Sensitive spectrophotometric determination of Co(II ...

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lates. Several techniques such as inductively coupled plasma/mass spectrometry (Martin-Camean et al. 2014), atomic absorption (Baghban et al. 2009; Ojeda.

Environ Monit Assess (2016) 188:265 DOI 10.1007/s10661-016-5263-x

Sensitive spectrophotometric determination of Co(II) using dispersive liquid-liquid micro-extraction method in soil samples Foroozan Hasanpour & Hassan Hadadzadeh & Masoumeh Taei & Mohsen Nekouei & Elmira Mozafari

Received: 4 October 2015 / Accepted: 23 March 2016 # Springer International Publishing Switzerland 2016

Abstract Analytical performance of conventional spectrophotometer was developed by coupling of effective dispersive liquid-liquid micro-extraction method with spectrophotometric determination for ultra-trace determination of cobalt. The method was based on the formation of Co(II)–alpha-benzoin oxime complex and its extraction using a dispersive liquid-liquid micro-extraction technique. During the present work, several important variables such as pH, ligand concentration, amount and type of dispersive, and extracting solvent were optimized. It was found that the crucial factor for the Co(II)–alpha benzoin oxime complex formation is the pH of the alkaline alcoholic medium. Under the optimized condition, the calibration graph was linear in the ranges of 1.0–110 μg L−1 with the detection limit (S/N = 3) of 0.5 μg L−1. The preconcentration operation of 25 mL of sample gave enhancement factor of 75. The proposed method was applied for determination of Co(II) in soil samples.

Keywords Dispersive liquid-liquid micro-extraction . Alpha-benzoin oxime . Cobalt(II) . Spectrophotometric determination F. Hasanpour (*) : M. Taei : M. Nekouei : E. Mozafari Chemistry Department, Payame Noor University, 19395-4697 Tehran, Iran e-mail: [email protected] H. Hadadzadeh Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran

Introduction Dispersive liquid-liquid micro-extraction (DLLME) is a novel sample preparation technique based on the ternary solvent system. This technique has several advantages over traditional sample pretreatment such as solid phase extraction and liquid-liquid extraction, including simplicity, low-cost, high enrichment factor, and ease of method development (Ojeda and Rojas 2009; Rastegarzadeh et al. 2013). Cobalt is an essential element in the human body in simulation of hemoglobin synthesis and metabolism of vitamins (Song et al. 2006). The small amount of this element is essential for oxygen transport and enzymatic activation in all mammals. However, it has adverse effects such as a mutagenic effect in plants and mammals at high concentration levels (Beyersmann and Hartwig 1992). Determination of Co(II) in water and soil matrix is very important since these media are the main pathways that Co(II) can enter in human food. The excess exposure of cobalt in humans or animals has resulted in adverse effects such as thyroid enlargement (Song et al. 2006), vasodilatation, flushing, and cardiomyopathy. The main target organ on inhalation exposure to cobalt is the respiratory system, with a higher risk of fibrosing alveolitis (Ojeda et al. 2013). The average content of cobalt in the soil is 8 mg kg−1. (Barałkiewicz and Siepak 1999). It is easily absorbed by organic substance and creates organic chelates. Several techniques such as inductively coupled plasma/mass spectrometry (Martin-Camean et al. 2014), atomic absorption (Baghban et al. 2009; Ojeda et al. 2012), neutron activation analysis (Garten et al.

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1992), HPLC (Yang et al. 2014), UV/Vis spectrophotometry (Paleologos et al. 2002; Wen et al. 2013), and adsorptive stripping voltammetry (Kajic et al. 2003; Rutyna and Korolczuk 2011) have been developed for determination of cobalt. In spite of recent advances in instrumental analysis, spectrophotometric determination of cobalt has advantages such as simplicity and inexpensive instrumentation. However, routine spectrophotometric methods are not sensitive enough to determine trace amount of this element lower than microgram per meter level. Therefore, a preconcentration step is of vital interest. This study introduces a spectrophotometric method for the determination of Co(II) using α-benzoin oxime (α-BO) as a chelating agent. α-BO has been employed in metal coordination by incorporation of its nitrogen and oxygen atoms as a σ-donor and a π-acceptor ligand. In most cases, the coordination actually occurs at nitrogen atom. α-BO was also used to extract some transition metals. The aim of the present work is combination of DLLME as a preconcentration step with a spectrophotometric method for the determination of cobalt and demonstrating its applicability for an effective extraction of Co(II) from soil samples. The response characteristic of the proposed method is compared with recently reported DLLME methods (Yousefi and Ahmadi 2011; Shokoufi et al. 2007; Baliza et al. 2009; Mirzaei et al. 2011; Gharehbaghi et al. 2008; Berton et al. 2012; Ranjbar et al. 2012; Farajzadeh et al. 2009) for the determination of Co(II) with various detection system (Table 1).

