1858
Anal. Chem. 1985, 57, 1658-1662
Raman Measurements of Tributyl Phosphate after Adsorption on Silver Hydrosols Erwin Gantner,* Dieter Steinert, and J o h a n n Reinhardt Kernforschungszentrum Karlsruhe GmbH, Institut fur Radiochemie, Postfach 3640, 0-7500 Karlsruhe, West Germany
Mlxtures of sllver hydrosols and trlbutyl phosphate saturated diluted nltrlc acld exhlblt an Intense Stokes-Raman spectrum between 600 cm-' and 1700 cm-' which Is possibly attrlbutable to surfaceenhanced Raman scattering but whose bands, however, cannot be assigned to trlbutyl phosphate. The observed effect which grows wlth Increasing wavelength of the excitlng llght (Ar' laser) depends on the acld concentratlon and attalns a maximum for 1 M "Os, whereas in weakly acidic and neutral solutlons It cannot be detected. Tributyl phosphate In dlluted HC104and H2S04glves the same spectrum. As the sllver sol Is rapidly destroyed by the aclds the Raman bands can be observed for only about 1-3 h. No linear relationship between the Raman slgnal and the trlbutyl phosphate concentratlon allowlng quantltatlve work could be obtalned whlch Is possibly due to the strong instability of the hydrosol In the presence of aclds.
Tributyl phosphate (TBP),which-diluted with alkanes-is used as an extractant for uranium and plutonium in the Purex process, is soluble a t room temperature up to about 450 M, mg/mL, equivalent to a concentration of the order of in diluted nitric acid dependent on the acid concentration (1). The determination of TBP whose concentration can be as low as 5 mg/L in aqueous Purex process and waste solutions is usually performed by gas chromatography after extraction with CHC13 with accuracies around 5% and better (2). Within the framework of our own studies on the applicability of spontaneous laser Raman spectrometry (LRS) in the nuclear fuel cycle (3),the possibility of direct determination of TBP in nitric acid solutions using this method was also examined. However, this method exhibited a lack of sensitivity, even in T B P saturated solutions. In recent years quite a number of papers have been published demonstrating the detectability of low molecule concentrations M) in a number of compounds following adsorption on metal surfaces thanks to a substantial enhancement of their Raman signals by surface-enhanced Raman scattering (SERS) which was observed for the first time in 1974 for pyridine on electrochemically roughened silver electrodes ( 4 ) and in 1979 also on colloidal silver and gold particles (5); it gives, related to the number of the adsorbed molecules, Raman scattering cross sections amplified by up to the factor lo6. Several theoretical interpretations of SERS have been proposed to this day which, in extreme cases, are based on either a purely elecrodynamic or a purely molecular model and are capable of explaining a t least the degree of amplification. Summarizing representations of the experimental material collected so far and of the theoretical attempts of interpreting them are contained, e.g., in ref 6-8. SERS studies involving metal hydrosols are particularly simple to perform because they can be conveniently prepared and the measurement is obtained simply by mixing the sol with the solution containing the compound. To verify whether TBP likewise shows Raman enhancement which has not yet been described in the literature, some preliminary tests were
made with silver hydrosol which yielded an intense Raman spectrum for TBP containing nitric acid solutions. This result gave the initiative to study in more detail the possible origin of this spectrum as well as the conditions of its occurrence and, if possible, to elaborate a method based on the effect observed which could be used to determine low TBP contents in aqueous Purex solutions. EXPERIMENTAL SECTION Preparation of the Silver Hydrosols. Two different hydrosols were used in the studies which had been obtained by reduction of silver nitrate solution with ethylenediaminetetraacetic acid (EDTA) and with NaBH, in conformity with the rules described in ref 9 and 5, respectively. Reduction with EDTA. A 0.8-mL EDTA solution (0.1 M) and 4 mL of NaOH (0.1M) are added to 100 mL of water prepared by simple distillation. This solution is heated until boiling while stirring and after addition of 1.3 mL of AgN03 solution (0.1 M) kept boiling for 90 s (interrupt heating in time because otherwise you obtain a dark gray useless sol!). The hydrosol having an intensive yellow brownish color, if properly prepared, remains stable for 1to 2 months without variations in color; its absorption spectrum (measured with a UV-vis spectral photometer, Model 635, Varian-Techtron) has an extinction maximum at 410 nm. Reduction with NaBH4. A 5-mL AgN03 solution (5 X M) is added to 30 mL of an ice-cooled NaBH4solution in distilled M). Then the mixture is violently agitated for water (2 X some minutes. The color and the stability of this hydrosol, which can be prepared without difficulties, are comparable to those of the product obtained with EDTA; the extinction maximum lies at approximately 390 nm. Sample Preparation. To prepare the solutions saturated with TBP, we agitated 2 mL of TBP (Merck; p.a.) with 20 mL of water and acid, respectively (0.1 