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Zirconium and hafnium determination by energy dispersive X-ray fluorescence with solid phase preconcentration
Talanta ELSEVIER
Talanta 44 (1997) 811 816
Zirconium and hafnium determination by energy dispersive X-ray fluorescence with solid phase preconcentration Patricio Peralta-Zamora *, Lorena Cornejo-Ponce, Maria Izabel Maretti S. Bueno, Jos6 Walter Martins Universidade Esta&lal de Campinas, lnstituto de Quhnica, CP. 6154. CEP 13083-970. Campinas-SP, Brazil
Received 20 May 1996; accepted 25 September 1996
Abstract
Two preconcentration methods has been developed for simultaneous determination of zirconium and hafnium by energy dispersive X-ray fluorescence (EDXRF). The first method is a liquid-solid extraction procedure with the use of an anionic exchange resin modified with xylenol orange. The second is a precipitation procedure carried out in the presence of lanthanum. Both methods permit significant enhancement of sensitivity in comparison with direct measurement in the aqueous phase. The applicability of both procedures for the preconcentration of Zr and Hf prior to their determination by EDXRF was demonstrated by analyzing synthetic mixtures and a sample of zirconium ore. The results obtained with the use of the modified resin show relative standard deviation of about 4% and good agreement with those obtained by spectrographic analysis. © 1997 Published by Elsevier Science B.V. Kevwordx: Hafnium; Preconcentration; X-ray fluorescence; Zirconium
I. Introduction
The simultaneous determination of zirconium and hafnium is one of the most difficult problems of analytical chemistry, because the close similarity of their chemical properties hinders the use of classical analytical methods [1-5]. Colorimetric techniques do not have any interesting application due to unavailability of specific chromogenics agents. The principal contribution of these techniques is reduced to the existence of several differential methods, based on differences between *Corrcsponding author.
zirconium and hafnium complexes under different acidic conditions [6] or in the presence of specific masking agents [7]. The classic configuration of flame atomic absorption or emission techniques shows little application for this determination due to the heat-resistant characteristics of both species and the high complexity of the emission spectra [1-4]. The use of more energetic atomization systems (i.e., plasma, arc and spark) permits more suitable results only for determination of the concentration ratios [1]. Absolute determinations can be performed by using special calibration methods, typically chemometric tools for correction of mutual spectral interferences [1]. The individual de-
0039-9140/97/$17.00 ~" 1997 Published by Elsevier Science B.V. All rights reserved. PII S00 39-9140(96 )02117-0
812
P. Peralta-Zamora et al./ Talanta 44 (1997) 811 816
termination of zirconium in silicate rocks by inductively-coupled plasma emission spectrometry were reported by Uchida et al. [8]. However, the results show poor precision (relative standard deviation, RSD, between 14 and 38%). Due to the great spectral resolution of the instrumental system, energy dispersive X-ray fluorescence permits the determination of Zr-Hf mixtures without important interference problems. However, the low sensitivity of the technique can be resolved only by the use of preconcentration methods [9-21]. The determination of a great number of metallic species by EDXRF is usually preceded by concentration steps which involve the use of ion exchange resins [9,10,12,16], activated charcoal [9,10,17], naphthalene [14], polyurethane [15], modified silica [21] and coprecipitation procedures using ferric hydroxide as a collector [18-20]. The great selectivity reached by the use of chelating resins [22] has contributed to its popularization as solvents system. Xylenol orange has been used for modification of ionic exchanger resins, and for determination of zirconium by solid phase spectrophotometry [23-25]. In this work we propose the use of an ion exchange resin modified with xylenol orange or a lanthanum coprecipitation procedure for preconcentration and simultaneous determination of zirconium-hafnium mixtures by EDXRF.
