Selective Determination of Europium(III) Oxide and

Selective Determination of Europium(III) Oxide and Hydroxide Colloids in Aqueous Solution by Laser-Induced Breakdown Spectroscopy JONG-IL YUN,* TOBIAS BUNDSCHUH, VOLKER NECK, and JAE-IL KIM Forschungszentrum Karlsruhe GmbH, Institut fu¨ r Nukleare Entsorgung, P.O. Box 3640, D-76021 Karlsruhe, Germany

Laser-induced breakdown spectroscopy (LIBS) was applied to selectively analyze the aqueous suspension of Eu 2 O 3 (s) particles in the presence of the Eu 3 1 aquo ion. A plasma was generated by focusing a pulsed Nd:YAG laser beam (l 5 532 nm) into the sample. The light emission from the plasma was detected by a spectrograph equipped with a gated intensi ed charge-coupled device (ICCD) in the wavelength range of 275–525 nm. The atomic emission intensity of the Eu 2 O 3 (s) suspension was about two orders of magnitude higher than that of the Eu 3 1 aquo ion. The detection limits for Eu 3 1 (aq) and Eu 2 O 3 (s) were found to be 3.3 3 10 2 5 mol/L and 2.0 3 10 2 7 mol/L, respectively. Such a difference allows the selective determination of colloidal europium particles. This capability of LIBS was used to study the form ation of Eu(OH) 3 (s) colloids in the aqueous Eu 3 1 solution by varying pH until the solubility limit was exceeded. The appraisal of the threshold pH for the solubility limit led to the determination of the solubility product of colloidal Eu(OH) 3 (s), which was then calculated to be log K 0sp 5 225.5 6 0.4. Index Headings: Laser-induced breakdown spectroscopy; LIBS; Plasm a em ission; Europium ; Eu 3 1 aquo ion; Oxide and hydroxide colloids; Solubility product.

INTRODUCTION The spectrochem ical method for analyzing materials using the laser spark that is produced by focusing a laser beam onto or into a sample is known as laser-induced breakdown spectroscopy (LIBS). The laser spark is a result of the dielectric breakdown of a m edium induced by the laser pulse. The medium is vaporized and electronically excited. The neutral and ionized atoms in excited states produce the light emission from a plasm a, which is then analyzed to determine elem ental com positions of a sample. The basic principles of LIBS are described in detail in several reviews.1–3 Since the early 1980s,4 LIBS has been developed as a quantitative analytical m ethod of great interest for scienti c and industrial applications.5–11 LIBS has been applied to the analysis of solids,12–16 gases,17,18 and aerosols.19,20 However, there are few publications on the analytical application of LIBS for aqueous solutions. 21–26 Cremers and co-workers reported the enhanced detection lim its for several elem ents of main groups I to III in aqueous solutions with the use of a double-pulse technique 21 and the determ ination of uranium in aqueous solution by surface excitation. 22 LIBS was also used for the spectrochem ical analysis of aqueous solutions containing nickel and chlorinated hydrocarbons 23 and the determination of colloidal iron in water. 24 Archontaki and Crouch enhanced the detectability of LIBS by individual single Received 22 June 2000; accepted 3 November 2000. * Author to whom correspondence should be sent.

