Speciation of Chromium after Coprecipitation with ... - Ingenta Connect

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Current Analytical Chemistry, 2012, 8, 358-364

Speciation of Chromium after Coprecipitation with Cu-Violuric Acid and Determination by Flame Atomic Absorption Spectrometry S. Saracoglu1*, E. Yılmaz2 and M. Soylak2 1

Erciyes University, Faculty of Education, Department of Elementary Education, 38039 Kayseri-Turkey

2

Erciyes University, Faculty of Science, Department of Chemistry, 38039 Kayseri-Turkey Abstract: A method based on coprecipitation of Cr (III) with copper-violuric acid for the preconcentration/speciation of Cr (III) and Cr (VI) has been developed. While Cr (III) was quantitatively recovered, Cr (VI) was recovered under 15 % level at pH 6. Total Cr was determined with reducing of Cr (VI) to Cr (III) with concentrated sulphuric acid and ethanol. The concentration of Cr (VI) was calculated by the concentration difference between the total Cr and Cr (III). The influences of analytical parameters such as pH, amount of coprecipitant, sample volume were examined. The detection limit was obtained as 1.17 μg L-1 for Cr (III). The accuracy of presented method was checked by analyte addition and analysis of certified reference materials. The method for determination with atomic absortion spectrometry after preconcentration with coprecipitation of Cr (III), Cr (VI) and total Cr has been applied to tap water, waste water and soil samples.

Keywords: Atomic absorption spectrometry, Cu-Violuric acid, chromium, coprecipitation, preconcentration, speciation. 1. INTRODUCTION Chromium is known to be one of the trace metals of environmental concern. Metallurgical (steel, ferrous and nonferrous alloys) and chemical (e.g. pigments, electroplating, tanning, etc) industries are the major anthropogenic sources of this element [1-4]. Cr (III) and Cr (VI) are the two common species found in the natural environment. Cr (VI) differs from Cr (III) in that: i) Cr (VI) exerts higher toxic effects more than Cr (III) on biological systems. [1], ii) Cr (VI) compounds are usually highly soluble, mobile and bioavailable compared to sparingly soluble trivalent Cr species [5], iii). Cr (VI) has carcinogenic properties [6], and IV) Cr (III) is unable to enter cells but Cr (VI) enters into cells through membrane anionic transporters [7]. Thus, knowledge of each species rather than the total Cr level is required to properly evaluate physiology and toxicological effects of Cr, its chemical transformations in water, soil, and atmosphere, as well as its distribution and transport in the environment [5]. Speciation analysis of Cr (III) and Cr (VI) is therefore of great importance and much research has been devoted to this area. Many different analytical techniques have been developed to quantify the various Cr forms present in the natural environment. Only a few, very sensitive instrumental methods (e.g., ICP-MS, ET-AAS and stripping voltammetry) can directly determine low-concentration total Cr [8]. The separation and preconcentration methods are usually required to determine the individual Cr species because of usually low levels of analyte and interferences of the concomitant ions. The separation/preconcentration methods reported in the

*Address correspondence to this author at the Erciyes University, Faculty of Education, Department of Elementary Education, 38039 Kayseri-Turkey; Tel/Fax: +90 352 4374933; E-mail: [email protected] 1875-6727/12 $58.00+.00

literature for Cr speciation are usually based on chromatographic method [9, 10], electrochemical methods [8], solid phase extraction [11-13], separation using chelating resin [14], solvent extraction [15, 16], cloud point extraction [2, 17]. Among these techniques, coprecipitation is particularly recommended and widely used for separation and preconcentration of trace metal ions in various samples [18-20]. Advantages of coprecipitation include simplicity, rapidity, and ability to attain a high pre-concentration factor and sufficient separation factors for alkali and alkaline earth elements in various samples analyses. Various procedures involving the use of metal chelating precipitates are well documented in the literature [21, 22]. The chelates of metal ions including copper with violuric acid (5-isonitrosobarbituric acid) have been used for spectrophotometric determinations [23, 24]. Violuric acid contains an isonitroso group as a substituent and is known to form many kinds of metal complexes in solution. The coppervioluric acid as coprecipitant was used for coprecipitation of lead and iron at trace levels in various samples in our previous study [25]. In that study, we found that copper-violuric acid has a good collecting ability for the studied trace metals and sufficient separation factor for matrix ions, such as alkali and alkaline earth metals. To the best of our knowledge, the copper-violuric acid coprecipitation system for preconcentration/speciation of Cr species prior to flame atomic absorption spectrometry has not been reported. Hence, Cr speciation by the coppervioluric acid coprecipitation system is a novel study. The aim of the present study is therefore to establish a coprecipitation method by using copper-violuric acid for speciation of Cr (III) and Cr (VI) prior to their flame atomic absorption spectrometric determination and demonstrate its potential application to the speciation of Cr in different environmental samples. © 2012 Bentham Science Publishers