2. Experimental 2.1. Instrumental
Spectrophotometric determinations were performed with an Intralab DMS-100 UV-Vis Spectrophotometer. X-ray fluorescence measurements were executed on a Spectrace 5000 Energy Dispersive X-ray Fluorescence Spectrometer, equipped with a rhodium tube and a Si(Li) semiconductor crystal detector (irradiation time, 60 s; tube voltage, 30 kV; tube current, 0.20 mA; atmosphere, air; filter, 0.127 mm rhodium film). The measurements were realized at 15.744 KeV for Zr (Kal) and 7.898 KeV for Hf (Lal). The samples were introduced on a cylindrical plastic device
from Chemplex, of about 2 cm of height and 3 cm of internal diameter, using a X-ray Mylar film as sample support. Spectrographic measurements were carried out on Carl Zeiss PGS-2 Emission Spectrometer, equipped with a linear array of photodiodes [26]. 2.2. Chemicals and solutions
The zirconium standard solution was prepared by direct dissolution of ZrOC12. 8H20 (99.5%, Riedel-de Haen) in H2SO 4 (1:5 v/v). Zirconium hydroxide was precipitated with an aqueous solution of NH 3. The solid was recovered by filtration, thoroughly washed with deionized water and dissolved in HC1 (final concentration of HC1, about 2 mol 1-l). The hafnium standard solution was prepared from H f O 2 (Specpure, Johnson, Mathey and Co.). The oxide was dissolved with a HFH2SO4 (1:1 v/v) mixture, heating several times almost to dryness with successive additions of concentrated H2SO 4. The solid residue was dissolved in H2SO 4 and the hydroxide precipitated with an aqueous solution of NH 3. The solid was recovered by filtration, thoroughly washed with deionized water and dissolved in HC1 (final concentration of HC1, about 2 mol 1 ~). Both solutions were standardized by a gravimetric procedure using mandelic acid [27]. The lanthanum standard solution (40 g I ~) was prepared from La203 (99%, Carlo Erba). Amberlite IRA-400 anion resin (Koch-Light Laboratories, 100-200 mesh) and Xylenol orange (Fluka A.G., Bucks S.G.) were used without further purification. The natural sample, purchased from the Energetic and Nuclear Research Institute (IPEN, $5,o Paulo-Brazil), corresponds to a mixture of zirconium and hafnium hydroxides obtained by physical and chemical treatment of a zirconium ore. 2.3. Preparation o f modified resin
Dried resin, 40 g, were added to 400 ml of an aqueous solution of xylenol orange (4.3 x 10 - 3 tool 1- ~, pH 7.0) and the mixture was vigorously stirred for 6 h. After this impregnation time, the
813
P. Peralta-Zamora et al./ Talanta 44 (1997) 811 816
solid mass was filtered, thoroughly washed with deionized water, dried at 60°C and stored in amber glass flasks.
2.4. Extraction with modified resin
Suitable volumes of solution of Zr and H f were placed in 200 ml beakers, diluted up to 50 ml with deionized water and adjusted to adequate pH values with aqueous solutions of HC1 or NH3. To these solutions were added 0.5 g of the modified resin. The mixture was stirred for a convenient time and the solid was recovered by filtration in Whatman 41 filter paper, washed with deionized water and dried at 60°C for 2 h. In preliminary studies, the Zr and H f recuperation was evaluated by colorimetric determination [28] of the remained amounts in the filtrate. For this determination, the filtrate (approximately 50 ml) was led to almost dryness in a water bath, dissolved in 3.0 ml of 0.3 m o l l i HC1, and added of 2.0 ml of xylenol orange 0.05% in HC1 0.3 mol 1 ~. The concentration of zirconium or hafnium was determined spectrophotometrically at 550 nm, using analytical curves obtained between 0.2 and 2.0 mg 1 ~. Further determinations were carried out by direct analysis of the solid mass by EDXRF. The concentration of Zr and H f was determined through external calibration using standard solutions submitted to analogous extraction procedures.
3. Results and discussion 3.1. Impregnation o f the resin
Colorimetric determination of the remaining amount of xylenol orange after the impregnation procedure verified that the efficiency of the impregnation process is very close to 100%. Relating the amounts used of resin (0.5 g) and the xylenol orange amount present in this solid fraction (0.043 ml tool!, it is possible to estimate a maximum extraction capacity of about 2000 ~tg for Zr or 3500 tag for Hf. 3.2. Extraction with modified resin
The extraction of both species is almost complete at similar pH values (pH 4.0 8.0). Although the zirconium and hafnium hydrolysis is favored at these pH values, precipitation was not observed probably due to the low concentration of the analytes used in this study (approximately 2 tag ml- 1) and due to the presence of large amounts of sulfate ion, which can displace the hydroxyl groups from hydrolyzed species [1]. The procedure is not selective in this pH region, a result resembling those obtained by the application of other extraction procedures [29,30]. For subsequent determinations, a pH of 7.0 was selected. A X-ray fluorescence spectrum of zirconium and hafnium on the xylenol orange modified resin is presented in Fig. 1.