Volume 55, Number 3, 2001

droplets. 25 Knopp et al. reported the detection lim its for metal ions in aqueous solutions and for suspended ErBa 2 Cu 3O x particles, in which the particles are detectable at m uch lower concentrations than the Er 3 1 aquo ion. 26 Their results indicate that this technique can be used, for example, for m onitoring colloid generation in aqueous solutions. The focus of the present work was directed toward the selective analytical application of LIBS for colloidal particles in aqueous solutions. The spectroscopic features, sensitivity, and detection lim its were investigated for europium, either dissolved as Eu 31 (aq) or as suspension of Eu 2 O 3(s) particles. In addition, the formation of europium hydroxide colloids was monitored as a function of pH in order to determ ine the solubility product of Eu(OH) 3(s). EXPERIMENTAL LIBS Apparatus. The experim ental setup for laserinduced breakdown spectroscopy is schematically depicted in Fig. 1. The second harmonic of a Q-switched, pulsed Nd:YAG laser (Continuum, Surelite-20, l 5 532 nm) was used, and the laser pulse at 5.0 6 0.2 mJ with a pulse duration of 12 ns was focused into the sample cell with a plano-convex silica lens (Spindler & Hoyer) with a focal length of 20 mm. The pulse-to-pulse uctuation of the Nd:YAG laser was 4% . The sample was contained in a rectangular silica cuvette with dim ensions 20 (length) 3 10 (width) 3 35 (height) mm (Hellm a). Details of the experim ental apparatus and conditions are given in Table I. The plasma emission light was m onitored perpendicularly to the laser pulse by a ber-optic cable, which was directly connected to a quartz window of the sam ple cell, as shown in Fig. 1. The ber-optic cable was mounted on a precision translation stage for ne adjustm ent control of the plasma emission intensity, and the wide acceptance angle also allowed it to be placed at a distance from the sam ple cell. The 9 m long, UV-enhanced beroptic cable transferred the emission light into a Czerny– Tu rner spectrograph (A cton R esearch C orpo ration , SpectraPro-275, f /3.8) that was equipped with a gated, image intensi ed charge-coupled device (ICCD) (Princeton Instruments, EEV 1024 3 256 photodiode array). Note that the extraordinary length of ber-optic cable, despite the loss of the detectability, was used for further rem ote experim ents in an inert alpha glove box. In the present work, a 500 nm blazed grating with 300 grooves/ mm was used for the simultaneous detection of many strong emission lines of europium over a wide wavelength range.

0003-7028 / 01 / 5503-0273$2.00 / 0 q 2001 Society for Applied Spectroscopy

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F IG . 1.

Schematic illustration of the LIBS apparatus.

By the use of a beamsplitter, a small fraction of the laser pulse was re ected onto a fast PIN photodiode (Hamamatsu), whose output signal provided the trigger signal for the cam era controller. The system components were synchronized to the incident laser pulse and the opening of the ICCD camera shutter by a digital delay/ pulse generator (Princeton Instrum ents, PG-200). Reading out and digitizing the data from the ICCD detector was carried out with a cam era controller (Princeton Instruments, ST-138). A PC-based software program (Spectroscopy Instruments, WINSPEC 1.6) provided complete support for LIBS experiments from the instrum entation control to the data acquisition to data analysis. A series of survey studies were conducted to determine the experimental conditions for recording the emission features of Eu. The time-resolved emission intensities

were monitored in combination with the gate width and delay time (i.e., tim e period between the laser pulse and emission detection). The optim al signal-to-noise ratio was obtained at a gate width of 1000 ns with a gate delay time of 100 ns. These timing parameters were used constantly in the present work. The emission spectra of bulk excitation were averaged over 1000 laser pulses. This considerable amount of averaging was used to obtain suf cient signal-to-noise ratios and reliable results on the measurement of Eu 2 O 3 suspension, which was partly agglomerated in different sizes. Chemicals. High-purity Eu 2 O 3 (99.9%) was used to prepare a 0.1 M Eu stock solution (15 200 mg Eu/L) in 0.7 M nitric acid, which was subsequently diluted with 0.7 M nitric acid to obtain the Eu concentrations in the range from 3 to 3000 mg/L. The suspension of Eu 2O 3(s) particles with a size of 150 –200 nm, as determined by scanning electron m icroscopy (SEM), was prepared by dispersion in ultra-pure water from a MilliQ-UV-Plus apparatus (M illipore). The titration experiment to m onitor the generation of Eu hydroxide colloids was perform ed at room temperature under atm ospheric conditions. The initial solution (1.3 3 10 2 4 M Eu perchlorate in 10 2 4 M HClO 4 /NaClO 4 , pH 5 4.82) was prepared from a Eu 2O 3 stock solution in perchloric acid. The pH was increased in small steps by adding 20 mL aliquots of carbonate-free NaOH and measured by a combined ROSS glass electrode calibrated against standard buffer solutions from Merck. RESULTS AND DISCUSSIO N Spectroscopic Features of Europium in Aqueous Solution. To improve the detection sensitivity of LIBS it

