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Journal of the Chilean Chemical Society

On-line version ISSN 0717-9707

J. Chil. Chem. Soc. vol.49 no.2 Concepción June 2004

http://dx.doi.org/10.4067/S0717-97072004000200011 

 

J. Chil. Chem. Soc., 49, N 2 (2004), pags.:163-168

SCREENING OF SOME TRANSITION METAL IONS AND QUANTITATIVE DETERMINATION OF COPPER, CADMIUM AND ZINC BY SOLID PHASE DERIVATIVE SPECTROPHOTOMETRY

 

M. Inés Torala, Nelson Larac, Jéssica Narváeza,b, and Pablo Richterb,*

a Faculty of Sciences, University of Chile, PO. Box 653, Santiago, Chile.
b Faculty of Chemical and Pharmaceutical Sciences, University of Chile, Santiago, Chile.
c University of Antofagasta, Antofagasta, Chile, e-mail: prichter@ciq.uchile.cl


ABSTRACT

A simple solid phase spectrophotometric method for both screening of eight transition metal ions plus lead and quantitative determination of copper, cadmium and zinc is described. The method is based on the preparation of a sensitive analytical zone by immobilization of the organic reagent 1-(2-pyridylazo)-2-naphthol (PAN) in a Dowex 50WX2-100 resin, in which Cd, Cu, Zn, Co, Fe, Ni, Ag, Pb and Hg react at pH 10, to form colored complexes on the surface of the resin. Absorbance can be measured directly on the solid phase at 550 nm, to detect the presence or absence of these cations in a solution sample. Physical-chemical variables of the method were optimized in order to find suitable analytical conditions for the simultaneous determination of Cu, Cd and Zn by solid phase derivative spectrophotometry. Under the selected conditions the three analytes can be accurately determined between 1 and 100 ng·mL-1, with a detection limit (3s criterion) of 0.9, 0.5, and 0.4 ng·mL-1, for Cu, Cd and Zn, respectively. The repeatability, expressed as the relative standard deviation (RSD), was lower than 2 %. The percentage recoveries in the determination of Cu, Cd, and Zn in a quality control standard were between 98 and 103 %.

Keywords: Simultaneous determination, Screening, transition metal ions, solid phase, derivative spectrophotometry.


INTRODUCTION

Toxic heavy metals in the environment is a global problem that causes growing concern to humanity. There are hundreds of anthropogenic sources of heavy metal pollution, including the mining, coal, natural gas, paper, and chloro-alkali industries. In response to the growing problems, governments have set up environmental regulations to protect the quality of surface and ground water from heavy metal pollutants.

Among the most common analytical methods for the determination of metal ions are spectrochemical techniques such as atomic absorption or inductively coupled plasma optical emission spectrometry. These techniques may not be readily available in all laboratories and these types of systems do require significant space.

The use of rapid screening systems (1) for the identification of one or several analytes in a sample is increasingly recurrent in analytical chemistry. A binary qualitative response (yes/no) can be helpful before using complicated quantitative analytical techniques which are time consuming in generating information. Often, a binary response avoids the application of long analytical processes, providing rapid and opportune information to make decisions. After a screening test, if further detailed level of information is required the application of a quantitative analytical method is mandatory.

The Toxicity Characteristic Leaching Procedure (TCLP) is an EPA SW-846 analytical method that simulates sanitary landfill contaminant leaching in waste samples. Based upon concentrations of the TCLP constituents and guidelines set forth in 40 CFR 261.4, the solid waste samples can be considered hazardous or non-hazardous. Considering that the TCLP and other leaching procedures are time consuming analytical methods for the determination of the toxicity characteristics from metallic analytes in wastes, the screening method development can be useful to determine the absence of metals in the leaching, thus avoiding the use of a spectroscopic technique to determine the absence of each analyte.

The chemical reactivity of metals with the organic reagent 1-(2-pyridylazo)-2-naphthol (PAN) has been previously used in the development of analytical methods (2-6). On the other hand, it has been observed that the reagent PAN can be easily and strongly immobilized on solid supports, like ion exchange resins, generating a sensitive surface for preconcentration and reaction in the solid phase with metals. This reaction between PAN and the metallic analyte renders a change in the color of the solid surface, which can be screened directly by spectrophotometric measurement in the solid phase.

