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Preconcentration and speciation of chromium in waters using solid-phase extraction and atomic absorption spectrometry
Talanta 51 (2000) 531 – 536 www.elsevier.com/locate/talanta
Preconcentration and speciation of chromium in waters using solid-phase extraction and atomic absorption spectrometry D.M. Adria´-Cerezo, M. Llobat-Estelle´s, A.R. Maurı´-Aucejo * Department of Quı´mica Analı´tica, Faculty of Chemistry, Uni6ersity of Vale`ncia Estudi General, Dr. Moliner 50, 46100 Burjassot, Vale`ncia, Spain Received 10 June 1999; received in revised form 12 October 1999; accepted 14 October 1999
Abstract A method for the preconcentration and speciation of chromium was developed. After formation of an anionic compound with ethylenediaminetetraacetic acid (CrY−), Cr (VI) and Cr (III) are retained on a strong anionic phase (SAX) and controlled elution with 0.5 M NaCl permits their speciation. The retention and elution conditions were optimised, and interferences due to the presence of other ions such as Mg(II), Mn(II), Sn(II), Fe(III), Ba(II), Al(III), Ca(II), chloride, iodine, bromide, fluoride, sulphate, phosphate, bicarbonate and nitrate were studied. The detection limits were 0.4 mg l − 1 and 1.1 mg l − 1 for Cr(III) and Cr(VI), respectively, and reproducibility was 9%. The results obtained for speciation of chromium by the proposed method in wastewaters are in agreement with the values obtained by a reference method for a 95% confidence level. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Chromium; Speciation; Water; Preconcentration
1. Introduction Toxicological studies have shown that the degree of toxicity of some elements depends on the chemical form in which the element is present. Chromium(III), for example, is considered an essential micronutrient for humans, whereas chromium(VI) is a potentially carcinogenic agent
* Corresponding author. Tel.: +34-96-386-4497; fax: +3496-386-4436. E-mail address: [email protected] (A.R. Maurı´-Aucejo)
[1]. It is therefore necessary to control the level of chromium in wastewater, natural water and drinking water. Many countries have developed laws along this line, but since the legislation permits a larger content of Cr(III) than Cr(VI), speciation of chromium in environmental samples is very important. Several methods for speciation of chromium using atomic spectroscopy have been investigated. These methods include previous preparation of a sample by liquid-liquid extraction after formation of a complex [2,3], solid-liquid extraction [4–7] or
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532
D.M. Adria´ -Cerezo et al. / Talanta 51 (2000) 531–536
a direct coupling of liquid chromatography with atomic spectroscopy [8 – 16]. The use of solid phase extraction permits simultaneous preconcentration and separation of Cr(III) and Cr(VI). Several studies have used a solid phase for retention of Cr(III) or Cr(VI) and determination of total chromium by a previous oxidation or reduction [2 – 4,6]. Other authors have proposed the use of two different solid phases [7]. The use of an anionic phase for retention of Cr(III) implies a previous step to form an anionic compound. Ethylenediaminetetraacetic acid (EDTA) is an excellent chelating reagent that combines with Cr(III) at a 1:1 ratio. Jen-Fon Jen et al. [16] propose a method for speciation of chromium by reversed-phase ionicpair HPLC after formation of a complex with EDTA (CrY−) and formation of an ionic-pair with tetrabutylamonium. However, the method does not involve preconcentration and the detection limits are 0.02 and 0.08 mg l − 1 for Cr(III) and Cr(VI), respectively. In the present study, we have developed a method for speciation of chromium using a strong anionic phase to separate Cr(VI) and Cr(III) as CrY−. We have optimised the retention and elution conditions for simultaneous preconcentration and fractionated elution of Cr(III) and Cr(VI) and determination by AAS.
2. Experimental
2.1. Apparatus and reagents A Perkin Elmer 5000 atomic absorption spectrometer equipped with a chromium hollowcathode lamp was used for the absorbance measurements. A Vac elut 20 connected to a diaphragm vacuum pump (KNF model N 026.3 AN.18) was employed for treatment of samples. The solid-phase extraction was carried out using Lichroelut Cartridges (Merck) containing 500 mg of trimethylaminopropyl chloride (SAX). All reagents were analytical grade.
2.2. Instrumental parameters Absorbance measurements were carried out at 357.9 nm in the instrumental conditions that provide the best sensitivity and no difference between the Cr(III) and Cr(VI) signals. These conditions were 15 mm of burner height and a Qacetylene/Qair ratio of 0.09. The flow rate of samples through cartridges was 3 ml min − 1 with a vacuum of − 0.18 Bar.