TABLE I. Experimental apparatus and conditions. Laser Pulse energy Wavelength Pulse duration Repetition rate Beam diameter

Continuum , SLI-20, Nd:YAG 5.0 6 0.2 mJ/pulse 532 nm 12 ns 20 Hz 5 mm

Detection system Spectrograph Gratings Wavelength operating range Slit width Slit height Spectral order

ARC, SpectraPro-275, Czerny–Turner 300 grooves/mm , 500 nm blaze 185 nm to 1.4 mm 10 mm (10 mm up to 3 mm adjustable) 4 mm rst

ICCD camera Operating temperature Removal of water condensation

Princeton Instruments, 1024 3 256 diode array Peltier cooling, 236 8C with water circulation Nitrogen gas

Detector controller A/D converter rate Readout rate

Princeton Instruments, ST-138 16 bits 150 kH z

Delay/pulse generator Gate delay/gate width Trigger signal

Princeton Instruments, PG-200 100 ns/1000 ns (3 ns–80 ms in 1 ns time increment) External, signal supplied by a fast photodiod e monitoring laser output

Fiber-optic cable

Spectroscopy & Imaging (S& I) GmbH, quartz ber optic, 9 m length

Personal computer Software program

IBM com patible PC WinSpec 1.6

274


F IG . 2. Comparison of a Eu spectrum calculated according to the NIST reference database 27 (assum ing an FWHM of 0.5 nm with a Gaussian line pro le) with a measured LIBS spectrum of Eu 3 1 (aq) (6.6 3 10 2 3 mol/L in 0.7 M nitric acid) and the LIBS spectrum of ultra pure water. The spectra are recorded at a signal integration time of 100 0 ns averaged over 1000 laser pulses.

is advantageous to select a wavelength region which permits the sim ultaneous detection of the strong emission lines of Eu listed in the reference database. 27 The most intense emission lines of neutral europium atoms (I) and singly positively charge ions (II) in the plasma were taken into account in the present study: Eu(I) lines at 459.4, 462.7, and 466.1 nm; Eu(II) lines at 372.4, 381.9, 390.7, 393.0, 397.1, 412.9, 420.5, 443.5, and 452.2 nm . Higher ionized species could not be monitored because of their high ionization potential. Figure 2 shows a typical emission spectrum calculated for the identi cation of Eu. The most intense lines of Eu were calculated by using the emission intensities and spectral positions from the reference database, 27 assum ing a full width at half-maximum (FW HM) of 0.5 nm with a Gaussian line pro le. The emission spectra of nitric acid solution and ultrapure water were measured as blank samples to test whether they interfere or overlap with Eu emission lines in the spectral region investigated (275–525 nm). The spectroscopic features for pure water and nitric acid resulted from ablated water vapor. They were qualitatively identical. As shown in Fig. 2, pure water shows a broad emission band at 306 nm. This m olecular band is attributed to the O–H rotation–vibration transitions of H 2O molecules at 306.4 nm (A 2 S1 –X 2P) associated with (n9, n 0) 5 (0, 0). 28 It mainly contributes to the background emission shape over the investigated wavelength range and does not overlap with the Eu emission lines. The measured emission spectrum in Fig. 2, which originated from the excited states of Eu 31 (aq) ions in nitric acid at a relatively high concentration of 6.6 3 10 2 3 mol/ L, is similar to the calculated emission spectrum of Eu. The O–H m olecular band is from the solvent water. The discrepancy between the measured and calculated emission intensity, especially for the Eu(I) lines at 459.4,

F IG . 3. LIBS spectra of the Eu concentration at 2.0 3 10 2 4 mol/L. (a) Eu 3 1 (aq) in 0.7 M nitric acid, and (b) suspension of Eu 2 O 3 (s) particles in water.

