y.; Kato, Y.; Yoshida, Z. Radiochim. Acta 1993, 60, 115-119. (23) Riglet, B.; Robouch, P.; Vitorge, P. Radiochim. Acta 1989, 46, 85-94. (24) Sjoblom, R.; Hindman, J. C. J. Am. Chem. Soc. 1951, 73, 1744-1751. (25) Lieser, K. H.; R.Hill; Muhlenweg, U.; Singh, R. N.; Shu-De, T.; Steinkopff, T. J. Radioanal. Nucl. Chem. Articles 1991, 147, 117-131. (26) Silva, R. J.; Nitsche, H. Radiochim. Acta 1995, 70/71, 377-396. (27) Lieser, K. H. Radiochim. Acta 1995, 70/71, 355-375. (28) Clark, D. L.; Hobart, D. E.; Neu, M. P. Chem. Rev. 1995, 95, 25-48. (29) Fahey, J. A. In The Chemistry of Actinide Elements; Katz, J. J., Seaborg, G. T., Morss, L. R., Eds.; Chapman and Hall: London, 1986; Vol. 1, pp 443498.

Figure 1. Electrochemical sample cell used for the optical and XANES experiments. The working electrode (W) is a 52-mesh Pt gauze and the auxiliary electrode (A) is Pt wire. The reference electrode is a Ag/AgCl (3 M NaCl) BAS RE-5B. The cell is sparged (N2) to promote mixing. The windows are changeable, with Kapton chosen for these experiments. This sample cell is held in an outer cell described elsewhere.18

We demonstrate the utility of XANES as a quantitative probe for the determination of formal potentials in solution. Our in situ technique is first verified by optical spectroscopy. The XANES of a Np solution are measured as a function of applied potential in the X-ray beam. The spectra obtained after bulk electrolysis at the various potentials are analyzed by PCA. Nernst plots of these data provide formal potentials that are consistent with literature values determined by other more established methods. It is expected that this technique will provide information that is not currently available from complex systems using established techniques. EXPERIMENTS A solution of approximately 7 mL of freshly cleaned 237Np in 1 M perchloric acid (5 mM Np), with a pH of approximately zero, was used for these experiments. (CAUTION: Neptunium, Z ) 93, is radioactive and requires special handling.) Bulk electrolyses were performed with a BAS 100B/W electrochemical workstation using a Ag/AgCl reference electrode (3 M NaCl, BAS RE-5B). All subsequent potentials are given with respect to this reference electrode, which has a redox potential of +0.196 V vs NHE at 25 °C.31 A general description of the electrochemical cell used for both the in situ optical and XANES experiments is given elsewhere.18 The carbon working electrode used previously was replaced in these experiments by a 52-mesh Pt gauze (Alfa Aesar, No. 10283), 50 mm × 16 mm. It is bent in a U shape and inserted into the cell as shown in Figure 1. A 0.032-in.-diameter Pt wire, which is spot welded to the gauze, provides the electrical feedthrough into the sealed cell. The auxiliary electrode consists of a tightly wound coil of 0.032-in.-diameter Pt wire. Kalrez O-rings (K 031, AS-568A, compound 4079) and Kapton film (1 mil) are used to assemble the cell. To ensure no loose contamination in the experimental hutch, the samples were contained within a (30) Hindman, J. C.; Sullivan, J. C.; Cohen, D. J. Am. Chem. Soc. 1958, 80, 18121814. (31) Bott, A. W. Curr. Sep. 1995, 14, 64-68.