M to 3 M "OB; 1 M HClO, and 0.5 M H,SO,), in the agitating funnel for 10 min. In the same way, a saturated solution of tripropylphosphate (Ventron;T P P further The information not available) was prepared in 1 M "0,. mixtures were allowed to settle overnight to have the phases separated, and the aqueous phases were subsequently used either directly or diluted first. An additional sample was prepared by dissolution of 10 pL of TBP in 20 mL of 1 M "OB (maximum amount of TBP soluble), thus avoiding the enrichment of easily water extractable impurities which might be present in the TBP. Approximately lod3M solutions of the homologous esters triethyl phosphate (TEP) and of trimethyl phosphate (TMP; both from Fluka; pure), which are well soluble in diluted acids, and also of some important hydrolysis products of TBP such as dibutyl phosphate (DBP;Fluka; technical grade), 1-butanol (Merck;extra pure), and inorganic phosphate (diammoniumphosphate; Merck p.a.) were obtained by adding appropriate aliquots to 1 M "OB. To prepare a sample for measurement, an aliquot (1-2 mL) of the solution to be investigated was added to 1mL of the silver hydrosol and shortly agitated in a 10 x 10 mm standard cuvette. The mixture instantaneously adopted a deep gray-blue color, and after some seconds it brightened again and adopted the yellow brownish color of the sol. The sample so prepared was then placed into the sample chamber of the Raman spectrometer, and the measurement started about 1-2 minutes after mixing. Measurement and Evaluation of the Raman Spectra. For the investigationsan automated Raman spectrometer, type U-loo0 of Instruments S.A., was used (holographic plane gratings with 1800 grooves/mm; photomultiplier with GaAs photocathode) together with a Columbia computer, type 964, for spectrometer
0003-2700/85/0357-1658$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
Table I. Raman Bands (crn-') of TBP in 1 M HNOI after Adsorption on Silver Hydrosol and of Pure TBP' TBP/Ag-Sol
pure TBP
TBP/Ag-Sol
768
909
1311 1363
pure TBP
1304
1394
1420 1437
929
1089 1126
(0)
1236 1265 1284
613 723 772 817 835 887
1659
960 979 1063 1091
1599
1123
1650
1454 1511 1576 1500
1152
1250
1000
750
1250
1000
750
1180
OWavenumber region 600 cm-l to 1700 cm-'. bands are in italic.
Most intense
control, data acquisition, and evaluation, and a digital plotter and printer as the output devices. The measurements were made at a 90° angle with respect to the exciting laser line with backward coated collecting mirrors for laser and scattered light. The samples were excited at 1000 mW power each with the 514.5-nm line of a 12-W Ar+ laser (Spectra Physics) and-to investigate the dependence on the wavelength-also with the other lines (457.5 nm, 476.5 nm, 488 nm, 496.5 nm, and 501.7 nm) of the Ar' laser. All measurements were made with spectrometer slit widths of 500 pm (about five wavenumbers) with an integration time of 0.2 s per point of measurement in increments of one wavenumber; the range of wavenumbers of 600 cm-l to 1700 cm-' was recorded on the basis of results from preliminary tests. The duration of the measurement and data storage (disk system of the Columbia computer) was about 10 min per spectrum. To determine the location of the band maxima and the band intensities corrected to take into account a linear background, the software of the manufacturer of the spectrometer was used for evaluation. In the quantitative investigations the three most intense SERS bands at 1363 cm-l, 1511cm-l, and 1650 cm-' were mostly used as well as the nitrate band at 1048 cm-' which served as an internal standard. Depolarization Ratio Measurements. For the determination of depolarization ratios of the Raman bands observed in the phase-sol mixtures, a half-wave plate was simply inserted in the exciting 514.5-nm laser beam which will at least show whether a band is strongly polarized or not (10). By use of a sample of TBP saturated 1 M HN03, three sequential measurements (without, with, and without half-wave plate) were performed from which the band intensities can be corrected for decay by linear interpolation. The intensities corrected in this manner were then used to calculate the depolarization ratios p = I L / 1 , ,of the prominent bands. RESULTS AND DISCUSSION Mixtures of TBP-saturated 1M HN03 with silver hydrosol give a Stokes-Raman spectrum with bands between 600 cm-l and 1700 cm-' which is possibly attributable to SERS but does not resemble the spectrum of pure TBP;this can be seen from Figure 1. As a reliable relationship does not exist with respect to the normal Raman spectrum of TBP (which, in addition, cannot be measured in aqueous solution with the spectrometer used), no enhancement factor can be indicated for the effect observed. Both hydrosol types yield the same Raman spectrum whose bands of highest intensities together with the bands of pure TBP have been listed in Table I for the range of wavenumbers investigated. For the more intense bands depolarization ratios p > 0.6 could be deduced from the depolarization measurements which indicates that these bands are depolarized, which is also typical for SERS bands (11). For the 1048-cm-l nitrate band a depolarization ratio p < 0.1 was found as expected.
