Specific Ultraviolet Spectrophotometric Determination of Europium Oxide as a Complex in Mixturesof Rare Earth Oxides Harvey Pobiner Princeton Laboratory, American Can Co., Princeton, N. J. 08540 A new analytical method for europium oxide is described. Of the 17 rare earth oxides, including scandium, yttrium, and thorium, only europium reacts with certain dipolar, aprotic solvents, such as dimethylformamide in hydrochloric acid solution, to produce a measurable ultraviolet absorption spectrum. A charge-transfer mechanism is indicated. The specificity of this reaction serves as a quantitative measure of the europium oxide and is useful for determining europium oxide in mixtures of rare earth oxides. The complex is stable at pH levels of 1to 2, but undergoes an alkaline decomposition which can be followed by ultraviolet spectrophotometry and turbidimetry. The analytical method was subsequently developed into a potential process for the recovery of europium oxide from mixtures of rare earth oxides and from certain phosphors.
the coordination complexes of the transition element salts with hexamethylphosphoramide (3), dimethylformamide, and dimethylacetamide (+-are known. The complexes of the rare earth metal acetates and iodides with dimethylformamide have been described (5). More recently, the complexes of europium and other lanthanide perchlorates with octamethylpyrophosphoramide (6) were reported. The spectrophotometry of rare earths is often nonselective. Vickery (7) stated that complexation is often the means to improve selectivity and minimize interferences in determining individual rare earths in solution. This paper discusses one such complexometric approach for determining europium oxide in mixtures of rare earth oxides. EXPERIMENTAL
A SPECIFIC ANALYTICAL METHOD for europium oxide in mixtures of rare earth oxides is based on the formation of the complex resulting from the reaction of europium oxide, hydrochloric acid, and dipolar, aprotic solvents which serve as both the ligand and the solvent for the complexation. Dimethylformamide was selected as the ligand, although one can also use dimethyl sulfoxide, dimethylacetamide, or hexamethylphosphoramide. The ultraviolet absorption spectrum of the complex in the region 260 to 280 mp serves as a measure of the europium oxide. The analytical data suggest that the absorption spectrum is associated with a charge-transfer complex between trivalent eurqpium and the dipolar, aprotic solvent. Trivalent europium in aqueous solution does not show this band. The absorptivity of this band is 50 times as intense as that of the strongest europium band at 394 mp. In addition, a solid product can be separated from solution and shown by infrared spectroscopy to be a coordination compound containing dimethylformamide. This product, containing europium, shows the new absorption band in dimethylformamide, but not in aqueous solution. Conditions were established to render analytical specificity for europium oxide among the 17 rare earth oxides. Moeller ( I ) classified the known types of lanthanide complexes as ion pair associations, nonchelated and chelated species. Two of these species may apply in this paper. Ion pair spectra are a type of charge-transfer spectra often found in the near-ultraviolet spectrum (2), and in this system could involve electron transfer between the dipolar solvent and ionic or coordinated europium. The isolable product would be classified as the nonchelated species, and in the absence of the donor solvent does not produce the ultraviolet band. The literature reports on the properties of nonchelated complexes of transition elements in which the anion of the original metal salt remains coordinated in the complex-for example, (1) T. Moeller, “The Chemistry of the Lanthanides,” Reinhold, New York, 1963, pp 53, 54. (2) J. Lewis and R. G. Wilkins, “Modern Coordination Chemistry,” Interscience, New York, 1960, pp 272,281.
