theoretical absorbed dosage required for complete oxidation or reduction of the species of interest based on G values taken from the literature. Comparison was made using the average of several titrations performed at a concentration level in about the middle of the useful range for iron(II), cerium(1V) without thallium, cerium(1V) with thallium, and chromium(V1) with thallium. Uranium was not compared because the mechanism for oxidation of uranium(1V) in the presence of iron(II1) has not been established unequivocally. The results are summarized in Table 111. The agreement
between experimental and theoretical dosages is only fair and emphasizes the desirability of using a calibration curve for each substance to be determined; however, a rough estimate of concentration can be obtained using experimental dosages and known G values. Exceptional agreement would not be expected because different workers commonly report G values that differ by several percentages (2). RECEIVED for review May 19, 1969. Accepted September 16,1969.
Vibrational Spectrometric and Electrochemical Lanthanum(lll)=Nitrate Complexes in Aqueous John Knoeckl Institute for Materials Research, National Bureau of Standards, Washington, D. C. 20234
NUMEROUS ION EXCHANGE (1-3) and solvent extraction (4, 5 ) schemes have been proposed for separating lanthanides in aqueous nitrate media. These mechanisms are usually explained in terms of the formation of nitrate complexes. In addition, nitrate is reduced polarographically to ammonia and hydroxylamine at the DME in the presence of lanthanum (111) and other rare earths at potentials considerably less cathodic than those at which ordinary nitrate reduction occurs (6). Wharton (7) has interpreted similar nitrate reduction in the presence of zirconium(II1) to imply the formation of a zirconium-nitrate complex. In the present work the aqueous lanthanum-nitrate system has been investigated polarographically, with a nitrate ionselective electrode, and by infrared and Raman spectroscopy. The appearance of the normally infrared forbidden y1 (A1') symmetric nitrate stretching mode at about 1050 cm-1 in lanthanum nitrate solutions is consistent with a lowering of nitrate symmetry to Cz, due to coordination. Similarly the appearance of four bands in the 1400-cm-1 region of both the infrared and Raman spectra is consistent with the splitting of the degenerate Y@') asymmetric stretching modes due to coordination and to solvent perturbation of the free nitrate (8).
The Raman depolarization ratio of the 1 4 0 0 - ~ m -spectral ~ envelope is indicative of bidentate nitrate coordination. Nitrate ion-selective electrode data indicate at least two complexes of 1:1 and 1 :3 lanthanum to nitrate ratio exist in aqueous solution. The 1:1 complex appears to be reduced polarographically with diffusion control. Present address, Department of Chemistry, North Dakota State University, Fargo, N. D. 58102 (1) J. Alstad and A. 0. Brunfeldt, Anal. Chim. Acta, 38,185 (1967). ( 2 ) T. Arends and M. L. Gallango, Brit. J . Hoematol., 11, 350 (1965). (3) J. Korksih, I. Hazan, and G. Arrhenius, Talanra, 10,865 (1963). (4) Z. Kolarik, Collection Czech. Chem. Commun., 32, 350 (1965). (5) E. B. Mikhlin and G. V. Korpusov, Zh. Neorgan. Khim., 10, 2780 (1965). (6) J. W. Collat and J. J. Lingane, J. Am. Chem. SOC.,76, 4214 (1954). (7) H. W. Wharton, J. Electroanal. Chem., 9, 134 (1965). (8) D. E. Irish and A. R. Davis, Can. J. Chem., 46,943 (1968).
1 1600
1
1 I500
1 1400
l 13CO
cm-1
I
l l 1200
I 1 1100
l 000
Figure 1. Infrared (top) and Raman (bottom) spectra of 1.5Mlanthanum nitrate 1400-cm-1 region of Raman spectrum resolved into four Gaussian components
EXPERINIEKTAL
Solutions. Lanthanum stock solutions were prepared by dissolving commercially available lanthanum nitrate hexahydrate or lanthanum chloride hexahydrate in distilled water or by dissolving lanthanum oxide in nitric or perchloric acid. The lanthanum content was determined in these solutions by titration with standard sodium fluoride solution. A cornmercial fluoride ion-selective electrode was used for end point detection. Nitrate stock solutions were prepared by dissolving accurately weighed portions of dried reagent grade potassium nitrate in distilled water. All other chemicals were reagent grade and used as received. Apparatus. Commercially available instrumentation was used throughout. Infrared spectra were obtained from samples run as smears between silver chloride plates. Raman spectra were obtained using a multipass reflecting cell and 6328-A He-'Ne laser excitation.
