Abstract

Chromium is one of the most notorious environmental pollutants. The development of a sensitive and selective chromium detection method is an important issue. In this paper, cation recognition properties of two benzyl-functionalized benzimidazolylidene ligands towards metal ions were investigated by UV–vis spectrophotometry. The results showed that the receptors had a higher selection of Cr3+. The important analytical parameters, such as pH, quantity of the reagents, and their effects on the studied system were investigated. Under the optimum conditions the absorbance of the Cr3+-L complex obeys Beer's law in the Cr3+ concentration range of 0.05–2.60 μg/mL with the limit of detection of 0.026 and 0.034 μg/mL for L1 and L2, respectively. Infrared (IR) spectrum and density functional theory (DFT) calculations were used to explore the coordinating sites and the complex strength of two ligands towards Cr3+. The thermodynamic parameters showed that complex formation is a spontaneous exothermic process. The proposed method was successfully applied to the determination of Cr3+ content in water and alloy samples. The proposed method is seen as a simple and effective way of determining Cr3+ concentration.

INTRODUCTION

The necessity of selective determination of transition and heavy metal ions has increased immensely during the last few decades due to increasing environmental issues. Among these metal ions, chromium (III), at trace levels, is a micronutrient that plays an important role in the activation of insulin to maintain the correct levels of glucose in the blood. But its accumulation in the human body results in toxicity. Chromium is commonly used in numerous industrial processes including pigment production, electroplating and tanning. Thus, large quantities of chromium compounds can be released into the environment and drinking water (Kumar & Shim 2008). One of the pathways by which chromium enters the human body is through the intake of water. The threshold limit for chromium uptake is 0.1 mg·m−3 in air and 0.05 mg·dm−3 in water (Zhao et al. 2013). Therefore, the determination of chromium ion concentration is important in water samples, and also in environmental and industrial applications (Memon et al. 2005; Zhang et al. 2008).

Detection of chromium in water samples has been measured by a variety of techniques including inductively coupled plasma–mass spectroscopy, electrothermal atomic absorption spectrometry, atomic emission spectroscopy, fluorescence imaging, potentiometric membrane sensor, voltammetry, etc. Although these methods are accurate and sensitive, they are expensive, time consuming, and necessitate complicated sample pretreatment procedures. The identification and quantitative analysis of metal ions by ultraviolet–visible (UV–vis) spectroscopy have been proven to be an attractive technique (Desvarge & Czarnik 1997; Murkovic & Wolfbeis 1997), due to its simplicity and the low cost of rapid analysis without requirement of any sophisticated instrumentation (Safavi & Bagheri 2004; Kuswandi et al. 2006). As a routine quantitative method, UV–vis spectrometry is widely applied to metal complexes in solution to determine metal concentration. Emphasis is focused on the development of new molecules with special organic structures whose donor groups provide a suitable coordinating site, aiming at enhancing the selectivity of the method. Although there are many reports about the determination of metal ions using UV–vis spectrometry, relatively few studies have been carried out for the Cr3+ ion. Chromium (III) recognition properties of organic glutamine-based derivatives (Zhao et al. 2013), calixarene derivatives with coumarin, benzoxazole or benzothiazole units (Wang et al. 2009), branched polyethyleneimine (Jia et al. 2014), azocalix[4]arene (Lu et al. 2005) and rhodamine spirolactam derivative (Bao et al. 2015) have been reported. However, there needs to be improved sensitivity and recognition selectivity for the Cr3+ ion over other transition metal ions.

In recent years, we have been testing suitable methods for measuring contaminants in water, which is important for environmental assessment and protection. Herein, cation recognition properties of two benzyl-functionalized benzimidazolylidene ligands towards different metal ions were investigated by UV–vis spectrophotometry. The ligand is a class of N-heterocyclic carbene (NHC) ligand. NHC is an electron-rich nucleophilic scaffold, in which the carbene center enjoys the benefits from stabilization related to both the σ-electron-withdrawing and π-electron-donating nature of the nitrogen centers. Because of their unique electronic dedication, they can bind to a broad range of transition metals (Adhikary et al. 2012). Some silver, gold and platinum complexes of these ligands have been synthesized and structurally characterized, and their potential uses as catalysts, medicine, and luminescent materials have also been studied (Zhang et al. 2009; Adhikary et al. 2012). However, research on the spectroscopic properties of these ligands and their interactions with heavy metal ions has been limited. The objective of this work is to investigate UV–vis absorption characteristics of two benzyl-functionalized benzimidazolylidene ligands (L1 and L2, Figure 1) and their complexes coordinated with heavy metal ions. The initial results along with their application in sensing Cr3+ ions are reported in this paper.