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purpose built box that permitted triple encapsulation of the sample throughout the experiment. The H2O-saturated N2 sparging gas was exhausted through an EDTA solution before venting into the outer sample box. The exhaustive electrolyses were achieved in less than 30 min, as determined from the coulometry, by the end current ratio (which was less than 1%) and by the reproducibility of the optical or XANES spectra. The shapes of the charge vs time and current vs time plots suggest no chemical reactivity for the electrolyzed species. The reaction that couples (1) and (2), i.e., Np(IV) f Np(V), showed evidence of chemical reactivity that is attributed to the formation of the neptunyl moiety. The temperature during the experiment was neither monitored nor controlled, but is estimated to be 22 ( 2 °C. There were no differences in the redox behaviors that were observable when the cell was in or out of the X-ray beam, indicating that hydrolysis by the X-ray beam is not significant. The optical data were obtained on an Olis-converted Cary-14 spectrophotometer using the cell outlined above. Kapton windows were used for these measurements. The Np L3 edge (17610 eV32) X-ray data were collected on 12BM-B, the BESSRC bending magnet beam line at the Advanced Photon Source. The beam line is equipped with Si〈111〉 crystals in a double-crystal configuration. Harmonic rejection was accomplished using a Pt mirror, set to reject energies higher than 25 keV. The use of harmonic rejection at these energies is necessary at the APS because of the relatively high flux of high-energy photons. The energy was calibrated by setting the inflection point of the first derivative from the Zr K edge to 17 998 eV. All data were taken in the fluorescence mode, using a flow-type ion chamber detector (The EXAFS Co.), which was purged with xenon and used without slits or a scattering radiation filter. This is a common approach with the conventional 45°-incident/45°-exit fluorescence XAFS configuration, which minimizes the scattered radiation to the detector. No time-dependent spectral changes were observed over multiple scans for each potential, and with the repetition of the experiment in two different synchrotron runs. The acquired data were independent of ring current or other details of the synchrotron operations. The stability of incident X-rays necessary for this reproducibility is a criterion for these experiments that is met by the BESSRC beam line at the APS. RESULTS AND DISCUSSION Optical Spectra. To verify our electrochemical technique, optical spectra were obtained from the Np solution under applied potential using the same cell and conditions as those used for the XANES experiment. Spectra representing the end points for the bulk electrolyses of the four different redox species are shown in Figure 2. These spectra were obtained by starting with a solution of Np(V) at rest potential (+0.689 V vs Ag/AgCl) and applying an oxidizing potential of +1.20 V until coulometry and the optical signal indicated an exhaustive oxidation to Np(VI). After reduction back to Np(V), under an applied potential of +0.400 V, which reproduced the initial Np(V) optical spectrum, a potential of -0.200 V was applied to reduce the Np(V) to Np(III). At intermediate electrolysis, the optical spectra showed evidence for the presence Np(IV), which disappeared as the reduction (32) Bearden, J. A.; Burr, A. F. Rev. Mod. Phys. 1967, 39, 125-142.

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Figure 2. Optical spectra obtained for Np(III), Np(IV), Np(V), and Np(VI) with the sample in the spectroelectrochemical cell. The data were taken with the solutions under electrochemical control at potentials of -0.2, +0.4, +0.689, and +1.2 V, respectively, following the sequence outlined in the text.

Figure 3. XANES spectra from the pure Np(III), Np(IV), Np(V), and Np(VI) states under the same conditions and sequences used to obtain the optical spectra shown in Figure 2.

proceeded. The transient presence of Np(IV) and further reduction to Np(III) is expected and is the result of the overpotential required by the reaction kinetics for the Np(V) reduction.30 Finally, Np(III) was oxidized to Np(IV) at an applied potential of +0.400 V. The subsequent reduction of Np(IV) at -0.200 V produced a Np(III) spectrum that was indistinguishable from that produced by the direct reduction from Np(V). It can be seen that although Np(IV) and Np(V) provide sharp absorption lines, those corresponding to Np(III) and Np(VI) are much broader and have much lower absorption coefficients, which renders them more difficult to use quantitatively. The broad lines observed from Np(III) and Np(VI) in solution make it difficult to use these lines for lower concentrations. In other words, determining reliable concentrations from reactions 1 or 2 using optical spectroscopy necessitates using only the disappearance of the lines arising from Np(IV) and Np(V). The optical data obtained here are consistent with previously published spectra33 that were used for the quantitative determination of Np oxidation states in solution. These data confirm that the electrochemical cell is working as expected.

Table 1. Parameters Obtained by Fitting Np L3 Edge XANES Dataa Np valence

inflection point (eV)

arctan position (eV)

Lorentzian position (eV)

Lorentzian half-width

edge intensity

III IV V VI

17613.8 17618.2 17616.2 17619.3

17619.9 17624.4 17624.3 17626.4

17617.0 17621.5 17620.0 17622.4

4.9 5.6 5.8 5.1

1.796 1.904 1.371 1.404

a The edge positions, determined from the XANES data by the inflection point of the first derivative, are consistent with the positions derived from fitting the XANES data to a Lorentzian and arctangent. The error in the edge positions is (2 eV. These results are similar to those previously reported for Pu35 except that they are shifted in energy.

