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Novel preconcentration technique for trace metal ions by aggregate film formation. Spectrophotometric determination of copper(II)
Analytica Chimica Acta 364 (1998) 13±17
Novel preconcentration technique for trace metal ions by aggregate ®lm formation. Spectrophotometric determination of copper(II) Masatoshi Endo*, Keigo Suzuki, Shigeki Abe Department of Materials Science and Engineering, Yamagata University, Yonezawa 992, Japan Received 22 September 1997; received in revised form 9 January 1998; accepted 1 February 1998
Abstract A highly sensitive method for the determination of copper(II) based on preconcentration of copper(II)-N,N± diethyldithiocarbamate as a aggregate ®lm is described. Traces of copper(II) were quantitatively collected into a thin ®lm consisting of hexadecyltrimethylammonium chloride (cetyltrimethylammonium chloride) and sulfosalicylic acid. The ®lm is easily detached from a membrane ®lter support and the copper(II) complex was determined spectrophotometrically by dissolving in N,N-dimethylformamide. The calibration graph linear from 1.610ÿ9 to 810ÿ8 mol copper(II) in 14 ml of test solution. The relative standard deviation was 3.1% (n5) at 310ÿ6 M copper(II). The membrane ®lter could be used repeatedly. The method was applied to the determination of trace copper(II) added to river water. # 1998 Elsevier Science B.V. Keywords: Preconcentration into the aggregate ®lm; UV±visible; Spectrophotometry copper(II); Copper(II)±N,N-diethyldithiocarbamate; Cetyltrimethylammonium chloride; Sulfosalicylic acid
1. Introduction Preconcentration is often a prerequisite for the determination of trace elements, and many simple enrichment methods including extraction and adsorption have been proposed. The approaches that have been investigated are solid-phase spectrophotometry of the trace metal after its collection on a ®lter [1,2], and ion-pair adsorption ®lm colorimetry [3,4]. Recently the use of solvent-soluble membrane ®lters [5,6] has received much interest for the collection of trace elements. Thus insoluble metal *Corresponding author. Fax: +81-238-26-3413; e-mail: [email protected] 0003-2670/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0003-2670(98)00130-5
chelates such as copper(II)-N,N±diethyldithiocarbamate (Cu(II)±DDTC) were successfully collected on a solvent-soluble membrane ®lter without the presence of carrier [7]. These complexes as well as the membrane ®lter were dissolved in dimethylformamide (DMF) for subsequent spectrophotometry of copper (II). A direct method for the determination of copper(II) by atomic absorption spectrophotometry (AAS) was also proposed; a portion of membrane ®lter with collected Cu(II)±DDTC complex was transferred to a graphite furnace [8]. The adsorption of Cu(II)± DDTC on the quartz plate of the electrode-separated piezoelectric quartz crystal has been applied for the determination of copper(II) [9]. The frequency change
14
M. Endo et al. / Analytica Chimica Acta 364 (1998) 13±17
is proportional to the concentration of copper(II) in the solution. The phase separation of a thermally reversible polymer was also applied to the selective extraction of hydrophobic substances in water [10]. When a homogeneous solution of poly(N-isopropylacrylamide) was heated, the polymer was deposited on the wall of the reaction vessel; the coexisting bulky ion associates such as 12-molybdophorphate±Malachite Green are effectively concentrated in the polymer. In the present work, an ion-associated ®lm preconcentration method has been developed to collect hydrophobic metal complexes such as Cu(II)±DDTC. Our preliminary experiments revealed that a cationic surfactant and sulfosalicylic acid form a suspended aggregate on mixing and uncharged Cu(II)±DDTC are quantitatively collected in the aggregate. As the aggregate on a membrane ®lter support is easily detachable as a ®lm, the Cu(II)±DDTC in the ®lm can be determined directly or after dissolution. Dissolution in dimethyl formamide (DMF) is convenient and a high preconcentration factor is obtained. 2. Experimental 2.1. Apparatus A Hitachi model U-3000 double-beam spectrophotometer with 1 cm micro-quartz cells was used for measuring the absorbance. A Horiba model F-7LC pH meter was used for pH measurements. 2.2. Reagents All chemicals used were of analytical-reagent grade. A standard copper(II) solution (210ÿ4 M) was prepared by dissolving copper(II) sulfate in water. A DDTC solution (610ÿ3 M) was prepared by dissolving sodium N,N-diethyldithiocarbamate (Wako) in water. A sulfosalicylic acid solution (0.1 M) was prepared by dissolving sulfosalicylic acid (Wako) in water. A 0.02 M solution of cetyltrimethylammonium chloride (CTAC) (Tokyo Chemical) was prepared in water. A Tiron solution (0.05 M) was prepared by dissolving 1,2-dihydroxybenzene-3,5-disulfonic acid sodium salt (Dojindo Laboratories, Kumamoto) in water.
