DERIVATIVE SPECTROPHOTOMETRIC METHOD FOR COPPER DETERMINATION M. INI~S TORAL* PABLO RICHTER and BRAULIO MUlqOZ Department of Chemistry, Faculty of Sciences, University of Chile, Las Palmeras no. 3425, P O. Box 653, Santiago, Chile (Received: November 1994; revised: March 1995)
Abstract. A simple, sensitive and selective method by solvent extraction-first derivative spectrophotometry is described for the determination of microamounts of copper in water by means of its reaction and extraction at pH 8.0 with 3-(4-phenyl-2-pyridinyl-5-phenyl-l,2,4-triazine)(PPT) and picrate (2,4,6-trinitro-phenol) into 1,2-dichloroethane. Copper was thus determined in the range 7.5350 ng/ml with a detection limits (3~) of 2.3 ng/ml. The relative standard deviations were in all instances less than 2.0%. The proposed method was successfully applied for the determination of copper in several kinds of water.
1. Introduction
Copper salts are usually used in water supply systems to control biological growths and to catalyze the oxidation of manganese. Corrosion of copper-containing alloys in pipe fittings of the distribution systems may introduce measurable amounts of copper into the water in a pipe system (Standard Methods For Examination of Water and Wastewater, 1992). On the other hand, it is well known that copper is an important trace metal in many biological and environmental systems. However, this metal is also highly toxic to fish and other organisms in water, whether it exists as free Cu(II) ion or as lipid-soluble copper complexes (Florence, 1986). North-central Chile is characterized by heavy copper mining production and consequently copper contents in several kinds of water may be expected, white, at the same time, becoming important pollutants of the environment. In this context, the detection and determination of copper in water samples are mandatory in order to establish the copper salts concentrations used in water supply systems, to determine the quality and contamination grade of several kinds of water. Reagents of the cuproine type are used for spectrophotometric determination of copper. However, other organic ligands have been proposed as chromogenic reagents for copper (Barua et al., 1984; Katani et al., 1984; Zhang et al., 1985; Reddy et aI., 1986; Evtimova et al., 1987; Arya et al., 1987). On the other hand, methods by extractive-spectrophotometry (Wasey et al., 1986) and derivativespectrophotometry (Gallardo Melgarejo et al., 1989) have been proposed. * To whom correspondence should be addressed.
Environmental Monitoring and Assessment 38: 1-10, 1995. (~) 1995 KIuwer Academic Publishers. Printed in the Netherlands.
2
M. INt~S TORAL ET AL.
Porphyrin chromogens were found to be highly sensitive but these complexes are formed slowly at room temperature and the formation of the colored complex is only possible after heating at 100 °C for 15 min. (Huang et al., 1985a,b). The substituted 1,2,4-triazine derivatives have also been used for the spectrophotometric determination of copper, 3- (4-phenyl-2-pyridinyl-5-phenyl- 1,2,4-triazine) (PPT) being the most sensitive for this analyte. However, the principal drawback of these reagents are their low specificity for copper because other transition metals (Fe, Co and Ni) lead to the same analytical reaction. In this work, a simple, sensitive and selective extraction-derivative spectrophotometric method for the determination of trace amounts of copper as the ionassociated Cu(I)-PPT-Picrate complex extracted into 1,2-dichloroethane is proposed. The absorbance of the resulting copper (I)-PTT-picrate ion-association complex and the derivative spectra were measured directly in the 1,2-dichloroethane. The selectivity is markedly enhanced by the use of the derivative spectrophotometric technique. This method is applied to the determination of copper in different water samples with low copper content, in these cases the atomic absorption spectrometry technique is not adequately sensitive (sensitivity = 25 ng/ml). Hence, the proposed method is a good alternative compared to the more expensive and less sensitive atomic absorption spectrometry method commonly used.
