Spectrophotometric Determination of Trace Concentrations of Copper in Waters Using the Chromogenic Reagent 4-Amino-3-Mercapto-6-[2-(2-Thienyl)Vinyl]-1,2,4-Triazin-5(4H)-One: Synthesis, Characterization, and Analytical Applications

A simple, selective, and inexpensive spectrophotometric method is described in the present study for estimation of trace concentrations of Cu2+ in water based on its reaction with chromogenic reagent namely 4-amino-3-mercapto-6-[2-(2-thienyl)vinyl]-1,2,4-triazin-5(4H)-one (AMT). The reaction between copper(II) ions and AMT reagent gives [Cu(L)(NO3)(H2O)2]•H2O complex, where L represents AMT molecule with NH group. The formed complex exhibits a sharp, and well-defined peak at λmax = 434 nm with a molar absorptivity (ε) of 1.90 × 104 L mol−1 cm−1, and Sandell’s factor of 0.003 μg mL−2. Absorbance of the [Cu(L)(NO3)(H2O)2]•H2O follows Beer’s law over a 0.7–25 μg mL−1 range with a detection limit of 0.011 μg mL−1. Validation of the submitted method was established by estimating Cu2+ in certified reference materials and actual sea and tap water samples. The results are compared with data obtained from copper concentration measurements using ICP-OES. The chemical structure of the Cu(II)-AMT complex was fully characterized by FT-IR, SEM, EDX, TGA, and ESR techniques.


Estimation of Cu(II) in Waters Using Recommended Method
Different volumes of standard copper(II) solutions (0.7-25.0 μg mL −1 ) were added to 8 mL of a 0.042% (w/v) AMT solution in 10 mL of acetate buffer (pH [4][5][6]. Deionized water was used to dilute all solutions to 50 mL. Absorbance was measured at 434 nm against a reagent blank. 100 mL of tap or sea water was sampled, filtered, and transferred into clean polyethylene bottles. All samples were analyzed within 6 h of collection. Appropriate quantities were treated according to the recommended procedure before and after spiking with known concentrations of Cu(II).

Evaluation of Proposed Method Using Certified Reference Materials
Multielement standard solution (# 90243) purchased from Sigma-Aldrich was used as a certified reference material to evaluate the proposed method. According to product catalog, this solution contains 10 mg L −1 of Cu, in addition to K, Bi, and Pb, with concentration of 100 mg L −1 ; Al, B, Cr, Li, Mo, Na, Ni, and Tl at concentration level of 50 mg L −1 ; while, Ba, Ca, Cd, Co, Fe, Mg, Mn, Sr, and Zn are in concentrations equal to Cu. An aliquot of the solution was analyzed by recommended procedure for estimating Cu after addition of few drops of KF . 2H2O (0.1% m/v) as a masking agent.
The steel sample (No.21899) obtained from Analytical Chemistry Laboratory Service, (München, Germany) was also used as a certified reference material to evaluate the proposed method. The copper content in the sample is 0.519% (w/w). The main components are Cr, Ni, Mn, and Zn, in addition to ultra-traces concentrations of Mo, P, C, S, Si, Tl, V, and Co. 0.2 g of steel sample was heated with 10 mL of aqua regia to near dryness. The sample was then mixed with 5 mL of concentrated H2SO4 and heated for 30 min. The resulting solution was neutralized by NaOH and diluted using doubly distilled water in a 100 mL calibrated flask. A part of aqueous solution was subjected to recommended procedure to estimate copper in steel sample.

Results and Discussion
A brown product was observed immediately upon mixing the aqueous copper(II) and AMT solutions. The electronic spectrum of the free ligand solution (1.2 × 10 −4 mol L −1 ) recorded against deionized water as a blank showed two peaks at 254, and 376 nm assigned to π-π* and n → π* transitions, respectively (Figure 1a). The peak at 376 nm shifted to 434 nm after reaction of the AMT Scheme 1. Synthesis of AMT.

Estimation of Cu(II) in Waters Using Recommended Method
Different volumes of standard copper(II) solutions (0.7-25.0 µg mL −1 ) were added to 8 mL of a 0.042% (w/v) AMT solution in 10 mL of acetate buffer (pH 4-6). Deionized water was used to dilute all solutions to 50 mL. Absorbance was measured at 434 nm against a reagent blank. 100 mL of tap or sea water was sampled, filtered, and transferred into clean polyethylene bottles. All samples were analyzed within 6 h of collection. Appropriate quantities were treated according to the recommended procedure before and after spiking with known concentrations of Cu(II).