Table 2. Intermediate Applied Potentials Used for Exhaustive Electrolysisa applied potential (V) -0.200 -0.100 -0.080 -0.061 -0.040 -0.020 0.00 +0.020 +0.100

Np(III) 1.0 0.887 0.736 0.542 0.364 0.229 0.127 0.046

relative concentration Np(IV) Np(V)

Np(VI)

0.113 0.264 0.458 0.636 0.771 0.873 0.954 1.0

+0.689

1.0

+0.800 +0.850 +0.876 +0.900 +0.921 +0.950 +0.957 +1.000 +1.200

0.953 0.943 0.934 0.759 0.515 0.312 0.304 0.116

Figure 4. (Top) XANES from intermediate Np(VI)/(V); (bottom) XANES for the intermediate Np(IV)/(III). The intermediate potentials used to generate these spectra are shown in Table 2. 0.047 0.057 0.066 0.241 0.485 0.688 0.696 0.884 1.0

a Errors in these potentials are (0.01 V. The relative concentrations of Np oxidation states as determined by a principal component analysis. Errors in the relative concentrations are estimated at (5%.

XANES Spectroelectrochemistry. The redox sequence used for the optical data was repeated, with the Np solution under potentiometric control, in the X-ray beam. There was no evidence that the X-ray beam interferes with or alters the electrochemistry. The Np L3 edge XANES data, taken at the same applied potentials as those used to obtain the optical spectra, are shown in Figure 3. The edges of the normalized XANES are modeled with a combination of Lorentzian and arctangent functions following standard methods.34 The results are displayed in Table 1. The nonlinear shift in edge energies with increasing valence and the differences in edge intensities for the different oxidation states have been previously observed from Pu solutions stabilized in the III, IV, V, and VI oxidation states.35,36 Slightly larger maximum intensities from the spectra of the more oxidized species have been reported from similar experiments on Eu.18 The change in maximum intensity with changing valence points out the need (33) Hagan, P. G.; Cleveland, J. M. J. Inorg. Nucl. Chem. 1966, 28, 2905. (34) Lytle, F. W. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 1251-1257.

for careful measurement of the valence-pure spectra. The Np(III) and Np(IV) spectra are qualitatively similar, as are the Np(V) and Np(VI) spectra. The shoulder to higher energy seen for the latter two spectra has been previously observed in uranyl,37,38 neptunyl,13 and plutonyl35,36 spectra, where it was attributed to multiple scattering from the linear OdMdO moiety. The same linear species is present for Np(V) and Np(VI) but not Np(III) and Np(IV). Regardless of the origins of the spectroscopic features, the combination of the shift in edge energies and the shape of the resonance are sufficient to distinguish the Np speciation for a single species in solution. The redox scheme used to obtain the XANES data for the isolated Np species, that is, Np(V) f Np(VI) f Np(V) f Np(III) f Np(IV) f Np(III), was repeated with exhaustive electrolysis at the intermediate potentials listed in Table 2. The only change made to the experimental setup during this electrolysis was to change the applied potential. No changes in the solution, its components, its position in the beam, the detection scheme, or any other experimental parameters were made during this procedure. This reproducibility permits direct comparisons between the different XANES spectra that could not be made if (35) Conradson, S. D.; Mahamid, I. A.; Clark, D. L.; Hess, N. J.; Hudson, E. A.; Neu, M. P.; Palmer, P. D.; Runde, W. H.; Tait, C. D. Polyhedron 1998, 17, 599-602. (36) Ankudinov, A. L.; Conradson, S. D.; MustredeLeon, J.; Rehr, J. J. Phys. Rev. B 1998, 57, 7518-7525. (37) Hudson, E. A.; Rehr, J. J.; Bucher, J. J. Phys. Rev. B 1995, 52, 1381513826. (38) Kalkowski, G.; Kaindl, G.; Brewer, W. D.; Krone, W. Phys. Rev. B 1987, 35, 2667-2677.

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Figure 5. The six most important spectral components obtained from the PCA of the Np(VI)/(V) couple. The total number of components must equal the number of original spectra (vectors) included in the analysis, which is 14 in this case, including the standards. The percent of the total variance (relative eigenvalue) is included for each component. Only the first six components are shown, in order of decreasing spectral contribution. The first two components represent 99.67% of the variance of the original spectra. The other 12 components are considered to be noise.