The solid phase spectrophotometry approach has been applied since 1976 (7), and it presents the advantage that the sensitivity is considerably higher than the corresponding spectrophotometry in liquid phase (8-14). Further the selectivity is also increased because the implicit separation of the analyte from the matrix can be reached selecting the appropriate conditions.

Classic spectrophotometry is not an appropriate technique for simultaneous determination of groups of analytes because overlapping bands are not resolved. In order to enhance the detection of minor spectra features and to carry out simultaneous determination without previous separation, the derivative spectrophotometry, which consists in the differentiation of a normal spectrum, appears useful in several matrixes (11-14).

The aim of this work was to develop a simple, reliable and inexpensive preconcentration and reaction system based on solid phase spectrophotometry, which can be used both, for metal screening purposes (Cd, Cu, Zn, Co, Fe, Ni, Ag, Pb and Hg) and for quantitative determination of copper, cadmium, and zinc in liquid samples by derivative spectrophotometry.

EXPERIMENTAL

Apparatus

A Shimadzu UV-PC 1603 spectrophotometer with 1-mm cells was used for absorbance measurements and to obtain derivative absorption spectra. An Orion Research Digital Ion - Analyzer 701 with glass and saturated calomel electrodes was used for pH determinations. A magnetic stirrer HI 190 M, Hanna instruments, was used for the preconcentration step.

Reagents

All reagents were of analytical grade and the solutions were prepared with high-purity water from a NANOpure Barnstead ultrapure water system device.

All standards of metals [Hg(II), Pb(II), Cd(II), Zn(II), Co(II), Ni(II), Ag(I), Fe(II) and Cu(II)] were Merck Titrisol solutions, 1000 mg·mL-1 .

Analyte solutions of 10 and 100 mg·mL-1 were prepared by dilution of the standard solutions. Other concentration ranges were prepared by appropriate dilution of the standard solutions as well. All the diluted solutions were stored in polyethylene containers at 4°C for a maximum of 48h.

Solution of 0.1 % 1-(2-pyridylazo)-2-naphthol (Merck), (PAN). This solution was prepared by dissolving 10 g of compound and diluted to 100 mL with methanol.

Solution of 1.0 x 10-3 molL-1 of dimethylglyoxime (DMG) dissolved in 1,2-dichloroethane. This solution was prepared by dissolving 0.029 g of compound and diluted to 250 mL with 1,2-dichloroethane as solvent.

Sodium borate - boric acid buffer (pH = 10,0). This solution was prepared by dissolving 12.78 g of sodium borate (Merck) in 100 mL of water, followed by the addition and dissolution of 6.18 g of boric acid (Merck) and was diluted to 1000 mL.

Resin, Dowex 50WX2-100 cation exchanger, p.a. Sigma, was used for immobilization of the reagent PAN and preconcentration of the analytes.

Leaching Solutions of pH=4.93 (L-1) and pH=2.88 (L-2).

L-1: Add 5.7 mL glacial acetic acid to 500 mL of water, add 64.3 mL of 1 molL-1 NaOH solution, and dilute to 1000 mL. When properly prepared, the pH of this solution was 4.93 ± 0.05.

L-2: Dilute 5.7 mL glacial acetic acid with water to 1000 mL. When correctly prepared, the pH of this solution was 2.88 ±0.05.

In the validation of the quantitative and screening method a Quality Control Standard 19 (QCS 19) (High Purity Standards) was used. This QCS 19 sample contains 100 mg·mL-1 of Sb, As, Be, Cd, Ca, Cr, Co, Cu, Fe, Pb, Mg, Mn, Mo, Ni, Se, Tl, Ti, V and Zn in 5% HNO3 and trace of HF.

