A novel spectrophotometric method for the determination of copper ion by using a salophen ligand, N,N′-disalicylidene-2,3-diaminopyridine

Peer review under responsibility of Taibah University.

Abstract

In this study, a novel, selective and highly sensitive spectrophotometric method has been developed for the determination of copper(II) by using a new chromogenic reagent N,N′-bissalicylidene-2,3-diaminopyridine (H₂IF). The absorbance was measured at λmax = 414 nm. Experimental conditions were optimized, and Beer’s law was observed in the concentration range of 6.35–318 μg L⁻¹ of copper. The molar absorptivity of the complex was calculated and found to be 1.46 × 10⁵ L mol⁻¹ cm⁻¹. The detection and quantification limits were calculated and found to be 6.38 μg L⁻¹ and 21.27 μg L⁻¹, respectively. The calculated relative standard deviation (n = 5) was 0.62% for a standard solution including 63.5 μg L⁻¹ copper(II). The interference effects of the common ions have been studied and the statistical assessment of the obtained results is presented. The developed method was applied for the determination of copper in water samples. The reliability and accuracy of the developed method was proven by the analysis of a waste water standard.

Keywords:

• Spectrophotometry
• Salophen
• Copper(II) determination

1 Introduction

Currently, the selective and sensitive monitoring of trace metal ions from different sources, especially in water, have become more important due to their toxic effects on environmental and biological systems. Copper and its salts are used in industries, laboratories, medicines, foods and beverages. Therefore, it is of significant importance to develop uncomplicated, rapid and effective methods for quantitative analysis of copper in food, industrial and environmental samples.

For copper(II) determination, several analytical techniques have been used such as atomic absorption spectrometry [1–3], voltammetry [4,5], potentiometry [6–10], inductive coupled plasma-emission spectrometry [11] and inductive coupled plasma-mass spectrometry [12]. Spectrophotometry is the most widely used analytical technique for analysis because it is simple, economic, and easily available to most laboratories. Many spectrophotometric reagents have been used for determination of Cu(II); however, most of these reagents have various limitations such as requiring more time for colour development, heating, a narrow Beer’s range, or interferences from many ions [13–20].

H₂IF is among the foremost salophen ligands that have many fruitful applications and was first proposed by Bosner and co-worker [21]. The neutral Cu(II) complex ([CuL]) of H₂IF has been successfully crystallized by Atakol et al. [22] and characterized by Asadi et al. [23]. Recently, the potential DNA binding features and photo physical behaviours have been investigated by Kocak et al. [24]. In one of its recent applications among numerous studies throughout the past few decades, the evaluation of potentiometric cation recognition characteristics of the ligand has been proposed by our group [25,26]. In our up-to-date studies, addition of Cu(II) to the solution of H₂IF resulted in a rapid colour change from yellow to light brown with an accompanying new band appearing at 414 nm in the absorption profile. However, the compound has not been previously applied for the spectrophotometric determination of Cu(II).

Herein, we have proposed a novel, highly sensitive, simple and feasible method for the spectrophotometric monitoring of Cu(II) ion with a H₂IF in a methanol–water medium. Various experimental conditions, e.g., the amount of chromogenic agent, the effect of coexistence ions, the effect of pH, the ranges of applicability of Beer’s law and limit of detection on the spectrophotometric determination of copper have been considered. The method was successfully performed for the analysis of copper(II) in water samples and reference materials C12X3500 and SPS-WW1 (waste water).

2 Experimental

2.1 Reagents, solutions and samples

Throughout the study, analytical-reagent grade chemicals were used.

The solutions of metal salts were prepared with the relevant cations. For each cation, a 10⁻¹ M stock solution was prepared and then the dilute solutions (1 × 10⁻² to 1 × 10⁻⁹ M) were prepared by serial dilution of the stock solutions.

Acetate buffer solutions were prepared in the pH range of 2.20–5.60 by mixing appropriate proportions of sodium acetate solution (0.1 M) and acetic acid (0.2 M).