2.3. Procedure A 14 ml sample solution containing 1.610ÿ9± 810ÿ8 mol of copper(II) was placed in a 50 ml beaker. 1 ml of 610ÿ4 M DDTC solution, 4 ml of 0.1 M sulfosalicylic acid solution and 1 ml of 0.02 M CTAC solution were added in order so that the ®nal volume was 20 ml. The solution was mixed gently and was made to stand for 10 min at room temperature. The Cu(II)±DDTC complex was concentrated into the aggregate of CTAC and sulfosalicylic acid. The aggregate was collected on a cellulose nitrate membrane ®lter with 0.45 mm pore size by ®ltration under suction and washed with 10 ml of sulfosalicylic acid solution. The ®lm was detached and dissolved in exactly 1 ml of DMF for subsequent absorbance measurement at 436 nm. 3. Results and Discussion 3.1. Formation of aggregate film Cationic surfactant CTAC forms micelles above its critical micelle concentration (cmc). The positive charge of the micelle was neutralized by the addition of sulfosalicylic acid and ion-association aggregates precipitated. These aggregates can be collected on a cellulose nitrate membrane ®lter by suction. The ®ltrated aggregate was easily detached from the membrane as a thin ®lm. Thus insoluble Cu(II)±DDTC complexes were quantitatively collected into the aggregate ®lm. A 13.7 mm diameter and 0.1 mm thick ®lm is obtained. When salicylic acid and sodium 1,3-benzenedisulfonic acid were added, the solution turned viscous. The formation of aggregates required 1 h in the sodium 3-sulfobenzoic acid system; no aggregate was observed by addition of sulfanilic acid. Pyromellitic acid, sodium 2,3-dihydroxynaphthalene-6-sulfonic acid and sodium 2,6-naphthalene disulfonic acid were unsuitable as association reagents because adhesion of the ®lm on the support occurs. Sulfosalicylic acid and Tiron forms the most favorable ®lms. It is concluded that the benzene derivatives having at least three negatively dissociable functional group, two of which are located adjacently, are effective as reagents for aggregate ®lm formation.
M. Endo et al. / Analytica Chimica Acta 364 (1998) 13±17
Fig. 1. Effect of cationic surfactants on the collection of copper(II)-DDTC. Total volume: 20 ml, [Cu(II)]: 310ÿ6 M, [DDTC]: 310ÿ5 M, [sulfosalicylic acid]: 210ÿ2 M (pH 1.8), surfactant: (a) C18-TMAC, (b) CTAC, (c) C14-TMAC, and (d) C12TMAC. The aggregate film was dissolved in 1 ml of DMF for absorbance measurement.
3.2. Effect of surfactant and sulfosalicylic acid Aggregates are found to precipitate at near the cmc; cationic surfactants suitable for present purpose are octadecyltrimethylammonium bromide (C18-TMAC, cmc: 3.410ÿ4 M), CTAC (cmc: 1.310ÿ3 M), tetradecyltrimethylammonium bromide (C14-TMAC, cmc: 4.010ÿ3 M), dodecyltrimethylammonium bromide (C12-TMAC, cmc: 1.610ÿ2 M). As shown in Fig. 1, CTAC and C18-TMAC were most satisfactory for the collection of Cu(II)±DDTC. Benzyldimethyltetradecylammonium chloride (Zephiramine) was unsuitable because it forms ®ne precipitates. With increase of CTAC concentration, the quantity of suspended matter increased, and its aggregation proceeds rapidly. A CTAC concentration of 1 10ÿ3 M was employed in the following experiments. The quantity of Cu(II)±DDTC adsorbed to the membrane ®lter increased when the concentration of CTAC was low. On the contrary, the amount of Cu(II)-complex adhering to the glassware remarkably increased above 510ÿ3 M. Trace aggregate on the wall of the reaction vessel was effectively recovered by washing with 0.1 M sulfosalicylic acid solution. The effect of sulfosalicylic acid concentration on the collection of Cu(II)±DDTC is shown in Fig. 2. No
15
Fig. 2. Effect of ion-association reagents on the collection of copper(II)-DDTC. [CTAC]: 110ÿ3 M, (a) sulfosalicylic acid (pH 1.8), and (b) Tiron (pH 6.0). The other conditions are as in Fig. 1.