Experimental Apparatus and materials T80 double beam UV/Vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd, China) with fixed bandwidth from 190 to 1100 nm and a quartz cell (0.3– 1.0 mL) were used. A laboratory centrifuge (Hettich, Germany) was used to accelerate the phase separation. All chemicals were analytical grade and purchased from Merck Company. Double-distilled water was used for the preparation of the solutions. A Stock solution of cobalt(II) nitrate hexahydrate (1000 μg mL−1) was prepared. The α-benzoin oxime standard solution (1 × 10−2 mol L−1) was prepared by dissolving the reagent in ethanol. The pH of the α-BO solution was

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adjusted using NaOH standard solution (5.0 × 10−1 mol L−1).

DLLME general procedure The proposed procedure for DLLME is according to Silva et al. 2012 with some modifications. The standard solutions of Co(II) were prepared in the range of 0.01– 200 μg L−1. Then, 0.5 mL of 1 × 10−3 mol L−1 of α-BO was added to a 25.0-mL volumetric flask containing Co(II). Finally, 300 μL of CHCl3 was injected by means of microsyringe and the resulting mixture was shaken for few seconds. The resulting cloudy solution (water, ethanol, and chloroform) was then centrifuged for 5 min at 5000 rpm. The small volume of the organic phase (300 μL) in the bottom of the tube was transferred to a quartz cell

Analysis of the soil samples The studied area located in the north and north-west of Isfahan near the Isfahan Steel Company (latitude 32° 24′ to 32° 26′ N and longitude 51° 18′ to 51° 23′ E). Four soils were sampled from four different points in the above place. The humidity and heat regime of the soil were aridic and thermic, respectively. The sampling depth was 20 cm. Some characteristics of these soils such as electrical conductivity (EC), pH, and concentrations of Ca2+, Na+, and Mg2+ ions were determined: Soil 1 (EC = 5.6 mS cm−1, pH = 7.2, Ca2+ = 506 ppm, Na + = 374 ppm, and Mg 2 + = 707 ppm), soil 2 (EC = 5.9 mS cm −1 , pH = 7.8, Ca 2+ = 492 ppm, Na + = 346 ppm, and Mg 2 + = 612 ppm), soil 3 (EC = 4.3 mS cm −1 , pH = 8.4, Ca 2+ = 503 ppm, Na+ = 415 ppm, and Mg2+ = 536 ppm), soil 4(EC = 6.6 mS cm −1 , pH = 7.9, Ca 2+ = 468 ppm, Na+ = 409 ppm, and Mg2+ = 680 ppm). A 5.0 ± 0.001 g of each soil sample was thoroughly grounded, dried at 100 °C and treated with 15 mL of concentrated HNO3, and then evaporated to dryness. The residue was dissolved in 60 mL H2SO4 and heated at 60 °C until dryness. Then, 100 mL of boiling water was added and the resulting suspension was digested for 30 min at 60 °C. After filtering, the filter paper was washed with water and the residue was diluted with double distilled water up to 100 mL. The solution was neutralized using NaOH and analyzed by general procedure (Amin 2014).

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Table 1 Comparison of recently DLLME with various detection methods and the proposed method for the determination of Co(II) Method

Linear range (μg L−1)

LOD (μg L−1)

Enhancement factor

FAASa

0.4–120

0.1

118

FO-LADSb

1–70

0.2

165

FAASc

3–100

0.9

16

GFAASd

0.005–0.055

0.001

800

UV-Vis spectrophotometrye

2–50

0.5

125

EAAS

0.05–3

0.005

60

ICP-OESg

0.0002–0.2

0.0001

79

f

UV-Vis spectrophotometryh

20–4000

3

94

Proposed method

1.0–110

0.5

75

a

Flame atomic absorption spectroscopy (Yousefi and Ahmadi (2011)

b

Fiber optic-linear array detection spectrophotometry (Shokoufi et al. (2007)

c

Baliza et al. (2009)

d

Graphite furnace atomic absorption spectrometry (Mirzaei et al. (2011)

e

Gharehbaghi et al. (2008)

f

Electrothermal atomic absorption spectrometry (Berton et al. (2012)

g

Inductively coupled plasma optical emission spectrometry (Ranjbar et al. (2012)

h

Farajzadeh et al. (2009)