462.7, and 466.1 nm, m ust be mainly attributed to the distinct degree of ionization in the plasma. Calibration and Detection Limits for Europium Sam ples. In the present study, the emission intensity was evaluated for the most intense Eu(I) lines at 459.4, 462.7, and 466.1 nm, which overlap to a small extent. The total peak area of these three emission lines, above the offline background level, was calculated as the emission intensity of Eu. As expected, the calibration of the LIBS equipment shows a linear relation between the element concentration and the emission intensity. The detection limit (c L ) is given by cL 5

3s S

(1)

where S is the slope of a linear calibration relation, and s is the noise signal taken to be the standard deviation of the emission signal at a lower analytical concentration below the limit of detection. Figure 3 shows that the detection sensitivity of LIBS depends on the physico-chemical state of Eu. The emission spectrum of the Eu 2O 3 (s) suspension shows a considerably higher signal intensity than the Eu 31 (aq) ion in nitric acid, although the Eu concentration of both solutions is the sam e (2.0 3 10 2 4 m ol/L). This observation indicates that the detection sensitivity of LIBS strongly depends on a microscopic local concentration in the focal area. In order to calibrate the emission intensity vs. the Eu concentration, the emission spectra were m easured for Eu concentration ranging from 6.6 3 10 2 7 mol/L to 6.6 3 10 2 4 mg/L for the Eu 2O 3 (s) suspension and from 6.6 3 10 2 5 mol/L to 2.0 3 10 2 2 m ol/L for the Eu 3 1 (aq) ion. As shown in Fig. 4, the detection sensitivity for the suspension of Eu 2 O 3 (s) particles is more than two orders of magnitude better than that for the Eu 31 (aq) ions. The detection lim its for the Eu 2 O 3 (s) suspension and Eu 3 1 (aq) were calculated to be 2.0 3 10 2 7 mol/L and 3.3 3 10 2 5 mol/L, respectively. For the determination of the detection limits, the standard deviation of the noise signal was APPLIED SPECTROSCOPY

275

F IG . 5. LIBS spectra of 1.3 3 10 2 4 M Eu in perchlorate solutions at different pH.

F IG . 4. Emission intensity (a) and intensity of scattered light (b) of aqueous Eu 3 1 (aq) solutions and Eu 2 O 3 (s) suspensio n as a function of the Eu concentration.

obtained by estimating the emission intensity of Eu 31 (aq) at a concentration of 2.0 3 10 2 5 mol/L in the sam e wavelength region. In the double logarithm ic calibration plot in Fig. 4a, a linear relation between the Eu concentration and emission intensity (correlation coef cient 5 0.99) was obser ved for both Eu 2 O 3(s) particles and Eu 31 (aq) ions. However, the slope for the Eu 2 O 3 (s) particles is slightly lower than 1. This result is m ainly ascribed to a gradual reduction of the laser power density at the focal area with increasing Eu 2 O 3 (s) concentration. In particular, at the higher concentration, the loss of the laser pulse energy in turbid solution could be clearly observed. The reduction is due to light scattering (LS) and light absorption in the light path of the focal area. In order to ascertain this effect, the detection of the light scattering was performed by the sam e instrumentation. The laser pulse energy was adjusted so that no breakdown in solution occurs. Scattered laser light of 532 nm was detected by the same detector system . As expected for the Eu 31 solution in nitric acid, the intensity of scattered light is negligible, independent of the concentration (Fig. 4b). Contrary to that observation, the intensity of light scattering from the suspension 276


of Eu 2O 3 (s) particles increases linearly with the concentration. Formation of Eu(O H) 3 (s) Colloids and Determination of Its Solubility Product. Because of the different detection sensitivity of LIBS for colloidal particles and aqueous species of Eu, it is possible to selectively detect its colloidal particles in aqueous solution. This capability of LIBS was applied to m onitor the form ation of Eu(OH) 3(s) colloids, when the pH of the Eu solution (1.3 3 10 2 4 m ol/L of Eu 31 in 10 2 4 M HClO 4 /NaClO 4 , pH 5 4.82) was increased by titration with NaOH. Figure 5 shows the emission spectra taken at different pH, and Fig. 6 the emission intensity of Eu as a function of pH. At the initial pH of 4.82, the emission intensity was in accord with the value for a Eu 3 1 concentration of 1.3 3 10 2 4 m ol/L in nitric acid (cf. Fig. 4a). Up to pH 5 6.89, the emission intensity remains constant. At pH $ 7.10,

F IG . 6. Emission intensity of 1.3 3 10 2 4 M Eu in perchlorate solutions as a function of pH.

