Determination of Trace Amounts of Copper in River and Sea Water Samples by Flame Atomic Absorption Spectrometry ( FAAS ) after Cloud-point Preconcentration

Este trabalho propõe um novo método de pré-concentração baseado em ponto nuvem para a determinação de cobre. O reagente 1,5-difenil-benzoína (Cupron) foi usado como complexante e Triton X-114 foi adicionado como surfactante. Após a separação das fases e diluição da fase rica em surfactante com metanol em meio acidificado foi feita a determinação do cobre por espectrometria de absorção atômica com chama (FAAS). O fator de enriquecimento do cobre, após a otimização das condições de complexação e otimização, foi igual a 88. O procedimento proposto possibilitou a determinação de cobre com limite de detecção igual a 0,04 μg L-1. O procedimento foi aplicado com sucesso na determinação de cobre em amostras de água de rio e de mar. O desvio padrão relativo foi de 1,9 % para o Cu. Também foram investigados efeitos de interferência de alguns ânions e cátions.


Introduction
Trace elements play important roles in biological processes, the compounds of which serve both as essential compounds and as toxins.Copper is both vital and toxic for many biological systems.Thus, the determination of trace amounts of copper is becoming increasingly important because of the growing interest in environmental pollution.Much interest and effort have been devoted to the studies of copper determination in water and biological matrices because it is a good tool for environmental and toxicological monitoring.Removal of copper from aqueous mediums such as effluents is also of great interest for environmental and human health purposes. 1Copper is an essential element for enzymes but above a healthy limit it accumulates in liver, causing dizziness, vomiting, diarrhea, transpiration and, depending on its concentration, death from bleeding. 2 Generally, determination of an extremely low concentration of this element is associated with separation and preconcentration steps due to insufficient sensitivity or matrix interference.Moreover, aqueous micellar solutions can replace the more dangerous and toxic organic solvents allowing us to perform the analysis under mild conditions. 3,4Quantification of low concentrations of metals requires either very sensitive instrumental techniques or preconcentration to achieve detection limits within the range of the available equipment. 5This procedure is attractive when flame atomic absorption spectrometry (FAAS), a relatively simple and available technique in many laboratories, is used.Classical liquid-liquid extraction and separation methods are usually time consuming and labor extensive and require relatively large volumes of high-purity solvents.Of additional concern is the disposal of the solvent following use, which creates a severe environmental problem.][8][9][10][11][12][13][14] Cloud-point extraction is probably the most versatile and simplest method for the preconcentration and extraction of hydrophobic species from water.The technique is based upon a property of most non-ionic surfactants in aqueous solutions: to form micelles and become turbid when heated to a temperature known as the cloud-point temperature (CPT).Above this temperature, the micellar solution separates in a surfactantrich phase, in which the surfactant concentration is close to the critical micellar concentration (CMC). 15This phenomenon, which is especially observable with polyoxyethylene surfactants, could be attributed to the ethyl oxide segments in the micelle, which repel each other at low temperatures and attract each other at high temperatures. 16he cloud point phenomenon is reversible and when the temperature falls below the CPT, a single phase appears again.Compared with the recent developments in preconcentration and determination of Cu, the proposed method is simple and sensitive. 17,18The phase separation phenomenon has also been used for the extraction and preconcentration of metal ions after the formation of sparingly water-soluble complexes. 19,200][31][32][33][34] In the present work we report the results obtained from studying the CPE to determine the Cu content of sea and river waters by using Cupron as a complexing agent and Triton X-114 as a surfactant.FAAS was used for the detection.

Apparatus
A Perkin-Elmer model 232-A atomic absorption spectrometer equipped with a copper hollow-cathode lamp as a radiation source was used throughout the measurement made at 324.7 nm.The acetylene flow rate and the burner height were adjusted in order to obtain the maximum absorbance signal, while aspirating the analyte solution in methanol.A thermostated bath maintained at the desired temperature was used for cloud point preconcentration experiments and phase separation was assisted using a centrifuge.

Materials
All reagents used were of analytical-reagent grade.Stock solutions of Cu (1000 mg L -1 ) and those used for the interference study (100 mg L -1 ) were prepared by dissolving appropriate amounts of their respective salts in doubly distilled water.The non-ionic surfactant Triton X-114 (Fluka Chemie AG, Switzerland) was used without further purification.1,5-diphenyl-benzoin (Cupron) (Fluka Chemie AG, Switzerland) was dissolved in 99.8 % methanol.A stock buffer solution (1 mol L -1 ) was prepared by dissolving appropriate amounts of acetic acid and sodium acetate in water.The pipettes and vessels used for the trace analysis were kept in sulfochromic acid mixture for at least 1 hour and subsequently washed four times in succession with water.The copper stock solution was prepared by dissolving 1.0000 g copper powder in concentrated nitric acid and then diluted to 1.0 L in a volumetric flask.The other Cu solutions were prepared by dilution of appropriate volumes of the stock solution.