2. Experimental 2.1. APPARATUS For measurements of the absorbance and the derivative absorption spectra, a Shimadzu UV-160 with 10 mm cells was used. The pH measurements were made with a Orion Research Digital Ion-Analyzer 701 with glass and saturated calomel electrodes. 2.2. R E A G E N T SOLUTION All reagents used were of analytical-reagent grade. If necessary, any copper present as an impurity was removed. All solutions were prepared with distilled demineralized water. 2.2.1. Purification of Sodium Acetate Solution 100 g of sodium acetate was dissolved in 500 ml of water. Any copper present as an impurity was removed in the following manner: 1 ml of 10% (w/v) hydroxylammonium chloride solution and 1 g of sodium perchlorate in a separating funnel was added to the sodium acetate solution. 5 ml of 5 × 10 -3 M PPT solution was then added in dichloroethane and the funnel was shaken for 2 min. The phases were allowed to separate and the organic layer allowed to run. This extraction was repeated until the organic phase became colorless.
DERIVATIVE SPECTROPHOTOMETRIC METHOD FOR COPPER DETERMINATION
3
2.2.2. Hydroxylammonium Chloride Solution (10% w/v) 100 g of the salt was dissolved in 1000 ml of distilled-demineralized water. This solution was purified as described for the purification of sodium acetate. 2.2.3. Copper (II) Standard Solution (Titrisol Merck, 1000 #g/ml) 2.2.4. 3-(4-Phenyl-2-Pyridinyl)-5-Phenyl-l,2,4-Triazine (PPT) Solution in
Dichlorethane (5.10 -3 M) 0.1552 g of the compound was dissolved in 100 rnl of dichloroethane (DCE). 2.2.5. Picric Acid Solution (0.01 M) Solution 2.291 g of purified picric acid was dissolved in 1000 ml of distilled-demineralized water. 2.2.6. Foreign Ion Solutions Solutions of diverse ions for interference studies were prepared by dissolving the amount of each compound needed to give 10-1000 #g/ml concentrations of the ion concerned. All these solutions were stored in polyethylene containers. 2.2.7. 1,2-Dichloroethane (DCE) Extrapure (sp. gr. 1.25)
2.3. RECOMMENDED PROCEDURE 25 ml of the water sample containing less than 350 #g of copper was placed in a 250 ml separating funnel and 1.0 ml of the sodium acetate solution, 1.0 ml of hydroxylammonium chloride solution, 4.0 ml of picric acid solution was added and the total volume was adjusted to 100 ml. It was then mixed and set aside for 2 rain. The mixture was shaken for 3 min with 5.0 ml of 1.10 -3 tool/1 PPT solution in 1,2-dichloroethane. The phases were allowed to separate, and the organic layer run into a dry flask. The first derivative spectra was recorded over the range of 700-350 nm against a reagent blank at a scan speed of 480 nm/min with AA 12 nm using 10 mm cells.
3. Results and Discussion 3.1.
S PECTRAL CHARACTERISTICS
The absorption spectra of the binary Copper(I)-PPT and of the ternary Cu(I)-PPTpicrate combinations extracted into DCE were measured against DCE (Figure 1). It is evident that the Cu(PPT)2+ binary complex is not extracted into DCE. However, the introduction of picric acid into the system leads to the formation of a ionassociated complex, which is easily extracted into the organic phase.
4
M. INI~STORALET AL.
1.0, A
A 0.5-
0.0 /-,00
600
500
700
/nlTI Fig. 1. Absorption spectra of DCE extract of Cu(I)-PPT-picrate and Cu(I)-PPT complexes measured against DCE. A:-Cu(l)-PPT-picrate; B:-Cu(I)-PPT. Copper concentration = 100 ppb. All other conditions as in text.
It is known that the molar ratio of PPT to copper is 2 : 1 (Schilt et al., 1974), hence we assumed that the stoichiometry of this ion-association complex would be Cu(PPT)2picrate. The spectrum of the Cu(PPT)2picrate complex shows three absorption bands at 380, 505 and 600 nm. The first band can be assigned to the absorption of picrate and PPT and depends on the reagent concentrations. The other two bands correspond to the absorption of the chromophore Cu(I)-PPT and is a function only of the copper concentration. The more sensitive band at 505 nm was selected for analytical purposes. In order to make the determination more selective and reproducible, firstderivative spectrophotometry was adopted and the value of the vertical distance from the base line to peak values at 545 nm or from through to peak values between 470 and 560 nm can be used for analytical measurements (Figure 2a). The second derivative spectrum shows a lower signal-to-noise ratio; it was then discarded for analytical purposes (Figure 2b). 3.2. STUDY OF THE EXPERIMENTAL VARIABLES Variables were optimized by the univariate method. Table I shows the optimum values found for the chemical and spectral variables and the range over which they were studied. It can be seen in Table I that the working values for the concentration of PTT and picric acid were chosen by taking into account that the analyte can be present together with a number of foreign cations in the sample, which implies an evident
DERIVATIVE SPECTROPHOTOMETRIC METHOD FOR COPPER DETERMINATION
0.12-
5
3.