2.3. Proposed procedure The following procedure for speciating chromium is proposed: First, HAc/Ac− buffer (1:10 v/v) and 0.3 ml of EDTA 0.1 M are added to an adequate aliquot of sample containing 1.5 mg of Cr(III) and/or 3 mg of Cr(VI). On the other hand, solutions containing 0–3 mg of Cr(III) and 0–6 mg of Cr(VI) with conductivity identical to the sample are prepared by addition of NaCl. Then, HAc/ Ac− buffer (1:10 v/v) and 0.3 ml of EDTA 0.1 M are added to volumes of these solutions identical to the aliquots of considered samples. All solutions (standards and samples) are heated in a bath at 80°C for 10 min or in a microwave oven for 75 s and introduced into the cartridges, previously activated with 2 ml of methanol and washed with 2 ml of buffer HAc/ Ac−. By using NaCl 0.1 M Cr(III) is eluted in the first millilitre, the second millilitre is discarded and Cr(VI) is eluted in the third and fourth millilitre. Alternatively, a method for preconcentrating and determining Cr(VI) without interference of Cr(III) is possible. The procedure is identical to the speciation procedure but EDTA is not added and the solutions are not heated. In this one, the 1st and 2nd ml of eluent is discarded because part of Cr(III) can be in another anionic form [17] and also to avoid dilution of Cr(VI).
D.M. Adria´ -Cerezo et al. / Talanta 51 (2000) 531–536
533
Table 1 Tolerance limits Interferent
I−
Br
Interferent /chromium molar ratio
41
65
−
F
−
2.7
PO3− 4
HCO− 3
SO2− 4
NO− 3
Cl−
Mg(II)
Mn(II)
Sn(II)
Al(III)
Fe(III)
Ca(II)
Ba(II)
55
128
95
58
427
214
95
0.44
19
9
390
0.38
3. Results and discussion
3.1. Retention and elution conditions 3.1.1. Formation of CrY− The use of anionic cartridges to retain Cr(III) implies the formation of an anionic form of Cr(III) and control of the pH.
Fig. 1. Variation of recovery with concentration of NaCl by elution with 4 ml and with volume of NaCl 0.5 mol l − 1.
The influence of pH, EDTA concentration, time and temperature of warming on the formation of CrY− was evaluated. The procedure used was as follows: different amounts of EDTA and 2 ml of buffer were added to 20 ml of sample containing 60 mg l − 1 of chromium. Then the mixture was warmed in a bath and introduced into the cartridge previously activated with 2 ml of methanol and washed with 2 ml of buffer solution. Finally, chromium was eluted with 4 ml of NaCl 0.5 M, and the absorbance value obtained. The amount of EDTA plays an important role because in its absence, Cr (III) is not retained. A molar EDTA/Cr(III) relation between 400 and 1300 is needed for quantitative formation of CrY−. On the other hand, the variation in the pH between 4 and 9 does not affect the recovery of Cr(III). The reaction between Cr(III) and EDTA is very slow at room temperature, but the formation of the complex was favoured at higher temperatures. If the sample was warmed at 80°C at pH 4.7, only 10 min was necessary for the quantitative formation of CrY−. In order to decrease the time analysis, the use of a microwave oven for the formation of CrY− was studied. In this case only 75 s at medium power were needed for the quantitative formation of the complex.
3.1.1.1. Stability of Cr(VI). To test the possible reduction of Cr(VI) during complex formation, the same study was applied to Cr(VI). Our results show that Cr(VI) is not altered during the derivatization process (warming with a bath and with microwave oven). On the other hand, the recovery of Cr(VI) is identical between pH 4 and 9.