Figure 1

Structure of ligands.

Figure 1

Structure of ligands.

METHODS

Reagents and solutions

L1 and L2 were synthesized according to the procedures in the literature (Adhikary et al. 2012; Liu et al. 2012). Other chemical reagents were of analytical grade. Nitrate and chloride salts of all metals were purchased from Shanghai Chemical Company and used without further purification. Double distilled water was used throughout the experiments.

Standard stock solutions of ligand (1.0 × 10−3 M) and metal ion salt (1.0 × 10−4 M) were prepared with C2H5OH/H2O (1:1 by volume). Working solutions were prepared by appropriate dilution of the stock solutions. The pH of the metal salt solution was adjusted by 1.5 mM HNO3 solution.

Apparatus

The UV–vis spectroscopy measurements were carried out on a spectrophotometer (Model HP Agilent 8453). The pH values were measured using a pH meter of model pHS-3C (Shanghai Honggai Instrument Factory) with a combined glass–calomel electrode of sensitivity (±0.01) pH units. The conductivity of each sample was measured with a conductivity meter (Model 145A +, Thermo Orion), using a conductivity cell (Model 011510, Thermo Orion). The conductivity cell was equipped with a water circulating jacket, and the temperature was controlled within ±0.02 K with a low temperature thermostat (Model DC-2006, Shanghai Hengping Instrument Factory). IR spectra were performed using a Nicolet NEXUS-470 Fourier transform infrared (FT-IR) spectrometer.

General procedure for UV–vis titrations

The UV–vis absorbance study was performed as per the following procedure (Jia et al. 2014; Yan et al. 2017). (i) Mixture solutions of appropriate amounts of 1 × 10−4 M Cr3+ and 0.8 mL 1 × 10−3 M ligand solution were added to a 10 mL volumetric flask and completed to the mark with C2H5OH/H2O. The pH of the solution was adjusted as a proper value. (ii) The mixture solutions were put in an ultrasonic bath for 10 min, and then placed in room temperature for 4 h to reach coordination equilibrium. (iii) The adsorption spectrum scanning (190–350 nm) of mixture solutions was measured by UV–vis spectrometer.

Conductometric measurements

The conductance of the metal ions was measured as a function of ligand concentration in solution. The experimental procedure was as follows: a solution of the metal ion (about 3 × 10−5 M, 25 mL) was placed in the cell (volume 100 mL) and the conductance of the solution was measured. A ligand solution was added into the solution of the metal ion step by step until the total concentration of the ligand was approximately five times as large as that of the metal ions. The concentration of ligand increased from 1.5 × 10−5 to 1.5 × 10−4 M.

Determination of Cr3+ in real samples

The proposed method was applied to determine trace amounts of Cr3+ in water samples (tap water and pond water) and in the alloy. Water samples were collected without adding any preservative in polyethylene bottles and analyzed within 6 h. Water samples were filtered through filter paper before use. The alloy sample was prepared as follows: 0.5–1.0 g alloy was completely dissolved in 20–40 mL of hydrochloric acid (1 + 1) by water-bath heating, and then 2–3 mL of 30% hydrogen peroxide was added. The excess of peroxide was decomposed by heating and the mixture was cooled and filtered. The filtered mixture was diluted to 500 mL with distilled water in a calibrated flask.

An amount of 0.1 mL stock solution of ligand was mixed with 5 mL sample solution in a volumetric flask and then the solution was spiked with different known concentrations of Cr3+. The solution was analyzed via a UV–vis spectrometer. The measured absorbance values were calculated for the Cr3+ concentrations using the standard addition method.

RESULTS AND DISCUSSION

Selectivity of two ligands towards different metal ions

The UV–vis absorption spectra of the two ligands in C2H5OH/H2O at 1.0 × 10−5M concentration are shown in Figure 2. The ligands are characterized by a strong band at 197 nm. The UV–vis absorption behaviour of the ligand/metal ions was investigated with the addition of 1.0 × 10−5 M of various metal ions such as K+, Mn2+, Ca2+, Co2+, Ag+, Cu2+, Hg2+, Fe3+, Cr3+, etc. The observed changes in UV–vis absorption spectra are also shown in Figure 2. Figure 2 indicates that the addition of equal moles of Cr3+ into the ligand solutions caused a significant increase of the absorbance at 197 nm, accompanied by a 10 nm red shift. The different maximum absorbance of the Cr3+-L complex may be attributed to changes in the conjugated system of ligand molecules. A trivalent chromium ion was connected with the nitrogen atoms that have a lone pair of electrons and formed a coordination bond. Therefore, as the conjugated system increased, the maximum absorbance of the Cr3+-L complex was shifted to longer wavelengths. This finding suggests a binding of the ligand with the Cr3+ ion.

