A new spectrophotometric reagent for copper: 3,3′-(1,3-propanediyldiimine)bis-[3-methyl-2-butanone]dioxime

Analytica Chimica Acta 408 (2000) 163–168

A new spectrophotometric reagent for copper:

3,30 -(1,3-propanediyldiimine)bis-[3-methyl-2-butanone]dioxime Serdar Karaböcek ∗ , Sinan Nohut, Ömer Dalman, Saadettin Güner Department of Chemistry, Karadeniz Technical University, 61080 Trabzon, Turkey Received 2 July 1999; received in revised form 21 October 1999; accepted 23 October 1999

Abstract A new reagent 3,30 -(1,3-propanedyldiimine)bis-[3-methyl-2-butanone]dioxime (PnAO) has been prepared and used for developing a simple spectrophotometric method for the determination of copper which is based on a 1 : 1 complex formation between copper(II) and PnAO in aqueous solution. The method is optimized in terms of pH, stability of the complex, amount of reagent required, sensitivity, linearity and tolerance limits of various foreign ions. The new photometric method was also applied for the determination of copper in environmental samples. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Copper(II) determination; Spectrophotometry; vic-dioxime; Environmental sample

1. Introduction It is well known that most of the transitional metals and heavy metals form complexes with vic-dioximes. The complexing properties of dioximes with respect to transitional metal cations have been studied and these complexes have been isolated and characterized [1–7]. However, few studies have been carried out for direct colorimetric determination of copper in aqueous solution [8,10]. Its determination in environmental and biological samples such as sediment, tea, wastewater, industrial effluent, etc., can be of interest in biochemical research since copper traces promote rancidity and off-flavors in foods and beverages. Hence there is a need for rapid and sensitive methods for its determination. Atomic emission or absorption and colorimetric methods are most commonly used, however, colori∗ Corresponding author. Tel.: +462-325-3195; fax: +462-325-3196. E-mail address: [email protected] (S. Karaböcek).

metric methods are often preferred, as they involve less expensive instrumentation and provide better sensitivity when appropriate chromogenic reagents are available. Most of the extractive spectrophotometric methods developed for copper are based on reactions with suitable color producing reagents such as sodium diethyldithiocarbamate [9], 5-bromosalicyl-aldoxime [10], N-m-tolyl-p-methoxybenzo-hydroxamic acid [11], picolinamidoxime [12], dithizone [13], 2,9-dimethyl-1,10-phenanthroline (neocuproin) [14,15], 2,9-dimethyl-4,7-diphenyl-1,10-phenantroline disulphonic acid (bathocuproine disulphonic acid) [16], biscyclohexanon-oxalyldihydrazone (cuprizon) [17, 18], tetraethyl-thiuramdisulfide (dicupral) [19] etc. Many of these reagents are the product of difficult and tedious organic syntheses. In the present method, the preparation of a new ligand, 3,30 -(1,3-propanedyldiimine)bis-[3-methyl-2butanone]dioxime (PnAO) is described and its use by a simple spectrophotometric method for the determination of copper is presented. Various factors