0.08 A~.~ A o.o~ 0.00. 0.0/-, - 400
+
!
.~
500
600
700
800
.~/nrn
O.OL,
A2A 0.00
-0.0~ 4
~00
500
600
¢'---------------
700
800
.~/nm Fig. 2. Derivative spectra of DCE extract of Cu(I)-PPT-picrate complex measured against DCE. a: First derivative spectrum; b: Second derivative spectrum. Copper concentration = 100 ppb. A ~ 12 nm. Scan rate, 480 nm/min. All other conditions as in text.
6
M. INt~S TORAL ET AL.
TABLEI Study of variables Variable
Range studied
Chemical pH PPT solution/
Optimum Working range value
1.0-12.0 6.0-10.0 0.1-12.0 0.3-12.0
8.0 10.0
1 • 10 -4 M
0.01 M picric acid solution/ml Aqueous/organic phase ratio
0.5-8.0
1.6-8.0
2-25
2-25
Spectral Classic spectrophotometry Analytical wavelength 350-700 505,600 Derivative spectrophotometry Derivative 0-4 1-2 order AA/nm 4.0-36.0 4.0-21.0
4.0 20
505
1 12.0
consumption of reagents. A pH value of 8 was chosen in order to keep the pH in the center of the optimum range. The aqueous/organic phase volume ratio was optimized taking into account a higher enrichment factor. The optimum values for the order of the derivative spectrum and the AA value for derivation were selected by considering the best signal-to-noise ratio. 3.3. FEATURESOF THE METHOD In the classic spectrophotometric mode, the calibration graph was obtained by plotting the absorbance (A) at 505 nm versus the analyte concentration. The linear regression equation calculated was: Cu (ppb) = 182A
(r = 0.9998)
On theother hand, in the derivative mode the calibration graph was obtained by plotting the first-derivative value h (the peak to the base line) at 545 nm with AA =
DERIVATIVE SPECTROPHOTOMETRIC METHOD FOR COPPER DETERMINATION
7
TABLE II Effect of foreign ions Tolerance limit, ng/ml Foreign
Classic
Derivative
species
spectrophotometry
spectrophotometry
CI-, SO42- , S032- , NO3-, CH3COO-, 52032-, tartrate, Br-, oxalate, citrate, NO2-, F-, SCN-
106a
106
P043-
105
106
A1+++, Mg++ , Na+ , Ca++ , Sr++, K+ , Mn++, Zn ++, Hg++, Cd++, Cu++, Bi+++
10 4a
104
Ni ++
5 × 102
6 × 103
Maximum tested. Copper 100 ng/ml.
a
12 nm, versus the analyte concentration. The linear regression equation calculated was: h = 1 . 1 4 x l 0 - 3 C ( p p b ) + 0 . 8 x 1 0 -4
( r = 0.9989)
where h is expressed as the derivative unit (DU), The standard deviation (~r) of the regent blank was 4.71 x 10 -4 DU. The detection limit, calculated by using the 3or recommendation, was found to be to 2.3 ng/ml and the determination range was between 7.7 and 350 ng/ml respectively. Ten standard solutions containing 0.10 #g/ml copper were analysed and the results gave a relative standard deviation of 0.94%.
3.4. EFFECT OF FOREIGN SUBSTANCES Solutions containing 0.1 #g/ml of copper and various amounts of other metal ions that can present in water were prepared and the recommended procedure was followed. An error of -4- 2% in the analytical signal was considered tolerable. The results o f the effect o f foreign ions on the determination of copper by the proposed method are summarized in Table II, from which it can be seen that this method is nearly specific for copper. The iron, nickel metal ions are tolerated, because in
