A novel voltammetric sensor for mercury(II) based on mercaptocarboxylic acid intercalated layered double hydroxide nanoparticles modified electrode

A novel voltammetric sensor for mercury(II) based on mercaptocarboxylic acid intercalated layered double hydroxide nanoparticles modified electrode

Accepted Manuscript Title: A novel voltammetric sensor for mercury(II) based on mercaptocarboxylic acid intercalated layered double hydroxide nanopart...

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Accepted Manuscript Title: A novel voltammetric sensor for mercury(II) based on mercaptocarboxylic acid intercalated layered double hydroxide nanoparticles modified electrode Authors: Karim Asadpour-Zeynali, Roghayeh Amini PII: DOI: Reference:

S0925-4005(17)30367-2 http://dx.doi.org/doi:10.1016/j.snb.2017.02.141 SNB 21872

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

7-10-2016 3-2-2017 22-2-2017

Please cite this article as: Karim Asadpour-Zeynali, Roghayeh Amini, A novel voltammetric sensor for mercury(II) based on mercaptocarboxylic acid intercalated layered double hydroxide nanoparticles modified electrode, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2017.02.141 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A novel voltammetric sensor for mercury(II) based on mercaptocarboxylic acid intercalated layered double hydroxide nanoparticles modified electrode

Karim Asadpour–Zeynali*, Roghayeh Amini

Department of Analytical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz 51666– 16471, Iran

* Corresponding author. Tel: +98 41 33393113 E–mail address: [email protected] (K. Asadpour–Zeynali).

Graphical abstract

Highlights  A highly sensitive and selective sensor for stripping voltammeric determination of Hg(II) was developed based on thioglycolic acid intercalated Mg–Al LDH modified electrode.  The fabricated sensor showed low detection limit (0.8 nM) and wider linear range (2.0– 800 nM).  The sensor was applied to determination of Hg(II) in real water samples with good recovery (97%–104%).

Abstract A sensitive and selective electrochemical sensor based on thioglycolic acid intercalated Mg–Al LDH (Mg–Al–TGA LDH) modified electrode was described for the determination of trace

mercury using square wave anodic stripping voltammetry (SWASV). The morphology and structure of the synthesized LDHs were characterized using transmission electron microscopy, Fourier transform IR and X–ray diffraction. Several variables affecting the stripping peak current such as the supporting electrolytes, pH value, deposition potential and deposition time, were carefully optimized. Under optimal conditions, the proposed sensor exhibited a wider linearity range from 2.0 to 800 nM Hg(II) with a detection limit of 0.8 nM (S/N = 3), which was below the guideline values in drinking water given by the World Health Organization. The effect of some potentially interfering ions were also investigated and results indicated that the presence of major cations and anions has no obvious influence on the determination of Hg(II). This newly prepared sensor was successfully applied to determination of Hg(II) in water samples with good recovery, ranging from 97–104%.

Keywords: Mercury(II); Layered double hydroxide; Anodic stripping voltammetry; Thioglycolic acid

1. Introduction Heavy metals pollution has proven to be a major threat to human health and the environment because of the high toxicity of these metals [1]. Mercury, one of the most hazardous heavy metals, causes severe health problems such as

Layered double hydroxides (LDHs) are a class of inorganic nano–materials that have the general formula of [MII1−xMIIIx (OH)2]x+[An−x/n· mH2O]x, where MII and MIII represent divalent and trivalent metal cations, respectively, and A is an exchangeable n–valent anion in the interlayer space [20]. Because of their many advantages such as high adsorption capacity, exchangeable interlayer anions, ease of preparation, catalytic activity and low cost, LDHs have been widely