Figure 2

UV–vis absorption spectra of L1 and L2 (1.0 × 10−5 M) upon addition of different metal ions (1.0 × 10−5 M) in C2H5OH/H2O solution.

Figure 2

UV–vis absorption spectra of L1 and L2 (1.0 × 10−5 M) upon addition of different metal ions (1.0 × 10−5 M) in C2H5OH/H2O solution.

In order to further illuminate the interaction between the ligands and metal ions, the relative reduction in the absorbance ((AA0)/A0) was calculated. As shown in Figure 3, among the examined metal ions, Cr3+ addition caused the largest relative reduction in the absorbance. Therefore, it can be concluded that the two ligands possess excellent selectivity in binding with Cr3+ and can be used for detection of Cr3+.

Figure 3

Relative reduction in UV–vis absorbance at λmax of L1 and L2 upon addition of different metal ions.

Figure 3

Relative reduction in UV–vis absorbance at λmax of L1 and L2 upon addition of different metal ions.

Conductometric titration

According to the method in the literature (Takeda et al. 1980), the complex formation constants (Kf) of the two ligands with many metal ions were calculated using the measured conductivity data at 298.15 K. The results in Table 1 show that among the obtained complex formation constants, the Kf values for the two ligands with Cr3+ ions are the largest ones. It indicates that L1 and L2 have high binding selectivity towards Cr3+ ions and are expected to act as suitable ligands for the recognition of Cr3+ ions.

Table 1

Complex formation constants (log Kf) of two ligands with metal ions

LigandCr3+Pb2+Ni2+Cu2+Zn2+Cd2+Co2+Hg2+Ag+Na+K+Mn2+Ca2+Fe3+Mg2+
L1 3.85 3.13 2.91 3.15 2.78 2.84 2.86 3.00 3.09 2.83 2.82 2.74 2.83 3.18 2.87 
L2 3.68 3.02 2.70 2.96 2.92 2.73 2.69 2.99 2.89 2.68 2.64 2.73 2.80 3.07 2.95 
LigandCr3+Pb2+Ni2+Cu2+Zn2+Cd2+Co2+Hg2+Ag+Na+K+Mn2+Ca2+Fe3+Mg2+
L1 3.85 3.13 2.91 3.15 2.78 2.84 2.86 3.00 3.09 2.83 2.82 2.74 2.83 3.18 2.87 
L2 3.68 3.02 2.70 2.96 2.92 2.73 2.69 2.99 2.89 2.68 2.64 2.73 2.80 3.07 2.95 
Using the obtained formation constants at 293.15, 298.15, 303.15 and 308.15 K, the thermodynamic function of complex formation was evaluated from Equation (1):  
formula
(1)
where ΔfG, ΔfH and ΔfS are the standard free energy, enthalpy and entropy of complex formation, respectively. The obtained values are presented in Table 2. The negative ΔfG value in all cases indicates that complex formation is a thermodynamically favourable process. The negative enthalpy ΔfH of coordination reactions suggests that the process is exothermic.
Table 2

Thermodynamic parameters of the two ligands coordinating with Cr3+

 ΔfG (kJ/mol)
ΔfH (kJ/mol)ΔfS (J/mol·K)
293.15 K298.15 K303.15 K308.15 K
Cr3+-L1 −21.83 −21.98 −22.13 −22.29 −12.86 30.60 
Cr3+-L2 −20.21 −20.63 −21.00 −21.16 −1.40 64.40 
 ΔfG (kJ/mol)
ΔfH (kJ/mol)ΔfS (J/mol·K)
293.15 K298.15 K303.15 K308.15 K
Cr3+-L1 −21.83 −21.98 −22.13 −22.29 −12.86 30.60 
Cr3+-L2 −20.21 −20.63 −21.00 −21.16 −1.40 64.40 

The binding ratios and constants of the two ligands with Cr3+

In order to study the sensitivity and binding mechanism of the two ligands, quantitative spectrometric titration was conducted in the presence of Cr3+. It can be seen from Figure 4 that upon addition of 0–5 equiv. of Cr3+, the intensity of the absorption band of the ligands at 197 nm increased accompanied with a continuous red shift from 197 nm to 207 nm. The binding ratios and constants of the ligands with Cr3+ were calculated using the Benesi–Hildebrand (B-H) equation. The Benesi–Hildebrand method (Benesi & Hildebrand 1949) is a widely used approach for determining the binding stoichiometry and equilibrium constants of non-covalent binding interactions, particularly for 1:1 and 1:2 binding stoichiometry. The association constant Ka was evaluated graphically by plotting 1/ΔA vs 1/[MZ+]n according to the B-H equation:  
formula
(2)
where A0 is the absorbance before metal ion addition; A is the absorbance after metal ion addition; Amax is the absorbance after adding an excess amount of metal ions. When there is a linear relationship between 1/ΔA and 1/[MZ+], the binding stoichiometry is 1:1, and the B-H equation is given as below:  
formula
(3)
Figure 4

