INTRODUCTION:

Copper is a nutritionally essential metal and is widely distributed in nature [1-3]. Copper occurs naturally in most vegetables, meat and fruits. In general, a daily copper intake of 1.5-2.0 mg is essential. A variety of clinical disorders have been reported due to dietary deficiency of copper [4]. Copper is an industrially important metal, it is used in coin making, wire making, medicine, alloys, fashioning metal products, transportation industry and thermal conductance [5]. On the other hand, toxic role of the metal ion is well recognized [6]. Increasing accumulation of copper (II) in the environment through numerous industrial sources, poses danger to public health. Thus the determination of

Cu (II) at trace level is important due to the harmful effects of the heavy metal on humans and the environment. But the determination of trace concentration of metal ions in environmental and biological samples is difficult because of various factors, particularly low concentrations of metal ions and/or matrix effects etc. Therefore new techniques for preconcentration and/or cleaning (to remove or decrease the matrix effects) are required to achieve detection limits within the range of the available equipment [7-9].

Recently, the cloud point extraction (CPE) using non-ionic surfactants has captured considerable attention as an alternative to the conventional extraction techniques for separation and preconcentration due to its simplicity, rapidity and for being environment friendly method [10-12]. The experimental conditions required in CPE methods for phase separation generally depend on the surfactants nature. Non-ionic surfactants normally used in conventional CPE are Triton X-100 and Triton X-114. But the only hitch in the cloudy separation occurred in these CPE methods by the surfactants is that, they require high temperatures and long extraction times [13,14]. To avoid this, instead of non-ionic surfactants, the cloudy separation can be brought about by using anionic surfactants [15-18]. Micelle mediated extraction using anionic surfactant has proved to be more advantageous than conventional method of CPE as it can be performed at low temperatures which provide high recovery and low limit of detection and also this method uses salting-out phenomenon rather than temperature change.

In the present study, an attempt has been made to establish a new procedure for the separation and extraction of Cu (II) by CPE methodology and its detection spectrophotometrically. The method is based on the complex formation between Cu (II) and isonicotinohydroxamic acid (INHA) as a new reagent in basic medium and subsequently extracted into the surfactant, sodium dodecyl sulfate (SDS) and NaCl solution at optimum conditions. The separated surfactant-rich phase is diluted with minimum amount of ethanol and Cu (II) is determined by UV-Vis spectromphotometry at λ_max of 800 nm. The proposed method was applied for the determination of Cu (II) in water samples.

EXPERIMENTAL:

Apparatus

A Systronics UV/Vis spectrometer was used for recording absorbance spectra with 1.0 mm glass cell. All spectral measurements were performed using the blank solution as a reference. An Elico Li 127 digital ionalyzer pH meter was used for checking the pH of solutions. A water bath with good temperature control and a centrifuge with 25 mL calibrated centrifuge tubes were used to accelerate the phase separation process.

Reagents and solutions

All the chemicals used in this experiment were of analytical grade and were used without further purification. All solutions were prepared with doubly distilled water. Stock solution of Cu (II) (1000 μg mL^-1) was prepared by dissolving appropriate amount Cu(NO₃)₂.3H₂O in distilled water and working standard solutions were obtained by appropriate dilution of the standard stock solution. An aqueous (2% w/v) solution of sodium dodecyl sulfate (SDS) was prepared by dissolving 2g of SDS in final volume of 100 mL distilled water. Borax buffer was prepared by adjusting the pH of 5×10^-2 M borax solution with 0.1 M potassium hydrogen phosphate to the desired pH value. Isonicotinohydroxamic acid (INHA) was synthesized according to the method described previously [19]. INHA standard solution (2.5×10^-3 mol L^-1) was prepared daily by dissolving the reagent in ethanol solution. An aqueous (1% w/v) solution of sodium chloride was prepared by dissolving 1g NaCl in 100 mL distilled water.

Recommended procedure

An aliquot of the solution containing 0.25-2.5 μg of Cu (II) standard solution was transferred to a 25 mL centrifuge tube, 1.0 mL of the 2.5×10⁻³ M INHA solution and 2.0 mL buffer solution of pH 8.0 were added. This was followed by the addition of 2.0 mL of 2.0 % SDS solution and 2.5 mL of 1.0 % NaCl solution. The solution was taken up to the mark with bidistilled water and allowed to stand for 5.0 min in room temperature. For the separation of two phases, the turbid solution was accelerated by centrifugation for 5 min at 2000 rpm. The mixture was cooled in an ice-salt bath to increase the viscosity of the surfactant-rich phase, and the aqueous phase was easily decanted by simply inverting the tube. The surfactant rich phase of this procedure was dissolved and diluted to 1.0 mL with a water-ethanol mixture (50:50) and transferred to a 1.0 mL glass cell for absorbance measurement at 800 nm against a reagent blank as the reference. The blank solution was prepared in the same way as the sample solution except that distilled water was used instead of the Cu (II) solution.