Z.r (~,~)
2.5. Preconcentration by precipitation in the presence o f lanthanum 1500
Aqueous solutions, 50 ml, containing adequate amounts of Zr and H f were added to 10 ml of the standard lanthanum solution. The hydroxides were precipitated by addition of an aqueous ammoniacal solution. After 5 min of stirring the precipitate was recovered by filtration and dried at 100°C for 2 h. The recuperation degree was evaluated by colorimetric analysis in the filtrate. Further determinations by E D X R F were directly carried out on the solid fraction.
1000
' ..~J+'] 500 Hf
6
(L~) 8
1
12
1
16
Energy(keY) Fig. 1. X-ray fluorescencespectrum of zirconium and hafnium on the xylenol orange modified resin.
814
P. Peralta-Zamora et al./ Talanta 44 (1997) 811-816
Table 1 Zr-Hf determination from synthetic samples with modified resin Added (rag)
Found (mg)
Zr
Hf
Zr (difference, %)
Hf (difference, %)
10.0 50.0 100.0 100.0 100.0 500.0 Average difference RSD (n = 5)
50.0 50.0 50.0 100.0 300.0 100.0
9.7(3.0j 49.1(1.8) 100.8(0.8) 101.2(1.2) 101.9(1.9) 497.3(0.5) 1.5% 3.0%
51.5(3.0) 52.3(4.6) 52.5(5.0) 100.7(0.7) 303.1(1.0) 105.9(5.9) 3.4% 4.O%
The quantitative recuperation of Zr and Hf is completed with stirring times of about 15 min. The complete extraction of the analytes in a short time like this, and the great convenience of the involved operations, implies a very important economy of time, mainly when compared with the times required for other preconcentration procedures. Liquid-liquid extraction procedures and liquid-solid methodologies executed in column systems, for example, are slower techniques because of the difficulty in reaching equilibrium conditions and long time elution stages, respectively. For posterior determinations stirring times of 20 min were selected. Using sample volumes between 50 and 200 ml, the extraction of Zr and Hf is quantitative. With higher volumes the recuperation of the analytes is decreased, presumably due to excessive dilution of the solid extractor and consequently inefficient contact of the phases during the selected stirring time. If we consider the complete transference of the Zr or Hf amounts present in this high volume of aqueous phase for a small mass of resin, it is possible to perceive the great preconcentration potentiality of the proposed method. For subsequent determinations maximum volumes of 200 ml were selected. Is known that the particle size have a great importance on the X-ray fluorescence measurements, nevertheless preliminary studies shown that for ours experimental conditions insignificant differences are obtained by using resin of size equivalent to 41-60 or 150-190 mesh.
Executing the extraction procedure in the optimized conditions and measuring the EDXRF emission directly on the solid mass, linear calibration curves are obtained for a large concentration range (10-800 lag for zirconium, and 50 800 lag for hafnium) with typical correlations higher than 0.999. Considering the sensibility of the determination as a function of the inclination of the analytical curves, we observed that the proposed method permits sensitivity increases of about 25 times for Zr and 40 times for Hf, in relation to direct measurement in the aqueous phase.
3.3. Analysis of synthetic samples The results obtained for synthetic mixtures of diverse composition (Table l) indicate high concordance between added and found amounts. The precision of the methodology, expressed as relative standard deviation for five determinations (about 3% for Zr and 4% for Hf), is very similar to that obtained by other techniques [9,10].