aggregate was found below 510ÿ4 M sulfosalicylic acid; almost all the complex was adsorbed on the ®lter. A portion of complex remained as solubilized forms in the ®ltrate at lower concentration of sulfosalicylic acid. The rate of aggregate formation increased with increasing sulfosalicylic acid concentration above 510ÿ3 M. A sulfosalicylic concentration of 210ÿ2 M was adopted in the recommended procedure. 3.3. Effect of experimental variables The effect of pH on the collection of the Cu(II)complex is shown in Fig. 3. The Cu(II)±DDTC complex was quantitatively collected at pH 1.8. The pH of solution was controlled by the sulfosalicylic acid; no addition of buffer was needed. The Cu(II)-complex could not be collected from strong acidic solution because the DDTC decomposes. It was found that the ®ltration time increased at high pH. Presumably the rate of aggregate formation slowed down owing to the dissociation of the carboxylic proton in the case of sodium 3-sulfobenzoic acid. The half-life for the decomposition of DDTC was reported to be 7 s at pH 2 [11]. Therefore the order of addition of the reactants must be considered. No adverse effect was observed for the collection of Cu(II)±DDTC complex when sulfosalicylic acid was added after the completion of complex formation.
16
M. Endo et al. / Analytica Chimica Acta 364 (1998) 13±17
mended values, the sample volume can be scaled up to 100 ml. The use of Tiron as an alternative collector was examined. Good results were obtained as in the case of sulfosalicylic acid. CTAC was most suitable as a cationic surfactant in the Tiron system. Constant absorbance of Cu(II)±DDTC was obtained when the concentration of Tiron was 210ÿ3±510ÿ2 M (cf. Fig. 2). The pH of the Tiron solution (110ÿ2 M) was 5.9 and the collection of the Cu(II)-complex was quantitative from pH 3 to 8 (cf. Fig. 3). The ®ltration time increased slightly. 3.4. Determination of copper(II) Fig. 3. Effect of pH on the collection of copper(II)-DDTC. [CTAC]: 110ÿ3 M, (a) sulfosalicylic acid (210ÿ2 M), and (b) Tiron (110ÿ2 M). The other conditions are as in Fig. 1.
As the decomposition of free DDTC proceeds rapidly, there is little interference by DDTC. A DDTC concentration of 310ÿ5 M was selected as optimal. The effect of standing time on copper(II) collection was examined. The Cu(II)±DDTC complex codeposited with the ion-association aggregate and quantitative collection was attained by 5 min standing. Various membrane ®lters were examined as a support. The aggregate was recovered as a ®lm when membrane ®lters such as cellulose nitrate, cellulose acetate, regenerated cellulose and polyether sulfone were employed. The ®lm, however, could not be completely detached from the ®lter except in the case of cellulose nitrate. No ®lm formation was observed when a hydrophilic PTFE-type membrane ®lter was used. The pore size of membrane did not signi®cantly in¯uence the collection ef®ciency of Cu(II)±DDTC at pore sizes ranging from 0.45 to 3 mm. Though the determination of copper(II) as an aggregate ®lm was possible visually, the absorbance measurement of Cu(II)±DDTC dissolved with the ®lm was advantageous for precise analysis. Solvents such as DMF, dimethyl sulfoxide and 2-methoxyethanol can be used to dissolve the aggregate ®lm; the ®lm was dissolved in 1 ml of DMF for subsequent absorbance measurement. The Cu(II)±DDTC complex was insoluble in ethanol. The ®ltration time was about 10 min at a sample volume of 20 ml. When the concentrations of CTAC and sulfosalicylic acid were at the recom-