Speciation of Chromium after Coprecipitation with Cu-Violuric Acid

2. EXPERIMENTAL 2.1. Instrumentation A Perkin-Elmer Model 3110 (Norwalk, CT, USA) atomic absorption spectrometer equipped with Perkin-Elmer singleelement hollow cathode lamps and a 10-cm air-acetylene burner was used for the determination of the Cr species. Other instrumental parameters for Cr were adjusted according to the recommendations in the manufacturer’s manual. The pH in the aqueous phase was measured with a pH meter (Nel pH-900 Model, Ankara, Turkey) equipped with a glasselectrode. A centrifuge (ALC PK 120 Model, Buckinghamshire, England) was used during centrifugation processes. 2.2. Reagents and Solutions Distilled water was used throughout all the experiments. All chemicals were analytical reagent grade unless otherwise stated. Cr (III) and Cr (VI) stock solutions (1000 mg L-1) were prepared from Cr (NO3)3.9H2O (E. Merck, Darmstadt, Germany) and K2Cr2O7, (Merck) respectively. Stock solutions were diluted daily for obtaining reference and working solutions. The calibration curve was established using the standard solutions prepared in 1 M HNO3 by dilution from stock solutions. The calibration standards were not submitted to the coprecipitation procedure. The range of calibration standards for Cr on flame atomic absorption spectrometric measurements were 1.0-10.0 mg L-1. Stock solutions of diverse ions were prepared from the high purity compounds and diluted daily for obtaining working solutions. A solution of 1000 mg L-1 Cu (II) was prepared by dissolving of proper amount of copper (II) nitrate (Merck) in water. A solution of violuric acid (0.1 % m/v) was prepared by dissolving of solid violuric acid (Fluka, Austria) in small amounts of ethyl alcohol (Merck) and diluting to 100 mL with the water. HNO 3 (Merck) used for preparing of diluted acid solution in this work was analytical grade and used without further purification. Sodium hydroxide (Merck) and hydrocloric acid (Merck) used for adjustment of pH were supra pure grade. The concentrated sulphuric acid (Merck) and ethanol (Merck) were used for reducing of Cr (VI) to Cr (III) were analytical reagent grade. All the plastic and glassware prior to use were cleaned with detergent, doubly distilled water, diluted nitric acid and doubly distilled water, respectively. 2.3. Method Optimization The proposed separation-preconcentration system was optimized using model solutions to reach the best conditions for speciation of Cr (III) and Cr (VI). The optimum conditions (sample pH, amount of copper, amount of violuric acid, sample volume and matrix ions) were determined by way of univariate optimization. The effect of pH on recoveries of Cr (III) and Cr (VI) has been investigated separately. The pH of sample solutions was adjusted with 1 M HCl or 1 M NaOH in the range 3–8. The influence of the amount of copper on the recoveries of Cr (III) and total Cr was investigated in the range of 0–2.0 mg of copper in the presence of 1.5 mg of violuric acid at pH 6. The effect of amount of violuric acid in the range of 0-2.5 mg for the quantitative recovery of Cr (III) and total Cr was also studied in the presence of 1.5 mg of