applied in different fields such as anion exchanger, adsorbent and catalysis [21-23]. Furthermore, LDHs have gained more attention of electrochemists, owing to their electrocatalysis properties, and extensively used for construction of electrochemical sensors and biosensors [24, 25]. Considering their potentially good electrochemical properties with easily manipulated properties and high chemical stability, it is of great significance to develop electrochemical sensors based on LDHs nanoparticles for detection of Hg(II) in low concentrations. Immobilization of an appropriate reagent in the LDH structure, offers enhanced selectivity and sensitivity to the sensors [26]. Previous studies have indicated that the thiol group is an excellent functional group for Hg(II) chelation [27-29]. Some electrochemical sensors based on material with reagent containing thiol group such as, thiol–functionalized silica [30], thiophenol functionalized single– walled carbon nanotubes [31], thiol functionalized chitosan–multi–walled carbon nanotubes [32] and gold micro–/nanopore arrays containing 2 mercaptobenzothiazole [33] have been reported for analysis of Hg(II). With regards to these facts, mercaptocarboxylic acid intercalated LDHs are promising choice as electrode material for selective and sensitive electrochemical monitoring of mercury. Herein, we report the synthesis of thioglycolic acid intercalated Mg–Al LDH (Mg–Al–TGA LDH) using anion exchange method. The as–synthesized nano–structured LDH structurally and morphologically was characterized by scanning electron microscopy (SEM), X–ray diffraction (XRD), and Fourier transform IR (FT–IR). Then, the Mg–Al–TGA LDH modified glassy carbon electrode (GCE) was applied for selective detection of trace mercury ions by square wave anodic stripping voltammety (SWASV). To the best of our knowledge, this is the first time that Mg–Al– TGA LDH is used for stripping analysis of Hg(II). The developed sensor was applied for

determination of Hg(II) under optimized condition in different water samples with satisfactory results.

2. Experimental 2.1. Apparatus and instruments The

electrochemical

measurements

were

carried

out

using

a

Metrohm

757

VA

Potentiostat/Galvanostat with a conventional three–electrode system comprising a platinum wire as the auxiliary electrode, a saturated calomel electrode (SCE) as the reference and a modified or bare GCE (2 mm in diameter) as the working electrode, (all electrodes were obtained from Azar Electrode Co, Urmia, Iran). In order to study the structure of the synthesized LDHs, FT–IR spectra were recorded on Shimadzu 8400 series (Japan) Fourier transform infrared spectrometer, and XRD measurements were performed on a Bruker AXS (D8 Advance) X–ray powder diffractometer (Cu Kα radiation source, λ=0.154056nm) between 5° and 70° generated at 40 kV and 35 mA at room temperature. The morphology of the nanoparticles was studied by field emission scanning electron microscope (FESEM; TESCAN, model MIRA3). Chemical composition analysis of the synthesized nano–materials was carried out on an electron dispersive X–ray analyser (EDAX) attached to the FESEM. A Metrohm model 827 pH–meter was used for pH measurments. Hot plate stirrer model Gerhaldt, centrifuge (Raymand. T.N.Co, Iran) and nitrogen gas (99.9995%, Azaroxide Co., Iran) were used in the LDH preparation process. An ultrasonic bath (FALC, Italy) operated at 32 kHz is used to clean GC electrode surface.

2.2. Chemicals

All chemicals used were of analytical reagent grade and all solutions were prepared with doubly distilled water. Stock solutions of Hg(II) were prepared by dissolving mercury(II) nitrate monohydrate (Merck) in the minimum required amount of nitric acid. The working standard solutions were prepared daily by suitable dilution of above stock solution. A 0.2 M acetate buffer solution (pH 4.0) was used as supporting electrolyte. All salts used for the interference study, sodium acetate, LDH precursors, i.e., Mg(NO3)2.6H2O, Al(NO3)3.9H2O, thioglycolic acid (TGA ≥ 99.0%) and NaOH were purchased from Merck (Germany). All the plastic and glassware were cleaned by soaking in 10 % (v/v) HNO3 and were rinsed with doubly distilled water prior to use.

2.3. Synthesis of Mg–Al–TGA LDH The synthesis of the Mg–Al–TGA LDH contains two steps: the preparation of the precursor Mg– Al LDH and subsequently anion exchange reaction between as–synthesized Mg–Al LDH and thioglycolic acid (TGA) to obtain Mg–Al–TGA LDH nanoparticles. Mg–Al LDH was prepared using co–precipitation method. Typically, 30 mL of a solution containing 6.0 mmol of Mg (NO3)2.6H2O and 3.0 mmol of Al(NO3)3.9H2O was added dropwise into 40 mL of 0.5 mol L−1 NaOH solution under vigorous stirring and nitrogen purging. The reaction mixture pH was adjusted to 10, and the obtained slurry was aged at 65 °C for 18 h. Then, the resulting precipitate was separated by centrifugation at 4000 rpm for 10 min and washed three times with doubly distilled water, and dried at 60 °C for about 15 h. The Mg–Al–TGA LDH was synthesized according to the procedure described elsewhere [27]. Firstly, 0.5 g of the prepared Mg–Al LDH was added into 20 mL of the doubly distilled water and stirred for 4 h under nitrogen atmosphere. Afterward, 50 mL of 50 mmol L−1 TGA solution was added dropwise into the above Mg–Al LDH suspension and resulting mixture was refluxed

