Chromium belongs to trace elements for which strict maximum permissible concentrations in natural water are prescribed. Highly toxic Cr(VI) (MPC = 0.05 mg/L) ...
Journal of Analytical Chemistry, Vol. 57, No. 3, 2002, pp. 194–197. Translated from Zhurnal Analiticheskoi Khimii, Vol. 57, No. 3, 2002, pp. 240–243. Original Russian Text Copyright © 2002 by Eksperiandova, Fokina, Blank, Grebenyuk.
ARTICLES
Simultaneous Preconcentration and Separation of Chromium(III) and Chromium(VI) at the Low-Temperature Directed Crystallization of Natural Water L. P. Eksperiandova, I. I. Fokina, A. B. Blank, and N. N. Grebenyuk Institute for Single Crystals, National Academy of Sciences of Ukraine, pr. Lenina 60, Kharkov, 61001 Ukraine Received May 29, 2001
Abstract—It was shown that low-temperature directed crystallization of natural water can be used for separating chromium(III) and chromium(VI). At a moderate crystallization rate (6–10 mm/h), the final part of the ingot (concentrate) is a suspension containing chromium(III) and chromium(VI) in the solid and liquid phases, respectively. It is proposed to separately determine these chromium species by atomic absorption spectrometry after separating the phases by centrifugation and dissolving the precipitate in acids.
Chromium belongs to trace elements for which strict maximum permissible concentrations in natural water are prescribed. Highly toxic Cr(VI) (MPC = 0.05 mg/L) and less toxic Cr(III) (MPC = 0.5 mg/L) are its most stable forms. Because contaminants undergo constant transformations in ecosystems, it is important to control the ratio of these chromium forms. Total chromium is usually determined by atomic absorption spectrometry (AAS) [1] and mass spectrometry (MS) [2]; voltammetry [3] and UV and visible spectrophotometry [4] are used for the separate determination of chromium forms. In the latter method, diphenyl carbazide is usually used [5]. However, the method is laborious and involves the successive determination of Cr(VI), oxidation of Cr(III) to Cr(VI), and the determination of total Cr(VI). Preconcentration of impurities is used to enhance the sensitivity of chromium determination. Chemical methods of group preconcentration (chromatography [2, 6], sorption [7, 8], coprecipitation [9], solvent extraction [10], and capillary electrophoresis [11]) are prevalent in the analysis of waters. Chromatography is usually combined with MS [2] or spectrophotometry [6], sorption with spectrophotometry [8], coprecipitation with AAS [9], extraction with spectrofluorimetry [10], and capillary electrophoresis with spectrophotometry [12]. In some cases, chromium preconcentration is accompanied by the separation of its forms (e.g., when sorption on alumina [13] or zinc oxide [14], chromatography [15], or extraction [16] is used). In the above works, the concentrate was analyzed by AAS, which was used for determining total chromium. AAS is suitable, because dissimilar chromium forms can be determined from different parts of the same concentrate. Chemical preconcentration methods have some disadvantages. Physical methods are more preferable, because they are not selective to different impurities and their background level is low because of the
absence of reagents. Among these techniques is preconcentration by directed crystallization [17, 18]. In this work, the behavior of Cr(III) and Cr(VI) was studied in the low-temperature directed crystallization of water, and a combination of this technique with the subsequent atomic-absorption determination of chromium was used. EXPERIMENTAL Crystallization preconcentration. Directed crystallization of water samples was performed from bottom to top, beginning from seeding, using a unit described previously [18]. A water sample was placed in a glass container (~100 mL), which was fixed in a holder and moved down at a speed of 10 mm/h from a heated zone to a cold thermostated chamber with a liquid antifreeze cooled to –15°C with the evaporator of a compression-type refrigerator. The solution over the growing ice ingot was stirred by the reversing rotation of the container around its vertical axis. When the crystallization was completed, 1 mL of boiling twice-distilled water was rapidly placed in the container with the ingot to sample the concentrate. When the upper part of the ingot melted, the concentrate obtained was transferred to a centrifuge tube and weighed. The components of the suspension were separated by centrifugation, and the precipitate and supernatant were analyzed separately (the precipitate was predissolved in 1 mL of HNO3 (1 : 1) and diluted with distilled water to 10 mL). Concentrate analysis. Electrothermal atomic absorption spectrometry was used for determining chromium. Measurements were performed on a Saturn spectrophotometer with a Grafit-2 atomizer. Standard graphite furnaces were used; a Perkin-Elmer hollow cathode lamp served as the light source; amplitudes of absorption signals were recorded at 357.9 nm. A 10-µL
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SIMULTANEOUS PRECONCENTRATION AND SEPARATION Intensity
RESULTS AND DISCUSSION We have shown previously that the low-temperature directed crystallization of water and weakly saline aqueous solutions can be used for the efficient group preconcentration of different impurities (cations of alkali, alkaline-earth, and heavy metals and their complexes; anions; organic impurities; and particles of heterogeneous systems) [21–23]. The distribution of impurities is described by the Rayleigh–Pfann equation, and the efficiency of their extrusion is estimated by the characteristic partition coefficient kL [18]. In pure water at a moderate crystallization speed (10 mm/h), kL varies between 0.05 and 0.10 for all impurities, and a 100-fold concentration can be attained. In natural water, kL depends on the total salt content; it is about 0.20–0.25 at a crystallization speed of 10 mm/h, which corresponds to the 40-fold concentration.