D.M. Adria´ -Cerezo et al. / Talanta 51 (2000) 531–536
534
3.1.2. Elution conditions A solution of NaCl was chosen as eluent. The volume and concentration of the eluent were optimised to obtain a maximum recovery of chromium. For this study, aliquots of 25 ml containing 0.24 mg l − 1 of chromium were retained using the above conditions Fig. 1 shows the variation in the recovery of chromium depending on the volume and concentration of the eluent. As can be seen, Cr(III) is eluted faster than Cr(VI). Therefore, it is possible to determine Cr(III) in the first millilitre of eluent, and Cr(VI) in the third and fourth millilitre without interference of Cr(III). 3.2. Study of ion interferents The interference of different ions, which is common in water samples, in the recovery of Cr(III) and Cr(VI) has been studied. The influence of anions on retention was studied because the solid phase is a strong anionic exchanger. On the other hand, the influence of several cations, which in the presence of EDTA at pH 4.7 are in the anionic form, was also tested. The interferents studied are: Mg(II), Mn(II), Sn(II), Fe(III), Ba(II), Al(III), Ca(II), chloride, iodine, bromide, fluoride, sulphate, phosphate, biTable 2 Composition of samplea Compound
Concentration (mol l−1)
CaCO3b MgCl2 Feb SnCl2b Al(NO3)3 NaI KBr NaCl NaNO3 NaHCO3 Na2SO4 BaCO3b K2CrO4 Cr(NO3)3
4.9×10−3 4.1×10−2 1.8×10−5 8.4×10−7 3.7×10−5 7.9×10−5 1.2×10−4 4.0×10−2 8.05×10−4 2.3×10−3 3.1×10−3 7.3×10−7 1.77×10−6 1.15×10−6
a b
Conductivity 9.64 mS. Disolved in HCl.
Fig. 2. Variation of recovery with conductivity and volume of sample. Cr(III) measured in the 1st ml and Cr(VI) in the 3rd and 4th of eluent. (a) 50 ml of sample (b) conductivity of 2800 mS.
carbonate and nitrate. Solutions containing 1 mg l − 1 of chromium and several molar Cr/interferent ratios were prepared, retained into the SAX cartridge and eluted with 4 ml of NaCl 0.5 M. The interferent species tested do not interfere with Cr(III) and Cr(VI) retention at the studied concentrations. Table 1 shows the tolerance limits of the interferents studied. A synthetic sample (composition in Table 2) containing amounts of interferents below the tolerance limits but with a high conductivity (9.64 mS) was analysed by determining of Cr(III) in the
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535
Table 3 Results obtained from standards with and without control of conductivity Volume of sample (ml) Chromium added (mg)
Results with conductivity control 1
−2
yˆ 9ts n
Results without conductivity control yˆ 9tsn
(mg)a
1
−2
(mg)a
20
Cr(VI) 4.80 Cr(III) 1.44
Cr(VI) 4.9 90.8 Cr(III) 1.8 9 0.2
Cr(VI) 3.58 9 0.08 Cr(III) 1.05 90.17
25
Cr(VI) 2.25 Cr(III) 1.50
Cr(VI) 2.23 90.16 Cr(III) 1.34 90.14
Cr(VI) 1.03 90.13 Cr(III) 1.02 90.14
a
95% of confidence level.
1st ml and Cr(VI) in the 3rd and 4th millilitre of the eluted solution. The percentage of recovery from 50 ml of sample was 15% and 0% for Cr(III) and Cr(VI), respectively. To determine the limit of total ions that permits the use of 500 mg of solid phase for sample treatment, different solutions containing chromium and several amounts of sodium chloride (which provides different conductivity) were prepared, and chromium was analysed under the above conditions. The variation in the recovery of Cr(III) and Cr(VI) depending on the conductivity and the amount of sample can be seen in Fig. 2. As can be observed, the recovery of chromium depends on the total ions present in the sample and then, it is necessary to obtain the calibration graphs from solutions of standards with the same conductivity
as the sample and using identical volumes of standards and sample. On the other hand, when samples have high conductivity, speciation can be done using 500 mg of solid phase, but preconcentration cannot because the recovery of chromium decreases when volume of sample increases. The use of standards with the same conductivity as the samples, by addition of NaCl, was evaluated by analysing a synthetic tap water sample (conductivity was 913 mS) and a synthetic sample with a conductivity of 1050 mS was provided by a solution of ammonium nitrate. The samples were analysed using standards of Cr(VI) and Cr(III) with the same conductivity provided by a solution of NaCl, and standards of Cr(VI) and Cr(III) without conductivity control. As can be seen in Table 3, controlling the conduc-
Table 4 Analysis of samples Sample
Conductivity (mS)
Volume of sample (ml)
Reference method (mg) 1 − yˆ 9ts n 2
Proposed method for 1spe- Proposed method for de− termination of1 Cr(VI) ciation (mg) yˆ 9ts n 2a − (mg) yˆ 9 ts n 2a
Waste water 1
1023
0.2
Cr(VI) 2.2 90.6
–
Cr(VI) 1.8 9 0.3
Waste water 2
1857
0.4
Cr(VI) 2.6 90.7 Cr(III) 0.9 9 0.7
Cr(VI) 2.36 90.12 Cr(III) 0.7 90.3
–
Waste water 3
2220
25
Cr(VI) BLD Cr(III) 0.6 90.2
Cr(VI) BLD Cr(III) 0.74 90.12
–
Tannery waste water
78900
0.05
Cr(VI) 2.3 90.4
Cr(VI) 2.5 9 0.4
–
Cr(III) 71 9 4
Cr(III) 83 94
a
95% of confidence level.