Procedures
For CPE, aliquots of 60 mL solutions containing the analyte, 0.1% (m/v) Triton X-114 and 1 × 10 -4 mol L -1 Cupron buffered at a suitable pH were placed for 10 min in a thermostatic bath maintained at 50 °C.Phase separation was achieved by centrifugation at 3500 rpm for 10 min.The phases were cooled in an ice bath to increase the viscosity of the surfactant-rich phase.The bulk aqueous phase was easily decanted.To reduce its viscosity, the remaining micellar phase (200 μL) was dissolved in 800 μL of methanolic solution of 0.1 mol L -1 HNO 3 .The final solution (approximately 1.0 mL) was introduced into air acethylene flame by conventional aspiration.

pH effect
The pH was the first parameter evaluated in the determination of the copper content.For this study, acetate, borate and ammoniacal buffers were used at different pH values.The effect of the sample pH on the copper response was investigated within the range 2.0-9.0.As can be seen in Figure 1, the best pH range for Cu (II) maximum extraction efficiency was 5.8-6.2.Therefore, the sample pH was maintained at 6.0 using acetate buffer in this study.J. Braz.Chem.Soc.

Triton X-114 concentration effect
The effect of surfactant concentration was studied within the Triton X-114 concentration range from 5 × 10 -3 to 2 × 10 -1 % (v/v).The surfactant Triton X-114 was chosen due its commercial availability and low cloud point temperature and high density of the surfactant rich phase, which facilitates phase separation by centrifugation.Figure 2 shows the effect of the surfactant concentration on the analytical signal.The maximum signal was observed when Triton X-114 concentration was 0.05 % (v/v).At concentrations higher than 0.05 % (v/v), the analytical signal decreased, probably due to the increase in the surfactant volume, which deteriorates the FAAS signal.At concentrations below this value, the extraction efficiency of complexes was low because there are few surfactant molecules to entrap the Cu-Cupron complex quantitatively.Accordingly, a concentration of 0.05 % Triton X-114 (v/v) was chosen for use in the subsequent experiments.

Effect of Cupron concentration
In this part the effect of different concentrations of the chelating agent Cupron on the analytical response was studied.Under the optimum pH, the effect of the cupron concentration as a chelating agent was studied on the analytical signal and the results were shown in Figure 3.It is clear from Figure 3 that the signal increases up to the concentration of 0.8 × 10 -3 mol L -1 , reaching a plateau, considered as complete extraction.A concentration of 1×10 -4 mol L -1 was chosen as the optimum concentration for the subsequent experiments.

Equilibration temperature and time effect
The equilibration temperature above the cloud point and equilibration time was thoroughly optimized.It was desirable to employ the shortest equilibration time and the lowest possible equilibration temperature, a compromise between completion of extraction and efficient separation of the phases.Figure 4 clearly shows that a temperature of 50 °C is adequate for the experiment.At lower temperatures separation of the two phases is not complete.The dependence of extraction efficiency on equilibration time was studied for a time interval of 5-25 min.An equilibration time of 10 min was chosen to be optimal to achieve a quantitative extraction.

Calibration, precision and detection limits
Calibration graphs were obtained by preconcentration of 60 ml of the sample in the presence of 0.05% Triton X-114 (v/v) under optimum experimental conditions.Table 1 gives the parameters for the calibration graphs, the relative standard deviations obtained for ten samples subjected to the complete procedure and the detection limits.The preconcentration factor calculated as the ratio of the concentration of the analyte after preconcentration to that before preconcentration which gives the same absorbance peak area was 88 for Cu.However, the preconcentration factors reported by other studies were usually obtained by using larger sample volumes.Furthermore, the need-to-handle volume of the final methanolic solution was estimated to be sufficient for the analysis of the Cu content and therefore it had to be increased to allow multiple runs for the same samples.The limits of detection were satisfactory, although further improvement is feasible, either by preconcentrating larger amounts of sample solution using higher concentrations of both the surfactant and chelating agent or by diluting the surfactant-rich phase in a smaller volume of methanolic solution.The precision of this method was established by repeatedly (n = 10) using 10.0 μg L -1 solutions of Cu.The relative standard deviation was found to be 1.9 for Cu.

Interferences effects
In view of the high selectivity provided by flame atomic absorption spectrometry, the only interferences studied were those related to the preconcentration step.The results shown in Table 2 show that the Cu recoveries are almost quantitative in the presence of the interfering cations.

Preconcentration and determination of Cu in river and sea water samples
To test the reliability of the proposed method for the assay of the Cu ion, it was applied to an analysis of sea and river water samples.Water samples were filtered using a 0.45 μm pore size membrane filter to remove the suspended particulate matter.As shown in Table 3, the proposed method was applied successfully to the preconcentration of trace amounts of Cu in sea and river water samples and spiked water samples.

Conclusions
We proposed the use of cloud-point extraction as an alternative method for the preconcentration of Cu before determination by FAAS.The surfactant-rich phase could be introduced into the nebulizer of a flame atomic

Table 1 .
Analytical characteristics of the methodThe limit of detection was calculated from 3S b /m equation, which m is slope of calibration curve and S b is standard deviation of blank sample (S b is obtained from ten consecutive measurements of blank). a

Table 3 .
Determination of Cu in sea and river water samples Sample volume 60 mL. a Mean of three determinations.

Table 2 .
Effect of foreign ions on the preconcentration and determination of the Cu content (50 ng mL -1 )