Evaluation of Proposed Method Using Certified Reference Materials
Multielement standard solution (# 90243) purchased from Sigma-Aldrich was used as a certified reference material to evaluate the proposed method. According to product catalog, this solution contains 10 mg L −1 of Cu, in addition to K, Bi, and Pb, with concentration of 100 mg L −1 ; Al, B, Cr, Li, Mo, Na, Ni, and Tl at concentration level of 50 mg L −1 ; while, Ba, Ca, Cd, Co, Fe, Mg, Mn, Sr, and Zn are in concentrations equal to Cu. An aliquot of the solution was analyzed by recommended procedure for estimating Cu after addition of few drops of KF•2H 2 O (0.1% m/v) as a masking agent.
The steel sample (No.21899) obtained from Analytical Chemistry Laboratory Service, (München, Germany) was also used as a certified reference material to evaluate the proposed method. The copper content in the sample is 0.519% (w/w). The main components are Cr, Ni, Mn, and Zn, in addition to ultra-traces concentrations of Mo, P, C, S, Si, Tl, V, and Co. 0.2 g of steel sample was heated with 10 mL of aqua regia to near dryness. The sample was then mixed with 5 mL of concentrated H 2 SO 4 and heated for 30 min. The resulting solution was neutralized by NaOH and diluted using doubly distilled water in a 100 mL calibrated flask. A part of aqueous solution was subjected to recommended procedure to estimate copper in steel sample.

Results and Discussion
A brown product was observed immediately upon mixing the aqueous copper(II) and AMT solutions. The electronic spectrum of the free ligand solution (1.2 × 10 −4 mol L −1 ) recorded against deionized water as a blank showed two peaks at 254, and 376 nm assigned to π-π* and n → π* transitions, respectively (Figure 1a). The peak at 376 nm shifted to 434 nm after reaction of the AMT reagent with copper(II) (Figure 1b-d). Figure 1 also demonstrates the increase in absorbance with increasing Cu 2+ concentration. Moreover, the electronic spectrum of Cu(II)-AMT complex recorded in solid state using the Nujol mulls method and shown in Supplementary Materials ( Figure S3) exhibited a broad band at 630 nm corresponding to T 2g ( 2 D) ← E g in octahedral environment [30]. The effective magnetic moment (µ eff ) of copper complex with AMT calculated from the Equation (1) was 1.82 B.M confirming the distorted octahedral symmetry of this complex.
where, χ M is molar susceptibility modified using Pascal's constants of all atoms diamagnetism, while T is absolute temperature.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 17 reagent with copper(II) (Figure 1b-d). Figure 1 also demonstrates the increase in absorbance with increasing Cu 2+ concentration. Moreover, the electronic spectrum of Cu(II)-AMT complex recorded in solid state using the Nujol mulls method and shown in Supplementary Materials ( Figure S3) exhibited a broad band at 630 nm corresponding to T2g( 2 D) ← Eg in octahedral environment [30]. The effective magnetic moment (μeff) of copper complex with AMT calculated from the Equation (1) where, χM is molar susceptibility modified using Pascal's constants of all atoms diamagnetism, while T is absolute temperature. The stoichiometry of Cu(II)-AMT complex was estimated using the mole ratio method, wherein the absorbance at 434 nm was plotted versus the [AMT]/[Cu 2+ ] ratio as shown in Figure 2. The results indicate that the AMT ligand and Cu 2+ reacted in a 1:1 ratio. The stoichiometry of Cu(II)-AMT complex was estimated using the mole ratio method, wherein the absorbance at 434 nm was plotted versus the [AMT]/[Cu 2+ ] ratio as shown in Figure 2. The results indicate that the AMT ligand and Cu 2+ reacted in a 1:1 ratio. Figure 1. Electronic spectra recorded in aqueous media of (a) 4-amino-3-mercapto-6-[2-(2thienyl)vinyl]-1,2,4-triazin-5(4H)-one (AMT) ligand at concentration of 1.2 × 10 −4 mol L −1 and following addition of the Cu(II)-AMT complex at Cu 2+ concentrations of (b) 0.70 µ g/mL, (c) 3.50 µ g/mL, and (d) 5.50 µ g/mL.