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D.M. Adria´ -Cerezo et al. / Talanta 51 (2000) 531–536
tivity of the standards by adding NaCl provides accurate results and makes it possible to speciate chromium following this procedure.
3.3. Analytical figures of merit These parameters were obtained from 50 ml samples containing Cr(III) and Cr(VI) with a conductivity that provides recoveries of 100 and 80% for Cr(III) and Cr(VI), respectively. The dynamic ranges obtained were 0 –2.4 and 0–4.8 mg l − 1 for Cr(III) and Cr(VI), respectively. Reproducibility was 9% evaluated as the coefficient of variation of five independent extractions of a sample containing Cr(III) and Cr(VI). The detection and quantification limits were determined as the ratio between three and ten times the standard deviation of the blank and the slope of the calibration graph. The results obtained were 0.4 mg l − 1 of Cr(III) and 1.1 mg l − 1 of Cr(VI) for the detection limit and 1.3 mg/l of Cr(III) and 3.6 mg/l of Cr(VI) for the quantification limit.
3.4. Analysis of samples Samples of industrial wastewater in the Comunitat Valenciana were analysed following the proposed procedures. The results are compared with values obtained by a reference method. The reference method involves reaction with APDC and extraction with IBMK [18]. Table 4 shows the results obtained. As can be seen, in all cases the results are in agreement with the reference values.
.
Acknowledgements The authors thank the AIMME for providing samples and Generalitat Valenciana for financial support (project AE97-11).
References [1] J.O. Nriagu, E. Nieboer, Chromium in the Natural and Human Environment, Wiley, New York, 1988. [2] E. Beceiro Gonzalez, J. Barciela Garcia, P. Bermejo Barrera, A. Bermejo Barrera, Fresenius J. Anal. Chem. 344 (1992) 301. [3] K.S. Subramanian, Anal. Chem. 60 (1988) 11. [4] R.M. Crespo´n Romero, M.C. Yebra Biurrun, M.P. Bermejo Barrera, Anal. Chim. Acta 327 (1996) 37. [5] M.C. Pannain, R.E. Santelli, Talanta 42 (1995) 1609. [6] B. Pasullean, C.M. Davidson, D. Littlejohn, J. Anal. Atom. Spectrom. 10 (1995) 241. [7] M.S. Jime´nez, I. Martı´n, J.M. Mir, J.R. Castillo, Atom. Spectroscopy 17 (5) (1996) 201. [8] T.M. Vickrey, W. Eue, J. Autom. Chem. 1 (4) (1979) 198. [9] P.C. Uden, I.E. Bigley, F.H. Walters, Anal. Chim. Acta 100 (1978) 555. [10] I.S. Krull, K.W. Panaro, L.L. Gershman, J. Chromatogr. Sci. 21 (10) (1983) 460. [11] K.E. Lawrence, G.W. Rice, V.A. Fassel, Anal. Chem. 56 (2) (1984) 289. [12] J.C. Van Loon, B. Radzink, in: G.W. Ewing (Ed.), Environmental Analysis, Academic Press, New York, 1977. [13] A. Ga´spa´r, J. Posta, R. To´th, J. Anal. Atom. Spectrom. 11 (1996) 1067. [14] J. Posta, A. Ga´spa´r, R. To´th, L. Ombo´di, Fresenius J. Anal. Chem. 355 (1996) 719. [15] C.M. Andrle, J.A.C. Broekaert, Fresenius J. Anal. Chem. 346 (6 – 9) (1993) 653. [16] J.-F. Jen, G.-L. On-tang, C.-S. Chen, S.-M. Yang, Analyst 118 (1993) 1281. [17] T.M. Florence, Talanta 29 (1982) 345. [18] A.E. Greenberg, L.C. Clesceri, A.D. Eaton, Standard Methods for the Examination of Water and Wastewater, American Public Health Association, Washington, DC, 1992.