for 18 h at 60 °C. The pH value was kept at 8.0 throughout the synthesis process. The obtained precipitate was separated by centrifugation, washed and dried.

2.4. Preparation of the Mg–Al–TGA LDH/GC electrode Prior to modification, the surface of GC electrode was polished with alumina powder on fine abrasive paper and then sonicated for 10 min in 1:1 water/ ethanol solution to remove the remaining amounts of alumina from the electrode surface. 4.0 mg of the synthesized LDHs was added into 1.0 mL of doubly distilled water, and sonicated for 30 min to obtain a uniform dispersion. Then, 5 μL of fresh prepared above suspension was dropped on the surface of pre– treated GC electrode and dried at room temperature.

2.5. Preparation of real samples Water samples i.e., tap water and spring water, were collected in pre–washed (with detergent, distilled water, dilute HNO3 and distilled water, respectively) polyethylene bottles from local sources (Tabriz, Iran). The water samples were filtered through a cellulose membrane filter (Millipor) of 0.45 μm pore size and then directly analyzed or stored at 4 °C for use. All the water samples were spiked with Hg(II) at different concentration levels and then analyzed by the procedure described in Section 2.6

2.6. Analytical procedure An appropriate volume of Hg(II) work solution was added into electrochemical cell containing 10 ml of acetate buffer solution of pH 4.0 and deaerated with pure nitrogen for 5 min. The accumulation step was carried out under potential of −0.7 V for 400 second while stirring

solution. Then the stirring was stopped and after 30 s rest period, the square wave voltammograms were recorded in the potential range from −0.7 to +0.70 V with a frequency of 20 Hz, amplitude of 50 mV, and a potential step of 5 mV. After each determination, for regenerating the working electrode surface, potential was held at 0.70 V for 100 s to remove previous deposits.

3. Results and discussion 3.1. Characterization of Mg–Al–TGA LDH The XRD patterns of the synthesized LDHs exhibit a single crystalline phase with the characteristic reflections related to hydrotalcite–like compounds [34]. XRD is a valuable tool to monitor intercalation because the basal spacing increases or decreases depending on the size of the intercalated ion [35]. During the intercalation of TGA, the layers of LDH swell to accommodate the TGA anions, and this expansion is reflected by the 10.1 Å value of its d003. The basal spacing corresponding to diffraction by the (003) plane is larger than that for the Mg–Al LDH (7.6 Å). Taking into account that the thickness of the LDH layer is 4.8 Å and the van der Waals size of the TGA is 5 Å (determined by the software HyperChem), it was confirmed that TGA were successfully intercalated in the interlayer of Mg–Al LDH. Fig. 1 shows the FT–IR spectra of the Mg–Al–TGA LDH (a) and the Mg–Al LDH (b). The broad band around 3450 cm−1 in the spectra of the Mg–Al LDH and intercalated compound is attributed to the stretching mode of hydroxyl groups of LDH layers and interlayer water molecules. The presence of water molecules is also responsible for the medium intensity band close to 1639 cm−1 and 1649 cm−1 (bending mode) in the Mg–Al LDH and intercalated compound, respectively. A sharp and intense band at 1377 cm−1 in the spectrum of the Mg–Al

LDH is due to stretching vibration of interlayer NO3− ions. The FT–IR spectrum of the Mg–Al– TGA LDH exhibited two sharp bands at 1583 and 1373 cm−1 can be attributed to the anti– symmetric and symmetric stretching vibrations of carboxylate group (COO−) , and the bands at 2931 and 2869 cm−1 were ascribed to CH vibrational modes [35, 36]. FESEM image of Mg–Al–TGA LDH (Fig. 2A) shows an aggregate that consists of crystallites that are collected as small pseudo–spherical particles with particle size less than 100 nm and stacking with each other, which makes plate–like morphology. In order to investigate the chemical composition of the synthesized intercalated compound, the EDAX experiment was performed. The EDAX pattern of Mg–Al–TGA LDH exhibits the presence of the used metal ions (magnesium and aluminum) in synthesis of LDH. The presence of TGA molecules in the LDH structure is responsible for appearance of the Sulfur, oxygen and carbon elements in EDAX pattern (Fig. 2B).