Current Analytical Chemistry, 2012, Vol. 8, No. 3

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copper at pH 6. The effect of sample volume on the recoveries of Cr (III) and total Cr was also investigated using model solutions prepared in the range of 25-300 mL and optimum values for the other variables. Therefore, the set of optimum values chosen for subsequent speciation of Cr (III) and Cr (VI) were as follows: 6 (pH), 1.5 mg (Cu and violuric acid). 2.4. Separation and Preconcentration Procedure The separation and preconcentration system was optimzed with model solutions prior to application of the method to real samples. For that purpose, 1.5 mL of 1000 mg L-1 Cu (II) solution and 1.5 mL of 0.1 % (m/v) violuric acid solution were added to 25 mL of solution containing 10 g of Cr (III) ion. Then, the pH of the solution was adjusted to related pH by the addition of 1 M NaOH solution. After 10 min, the solution was centrifuged at 4000 rpm for 7 min. The supernatant was removed. The precipitate remained adhering to the tube was dissolved with 5 mL of 1 M HNO3. The Cr content of the final solution was determined by AAS. Reduction of Cr (VI) to Cr (III) has been performed by using the procedure given in literature [13]. 0.5 mL of concentrated sulphuric acid and 0.5 mL of ethanol were added to model solution for this purpose. After reduction of Cr (VI) to Cr (III), the method described above was applied to the determination of the total Cr. Total Cr was determined by AAS. The level of Cr (VI) was calculated by difference between the total Cr and Cr (III) concentrations. The optimum conditions for separation of Cr (III) from Cr (VI) and for preconcentration of Cr (III) have been determined by using the test procedure given above. 2.5. Analysis of Real Samples The tap water samples were collected from Kayseri and Ordu, Turkey. Waste water sample was collected from a factory located in Kayseri organized industrial zone. The samples were filtered through a millipore cellulose membrane of pore size 0.45 μm. The samples were acidified to 1 % with nitric acid and stored at 4 oC in a refrigerator. 200 mL sample for analysis of water samples was taken. The preconcentration procedure given above was applied to water samples. The levels of investigated Cr species in the water samples were determined by AAS. Various amounts of Cr ions were also spiked to tap water sample for the validity of procedure. In the analysis of soil sample taken from Yozgat and Nevehir, Turkey, 1.0 g of soil sample was transferred to a beaker and 20 mL of aqua regia was added. Afterward, the solution was evaporated to dryness in a hood. This process was repeated twice. 10 mL of distilled water was added to the residue. The suspension was filtered through a blue band filter paper (Macherey-Nagel, Düren, Germany), and the insoluble part was washed with distilled water. Then, the preconcentration procedure given above was applied to the final solutions and the Cr contents in solutions were determined by AAS. The method was also applied to certified reference materials. For that purpose TMDA-54.4 fortified lake water, GBW 07402 soil and BCR-032 Moroccan phosphate rock were used. The proposed method was applied to 10.0 mL of

360


Saracoglu et al.

120

Recovery, %

100 80 Cr (III) Cr (VI)

60 40 20 0 3

4

5

6

7

8

pH Fig. (1). Effect of pH on the recoveries of Cr (III) and Cr (VI).

certified water sample. The Cr content in solution was determined by AAS. An amount of sample for the soil and rock samples (0.1 g amount of GBW 07402 soil, 0.025 g amount of BCR-032 Moroccan phosphate rock) was decomposed with 20 mL of aqua regia, and the solution was evaporated to dryness. This process was repeated twice. Ten millilitre of distilled water was added to the residue. The suspension was filtered through a blue band filter paper (MN 640de, diameter 110 mm, Macherey-Nagel, Düren, Germany), and the insoluble part was washed with distilled water. Then, the preconcentration procedure given above was applied to the final solutions. The levels of Cr species were determined by AAS. The total Cr in real samples was determined as Cr (III) after the reduction of Cr (VI) to Cr (III) in the samples. The reduction of Cr (VI) to Cr (III) was performed by the procedure given in literature [13]. 3. RESULTS AND DISCUSSION Initially the proposed separation-preconcentration system was optimzed to reach the best conditions for speciation of Cr (III) and Cr (VI). For this purpose, the effects of some analytical parameters effect on recoveries of Cr species were investigated. 3.1. Effect of pH pH plays a important role on the formation of hydrophobic metal complex. The effect of pH on recoveries of Cr (III) and Cr (VI) has been investigated separately. pH values of sample solutions were adjusted to a range of 3-8 with 1 M HCl or 1 M NaOH. The results of the effect of pH on the recoveries were shown in Fig. (1). It can be seen that, the recoveries were quantitative (> 95 %) for Cr (III) at the pH 6 and 7, and the recovery of Cr (VI) was rather low (below 15 %) in these pH levels. These result shows that separation of Cr (III) from Cr (VI) by presented coprecipitation procedure is possible with the control of pH of the solution. In order to speciation of Cr (III) and Cr (VI), a pH of 6.0 was selected for subsequent work. The pH for the coprecipitation of these anaytes is lower than other some studies [6, 13, 18, 20]. This is probably due to the lower solubility product of this compound. The lower