8
M. INI~STORAL ET AL.
TABLE III Determination of copper in several kind of water Sample
Proposed method founda ppb (RSD)
AAS method founda ppb (RSD)
12.1 (0.9) 12.5 (0.8) 13.8 (0.7) 12.7 (O.9)
12.3 (1.9) 12.7 (2.0) 13.2 (2.0) 12.9 (2.0)
River water 1
2 3 4
Tap water 1
2 3 4
12.1
12.5
(1.5)
(].9)
13.2 (0.8) 12.3 (0.9) 15.3 (O.7)
13.8 (2.0) 12.6 (2.0) 15.8 (2.O)
12.4 (1.1) 26.2 (0.8) 30.1 (0.9) 25.1 (0.7)
12.5 (1.9) 26.8
Well-water 1
(2.0) 29.8 (2.0) 25.5 (2.0)
a Average of eight determinations. RSD = relative standard deviation.
this condition these ion-associated complexes are not formed. Thus, most common cation and anions are tolerated even when present in large amounts. 3.5.
APPLICATION OF THE METHOD
The method proposed was applied to the determination of trace amounts of copper in tap, river and well-waters.
DERIVATIVESPECTROPHOTOMETRICMETHODFOR COPPERDETERMINATION
9
Copper in river and well waters. The water samples were collected from the Maipo river in Santiago, Chile, in August, 1993. The well water samples were obtained from four different wells in Santiago, Chile, in August, 1993. All the samples were instantly drawn through a membrane filter (pore size 0.45 #m), the determination o f copper ion of the filtrate was then carried out using the method proposed. The results are shown in Table III). C o p p e r i n tap water. The determination was carried out taking an alliquot of 25.0 ml of the sample by the method already described. The samples were preserved by the addition of HNO3 until a pH of 2.0 was achieved. The results are shown in Table III. The results obtained were compared with those obtained by atomic absorption spectrometry, as can be seen in Table III. These determinations were carried out using a solution which was preconcentrated four times by evaporation.
Acknowledgments The authors are grateful to the Department of Investigation (DTI) of the University of Chile (Project Q-3285) for financial support. Thanks are also extended to Irlg. E m m a Contreras, Profs. Feliz Blu and Luis Tortes of the Empresa Metropolitana de Obras Sanitarias E M O S S.A. for the facility of comparing results by atomic absorption spectrometry.
References Arya, S. E, Malla, J. L. and Slathia, J.: 1987, 'A Selective Spectrophotometric Method for Determination of Copper with Ferron', Talanta 34(2), 293-295. Barua, S., Varma, Y. S., Gard, B. S., Singh, R. E and Singh, I.: 1981, 'Spectrophotometric Determination of Copper in Blood Serum with 4-(2-Quinolylazo)phenol', Analyst 106, 799-802. Evtimova, B. E.: 1986, 'Spectrophotometric Determination of Traces of Copper (II) with Chrome Azurol. S (C.I. Mordandant Blue 29) and Cationic Surfactants', Dokl. Bolg. Akad. Nauk. 39(8), 61-64. Florence, T. M.: 1986, 'Electrochemical Approaches to Trace Element Speciation in Waters. A Review' Analyst 111, 489-555. Gallardo Melgarejo, A., Gallardo C6spedes, A. and Cant Pav6n J. M.: 1989, 'Determination of Niquel, Zinc and Copper by Second-Derivative Spectrotometry using 1-(2-Pyridylazo)-2-Naphthol as Reagent' Analyst 114, 109-111. Huang, Z., He, S., Wang, S., Xirn, M., Cheng, C. and He, Y.: 1985a, 'Synthesis of 5,10,15,20-tetrakis(3carboxyethyl-2-furyl)-porphine and spectrophotometric study of its reaction with copper', Huaxue Shiji 7(2) 73-75 (Ch); Anal. Abstrac. 1986, 48, 4B27. Huang, Z., He, S., Li, H., Xio, J. and He, Y.: 1985b, 'Synthesis of 5.10,15,20-tetrakis-(4aminophenyl)porphine and Spectrophotometric Study of its Reaction with Copper', Fenxi Huaxue 13(10), 773-775; Anal. Abstrac. 1986, 48, 7B39. Reddy, K. G., Reddy, K. H. and Reddy, D. V.: 1986, '2,4-Dihydroxybenzophenone Semicarbazone and Thiosemicarbazone as New Chromogenic Reagents for the Rapid Speetrophotometric Determination of Copper', Indian J. Chem., Sect A, 25A(10), 982-984. Schilt, A. A., Chriswell, C. D. and Fang, T. A.: 1974, 'New Chromogens of ferroin Type-VII. Some 3-substituted-1,2,4-Triazines, and Triazole and 2,4- and 2,6-bis Triazinyl and Triolinyl Substitute Pyridines', Talanta 21,831-836.