Procedures

Activation of the resin Dowex 50WX2-100

Place in a 1000 mL beaker 300 mL of ultrapure water, add 30 g of the resin Dowex 50WX2-100 and stir for one hour and discard the excess of water. Then add 150 mL of 0.1 molL-1 hydrochloric acid solution and stir for 5 minutes. Wash out the resin with water to obtain pH 6.0. Then dry the resin at 45°C for 24 hours.

Immobilization of the reagent on the resin

Transfer a portion of 15 mL of PAN solution into a 100 ml beaker and add 5.0 g of resin Dowex 50WX2-100 free of water. Stir the mixture for 1h. Collect the resin beads by filtration, wash twice with 10 mL of high purity water and finally dry the resin at 45 C.

Procedure for the screening method (Cd, Cu, Zn, Co, Fe, Ni, Ag, Pb and Hg)

Place in a 500 mL beaker an aliquot of 10 mL of sample solution containing the metals in study, add 10 mL of 0.1 molL-1 boric/borate buffer solution (pH 10.0) and adjust the total volume to 250 mL with water. Add 60 mg of the resin Dowex 50WX2-100 containing the PAN immobilized (PAN-resin) and stir for 15 minutes. After, discard the solution and wash twice with high purity water. Then pack the resin by using a pipette into 1mm spectrophotometric cell and thereafter evaluate the absorbance by classic and derivative spectrophotometry against a reagent blank between 400 and 800 nm using a smoothing factor of 20 and a scale factor of 104.

Procedure for quantitative determination of Cu, Cd and Zn

Place in a 500 mL separating funnel an aliquot of 200 mL sample solution containing the metals in study. Add 15 mL of 1.0 x 10-3 molL-1 DMG dissolved in 1,2-dichloroethane. Shake the funnel for 3 minutes, and allow separating the phases and discard the organic layer.

Place in a 500 mL glass beaker the aqueous layer and add 1.0 g of KF dissolved in 4 molL-1 HNO3 solution and heat for ten minutes. Then cool down the solution and add 10 mL of boric acid/sodium borate buffer solution (pH=10), 1g of KI and adjust the total volume to 250 mL, stir for two minutes and add 60 mg of the PAN-resin and stir again for 60 minutes. Then discard the solution and wash the solid phase twice with ultrapure water. Finally pack the resin using a pipette into 1-mm spectrophotometric cell. Record the zero-order spectra of the solid phase between 800 and 400 nm against a reagent blank resin prepared under the same experimental conditions. Obtain the fourth digital derivative spectra using a smoothing factor of 40 and a scale factor of 104.

RESULTS AND DISCUSSION

The organic reagent 1-(2-pyridylazo)-2-naphthol, (PAN) reacts with Cd, Cu, Zn, Co, Fe, Ni, Ag, Pb and Hg at pH=10 to form colored chelates, which absorb near to 550 nm. Considering that all complexes are cationic chelates, these can be retained on an cation exchange resin, which can be used to introduce a preconcentration step previous to the instrumental measurement in order to increase the sensitivity of the analytical signals.

In order to obtain an appropriate solid phase for the retention of these analytes, retention of M-PAN chelates at pH=10, were checked in the following solid supports: Chelex-100, Dowex 50WX2-100, Sephadex sp C-25, Permutit Decalso, Permutit HPF and Permutit H-70. The cation-exchanger Dowex 50WX2-100 was found to be the best for the retention of these complexes, considering the relative sensitivity.

The retention of the analytes was also studied on the same resins, but the PAN was previously immobilized on them. In this case the resin Dowex 50WX2-100 was also found to be the most satisfactory for the retention of the analytes.

It was observed that in both cases the retention of the target species is practically irreversible. Different acid eluents and solvents were tested, but it was not possible to remove completely from the solid phase neither the complexes nor the cations. Thus the aromatic structure of the PAN molecule must be important for its retention on the resin by interaction with the organic structure of the polymer. The complexes retention is favored for a similar interaction, together with the electrostatic attraction and the high stability of the complexes.

Considering that the retention of the analytes is possible by these two approaches, PAN immobilized on the resin was selected as the solid phase due to better reproducibility in results, lower base line and a fast procedure, because the addition of PAN solution to each sample is eliminated.