Characterization
Techniques including FT-IR, SEM, EDX, TGA, and ESR were used to characterize the complex formed between Cu(II) and AMT.  Figure 3. For AMT, a peak due to NH 2 was observed at 3291-3195 cm −1 , aromatic C-H stretching peak was located at 3064-3091 cm −1 , and the aliphatic C-H stretching peak was observed at 2936 cm −1 . A peak corresponding to S-H was observed at 2665 cm −1 . Peaks of C=O, C=C, and C=N were observed at 1660, 1593, and 1618 cm −1 , respectively. Following differences were observed between the spectrum of AMT and that of its complex with Cu 2+ . The NH 2 peak in Cu(II)-AMT shifted to approximately 3142 cm −1 and decreased significantly in intensity. This indicates that the NH 2 group participated in complex formation. The C=O frequency shifted from 1660 to 1705 cm −1 in reference to coordination of AMT to Cu 2+ through the carbonyl oxygen atom. Peaks due to ν(Cu-N), and ν(Cu-O) were observed at 530, and 518 cm −1 , respectively. The coordination behavior of the nitrate ion was confirmed through the appearance of characteristic frequencies of bidentate nitrate group at 1545, 1353, and 820 cm −1 and the absence of the distinctive band of ionic nitrate group at 1766 cm −1 [31]. Therefore, the nitrate group served as a bidentate ligand with C 2v symmetry.

Characterization
Techniques including FT-IR, SEM, EDX, TGA, and ESR were used to characterize the complex formed between Cu(II) and AMT.

FT-IR Technique
The FT-IR spectra of AMT and the [Cu(L)(NO3)(H2O)2] . H2O complex are demonstrated by traces A and B, respectively, in Figure 3. For AMT, a peak due to NH2 was observed at 3291-3195 cm −1 , aromatic C-H stretching peak was located at 3064-3091 cm −1 , and the aliphatic C-H stretching peak was observed at 2936 cm −1 . A peak corresponding to S-H was observed at 2665 cm −1 . Peaks of C=O, C=C, and C=N were observed at 1660, 1593, and 1618 cm −1 , respectively. Following differences were observed between the spectrum of AMT and that of its complex with Cu 2+ . The NH2 peak in Cu(II)-AMT shifted to approximately 3142 cm −1 and decreased significantly in intensity. This indicates that the NH2 group participated in complex formation. The C=O frequency shifted from 1660 to 1705 cm −1 in reference to coordination of AMT to Cu 2+ through the carbonyl oxygen atom. Peaks due to ν(Cu-N), and ν(Cu-O) were observed at 530, and 518 cm −1 , respectively. The coordination behavior of the nitrate ion was confirmed through the appearance of characteristic frequencies of bidentate nitrate group at 1545, 1353, and 820 cm −1 and the absence of the distinctive band of ionic nitrate group at 1766 cm −1 [31]. Therefore, the nitrate group served as a bidentate ligand with C2v symmetry. Table 1 lists the different FT-IR frequencies of AMT and the [Cu(L)(NO3)(H2O)2] . H2O complex.  Table 1. FT-IR frequencies (cm −1 ) and tentative assignments of AMT and the Cu(II)-AMT complex.

AMT Reagent Tentative Assignments Cu(II)-AMT Complex Tentative Assignments
Nitrate group

SEM and EDX Analysis
The SEM images of AMT and its complex with copper(II) shown in Figure 4 provide useful information on surface morphology and particles size. The surface morphology of AMT changed dramatically after its reaction with Cu 2+ . Free AMT molecules are visualized as platelet-like particles with a clear, and smooth surface ( Figure 4A-D). The AMT particles size calculated using image J program ranged from 4.6 to 8.2 µm. Agglomerated particles with a rough surface are observed in the SEM image of Cu(II)-AMT as shown in Figure 4E-H. The particles size of Cu(II)-AMT complex were in the range of 18.8-31.25 µm. However, the SEM images of Cu(II)-AMT complex displayed a few particles with size less than 10 µm that may have combined to form agglomerates with larger size. The EDX analysis of the AMT ligand revealed three signals corresponding to carbon, oxygen, and sulfur elements ( Figure 5A), while the EDX spectrum of copper complex with AMT confirmed the presence of copper in the complex as indicated by the signals corresponding to this element in Figure 5B. Such signals were not found in the EXD spectrum of free AMT ( Figure 5A). The elemental analysis of AMT ligand and its complex with Cu(II) is displayed in Table 2