The calibration graph was linear from 810ÿ8 to 410ÿ6 M copper(II) in 20 ml of the solution before preconcentration. The relative standard deviation was 3.1% for ®ve replicate determinations of 310ÿ6 M of copper(II). Nickel(II) (510ÿ7±910ÿ6 M) can be determined as its DDTC complex by a similar procedure. The membrane ®lter could be re-used more than 10 times. An optimum range for visual determination of copper(II) with the ®lm was 210ÿ7±210ÿ5 M. The effect of foreign ions was examined. As shown in Table 1, a 300-fold molar excess of manganese(II) and zinc(II) showed no interference. Cobalt(II), nickel(II) and iron(II) interfered at an equivalent molar level of copper(II). The effect of coexisting ions was suppressed by lowering the pH of the solution, though appreciable decomposition of DDTC occurs by acid attack. To circumvent this problem, the use of more acid-resistant ammonium pyrrolidinedithiocarbamate (APDC) is recommended; the Cu(II)±APDC complex was effectively collected at pH 0.5. A 100-fold molar excess of cobalt(II) and iron(II) did not interfere in the Table 1 Tolerance of foreign ions for the determination of copper(II) Foreign ions
Mn(II) Zn(II) Fe(II) Co(II) Ni(II)
Max. tolerated concentration (M) DDTC (pH 1.8)
APDC (pH 0.6)
1.510ÿ3 9.010ÿ4 3.010ÿ6 6.010ÿ7 1.510ÿ6
Ð Ð 3.010ÿ4 3.010ÿ4 3.010ÿ6
[Cu(II)]: 3.010ÿ6 M, [DDTC]: 310ÿ5 M, [APDC]: 310ÿ5 M.
M. Endo et al. / Analytica Chimica Acta 364 (1998) 13±17 Table 2 Recovery of copper(II) from river watera Sample
A A A B C a
Copper(II) (10ÿ8 mol)
Recovery (%)
b
Added
Found
3.00 6.00 9.00 6.00 6.00
3.000.03 5.900.02 9.340.02 5.820.01 6.180.09
100 98.3 104 97.0 103
Samples (14 ml) were taken at Ohta river, Yonezawa-shi, Japan. Mean standard deviation (n5).
b
APDC preconcentration system. The presence of nickel(II) at an equivalent molar concentration to copper(II) was permitted. A 100-fold molar excess of nickel(II), however, gave a 15% error. The proposed method was applied to the determination of copper(II) added to river water. The results of recovery tests are shown in Table 2. Satisfactory results were obtained. 4. Conclusion Trace amounts of copper(II) were quantitatively collected as the Cu(II)±DDTC complex into an aggregate ®lm of the CTAC±sulfosalicylic acid ion-associ-
17
ate. This preconcentration method was effective for the separation of trace metals because the analyte was recovered in the form of a ®lm. The ®lm was easily detached from a membrane ®lter support, making possible the repeated use of the ®lter. References [1] K. Matsuhisa, K. Ohzeki, Analyst 111 (1986) 685. [2] I. Nukatsuka, T. Ohba, H. Ishida, H. Satoh, K. Ohzeki, R. Ishida, Analyst 117 (1992) 1513. [3] E. Kaneko, H. Tanno, T. Yotsuyanagi, Mikrochim. Acta III (1988) 333. [4] E. Kaneko, H. Tanno, T. Yotsuyanagi, Mikrochim. Acta I (1991) 37. [5] S. Taguchi, E. Ito-oka, K. Goto, Bunseki Kagaku 33 (1984) 453. [6] S. Taguchi, E. Ito-oka, I. Masuyama, I. Kasahara, K. Goto, Talanta 32 (1985) 391. [7] M. Kan, H. Sakamoto, T. Nasu, M. Taga, Anal. Sci. 7 (1991) 913. [8] Y. Morita, M. Yoshikawa, A. Isozaki, Bunseki Kagaku 45 (1996) 909. [9] T. Nomura, M. Kumagai, A. Sato, Anal. Chim. Acta 343 (1997) 209. [10] C. Matsubara, S. Izumi, K. Takamura, H. Yoshioka, Y. Mori, Analyst 118 (1993) 553. [11] K.I. Aspila, V.S. Sastiri, C.L. Chakrabarti, Talanta 16 (1969) 1099.