10
M. INI~STORALET AL.
Greenberg, A. E., Clesceri, L. S., Eaton, A. D. (eds): 1992, Standard Methods For Examination of Water and Wastewater, 18 th ed, pp. 3-61. Wasey, A., Puri, B. K., Katyal, M. and Satake, M.: 1986, 'Spectrophotometric Determination of Copper in Environmental Samples by Solid-Liquid Extraction of its Phenanthrene-9,10-quinone Monoximate Complex into Molten Naphthalene', Int. J. Environ. Anal. Chem. 24(3), 169-182. Zhang, G., Hu, Y., Xu, G. and Lou, G.: 1985, 'Spectrophotometric Determination of trace Copper with 5-(5-Chloropyridylazo)-toluene-2,4-diamine', Fenxi Huaxue 13(2), 124-127 (Ch); Anal. Abstrac. 1986, 48, 1B43. Katani, T., Hayakawa, T., Furukawa, M. and Shibata, S.: 1984, 'Spectrophotometric Determination of Copper with 2-(2-Benzothiazolylazo)-5-dimethylaminobenzoic Acid', Analyst 109(11) 15111512.
Abstract. A simple, sensitive and selective method by solvent extraction-first derivative spectrophotometry is described for the determination of microamounts of copper in water by means of its reaction and extraction at pH 8.0 with 3-(4-phenyl-2-pyridinyl-5-phenyl-l,2,4-triazine)(PPT) and picrate (2,4,6-trinitro-phenol) into 1,2-dichloroethane. Copper was thus determined in the range 7.5350 ng/ml with a detection limits (3~) of 2.3 ng/ml. The relative standard deviations were in all instances less than 2.0%. The proposed method was successfully applied for the determination of copper in several kinds of water.
1. Introduction
Copper salts are usually used in water supply systems to control biological growths and to catalyze the oxidation of manganese. Corrosion of copper-containing alloys in pipe fittings of the distribution systems may introduce measurable amounts of copper into the water in a pipe system (Standard Methods For Examination of Water and Wastewater, 1992). On the other hand, it is well known that copper is an important trace metal in many biological and environmental systems. However, this metal is also highly toxic to fish and other organisms in water, whether it exists as free Cu(II) ion or as lipid-soluble copper complexes (Florence, 1986). North-central Chile is characterized by heavy copper mining production and consequently copper contents in several kinds of water may be expected, white, at the same time, becoming important pollutants of the environment. In this context, the detection and determination of copper in water samples are mandatory in order to establish the copper salts concentrations used in water supply systems, to determine the quality and contamination grade of several kinds of water. Reagents of the cuproine type are used for spectrophotometric determination of copper. However, other organic ligands have been proposed as chromogenic reagents for copper (Barua et al., 1984; Katani et al., 1984; Zhang et al., 1985; Reddy et aI., 1986; Evtimova et al., 1987; Arya et al., 1987). On the other hand, methods by extractive-spectrophotometry (Wasey et al., 1986) and derivativespectrophotometry (Gallardo Melgarejo et al., 1989) have been proposed. * To whom correspondence should be addressed.
Environmental Monitoring and Assessment 38: 1-10, 1995. (~) 1995 KIuwer Academic Publishers. Printed in the Netherlands.
2
M. INt~S TORAL ET AL.
Porphyrin chromogens were found to be highly sensitive but these complexes are formed slowly at room temperature and the formation of the colored complex is only possible after heating at 100 °C for 15 min. (Huang et al., 1985a,b). The substituted 1,2,4-triazine derivatives have also been used for the spectrophotometric determination of copper, 3- (4-phenyl-2-pyridinyl-5-phenyl- 1,2,4-triazine) (PPT) being the most sensitive for this analyte. However, the principal drawback of these reagents are their low specificity for copper because other transition metals (Fe, Co and Ni) lead to the same analytical reaction. In this work, a simple, sensitive and selective extraction-derivative spectrophotometric method for the determination of trace amounts of copper as the ionassociated Cu(I)-PPT-Picrate complex extracted into 1,2-dichloroethane is proposed. The absorbance of the resulting copper (I)-PTT-picrate ion-association complex and the derivative spectra were measured directly in the 1,2-dichloroethane. The selectivity is markedly enhanced by the use of the derivative spectrophotometric technique. This method is applied to the determination of copper in different water samples with low copper content, in these cases the atomic absorption spectrometry technique is not adequately sensitive (sensitivity = 25 ng/ml). Hence, the proposed method is a good alternative compared to the more expensive and less sensitive atomic absorption spectrometry method commonly used.