The change in the color of the resin surface when metals are retained and react with PAN into its surface can be used for analytical purposes, leading to a solid phase spectrophotometric screening method by monitoring the absorbance at 550 nm (Figure 1). This method, for instance, could be used to detect the presence or absence of transition metals in a solid waste leachate prior to perform all determinations established in the US-EPA toxicity characteristic leaching procedure (TCLP).


 
Fig. 1. Solid phase spectra of PAN complexes of Hg(II), Pb(II), Cd(II), Zn(II), Co(II), Ni(II), Ag(I) and Cu(II) retained in a DOWEX 50WX2-100 resin at pH 10.(A) Hg++, 200 mg·L-1 ;(B) Pb++, 120 mg·L-1; (C) Cd++, 10 mg·L-1; (D) Zn++, 10 mg·L-1; (E) Co++, 10 mg·L-1; (F) Ni++, 10 m·L-1; (G) Ag+, 600 mg·L-1 and (H) Cu++, 10 mg·L-1.

In order to establish the possibility to use this methodology, individual solutions for each analyte were prepared at the same concentration but using two leaching solutions of the TCLP both adjusted to pH 10 . The obtained spectra are equivalent to those attained starting from standard aqueous analyte solutions, probably indicating that leachate composition does not alter the formation of these complexes.

In this context, and in view of the TCLP rules for these analyte concentrations, the effect of preconcentration (stirring time) on their retention was studied. The results show that in all cases with 60-min stirring the formation and retention of these complexes was quantitative because in this condition the absorbances were maxima and constant.

The effect of the resin amount on the analytical signal was optimized (between 60 and 400 mg) following the general procedure. The results show that upon reducing the resin mass the analytical signal increased because the preconcentration factor was higher. A mass of 100 mg was selected to ensure a good packaging of cell.

To improve the formation and the retention of the complexes by the resin, and thus to decrease the time of analysis for screening purposes, the study of the temperature effect was performed. It was found that the increase in temperature does not favor this process.

To evaluate the efficiency of retention, using a short stirring time, a calibration graph was carried out with two stirring times: 60 and 15 min. As is shown in Table 1, when the preconcentration time is short the sensitivity of the method is lower, but the precision is satisfactory for screening purposes and the sensitivity achieved is also adequate to detect concentrations near the TCLP limits.


Table 1. Effects of the stirring time on the sensitivity for different metal ions at pH=10

Metal l max.
nm
Sensitivitya
at 15 min (S1)
Sensitivity
at 60 min (S2)
S1/ S2 Ratio

Hg 564.5 1.5 x10-3 1.8 x10-3 1.20
Pb 558.5 1.2 x10-3 1.7 x10-3 1.41
Cd 556.5 1.1 x10-2 2.6 x10-2 2.36
Zn 556.0 1.4 x10-2 2.6 x10-2 1.86
Co 587.0 1.6 x10-2 2.8 x10-2 1.75
Ni 567.0 5.1 x10-2 5.9 x10-2 1.16
Ag 559.5 3.6 x10-4 5.0 x10-4 1.39
Cu 561.0 8.9 x10-3 3.0 x10-2 3.37
Fe 587.0 3.7 x10-4 9.0 x10-4 2.43

a Units: A mL ug-1

On the other hand, the effect of pH on the analytical signal of these cations was studied between pH 2 and 12. These effect is different depending on the cation involved. This fact together with the use of appropriate masking agents and liquid-liquid extraction allowed the development of a quantitative method for simultaneous determination of Cu, Cd, and Zn.

In this way, if Ni and Co are present in the sample they are eliminated by extraction from the sample using 15 mL of 1.0 x 10-3 molL-1 DMG solution in 1,2-dichloroethane. If the aqueous phase contains mercury and iron, 1g of KI was added as masking agent for Hg and 1.0 g of KF dissolved in 4 molL-1 HNO3 solution was used to preclude the interference of iron. In these conditions 60 mg of PAN-resin (solid phase) was used for the preconcentration and determination of Cd, Cu and Zn by derivative spectrophotometry.