. SEM and EDX Analysis
The SEM images of AMT and its complex with copper(II) shown in Figure 4 provide useful information on surface morphology and particles size. The surface morphology of AMT changed dramatically after its reaction with Cu 2+ . Free AMT molecules are visualized as platelet-like particles with a clear, and smooth surface ( Figure 4A-D). The AMT particles size calculated using image J program ranged from 4.6 to 8.2 μm. Agglomerated particles with a rough surface are observed in the SEM image of Cu(II)-AMT as shown in Figure 4E-H. The particles size of Cu(II)-AMT complex were in the range of 18.8-31.25 μm. However, the SEM images of Cu(II)-AMT complex displayed a few particles with size less than 10 μm that may have combined to form agglomerates with larger size. The EDX analysis of the AMT ligand revealed three signals corresponding to carbon, oxygen, and sulfur elements ( Figure 5A), while the EDX spectrum of copper complex with AMT confirmed the presence of copper in the complex as indicated by the signals corresponding to this element in Figure  5B. Such signals were not found in the EXD spectrum of free AMT ( Figure 5A). The elemental analysis of AMT ligand and its complex with Cu(II) is displayed in Table 2.

Thermal Analysis
Thermogravimetric analysis of the ligand occurred in two stages as shown in Figure 6A. The first stage which was because of the loss of one molecule each of NH 3 and CO covered the range 60-200 • C. Complete decomposition of ligand occurred throughout the second stage at 200-600 • C. No residue was observed. Scheme 2 shows the proposed path of ligand thermolysis. Thermogravimetric analysis of Cu(II)-AMT involved four stages as shown in Figure 6B and Scheme 3. The loss of one molecule of uncoordinated water was in the first stage and occurred at 27-104 • C with a 4.45% weight loss. Two molecules of coordinated water were lost in the second stage at 120-200 • C with a weight loss of 8.25%. The third stage at 200-528 • C involved partial fragmentation of the ligand moiety and the release of nitrogen oxides amounting to a 63.02% weight loss. Complete decomposition of the complex leaving a residue of copper oxide occurred in the final stage at 528-1000 • C (24.64% weight loss).

Thermal Analysis
Thermogravimetric analysis of the ligand occurred in two stages as shown in Figure 6A. The first stage which was because of the loss of one molecule each of NH3 and CO covered the range 60-200 °C. Complete decomposition of ligand occurred throughout the second stage at 200-600 °C. No residue was observed. Scheme 2 shows the proposed path of ligand thermolysis. Thermogravimetric analysis of Cu(II)-AMT involved four stages as shown in Figure 6B and Scheme 3. The loss of one molecule of uncoordinated water was in the first stage and occurred at 27-104 °C with a 4.45% weight loss. Two molecules of coordinated water were lost in the second stage at 120-200 °C with a weight loss of 8.25%. The third stage at 200-528 °C involved partial fragmentation of the ligand moiety and the release of nitrogen oxides amounting to a 63.02% weight loss. Complete decomposition of the complex leaving a residue of copper oxide occurred in the final stage at 528-1000 °C (24.64% weight loss).