2. Experimental 2.1. APPARATUS For measurements of the absorbance and the derivative absorption spectra, a Shimadzu UV-160 with 10 mm cells was used. The pH measurements were made with a Orion Research Digital Ion-Analyzer 701 with glass and saturated calomel electrodes. 2.2. R E A G E N T SOLUTION All reagents used were of analytical-reagent grade. If necessary, any copper present as an impurity was removed. All solutions were prepared with distilled demineralized water. 2.2.1. Purification of Sodium Acetate Solution 100 g of sodium acetate was dissolved in 500 ml of water. Any copper present as an impurity was removed in the following manner: 1 ml of 10% (w/v) hydroxylammonium chloride solution and 1 g of sodium perchlorate in a separating funnel was added to the sodium acetate solution. 5 ml of 5 × 10 -3 M PPT solution was then added in dichloroethane and the funnel was shaken for 2 min. The phases were allowed to separate and the organic layer allowed to run. This extraction was repeated until the organic phase became colorless.
DERIVATIVE SPECTROPHOTOMETRIC METHOD FOR COPPER DETERMINATION
3
2.2.2. Hydroxylammonium Chloride Solution (10% w/v) 100 g of the salt was dissolved in 1000 ml of distilled-demineralized water. This solution was purified as described for the purification of sodium acetate. 2.2.3. Copper (II) Standard Solution (Titrisol Merck, 1000 #g/ml) 2.2.4. 3-(4-Phenyl-2-Pyridinyl)-5-Phenyl-l,2,4-Triazine (PPT) Solution in
Dichlorethane (5.10 -3 M) 0.1552 g of the compound was dissolved in 100 rnl of dichloroethane (DCE). 2.2.5. Picric Acid Solution (0.01 M) Solution 2.291 g of purified picric acid was dissolved in 1000 ml of distilled-demineralized water. 2.2.6. Foreign Ion Solutions Solutions of diverse ions for interference studies were prepared by dissolving the amount of each compound needed to give 10-1000 #g/ml concentrations of the ion concerned. All these solutions were stored in polyethylene containers. 2.2.7. 1,2-Dichloroethane (DCE) Extrapure (sp. gr. 1.25)
2.3. RECOMMENDED PROCEDURE 25 ml of the water sample containing less than 350 #g of copper was placed in a 250 ml separating funnel and 1.0 ml of the sodium acetate solution, 1.0 ml of hydroxylammonium chloride solution, 4.0 ml of picric acid solution was added and the total volume was adjusted to 100 ml. It was then mixed and set aside for 2 rain. The mixture was shaken for 3 min with 5.0 ml of 1.10 -3 tool/1 PPT solution in 1,2-dichloroethane. The phases were allowed to separate, and the organic layer run into a dry flask. The first derivative spectra was recorded over the range of 700-350 nm against a reagent blank at a scan speed of 480 nm/min with AA 12 nm using 10 mm cells.
3. Results and Discussion 3.1.
S PECTRAL CHARACTERISTICS
The absorption spectra of the binary Copper(I)-PPT and of the ternary Cu(I)-PPTpicrate combinations extracted into DCE were measured against DCE (Figure 1). It is evident that the Cu(PPT)2+ binary complex is not extracted into DCE. However, the introduction of picric acid into the system leads to the formation of a ionassociated complex, which is easily extracted into the organic phase.
4
M. INI~STORALET AL.
1.0, A
A 0.5-
0.0 /-,00
600
500
700
/nlTI Fig. 1. Absorption spectra of DCE extract of Cu(I)-PPT-picrate and Cu(I)-PPT complexes measured against DCE. A:-Cu(l)-PPT-picrate; B:-Cu(I)-PPT. Copper concentration = 100 ppb. All other conditions as in text.