Preconcentration and speciation of chromium in waters using solid-phase extraction and atomic absorption spectrometry D.M. Adria´-Cerezo, M. Llobat-Estelle´s, A.R. Maurı´-Aucejo * Department of Quı´mica Analı´tica, Faculty of Chemistry, Uni6ersity of Vale`ncia Estudi General, Dr. Moliner 50, 46100 Burjassot, Vale`ncia, Spain Received 10 June 1999; received in revised form 12 October 1999; accepted 14 October 1999
Abstract A method for the preconcentration and speciation of chromium was developed. After formation of an anionic compound with ethylenediaminetetraacetic acid (CrY−), Cr (VI) and Cr (III) are retained on a strong anionic phase (SAX) and controlled elution with 0.5 M NaCl permits their speciation. The retention and elution conditions were optimised, and interferences due to the presence of other ions such as Mg(II), Mn(II), Sn(II), Fe(III), Ba(II), Al(III), Ca(II), chloride, iodine, bromide, fluoride, sulphate, phosphate, bicarbonate and nitrate were studied. The detection limits were 0.4 mg l − 1 and 1.1 mg l − 1 for Cr(III) and Cr(VI), respectively, and reproducibility was 9%. The results obtained for speciation of chromium by the proposed method in wastewaters are in agreement with the values obtained by a reference method for a 95% confidence level. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Chromium; Speciation; Water; Preconcentration
1. Introduction Toxicological studies have shown that the degree of toxicity of some elements depends on the chemical form in which the element is present. Chromium(III), for example, is considered an essential micronutrient for humans, whereas chromium(VI) is a potentially carcinogenic agent
* Corresponding author. Tel.: +34-96-386-4497; fax: +3496-386-4436. E-mail address: [email protected] (A.R. Maurı´-Aucejo)
[1]. It is therefore necessary to control the level of chromium in wastewater, natural water and drinking water. Many countries have developed laws along this line, but since the legislation permits a larger content of Cr(III) than Cr(VI), speciation of chromium in environmental samples is very important. Several methods for speciation of chromium using atomic spectroscopy have been investigated. These methods include previous preparation of a sample by liquid-liquid extraction after formation of a complex [2,3], solid-liquid extraction [4–7] or
0039-9140/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 9 9 ) 0 0 3 0 9 - 4
532
D.M. Adria´ -Cerezo et al. / Talanta 51 (2000) 531–536
a direct coupling of liquid chromatography with atomic spectroscopy [8 – 16]. The use of solid phase extraction permits simultaneous preconcentration and separation of Cr(III) and Cr(VI). Several studies have used a solid phase for retention of Cr(III) or Cr(VI) and determination of total chromium by a previous oxidation or reduction [2 – 4,6]. Other authors have proposed the use of two different solid phases [7]. The use of an anionic phase for retention of Cr(III) implies a previous step to form an anionic compound. Ethylenediaminetetraacetic acid (EDTA) is an excellent chelating reagent that combines with Cr(III) at a 1:1 ratio. Jen-Fon Jen et al. [16] propose a method for speciation of chromium by reversed-phase ionicpair HPLC after formation of a complex with EDTA (CrY−) and formation of an ionic-pair with tetrabutylamonium. However, the method does not involve preconcentration and the detection limits are 0.02 and 0.08 mg l − 1 for Cr(III) and Cr(VI), respectively. In the present study, we have developed a method for speciation of chromium using a strong anionic phase to separate Cr(VI) and Cr(III) as CrY−. We have optimised the retention and elution conditions for simultaneous preconcentration and fractionated elution of Cr(III) and Cr(VI) and determination by AAS.
2. Experimental
2.1. Apparatus and reagents A Perkin Elmer 5000 atomic absorption spectrometer equipped with a chromium hollowcathode lamp was used for the absorbance measurements. A Vac elut 20 connected to a diaphragm vacuum pump (KNF model N 026.3 AN.18) was employed for treatment of samples. The solid-phase extraction was carried out using Lichroelut Cartridges (Merck) containing 500 mg of trimethylaminopropyl chloride (SAX). All reagents were analytical grade.
2.2. Instrumental parameters Absorbance measurements were carried out at 357.9 nm in the instrumental conditions that provide the best sensitivity and no difference between the Cr(III) and Cr(VI) signals. These conditions were 15 mm of burner height and a Qacetylene/Qair ratio of 0.09. The flow rate of samples through cartridges was 3 ml min − 1 with a vacuum of − 0.18 Bar.