TABLE II. Literature data for the hydrolysis constants of Eu(III) and the solubility product of Eu(OH ) 3 (s) at 20–25 8 C. Hydrolysis constants a Authors

Medium

Baes, Mesmer Usherenko, Skorik 33 Frolova et al.34 Lundqvist 35 Nair et al.36 Caceci, Choppin 37 Jime´ nez-Reyes et al.38

I 50 I 5 0.05 M 0.3 M NaClO 4 1 M NaClO 4 1 M NaClO 4 0.7 M NaCl 2 M NaCl

Bernkopf 40

0.1 M NaClO 4

32

Solubility product Authors Present work Baes, Mesmer 32 Smith, Martell43 Bernkopf 39

M ethod b

log K91

log K 01

log b01

titr titr extr titr extr titr extr

28.03 6 0.03 28.33 28.1 6 0.4 28.12 27.3 6 0.2 28.29 6 0.02 28.45 6 0.23

27.8 27.7 27.7 27.2 27.2 26.7 27.6 27.8

6.2 6.3 6.3 6.8 6.8 7.3 6.4 6.2

sol n 5 n 5 n 5 n 5

1 2 3 4

Medium I 5 0.001 M I 50 I 50 0.1 M NaClO 4

log b9n 5.34 (5.84 6 11.61 (11.88 6 11.61 (11.88 6 18.30 (18.57 6

log b0n 6.0 (6.5) 12.7 (12.9) 17.9 (18.3) 19.5 (19.8)

0.47) c 0.21) c 0.21) c 0.09) c

log K9s p

log K 0sp

225.3 6 0.4

225.5 6 0.4 224.5 225.6 226.7 (227.1)

225.45 (225.84 6 0.3) c

Log K 9n values refer to the equilibria Eu 3 1 1 nH 2 O Û Eu(OH)3n 2 n 1 nH 1 and log b9n values to the equilibria Eu 3 1 1 nOH 2 Û Eu(OH)3n 2 n . The constants marked with a dash denote conditional concentration constants in the given medium. The constants at I 5 0 are calculated with activity coef cients based on ion interaction coef cients for the correspond ing Am(III) species in NaClO 4 32 and NaCl41 solution. b titr 5 potentiom etric titration; extr 5 solven t extraction; sol 5 solubility. c The values in parentheses refer to a chemical model, which includes additional dimeric and trimeric species. a

the emission intensity was increased by more than 3s of the emission intensity observed at pH , 7. This result is an unambiguous indication of the form ation of solid Eu particles, which rem ain colloidal in solution, devoid of precipitation. As shown in the recent laser-induced breakdown detection (LIBD) studies, 29,30 the investigation of the initial generation of colloidal particles, when the m etal ion concentration reaches or just exceeds the solubility of the corresponding hydroxide or oxide at given pH, provided the possibility of determ ining the therm odynamic solubility. In the present experiment with a given Eu concentration of 1.3 3 10 2 4 m ol/L at a low ionic strength of I ø 10 2 3 mol/L, the solubility of Eu hydroxide was exceeded in the range of pH 5 7.0 6 0.1. The conditional solubility product K 9sp of Eu(III) hydroxide is related to the therm odynamic constant K 0sp at I 5 0 by the activity coef cients g i of the Eu 31 and OH 2 ions:

ble II, and the m ean value of log b10 5 6.5 6 0.4 was used for EuOH 2 1 . This value is som ewhat larger than the value of log b10 5 6.2 selected by Baes and Mesmer32 from the literature published prior to 1976. The formation constants of the Eu(III) carbonate complexes are known rather accurately, as pointed out in the reviews of Byrne and Sholkovitz 40 and Neck et al.: 41 log b10 5 8.0 6 0.1 for EuCO 13 , log b20 5 13.1 6 0.3 for Eu(CO 3) 22 at I 5 0 and 25 8C. On the basis of these constants converted to I 5 10 2 3 mol/L, the following species distribution was calculated for pH 5 7.0 and pCO 2 5 10 2 3.5 bar: 34% Eu 3 1 , 9% EuOH 2 1 , 1.3% Eu(OH) 12 , 56% EuCO 13 , and 0.2% Eu(CO 3 ) 22 . With the corresponding concentration of log[Eu 31 ] 5 24.36 6 0.11, the solubility product of Eu(OH ) 3 (s) at I 5 0 was calculated to be

K 9sp 5 [Eu 31 ][OH 2 ] 3

(2)

0 5 K 9 g 3 1 (g 3 K sp sp Eu OH 2 )

(3)

It is of note that the present value refers to freshly formed colloidal particles of small but unknown size. According to Schindler,42 a somewhat lower solubility product has to be expected for a bulk precipitate consisting of larger particles. On the other hand, the effect of the carbonate complexation m ight be overestimated in the above calculation, because the solutions in contact with air were probably not in equilibrium with the CO 2 partial pressure. Neglecting the form ation of EuCO 13 in calculating the species distribution [77% Eu 3 1 , 20% EuOH 21 , 3% Eu(OH)12 ] would increase the calculated solubility product by 0.4 log units, which m ight approximately compensate for the particle size effect. Nevertheless, the present value was in reasonable agreem ent with the solubility products selected in the reviews of Baes and M esmer 32 and Smith and Martell 43 (log K 0sp 5 224.5 and 225.6, respectively). These recom mendations are based on a few experim ental data for fresh precipitates of amorphous Eu(OH) 3(s) 44,45 under not properly de ned condi-

The extended Debye–Hu¨ ckel equation of the speci c ion interaction theory (SIT), com bined with the interaction coef cients recom mended in the Nuclear Energy Agency–Thermodynamic Data Base (NEA–TDB),31 were used for the calculation of activity coef cients. The OH 2 concentration at pH 5 7.0 (log[OH 2 ] 5 log[H 1 ] 5 26.985) was calculated from the ion product of water (log K 0w 5 214.00 31 and log K 9w 5 213.97 at I 5 10 2 3 mol/L). In order to calculate the concentration of the Eu 3 1 aquo ion from the total Eu concentration of [Eu] 5 1.3 3 10 2 4 mol/L under the given condition (pH 5 7.0, solution in contact with air, i.e., with the carbon dioxide partial pressure of 10 2 3.5 bar), particularly the contributions of the hydrolysis species EuOH 21 and the carbonate complex EuCO 13 have to be taken into account. The literature data 32–39 for Eu(III) hydrolysis constants are listed in Ta-