It is known that the molar ratio of PPT to copper is 2 : 1 (Schilt et al., 1974), hence we assumed that the stoichiometry of this ion-association complex would be Cu(PPT)2picrate. The spectrum of the Cu(PPT)2picrate complex shows three absorption bands at 380, 505 and 600 nm. The first band can be assigned to the absorption of picrate and PPT and depends on the reagent concentrations. The other two bands correspond to the absorption of the chromophore Cu(I)-PPT and is a function only of the copper concentration. The more sensitive band at 505 nm was selected for analytical purposes. In order to make the determination more selective and reproducible, firstderivative spectrophotometry was adopted and the value of the vertical distance from the base line to peak values at 545 nm or from through to peak values between 470 and 560 nm can be used for analytical measurements (Figure 2a). The second derivative spectrum shows a lower signal-to-noise ratio; it was then discarded for analytical purposes (Figure 2b). 3.2. STUDY OF THE EXPERIMENTAL VARIABLES Variables were optimized by the univariate method. Table I shows the optimum values found for the chemical and spectral variables and the range over which they were studied. It can be seen in Table I that the working values for the concentration of PTT and picric acid were chosen by taking into account that the analyte can be present together with a number of foreign cations in the sample, which implies an evident
DERIVATIVE SPECTROPHOTOMETRIC METHOD FOR COPPER DETERMINATION
0.12-
5
3.
0.08 A~.~ A o.o~ 0.00. 0.0/-, - 400
+
!
.~
500
600
700
800
.~/nrn
O.OL,
A2A 0.00
-0.0~ 4
~00
500
600
¢'---------------
700
800
.~/nm Fig. 2. Derivative spectra of DCE extract of Cu(I)-PPT-picrate complex measured against DCE. a: First derivative spectrum; b: Second derivative spectrum. Copper concentration = 100 ppb. A ~ 12 nm. Scan rate, 480 nm/min. All other conditions as in text.
6
M. INt~S TORAL ET AL.
TABLEI Study of variables Variable
Range studied
Chemical pH PPT solution/
Optimum Working range value
1.0-12.0 6.0-10.0 0.1-12.0 0.3-12.0
8.0 10.0
1 • 10 -4 M
0.01 M picric acid solution/ml Aqueous/organic phase ratio
0.5-8.0
1.6-8.0
2-25
2-25
Spectral Classic spectrophotometry Analytical wavelength 350-700 505,600 Derivative spectrophotometry Derivative 0-4 1-2 order AA/nm 4.0-36.0 4.0-21.0
4.0 20
505
1 12.0
consumption of reagents. A pH value of 8 was chosen in order to keep the pH in the center of the optimum range. The aqueous/organic phase volume ratio was optimized taking into account a higher enrichment factor. The optimum values for the order of the derivative spectrum and the AA value for derivation were selected by considering the best signal-to-noise ratio. 3.3. FEATURESOF THE METHOD In the classic spectrophotometric mode, the calibration graph was obtained by plotting the absorbance (A) at 505 nm versus the analyte concentration. The linear regression equation calculated was: Cu (ppb) = 182A
(r = 0.9998)
On theother hand, in the derivative mode the calibration graph was obtained by plotting the first-derivative value h (the peak to the base line) at 545 nm with AA =
DERIVATIVE SPECTROPHOTOMETRIC METHOD FOR COPPER DETERMINATION
7
TABLE II Effect of foreign ions Tolerance limit, ng/ml Foreign
Classic
Derivative
species
spectrophotometry
spectrophotometry
CI-, SO42- , S032- , NO3-, CH3COO-, 52032-, tartrate, Br-, oxalate, citrate, NO2-, F-, SCN-
106a
106
P043-
105
106
A1+++, Mg++ , Na+ , Ca++ , Sr++, K+ , Mn++, Zn ++, Hg++, Cd++, Cu++, Bi+++
10 4a
104
Ni ++
5 × 102
6 × 103
Maximum tested. Copper 100 ng/ml.
a
12 nm, versus the analyte concentration. The linear regression equation calculated was: h = 1 . 1 4 x l 0 - 3 C ( p p b ) + 0 . 8 x 1 0 -4
( r = 0.9989)
where h is expressed as the derivative unit (DU), The standard deviation (~r) of the regent blank was 4.71 x 10 -4 DU. The detection limit, calculated by using the 3or recommendation, was found to be to 2.3 ng/ml and the determination range was between 7.7 and 350 ng/ml respectively. Ten standard solutions containing 0.10 #g/ml copper were analysed and the results gave a relative standard deviation of 0.94%.