2.3. Proposed procedure The following procedure for speciating chromium is proposed: First, HAc/Ac− buffer (1:10 v/v) and 0.3 ml of EDTA 0.1 M are added to an adequate aliquot of sample containing 1.5 mg of Cr(III) and/or 3 mg of Cr(VI). On the other hand, solutions containing 0–3 mg of Cr(III) and 0–6 mg of Cr(VI) with conductivity identical to the sample are prepared by addition of NaCl. Then, HAc/ Ac− buffer (1:10 v/v) and 0.3 ml of EDTA 0.1 M are added to volumes of these solutions identical to the aliquots of considered samples. All solutions (standards and samples) are heated in a bath at 80°C for 10 min or in a microwave oven for 75 s and introduced into the cartridges, previously activated with 2 ml of methanol and washed with 2 ml of buffer HAc/ Ac−. By using NaCl 0.1 M Cr(III) is eluted in the first millilitre, the second millilitre is discarded and Cr(VI) is eluted in the third and fourth millilitre. Alternatively, a method for preconcentrating and determining Cr(VI) without interference of Cr(III) is possible. The procedure is identical to the speciation procedure but EDTA is not added and the solutions are not heated. In this one, the 1st and 2nd ml of eluent is discarded because part of Cr(III) can be in another anionic form [17] and also to avoid dilution of Cr(VI).
D.M. Adria´ -Cerezo et al. / Talanta 51 (2000) 531–536
533
Table 1 Tolerance limits Interferent
I−
Br
Interferent /chromium molar ratio
41
65
−
F
−
2.7
PO3− 4
HCO− 3
SO2− 4
NO− 3
Cl−
Mg(II)
Mn(II)
Sn(II)
Al(III)
Fe(III)
Ca(II)
Ba(II)
55
128
95
58
427
214
95
0.44
19
9
390
0.38
3. Results and discussion
3.1. Retention and elution conditions 3.1.1. Formation of CrY− The use of anionic cartridges to retain Cr(III) implies the formation of an anionic form of Cr(III) and control of the pH.
Fig. 1. Variation of recovery with concentration of NaCl by elution with 4 ml and with volume of NaCl 0.5 mol l − 1.
The influence of pH, EDTA concentration, time and temperature of warming on the formation of CrY− was evaluated. The procedure used was as follows: different amounts of EDTA and 2 ml of buffer were added to 20 ml of sample containing 60 mg l − 1 of chromium. Then the mixture was warmed in a bath and introduced into the cartridge previously activated with 2 ml of methanol and washed with 2 ml of buffer solution. Finally, chromium was eluted with 4 ml of NaCl 0.5 M, and the absorbance value obtained. The amount of EDTA plays an important role because in its absence, Cr (III) is not retained. A molar EDTA/Cr(III) relation between 400 and 1300 is needed for quantitative formation of CrY−. On the other hand, the variation in the pH between 4 and 9 does not affect the recovery of Cr(III). The reaction between Cr(III) and EDTA is very slow at room temperature, but the formation of the complex was favoured at higher temperatures. If the sample was warmed at 80°C at pH 4.7, only 10 min was necessary for the quantitative formation of CrY−. In order to decrease the time analysis, the use of a microwave oven for the formation of CrY− was studied. In this case only 75 s at medium power were needed for the quantitative formation of the complex.
3.1.1.1. Stability of Cr(VI). To test the possible reduction of Cr(VI) during complex formation, the same study was applied to Cr(VI). Our results show that Cr(VI) is not altered during the derivatization process (warming with a bath and with microwave oven). On the other hand, the recovery of Cr(VI) is identical between pH 4 and 9.