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ANALYTICAL CHEMISTRY
Reagents. Scandium oxide and thorium oxide were obtained from Alfa Inorganics, Beverly, Mass. Yttrium oxide and the remaining rare earth oxides, all 99.9% purity, were obtained from Matheson, Coleman and Bell, East Rutherford, N. J. Solvents were of the highest purity available from the Fisher Scientific Co. and included N,N-dimethylformamide (No. D-133), dimethyl sulfoxide (No. D-136), N,Ndimethylacetamide (No. D-1351, and hexamethylphosphoric triamide (No. 8894). Calibration with Europium Oxide. Weigh out the following amounts of europium oxide in 150-ml beakers: 0.005, 0.01, 0.02, 0.025, 0.03, 0.035, 0.04, and 0.05 gram. Into each beaker pipet 5 ml of concentrated hydrochloric acid and add about 75 ml of dimethylformamide. Heat the turbid solutions on a hot plate to about 90” C for 1 to 2 minutes to effect solution. Cool to room temperature and adjust to pH 1.0 with HC1 at the p H meter. Finally dilute to 100 ml with dimethylformamide in volumetric flasks. Run an Io with dimethylformamide from 360 to 260 mk. Run the ultraviolet absorption spectrum of each solution us. dimethylformamide from 360 to 260 mp. Use 1-cm cells and a recording spectrophotometer, such as the Beckman DK-2A. Determine the absorbance at the maximum, 272 mk. Read this absorbance relative to a reference zero reading at 360 mp in the calibration and in the analysis to compensate for any background interference due to light scattering in subsequent sample solutions. Plot the grams of europium oxide against the corrected absorbance. A smooth curve is obtained as in Figure 1. Analysis of Samples. Complex 0.03 to 0.05 gram of mixed rare earth oxides with HCl and D M F and heat. Adjust to pH 1.0 and dilute as in the calibration procedure. If insolubles persist after quantitative dilution, filter on dry Whatman No. 42 paper before obtaining the absorption (3) J. T. Donoghue and R. S . Drago, Znorg. Chem., 2, 572 (1963). (4) W. E. Bull, S . K. Madan, and J. E. Willis, Zbid.,2, 303 (1963). (5) T. Moeller, V. Galasyn, and J. Xavier, J. Znorg. Nucl. Chem.,
12,259 (1960). (6) M. D. Joesten and R. A. Jacob, 152nd Meeting, American Chemical Society, Lanthanide and Actinide Symposium, September 1966, Paper 98. (7) R. C. Vickery, “Analytical Chemistry of the Rare Earths,” Chaps. V, VI, Pergamon Press, New York, 1961.
I 8
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Figure 1. Absorbance of band at 272 mp us. grams of Euz03complexed per 100 ml. of solution spectrum. Calculate the percentage of EuzO3after converting the absorbance reading to grams of Euz03with the aid of the calibration curve. Mixtures of rare earth chelates or complexes can also be analyzed for europium after an ignition to the oxide phase. Decomposition Study. The alkaline decomposition of the absorbing species was followed by a turbidimetric titration of the solution with 1 N NaOH. The Brice-Phoenix Model 2000-D light-scattering photometer equipped with a standard square turbidity cell (No. T-101) was used. A 1-minute nitrogen stirring of the solution preceded the recording of each scattering ratio. The ordinate in Figure 2 is the standard scattering ratio multiplied by the appropriate neutral filter factor for the Hg line source (546 mp). Incremental increases in pH were measured with a pH meter. The Beckman DK-2A spectrophotometer was also used to measure the decrease in absorbance with increase in pH as the complex decomposed. Preparation and Isolation of a Coordination Complex. This separation requires complexation followed by a recovery similar to that of Bull and coworkers ( 4 ) . Weigh out 0.3 gram of europium oxide. Add about 5 ml of HC1 and 25 ml of D M F and heat while magnetically agitating. Avoid decomposition of solvent by the use of moderate heat (about 130” C). Continue heating for about 2 hours to expel moisture from the clear solution. Cool and add a 12 to 1 volume excess of anhydrous ethyl ether. Cool further in an ice bath to precipitate the complex. Filter through a sintered glass crucible and wash with ethyl ether. Redissolve in D M F and recrystallize with ethyl ether. Filter, dry at 105’ C, and store the hygroscopic complex in the desiccator. Determine europium content by the procedure described for the analysis of rare earth oxides. Obtain the infrared spectrum as a KBr pellet and identify the coordinating atom of the donor as described (2, 8). A Perkin-Elmer Model 521 infrared spectrophotometer was used. (8) L. J. Andrews and R. M. Keefer, “Molecular Complexes in Organic Chemistry,” Holden-Day, San Francisco, 1964, p 26.