Sample Preparation

Water samples were collected from taps and effluents near a tannery and were preconcentrated. All the collected samples were spiked with a suitable amount of standard solution of Cu (II). All the samples were filtered through a 0.44 μm membrane to remove the suspended and floated particles. Standard addition method was applied so as to calculate recovery values and check accuracy of results.

RESULTS AND DISCUSSION:

The UV-VIS spectrum (Fig.1) of the extracted solvated species, obtained by CPE method showed maximum absorbance at wavelength λ_max- 800 nm.

Figure 1 The absorption spectrum of copper (II)-INHA complex

Optimization of the system

To take full advantage of the procedure, the reagent concentrations and reaction conditions must be optimized. Various experimental parameters were studied in order to obtain optimized system. The investigations are presented in table 1.

Table 1

Optimum condition in determination of Cu (II) by cloud point extraction

Parameter	Range study	Optimum
pH	3.0-11.0	8.0
INHA (M)	1.0×10^-3- 3.5×10^-3	2.5×10^-3M
SDS (%)	1.5-2.0	2.0 %
NaCl(%)	0.5-3.0	1.0 %
Time of bath (min)	1-30	5 min
Time of centrifugation (min)	5-25	5 min

Effect of SDS concentration

The concentration of anionic surfactant, SDS can affect the extraction of complex and sensitivity of the method, therefore the effect of SDS concentration on the absorbance of the extracted phase was investigated. The absorbance of the surfactant-rich phase increased by increasing SDS concentration between 1.5 % and 2.0 % (w/v) and remained nearly constant at higher concentrations. Therefore, 2.0 % (w/v) SDS was used as optimum concentration throughout the studies.

Effect of ionic strength

The influence of ionic strength was examined by studying the extraction efficiency for NaCl concentration in the range 0.5-3.0 %. Ionic strength had no significant effect upon recovery percentage and sensitivity up to 3%. So a concentration of 1% (w/v) as the optimum NaCl concentration was chosen for the highest possible extraction efficiency.

Effect of pH

The pH is crucial factor influencing both the reaction between metal ions and chelating molecules and CPE procedure. Cu (II) reacts with INHA to form intense green coloured complex and in a previous study, the characteristics of this chelation is described [19]. The impact of pH on the extraction of 1.0 μg mL^-1of Cu (II) was investigated in the range of 3.0-11.0. Fig. 2 shows that the maximum absorbance and efficiency of CPE method was obtained in the pH range of 7.5 - 8.5 by the addition of borax buffer. The effect of different volumes of the buffer solution (0.5 - 6.0 mL) was investigated. The results showed that the extraction efficiency of the solution increased with buffer volume up to 2.0 mL and remained constant above that. Hence, 2.0 mL of borax buffer was added to the sample solutions to maintain the pH at 8.0.

Figure 2 Effect of pH on cloud point extraction of copper

Effect of INHA concentration

In order to determine the optimal reagent concentration, all other experimental variables, except reagent concentration, were kept constant. The variation of the analytical signal as a function of the concentration of INHA in the range of 1.0×10^-3- 3.5×10^-3M was studied, and the experimental results in Fig. 3 showed that the signal intensity of the analyte was practically constant by INHA at concentrations up to about 3.0×10^-3M. A 2.5×10^-3M INHA concentration was selected for all the proposed research work.

Figure 3 Effect of INHA concentration on CPE of copper

Choice of solvent

It was found that ethanol is the best solvent for dissolving INHA reagent. The study revealed that, the extracted complex exerted maximum absorbance in the presence of ethanol up to 20%. However, 10% ethanol content was used in the proposed procedure.

Time required for color development

The formation and stability of the Cu (II)-INHA complex with respect to time was investigated using the optimum experimental conditions of determination. The study showed that, the maximum absorbance takes place 5 minutes after mixing. The order of addition of the reagent and surfactant has no effect on the stability of the formed complex.

Selection of the dilution solvent for the surfactant rich phase

Different solvents such as acetonitrile, ethanol, methanol, THF, mixture of acetonitrile-water and mixture of acetonitrile-ethanol were tested. The aim was to select a solvent or a mixture so that the surfactant rich phase and the extracted compound could be dissolved completely. The results showed that ethanol is the best solvent to dissolve the extracted phase. Therefore, ethanol was selected as the diluent for further experiments.