UV–vis spectra of ligand (1.0 × 10−5 M) with gradual addition of Cr3+ ion. Insert: Beer's law plot.

Figure 4

UV–vis spectra of ligand (1.0 × 10−5 M) with gradual addition of Cr3+ ion. Insert: Beer's law plot.

It can be seen from Figure 5 that there is a linear relationship between 1/ΔA and 1/[Cr3+] with the linear correlation coefficients of 0.9992 and 0.9999 for Cr3+-L1 and Cr3+-L2, respectively. The excellent linear relationship indicates a 1:1 stoichiometric binding between ligand and Cr3+. From the slope and the intercept values, the Ka values were determined to be 1.45 × 104 and 8.36 × 103L·mol−1 for Cr3+-L1 and Cr3+-L2, respectively.

Figure 5

Changes in UV–vis spectrum with addition of different amounts of Cr3+ in C2H5OH/H2O.

Figure 5

Changes in UV–vis spectrum with addition of different amounts of Cr3+ in C2H5OH/H2O.

Effect of pH

As pH is an important factor in influencing the coordination process, the addition of 1.5 mM nitric acid in 10 ml mixed solution was studied. Since chromium (III) tends to form hydrate precipitate in basic and weak acidic environments, the effect of pH on the interaction between the two ligands and Cr3+ was scrutinized in the pH range of 3.4–4.8. The results are shown in Figure 6. From the results, it is observed that the complex exhibits maximum absorbance in the pH range of 3.4–3.7. The reason was possibly that complete chelation required a proper acidity. In more alkaline media the absorbance reduced because of complex hydrolysis and hydroxide formation. Thus, all samples discussed in the following section were prepared around pH 3.5.

Figure 6

The variation of the absorbance of the Cr3+-L complex vs pH at 207 nm. [Cr3+] = 1.0 × 10−5 M, [L] = 1.0 × 10−5 M, blank reagent.

Figure 6

The variation of the absorbance of the Cr3+-L complex vs pH at 207 nm. [Cr3+] = 1.0 × 10−5 M, [L] = 1.0 × 10−5 M, blank reagent.

Effect of reagent concentration

The effect of reagent concentration on the absorbance of the complex was investigated by varying the reagent concentration at constant Cr3+ concentration (1.0 × 10−5M). It is clear from Figure 7 that the maximum absorbance was attained with 0.8 mL of 1.0 × 10−5M ligand; above this volume up to 0.9 mL the absorbance remained unchanged. Therefore, 0.8 mL of 1.0 × 10−5 M ligand was used in all further measurements.

Figure 7

Effect of the volume of 1.0 × 10−5 M ligand on the complex with Cr3+ (1.0 × 10−5 M).

Figure 7

Effect of the volume of 1.0 × 10−5 M ligand on the complex with Cr3+ (1.0 × 10−5 M).

Analytical data

A working curve was constructed at optimum conditions by conducting a set of similar experiments at various concentrations of Cr3+, and the respective results are depicted in the inset of Figure 4. The ranges of linearity of absorbance as a function of Cr3+ concentration, i.e. obeying Beer's law, provide a satisfactory measure of the sensitivity of the method. Under the optimum conditions, the absorbance of both Cr3+-L complexes obeys Beer's law in the Cr3+ concentration range of 0.05–2.60 μg/mL. The limit of detection (LOD) was estimated using the normalized response of the UV–vis absorbance calibration value (AminA)/(AminAmax) as a function of log[Cr3+] (Shortreed et al. 1996; Kim et al. 2009). As shown in Figure 8, the lowest detectable limits (LOD) for Cr3+ using this method were 0.026 and 0.034 μg/mL, respectively. This is comparable to previously reported methods. The obtained optical parameters are given in Table 3.