Novel preconcentration technique for trace metal ions by aggregate ®lm formation. Spectrophotometric determination of copper(II) Masatoshi Endo*, Keigo Suzuki, Shigeki Abe Department of Materials Science and Engineering, Yamagata University, Yonezawa 992, Japan Received 22 September 1997; received in revised form 9 January 1998; accepted 1 February 1998
Abstract A highly sensitive method for the determination of copper(II) based on preconcentration of copper(II)-N,N± diethyldithiocarbamate as a aggregate ®lm is described. Traces of copper(II) were quantitatively collected into a thin ®lm consisting of hexadecyltrimethylammonium chloride (cetyltrimethylammonium chloride) and sulfosalicylic acid. The ®lm is easily detached from a membrane ®lter support and the copper(II) complex was determined spectrophotometrically by dissolving in N,N-dimethylformamide. The calibration graph linear from 1.610ÿ9 to 810ÿ8 mol copper(II) in 14 ml of test solution. The relative standard deviation was 3.1% (n5) at 310ÿ6 M copper(II). The membrane ®lter could be used repeatedly. The method was applied to the determination of trace copper(II) added to river water. # 1998 Elsevier Science B.V. Keywords: Preconcentration into the aggregate ®lm; UV±visible; Spectrophotometry copper(II); Copper(II)±N,N-diethyldithiocarbamate; Cetyltrimethylammonium chloride; Sulfosalicylic acid
1. Introduction Preconcentration is often a prerequisite for the determination of trace elements, and many simple enrichment methods including extraction and adsorption have been proposed. The approaches that have been investigated are solid-phase spectrophotometry of the trace metal after its collection on a ®lter [1,2], and ion-pair adsorption ®lm colorimetry [3,4]. Recently the use of solvent-soluble membrane ®lters [5,6] has received much interest for the collection of trace elements. Thus insoluble metal *Corresponding author. Fax: +81-238-26-3413; e-mail: [email protected] 0003-2670/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0003-2670(98)00130-5
chelates such as copper(II)-N,N±diethyldithiocarbamate (Cu(II)±DDTC) were successfully collected on a solvent-soluble membrane ®lter without the presence of carrier [7]. These complexes as well as the membrane ®lter were dissolved in dimethylformamide (DMF) for subsequent spectrophotometry of copper (II). A direct method for the determination of copper(II) by atomic absorption spectrophotometry (AAS) was also proposed; a portion of membrane ®lter with collected Cu(II)±DDTC complex was transferred to a graphite furnace [8]. The adsorption of Cu(II)± DDTC on the quartz plate of the electrode-separated piezoelectric quartz crystal has been applied for the determination of copper(II) [9]. The frequency change
14
M. Endo et al. / Analytica Chimica Acta 364 (1998) 13±17
is proportional to the concentration of copper(II) in the solution. The phase separation of a thermally reversible polymer was also applied to the selective extraction of hydrophobic substances in water [10]. When a homogeneous solution of poly(N-isopropylacrylamide) was heated, the polymer was deposited on the wall of the reaction vessel; the coexisting bulky ion associates such as 12-molybdophorphate±Malachite Green are effectively concentrated in the polymer. In the present work, an ion-associated ®lm preconcentration method has been developed to collect hydrophobic metal complexes such as Cu(II)±DDTC. Our preliminary experiments revealed that a cationic surfactant and sulfosalicylic acid form a suspended aggregate on mixing and uncharged Cu(II)±DDTC are quantitatively collected in the aggregate. As the aggregate on a membrane ®lter support is easily detachable as a ®lm, the Cu(II)±DDTC in the ®lm can be determined directly or after dissolution. Dissolution in dimethyl formamide (DMF) is convenient and a high preconcentration factor is obtained. 2. Experimental 2.1. Apparatus A Hitachi model U-3000 double-beam spectrophotometer with 1 cm micro-quartz cells was used for measuring the absorbance. A Horiba model F-7LC pH meter was used for pH measurements. 2.2. Reagents All chemicals used were of analytical-reagent grade. A standard copper(II) solution (210ÿ4 M) was prepared by dissolving copper(II) sulfate in water. A DDTC solution (610ÿ3 M) was prepared by dissolving sodium N,N-diethyldithiocarbamate (Wako) in water. A sulfosalicylic acid solution (0.1 M) was prepared by dissolving sulfosalicylic acid (Wako) in water. A 0.02 M solution of cetyltrimethylammonium chloride (CTAC) (Tokyo Chemical) was prepared in water. A Tiron solution (0.05 M) was prepared by dissolving 1,2-dihydroxybenzene-3,5-disulfonic acid sodium salt (Dojindo Laboratories, Kumamoto) in water.