DISCUSSION
Dimethylformamide (DMF) serves as the donor solvent for the formation of an apparent charge-transfer band with trivalent europium in the ultraviolet spectrum. This is the basis for the quantitative analysis. Furthermore, the isolation of a solid complex containing D M F and a europium salt indicates that the rare earth is probably present as a coordination complex in solution. Rare earth ions are known to form complexes with ligands in which oxygen is the donor atom (4-6). In DMF, the dimethylamino group exerts an electron-donating effect, helping to induce dipoles in the solvent molecule for complexation. This effect, coupled with the withdrawal of electron density from the carbonyl carbon toward the oxygen, provides for possible coordination between europium and oxygen. Investigation of Absorption Mechanism. Several mechanisms for the formation of the ultraviolet band were considered. The possibility of an enhancement of the D M F spectrum was suggested. However, D M F does not have a significant absorption maximum above 197 mp, and no apparent enhancement of its absorption in the presence of other lanthanide ions was observed. The possibility of a reduction to divalent europium was ruled out, since an absorption band at 268 mp is obtained in the presence of an oxidizing acid, such as dilute nitric, with the dipolar solvent. This suggested that trivalent europium is present in the complex. Intensification of trivalent europium absorption was then investigated. The absorption spectra of aqueous solutions of EuC13 showed no band formation at 272 mp nor enhancement of absorption as a result of blending in 1 to 10 volume % concentrations of DMF. This contrasts with the formation of a new absorption band at 272 mp in solutions of EuC13 in VOL, 40, NO. 3, MARCH 1968
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DMF. The absorptivity for europium in the DMF-HCl system at 272 mp is 1.05, compared with 0,019 in an aqueous system a t 394 mp. Charge-transfer spectra can be a n order of magnitude larger in intensity than the electronic transitions within the energy levels of the metal atom (2). A charge-transfer absorption mechanism was suggested for the formation of this band, which is not found in the spectrum
Table I. Reaction of Rare Earth Oxides in a Dimethylformamide-HydrochloricAcid System
Atomic NO.^
21 39 57 58 59 60 62 63 64 65 66 67 68 69 70 71 90
Oxide used Scz03 Y203 Laz03 CeOz Pr6OI1 Nd203 Smz03 Eu203 Gdz03 Tb407 DyzOs
Hoz03 Erz03 Tmz03 Yb203 Luz03 Thos
Result of DMF-HCI complexation Insoluble oxide Insoluble oxide Homogeneous s o h , no UV maximum Insoluble oxide Insoluble oxide Homogeneous soln., no UV maximum Homogeneous soln., no UV maximum Homogeneous soln., UV maximum at 272 rnp Homogeneous s o h , no UV maximum Insoluble oxide Homogeneous soln., no UV maximum Homogeneous soln., no UV maximum Homogeneous s o h , no UV maximum Insoluble oxide Insoluble oxide Insoluble oxide Insoluble oxide
Element 61, promethium, is a radioactive fission product of uranium. Since it is not a naturally occurring rare earth metal, it was omitted from the investigation. 0
Table 11. Determination of E11203in Rare Earth Oxide Mixtures EUz03 detn., Mixture Wt. ratio of theory EUZO~/SCZO~ 1/0.65 97.2 E~z031Yz03 114.07 101.4 Eu203/La203 1/0.93 95.4 EuzO3/CeO2 l / O . 89 100.0 E ~ ~ 0 ~ l P r ~ 0 ~ 1 110.99 94.3 Euz031Ndz03 110.67 101.3 Eu~O~lSm~03 110.70 100.0 E~z031Gdz03 110.76 100.8 Euz03/Tb@7 110.77 100.0 EuzOdDyzOa 110.99 100.6 EuzO~/HOZO~ 110.92 97.4 EuzO3IEr2O3 I/O. 89 95.9 Eu~OdTm~0~ l / l . 13 103.1 E~z08/Ybz03 111.25 103.5 E U ~ O ~ ~ L U Z O ~ l / l .05 101.2 Euz031Th02 111.93 101.0
Table 111. Extent of Complexation as Measured by Absorbance-pH Relationship Eu203apparently A272 mpb complexed PHG 1.0 0.440 100 2.0 0.438 100 4.2 0.315 78.3 5.5 0.188 49.4 6.1 0.079 22.7 8.1 0.023 7.6 10.5 O.OO0 0.0 a
Adjusted from pH 1.0 with ",OH.