Table 3

Optical parameters for the determination of Cr3+ with two ligands

ParametersCharacteristic (L1)Characteristic (L2)
Beer's law range (μg/mL) 0.05–2.60 0.05–2.60 
Molar absorptivity (L/(mol·cm)) 7.11 × 104 1.03 × 105 
LOD (μg/mL) 0.026 0.034 
Regression equationa y = 0.728 + 0.2896x y = 0.6924 + 0.3077x 
Correlation coefficient 0.997 0.995 
Standard deviationb 0.0063 0.0067 
Relative standard deviation (%)b 0.206 0.115 
ParametersCharacteristic (L1)Characteristic (L2)
Beer's law range (μg/mL) 0.05–2.60 0.05–2.60 
Molar absorptivity (L/(mol·cm)) 7.11 × 104 1.03 × 105 
LOD (μg/mL) 0.026 0.034 
Regression equationa y = 0.728 + 0.2896x y = 0.6924 + 0.3077x 
Correlation coefficient 0.997 0.995 
Standard deviationb 0.0063 0.0067 
Relative standard deviation (%)b 0.206 0.115 

ax is the concentration of Cr3+ and y is the absorbance.

bSix replicate measurements.

Figure 8

Normalized response of UV–vis absorbance calibration value as a function of Cr3+ concentration in C2H5OH/H2O solutions.

Figure 8

Normalized response of UV–vis absorbance calibration value as a function of Cr3+ concentration in C2H5OH/H2O solutions.

In addition, an exhaustive comparison is made in Table 4 for some important characteristics like linearity range and detection limit with previous reports by other researchers (Lu et al. 2005; Zhao et al. 2013; Bao et al. 2015; Sharif et al. 2015). We can get the result that the proposed method is superior to those sensors in a number of literature sources in some cases, such as low detection limits and wider linear ranges.

Table 4

Analytical features of some spectrophotometric methods employed for Cr3 determination based on different ligands

ReagentpHLOD (μg/mL)Beer's law range (μg/mL)
Azocalixarene derivative (Lu et al. 20053.0 – 48.6–231.4 
Rhodamine spirolactam derivative (Bao et al. 20157.2 0.378 2.6–10.4 
Glutamine with dansyl groups (Zhao et al. 20135.5 – 0–10.4 
Branched polyethyleneimine (Jia et al. 20145.0–6.0 – 0.50–20.0 
Silver nanoparticles (Sharif et al. 20156.0 0.023 0.05–2.60 
Present work L1 3.5 0.026 0.05–2.60 
Present work L2 3.5 0.034 0.05–2.60 
ReagentpHLOD (μg/mL)Beer's law range (μg/mL)
Azocalixarene derivative (Lu et al. 20053.0 – 48.6–231.4 
Rhodamine spirolactam derivative (Bao et al. 20157.2 0.378 2.6–10.4 
Glutamine with dansyl groups (Zhao et al. 20135.5 – 0–10.4 
Branched polyethyleneimine (Jia et al. 20145.0–6.0 – 0.50–20.0 
Silver nanoparticles (Sharif et al. 20156.0 0.023 0.05–2.60 
Present work L1 3.5 0.026 0.05–2.60 
Present work L2 3.5 0.034 0.05–2.60 

Interference effect

In order to investigate the selectivity of the proposed method, the effects of foreign species on the determination of Cr3+ were investigated. The interference study was performed by using 5.0 × 10−5 M of Cr3+ ions and variable concentrations of the interfering cations at pH 3.5. The tolerance limit was defined as the amount of added ions that caused less than 5% relative error in the determination of Cr3+. The results are depicted in Table 5. Table 5 shows that the concentration of the target ions can be selectively determined using the proposed method in the presence of excess amounts of the potential interferences examined.

Table 5

Effect of diverse species on the spectrophotometric determination of Cr3+ (5.0 × 10−5 M)