3.4. Precipitation in the presence of lanthanum This method appears as a very simple alternative for preconcentration of metallic species because it depends on but a few experimental variables. The added amount of lanthanum, for example, is not a critical parameter. The selected value was defined only to facilitate the filtration and transference operations and as a function of the capacity of the sample compartment. Prelimi-
815
P. Peralta-Zamora et al. / Talanta 44 (1997) 811 816
Table 2 Zr-Hf determination from synthetic samples by precipitation in the presence of lanthanum Added (rag)
Found (rag)
Zr
Hf
Zr (difference, %)
Hf (difference, %)
100.0 100.0 100.0 300.0 500.0 Average difference RSD (n = 5)
100.0 150.0 200.0 200.0 200.0
100.8(0.8) 98.7(1.3) 99.5(0.5 ) 297.4(0.9) 498.7(0.3) 0.8 4.0%
101.2(1.2) 150.9(0.6) 201.9(1.0) 203.4(1.7) 205.6(2.8) 1.5% 4.0%
nary experiments indicated 300 ml as a m a x i m u m sample volume; with higher volumes the recuperation of the metallic species is incomplete. The calibration curves show good linearity (r > 0.999) over a large concentration range (300-1000 gg ml %. However, the sensitivity is lower than that obtained by extraction on the modified resin. In any case, the sensitivity is enhanced about 4 times in relation to the directly measurement in the aqueous phase. The results for the Z r - H f determination from synthetic mixtures (Table 2) show high coherence with the added amounts, with typical standard deviations of about 4%. The accuracy of the proposed method is better than those obtained by Ricci [18] (5-10%) through a coprecipitation procedure using ferric hydroxide as a collector agent. 3.5. Z r - H J determination fi'om natural samples
Both proposed preconcentration procedures were evaluated for determination of Z r - H f mixtures from natural samples, which result from
physical and chemical treatments of zirconium ore. The results for zirconium determination using the modified resin and the coprecipitation procedure (Table 3) show a high coherence with those obtained by spectrographic analysis. By applying the statistical Student's test for 95% of confidence, we verify that for hafnium determination, only the modified resin provides comparable results. The results obtained by the coprecipitation procedure shows significant differences with those obtained by spectrographic analysis and by using the modified resin.
4. Conclusions Both preconcentration procedures permit the quantitative recuperation of Zr and H f from large volumes of aqueous samples. For this motive they represent a potential and simple alternative for determination of metallic species by E D X R F . The procedure based on the use of a modified resin permits a significant increase of the sensitivity of
Table 3 Zr-Hf determination from zirconium ore using the proposed methods Run
I 2 3 Average RSD (%)
Extraction with modified resin (%)
Precipitation (%)
Spectrography (%)
Zr
Hf
Zr
Hf
Zr
[~t"
95.4 95.2 95.6 95.4 + 0.4 0.2
4.6 4.8 4.4 4.6 + 0.4 4.3
96.5 97.1 96.7 96.8 i 0.6 0.3
3.5 2.9 3.3 3,2 _+0.6 9.5
95.0 95.3 95.2 95.2 ± 0.3 0.2
5.0 4.7 4.8 4.8 + 0.3 3.2
816
P. Peralta-Zamora et a l . / Talanta 44 (1997) 811-816
the determination and can be used in analyses of low concentration samples. The non-selectivity of the extraction procedure with the use of modified resin is not a significant inconvenient, because can be compensate for the great selectivity presented for the EDXRF technique.
References [1] A.K. Mukherji, Analytical Chemistry of Zirconium and Hafnium, Pergamon Press, Oxford, 1970. [2] R.J.H. Clark, D.C. Bradley and P. Thornton, The Chemistry of Titanium, Zirconium and Hafnium, Pergamon Press, Oxford, 1975. [3] V.V. Serbinovich, V.P. Antonovich and N.A.J. Pshetakovskaya, J. Anal. Chem. USSR, 41 (1986) 867. [4] A. Brookes and A. Townshend, Analyst, 95 (1970) 529. [5] U.G. Senin, A.M. Asavin, L.N. Lazutkina and N.V. Korsakova, J. Anal. Chem. USSR, 44 (1989) 1348. [6] S.V. Elinson and N.A. Mirzoyan (in A. Brookes and A. Townshend), Analyst, 95 (1970) 529. [7] K.L. Cheng, Talanta, 3 (1959) 81. [8] H. Uchida, K. Inasaki and K. Tanaka, Anal. Chim. Acta., 134 (1982) 375. [9] A.T. Ellis, D.E. Leyden, W. Wegscheider, B.B. Jablonski and W.B. Bodnar, Anal. Chim. Acta, 142 (1982) 73. [10] A.T. Ellis, D.E. Leyden, W. Wegscheider, B.B. Jablonski and W,B. Bodnar, Anal. Chim. Acta, 142 (1982) 89. [I1] I.F. Seregina, G.T. Tsizin, A.M. Shilnikov, A.A. Formanovski and Y.A. Zolotov, J. Anal. Chem. USSR, 48 (1993) 122. [12] A.N. Masi and R.A. Olsina, Talanta, 40 (1993) 931.