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534
3.1.2. Elution conditions A solution of NaCl was chosen as eluent. The volume and concentration of the eluent were optimised to obtain a maximum recovery of chromium. For this study, aliquots of 25 ml containing 0.24 mg l − 1 of chromium were retained using the above conditions Fig. 1 shows the variation in the recovery of chromium depending on the volume and concentration of the eluent. As can be seen, Cr(III) is eluted faster than Cr(VI). Therefore, it is possible to determine Cr(III) in the first millilitre of eluent, and Cr(VI) in the third and fourth millilitre without interference of Cr(III). 3.2. Study of ion interferents The interference of different ions, which is common in water samples, in the recovery of Cr(III) and Cr(VI) has been studied. The influence of anions on retention was studied because the solid phase is a strong anionic exchanger. On the other hand, the influence of several cations, which in the presence of EDTA at pH 4.7 are in the anionic form, was also tested. The interferents studied are: Mg(II), Mn(II), Sn(II), Fe(III), Ba(II), Al(III), Ca(II), chloride, iodine, bromide, fluoride, sulphate, phosphate, biTable 2 Composition of samplea Compound
Concentration (mol l−1)
CaCO3b MgCl2 Feb SnCl2b Al(NO3)3 NaI KBr NaCl NaNO3 NaHCO3 Na2SO4 BaCO3b K2CrO4 Cr(NO3)3
4.9×10−3 4.1×10−2 1.8×10−5 8.4×10−7 3.7×10−5 7.9×10−5 1.2×10−4 4.0×10−2 8.05×10−4 2.3×10−3 3.1×10−3 7.3×10−7 1.77×10−6 1.15×10−6
a b
Conductivity 9.64 mS. Disolved in HCl.
Fig. 2. Variation of recovery with conductivity and volume of sample. Cr(III) measured in the 1st ml and Cr(VI) in the 3rd and 4th of eluent. (a) 50 ml of sample (b) conductivity of 2800 mS.
carbonate and nitrate. Solutions containing 1 mg l − 1 of chromium and several molar Cr/interferent ratios were prepared, retained into the SAX cartridge and eluted with 4 ml of NaCl 0.5 M. The interferent species tested do not interfere with Cr(III) and Cr(VI) retention at the studied concentrations. Table 1 shows the tolerance limits of the interferents studied. A synthetic sample (composition in Table 2) containing amounts of interferents below the tolerance limits but with a high conductivity (9.64 mS) was analysed by determining of Cr(III) in the
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535
Table 3 Results obtained from standards with and without control of conductivity Volume of sample (ml) Chromium added (mg)
Results with conductivity control 1
−2
yˆ 9ts n
Results without conductivity control yˆ 9tsn
(mg)a
1
−2
(mg)a
20
Cr(VI) 4.80 Cr(III) 1.44
Cr(VI) 4.9 90.8 Cr(III) 1.8 9 0.2
Cr(VI) 3.58 9 0.08 Cr(III) 1.05 90.17
25
Cr(VI) 2.25 Cr(III) 1.50
Cr(VI) 2.23 90.16 Cr(III) 1.34 90.14
Cr(VI) 1.03 90.13 Cr(III) 1.02 90.14
a
95% of confidence level.
1st ml and Cr(VI) in the 3rd and 4th millilitre of the eluted solution. The percentage of recovery from 50 ml of sample was 15% and 0% for Cr(III) and Cr(VI), respectively. To determine the limit of total ions that permits the use of 500 mg of solid phase for sample treatment, different solutions containing chromium and several amounts of sodium chloride (which provides different conductivity) were prepared, and chromium was analysed under the above conditions. The variation in the recovery of Cr(III) and Cr(VI) depending on the conductivity and the amount of sample can be seen in Fig. 2. As can be observed, the recovery of chromium depends on the total ions present in the sample and then, it is necessary to obtain the calibration graphs from solutions of standards with the same conductivity
as the sample and using identical volumes of standards and sample. On the other hand, when samples have high conductivity, speciation can be done using 500 mg of solid phase, but preconcentration cannot because the recovery of chromium decreases when volume of sample increases. The use of standards with the same conductivity as the samples, by addition of NaCl, was evaluated by analysing a synthetic tap water sample (conductivity was 913 mS) and a synthetic sample with a conductivity of 1050 mS was provided by a solution of ammonium nitrate. The samples were analysed using standards of Cr(VI) and Cr(III) with the same conductivity provided by a solution of NaCl, and standards of Cr(VI) and Cr(III) without conductivity control. As can be seen in Table 3, controlling the conduc-
Table 4 Analysis of samples Sample
Conductivity (mS)
Volume of sample (ml)
Reference method (mg) 1 − yˆ 9ts n 2
Proposed method for 1spe- Proposed method for de− termination of1 Cr(VI) ciation (mg) yˆ 9ts n 2a − (mg) yˆ 9 ts n 2a
Waste water 1
1023
0.2
Cr(VI) 2.2 90.6
–
Cr(VI) 1.8 9 0.3
Waste water 2
1857
0.4
Cr(VI) 2.6 90.7 Cr(III) 0.9 9 0.7
Cr(VI) 2.36 90.12 Cr(III) 0.7 90.3
–
Waste water 3
2220
25
Cr(VI) BLD Cr(III) 0.6 90.2
Cr(VI) BLD Cr(III) 0.74 90.12
–
Tannery waste water
78900
0.05
Cr(VI) 2.3 90.4
Cr(VI) 2.5 9 0.4
–
Cr(III) 71 9 4
Cr(III) 83 94
a
95% of confidence level.