The river and sea water samples collected from various resources were filtered over a 0.45 m filter from Millipore (Milford MA).

Certified materials C12X3500 and SPS-WW1 were supplied from MBH Analytical Ltd. and LGC Group (UK).

2.2 Characterization of H₂IF

H₂IF was synthesized according to the method carried out in our prior studies using the same ratio of reactants [25,26]. Yield: 85%. Anal.Calc. C, 71.91; H, 4.76; N, 13.24. Found: C, 71.92; H, 4.82; N 13.67. IR (KBr, cm⁻¹): 3476 ν_as(OH); 3043 ν_ar(CH); 1615-1606 ν(C=N); 1553 ν(C=C). 1H-NMR (400 MHz, ppm, CDCl₃) 9.54 (s, 1H, –OH) 8.67 (s, 1H, imine) 8.45 (s, 1H, imine-pyridine nitrogen side) 7.28-7.75 (m, 4H, benzene) 6.89-7.15 (m, 3H, pyridine). ESI-MS (m/z, positive mode): 318 (H₂IF²⁺, 64%); 301 (H₂IF-OH²⁺, 100%). An optimized structure of the ligand based on density functional theory calculations is given in Fig. 1 (see Ref. [25] for computational details).

Fig. 1 Planar intramolecular H-bonded optimized structure of H₂IF.

A 1 × 10⁻³ M solution of the H₂IF was prepared by dissolving a proper weight of reagent in 10.0 mL methanol and then diluted with methanol to 50 mL in a calibrated flask.

2.3 Apparatus

A Thermo Array Evolution UV–Vis spectrophotometer (USA) was used for the observation of the absorption spectra.

A glass pH electrode (Schott) connected to a Jenway 3040 model Ion Analyzer was used for pH measurements of the buffer solutions.

2.4 Calibration procedure

A 1 mL of 5 × 10⁻⁴ M H₂IF and 1 mL of acetate buffer (pH 4.8) were added into a 4-mL glass tube. A 1 mL aliquot containing between 0.0 and 635 μg L⁻¹ Cu(II) was transferred into tube and mixed well. The absorbance was measured at 410 nm against a blank solution. The blank solution was prepared in a similar way, but without containing any copper. All absorbance measurements were implemented at room temperature. The amount of Cu(II) in the sample was determined from the calibration graph prepared in the same way.

3 Results and discussion

3.1 Spectral properties of ligand and complex

The H₂IF ligand reacts with a Cu(II) ion to form a stable square-planar neutral complex of 1:1 adduct, which is highly soluble in a methanol-water mixture. The electroneutral nature of the complex is derived from deprotonation of H₂IF induced upon complexation with Cu(II). The obtained absorption spectra of the free ligand and the Cu(II) complex is given in Fig. 2. The spectra were recorded in semi-aqueous media of 5 × 10⁻⁴ M H₂IF and Cu(II)–H₂IF complex in the range of 200–600 nm. The absorption of H₂IF occurs within the 200–400 nm range of the spectrum, whereas the Cu(II) complex further shows a distinct band along the 400–450 nm range with a 414-nm transition maximum. It is apparent from Fig. 2 that the 414-nm maximum makes spectrophotometric determination of the metal very feasible. Investigations were carried out to define the most appropriate conditions. The influence of each of the following variables such as the effect of pH and time, and the amount of chromogenic reagent on the reaction was tested. Using a univariate optimization method, the selected parameter was changed while the other variables were kept constant to obtain the optimum value for each of the variables.

Fig. 2 Absorption spectra of H₂IF (blue) and H₂IF-copper(II) complex (red).

3.2 Investigation of the appropriate experimental conditions

The workable pH range for the determination of Cu(II) was studied and ascertained. Several types of buffers such as universal, acetate and phosphate were prepared and tried. An acetate buffer was used to preserve the optimum buffer solution. This buffer gave the highest absorbance value. Keeping the other conditions constant, the pH values were varied in the range 2.20–5.60, as shown in Fig. 3. The inherent features of the spectral curves persisted nearly constant at pH 4.60–5.00. A pH of 4.80 was arbitrarily chosen for complete colour development. Therefore, all following studies were carried out at pH 4.80. It was observed that, under each condition, the absorbance persisted constant with the addition of 1 mL buffer for 1 mL sample.