pH for coprecipitation with copper-violuric acid is advantageous for the separation of the analytes from alkali and alkaline earth metals. 3.2. Effect of Amounts Copper and Violuric Acid The amount of coprecipitant is important variable affecting recovery of analytes. The influence of the amount of copper on the recoveries of Cr (III) and total Cr was investigated in the range of 0-2.0 mg of copper in the presence of 1.5 mg of violuric acid at pH 6. The results were depicted Fig. (2). The recoveries of Cr (III) and total Cr were nothing without copper. This point gives that copper is necessary for this speciation work. Quantitative recoveries for analytes were obtained after 1.0 mg of copper and the recovery values were quantitative in the range of 1.0-2.0 mg of copper for both Cr (III) and total Cr. In all subsequent work, 1.5 mg copper was used as carrier reagent. The effect of amount of violuric acid in the range of 02.5 mg for the quantitative recovery of Cr (III) and total Cr was also studied in the presence of 1.5 mg of copper at pH 6. As can be seen from Fig. (3), the recoveries of Cr species were not quantitative without violuric acid. The recoveries of Cr (III) were quantitative in the range of 1.0-2.5 mg of violuric acid. Quantitative recoveries for total Cr were obtained after 0.5 mg of violuric acid. All further experiments were performed with 1.5 mg of violuric acid for this study. 3.3. Effect of Sample Volume In order to investigate the possibility of enrichment low concentration of analyte from large volume, the effect of sample volume on the recoveries of Cr (III) and total Cr was also investigated. For this purpose, model solutions prepared in the range of 25-300 mL of sample volume in optimal conditions was used. The results are given in Fig (4). It was found that the recoveries of Cr (III) and total Cr up to 200 mL of the sample solution were quantitative. The recoveries decreased with increasing sample volume after 200 mL. The final volume of the presented study was 5.0 mL. The preconcentration factor is calculated by the ratio of the highest sample volume for Cr (200 mL) and the lowest final volume (5.0 mL). The highest preconcentration factor achieved in present study was 40.



361

120

100

Recovery, %

80

Cr (III)

60

Total Cr

40

20

0 0

0,5

1

1,5

2

2,5

Amount of Cu (II), mg

Fig. (2). Effect of amount of copper on the recoveries of Cr (III) and total Cr.

120

100

Recovery, %

80 Cr (III)

60

Total Cr

40

20

0 0

0,5

1

1,5

2

2,5

3

Amount of violuric acid, mg

Fig. (3). Effect of amount of violuric acid on the recoveries of Cr (III) and total Cr.

3.4. Effect of Matrix Ions

3.5. Charactetistics of the Method

The influences of matrix components of the sample analyzed are important factor in determination of metal ions. The effect of some foreign ions which interfere in the determination by the presented method of Cr species in various real samples was studied in the optimized contitions mentioned above. The effects of investigated matrix ions were tested separately. The tolerence limits of the coexiting ions are given in Table 1. The results showed that the possible interferent cations and anions in various samples have no obvious influence on the selective preconcentration and detemination of Cr (III) under the selected conditions.

Under the optimal experimental conditions, relative standard deviations (R.S.D.) of Cr (III) and Cr (VI) were 4.6 and 7.7 % (N=11), respectively. The detection limit calculated as three times the standard deviation of the blank signals (N=11) in this method was 1.17 μg L-1. 3.6. Sample Analysis The accuracy of the proposed method was examined by analyzing total Cr in water (TMDA-54.4 fortified lake water) and soil certified reference materials (GBW 07402 soil ve BCR-032 Moroccan phosphate rock). As can be seen Table

362


Saracoglu et al.

120

Recovery, %

100 80 Cr (III) Total Cr

60 40 20 0 0

50

100

150

200

250

300

350

Sample volume, ml Fig. (4). Effect of sample volume on the recoveries of Cr (III) and total Cr. Table 1. Tolerable Levels of Some Ions on Presented Method Matrix Ions

Added Compounds

Tolerable Levels (g mL-1)

Mg +2

Mg(NO3)2

2000

Na+

NaNO3

17500

PO4-3

Na3PO4 .12H2O

5000

Na2SO4

5000

Ca(NO3)2

5000

-2

SO4 Ca Pb

+2

+2

Ni+2 +2

Pb(NO3)2

50

Ni (NO3) 2.6H2O

50

MnSO4 .H2O

50

Fe+3

Fe(NO3)3 .9H2O

50

Cl-

NaCl

20000

Mn

Table 2. Total Cr in Certified Reference Material Samples Sample

Certified Value

Found Value

Recovery (%)