[13] L. Oi-Wah and H. Sing Yiu, Anal. Chim. Acta, 280 (1993) 269. [14] V. Bhagavathy, L.P. Reddy, R. Prasada and A.D. Damodaram, J. Radioanal. Nucl. Chem., 49 (1991) 35. [15] M.S. Carvalho, J.A. Medeiros, A.W. N6brega, J.L. Mantovano and U.P. Rocha, Talanta, 42 (1995) 45. [16] H. Knote and V. Krivan, Anal. Chem., 54 (1982) 1858. [17] V. Bhagavathy, L.P. Reddy, P.S.T. Sai, R. Prasada and A.D. Damodaram, Anal. Chim. Acta, 242 (1991) 215. [18] E. Ricci, Anal. Chem., 52 (1980) 1708. [19] V. Bhagavathy, P.S.T. Sai, R. Prasada and A.D. Damodaram, Anal. Lett., 22 (1989) 197. [20] H. Qing-Lie, T.C. Hughes, M. Haukka and P, Hannaker, Talanta, 32 (1985) 495. [21] C.L. Ponce, P.P. Zamora and M.I.M.S. Bueno, Quimica Nova, 19 (1996) 30. [22] C. Cordeiro, M. Gonzalez de la Barrera, J. Rosas, J. Ma. Pinilla and L. Hernandez, An. Quire., 89 (1993) 230. [23] L.F. Capit&n Vallvey, J.M. Bosque Sendra and M.C. Valencia, Analusis, 17 (1989) 601. [24] L.F. Capitfin, L.F. Capit~in Vallvey, M.C. Valencia, J.M. Bosque Sendra and I. de Orbe, Analusis, 19 (1991) 177. [25] G.D. Brykina, G.G. Lebedeva and G.F. Agapova, J. Anal. Chem. USSR, 45 (1990) 1322. [26] C.R. Bellato, J.J.R. Rohweder, I.M. Raimundo Jr. and C. Paquini, XVIII Reuni~.o Anual da Sociedade Brasileira de Quimica, Livro de Resumos, QA-47 (1995). [27] R.S. Barbi~ri, J.C. Rocha, V.R. Terra and A. Marques Neto, Eclat. Quire., 14 (1989) 101. [28] K.L. Cheng, Talanta, 2 (1959) 61. [29] F.L. Moore, Anal. Chem., 28 (1956) 947. [30] Z.P.G. Peralta, Tese de Doutorado, Instituto de Quimica Universidade Estadual de Campinas, Campinas-SP (Brazil), 1995.
Talanta 44 (1997) 811 816
Zirconium and hafnium determination by energy dispersive X-ray fluorescence with solid phase preconcentration Patricio Peralta-Zamora *, Lorena Cornejo-Ponce, Maria Izabel Maretti S. Bueno, Jos6 Walter Martins Universidade Esta&lal de Campinas, lnstituto de Quhnica, CP. 6154. CEP 13083-970. Campinas-SP, Brazil
Received 20 May 1996; accepted 25 September 1996
Abstract
Two preconcentration methods has been developed for simultaneous determination of zirconium and hafnium by energy dispersive X-ray fluorescence (EDXRF). The first method is a liquid-solid extraction procedure with the use of an anionic exchange resin modified with xylenol orange. The second is a precipitation procedure carried out in the presence of lanthanum. Both methods permit significant enhancement of sensitivity in comparison with direct measurement in the aqueous phase. The applicability of both procedures for the preconcentration of Zr and Hf prior to their determination by EDXRF was demonstrated by analyzing synthetic mixtures and a sample of zirconium ore. The results obtained with the use of the modified resin show relative standard deviation of about 4% and good agreement with those obtained by spectrographic analysis. © 1997 Published by Elsevier Science B.V. Kevwordx: Hafnium; Preconcentration; X-ray fluorescence; Zirconium
I. Introduction
The simultaneous determination of zirconium and hafnium is one of the most difficult problems of analytical chemistry, because the close similarity of their chemical properties hinders the use of classical analytical methods [1-5]. Colorimetric techniques do not have any interesting application due to unavailability of specific chromogenics agents. The principal contribution of these techniques is reduced to the existence of several differential methods, based on differences between *Corrcsponding author.