Determination of cadmium, copper and zinc Study of the preconcentration conditions

The preconcentration parameters were optimized separately by the univariate method. This study was performed at room temperature, because this process was independent of the temperature in the range between 15-40°C. The effect of the mass of PAN-resin on the analytical signal was optimized using the general procedure, but the mass of resin was varied between 50 and 400 mg. For the three analytes, the absorbance signals decrease exponentially with the mass increase of the PAN-resin. For the selection of this parameter it was also considered mandatory to have adequate mass of the resin to pack a cell of 1 mm. In this context, a mass of 60 ± 0.01 mg was selected. The effect of the stirring time on the retention efficiency for the three cations on Dowex 50WX2-100-resin was also studied. According to the Figure 2, all analytes show the same behavior, being 60 min an optimum stirring time for the 60 mg amount of PAN-resin, and was the time selected. These conditions were selected for adequate sensitivity and resolution of the derivative signals.


 
Fig. 2. Effect of stirring time on the retention of Cd (II), Cu(II) and Zn(II) on the PAN-resin. Analyte concentration;10 mg·L-1.

A study of the effect of the sample volume on the absorbance signal was done for each analyte, using a constant analyte amount of 400 ng and 60 mg PAN-resin. The absorbance signal was constant up to 2000 mL. This effect is interesting since it allows improving the detection and quantification limits of the proposed method.

Study by derivative spectrophotometry

The zero-order spectra of the Cd(II), Cu(II) and Zn(II) retained on PAN-resin vs a reagent blank resin, show one band between 500 and 600 nm, corresponding to the absorption of the M(II)-PAN complexes (Figure 1), which are strongly overlapped. Thus, the derivative spectrophotometric technique was used to enhance the spectral details and to distinguish among the three complexes enabling the assessment of the derivative of absorbances as a function of wavelength for each analyte at selected wavelengths.

Spectral parameters selection

The derivative of absorbance signals were evaluated directly in the solid phase. To minimize both the dispersion phenomena and the background noise the digital derivative spectrophotometry with smoothing was adopted. Traditionally, in the differentiation mode of the classic spectra the Dl value is modulated; a small value of this variable yields a high resolution of the spectra, but usually the background noise, increases too affecting the signal to noise ratio. Savitzky and Golay (15) introduced a least square procedure in digital derivative spectrophotometry in order to smooth and to differentiate the spectra numerical data. In this procedure, Dl is constant and the noise is minimized. In this work, this approach was used.

Derivative order selection

Beginning with the zero-order spectra of these complexes retained on the solid phase, the first to the fourth derivative spectra were obtained thereafter (Figure 3). According to Figures 3 and 4, the fourth derivative is more useful for the simultaneous determination of these analytes. The copper determination can be carried out directly at 566 nm because in this point the zinc and cadmium complexes present a zero crossing point. On the other hand, cadmium and zinc can be determined by means of an equation system at 557 nm and 574.5 nm (Table 2).


 
Fig. 3. First, second, third and fourth derivative spectra of the complexes Cd(II)-PAN, Cu(II)-PAN and Zn(II)­PAN retained on PAN-resin. (A) Cd (II) 10 mg·L-1; (B) Cu (II)10 mg·L-1 and (C) Zn(II) 10 mg·L-1.


 
Fig. 4. Zoom of the fourth derivative spectra of the complexes Cd(II) -PAN, Cu(II) - PAN and Zn(II) ­ PAN retained on PAN-resin. (A) Cd(II) 10 mg·L-1; (B) Cu (II)10 mg·L-1 and (C) Zn(II) 10 mg·L-1.

Tabla 2. Analytical figures of merit for the determination of cadmium, copper and zinc.