Foreign ionsL1
L2
Tolerance limit (M)Relative error (%) |ΔA|/A0 × 100Tolerance limit (M)Relative error (%) |ΔA|/A0 × 100
K+ 5.0 × 10−3 4.81 5.0 × 10−3 0.72 
Na+ 5.0 × 10−3 4.10 5.0 × 10−3 4.13 
Mn2+ 5.0 × 10−3 2.79 5.0 × 10−3 1.39 
Ni2+ 5.0 × 10−3 2.03 5.0 × 10−3 1.30 
Co2+ 5.0 × 10−3 2.00 5.0 × 10−5 0.91 
Ca2+ 1.0 × 10−3 0.76 1.0 × 10−3 4.75 
Cd2+ 1.0 × 10−4 1.54 5.0 × 10−4 0.84 
Ag+ 5.0 × 10−5 0.29 5.0 × 10−5 4.30 
Cu2+ 1.0 × 10−5 1.78 1.0 × 10−5 4.96 
Mg2+ 5.0 × 10−4 1.58 5.0 × 10−4 4.36 
Hg2+ 5.0 × 10−5 2.39 5.0 × 10−5 0.06 
Zn2+ 5.0 × 10−5 1.35 5.0 × 10−5 4.13 
Pb2+ 1.0 × 10−5 2.36 1.0 × 10−5 4.17 
Fe3+ 5.0 × 10−6 2.53 5.0 × 10−6 4.35 
Foreign ionsL1
L2
Tolerance limit (M)Relative error (%) |ΔA|/A0 × 100Tolerance limit (M)Relative error (%) |ΔA|/A0 × 100
K+ 5.0 × 10−3 4.81 5.0 × 10−3 0.72 
Na+ 5.0 × 10−3 4.10 5.0 × 10−3 4.13 
Mn2+ 5.0 × 10−3 2.79 5.0 × 10−3 1.39 
Ni2+ 5.0 × 10−3 2.03 5.0 × 10−3 1.30 
Co2+ 5.0 × 10−3 2.00 5.0 × 10−5 0.91 
Ca2+ 1.0 × 10−3 0.76 1.0 × 10−3 4.75 
Cd2+ 1.0 × 10−4 1.54 5.0 × 10−4 0.84 
Ag+ 5.0 × 10−5 0.29 5.0 × 10−5 4.30 
Cu2+ 1.0 × 10−5 1.78 1.0 × 10−5 4.96 
Mg2+ 5.0 × 10−4 1.58 5.0 × 10−4 4.36 
Hg2+ 5.0 × 10−5 2.39 5.0 × 10−5 0.06 
Zn2+ 5.0 × 10−5 1.35 5.0 × 10−5 4.13 
Pb2+ 1.0 × 10−5 2.36 1.0 × 10−5 4.17 
Fe3+ 5.0 × 10−6 2.53 5.0 × 10−6 4.35 

IR spectral studies

The interaction can also be qualitatively grasped by FT-IR spectra (Zhang et al. 2015). Figure 9 shows FT-IR spectra of the free ligand and the mixtures of ligand and Cr3+. As shown in Figure 9(a), the IR spectrum of ligand L1 exhibited three characteristic IR peaks at 1,617.27, 1,284.26, and 1,560.31 cm−1 corresponding to the stretch vibrations of –C = N, –C–N in the pyridine ring, and –C = N in the imidazole ring, respectively. Comparing the IR spectra of L1-Cr3+ with that of the free receptor, the above three characteristic IR peaks exhibit upward shifts to 1,621.73, 1,293.74 and 1,563.92 cm−1. These differences suggest that the carbon atom of the imidazole ring and the nitrogen atom in the pyridine ring took part in coordination with Cr3+ ions. For the free receptor L2 (Figure 9(b)), the peaks located at 1,612.82 and 1,285.91 cm−1 are assigned to –C = N and –C–N stretching in the benzimidazole ring. Comparing the IR spectra of L2-Cr3+ with that of L2, the stretch vibration of v(–C = N) at 1,612.82 cm−1 exhibited an upward shift to 1,616.51 cm−1 and v(–C–N) at 1,285.91 cm−1 was divided into two peaks at 1,315.62 and 1,276.88 cm−1. These results indicate that the carbon atom of one benzimidazole ring and the nitrogen atom in the other benzimidazole ring took part in complexation with Cr3+ ions.

Figure 9

FT-IR spectra of the two ligands and L-Cr3+ mixtures, (a) for L1 and (b) for L2.

Figure 9

FT-IR spectra of the two ligands and L-Cr3+ mixtures, (a) for L1 and (b) for L2.

DFT calculations

The two ligands have similar structures. However, the sensor based on L1 gives a low detection limit. To illustrate the difference of the interaction strength between the two ligands and Cr3+, the charge distribution and orbital energies of the HOMO and LUMO of the two ligands were calculated by quantum-chemical calculation by GaussView 5.0.8. The calculations were performed using the density functional theory (DFT) method and the three-dimensional molecular geometry optimizations were performed using the B3LYP and 6–31G basic sets. The results showed that the more negatively charged area of L1 is located at the nitrogen atom in the pyridine ring, the carbon atom in the imidazole ring and the Cl atom. The obtained atomic partial charges for these atoms are about −0.077, −1.38 and −0.570. For L2, the more negatively charged area is located at the nitrogen atom in the benzimidazole ring, the carbon atom in the middle benzimidazole ring and the Cl atom. The atomic partial charges for the three atoms are about −0.070, −0.730 and −0.525. The more negative charges is favourable for the complexation of L1 to Cr3+. The frontier molecular orbitals of the two ligands were also calculated (Figure 10). The calculated total energies and energy gaps for L1 and L2 are −3.80 × 104, −4.20 × 104 and −4.20, −4.34 eV, respectively. From these results, we can conclude that L1 has higher selectivity for Cr3+. The calculation is in good agreement with the results of UV, IR and conductometric titration.