0003-2670/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 8 2 5 - 9

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influencing the sensitivity of this method such as wavelength, pH, stirring time, the effect of foreign ions and the ranges of applicability of Beer’s law on the determination of copper are also included. 2. Experimental 2.1. Apparatus An Orion 601/A pH meter was used for pH measurements. A Unicam UV2-100 double beam UV–Visible spectrophotometer equipped with a quartz cell of 10 mm path length was used for the absorption spectra and the absorbance measurements. 2.2. Preparation of ligand The ligand, 3,30 -(1,3-propanedyldiimine)bis-[3methyl-2-butanone]dioxime (PnAO) was prepared according to the procedure reported previously [20]. The product is a white, air-stable, crystalline solid only slightly soluble in water, soluble in methanol, acetonitrile, and strong acids (with protonation), but its 1 : 1 copper complex is highly soluble in water. The purity of this reagent was checked by physical data; melting point range, 188–189◦ C, Anal. Calc. for C13 H28 O2 N4 : C, 57.32; H, 10.36; N, 20.57. Found: C, 57.40; H, 10.30; N, 20.45. The structural form is confirmed by the infrared spectra measured using a potassium bromide disc. IR (ν, cm−1 ) 3321 (–OH), 3225 (–NH), 1670 (–C=N). It was also characterized by the 1 H-NMR (ppm in DMSO-d6 relative to TMS): 1.25 (4 CH3 ), 1.80 (2 CH3 ); 4.95 (average triplet), 7.15 (average doublet); 12.10 (C=N–OH, singlet). 2.3. Chemicals and reagents All chemicals used were of analytical reagent grade (BDH and Merck). All solutions were prepared with distilled demineralized water. The stock standard copper(II) solution was prepared by dissolving 0.6393 g of copper(II) chloride dihydrate in distilled water and diluting to 250 ml. The solution was standardized titrimetrically by a known method [21]. The method basically depends on the titration of evolving iodine in the presence of starch by the reduction of Cu+2 to Cu+ in the mixtures containing

iodide and copper(II) salts. The working standard solutions were prepared by suitable dilution of the stock solution. A 0.0157 M stock complexing agent solution was prepared by dissolving 0.95 g PnAO in 20 ml dimethylsulfoxide and this solution was diluted with methanol to 250 ml. The buffer solution was prepared by dissolving 400 g of ammonium acetate in water and its pH was adjusted to 5.50 by addition of 350 ml concentrated ammonium hydroxide and was diluted to 1 l with demineralized water. Validation of the method described here was performed by using five Certified Reference Materials (CRMs) from the Community Bureau of Reference (BCR), International Atomic Energy Agency (IAEA), Office of Standard Reference Materials (NBS) and National Institute for Japan Environmental Agency (NIES). 2.4. Procedure For the determination of Cu(II), an aliquot containing an amount of copper(II) within the range recommended for the method was transferred into a 25 ml volumetric flask, 5 ml of 0.0157 M complexing agent solution was added after addition of 1 ml buffer solution. The absorbance change was measured after 10 min at the wavelength of maximum absorbance against a reagent blank as a reference. Copper contents were estimated by stirring the aqueous phase containing 20 mg/ml of copper and buffer solution with PnAO for varying time periods from 1 to 60 min. The absorbance in each case was measured. Interference studies were investigated by using a standard 20.00 mg/ml copper solution with varying concentrations of other species added as listed in Table 2.

3. Results and discussion 3.1. Absorption spectra The ligand (Fig. 1) reacts with divalent copper to form a stable red complex species which is highly sol-

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appears at around 1793 cm−1 after complexation with copper(II). This observation indicates that one of the hydrogen atoms is released as a free proton, and the other forms a hydrogen bridge (O· · · H–O). All these data support that the copper(II)–PnAO complex has an ionic character (Fig. 1). 3.2. The effect of pH Fig. 1. Proposed structure of the ligand (PnAO) (A), and its copper(II) complex (B).

uble in water and may be used for the direct determination of copper in aqueous media. The absorption spectrum of the copper(II)–PnAO complex in aqueous solution was studied over the wavelength range of 300–600 nm. This complex exhibited absorption maxima at 525 nm (Fig. 2) The absorption maximum of the ligand is at 325 nm and that of complex is at 525 nm indicating the formation of a complex between the ligand and copper(II). The molar absorptivity of the complex calculated from the absorbance data was found to be 2.95 × 104 l mol−1 cm−1 at 525 nm. The energy observed at around 525 nm belonging to d–d transitions indicates that the geometry is square planar. The IR spectrum of the ligand shows a sharp band at 3321 cm−1 which can be attributed to the −OH groups, and this band disappears and a broad band

The effect of pH on the determination of copper in aqueous medium was investigated spectrophotometrically. For this purpose, the solution containing 20 mg/ml of Cu(II) was measured in the pH range of 0–14 at 525 nm. The results obtained are shown in Fig. 3. As it is clearly seen, the amount of copper can be determined quantitatively in the pH range of 4–10. 3.3. Nature and stability of the complex Job’s method of continuous variation and the molar-ratio method were applied to ascertain the stoichiometric composition of the complex. A 1 : 1 (Cu : PnAO) complex was indicated by both methods. The color of the copper (II) complex was attained within 10 min after the addition of copper(II) ion, and the intensity remained constant for at least 12 h.