1500
WAVENUMBERS
Figure 1. Raman spectrum of pure TBP (a)and of TBP in 1 M HNO, with added silver sol (b).
In order to be able to exclude other compounds which might be present in the system TBP-H20-HN03 as the origin of the spectrum observed, mixtures of silver hydrosol with nitric acid solutions containing DBP, 1-butanol, and inorganic phosphate, which, according to ref 12, constitute the major impurities and hydrolysis products of TBP, have also been measured. In no case could a Raman signal be found under the working conditions described with the exception of the 1048-cm-l band due to the nitrate ion. From these results we conclude that the spectrum observed should come from the TBP dissolved in the aqueous phase. This is also confirmed by the fact that the nitric acid phase equilibrated with 2 mL of TBP (in which impurities coming from the organic phase could eventually be enriched) and the sample prepared by dissolution of 10 pL in 1M HN03 show identical spectra with comparable band intensities. Similar tests for detection of a surface enhanced Raman effect were performed also with nitric acid solutions of the homologous esters TPP, TEP, and TMP (for preparation of these solutions, cf. Experimental Section). For the sample containing TPP, a spectrum was obtained which-although with about 10 times lower intensity-is surprisingly similar to the spectrum found for TBP (Figure 2) while in the spectra of the T E P and TMP containing solutions, respectively, no other bands appeared in addition to the nitrate band. These results cannot be interpreted on the basis of the experiments performed until now. Signal Dependence on Time. When nitric acid solution containing TBP or not is added to the yellow brownish silver hydrosol, the mixture instantaneouslyadopts a deep gray-blue color and brightens again some seconds later to become a yellow brownish solution from which within a few hours-
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
look
I\ \
m
-
N1
m
15cc
1250
1200
75c
WAVENLlMHERS
Flgure 2. Raman spectrum of TPP in 1 M HNO, with added silver sol. c
0 '
0.8
20
40
60
t Iminl
Flgure 4. Normalized 1363-cm-' band intensity as a function of time in 0.5 M HNO, (A) and 1 M HNO, (0).
0.6
0.4
0.2
350
450
550 wavelength (nrn)
Figure 3. Absorption spectrum of silver sol (NaBH,) 2 min (a), 30 min (b), and 60 mln (c) after adding TBP-saturated 1 M HNO,.