3.2. Electrochemical Behavior of Hg(II) at Mg–Al–TGA LDH/GCE Figure 3 shows the square wave anodic stripping voltammograms of 0.5 μM of Hg(II) in 0.2 M acetate buffer solution (pH 4.0) at the bare GCE, Mg–Al LDH/GCE and Mg–Al–TGA LDH/GCE. When SWASV is performed, Hg0, which was produced by reduction of the adsorbed Hg(II) in deposition step, reoxidized to Hg(II). As can be seen, a small stripping peak was obtained at bare GCE (Fig. 3a) and Mg–Al LDH/GCE (Fig. 3b). In the case of Mg–Al–TGA LDH/GCE, a sharper and higher peak current for anodic stripping of Hg(II) at 0.32 V was obtained (Fig. 3c). The increase in anodic current at the Mg–Al–TGA LDH modified electrode demonstrates that the TGA in the structure of LDH plays an important role in the accumulation process of Hg(II) ions on the electrode surface. This phenomenon is due to the strong chelating

ability of thiol groups to Hg(II), which cause Hg(II) ions can be strongly absorbed on the surface of modified electrode.

3.3. Optimization of experimental parameters In order to obtain the best voltammetry behavior of the Mg–Al–TGA LDH/GC electrode toward Hg(II), some variables affecting the peak current including supporting electrolyte, pH, deposition potential and deposition time for a 0.5 μM Hg(II) solution were studied. 3.3.1. Supporting electrolyte and pH Effect of the some supporting electrolytes, such as phosphate buffer solution (PBS), acetate buffer solution, Britton–Robinson buffer solution, KNO3 solution and KCl solution on the voltammetric behavior of the proposed modified electrode was studied. The results showed that voltammetric peaks were observed in all these electrolytes, but when the measurements were performed in acetate buffer solution, the best shape of the peaks, the largest stripping peak current and the lowest background current were obtained. Therefore, this supporting electrolyte was employed in the subsequent experiments. Also, the effect of the solution pH was studied in 0.2 M acetate buffer in the pH range from 3.5 to 6.0. Fig. 4A shows dependence of the stripping peak response of Hg(II) on solution pH. Stripping peak current increases with increasing of the solution pH up to 4.0, and then decreases with a further increasing of solution pH. Decreasing of peak current at higher pH can be attributed to the hydrolysis of the metal ions, and instability of LDHs in low pH values causes the decreasing stripping peak current at pH lower than 4.0. Therefore, 0.2 M acetate buffer solution at pH 4.0 was used in the further studies. 3.3.2. Deposition potential

In stripping analysis, an appropriate deposition potential is very important to obtain the best sensitivity. Thus, the effect of the deposition potential on the Hg(II) stripping peak current was studied in the potential range from –0.10 to –0.90 V versus SCE. As shown in Fig. 4B, when the deposition potential shifted from –0.10 to –0.70 V, the stripping peak currents increased sharply. However, at the deposition potential more negative than –0.7 V, the stripping current decreased. This is due to the reduction of hydrogen along with Hg(II) ions at more negative potentials which must be leading to desorption of modified active film during hydrogen evolution [37]. Thus, –0.7 V was selected as deposition potential in the following experiments. 3.3.3. Deposition time Finally, the influence of deposition time on the stripping peak current was investigated (data not shown). It is found that the stripping peak currents increase with an increase in the deposition time up to 400 s and remains nearly constant at higher times owing to the saturation loading of active sites at the electrode surface. Therefore, 400 s is selected as optimal deposition time for further studies.