Fig. 2. Visible absorption spectra of the ligand (PnAO) (A), and its copper(II) complex (B).

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Fig. 3. Effect of pH on the formation of copper–PnAO complex.

3.4. Beer’s law and sensitivity

y = 0.002946x − 0.00084

A calibration graph for the determination of copper was prepared under optimum experimental conditions (1 ml buffer solution, 0.0157 M PnAO in aqueous medium). Beer’s law is obeyed within a wide range of 0.5–350 mg/ml of copper at 525 nm (Table 1). The concentration range observed with this new photometric reagent is much larger than those observed with the existing ones [22–24]. The calibration graph can be represented by a linear regression equation:

Here, y is the absorbance and x the concentration of Cu(II) in mg/ml. The molar absorptivity is 2.95 × 104 l mol−1 cm−1 and the Sandell’s sensitivity calculated on the basis of total copper present is 0.02 mg cm−2 .

Table 1 Conditions for the spectrophotometric determination of copper(II) Wavelength of maximum absorbance (nm) Limit of detection (LOD) (␮g/l) Limit of quantification (LOQ) (␮g/l) LOD/LOQ Range of linearity (␮g/l) Molar absorptivity (l mol−1 cm−1 ) Optimum pH Calibration graph Slope Intercept Correlation coefficient (r) Relative standard deviation (%)

525 10 40 0.25 0.5–350 2.95 × 104 4–10 0.002946 −0.00084 0.999 0.74

(r 2 = 0.999)

3.5. Precision and accuracy The precision of the method was checked by taking 10 replicate measurements on solutions each containing 2 mg of Cu(II). The relative standard deviation (10 determinations with 2 mg of Cu(II), 95% confidence level) is ±0.67%. The detection limit (LOD) is 10 ␮g Cu/l, limit of quantification (LOQ) is 40 ␮g Cu/l, and the LOD/LOQ ratio is 0.25. Altogether these data indicate the robustness of the procedure. The accuracy of the method was checked by bathocuproine disulphonic acid method [25] and using five International Certified Reference samples; river sediment (NBS-SRM-1645), sewage sludge (BCR-CRM-145), lake sediment (IAEA-SL-1), tomato leaves (NBS 1573), and tea leaves (NIES No.7). The experimental data (Table 2) show a good agreement between the results obtained by the two methods.

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Table 2 Determination of copper in environmental samples and foodstuffs Sample

River sediment (NBS-SRM-1645) Degirmendere sediment (Black Sea) Sewage sludge (BCR-CRM-145)c Sewage sludge (Trabzon)c Lake sediment (IAEA-SL-1) Tomato leaves (NBS 1573) Tea leaves (NIES No.7) Turkish tea leaves Hazelnut (C. avellena var. Pontica) Turkish tobacco Supradyn (Roche, Turkey)c

Certified Cu content ␮g/g (X ± SD)

109 ± 19 – 394 ± 12 – 30 ± 5.6 11 ± 1 7.0 ± 0.3 – – – –

Cu found (␮g/g)a Proposed method (X ± RSD%)

Bathocuproin methodb (X ± RSD%)

115 ± 5.2 264 ± 7.4 403 ± 5.3 468 ± 5.7 24 ± 3.1 13 ± 2.4 7.1 ± 2.7 68.4 ± 3.8 13.8 ± 2.1 24.3 ± 2.8 1.1 ± 8.7

118 ± 7.3 277 ± 8.2 365 ± 6.3 451 ± 7.8 26 ± 5.4 14 ± 3.8 6.8 ± 4.6 76.1 ± 6.2 12.2 ± 4.0 26.4 ± 3.6 1.0 ± 8.5

a

Average of five determinations. Spectrophotometric determination of copper with bathocuproine disulfonic acid was performed as described in [25]. c mg/g. b