Fig. 3 Dependence of the absorbance of H₂IF-copper(II) complex on pH.

The concentration of H₂IF was another important variable that could affect the absorbance intensity of the complex in the solution. The influence of the H₂IF concentration was studied in the range 1 × 10⁻⁵ to 1 × 10⁻⁴ M. Increasing the chromogenic reagent concentration resulted in an increment in the absorbance of the complex. It was observed that the concentration of the reagent should be in the range of 5.0 × 10⁻⁵ to 5.2 × 10⁻⁵ M. The maximum absorbance of the coloured complex incorporated with minimum blank reading was found to correspond to 5 × 10⁻⁵ M H₂IF. In the subsequent studies, the volume of 5 × 10⁻⁵ M H₂IF was fixed at 1.0 mL.

The effect of time on the complex formation reaction between Cu(II) and H₂IF was studied. The solution colour changed immediately from light yellow to light brown upon addition of Cu(II) to the solution. In only one minute, the absorbance of the system at 414 nm reached the maximum with the addition of Cu(II) to the solution and did not increase more with a longer time duration. This result depicts a rapid interaction of H₂IF with Cu(II) in a one-minute period under experimental conditions. This method could be applied for real-time detection of Cu(II) with no long interaction time required to obtain the whole absorbance before measurement of the signal.

In our previous potentiometric studies, interferences from other coexisting ions were computationally demonstrated through the comparisons of the calculated host-guest interaction energies in the framework of conceptual DFT and only the interference of Cr³⁺ was found to be noteworthy [25].

In the study herein, the effect of common interfering ions on the analysis of Cu(II) was also studied in detail. The tolerance limit was taken as the amount that caused a ±5% absorbance error in the determination of 63.5 μg L⁻¹ Cu(II). Great amounts of Na⁺, K⁺, NH₄⁺, Ca²⁺, ClO₄⁻, SO₄²⁻, CO₃²⁻, NO₃⁻, 10000-fold excess of Cd²⁺, Ba²⁺, F⁻, Br⁻, 5000-fold excess of Pb²⁺, Mn²⁺, 2000-fold excess of Zn²⁺ and Mg²⁺, and 1000-fold excess of Cr³⁺, Co²⁺ and Ni²⁺ did not interfere in the analysis of copper(II).

3.3 Applicability of Beer’s Law and sensitivity

Under the optimum conditions, the calibration curve for the determination of copper(II) was obtained and is depicted in Fig. 4. The Beer’s law was observed in the range of 6.35–318 μg L⁻¹ copper(II) concentration. The obtained linear regression equation was: A = 0.002 + 0.010 CCu (μg L⁻¹). The correlation coefficient was 0.997. The apparent molar absorption coefficient (ε) was 1.46 × 10⁵ L mol⁻¹ cm⁻¹, calculated from the slope of calibration curve. The detection (LOD = 3S_a/b) and quantification (LOQ = 10S_a/b) limits were calculated and found to be 6.38 μg L⁻¹ and 21.27 μg L⁻¹, respectively. (where S _a is the standard deviation of the response and b is the slope of the calibration curve). The calculated relative standard deviation (n = 5) was 0.62% for a standard solution containing 63.5 μg L⁻¹ Cu(II).

Fig. 4 Calibration curve for spectrophotometric determination of copper(II).

3.4 Application for real samples

The analytical applicability of the developed method was evaluated by performing it for the determination of copper in sea and river water samples. The samples collected from various resources were filtered, spiked with 25 or 50 μg L⁻¹ Cu and reanalysed. The obtained results of these spike/recovery experiments are given in Table 1. Good recoveries (98.5–102.96%) were obtained for Cu(II) in these samples, demonstrating the accuracy of the developed method. As another validation protocol, reference standards C12X3500 and SPS-WW1 were tested. The results were found to be in good agreement with the certificated copper concentrations (Table 1).