RESULTS AND DISCUSSION Vibrational Nitrate Spectra. Table I correlates the number and assignment of the infrared and Raman active frequencies which should be observed for free nitrate, Dah symmetry, and symmetry. The partial infrared spectrum bound nitrate, C2,,
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
0
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Point group
Table I. Correlation Table for Free and Complexed Nitrate Symmetries Stretch (sym.) Bend Stretch (asym.) Yl
Yz
D3h
Ai'W
A2"Il)
CzY
Yz AiVJ3
Bl(Z,R)
(9) F. H. Spedding and S . Jaffe, J. Am. Chem. SOC.,76, 884 (1954). (10) J. T. Miller and D. E. Irish, Can. J. Chem., 45, 147 (1967). (11) €3. Strauch and L. N. Komissarova, Z . Chem., 6,474 (1966). (12) R. P.Qertel and R. A. Plane, Inorg. Chem., 7, 1192 (1968). (13) 6. B. Merini-Bettolo and L. Paolini, Guzz. Chim. Itul., 75, 78 (1945). (14) T. A. Beineke and J. DelGaudio, Znorg. Chem., 7, 715 (1968). e
Yl
Deformation Y4
E'II,R)
Y0
of a 1.5M lanthanum nitrate solution is shown in Figure 1. This region of the infrared spectrum is free from water absorptions and, since both the symmetric and asymmetric stretching frequencies are observed in this region, a convenient test for nitrate complexes is provided. The symmetric stretching frequency of nitrate, which is infrared forbidden in D3h symmetry, occurs at about 1OSO cm-1 and the infrared active band at 1038 cm-I in Figure 1 is therefore consistent with bound nitrate. In addition, the correlation table predicts that the asymmetric stretching bands will lose their degeneracy and appear as two bands when nitrate acts as a ligand. In the 1 4 0 0 - ~ m -region ~ of the lanthanum nitrate spectrum, a spectral envelope which appears to be composed of several bands was found. Irish and Davis (8) have reported that the degeneracy of the asymmetric stretching bands of nitrate is lost even in dilute solutions of alkali metal nitrates, presumably because of ion-solvent interactions. The splitting of these bands for free nitrate was found to be on the order of 50 cm-1. However, when the loss of degeneracy arises from complex formation, the splitting of the asymmetric vibrational bands is very much larger. In solids containing the hexanitratocerate(IV) anion, splittings of about 160 cm-l have been observed in this laboratory. In solutions containing both free and complexed nitrate in equilibrium, such as the lanthanum nitrate solutions, a total of four bands would thus be predicted for the asymmetric stretch. The 1400-cm-1 region of the infrared spectrum shown in Figure 1 appears to be composed of several bands in agreement with this prediction. The Raman spectrum of the same solution is also shown in Figure 1. The 1250- to 1550-cm-1 region was resolved into four Gaussian components. The two central bands, separated by about 50 cm-1, are attributed to free nitrate, while the outer bands, separated by about 140 cm-l, are assigned to complexed nitrate. The Raman depolarization ratio of the 1400-cm-' spectral envelope was determined. Polarization of the highest frequency band in this region (1480 cm-l) together with depolarization of the lowest frequency band (1340 cm-l) implies bidentate coordination. The terminal N - 0 (symmetric and polarized) vibration should be at higher frequency than the antisymmetric NOz stretching motion (depolarized) for bidentate nitrate. If nitrate were monodentate in the lanthanum complexes, the frequencies would be reversed and the lowest frequency band would be polarized. The depolarization ratios do not rule out bridging nitraie. However, conductivity data (9) for rare earth nitrate solutions have not been interpreted in terms of polymeric species. In addition, similar Raman depolarization data have been reported for aqueous solutions of cerium(1V) nitrate (IO), scandium(II1) nitrate ( Z I ) , and bismuth(II1) nitrate (12). Both Bi(N03)3. 5H20 (13) and (NH4)2Ce(N03)e(14) have
2070
Ys Y4
Bz(Z,R)
E'(Z,R) Ys Y6 Ai(Z,R) &(Z,R)
0
%>
-
20
'I -40
W
u Lo
2 60 W
80
ld4 IdS Id' TOTAL NITRATE CONCENTRATION, moles/liter
Id'
Figure 2. Determination of free nitrate concentration in solutions containing lanthanum(II1) - Calibration curve for KNOa in 0.100M KCl A KN03 in 0.070M KCI, 0.0100M LaCls
shown to contain bidentate nitrates in the solid state. For these reasons, bridging nitrates in aqueous lanthanum(II1) solutions seem unlikely. Nitrate IonSelective Electrode Data. As shown in Figure 2, the free nitrate concentration in solutions containing both lanthanum(II1) and nitrate was considerably less than the total nitrate concentration. The free nitrate concentrations as determined from Figure 2 were used to calculate the conditional formation constant, (I