2.3. Procedure A 14 ml sample solution containing 1.610ÿ9± 810ÿ8 mol of copper(II) was placed in a 50 ml beaker. 1 ml of 610ÿ4 M DDTC solution, 4 ml of 0.1 M sulfosalicylic acid solution and 1 ml of 0.02 M CTAC solution were added in order so that the ®nal volume was 20 ml. The solution was mixed gently and was made to stand for 10 min at room temperature. The Cu(II)±DDTC complex was concentrated into the aggregate of CTAC and sulfosalicylic acid. The aggregate was collected on a cellulose nitrate membrane ®lter with 0.45 mm pore size by ®ltration under suction and washed with 10 ml of sulfosalicylic acid solution. The ®lm was detached and dissolved in exactly 1 ml of DMF for subsequent absorbance measurement at 436 nm. 3. Results and Discussion 3.1. Formation of aggregate film Cationic surfactant CTAC forms micelles above its critical micelle concentration (cmc). The positive charge of the micelle was neutralized by the addition of sulfosalicylic acid and ion-association aggregates precipitated. These aggregates can be collected on a cellulose nitrate membrane ®lter by suction. The ®ltrated aggregate was easily detached from the membrane as a thin ®lm. Thus insoluble Cu(II)±DDTC complexes were quantitatively collected into the aggregate ®lm. A 13.7 mm diameter and 0.1 mm thick ®lm is obtained. When salicylic acid and sodium 1,3-benzenedisulfonic acid were added, the solution turned viscous. The formation of aggregates required 1 h in the sodium 3-sulfobenzoic acid system; no aggregate was observed by addition of sulfanilic acid. Pyromellitic acid, sodium 2,3-dihydroxynaphthalene-6-sulfonic acid and sodium 2,6-naphthalene disulfonic acid were unsuitable as association reagents because adhesion of the ®lm on the support occurs. Sulfosalicylic acid and Tiron forms the most favorable ®lms. It is concluded that the benzene derivatives having at least three negatively dissociable functional group, two of which are located adjacently, are effective as reagents for aggregate ®lm formation.
M. Endo et al. / Analytica Chimica Acta 364 (1998) 13±17
Fig. 1. Effect of cationic surfactants on the collection of copper(II)-DDTC. Total volume: 20 ml, [Cu(II)]: 310ÿ6 M, [DDTC]: 310ÿ5 M, [sulfosalicylic acid]: 210ÿ2 M (pH 1.8), surfactant: (a) C18-TMAC, (b) CTAC, (c) C14-TMAC, and (d) C12TMAC. The aggregate film was dissolved in 1 ml of DMF for absorbance measurement.
3.2. Effect of surfactant and sulfosalicylic acid Aggregates are found to precipitate at near the cmc; cationic surfactants suitable for present purpose are octadecyltrimethylammonium bromide (C18-TMAC, cmc: 3.410ÿ4 M), CTAC (cmc: 1.310ÿ3 M), tetradecyltrimethylammonium bromide (C14-TMAC, cmc: 4.010ÿ3 M), dodecyltrimethylammonium bromide (C12-TMAC, cmc: 1.610ÿ2 M). As shown in Fig. 1, CTAC and C18-TMAC were most satisfactory for the collection of Cu(II)±DDTC. Benzyldimethyltetradecylammonium chloride (Zephiramine) was unsuitable because it forms ®ne precipitates. With increase of CTAC concentration, the quantity of suspended matter increased, and its aggregation proceeds rapidly. A CTAC concentration of 1 10ÿ3 M was employed in the following experiments. The quantity of Cu(II)±DDTC adsorbed to the membrane ®lter increased when the concentration of CTAC was low. On the contrary, the amount of Cu(II)-complex adhering to the glassware remarkably increased above 510ÿ3 M. Trace aggregate on the wall of the reaction vessel was effectively recovered by washing with 0.1 M sulfosalicylic acid solution. The effect of sulfosalicylic acid concentration on the collection of Cu(II)±DDTC is shown in Fig. 2. No
15
Fig. 2. Effect of ion-association reagents on the collection of copper(II)-DDTC. [CTAC]: 110ÿ3 M, (a) sulfosalicylic acid (pH 1.8), and (b) Tiron (pH 6.0). The other conditions are as in Fig. 1.