Ca
O Si
C
Na Mg
195
Ca S Cl
Cu Zn
Wave length Fig. 1. X-ray spectrum of a precipitate obtained after the centrifugation of a suspension (concentrate) and drying recorded on a JSM-820 scanning electron microscope combined with a Link AN10/85S system of X-ray microanalysis.
portion of a test or a calibration solution was introduced into the furnace using a micropipette. The measurement cycle involved the following steps: drying of the solution (at 80°C for 30 s with gradually increasing the temperature to 200°C for 30 s); preliminary thermal treatment (1100°C for 30 s); atomization (2600°C for 6 s); and cleaning of the furnace (2700°C for 3 s). A series of calibration solutions was prepared from the preconcentrated reference solution passes through all preconcentration steps. This solution contained macrocomponents (in amounts corresponding to those in the water to be analyzed) and chromium in the specified concentrations. The results obtained were statistically processed at the confidence level P = 0.95 in accordance with the recommendations published in [19, 20].
It was found that when pure water was crystallized, a small nontransparent zone containing dissolved gases, which were not released into the atmosphere, appeared at the end of the ingot. In this case, the upper part of the ingot (concentrate) became a suspension when melted. According to the data of X-ray microanalysis, the solid phase contained appreciable amounts of calcium and silicon (Fig. 1); X-ray diffraction analysis showed that the precipitate consisted mainly of calcium carbonate (in the form of calcite) deposited because of the extrusion of impurities dissolved in water by the crystallization front (Fig. 2). The liquid phase of the suspension represented a concentrate of impurities dissolved in water. The observed separation of the concentrate into the solid and liquid phases after the low-temperature direct crystallization can be used for the simultaneous preconcentration and separation of impurities. In particular,
Intensity CaCé3
CaCé3
Wave length Fig. 2. Diffraction pattern of a precipitate obtained after the centrifugation of suspension (concentrate) and drying recorded with a Siemens D 500 diffractometer (Cu Kα radiation). JOURNAL OF ANALYTICAL CHEMISTRY
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EKSPERIANDOVA et al.
Testing the procedure for the AAS determination of Cr(III) and Cr(VI) at different ratios after the preconcentration of natural water by low-temperature directed crystallization Cr(III), mg/L added, mg/L (xi) found, mg/L (yi) 0.040
0.075
0.19
0.34
0.044 0.044 0.043 0.068 0.078 0.078 0.17 0.18 0.18 0.32 0.31 0.32
Cr(VI), mg/L zi
added, mg/L (xi) found, mg/L (yi)
0.91 0.91 0.93 1.10 0.96 0.96 1.12 1.05 1.05 1.06 1.10 1.06
0.43
0.47
0.092
0.10
0.42 0.45 0.43 0.44 0.48 0.46 0.090 0.097 0.095 0.09 0.09 0.09
zi = xi /yi ; number of experiments n = 12; z = 1.02;
sr = 0.08;
sr = 0.06;
t = 1 – z n /s r = 0.79 < 2.20
t = 1 – z n /s r = 1.10 < 2.20
In the concentration ranges studied (0.04–1.6 mg/L for Cr(III) and 0.07–2.5 mg/L for Cr(VI)) at different ratios (from 1 : 10 to 3 : 1), kL in natural water was 0.24 and 0.26, respectively. This agrees with our previous conclusion about the dependence of kL on the salt content of water. If necessary, kL can be decreased by several times by performing the crystallization at a lower speed (1–2 mm/h). Note that the separation of the concentrate (suspension) into the solid and liquid phases is accompanied by the additional preconcentration of Cr(III). The accuracy and precision of the AAS procedure with preconcentration by crystallization were tested on model mixtures by the added–found method.
0.09
1.02 0.95 1.00 1.07 0.98 1.02 1.02 0.95 0.97 1.11 1.11 1.11
zi = xi /yi ; number of experiments n = 12; z = 1.02;
we showed that a mixture of Cr(III) and Cr(VI) was concentrated along with the matrix components of natural water and separated into Cr(III), which probably formed a solid solution with the precipitate, and Cr(VI), which was retained in the liquid phase of the concentrate. After the concentrate was separated into the solid and liquid phases and the precipitate was dissolved in a mineral acid, Cr(III) (after oxidation) and Cr(VI) could be determined separately using the known procedure based on the color reaction of Cr(VI) with diphenyl carbazide [5], or Cr(III) and Cr(VI) could be determined by AAS. The latter method is preferable, because it requires no additional chemical treatment of the concentrate. Note that, when low-temperature directed crystallization was used for preconcentration, the reduced temperature decelerated the chemical transformations of the impurities, including the interconversion of chromium species during sample preparation.