components were added to the solution, if the cell was moved, or if the experimental setup was changed in any way. The XANES spectra recorded after exhaustive electrolysis at each intermediate potential are shown in Figure 4 and are available as Supporting Information. A qualitative change can be seen at each intermediate potential. These data were analyzed for their spectral components using a standard linear regression analysis as well as PCA. Principal Component Analysis. The XANES spectra shown in Figures 3 and 4 are expressed mathematically in a PCA as vectors,15,16 using the absorption data within the energy range of 17 595-17 665 eV. Diagonalization of the resulting matrix provides eigenfunctions and eigenvalues. The eigenfunctions are the PCA components that are necessary to represent the collective XANES spectra. The eigenvalues represent the relative weight of each component within the series. In this example, when all spectra are treated together, the analysis reveals that all the data can be represented by four independent PCA components: two in the Np(VI)/(V) series and two in the Np(IV)/(III) series. These components constitute an alternative representation of the original 4626

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spectra. From these results we conclude that there are only four different Np species that contribute to all of the XANES spectra. This is confirmation that there were no unexpected side reactions or chemical transformations during the experiments. The four spectra shown in Figure 3, each representing a pure Np oxidation state, were used as standard spectra. More detailed quantitative analyses were done by treating the two redox couples separately. It is determined that the PCA coordinate system for each redox couple can be successfully rotated from the original components to the two standard spectra that represent the end points of the redox reactions. The success of this rotation confirms that the two standards chosen are necessary and sufficient to model the redox couple in each of the two cases. The PCA of the Np(IV)/(III) couple is straightforward because of the degree of spectral resolution between the two end points. The Np(VI)/(V) couple required further efforts because of the relatively minor differences between the two end points. In our analysis of the higher oxidation couple, five spectra

Figure 6. Nernst plots: (top) (VI)/(V) couple; (bottom) (IV)/(III) couple. The concentrations used in these plots were obtained from PCA of the data shown in Figure 4 and are listed in Table 2.

representing duplicate measurements on the two standards were included in the series subjected to PCA. This addition does not increase the number of components necessary to reproduce the spectra and provides an improved assessment of the effects of experimental noise. The spectral components determined from the analysis of the Np(VI)/(V) couple are shown in Figure 5. The first two components are able to account for 99.67% of the variance (intensity) of the original spectra (fits to individual spectra are provided in the Supporting Information). Inclusion of additional components does not significantly improve the fit. From a full analysis, the relative amounts of the two standards at each intermediate potential were determined. The results are listed in Table 2. The concentrations of each oxidation state as determined from the PCA were used to construct the Nernst plots shown in Figure 6. The reproducible manner in which the original data were obtained permitted the use of PCA without any preprocessing of the data, such as background subtraction or normalization. This eliminates a potential source of error normally present in XAS analysis. The errors in determining concentrations are slightly higher at lower concentrations of the more oxidized species; nevertheless, they are considered reliable to (5%. The more oxidized species have a slight shift of their spectra to higher

energy, as demonstrated for the standard spectra shown in Figure 3. Therefore, there is no independent, unresolved data for the more oxidized species at low concentrations. The information is buried under the response for the reduced species. The problem of resolving the two species of each intermediate spectrum is aggravated by the line width/energy separation ratio, which is particularly unfavorable for the Np(VI)/(V) couple because of the presence of the multiple scattering from the OdNpdO oxygen atoms. The use of PCA for the extraction of concentrations allows us to extract concentrations from unresolved spectra with lower errors than we were able to obtain from a conventional, linear regression analysis on the same data. In general, the results from a PCA agree with those obtained from a standard linear regression analysis. However, the error on the relative concentrations for the Np(VI)/(V) couple obtained from the latter technique is larger for low concentrations of Np(VI). The larger scatter in the data points using linear regression analyses over those determined from PCA can be seen from a simple error analysis of the best line determined by linear leastsquares fitting. Whereas the errors obtained from the two data reduction techniques are indistinguishable for the IV/III data, the VI/V data have an error in the determined y intercept of (0.007 from PCA, compared to (0.012 from the linear regression analyses. The difference in errors is even more pronounced for the determination of the slope, which has a least-squares error of (0.004 from PCA compared with (0.009 from the linear regression analysis. The reduced error for the PCA over standard linear regression is attributed to the removal of spectral noise that occurs when only the two most important PCA components are included in the fit to the data. In addition to lower errors in selected cases, PCA also provides a quantitative indication of the number of Np species required to fit the data and a confirmation that an appropriate set of standards has been used in the analysis. Such information is not available from a standard linear regression analysis. Formal Potentials. The formal potentials of the Np(VI)/Np(V) and Np(IV)/Np(III) couples are determined using the Nernst equation:4