zirconium and hafnium complexes under different acidic conditions [6] or in the presence of specific masking agents [7]. The classic configuration of flame atomic absorption or emission techniques shows little application for this determination due to the heat-resistant characteristics of both species and the high complexity of the emission spectra [1-4]. The use of more energetic atomization systems (i.e., plasma, arc and spark) permits more suitable results only for determination of the concentration ratios [1]. Absolute determinations can be performed by using special calibration methods, typically chemometric tools for correction of mutual spectral interferences [1]. The individual de-
0039-9140/97/$17.00 ~" 1997 Published by Elsevier Science B.V. All rights reserved. PII S00 39-9140(96 )02117-0
812
P. Peralta-Zamora et al./ Talanta 44 (1997) 811 816
termination of zirconium in silicate rocks by inductively-coupled plasma emission spectrometry were reported by Uchida et al. [8]. However, the results show poor precision (relative standard deviation, RSD, between 14 and 38%). Due to the great spectral resolution of the instrumental system, energy dispersive X-ray fluorescence permits the determination of Zr-Hf mixtures without important interference problems. However, the low sensitivity of the technique can be resolved only by the use of preconcentration methods [9-21]. The determination of a great number of metallic species by EDXRF is usually preceded by concentration steps which involve the use of ion exchange resins [9,10,12,16], activated charcoal [9,10,17], naphthalene [14], polyurethane [15], modified silica [21] and coprecipitation procedures using ferric hydroxide as a collector [18-20]. The great selectivity reached by the use of chelating resins [22] has contributed to its popularization as solvents system. Xylenol orange has been used for modification of ionic exchanger resins, and for determination of zirconium by solid phase spectrophotometry [23-25]. In this work we propose the use of an ion exchange resin modified with xylenol orange or a lanthanum coprecipitation procedure for preconcentration and simultaneous determination of zirconium-hafnium mixtures by EDXRF.
2. Experimental 2.1. Instrumental
Spectrophotometric determinations were performed with an Intralab DMS-100 UV-Vis Spectrophotometer. X-ray fluorescence measurements were executed on a Spectrace 5000 Energy Dispersive X-ray Fluorescence Spectrometer, equipped with a rhodium tube and a Si(Li) semiconductor crystal detector (irradiation time, 60 s; tube voltage, 30 kV; tube current, 0.20 mA; atmosphere, air; filter, 0.127 mm rhodium film). The measurements were realized at 15.744 KeV for Zr (Kal) and 7.898 KeV for Hf (Lal). The samples were introduced on a cylindrical plastic device
from Chemplex, of about 2 cm of height and 3 cm of internal diameter, using a X-ray Mylar film as sample support. Spectrographic measurements were carried out on Carl Zeiss PGS-2 Emission Spectrometer, equipped with a linear array of photodiodes [26]. 2.2. Chemicals and solutions
The zirconium standard solution was prepared by direct dissolution of ZrOC12. 8H20 (99.5%, Riedel-de Haen) in H2SO 4 (1:5 v/v). Zirconium hydroxide was precipitated with an aqueous solution of NH 3. The solid was recovered by filtration, thoroughly washed with deionized water and dissolved in HC1 (final concentration of HC1, about 2 mol 1-l). The hafnium standard solution was prepared from H f O 2 (Specpure, Johnson, Mathey and Co.). The oxide was dissolved with a HFH2SO4 (1:1 v/v) mixture, heating several times almost to dryness with successive additions of concentrated H2SO 4. The solid residue was dissolved in H2SO 4 and the hydroxide precipitated with an aqueous solution of NH 3. The solid was recovered by filtration, thoroughly washed with deionized water and dissolved in HC1 (final concentration of HC1, about 2 mol 1 ~). Both solutions were standardized by a gravimetric procedure using mandelic acid [27]. The lanthanum standard solution (40 g I ~) was prepared from La203 (99%, Carlo Erba). Amberlite IRA-400 anion resin (Koch-Light Laboratories, 100-200 mesh) and Xylenol orange (Fluka A.G., Bucks S.G.) were used without further purification. The natural sample, purchased from the Energetic and Nuclear Research Institute (IPEN, $5,o Paulo-Brazil), corresponds to a mixture of zirconium and hafnium hydroxides obtained by physical and chemical treatment of a zirconium ore. 2.3. Preparation o f modified resin
Dried resin, 40 g, were added to 400 ml of an aqueous solution of xylenol orange (4.3 x 10 - 3 tool 1- ~, pH 7.0) and the mixture was vigorously stirred for 6 h. After this impregnation time, the
813
P. Peralta-Zamora et al./ Talanta 44 (1997) 811 816
solid mass was filtered, thoroughly washed with deionized water, dried at 60°C and stored in amber glass flasks.