Cr(III)/Cr(IV)
zi
0.16
2.06
3.40
The results of tests for different Cr(III) : Cr(VI) ratios are given in the table. It can be seen that different Cr(III) : Cr(VI) ratios hardly affect the determination error; systematic errors are insignificant, and random errors are characterized by the RSD values admissible for the determination of small impurity concentrations. The fact that random errors were similar in the determination of Cr(III) and Cr(VI) allowed us to state that the results of AAS measurements for the solid and liquid phases belong to the same general population. The limit of the AAS determination of Cr(III) and Cr(VI) was 0.003 mg/L, which is about 20 and 150 times lower than the MPCs of these impurities in the natural water for Cr(VI) and for Cr(III), respectively. ACKNOWLEDGMENTS We thank T.S. Teplitskaya and P.V. Mateichenko for their assistance in X-ray diffraction analysis and X-ray microanalysis. REFERENCES 1. Rubio, R., Sahuqullo, A., Rauret, G., and Quevauviller, P., Int. J. Environ. Anal. Chem., 1992, vol. 47, no. 2, p. 99. 2. Pantsar-Rallio, M. and Manninen, P.K.G., Anal. Chim. Acta, 1996, vol. 318, no. 3, p. 335. 3. De-Souzo, E.M., De-Luca, A., Wagener R., and Farias, P., Croat. Chim. Acta, 1997, vol. 70, no. 1, p. 259. 4. Gao, R.M., Liu, H.G, and Zhao, Z.O. Lihua Jianyan, Xuaxue Fence, 1995, vol. 31, no. 5, p. 304.
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SIMULTANEOUS PRECONCENTRATION AND SEPARATION 5. Umland, F., Janssen, A., Thierig, D., and Wunsch, G., Theorie und Praktische Anwendung von Komplexbildnern, Frankfurt am Main: Akademische Verlagsgesellschaft, 1971. Translated under the title Kompleksnye soedineniya v analiticheskoi khimii, Moscow: Mir, 1975, p. 416. 6. Bittner, M. and Broekaert, J.A.C., Anal. Chim. Acta, 1998, vol. 364, nos. 1–3, p. 31. 7. Lysenko, A.A., Khramkova, N.V., and Turkin, E.I., Teor. Prakt. Sorbts. Protsess., 1998, no. 23, p. 59. 8. Manzoori J.L., Sorouraddin M.H., and Shemirani F., Anal. Lett., 1996, vol. 29, no. 11, p. 2007. 9. Ueda J., Satoh, H., and Kagaya, S., Anal. Sci., 1997, vol. 13, no. 4, p. 613. 10. Andres-Garcia, E. and Blanco-Gomis, D. Analyst (Cambridge, U.K.), 1997, vol. 122, no. 9, p. 899. 11. Jung, G.Y., Kim, Y.S., and Lim, H.B., Anal. Sci., 1997, vol. 13, no. 3, p. 463. 12. Himeno, S., Nakashima, Y., and Sano, K.I., Anal. Sci., 1998, vol. 14, no. 2, p. 369. 13. Manzoori, J.L., Sorouraddin, M.N., and Shemirani, F., Talanta, 1995, vol. 42, no. 8, p. 1151. 14. Sahauam, A.C., Arunachalam, J., and Gangadharan, S., Can. J. Anal. Sci. Spectrosc., 1998, vol. 43, no. 1, p. 4.
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15. Gaspar, A., Posta, J., and Toth, R., J. Anal. Atom. Spectrosc., 1996, vol. 11, no. 11, p. 1067. 16. Korenman, Ya.I., Kopach, S., Kalembkievich, Ya., et al., Zh. Anal. Khim., 2000, vol. 55, no. 1, p. 31. 17. Zolotov, Yu.A. and Kuz’min, N.M., Kontsentrirovanie mikroelementov (Preconcentration of Trace Elements), Moscow: Mir, 1982, p. 107. 18. Blank, A.B., Analiz chistykh veshchestv s primeneniem kristallizatsionnogo kontsentrirovaniya (Analysis of Pure Substances Using Crystallization Preconcentration), Moscow: Khimiya, 1986. 19. Doerffel, K., Statistik in der analytischen Chemie, Leipzig: Grundstoffindustrie, 1990, 5 ed. Translated under the title Statistika v analiticheskoi khimii, Moscow: Mir, 1994. 20. Komar’, N.P., Zh. Anal. Khim., 1952, vol. 7, no. 6, p. 325. 21. Eksperiandova, L.P., Blank, A.B., and Fokina, I.I., Fresenius’ J. Anal. Chem., 1998, vol. 361, p. 287. 22. Eksperiandova, L.P., Fokina, I.I., Blank, A.B., et al., Anal. Chim. Acta, 1999, vol. 396, p. 317. 23. Fokina, I.I., Eksperiandova, L.P., and Blank, A.B., Vestn. Khar’k. Univ., Ser. Khim., 1998. no. 420, issue. 2, p. 93.
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