3.4. Analytical Performance for the Detection of Hg(II). Under the optimized experimental conditions described above, Mg–Al–TGA LDH modified GC electrode was used to determine Hg(II) by SWASV. Fig. 5 shows the SWASV responses of Mg– Al–TGA LDH /GCE toward Hg(II) in 0.2 M acetate buffer solution (pH 4.0) over the concentrations range from 2.0 to 800 nM. As shown in the inset B, the stripping peak current was proportional to the concentration of Hg(II), with a good linear correlation (r = 0.998). The linear regression equation can be expressed as I (μA) = 90.443 C (μM) + 0.1275. The limit of detection was calculated to be 0.8 nM, based on signal to noise ratio (s/n) of 3, which was much lower than

the guideline value of Hg(II) in drinking water (30 nM) as recommended by the WHO. Table 1 lists the analytical performance of the proposed Mg–Al–TGA LDH /GC electrode compared to previously reported electrodes for stripping analysis of Hg(II). As it can be seen, Mg–Al–TGA LDH /GCE exhibits wide linear range and comparable or even lower detection limit. This results indicate proposed sensor provides a high–performance for Hg(II) detection. The repeatability of the electrode was investigated by repetitively measuring the solution containing 0.5 μM Hg(II) ion with the same modified electrode. The relative standard deviation was 3.2 % for

, which indicates good repeatability. The

reproducibility of the response of the proposed sensor was also studied. For this purpose, the determination of 0.5 μM Hg(II) was performed with five independently prepared electrodes under the same condition, and the RSD of 4.6 % was observed. We also investigated the stability of the Mg–Al–TGA LDH /GC electrode by storage of modified electrode in air. After 20 days storage period, the electrode retained 90 % of its initial current response toward Hg(II). These results indicate that the proposed sensor exhibits good stability and reproducibility.

3.5. Interference study In order to evaluate the possible analytical applications of the proposed sensor, the influences of some coexisting ions on the determination of Hg(II) ion were investigated under the optimal conditions. The tolerance limit was defined as the maximum concentration of the interfering substances causing a change in the analytical signal no higher than ± 5 %. In these experiments, different amounts of various individual cations and anions were added to the acetate buffer solution (pH 4.0) in the presence of 0.5 μM Hg(II) ion and then, followed according to general

procedure. The results (see Table 2) revealed that the fabricated sensor was fairly free from the interference resulting from the presence of different ions commonly found in natural water samples. As shown in fig. 6, the stripping peaks due to Cu(II), Bi(III), Sb(III) and Ag (I) appeared well separated from the Hg(II) peak, and the Hg(II) stripping peak remained unaffected up to 25–fold of these interfering ions. Therefore, Mg–Al–TGA LDH /GC electrode can be successfully applied to determination of Hg(II) in real samples without interferences effects.

3.6. Analysis of real samples In order to evaluate the sensor performance in practical analytical applications, the proposed method was utilized for the determination of Hg(II) in different water samples (tap water and spring water). The water samples were directly spiked with different concentrations of Hg(II) and the stripping analysis was performed under optimized experimental conditions mentioned above. As summarized in Table 3, the recoveries in the range of 97.0–104% were obtained, indicating that the proposed sensor can be applied to mercury analysis in environmental water samples.

Conclusions In this study, we have developed a novel electrochemical sensing platform for selective analysis of mercury(II) based on thioglycolic acid intercalated LDH modified electrode. The Mg–Al– TGA LDH /GC electrode exhibited excellent performance towards the stripping analysis of Hg(II) due to the strong chelating ability of thiol groups in the TGA to Hg(II). The fabricated sensor is simple and sensitive with low cost, and exhibits good stability and reproducibility. Additionally, the sensor demonstrated high selectivity over a wide range of other cations and

anions, and showed a low detection limit for Hg(II). The practical application of the proposed electrode was evaluated in real water samples, and good results were obtained.

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Karim Asadpour-Zeynali received his Ph.D. degree in Analytical Chemistry from University of Tabriz, Iran in 2005. Currently, he is Associate Professor of Analytical Chemistry at University of Tabriz where he is conducting research activities in the areas of preparation of new modified electrodes with nanoparticles and quantum dots for electroanalytical chemistry and spectroelectrochemistry applications. Also Asadpour-Zeynali’s research activities concern the development of chemometrics techniques for electroanalytical chemistry. His work has produced nearly 78 peer-reviewed scientific papers and 71 presentations in national and international conferences.