3.6. Effect of diverse ions The effect of diverse ions in the determination of 20 mg of Cu(II) was studied according to the procedure described. Most of the ions studied do not interfere in the determination of copper, as shown in Table 3. The tolerance limit of an ion was taken as the maximum amount (mg) causing an error not greater than 2%. Many transitional metals will react with dioxime ligand in competition with copper (Co+2 , λmax : 485 nm, εmax : 4.40 × 103 l mol−1 cm−1 ; Ni+2 , λmax : 438 nm, εmax : 4.10 × 103 l mol−1 cm−1 ; Fe+3 , λmax : 460 nm, εmax : 5.64 × 103 l mol−1 cm−1 ), but using an excess of reagent in the procedure, they did not affect the results. 3.7. Applications The proposed method was applied for the determination of copper(II) in environmental samples such as sediment, tea, hazelnut, wastewater, industrial effluent, etc. 20–50 g of materials were digested with an excess of perchloric and nitric acids. The mixture was centrifuged and filtered, and the filtrate was evaporated to dryness and the residue was dissolved in 0.1 M HCl and diluted to 100 ml with water. A 10 ml of sample solution was used for the determination of copper(II) (Table 2).

Table 3 Effect of diverse ions on the determination of copper(II) Ion

Added as

Tolerance limit of ion (interfering ion/mg)

Ni+2 Fe+3 Fe+2 Zn+2 Al+3 Co+2 Pb+2 Mn+2 Cr+6 Mg+2 Ca+2 Sr+2 Ba+2 Cd+2 Sn+2 Thiosulphate Phosphate Tartrate Oxalate Nitrate Chloride Sulphate Fluoride EDTA

NiSO4 ·7H2 O FeCl3 FeSO4 ·(NH4 )2 SO4 ·6H2 O ZnSO4 ·7H2 O Al(NO3 )3 Co(NO3 )2 ·6H2 O Pb(NO3 )2 MnSO4 ·4H2 O K2 Cr2 O7 MgSO4 CaCl2 SrCl2 ·6H2 O BaCl2 ·2H2 O CdSO4 SnCl2 ·2H2 O Na2 S2 O3 ·2H2 O Na3 PO4 Tartaric acid Oxalic acid NaNO3 NaCl Na2 SO4 NaF EDTA.2Na

0.5 0.5 1.0 2.5 5.0 0.5 2.5 2.5 1.0 5.0 15.0 20.0 10.0 7.5 5.0 2.5 7.5 5.5 7.0 5.0 5.0 10.0 15.0 1.0

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4. Conclusions

References

The number of reagents currently used in the copper determination in aqueous media are limited and most of them require a chromogen for spectrophotometric methods. The ligand used in this study has several advantages over other ligands currently utilized for photometric copper determination and it could be obtained at high yields by a simple ethanol-crystallization from the crude compound. This reagent forms highly stable 1 : 1 complexes with Cu(II) in aqueous media. Although some other transition metal ions form complexes with this reagent, their absorbance maxima are significantly different from the one formed with Cu(II) (λmax : 525 nm, εmax : 2.95 × 104 ), therefore, do not interfere with copper. It could be easily applied to various environmental and biological samples for determination of 0.5–350 mg/ml copper. Moreover, the method is more reproducible and precise than the bathocuproine disulphonic acid method as seen from the results. Therefore, the method could be regarded as rapid, simple, sensitive, reproducible and also appropriate for direct determination of copper. Our previous work describes a copper extraction method by organic solvents using a selective reagent, S,S0 -bis(2-aminophenyl)oxalate [26]. However, it suffers from the disadvantage of being quite insoluble in water. The new reagent presented in this work is highly soluble in water and therefore, providing the possibility of a direct spectrophotometric determination of copper in aqueous media. Moreover, the data obtained by both methods are consistent with each other. This indicates that the analytical application of the new reagent is much more practical than that of the previous one [26].

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Acknowledgements This work was supported by The Research Fund of Karadeniz Technical University (Trabzon, Turkey).