Thermal Analysis
Thermogravimetric analysis of the ligand occurred in two stages as shown in Figure 6A. The first stage which was because of the loss of one molecule each of NH3 and CO covered the range 60-200 °C. Complete decomposition of ligand occurred throughout the second stage at 200-600 °C. No residue was observed. Scheme 2 shows the proposed path of ligand thermolysis. Thermogravimetric analysis of Cu(II)-AMT involved four stages as shown in Figure 6B and Scheme 3. The loss of one molecule of uncoordinated water was in the first stage and occurred at 27-104 °C with a 4.45% weight loss. Two molecules of coordinated water were lost in the second stage at 120-200 °C with a weight loss of 8.25%. The third stage at 200-528 °C involved partial fragmentation of the ligand moiety and the release of nitrogen oxides amounting to a 63.02% weight loss. Complete decomposition of the complex leaving a residue of copper oxide occurred in the final stage at 528-1000 °C (24.64% weight loss). The powdered ESR spectrum of the Cu-AMT complex measured at 300 K showed two bands at 330 and 310 mT with g⊥ = 2.08306 and gıı = 2.2175, respectively (Figure 7). The g-values confirmed that the unpaired electron of Cu(II) occupies the orbital of dx2−y2, where gıı is greater than both g⊥ and g of free electron (2.0023) [32,33]. The shape of the ESR spectrum suggests a distorted octahedral symmetry for the Cu-ATM complex [32] in excellent agreement with µ eff value. The gav value calculated from Equation (2) was equal to 2.1279. Deviation of gav from g of free electron was because of the covalent property of bonding in Cu-ATM [33]. gav = 1/3(gıı + 2 g⊥) A molar conductivity of Λm = 8.2 Ω −1 cm 2 mol −1 measured at 1.00 × 10 −3 M in DMF solvent at room temperature indicated that the Cu-ATM complex is a non-electrolyte [34]. Full characterization of the complex formed between Cu 2+ and AMT suggests the chemical formula to be [Cu(L)(NO3)(H2O)2] . H2O. Figure 8 shows the proposed structure of the complex.

Electron Spin Resonance of [Cu(L)(NO 3 )(H 2 O) 2 ]•H 2 O
The powdered ESR spectrum of the Cu-AMT complex measured at 300 K showed two bands at 330 and 310 mT with g ⊥ = 2.08306 and g ıı = 2.2175, respectively (Figure 7). The g-values confirmed that the unpaired electron of Cu(II) occupies the orbital of d x2−y2 , where g ıı is greater than both g ⊥ and g of free electron (2.0023) [32,33]. The shape of the ESR spectrum suggests a distorted octahedral symmetry for the Cu-ATM complex [32] in excellent agreement with µ eff value. The g av value calculated from Equation (2) was equal to 2.1279. Deviation of g av from g of free electron was because of the covalent property of bonding in Cu-ATM [33]. g av = 1/3(g ıı + 2 g ⊥ ) (2) Scheme 2. Suggested thermal decomposition of AMT. The powdered ESR spectrum of the Cu-AMT complex measured at 300 K showed two bands at 330 and 310 mT with g⊥ = 2.08306 and gıı = 2.2175, respectively (Figure 7). The g-values confirmed that the unpaired electron of Cu(II) occupies the orbital of dx2−y2, where gıı is greater than both g⊥ and g of free electron (2.0023) [32,33]. The shape of the ESR spectrum suggests a distorted octahedral symmetry for the Cu-ATM complex [32] in excellent agreement with µ eff value. The gav value calculated from Equation (2) was equal to 2.1279. Deviation of gav from g of free electron was because of the covalent property of bonding in Cu-ATM [33]. gav = 1/3(gıı + 2 g⊥) (2) A molar conductivity of Λm = 8.2 Ω −1 cm 2 mol −1 measured at 1.00 × 10 −3 M in DMF solvent at room temperature indicated that the Cu-ATM complex is a non-electrolyte [34]. Full characterization of the complex formed between Cu 2+ and AMT suggests the chemical formula to be [Cu(L)(NO3)(H2O)2] . H2O. Figure 8 shows the proposed structure of the complex.  A molar conductivity of Λ m = 8.2 Ω −1 cm 2 mol −1 measured at 1.00 × 10 −3 M in DMF solvent at room temperature indicated that the Cu-ATM complex is a non-electrolyte [34]. Full characterization of the complex formed between Cu 2+ and AMT suggests the chemical formula to be [Cu(L)(NO 3 Figure 8 shows the proposed structure of the complex.

Analytical Application
The brown complex formed between Cu 2+ and AMT was used to develop an analytical procedure for monitoring the copper concentration in environmental samples.

Optimization of the Recommended Procedure
The relationship between solution pH and the absorbance of [Cu(L)(NO3)(H2O)2] . H2O at λmax = 434 nm was examined at various pH values using HCl/NaOH and acetate buffer (CH3COOH/CH3COONa). The results in Figure 9 show that the maximum absorbance of [Cu(L)(NO3)(H2O)2] . H2O occurred at pH 4-6 in acetate buffer. The absorbance is less at pH < 3, because the protonation equilibrium of the reagent shifted to the left as displayed in Scheme 4 producing protonated form (H2L). Therefore, the deprotonated form of the reagent (HL) is required for complex formation. The absorbance of the Cu(II)-AMT complex decreased dramatically above pH 6, a behavior most likely attributed to the hydrolysis of Cu(II)-AMT and the formation of colorless forms of copper(II) such as hydroxo species. Thus, the acetate buffer with pH 4-6 was chosen for further experiments.