* 0.03 gram of Euz03 complexed.
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ANALYTICAL CHEMISTRY
of any of the reactants. After the complex is isolated, it can be redissolved in D M F and shown to have the band at 272 mk, as well as trivalent europium spectra in the near ultraviolet. If the isolated complex is dissolved in aqueous solution, there is no formation of the 272-mp band; the weaker europium bands will persist, however. This was significant evidence of the charge-transfer mechanism. Apparently the donor, DMF, must be present in large excess of the acceptor, EuCl,, and the spectral measurements are made against blanks which are identical with the reaction mixture except that they contain no acceptor (8) nor acid. Interferences by other rare earth ions are minimal, possibly because of weaker charge-transfer absorptions at other wavelengths for oxides producing homogeneous solutions under the conditions of the method (Table I). Specificity. Specificity in this analytical method refers t o Eu203in mixtures of rare earth oxides. Undoubtedly the specificity is aided by the fact that nine of the 17 rare earth oxides remain insoluble, apparently d o hot form the chloride, and are thus unavailable for reaction with the donor-for example, T b 4 0 7does not complex in an HCI-DMF solvent system, but TbCI3and Tb(N03)3complex with D M F and produce a new ultraviolet spectrum. Thus the method is not to be used for a mixture of EuC13 and TbCI3, but is suitable for a mixture of Eu203and TbiOi. If other rare earth chlorides form in the homogeneous systems listed in Table I, they apparently do not interfere with the EuC1, solvent interaction, as shown by the analytical recoveries of Table 11. Stability and pH Control. The charge-transfer complex decomposes with increase in pH level and successively decreasing absorbances for a given concentration are obtained as the pH level is changed from 1.0 to 10.5 (Table 111). The decrease in absorbance with p H is taken as an indication of the alkaline decomposition of the species. It is thus necessary t o control the pH level for the calibration and analysis. A pH of 1.O ensures a strong absorbance band at 272 mp for the complex in DMF. Stability of the developed absorption band at pH 1.0 was checked for 30 days, with no resultant change greater than 1 of the initial absorbance. Alkaline decomposition of the complex allows recovery of the europium hydroxide at p H 10.5. Europium oxide is then recovered from the hydroxide by heating a t 800" C. The p H was adjusted by incremental addition of standard caustic and the decomposition of the complex was initially followed by a light-scattering photometer, the Brice-Phoenix Model 2000-D. As caustic was added, the absolute turbidity increased as the europium hydroxide began to precipitate. This turbidimetric titration (Figure 2) showed a marked increase in scattering a t one point, because of large particle formation, and finally a leveling of the turbidity independent of additional caustic. The measured p H at the change in slope was 6.5 and the plateau of maximum turbidity began after p H 8.0. The ultraviolet absorbance study proved more sensitive as a measure of the complex remaining in solution. The ultraviolet absorbance was eliminated at pH 10.5 and the europium hydroxide was then recovered from the D M F by filtration. When dimethyl sulfoxide was used as the ligand-solvent, recovery of the europium hydroxide was complete at p H 8.0. From these ultraviolet absorbance studies (Table 111), it was concluded that a p H of 1.0 provided for a reliable determination of complexed europium oxide. Use of Other Dipolar, Aprotic Solvent-Ligands. Besides the use of D M F as a ligand and solvent for the complexation, other dipolar, aprotic solvents were found acceptable. Dimethyl sulfoxide (DMSO), dimethylacetamide (DMA), and
Table IV. Analytical Recoveries of Cornplexed Eu20 3 E u ~ O d100 ~ , ml HCI-DMF Sample EuzOa,residue from ignited chelate
0.8
Theory 0.0204 0.0131 0.0073 0.0308 0.0278 0.0152 0.0370 0,0409
0.7
0.6
synthetic mix in HCl-DMF
U
f
Found 0.0202 0.0120 0.0073 0.0296 0.0290 0.0157 0.0380 0.0400
z
recovery 99.0 91.6 100.0 96.1 104.3 103.3 102.7 97.8
0.5
B
..