log K 0sp 5 225.5 6 0.4

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tions and an estim ation among other trivalent lanthanides. 46 The lower solubility product reported by Bernkopf 39 (cf. Table II) refers to a more stable, aged, or possibly crystalline Eu(OH) 3 (s). Similar differences in the solubility products of amorphous and crystalline hydroxides are known for Nd(III) 47 and Am (III).31 In the recently reported potentiom etric titration study 38 up to pH 13 with a Eu concentration of 2 3 10 2 4 mol/L, the authors interpreted the results for the formation of various aqueous hydrolysis species M(OH)3n2 n (aq). The present study, however, attests to the form ation of colloidal Eu(OH) 3 (s) at pH $ 7 for the Eu concentration under study. CO NCLUSION LIBS is found to be applicable for analyzing the suspension of Eu 2O 3 (s) particles selectively in the presence of the Eu 3 1 aquo ion. The detection sensitivity of LIBS depends on the m icroscopic local element concentration in the focal area. For this reason, the emission intensity for the suspended Eu 2 O 3(s) particles is about two orders of magnitude higher than for the aqueous Eu 3 1 ion. This effect allows quantitative m onitoring of the formation of europium hydroxide colloids. LIBS can be used as a selective analytical tool for colloids or particulate impurities. For analyzing the chemical com position and particle size of colloidal particles, a combination of LIBS with the laser-induced breakdown detection 48 can be a desirable tool. 1. D. A. Cremers and L. J. Radziemski, ‘‘Laser Plasmas for Chemical Analysis’’, in Laser-Induced Plasmas and Applications, L. J. Radziemski, R. W. Solarz, and J. A. Paisner, Eds. (Marcel Dekker, New York, 1987). 2. V. M ajidi and M . R. Joseph, Crit. Rev. Anal. Chem. 23, 143 (1992). 3. K. Song, Y. I. Lee, and J. Sneddon , Appl. Spectrosc. Rev. 32, 183 (1998). 4. T. R. Loree and L. J. Radziem ski, J. Plasma Chem. Plasma Proc. 1, 271 (1981). 5. D. A. Crem ers, Appl. Spectrosc. 41, 572 (1987). 6. W. Sdorra, B. Ju¨ rgen, and K. Niemax, Mikrochim. Acta 108, 1 (1992). 7. M. Ducreux-Zappa and J.-M. Mermet, Spectrochim. Acta, Part B 51, 321 (1996). 8. A. S. Eppler, D. A. Cremers, D. D. Hickmott, M . J. Ferris, and A. C. Koskelo , Appl. Spectrosc. 50, 1175 (1996). 9. A, Sircar, R. K. Dwivedi, and R. K. Thareja, Appl. Phys. B63, 623 (1996). 10. L. Paksy, B. Ne´ met, A. Lengyel, L. Kozma, and J. Czekkel, Spectrochim. Acta, Part B 51, 279 (1996). 11. C. J. Lorenzen , C. Carlhoff, U. Hahn, and M . Jogwich, J. Anal. At. Spectrom. 7, 1029 (1992). 12. A. V. Pakhomov, W. Nichols, and J, Borysow, Appl. Spectrosc. 50, 880 (1996). 13. D. E. Kim, K. J. Yoo, H. K. Park, K. J. Oh, and D. W. Kim , Appl. Spectrosc. 51, 22 (1997). 14. N. Andre´ , C. Geertsen, J.-L. Lacour, P. Mauchien, and S. Sjo¨ stro¨ m , Spectrochim. Acta, Part B 49, 1363 (1994). 15. C. M. Davies, H. H. Telle, D. J. M ontgomer y, and R. E. Corbett, Spectrochim. Acta, Part B 50, 1059 (1995). 16. J. A. Aguilera, C. Aragon, and J. Cam pos, Appl. Spectrosc. 46, 1382 (1992).