TMDA-54.4 fortified lake water, N=3

438 g L-1

444 ± 35 g L-1

96

GBW 07402 soil, N=5

47 ± 4 g g

45 ± 3 g g

-1

96

BCR-032 Moroccan Phosphate Rock, N=5

257 ± 16 g g

255 ± 13 g g

99

-1

-1

-1

Mean ± Standard deviation

2, the analytical values obtained with proposed method were in good aggrement with the certified value of Cr in reference materials. In order to expore feasibility of proposed preconcentration-speciation procedure, the proposed method has been applied to the determination of Cr (III), Cr (VI) and total Cr in tap water and waste water collected from Kayseri and Ordu. The standard addition method was used, and the reliability was checked by spiking experiments. The results were presented in Table 3. As shown from the Table 3, a good

agreement was obtained between the added and measured analyte amount, the accuracy of the results was quite satisfactory. Relative error was lower than 10 % for Cr species. The results indicated that the recoveries were reasonable for trace analysis. The proposed coprecipitation method could be applied succesfully for the separation, preconcentration and speciation of trace amounts of Cr (III) and Cr (VI) species in water samples. The method has been also succesfully applied to speciation and determination of Cr (III), Cr (VI) and total Cr in soil sample collected from Yozgat and Nevehir (Table 4).



363

Table 3. Recoveries of Cr (III) and Cr (VI) from Spiked Tap Water Sample (Vsample=200 mL, N=3) Added (g)

Found (g)

Calculated

Recovery (%)

Cr (III)

Cr (VI)

Cr (III)

Total Cr

Cr (VI)

Cr (III)

Cr (VI)

0

0

BDL

BDL

-

-

-

5

0

5.1 ± 0.2

5.1 ± 0.2

-

102

-

10

0

9.8 ± 0.1

9.8 ± 0.1

-

98

-

15

0

15.5 ± 0.2

15.5 ± 0.2

-

103

-

15

2.5

15.0 ± 0.0

17.4 ± 1.5

2.4 ± 0.2

100

96

10

5

9.6 ± 0.3

14.5 ± 0.6

4.9 ± 0.1

96

98

5

7.5

5.0 ± 0.0

12.5 ± 0.8

7.5 ± 0.3

100

100

0

15

BDL

14.9 ± 0.9

14.9 ± 0.4

-

99

Mean ± Standard deviation; BDL: Below the detection limit

Table 4. Determinations of Cr (III), Cr (VI) and Total Cr in Tap Water, Waste Water and Soil Samples (N=5) Concentrations Sample Cr (III)

Total Cr

Cr (VI)

Tap water from Kayseri

BDL

BDL

BDL

Tap water from Ordu

BDL

BDL

BDL

Waste water

22.3 ± 1.4 g mL-1

6280.3 ± 643 g mL-1

6258 ± 643 g mL-1

Soil from Yozgat

89.6 ± 1.3 g g-1

89.6 ± 1.3 g g-1

BDL

Soil from Nevehir

6.5 ± 0.1 g g

6.5 ± 0.1 g g

BDL

-1

-1

Mean ± Standard deviation; BDL: Below the detection limit

Chromium is a metallic element that which is present in soils, waters and rocks. Cr is originates from natural source. The average amount of this element in various kinds of soils ranges from 0.02 to 58 μmol g-1 [1]. The increase in Cr concentration in water or soil is caused by discharged of large quantities of chorium compounds in liquid and solid wastes into the environment. The determination of Cr species is important to control this element since it is both toxic and carcinogenic, especially Cr (VI).

REFERENCES

4. CONCLUSIONS

[5]

In this work, a sensitive and selective method for the separation/speciation and determination of Cr species in environmental samples using copper-violuric acid coprecipitation system coupled with AAS was developed. The developed method has been successfully applied to detemination of Cr in tap and waste water, soil samples. The developed method was simple, economic, rapid, accurate, and reproducible. The other basis advantages of the developed method were high salt tolerence, high preconcentration factor and low detection limit for Cr species, applicability to both oxidation states of Cr (i.e. directly to Cr (III), and indirectly to Cr (VI) after reduction with sulphuric acid and ethanol). The precision and accuracy of the method was satisfactory. The method can be used for the speciation of Cr in various water and soil matrices other than this sample.

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Received: August 15, 2011

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Accepted: December 31, 2011