U 0.4
:O
II .. I
sonance hybrid, H-C-N-R, to the ground state of the complex
0.3
0.2
0.1
0.0 260
280
300
320
340
Wavrlmsth, rnp
Figure 3. Absorbance curves of complexes of EuzOa with HCI, dipolar, aprotic solvent Calibration blend of 0.040 g EuZO3with 5 ml HCI in 100 ml of dimethylformamide B, C, D. Random concn. of EuZ03with 5 ml of HCI in 100 ml of dimethyl sulfoxide, dimethylacetamide, hexamethylphosphoramide, respectively A.
hexamethylphosphoric triamide (HMPA) all produce a new ultraviolet absorption spectrum for the reaction with europium oxide and hydrochloric acid. The location of the absorption maxima depends on the ligand-solvent (Figure 3). Although these dipolar solvents are nonideal ultraviolet solvents, their usable spectral ranges with the recording spectrophotometer extend to 260 mp and can be used in the analysis. Other sources of anions were investigated for the complexation, such as sulfuric, nitric, and acetic acids, in place of hydrochloric acid. Although complexes can be prepared with these acids, the apparent acidic degradation of the excess dipolar, aprotic solvent interferes with the ultraviolet analysis. Hydrochloric acid was thus established as the source of anions for the complex. Coordination through Oxygen. The infrared spectra of the isolated europium complexes support coordination through the oxygen atom. The literature (4,9) indicates that in those metal-amide complexes showing a shift of the carbonyl absorption to lower frequency, there is a decrease in the stretching force constant and coordination occurs through the .. R
:o:+/
oxygen via the resonance hybrid, H-C=N.
If coordina-
\
R tion to the nitrogen occurs, the contribution of another re(9) C. D. Schmulbach and R. 4484 (1960).
s. Drago, J.
Am. Chem. SOC.,82,
R causes an increase in the carbonyl frequency. The spectrum of the complex between EuC13 and D M F showed a carbonyl shift of 78 cm-l to lower frequencies and that between EuC13 and DMA showed a similar carbonyl shift of 43 cm-l. This compares with literature values ( 4 ) of 32 to 72 cm-1 for metallic complexes of DMA. The spectrum of the complex between EuC13 and HMPA showed a splitting of the P=O band and a shift of 84 and 40 cm-l to lower frequencies. This has also been associated with coordination through the oxygen as reported ( 4 ) . The ultraviolet spectrum of the isolated complex dissolved in a D M F solution at pH 1.O showed the charge-transfer spectrum. The ultraviolet analysis by the method described was 6.69% Eu. This compared with 6.60% Eu obtained by an ignition of the product to Eu200J. Precision, Relative Error, and Sensitivity. At a concentration of 0.03 gram of EU203 per 100 ml of HC1-DMF, the standard deviation for a series of 20 replicate determinations is 0,001 gram or 3% of the amount present. Tables I1 and IV reflect the accuracy of the determination. In a series of mixtures of E U 2 0 3 with rare earth oxides (Table II), relative error was 2.0%. In a series of analyses of europium oxide obtained from ignited chelates and from synthetic mixtures (Table IV), relative error was 3.2%. The detection limit is 0.005 gram of E U 2 0 3 under the established conditions. One can use a longer cell path and reduced volume to extend this limit if necessary. Applicability. The method is used as a routine test in this laboratory to measure europium oxide in the ash of rare earth chelates and in mixtures of rare earth oxides (Table I and 11), and for the determination of the separation factor realized in the analyses of rare earth mixtures containing europium. Certain features of the analytical method have process potential and were developed further. For example, the insolubility of some rare earth oxides and the separation of europium hydroxide by alkaline degradation of the complex, followed by conversion to the europium oxide, allow recovery of europium oxide from certain phosphors and rare earth mixtures, Eu203 was recovered from phosphor mixtures in substrates such as BaTiOo,A1203,CaW04,ZnS, CdS, and Na2B40,to the extent of 73 to 99% of theory. ACKNOWLEDGMENT The technical assistance of Hilja Treumut and Ramanlal R. Pate1 is acknowledged.
RECEIVED for review October 18, 1967. Accepted January 2, 1968. VOL. 40, NO. 3, MARCH 1968
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