E ) Eo′ + (RT/nF) ln([Ox]/[Red])

wherein [Ox] and [Red] correspond to the relative concentrations of oxidized and reduced Np species in solution, n is the number of electrons transferred in the reaction, and Eo′ is the formal potential. This equation is applicable for reversible reactions when both the oxidized and reduced species are soluble. The experimental conditions, i.e., concentration, electrolyte, and pH, were chosen to optimize Np solubility. There was no detectable change in the Np concentration as monitored by the height of the jump in absorption across the Np L3 edge. The Nernst plots for both (1) and (2) are shown in Figure 6. The formal potential of the Np(VI)/Np(V) couple in 1 M HClO4 determined here by in situ XANES spectroelectrochemistry is +0.931 ( 0.015 V. The relatively large error in our measured value is attributable to uncertainties in determining applied voltages31 and extracting concentrations, as well as other experimental variations. There are several literature reports for the formal potential of this couple in 1 M HClO420-23 with an average value Analytical Chemistry, Vol. 71, No. 20, October 15, 1999

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of +0.941 ( 0.001 V. The published results are determined by potentiometric and polarographic techniques. Our result from XANES spectroelectrochemistry agrees with this averaged literature value within the experimental error. The formal potential of the Np(IV)/Np(III) couple in 1 M HClO4 determined here by XANES spectroelectrochemistry is -0.053 ( 0.01 V. This result also compares favorably with published values,20,22,23 which average -0.045 ( 0.005 V. The number of electrons transferred in the redox couple can be determined experimentally from the slope of the Nernst plot. The slope is 0.062 ( 0.005 for the Np(VI)/(V) couple and 0.062 ( 0.002 for the Np(IV)/(III) couple. A one-electron transfer corresponds to a slope of 0.0585 at 22 °C. The observation of 0.94 electrons transferred for both reactions is lower than expected, but it is within error of 1 electron transfer for the Np(VI)/(V) case. From the plot shown in Figure 6, it is clear that there is not a measurable deviation from linearity across the measured range, which indicates that there is no significant irreversibility in the reactions. Deviations from expected slope are not uncommon in Nernstian analyses and do not necessarily support previous arguments that the two Np couples under investigation are not entirely reversible.19,22,39 Instead, the result could reflect other artifacts, such as small changes in activity coefficients over the range of the experiments. It should be noted that there is a difference in the integrated absorption intensities between the oxidized and reduced forms of Np in both redox couples that could influence the slope. These different intensities are attributed to slight differences in the X-ray absorption transition matrix element for the different oxidation states.40,41 Such differences have been previously observed in the XANES spectra obtained from Eu(II) and Eu(III).18 However, this change in total absorbance is automatically included in the PCA and therefore does not contribute a systematic error to the slope. (39) Niesse, U. Collect. Czech. Chem. Commun. 1981, 46, 3179-3182. (40) Rohler, J. In Handbook of the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Ed.; North-Holland: Amsterdam, 1987; Vol. 10, pp 453545. (41) Wortman, G. Hyperfine Interact. 1989, 47, 179-202.

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Analytical Chemistry, Vol. 71, No. 20, October 15, 1999

CONCLUSIONS Herein we have demonstrated that in situ XANES spectroelectrochemistry can be used to determine formal potentials. There are other techniques, such as potentiometry or polarography, that can provide a more accurate and precise determination of the formal potentials of the two Np redox couples that served as our examples. However, there are situations in which XANES spectroelectrochemistry will be the only method available for probing a redox couple. This is particularly true for complex samples that contain multiple redox-active species or for systems with some other interference, such as an overlying optical transition that prohibits the direct determination of appropriate concentrations. The work described herein is meant to serve as proof-of-concept. The errors in the formal potential could be minimized with improved control of experimental conditions. Under ideal conditions, and with well-resolved XANES spectra for the oxidation states under consideration, this technique should provide a competitive, and sometimes unique, method for determining formal potentials of ions in complex systems. SUPPORTING INFORMATION AVAILABLE Supporting Information available includes a two-dimensional plot of the data shown in Figure 4 that shows isosbestic points in the data and an exemplary fit of the Np(VI)/(V) primary XANES data using the first two PCA components shown in Figure 5. This material is available free of charge via the Internet at http:// pubs.acs.org. ACKNOWLEDGMENT We gratefully acknowledge the insightful contributions of J. C. Sullivan. We also acknowledge the technical assistance provided by S. Skanthakumar, M. P. Jensen, J. Linton, and the infrastructure provided by the Actinide Facility. This work is supported by U.S. DOE, BES-Chemical Sciences, under Contract W-31-109-ENG-38. Received for review January 27, 1999. Accepted August 7, 1999. AC990080T