Determination of chromium (III) ions in water samples by UV–vis spectrometry based on benzyl-functionalized benzimidazolylidene ligand

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.

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.

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.

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.

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.

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.

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.

In order to further illuminate the interaction between the ligands and metal ions, the relative reduction in the absorbance ((A–A₀)/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⁺.

According to the method in the literature (Takeda et al. 1980), the complex formation constants (K_f) 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 K_f 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.

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 K_a values were determined to be 1.45 × 10⁴ and 8.36 × 10³L·mol⁻¹ for Cr³⁺-L1 and Cr³⁺-L2, respectively.

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.

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.

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 (A_min−A)/(A_min−A_max) 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.

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.

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.

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.

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.

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.

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.

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