Effect of other experimental factors

The influence of reaction completion time, extraction time and centrifugation time on the extraction efficiency was optimized. The dependence of extraction efficiency on reaction completion and extraction time was studied within a range of 1-30 min and 0-10 min, respectively. The results showed that a reaction completion time of 5 min and an extraction time of 2 min are adequate and sufficient to quantitative extraction of Cu (II) based on the proposed method. The effect of the centrifugation time on the extraction efficiency was also studied within a range of 5-25 min. A centrifugation time of 5 min at 2000 rpm was selected for the entire procedure, since the analyte extraction in this time is almost quantitative.

Calibration curve and detection limit:

For determination of Cu (II) spectrophotometrically, a calibration curve (Fig.4) was prepared by carrying the extraction process as detailed under principal method with varying concentration of the metal ion and recording the absorption at 800 nm. The dynamic range of Cu (II) was 0.01-20.3 μg mL^-1 and the linear relationship (n=10) has been found to be 0.11-1.02 μg mL^-1with correlation coefficient (R²) of 0.999, showing good linearity of calibration curve. Based on the signals of ten blank solutions and the slope of calibration curve, it was found that the detection limit was 1 μg mL^-1. Table 2, summarizes the analytical characteristics of the optimized method.

Figure 4 Calibration curve of Cu (II) – INHA complex in SDS

Table 2

Analytical features of the proposed method

Parameter	Result
Regression equation (n = 10)^a	r = 0.999 ^b
Linear range (μg mL^-1)	0.11- 1.02
Molar absorptivity (L mol^-1cm^-1)	1.248 x 10⁴
Sandell's sensitivity (μg cm^-2)	0.0027
Limit of detection (n = 10, μg mL^-1)	1.06
Enhancement factor	33.3

a Concentration of Cu (II) inμg mL^-1

b Regression coefficient

Interference effect

The impact of potential inference of some cationic and anionic species on the preconcentration of Cu (II) was investigated. 10 μg mL^-1 of Cu (II) solutions were prepared and different amounts of foreign ions were spiked and examined according to the proposed method under optimal conditions, and the tolerance of the foreign ions, defined as a relative error of ±5% with respect to the absorbance difference for the Cu (II) solution, is shown in table 3.

Table 3

The effect of Foreign ions on the determination of 10 μgL^-1of Cu (II)

Foreign ions	Tolerance Limit (μg mL^-1)
K⁺, SO₄^2-, Al³⁺	400
NO^3-, NH₄⁺	250
Ba²⁺, CO₃^2-	100
Zn²⁺, Bi²⁺, Sn²⁺	6
Pb²⁺, Cd²⁺	20
Fe³⁺, Mn²⁺	15
Ni²⁺, Co²⁺	10

^*. Masked with suitable masking agents

APPLICATION OF THE METHOD

The developed procedure was applied to the determination of metal ions in tap water, effluent water samples and alloy sample by standard addition method. The results are presented in table 4.

Table 4

Determination of copper in water samples by proposed method

Sample	Cu (II) added (μg mL^-1)	Cu (II) found (μg mL^-1)	Recovery (%)
	-	n.d. ^a	-
Tap water	0.5	0.48 ± 0.004 ^b	96.6
	1.0	0.99 ± 0.009
	--	n.d.	--
Effluent water	0.5	0.49 ± 0.012	98.0
	1.0	0.95 ± 0.004	95.0

^a No detected.

^bmean ± S.D. (n=3).

CONCLUSION:

The proposed method gives a simple, very sensitive, and low-cost spectrophotometric procedure for determination of Cu (II) that can be applied to water samples. Anionic surfactant has been used for separation and preconcentration of Cu (II) in samples, and thus toxic solvent extraction, has been avoided. The proposed method is faster and simpler and that it provides a wider dynamic range and a lower limit of detection. The results of this study clearly shows the potential and versatility of this method, which could be applied to monitoring of Cu (II) spectrophotometrically in various samples.

ACKNOWLEDGEMENT:

The authors would like to thank the Principal and also the Head of the department of Chemistry of Sri Meenakshi Govt. Arts College for Women(A), Madurai, for providing the necessary facilities required for the research work. The authors are also very much grateful and thankful to UGC-SERO Hyderabad for their financial support.

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Received on 30.12.2017 Modified on 13.01.2018

Asian J. Research Chem. 2018; 11(2):355-359.

DOI:10.5958/0974-4150.2018.00064.0