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17. C. Lazzari, M. De Rosa, S. Rastelli, A. Ciucci, V. Palleschi, and A. Salvetti, Laser and Particle Beam s 12, 525 (1994). 18. C. Haisch, R. Niessner, O. I. Matveev, U. Panne, and N. Om enetto, Fresenius’ J. Anal. Chem. 356, 21 (1996). 19. D. K. Otteson, J. C. F. Wang, and L. J. Radziem ski, Appl. Spectrosc. 43, 967 (1989). 20. L. Xu, V. Bulatov, V. V. Gridin, and I. Schechter, Anal. Chem. 69, 2103 (1997). 21. D. A. Cremers, L. J. Radziem ski, and T. R. Loree, Appl. Spectrosc. 38, 721 (1984). 22. J. R. Wachter and D. A. Cremers, Appl. Spectrosc. 41, 104 2 (1987). 23. L. M. Berman and P. J. Wolf, Appl. Spectrosc. 52, 438 (1998). 24. Y. Ito, O. Ueki, and S. Nakamura, Anal. Chim . Acta 299, 401 (1995). 25. H. A. Archontaki and S. R. Crouch , Appl. Spectrosc. 42, 741 (1988). 26. R. Knopp, F. J. Scherbaum , and J. I. Kim, Fresenius’ J. Anal. Chem. 355, 16 (1996). 27. J. Reader and C. H. Corliss, ‘‘NIST Spectroscopic Properties of Atom s and Atomic Ions Database Version 1.0’’, NIST Standard Reference Database 38 (NIST, Gaithersburg, Maryland, 1992). 28. R. W. B. Pearse and A. G. Gaydon, The Identi cation of Molecular Spectra (Chapm an and Hall, London, 1976). 29. R. Knopp, V. Neck, and J. I. Kim, Radiochim. Acta 86, 101 (1999). 30. T. Bundschuh , R. Knopp, R. Mu¨ ller, J. I. Kim, V. Neck, and Th. Fangha¨ nel, Proceedings of the International Conference on Migration ‘99, Lake Tahoe, USA (1999), accepted for publication in Radiochim. Acta. 31. R. J. Silva, G. Bidoglio, M. H. Rand, P. Robouch, H. Wanner, and I. Puigdomenech (Organization for Economic Co-operatio n and Development, NEA-TDB), ‘‘Chemical Thermodyna mics of Americium’’, in Chemical Thermodynamics (Elsevier, North-Holland, Amsterdam, 1995), Vol. 2. 32. C. F. Baes and R. E. Mesm er, The Hydrolysis of Cations (Wiley, New York, 1976). 33. L. N. Usherenko and N. A. Skorik, Russ. J. Inorg. Chem. 17, 1533 (1972). 34. U. K. Frolova, V. N. Kumok, and V. V. Serebnikov, Khim. I. Khim Tekhnol. 9, 176 (1966). 35. R. Lundqvist, Acta Chem. Scand. A 36, 741 (1982). 36. G. M. Nair, K. Chander, and J. K. Joshi, Radiochim. Acta 30, 37 (1982). 37. M. S. Caceci and G. R. Choppin, Radiochim. Acta 33, 101 (1983). 38. M. Jime´ nez-Reyes, M. Solache-Rios, and A. Rojas-Herna´ ndez , Radiochim. Acta 87, 125 (1999). 39. M. F. Bernkopf, Thesis, Technisch e Universita¨ t Mu¨ nche n (1984), and M. F. Bernkopf and J. I. Kim, Report RCM 02884, Institut fu¨ r Radiochemie, TU Mu¨ nchen (1984). 40. R. H. Byrne and E. R. Sholkovitz, ‘‘Marine Chemistry and Geochemistry of the Lanthanides’’, in Handboo k on the Physics and Chemistry of Rare Earths, K. A. Gschneidner, Jr., and L. Eyring, Eds. (North Holland, Amsterdam , 1996) , Vol. 23, Chap. 158, p. 497. 41. V. Neck, Th. Fangha¨ nel, and J. I. Kim, ‘‘Aquatische Chemie und thermodynam ische Modellierung von trivalenten Actiniden’’, Report FZKA 6110, Forschungszentrum Karlsruhe, Germany (1998). 42. P. W. Schindler, Heterogeneo us Equilibria Involving Oxides, Hydroxides, Carbonates, and Hydroxide Carbonates, Advances in Chem istry Series 67 (American Chemical Society, Washington, D.C., 1967), p. 196. 43. R. M . Smith and A. E. M artell, Critical Stability Constants (Plenum Press, New York 1976), Vol. 4. 44. T. Moeller and N. Fogel, J. Am. Chem. Soc. 73, 4481 (1951) 45. T. Moeller and H. E. Kremers, J. Phys. Chem. 48, 395 (1944) 46. N. V. Aksel’rud, Russ. Chem. Rev. 32, 353 (1963) 47. I. I. Diakonov, B. R. Tagirov, and K. V. Ragnarsdottir, Radiochim. Acta 81, 107 (1998) 48. T. Bundschuh , R. Knopp, and J.-I. Kim, Colloids Surf. A: Physicochem . Eng. Aspects, paper accepted.