  Cadmium Copper Zinc

Detection limit
(ng·mL-1)
0.5 0.9 0.4
Quantitation limit
(ng·mL-1)
1.8 2.9 1.2
Determination
range (ng·mL-1)
1.8-100.0 2.9-100.0 1.2-100.0
       
Repeatibility (RSD/%) 2.0 1.6 2.0
       
Recovery in synthetic
mixtures (%)
97.8 101.9 102.6
       
Recovery QCS-19 (%) 98.0 100.3 102.9
       
Derivative order Fourth Fourth Fourth
       
Equation System
or Equation (C in ng/mL)
DU=0.054·C +
0.032 (l = 557 nm)
DU = 0.035·C +
0.075 (l=574.5 nm)

DU = 0,020·C +
0.082 (l = 566 nm)
DU=0,065·C + 0.153
(l = 557 nm)
DU=0,051·C +0.023
(l = 574,5 nm)

Smoothing and scale factors selection

Using the fourth derivative spectra, the smoothing factor was varied between 5 and 40. These values have a direct relation with the wavelength range scan, and the best signal to noise ratio was obtained when a smoothing factor of 40 was selected. The scale factor was varied from 10 to 104. In all this interval the distortion effects were not observed and the signal to noise ratio remains unaltered. The maximum value of 104 was selected for facility of absorbance lecture.

Features of the proposed method

In the selected conditions, calibration graphs were obtained by plotting the fourth derivative of the absorbance signal (DU, derivative unit) for copper, cadmium and zinc versus the concentration of the respective analyte. The linear regression equation and the correlation coefficient calculated for copper and the equations systems for the determination of cadmium and zinc in mixtures of these analytes are shown in Table 2. The detection limits (3s criterion), quantitation limits (10s criterion), the determination ranges, the repeatability, expressed as the relative standard deviation (RSD), for ten standard containing 40 ngmL-1 of each element, are also included. As can be seen these analytical features are quite comparable with conventional atomic absorption and emission methods, which involve the use of more expensive technology.

The accuracy of the proposed method was assessed by determination of the three analytes in synthetic solution mixtures. The recoveries obtained in standard solutions containing copper (1 mgmL-1), cadmium (1 mgmL-1), and zinc (5 mgmL-1) were between 97.8 and 102.6 % (n=6), as expected.

Validation and application

The Quality Control Standard 19 (QCS 19) was spiked with Ag+ and Hg++ solutions and diluted in water in order to obtain a final concentration of 0.2 mgmL-1 of each analyte. This solution was evaluated by following the screening procedure obtaining a high analytical response near to 550 nm, which indicates the presence of transition metal ions, however this method do not discriminate which cations are present in each sample.

The validation of the quantitative simultaneous derivative spectrophotometric method was done in the same reference QCS 19. Good recoveries (Table 2) and relative standard deviations lower than 3 % were obtained, in spite of the presence of concomitant ions such as As, Be, Ca, Cr, Mg, Mn, Mo, Se, Ti, Tl and V, which could interfere in these determinations.

The proposed method was also applied in the leaching solutions of pH=4.93 (L-1) and pH=2.88 (L-2), which were enriched with 100 uL of QCS 19 solution. The recoveries obtained for copper, cadmium and zinc were between 102.5 and 98.9 % (n=5). In all cases the RSD was lower than 3.0 %.

CONCLUSIONS

The proposed method is useful for the enrichment of trace levels of analytes previous to the solid phase spectrophotometric screening of transition metal ions or quantitative determination of copper, cadmium and zinc. These analytes can be easily screened at ngmL-1 level with preconcentration times less than 15 min.

The method can be used as screening to estimate the total concentration of transition metals present in a large number of samples thus avoiding the continuous use of expensive instrumentation for routine analysis.

The reproducibility and recovery of the quantitative simultaneous method were found to be satisfactory as can be seen in Table 2. The detection limits obtained for this method for copper, cadmium and zinc are quite comparable with conventional atomic absorption and emission methods, which involve the use of more expensive technology

 

ACKNOWLEDGEMENTS

The authors thank to Fondo Nacional de Desarrollo Científico y Tecnológico de Chile (FONDECYT, Projects 1000757 and 1020692), CONICYT (AT-403001) and University of Chile PG-68-02 for financial support.

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Received: October 17, 2003 ­ Accepted: January 20, 2004

 

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