Analytical Application
The brown complex formed between Cu 2+ and AMT was used to develop an analytical procedure for monitoring the copper concentration in environmental samples.

Optimization of the Recommended Procedure
The relationship between solution pH and the absorbance of [Cu(L)(NO 3 )(H 2 O) 2 ]•H 2 O at λ max = 434 nm was examined at various pH values using HCl/NaOH and acetate buffer (CH 3 COOH/ CH 3 COONa). The results in Figure 9 show that the maximum absorbance of [Cu(L)(NO 3 )(H 2 O) 2 ]•H 2 O occurred at pH 4-6 in acetate buffer. The absorbance is less at pH ≤ 3, because the protonation equilibrium of the reagent shifted to the left as displayed in Scheme 4 producing protonated form (H 2 L). Therefore, the deprotonated form of the reagent (HL) is required for complex formation. The absorbance of the Cu(II)-AMT complex decreased dramatically above pH 6, a behavior most likely attributed to the hydrolysis of Cu(II)-AMT and the formation of colorless forms of copper(II) such as hydroxo species. Thus, the acetate buffer with pH 4-6 was chosen for further experiments.

Analytical Application
The brown complex formed between Cu 2+ and AMT was used to develop an analytical procedure for monitoring the copper concentration in environmental samples.

Optimization of the Recommended Procedure
The relationship between solution pH and the absorbance of [Cu(L)(NO3)(H2O)2] . H2O at λmax = 434 nm was examined at various pH values using HCl/NaOH and acetate buffer (CH3COOH/CH3COONa). The results in Figure 9 show that the maximum absorbance of [Cu(L)(NO3)(H2O)2] . H2O occurred at pH 4-6 in acetate buffer. The absorbance is less at pH < 3, because the protonation equilibrium of the reagent shifted to the left as displayed in Scheme 4 producing protonated form (H2L). Therefore, the deprotonated form of the reagent (HL) is required for complex formation. The absorbance of the Cu(II)-AMT complex decreased dramatically above pH 6, a behavior most likely attributed to the hydrolysis of Cu(II)-AMT and the formation of colorless forms of copper(II) such as hydroxo species. Thus, the acetate buffer with pH 4-6 was chosen for further experiments.    (Figure 10). The absorbance of the complex was constant at [AMT] = 6.5 × 10 −5 to 2.2 × 10 −4 mol L −1 , but increased at concentrations above 2.2 × 10 −4 mol L −1 . A small decrease in absorbance was seen above 2.8 × 10 −4 mol L −1 , possibly because excess chromogenic reagent reduced the extent of complex formation by increasing the acidity of the solution [35]. Thus, the AMT concentration was fixed at 2.8 × 10 −4 mol L−1 for all measurements.

Investigation of Method Selectivity
The potential impact of common foreign ions on the estimation of copper was tested at optimized conditions and in the presence of 2 μg mL −1 Cu(II). The tolerance limit of interfering ions was expressed by the concentration of these ions or species that makes the relative error in the copper determination greater than ±5%. Al 3+ and Mn 2+ seriously interfered-probably due to complex formation with AMT-as shown in Table 3. However, these interferences were eliminated by addition of 0.5 mL KF . 2H2O (0.1 mol L −1 ), which reduced the relative error to an acceptable ±5% level at an interfering ion concentration of 100 µ g mL −1 .

Investigation of Method Selectivity
The potential impact of common foreign ions on the estimation of copper was tested at optimized conditions and in the presence of 2 μg mL −1 Cu(II). The tolerance limit of interfering ions was expressed by the concentration of these ions or species that makes the relative error in the copper determination greater than ±5%. Al 3+ and Mn 2+ seriously interfered-probably due to complex formation with AMT-as shown in Table 3. However, these interferences were eliminated by addition of 0.5 mL KF . 2H2O (0.1 mol L −1 ), which reduced the relative error to an acceptable ±5% level at an interfering ion concentration of 100 µ g mL −1 .