Roghayeh Amini received her M.Sc degree in analytical chemistry in 2012 from University of Azarbaijan Shahid Madani University, Tabriz, Iran. She is currently a Ph.D student under supervision of Dr. K. Asadpour‒Zeynali at department of chemistry, university of Tabriz, Iran. Her current research interest is synthesis of nanomaterials and their application in development of electrochemical sensors.

Figure captions

Fig. 1 FT–IR spectrum of (a) Mg–Al–TGA LDH (b) Mg–Al LDH.

Fig. 2 (A) FESEM image of Mg–Al–TGA LDH. (B) EDAX patterns of Mg–Al–TGA LDH.

Fig. 3 SWASV response of (a) bare GCE, (b) Mg–Al LDH/GCE and (c) Mg–Al–TGA LDH/GCE in 0.2 M acetate buffer solution (pH 4.0) containing 0.5 μM Hg(II). SWASV parameter: frequency 20 Hz; amplitude 50 mV and potential step 5 mV.

Fig. 4 Influence of (A) pH value and (B) deposition potential on the peak current of 0.5 μM Hg(II) at Mg–Al–TGA LDH/GCE. (Other parameters are the same as in Fig. 3)

Fig. 5 SWASV response of the Mg–Al–TGA LDH/GC electrode toward Hg(II) at different concentrations from 2.0 to 800 nM (other conditions are the same as in Fig. 3). Inset A represent voltammograms of Hg(II) at low concentrations (2.0–35 nM ) and inset B show calibration plot of the SWASV peak current versus the concentration of Hg(II) ranging from 2.0 to 800 nM.

Fig. 6 SWASV responses of 0.5 μM Hg(II) in absence(….) and presence (––) of 25–fold of (A) Cu(II), (B) Bi(III), (C) Sb(III) and (D) Ag (I). (Other conditions are the same as in Fig. 3).

Table 1 Comparison of voltammetric determination of Hg(II) at the proposed electrode with other electrodes. Electrode Method Linear range (nM) Detection limit Reference (nM) Graphene–Au nanocomposite/GCE DPASV 1–150 0.6 [10] N–PC–Au/Au electrode

SWASV

1–1000

0.35

[11]

rGO–Au nanocomposite /CPE

DPASV

5–40

2.04

[12]

Nf–Au(I)NTs/GCE

SWASV

1–1000

0.5

[13]

Au/SPE

SWASV

5–500

5.1

[14]

Au–TiO2 NPs /Au electrode

DPASV

5–400

1

[15]

Nano IIP/CPE

SWASV

1–8000

0.2

[16]

MWCNTs/CPE

SWASV

2–700

0.9

[17]

Thiol–functionalized silica/GCE

DPASV

10–100

4.3

[30]

SWCNT–PhSH/Au electrode

SWASV

5–90

3

[31]

Chit–SH/ MWCNTs/GCE

SWASV

10–140

3

[32]

Mg–Al–TGA LDH/GCE

SWASV

2–800

0.8

This work

Abbreviations: N–PC–Au: nitrogen–doped porous carbon–gold nanocomposite; rGO: reduced graphene oxide; Nf–Au(I)NTs: nafion stabilized Au(I)–alkanethiolate nanotubes; IIP: ion imprinted polymer; MWCNTs: multi–walled carbon nanotubes; Chit–SH: thiol functionalized chitosan

Table 2 Tolerance limits of coexisting ions in the determination of 0.5 µM of Hg(II) Coexisting ions Foreign ion to analyte ratio Na+ , K+, Li+, NH4+, Mg2+, Al3+, Mn2+, Cr3+ NO3−, NO2−, Cl−, Br–, CO32–, SO42–,PO43–

1000:1

Zn2+, Co2+, Ni2+, Fe2+, Fe3+

500:1

Cd2+, Pb2+

100:1

Cu2+, Bi3+, Sb3+, Ag+

25:1

Table 3 Determination of Hg(II ) in water sample by the proposed method (n=3) Hg(II) (nM) Recovery RSD Sample (%) (%) Added Found Tap water 20 19.4 97.0 2.2 50 51.0 102.0 3.0 Spring water 20 20.3 101.5 1.9 50 52.0 104.0 2.8