Table 1 Analytical results of water samples and the recovery of spiked analyte, and standard reference materials.

Sample	Added (μg L^−1)	Found^b (μg L^−1)	Recovery	Certified (μg L^−1)
River water^a	–	60.72(0.32)
	25.00	86.46(0.24)	102.96
Sea water^a	–	50.45(0.35)
	50.00	99.25(0.18)	98.50
C12X3500	–	24.82(0.27)		25.00
SPS-WW1	–	392.45(0.12)		400.00

a Samples were collected at Samsun, Turkey.

b Values in parentheses are the relative standard deviations for n = 3 with a confidence of 95%.

3.5 Determination of the stoichiometry of the complex

Job method was used for the determination of the composition of the Cu(II)–H₂IF complex. Equimolar (1 × 10⁻⁴ mol L⁻¹) solutions of Cu(II) and H₂IF were prepared. The metal(II) and reagent solutions were mixed in constantly ranging proportions, keeping the total volume of metal and ligand constant at 4.0 mL. The pH was kept at a constant value of 4.8 by the addition of acetate buffer. The absorbance originating from the complex which develops a light brown colour was measured at 414 nm. The obtained values are shown in Fig. 5. The maximum absorbance value was read at approximately 2 mL ligand volume and was assigned to a molar [Cu(II)]:[H₂IF] ratio of 1:1 adduct suggesting a Cu(II)–H₂IF stoichiometry.

Fig. 5 Job’s plot for the mixtures of H₂IF and Cu(II) solutions maximized at an approximately 1:1 metal to ligand ratio.

3.6 Comparison with the current literature

In summary, a relevant benchmark of spectrophotometric methods from the attached literature [27–32] and from the present study is compiled and given in Table 2. All parameters in Table 2 are nearly comparable within a reasonable data range and seem not superior to each other in general trend. Nonetheless, the shortness of reaction time which is favourable for practical application of spectrophotometric methods was found to be shortest one for the present method among existing values in Table 2.

Table 2 Comparison of the present spectrophotometric data to others in the determination of copper(II).

ε (L mol^−1 cm^−1)	Beer’s law range	λmax	Reaction time (min)	Ref.
2.7 × 10^4	6.6–330 μmol L^−1	502	2	1
0.4 × 10^5	2.0–14 μg mL^−1	375	5	2
4.37 × 10^5	4.0–115 ng mL^−1	608	10	3
0.87 × 10^3	10–80 μg mL^−1	445	15	4
1.08 × 10^4	0.31–6.35 mg L^−1	390	–	5
2.8 × 10^3	0.15–0.6 mg L^−1	240	–	6
0.158 × 10^4	1–10 mg L^−1	398	–	7
1.46 × 10^5	6.35–318 μg L^−1	410	1	Current work

4 Conclusion

In this work, a novel, highly selective and feasible method was developed for the spectrophotometric monitoring of Cu(II) by using N,N′-disalicylidene-2,3-diaminopyridine (H₂IF) as a new chromogenic reagent. The ligand has been found to be a highly sensitive and selective spectrophotometric reagent for Cu(II) ions. In the presence of Cu(II), H₂IF displays an obvious colour change from light yellow to light brown in a very short period. H₂IF also exhibits a good selectivity and sensitivity towards Cu(II) over other common competitive alkali, alkaline earth and transition metal ions. Experimental findings have indicated that the reagent can be readily suggested among one of selective spectrophotometric reagents and has a potential application for rapid, selective and sensitive detection of Cu(II) in aqueous media. Moreover, the spectrophotometric efficiency of H₂IF would pave the way for flow-injection spectrophotometric applications of the reagent.

Acknowledgement

Equipment support from the Ondokuz Mayıs University is gratefully acknowledged.

Notes

Peer review under responsibility of Taibah University.