dependent on the acid concentration-metallic silver is precipitated while the solution completely clarifies. Compared to the spectrum of the pure sol, the absorption of this mixture is characterized by an absorption maximum getting increasingly flatter within a 1-h interval, associated with a pronounced broadening toward longer wavelengths which is visible from Figure 3 by the example of the NaBH4 sol. As the destruction of the silver hydrosol, which, caused by the acid, occurs much faster than in the experiments described by Creighton (5),should be accompanied by the decrease and eventual disappearance of a SERS signal, the dependence on time of the observed Raman intensity was investigated for two acid concentrations (0.5 M and 1M "0,; both TBP saturated). The results obtained for the NaBH4 sol over a period of about 100 min have been represented in Figure 4 where the intensity of the 1363-cm-l band (related to the nitrate band as the internal standard) is plotted linearly vs. time. Then, in analogy to the stability in terms of time observed for the silver hydrosol added to the acid, the Raman signal decreases faster in the stronger acid than in the weaker acid; moreover, both plots, after about 30 min (1M "OB) and after 60 rnin (0.6 M HNOJ, respectively, turn into a segment with a slower slope. EDTA sol behaves in a similar way but it is slightly less stable than the NaBH4 sol. Approximately 2 h (1 M H NO3) and 5 h (0.5 M HNOJ, respectively, after preparation 01 the hydrosol-acid mixture, no more Raman activity at-
tributable to TBP can be observed. On the other hand, the intensity of the nitrate band (1048 cm-l) steadily increases over the period investigated which can be expected also from the fact that the solution clarifies optically with the time going on. Since the signal decreases already noticeably during the measurement interval (about 10 min), the relative intensities found of the strongest Raman bands (verified for the 1650-cm-I and 1511-cm-' bands and the 1363-cm-' band as reference band) are not constant but may vary by up to about 20% from measurement to measurement. In a few experiments larger deviations (more intense by up to a factor of 3) have been observed for the bands at 613 cm-l and 1126 cm-l. Signal Dependence on t h e Exciting Wavelength. As shown by Creighton (5) for pyridine after adsorption on silver hydrosols, the SERS excitation profile exhibits a fairly sharp maximum which moves, dependent on time and on the amount of pyridine added, with increasing particle size from about 500 nm to 620 nm thereby increasing in height and coinciding with the maximum of Mie scattering on the large silver particle fraction. From these results he concludes that for the observation of intense SERS spectra of molecules adsorbed on metal sols, it is essential that the sols show strong Mie scattering at the exciting wavelength. The results of our own measurements using the lines of the Ar+ laser (457.9 nm up to 514.5 nm) are represented in Figure 5 for the mixture of NaBH4 sol/TBP in 1 M HNO,; the intensity of the 1363-cm-I band (related to the nitrate band) has been plotted vs. the exciting wavelength. As can be seen from Figure 5, the Raman intensity increases considerably with growing wavelength without, however, exhibiting a maximum which, according to the studies described in ref 5, is expected to be beyond the range of wavelengths investigated in this work. Figure 5 also shows the course of the obviously wavelength dependent relative intensity of the 1650-~rn-~ band (related to the 1363-cm-I band) which increases by nearly the factor 3 with decreasing wavelength over the range of wavelengths investigated. This situation is illustrated also by Figure 6 in which the Raman spectrum obtained with the 476.5-nm line is represented. Practically the same results are obtained with the EDTA sol. Influence of t h e H+ Concentration on t h e Raman Signal. Preliminary experiments have shown a strong dependence of the observed Raman intensity on the nitric acid concentration. To investigate this influence TBP saturated
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
1661
'R
'R 1.5
1.0
0.5
0
1
3
2
~ IM1 ~
C ,
,
.
.
460
,
.
~
3
Figure 7. Dependence of 1363-cm-' band intensity on the nitric acid concentration.
.
500 wavelength ( n r n )
Figure 5. 1363 cm-' band intensity (A)and band intensity ratio 1650 cm-'/1363 cm-' (0)as a function of excitation wavelength. et
I
"1
,
CLOi
m
I?
!500
:so0
.253
1000
1250
750
750
dALENUMBERS
Figure 6. Raman spectrum of TBP In 1 M HNOBwith added silver soi; excitation wavelength 476.5 nm.
water and nitric acid solutions (0.1 M to 3 M "OB) were prepared, EDTA silver sol was added to them, and immediately after mixing the two phases the measurements was performed. To represent the results again the intensity of the 1363-cm-l band (related to nitrate as the internal standard) was selected which is plotted in Figure 7 vs. the HNO, concentration of these solutions. It can be seen from this figure that the intensity of the Raman signal, which is not detectable in neutral or weakly acidic (KO.1 M "OB) solutions, initially increases considerably with growing acid concentration,passes subsequently a maximum for 1M "Os, and decreases again with further augmenting acid concentration. All the rest of the bands occurring in the spectrum and caused by the presence of TBP basically show the same behavior. A significant dependence of the relative intensities of the bands a t 1650 cm-l, 1511 cm-', and 1363 cm-l (used as reference band) on the acid concentration was not found. From the lack of a Raman signal in the absence of a strongly acidic medium, one might conclude that the Raman active species in our experiments is a product of interaction between the organic and the H+ ions about which no information could be found in the literature. Complex compounds between TBP and extracted HN03 which are present with remarkable concentrations in the TBP phase (13)can be excluded as the origin of the spectrum observed because similar experiments
I : :500 1250 1000
I
-
750
WAVENUMBERS
Figure 8. Raman spectrum of TBP in 1 M HCiO, (a) and 0.5 M H,SO, (b) with added silver sol.