536
D.M. Adria´ -Cerezo et al. / Talanta 51 (2000) 531–536
tivity of the standards by adding NaCl provides accurate results and makes it possible to speciate chromium following this procedure.
3.3. Analytical figures of merit These parameters were obtained from 50 ml samples containing Cr(III) and Cr(VI) with a conductivity that provides recoveries of 100 and 80% for Cr(III) and Cr(VI), respectively. The dynamic ranges obtained were 0 –2.4 and 0–4.8 mg l − 1 for Cr(III) and Cr(VI), respectively. Reproducibility was 9% evaluated as the coefficient of variation of five independent extractions of a sample containing Cr(III) and Cr(VI). The detection and quantification limits were determined as the ratio between three and ten times the standard deviation of the blank and the slope of the calibration graph. The results obtained were 0.4 mg l − 1 of Cr(III) and 1.1 mg l − 1 of Cr(VI) for the detection limit and 1.3 mg/l of Cr(III) and 3.6 mg/l of Cr(VI) for the quantification limit.
3.4. Analysis of samples Samples of industrial wastewater in the Comunitat Valenciana were analysed following the proposed procedures. The results are compared with values obtained by a reference method. The reference method involves reaction with APDC and extraction with IBMK [18]. Table 4 shows the results obtained. As can be seen, in all cases the results are in agreement with the reference values.
.
Acknowledgements The authors thank the AIMME for providing samples and Generalitat Valenciana for financial support (project AE97-11).
References [1] J.O. Nriagu, E. Nieboer, Chromium in the Natural and Human Environment, Wiley, New York, 1988. [2] E. Beceiro Gonzalez, J. Barciela Garcia, P. Bermejo Barrera, A. Bermejo Barrera, Fresenius J. Anal. Chem. 344 (1992) 301. [3] K.S. Subramanian, Anal. Chem. 60 (1988) 11. [4] R.M. Crespo´n Romero, M.C. Yebra Biurrun, M.P. Bermejo Barrera, Anal. Chim. Acta 327 (1996) 37. [5] M.C. Pannain, R.E. Santelli, Talanta 42 (1995) 1609. [6] B. Pasullean, C.M. Davidson, D. Littlejohn, J. Anal. Atom. Spectrom. 10 (1995) 241. [7] M.S. Jime´nez, I. Martı´n, J.M. Mir, J.R. Castillo, Atom. Spectroscopy 17 (5) (1996) 201. [8] T.M. Vickrey, W. Eue, J. Autom. Chem. 1 (4) (1979) 198. [9] P.C. Uden, I.E. Bigley, F.H. Walters, Anal. Chim. Acta 100 (1978) 555. [10] I.S. Krull, K.W. Panaro, L.L. Gershman, J. Chromatogr. Sci. 21 (10) (1983) 460. [11] K.E. Lawrence, G.W. Rice, V.A. Fassel, Anal. Chem. 56 (2) (1984) 289. [12] J.C. Van Loon, B. Radzink, in: G.W. Ewing (Ed.), Environmental Analysis, Academic Press, New York, 1977. [13] A. Ga´spa´r, J. Posta, R. To´th, J. Anal. Atom. Spectrom. 11 (1996) 1067. [14] J. Posta, A. Ga´spa´r, R. To´th, L. Ombo´di, Fresenius J. Anal. Chem. 355 (1996) 719. [15] C.M. Andrle, J.A.C. Broekaert, Fresenius J. Anal. Chem. 346 (6 – 9) (1993) 653. [16] J.-F. Jen, G.-L. On-tang, C.-S. Chen, S.-M. Yang, Analyst 118 (1993) 1281. [17] T.M. Florence, Talanta 29 (1982) 345. [18] A.E. Greenberg, L.C. Clesceri, A.D. Eaton, Standard Methods for the Examination of Water and Wastewater, American Public Health Association, Washington, DC, 1992.