2.4. Extraction with modified resin
Suitable volumes of solution of Zr and H f were placed in 200 ml beakers, diluted up to 50 ml with deionized water and adjusted to adequate pH values with aqueous solutions of HC1 or NH3. To these solutions were added 0.5 g of the modified resin. The mixture was stirred for a convenient time and the solid was recovered by filtration in Whatman 41 filter paper, washed with deionized water and dried at 60°C for 2 h. In preliminary studies, the Zr and H f recuperation was evaluated by colorimetric determination [28] of the remained amounts in the filtrate. For this determination, the filtrate (approximately 50 ml) was led to almost dryness in a water bath, dissolved in 3.0 ml of 0.3 m o l l i HC1, and added of 2.0 ml of xylenol orange 0.05% in HC1 0.3 mol 1 ~. The concentration of zirconium or hafnium was determined spectrophotometrically at 550 nm, using analytical curves obtained between 0.2 and 2.0 mg 1 ~. Further determinations were carried out by direct analysis of the solid mass by EDXRF. The concentration of Zr and H f was determined through external calibration using standard solutions submitted to analogous extraction procedures.
3. Results and discussion 3.1. Impregnation o f the resin
Colorimetric determination of the remaining amount of xylenol orange after the impregnation procedure verified that the efficiency of the impregnation process is very close to 100%. Relating the amounts used of resin (0.5 g) and the xylenol orange amount present in this solid fraction (0.043 ml tool!, it is possible to estimate a maximum extraction capacity of about 2000 ~tg for Zr or 3500 tag for Hf. 3.2. Extraction with modified resin
The extraction of both species is almost complete at similar pH values (pH 4.0 8.0). Although the zirconium and hafnium hydrolysis is favored at these pH values, precipitation was not observed probably due to the low concentration of the analytes used in this study (approximately 2 tag ml- 1) and due to the presence of large amounts of sulfate ion, which can displace the hydroxyl groups from hydrolyzed species [1]. The procedure is not selective in this pH region, a result resembling those obtained by the application of other extraction procedures [29,30]. For subsequent determinations, a pH of 7.0 was selected. A X-ray fluorescence spectrum of zirconium and hafnium on the xylenol orange modified resin is presented in Fig. 1.
Z.r (~,~)
2.5. Preconcentration by precipitation in the presence o f lanthanum 1500
Aqueous solutions, 50 ml, containing adequate amounts of Zr and H f were added to 10 ml of the standard lanthanum solution. The hydroxides were precipitated by addition of an aqueous ammoniacal solution. After 5 min of stirring the precipitate was recovered by filtration and dried at 100°C for 2 h. The recuperation degree was evaluated by colorimetric analysis in the filtrate. Further determinations by E D X R F were directly carried out on the solid fraction.
1000
' ..~J+'] 500 Hf
6
(L~) 8
1
12
1
16
Energy(keY) Fig. 1. X-ray fluorescencespectrum of zirconium and hafnium on the xylenol orange modified resin.
814
P. Peralta-Zamora et al./ Talanta 44 (1997) 811-816
Table 1 Zr-Hf determination from synthetic samples with modified resin Added (rag)
Found (mg)
Zr
Hf
Zr (difference, %)
Hf (difference, %)
10.0 50.0 100.0 100.0 100.0 500.0 Average difference RSD (n = 5)
50.0 50.0 50.0 100.0 300.0 100.0
9.7(3.0j 49.1(1.8) 100.8(0.8) 101.2(1.2) 101.9(1.9) 497.3(0.5) 1.5% 3.0%
51.5(3.0) 52.3(4.6) 52.5(5.0) 100.7(0.7) 303.1(1.0) 105.9(5.9) 3.4% 4.O%
The quantitative recuperation of Zr and Hf is completed with stirring times of about 15 min. The complete extraction of the analytes in a short time like this, and the great convenience of the involved operations, implies a very important economy of time, mainly when compared with the times required for other preconcentration procedures. Liquid-liquid extraction procedures and liquid-solid methodologies executed in column systems, for example, are slower techniques because of the difficulty in reaching equilibrium conditions and long time elution stages, respectively. For posterior determinations stirring times of 20 min were selected. Using sample volumes between 50 and 200 ml, the extraction of Zr and Hf is quantitative. With higher volumes the recuperation of the analytes is decreased, presumably due to excessive dilution of the solid extractor and consequently inefficient contact of the phases during the selected stirring time. If we consider the complete transference of the Zr or Hf amounts present in this high volume of aqueous phase for a small mass of resin, it is possible to perceive the great preconcentration potentiality of the proposed method. For subsequent determinations maximum volumes of 200 ml were selected. Is known that the particle size have a great importance on the X-ray fluorescence measurements, nevertheless preliminary studies shown that for ours experimental conditions insignificant differences are obtained by using resin of size equivalent to 41-60 or 150-190 mesh.