with TBP saturated solutions of 1M HC104and 0.5 M H,S04 which both yield the same spectrum as that obtained in nitric acid media (Figure 8) show that the anion obviously does not contribute to the formation of this Raman active species. The decrease of the Raman signal with increasing nitric acid concentration in the range beyond 1 M HNO, can possibly be explained by faster coagulation of the silver sols under these conditions combined, according to ref 5, with a simultaneous shift of the excitation profile to longer wavelengths. Dependence on Concentration of the Raman Intensity. To examine whether the observed effect might be suitable to
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
by repeating measurements including also TBP containing phase-to-sol ratios differently from 21 but the band intensities obtained in the repetitions did vary by up to a factor of 2. Because of the nonlinear calibration curves and the insufficient sensitivity obtained with respect to the possible concentration range of TBP in Purex solutions (cf. introductory section), combined with the poor reproducibility of quantitative data, the determination of TBP in aqueous solutions, especially with concentrations much lower than its maximum solubility, seems not to be possible by making use of SERS. The main reasons for that should be the serious changes of the properties of the sols during the time interval of sample preparation and measurement. Registry No. TBP, 126-73-8;Ag, 7440-22-4.
R 2.0
1.0
LITERATURE CITED 0 50
100
C re1
Figure 9, 1363 cm-’ band intensky as a function of TBP concentration in 1 M HNO, (100 = saturation).
determine TBP in nitric acid solutions, calibration measurements were performed on samples with different TBP contents. For this purpose, aliquots of TBP-saturated 1 M HNOBwere diluted with the same acid, 1mL of NaBH4 sol was added to 2 mL of each of these solutions, and the Raman spectra were measured. The results obtained are plotted in Figure 9 vs. the degree of saturation in TBP (50% to 100%) again using the intensity of the 1363-cm-l band normalized to the nitrate band. This plot is not linear and has a maximum a t about 90% saturation; moreover, the Raman intensities found for TBP concentrations below 50% saturation (not entered in the figure) are already too low to be evaluated from the spectra. These results could in principle be confirmed
(1) Becker, R.; Stieglitz, L.; Bautz, H., Institut fur Heisse Chemie, Kernforschungszentrum Karlsruhe, unpubllshed work, 1981. (2) Stieglltz, L.; Becker, R.; Bautz, H.; Wunschel, A. Kernforschungszent. Karlsruhe, [Ser.] KfK 1078, KfK-2613, 18 pages. (3) Gantner, E.; Steinert, D.; Freudenberger, M.; Ache, H. J. Kernforschungszent. Karlsruhe, [Ber.] KfK 1084, KfK-3852, 34 pages. (4) Fleischmann, M.; Hendra, P. J.; McQulilan, A. J. Chem. Phys. Len. 1974, 26, 163-166. (5) Crelghton, J. A.; Blatchford, C. G.; Albrecht, M. G. J . Chem. SOC., Faraday Trans. 2 1070, 75, 790-798. (6) Chang, R. K., Furtak, T. E., Eds. “Surface Enhanced Raman Scatterlng”; Plenum Press: New York, 1982. (7) Ueba, H.; Ichimura, S.; Yamada, H. Surf. Sci. 1982, 119, 433-448. (8) Dornhaus, R. Festkorperpro6leme 1982, 22, 201-228. (9) Fabrikanos, A.; Athanassiou, S.; Lleser, K. H. 2. Naturforsch., 8 1063, 186, 612-617. (10) Ailemand, Ch. D. Appl. Spectrosc. 1970, 24, 348-353. (11) Birke, R. L.; Lombardi, J. R.; Sanchez, L. A. Adv. Chem. Ser. 1982, NO. 201, 89-107. (12) Schulz, W. W., Navratil, J. D., Eds. “Science and Technology of Tributyl Phosphate”; CRC Press: Boca Raton: FL, 1984; Vol. 1, Chapters 2 and 5. (13) Alcock, K.; Grlmely, S. S.; Healy, T. V.; Kennedy, J. Trans. Faraday SOC. 1956, 52, 39-47.
RECEIVED for review November 2, 1984. Accepted March 28, 1985.