aggregate was found below 510ÿ4 M sulfosalicylic acid; almost all the complex was adsorbed on the ®lter. A portion of complex remained as solubilized forms in the ®ltrate at lower concentration of sulfosalicylic acid. The rate of aggregate formation increased with increasing sulfosalicylic acid concentration above 510ÿ3 M. A sulfosalicylic concentration of 210ÿ2 M was adopted in the recommended procedure. 3.3. Effect of experimental variables The effect of pH on the collection of the Cu(II)complex is shown in Fig. 3. The Cu(II)±DDTC complex was quantitatively collected at pH 1.8. The pH of solution was controlled by the sulfosalicylic acid; no addition of buffer was needed. The Cu(II)-complex could not be collected from strong acidic solution because the DDTC decomposes. It was found that the ®ltration time increased at high pH. Presumably the rate of aggregate formation slowed down owing to the dissociation of the carboxylic proton in the case of sodium 3-sulfobenzoic acid. The half-life for the decomposition of DDTC was reported to be 7 s at pH 2 [11]. Therefore the order of addition of the reactants must be considered. No adverse effect was observed for the collection of Cu(II)±DDTC complex when sulfosalicylic acid was added after the completion of complex formation.
16
M. Endo et al. / Analytica Chimica Acta 364 (1998) 13±17
mended values, the sample volume can be scaled up to 100 ml. The use of Tiron as an alternative collector was examined. Good results were obtained as in the case of sulfosalicylic acid. CTAC was most suitable as a cationic surfactant in the Tiron system. Constant absorbance of Cu(II)±DDTC was obtained when the concentration of Tiron was 210ÿ3±510ÿ2 M (cf. Fig. 2). The pH of the Tiron solution (110ÿ2 M) was 5.9 and the collection of the Cu(II)-complex was quantitative from pH 3 to 8 (cf. Fig. 3). The ®ltration time increased slightly. 3.4. Determination of copper(II) Fig. 3. Effect of pH on the collection of copper(II)-DDTC. [CTAC]: 110ÿ3 M, (a) sulfosalicylic acid (210ÿ2 M), and (b) Tiron (110ÿ2 M). The other conditions are as in Fig. 1.
As the decomposition of free DDTC proceeds rapidly, there is little interference by DDTC. A DDTC concentration of 310ÿ5 M was selected as optimal. The effect of standing time on copper(II) collection was examined. The Cu(II)±DDTC complex codeposited with the ion-association aggregate and quantitative collection was attained by 5 min standing. Various membrane ®lters were examined as a support. The aggregate was recovered as a ®lm when membrane ®lters such as cellulose nitrate, cellulose acetate, regenerated cellulose and polyether sulfone were employed. The ®lm, however, could not be completely detached from the ®lter except in the case of cellulose nitrate. No ®lm formation was observed when a hydrophilic PTFE-type membrane ®lter was used. The pore size of membrane did not signi®cantly in¯uence the collection ef®ciency of Cu(II)±DDTC at pore sizes ranging from 0.45 to 3 mm. Though the determination of copper(II) as an aggregate ®lm was possible visually, the absorbance measurement of Cu(II)±DDTC dissolved with the ®lm was advantageous for precise analysis. Solvents such as DMF, dimethyl sulfoxide and 2-methoxyethanol can be used to dissolve the aggregate ®lm; the ®lm was dissolved in 1 ml of DMF for subsequent absorbance measurement. The Cu(II)±DDTC complex was insoluble in ethanol. The ®ltration time was about 10 min at a sample volume of 20 ml. When the concentrations of CTAC and sulfosalicylic acid were at the recom-