Results and discussion Cobalt has oxidation states from −1 to +4, but in the nature, it usually exists in the form of double-valenced cation. αBO behaves as a dibasic acid toward some transition metals such as Cu(II), Co(II), and Ni(II) in basic polar solvent. Figure 1 shows the absorbance spectrum of α-BO in chloroform which has a broad peak cover of a range from approximately 200 to 290 nm at λmax of 235 nm. In an ethanolic NaOH solution, it reacts with Co(II) to form Co(II)–α-BO red complex (λmax = 270 nm) which has a sufficient hydrophobicity to be extracted into a small vol-

ume of organic phase. Figure 2 shows the UV/Vis absorption spectra of the complex after DLLME with maximum absorbance at 310 nm. α-BO has an N-OH donating group and insoluble in water at neutral pH. The complexation occurs due to incorporation of O atoms with cobalt(II) as borderline hard acid. The mole ratio method using UV/Vis spectrophotometry confirmed a stoichiometric ratio 1:2 for Co(II) and α-BO. For the determination of Co(II) with DLLME technique, the important variables to attain high sensitivity and enrichment factors were optimized (such as the disperser extraction solvent, volumes, α-BO concentration, and pH of the solution).

O HO

N OH

2

+

2+

Co

N

O

Co OH

Alpha-Benzoin oxime in ethanolic NaOH

O

N

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20.0 mL of 10 μmol L−1 Co(II) and 3 × 10−4 mol L−1 of α-BO. The extraction recovery is defined as follows: ER ¼

Fig. 1 Absorbance spectrum of α-BO in chloroform

Effect of the dispersive and extracting solvent The miscibility of the dispersing solvent with extraction solvent and water phase is the main criteria for choosing the disperser solvent in DLLME. The extracting solvent has to fulfill several requirements including high extraction capability of compounds, higher density than aqueous solvent, and low solubility in water. Furthermore, it should be soluble in the dispersing solvent. Based on these characters, CH2Cl2, CHCl3, CCl4, and cyclohexane were tasted as the extraction solvent and three disperser solvents such as acetone, ethanol, and methanol were selected taking into account their miscibility with extraction solvent and α-BO. The extraction solvents (300 μL) were aspirated to a series of alkaline (0.001 mol L−1 NaOH) dispersing solvent containing

Fig. 2 Absorption spectra of the α-BO–Co(II) complex in an ethanolic solution (a) and the α-BO–Co(II) complex after extraction in chloroform (b), [OH − ] = 1.0 × 10 − 3 mol L − 1 , [Co(II)] = 50 μg L−1, and [α-BO] = 3.0 × 10−4 mol L−1

Cs  V s C0  V 0

where the Cs and C0 parameters are the concentration of the analyte in the sedimented phase and aqueous samples, respectively. The Vsed and V0 parameters are the volume of the sedimented phase and that of aqueous sample, respectively. According to the results (Fig. 3), the maximum extraction recovery (ER%) is obtained when ethanol and CHCl3 are used as the disperser and extraction solvent, respectively. Since the microcell provides a volume between 0.3 to 1 mL, the injection volume less than 300 μL for the extraction solvent could not be measured by the spectrophotometer fitted by the microcell. Therefore, the effect of the chloroform volume was investigated in the range of 300–1000 μL. The absorbance of the complex is decreased by increasing of CHCl3 volume due to the dilution effect. Therefore, 300 μL of CHCl3 was selected as the optimized volume for the extraction solvent. Effect of pH The existence of the ethanolic NaOH is a critical factor for the formation of the complex and considerably affects the extraction efficiency. The α-BO molecule, as a chelating ligand, has an oxime group which strongly binds to the metal ions. In an alkaline media, the α-BO molecule loses its proton easily and the negatively charged oxygen

Fig. 3 Effect of the dispersing and extracting solvents on the extraction recovery of Co(II). Conditions: 0.001 mol L−1 NaOH, 50 μg L−1 Co(II), and 3 × 10−4 mol L−1 of α-BO