3.4. EFFECT OF FOREIGN SUBSTANCES Solutions containing 0.1 #g/ml of copper and various amounts of other metal ions that can present in water were prepared and the recommended procedure was followed. An error of -4- 2% in the analytical signal was considered tolerable. The results o f the effect o f foreign ions on the determination of copper by the proposed method are summarized in Table II, from which it can be seen that this method is nearly specific for copper. The iron, nickel metal ions are tolerated, because in
8
M. INI~STORAL ET AL.
TABLE III Determination of copper in several kind of water Sample
Proposed method founda ppb (RSD)
AAS method founda ppb (RSD)
12.1 (0.9) 12.5 (0.8) 13.8 (0.7) 12.7 (O.9)
12.3 (1.9) 12.7 (2.0) 13.2 (2.0) 12.9 (2.0)
River water 1
2 3 4
Tap water 1
2 3 4
12.1
12.5
(1.5)
(].9)
13.2 (0.8) 12.3 (0.9) 15.3 (O.7)
13.8 (2.0) 12.6 (2.0) 15.8 (2.O)
12.4 (1.1) 26.2 (0.8) 30.1 (0.9) 25.1 (0.7)
12.5 (1.9) 26.8
Well-water 1
(2.0) 29.8 (2.0) 25.5 (2.0)
a Average of eight determinations. RSD = relative standard deviation.
this condition these ion-associated complexes are not formed. Thus, most common cation and anions are tolerated even when present in large amounts. 3.5.
APPLICATION OF THE METHOD
The method proposed was applied to the determination of trace amounts of copper in tap, river and well-waters.
DERIVATIVESPECTROPHOTOMETRICMETHODFOR COPPERDETERMINATION
9
Copper in river and well waters. The water samples were collected from the Maipo river in Santiago, Chile, in August, 1993. The well water samples were obtained from four different wells in Santiago, Chile, in August, 1993. All the samples were instantly drawn through a membrane filter (pore size 0.45 #m), the determination o f copper ion of the filtrate was then carried out using the method proposed. The results are shown in Table III). C o p p e r i n tap water. The determination was carried out taking an alliquot of 25.0 ml of the sample by the method already described. The samples were preserved by the addition of HNO3 until a pH of 2.0 was achieved. The results are shown in Table III. The results obtained were compared with those obtained by atomic absorption spectrometry, as can be seen in Table III. These determinations were carried out using a solution which was preconcentrated four times by evaporation.
Acknowledgments The authors are grateful to the Department of Investigation (DTI) of the University of Chile (Project Q-3285) for financial support. Thanks are also extended to Irlg. E m m a Contreras, Profs. Feliz Blu and Luis Tortes of the Empresa Metropolitana de Obras Sanitarias E M O S S.A. for the facility of comparing results by atomic absorption spectrometry.
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Greenberg, A. E., Clesceri, L. S., Eaton, A. D. (eds): 1992, Standard Methods For Examination of Water and Wastewater, 18 th ed, pp. 3-61. Wasey, A., Puri, B. K., Katyal, M. and Satake, M.: 1986, 'Spectrophotometric Determination of Copper in Environmental Samples by Solid-Liquid Extraction of its Phenanthrene-9,10-quinone Monoximate Complex into Molten Naphthalene', Int. J. Environ. Anal. Chem. 24(3), 169-182. Zhang, G., Hu, Y., Xu, G. and Lou, G.: 1985, 'Spectrophotometric Determination of trace Copper with 5-(5-Chloropyridylazo)-toluene-2,4-diamine', Fenxi Huaxue 13(2), 124-127 (Ch); Anal. Abstrac. 1986, 48, 1B43. Katani, T., Hayakawa, T., Furukawa, M. and Shibata, S.: 1984, 'Spectrophotometric Determination of Copper with 2-(2-Benzothiazolylazo)-5-dimethylaminobenzoic Acid', Analyst 109(11) 15111512.