The calibration graph was linear from 810ÿ8 to 410ÿ6 M copper(II) in 20 ml of the solution before preconcentration. The relative standard deviation was 3.1% for ®ve replicate determinations of 310ÿ6 M of copper(II). Nickel(II) (510ÿ7±910ÿ6 M) can be determined as its DDTC complex by a similar procedure. The membrane ®lter could be re-used more than 10 times. An optimum range for visual determination of copper(II) with the ®lm was 210ÿ7±210ÿ5 M. The effect of foreign ions was examined. As shown in Table 1, a 300-fold molar excess of manganese(II) and zinc(II) showed no interference. Cobalt(II), nickel(II) and iron(II) interfered at an equivalent molar level of copper(II). The effect of coexisting ions was suppressed by lowering the pH of the solution, though appreciable decomposition of DDTC occurs by acid attack. To circumvent this problem, the use of more acid-resistant ammonium pyrrolidinedithiocarbamate (APDC) is recommended; the Cu(II)±APDC complex was effectively collected at pH 0.5. A 100-fold molar excess of cobalt(II) and iron(II) did not interfere in the Table 1 Tolerance of foreign ions for the determination of copper(II) Foreign ions
Mn(II) Zn(II) Fe(II) Co(II) Ni(II)
Max. tolerated concentration (M) DDTC (pH 1.8)
APDC (pH 0.6)
1.510ÿ3 9.010ÿ4 3.010ÿ6 6.010ÿ7 1.510ÿ6
Ð Ð 3.010ÿ4 3.010ÿ4 3.010ÿ6
[Cu(II)]: 3.010ÿ6 M, [DDTC]: 310ÿ5 M, [APDC]: 310ÿ5 M.
M. Endo et al. / Analytica Chimica Acta 364 (1998) 13±17 Table 2 Recovery of copper(II) from river watera Sample
A A A B C a
Copper(II) (10ÿ8 mol)
Recovery (%)
b
Added
Found
3.00 6.00 9.00 6.00 6.00
3.000.03 5.900.02 9.340.02 5.820.01 6.180.09
100 98.3 104 97.0 103
Samples (14 ml) were taken at Ohta river, Yonezawa-shi, Japan. Mean standard deviation (n5).
b
APDC preconcentration system. The presence of nickel(II) at an equivalent molar concentration to copper(II) was permitted. A 100-fold molar excess of nickel(II), however, gave a 15% error. The proposed method was applied to the determination of copper(II) added to river water. The results of recovery tests are shown in Table 2. Satisfactory results were obtained. 4. Conclusion Trace amounts of copper(II) were quantitatively collected as the Cu(II)±DDTC complex into an aggregate ®lm of the CTAC±sulfosalicylic acid ion-associ-
17
ate. This preconcentration method was effective for the separation of trace metals because the analyte was recovered in the form of a ®lm. The ®lm was easily detached from a membrane ®lter support, making possible the repeated use of the ®lter. References [1] K. Matsuhisa, K. Ohzeki, Analyst 111 (1986) 685. [2] I. Nukatsuka, T. Ohba, H. Ishida, H. Satoh, K. Ohzeki, R. Ishida, Analyst 117 (1992) 1513. [3] E. Kaneko, H. Tanno, T. Yotsuyanagi, Mikrochim. Acta III (1988) 333. [4] E. Kaneko, H. Tanno, T. Yotsuyanagi, Mikrochim. Acta I (1991) 37. [5] S. Taguchi, E. Ito-oka, K. Goto, Bunseki Kagaku 33 (1984) 453. [6] S. Taguchi, E. Ito-oka, I. Masuyama, I. Kasahara, K. Goto, Talanta 32 (1985) 391. [7] M. Kan, H. Sakamoto, T. Nasu, M. Taga, Anal. Sci. 7 (1991) 913. [8] Y. Morita, M. Yoshikawa, A. Isozaki, Bunseki Kagaku 45 (1996) 909. [9] T. Nomura, M. Kumagai, A. Sato, Anal. Chim. Acta 343 (1997) 209. [10] C. Matsubara, S. Izumi, K. Takamura, H. Yoshioka, Y. Mori, Analyst 118 (1993) 553. [11] K.I. Aspila, V.S. Sastiri, C.L. Chakrabarti, Talanta 16 (1969) 1099.