Figure 10

Computational analysis of the HOMO and LUMO levels of L1 and L2.

Figure 10

Computational analysis of the HOMO and LUMO levels of L1 and L2.

Application of the method for determination of Cr3+ in water samples and alloy samples

To assess the feasibility of the proposed method, an attempt was made to determine Cr3+ ion content in real samples. As Cr3+ or its compounds are widely used in industry and industrial sewage enters into the environment and drinking water, water and alloy samples were chosen. An excess amount of Cr3+ in these samples has a great influence on our lives.

To ensure that the method is valid and has reasonable accuracy and precision, recovery of Cr3+ in these samples was determined. The pH of the sample solution was adjusted to 3.5. The results shown in Table 6 indicate good recoveries in the range of 92.3%–103%. The relative standard deviations (RSD) were found to be <5%. The calculated t-values listed in Table 6 were less than the critical (tabulated) one. Thus, there is no significant difference between averages and variances of results for the 95% level of significance. This indicates that the developed method was not affected by the matrices of the two types of water samples. The other constituents (Mn, Ni, Cu) in the alloy sample do not interfere significantly with the detection of Cr3+. These results suggest the capability of the method in the determination of Cr3+ in real samples.

Table 6

Analytic results of Cr3+ in water samples and alloy samplesa

SampleL1
L2
Cr3+ amount (μg/mL)Added (μg/mL)Found (μg/mL)Recovery (%)t valuesCr3+ amount (μg/mL)Added (μg/mL)Found (μg/mL)Recovery (%)t values
Tap water 0.14 0.26 0.38 92.3 3.91 0.13 0.26 0.39 99.1 1.18 
0.52 0.67 101 1.29 0.52 0.66 101 1.70 
1.04 1.18 100 0.66 1.04 1.17 99.7 1.74 
1.30 1.44 100 0.38 1.30 1.44 100 3.06 
1.56 1.71 101 1.57 1.56 1.69 100 1.84 
Pond water 0.17 0.26 0.43 100 0.10 0.16 0.26 0.41 98 2.68 
0.52 0.69 99.9 0.77 0.52 0.69 101 3.25 
1.04 1.21 100 0.77 1.04 1.20 100 0.81 
1.30 1.47 99.8 3.06 1.30 1.46 100 0.05 
1.56 1.73 100 3.06 1.56 1.72 99.8 1.29 
Alloy 1.20 1.20 – 1.59 1.14 1.14 – 0.67 
0.26 1.46 100 0.45 0.26 1.41 103 1.98 
0.52 1.72 99.8 1.81 0.52 1.65 98.9 1.28 
SampleL1
L2
Cr3+ amount (μg/mL)Added (μg/mL)Found (μg/mL)Recovery (%)t valuesCr3+ amount (μg/mL)Added (μg/mL)Found (μg/mL)Recovery (%)t values
Tap water 0.14 0.26 0.38 92.3 3.91 0.13 0.26 0.39 99.1 1.18 
0.52 0.67 101 1.29 0.52 0.66 101 1.70 
1.04 1.18 100 0.66 1.04 1.17 99.7 1.74 
1.30 1.44 100 0.38 1.30 1.44 100 3.06 
1.56 1.71 101 1.57 1.56 1.69 100 1.84 
Pond water 0.17 0.26 0.43 100 0.10 0.16 0.26 0.41 98 2.68 
0.52 0.69 99.9 0.77 0.52 0.69 101 3.25 
1.04 1.21 100 0.77 1.04 1.20 100 0.81 
1.30 1.47 99.8 3.06 1.30 1.46 100 0.05 
1.56 1.73 100 3.06 1.56 1.72 99.8 1.29 
Alloy 1.20 1.20 – 1.59 1.14 1.14 – 0.67 
0.26 1.46 100 0.45 0.26 1.41 103 1.98 
0.52 1.72 99.8 1.81 0.52 1.65 98.9 1.28 

aNumber of replicate measurements = 3; tabulated t value = 4.30 (at 95% confidence level with two degrees of freedom).