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donor group can bind to the metal ion. The pH of the solution was adjusted using NaOH. The effect of NaOH concentration was investigated in the range of 1.0 × 10−4– 0.1 mol L−1. The maximum absorbance due to a high extraction was obtained in 0.001 mol L−1 NaOH. Effect of the α-BO concentration The α-BO solutions at different concentrations in ethanol were prepared. The effect of α-BO concentration on the absorbance was examined in the range of 5.0 × 10−5 to 5.0 × 10−3 mol L−1. When the α-BO concentration is greater than 1 × 10−3 mol L−1, the absorbance becomes constant. Therefore, the 3 × 10−4 mol L−1 was selected as the optimized concentration for α-BO. Effect of the interferences α-BO can form complex with other heavy metal ions besides Co(II). The effect of some ions which present in the soil sample for the determination of Co(II) was investigated. A solution containing various amounts of each interfering ions was added to 10 μg L−1 of Co(II) and treated according to the DLLME procedure. The tolerance limit was considered as the largest amount of interfering ions, producing a variation less than 5 % on recovery of Co(II). The results show that there is no interference from Cr3+, Ce3+, Pb2+, Mg2+, Hg2+, Fe3+, Al3+, Cd2+, Ag+, Cu2+, and Fe2+ in the determination of Co(II). In the ethanolic NaOH solution, α-BO forms a dark green complex with Cu2+ ions. The λmax of α-BO– Cu(II) is completely different from the maximum wavelength of α-BO–Co(II) complex resulted in the prevention spectral overlap. However, Ni(II) interferes at about fivefold level. To eliminate the interference from Ni(II) in preliminary experiment, pH of the solution was adjusted to 9 and nickel was removed by adding dimethyl glyoxime (0.001 mol L−1) and extracted with chloroform. Calibration curve and detection limit At the optimized experimental condition, Beer-Lambert law is obeyed in the concentration range of 1– 110 μg L−1 with the equation of A = 0.011CCo + 0.087, where CCo is the cobalt(II) concentration in micrograms per liter with the R2 (correlation coefficient) of 0.98. The detection limit (defined as 3S b /m) was obtained 0.5 μg L−1 where Sb was standard deviation of blank and m was the slope of the calibration graph. The

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enrichment factor was considered as the ratio between the analyte concentration in the sedimented phase and the initial concentration of analyte within the sample1. Enhancement factor about 75 was achieved using the present procedure. The RSD of the method for 10.0 μg L−1 of Co(II) was obtained as 2.1 %.

Application The DLLME method was applied for the determination of Co(II) in the soil samples. The concentration of Co(II) was determined by the atomic absorption method as an official method (Pourreza et al. 2010). The cobalt content in the four different soils was determined using the proposed procedure (Table 2). The performance of the proposed method and official method was compared using two sample t tests (for accuracy) and F-test (for precision) at the 95 % confidence level. The statistical results show that there are good agreements between the Co(II) concentration in the proposed method and standard atomic absorption method.

Conclusions A simple, fast, and economic DLLME method as a preconcentration step has been employed to improve the analytical performance of UV/Vis spectrophotometric method in the ultra-trace determination of cobalt. Ethanol provided by the addition of an ethanolic NaOH acts as both analyte watermiscible solvent and disperser solvent which create binary component system. The proposed method effectively minimizes the use of hazardous Table 2 Co(II) content (μg g−1) in the soil samples determined using the proposed and atomic absorption official method Sample Proposed methoda

Atomic absorption method

Fexpb Spool

Soil 1

1.21 ± 0.07

2.04 0.086 1.28

Soil 2 Soil 3 Soil 4 a

1.3 ± 0.10 8.0 ± 0.40 12.0 ± 1.1 0.5 ± 0.02

8.3 ± 0.25

2.56 0.33

texpc

1.11

10.7 ± 0.9

1.49 1.0

1.6

0.65 ± 0.04

4.0

0.57

0.32

Mean value ± S.D. (n = 3)

b

F-test calculated, theoretical value = 39.0 (P = 0.05)

c

Student’s t test calculated, theoretical value = 2.78 (P = 0.05)

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reagents and solvents and allows the sample extraction and preconcentration to be done in a single step. The proposed method also has good reproducibility and accuracy and can be applied for the determination of Co(II) in the soil samples.

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