Executing the extraction procedure in the optimized conditions and measuring the EDXRF emission directly on the solid mass, linear calibration curves are obtained for a large concentration range (10-800 lag for zirconium, and 50 800 lag for hafnium) with typical correlations higher than 0.999. Considering the sensibility of the determination as a function of the inclination of the analytical curves, we observed that the proposed method permits sensitivity increases of about 25 times for Zr and 40 times for Hf, in relation to direct measurement in the aqueous phase.
3.3. Analysis of synthetic samples The results obtained for synthetic mixtures of diverse composition (Table l) indicate high concordance between added and found amounts. The precision of the methodology, expressed as relative standard deviation for five determinations (about 3% for Zr and 4% for Hf), is very similar to that obtained by other techniques [9,10].
3.4. Precipitation in the presence of lanthanum This method appears as a very simple alternative for preconcentration of metallic species because it depends on but a few experimental variables. The added amount of lanthanum, for example, is not a critical parameter. The selected value was defined only to facilitate the filtration and transference operations and as a function of the capacity of the sample compartment. Prelimi-
815
P. Peralta-Zamora et al. / Talanta 44 (1997) 811 816
Table 2 Zr-Hf determination from synthetic samples by precipitation in the presence of lanthanum Added (rag)
Found (rag)
Zr
Hf
Zr (difference, %)
Hf (difference, %)
100.0 100.0 100.0 300.0 500.0 Average difference RSD (n = 5)
100.0 150.0 200.0 200.0 200.0
100.8(0.8) 98.7(1.3) 99.5(0.5 ) 297.4(0.9) 498.7(0.3) 0.8 4.0%
101.2(1.2) 150.9(0.6) 201.9(1.0) 203.4(1.7) 205.6(2.8) 1.5% 4.0%
nary experiments indicated 300 ml as a m a x i m u m sample volume; with higher volumes the recuperation of the metallic species is incomplete. The calibration curves show good linearity (r > 0.999) over a large concentration range (300-1000 gg ml %. However, the sensitivity is lower than that obtained by extraction on the modified resin. In any case, the sensitivity is enhanced about 4 times in relation to the directly measurement in the aqueous phase. The results for the Z r - H f determination from synthetic mixtures (Table 2) show high coherence with the added amounts, with typical standard deviations of about 4%. The accuracy of the proposed method is better than those obtained by Ricci [18] (5-10%) through a coprecipitation procedure using ferric hydroxide as a collector agent. 3.5. Z r - H J determination fi'om natural samples
Both proposed preconcentration procedures were evaluated for determination of Z r - H f mixtures from natural samples, which result from
physical and chemical treatments of zirconium ore. The results for zirconium determination using the modified resin and the coprecipitation procedure (Table 3) show a high coherence with those obtained by spectrographic analysis. By applying the statistical Student's test for 95% of confidence, we verify that for hafnium determination, only the modified resin provides comparable results. The results obtained by the coprecipitation procedure shows significant differences with those obtained by spectrographic analysis and by using the modified resin.
4. Conclusions Both preconcentration procedures permit the quantitative recuperation of Zr and H f from large volumes of aqueous samples. For this motive they represent a potential and simple alternative for determination of metallic species by E D X R F . The procedure based on the use of a modified resin permits a significant increase of the sensitivity of
Table 3 Zr-Hf determination from zirconium ore using the proposed methods Run
I 2 3 Average RSD (%)
Extraction with modified resin (%)
Precipitation (%)
Spectrography (%)
Zr
Hf
Zr
Hf
Zr
[~t"
95.4 95.2 95.6 95.4 + 0.4 0.2
4.6 4.8 4.4 4.6 + 0.4 4.3
96.5 97.1 96.7 96.8 i 0.6 0.3
3.5 2.9 3.3 3,2 _+0.6 9.5
95.0 95.3 95.2 95.2 ± 0.3 0.2
5.0 4.7 4.8 4.8 + 0.3 3.2
816
P. Peralta-Zamora et a l . / Talanta 44 (1997) 811-816
the determination and can be used in analyses of low concentration samples. The non-selectivity of the extraction procedure with the use of modified resin is not a significant inconvenient, because can be compensate for the great selectivity presented for the EDXRF technique.
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