Investigation of Method Selectivity
The potential impact of common foreign ions on the estimation of copper was tested at optimized conditions and in the presence of 2 µg mL −1 Cu(II). The tolerance limit of interfering ions was expressed by the concentration of these ions or species that makes the relative error in the copper determination greater than ±5%. Al 3+ and Mn 2+ seriously interfered-probably due to complex formation with AMT-as shown in Table 3. However, these interferences were eliminated by addition of 0.5 mL KF•2H 2 O (0.1 mol L −1 ), which reduced the relative error to an acceptable ±5% level at an interfering ion concentration of 100 µg mL −1 .

Analytical Performance of the Recommended Procedure
The plot of Cu(II) concentration versus absorbance is linear over a range of 0.7-25 µg mL −1 . The linear equation that describes the calibration curve of our developed method is: where, A and C are absorbance, and concentration of copper(II) solution, respectively. Sandell's factor, and the molar absorptivity (ε) of the proposed method are 0.003 µg cm −2 , and 1.9 × 10 4 L mol −1 cm −1 , respectively. LOD calculated using the equations in [21] is 0.011 µg mL −1 (11.00 µg L −1 ). Relative standard deviation (RSD) and relative error (RE) of the spectrophotometric method calculated from ten replicate measurements of copper recovery at 2 µg mL −1 in distilled water were 1.4% and 1.2%, respectively. Variables demonstrating the effectiveness of the analytical method are listed in Table 4. Comparison of these analytical features with those of previously-published spectrophotometric methods in Table 5 establishes the good sensitivity and wide linear dynamic range of our method. Moreover, the analytical performance of the proposed method, in terms of selectivity, sensitivity, and dynamic concentration range, was compared with a wide variety of analytical techniques [5][6][7][8][9][36][37][38][39].
The spectrophotometric method developed in the present study provides better sensitivity and selectivity than some the methods mentioned in Table 6, without the need to use the extraction or preconcentration methodology. The dynamic concentration range of the proposed method covers a wide domain of concentrations including the maximum values of copper in drinking waters recommended by the U.S. Environmental Protection Agency [EPA] (1 mg L −1 ) and the WHO (2 mg L −1 ). Moreover, the proposed chromogenic reagent can operate at a wide range of pH values. Therefore, the suggested spectrophotometric method is appropriate for rapid and routine analyzes in many laboratories.

Evaluation of the Recommended Procedure
Our spectrophotometric procedure was effectively employed for the analysis of Cu in multielement standard solution (# 90243), and steel sample (No.21899) as certified reference materials, in addition to tap water (Taif City, Saudi Arabia) and sea water (Red Sea, Jeddah Governorate, Saudi Arabia). Copper concentrations in multielement solution and steel sample estimated by the proposed spectrophotometric procedure were 9.91 ± 0.27 mg L −1 and 0.475% (w/w), respectively, with small differences from certified values. However, Student's t-test revealed no significant differences between the concentrations determined by spectrophotometric method and certified values at the 95% confidence level since the tabulated t-value (2.78) was always greater than the calculated values (2.61, 2.55) for five replicate measurements. Water samples were spiked with a known concentration of copper as shown in Table 7, and the recovery method was determined. Acceptable recoveries were obtained, which confirms the accuracy and applicability of the method for copper determination in water samples. A comparison of the results of our spectrophotometric method with those of the standard ICP-OES indicates analytically-acceptable agreement between these procedures. Therefore, the proposed method can be used for the rapid and sensitive detection of Cu(II) in water samples. The rapid color development due to reaction of AMT with copper(II) can be detected by the naked eye, which makes the proposed method suitable for the qualitative analysis of copper(II) in a variety of samples.

Conclusions
The  Figure 8. The reaction between AMT and copper(II) was used to develop an effective spectrophotometric procedure for the determination of copper in water samples. The developed method is simple, low cost, and sensitive with LOD of 11.00 ppb. The proposed chromogenic reagent operates at a wide range of concentrations and pH values. On the other hand, the developed method can work as a sensitive chemo sensor for copper monitoring in water samples since color change due to reaction of AMT with Cu(II) appears rapidly within less than 10 s and remains stable for up to 2 h.

Conflicts of Interest:
The authors declare no conflict of interest.