CONCLUSIONS

Due to chromium's extensive applications, the chromium ion is considered to be an environmental pollutant. Therefore, the development of an accurate and reliable method for determination of chromium ions is very important. This work presents a simple, sensitive and selective UV–vis spectrophotometric method for determining trace amounts of Cr3+ ions using two benzyl-functionalized benzimidazolylidene ligands (L1 and L2) as receptors. The results of experiments showed Cr3+ ions can be qualitatively distinguished and accurately quantified. The stoichiometric ratio was 1:1. Beer's law was found to be obeyed in the concentration range of 0.05–2.60 μg/mL. The detection limits of L1 and L2 for Cr3+ were 0.026 and 0.034 μg/mL. The obtained results showed the proposed method is efficient for recognizing Cr3+ ions with possible analytical applications.

ACKNOWLEDGEMENTS

The project is financially supported by the Science and Technology Project of Henan Province (142102310336).

REFERENCES

REFERENCES
Bao
X. F.
,
Cao
Q. S.
,
Nie
X. M.
,
Zhou
Y. M.
,
Ye
R. L.
,
Zhou
B. J.
&
Zhu
J.
2015
Design and synthesis of a novel chromium(III) selective fluorescent chemosensor bearing a thiodiacetamide moiety and two rhodamine B fluorophores
.
Sensors and Actuators B: Chemical
221
,
930
939
.
Benesi
H. A.
&
Hildebrand
J. H.
1949
A spectrophotometric investigation of the interaction of iodine with aromatic hydrocarbons
.
Journal of the American Chemical Society
71
,
2703
2707
.
Desvarge
J. P.
&
Czarnik
A. W.
1997
Chemosensors of Ions and Molecule Recognition
.
Kluwer
,
Dordrecht, The Netherlands
.
Jia
J.
,
Wu
A. H.
&
Luan
S. J.
2014
Spectrometry recognition of polyethyleneimine towards heavy metal ions
.
Colloids and Surfaces A: Physicochemical and Engineering Aspects
449
,
1
7
.
Kim
M. H.
,
Jang
H. H.
,
Yi
S.
,
Chang
S.
&
Han
M. S.
2009
Coumarin-derivative-based off–on catalytic chemodosimeter for Cu2+ ions
.
Chemical Communications
32
,
4838
4840
.
Kuswandi
B.
,
Nuriman, Verboom
W.
&
Reinhoudt
D. N.
2006
Tripodal receptors for cation and anion sensors
.
Sensors
6
,
978
1017
.
Memon
S. Q.
,
Bhanger
M. I.
&
Khuhawar
M. Y.
2005
Preconcentration and separation of Cr(III) and Cr(VI) using sawdust as a sorbent
.
Analytical and Bioanalytical Chemistry
383
,
619
624
.
Murkovic
I.
&
Wolfbeis
O. S.
1997
Fluorescence-based sensor membrane for mercury (II) detection
.
Sensors and Actuators B: Chemical
39
,
246
251
.
Sharif
T.
,
Niaz
A.
,
Najeeb
M.
,
Zaman
M. I.
,
Ihsan
M.
&
Sirajuddin
,
2015
Isonicotinic acid hydrazide-based silver nanoparticles as simple colorimetric sensor for the detection of Cr3+
.
Sensors and Actuators B: Chemical
216
,
402
408
.
Shortreed
M.
,
Kopelman
R.
,
Kuhn
M.
&
Hoyland
B.
1996
Fluorescent fiber-optic calcium sensor for physiological measurements
.
Analytical Chemistry
68
,
1414
1418
.
Takeda
Y.
,
Yano
H.
,
Ishibashi
M.
&
Isozumi
H.
1980
A conductance study of alkali metal ion-15-crown-5, 18-crown-6, and dibenzo-24-crown-8 complexes in propylene carbonate
.
Bulletin of the Chemical Society of Japan
53
,
72
76
.
Zhang
X. M.
,
Gu
S. J.
,
Xia
Q. Q.
&
Chen
W. Z.
2009
New structural motifs of silver and gold complexes of pyridine-functionalized benzimidazolylidene ligands
.
Journal of Organometallic Chemistry
694
,
2359
2367
.
Zhang
L.
,
Tu
Z. C.
,
Wang
H.
,
Kou
Y.
,
Wen
Q. H.
,
Fu
Z. F.
&
Chang
H. X.
2015
Response surface optimization and physicochemical properties of polysaccharides from Nelumbo nucifera leaves
.
International Journal of Biological Macromolecules
74
,
103
110
.
Zhao
M. L.
,
Ma
L. G.
,
Zhang
M.
,
Cao
W. G.
,
Yang
L. T.
&
Ma
L. J.
2013
Glutamine-containing ‘turn-on’ fluorescence sensor for the highly sensitive and selective detection of chromium (III) ion in water
.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
116
,
460
465
.