In these conditions, the sharp peak of DNP appeared at -0.348 V. The peak current of DNP was proportional to the concentration of DNP in the range of 0.03 - 0.75 mg/L. The limit of detection of DNP in water sample was 0.00917 mg/L and its limit of quantity was 0.03057 mg/L.
Trang 1This paper is available online at http://stdb.hnue.edu.vn
DETERMINATION OF 2,4-DINITROPHENOL (DNP) BY VOLTAMMETRY USING A HANGING MERCURY DROP ELECTRODE (HMDE)
Tran Duc Luong1, Tran Van Tai1, Nguyen Hoai Nam2,
Ho Viet Quy1, Tran Van Chung3 and Nguyen Xuan Thanh4
1Faculty of Chemistry, Hanoi National University of Education
2Institute of Materials Science, Vietnam Academy of Science and Technology
3Institute of Chemistry and Material, Academy of Military Science and Technology
4College of Urban Works Construction
Abstract.The determination of 2,4-dinitrophenol (DNP) by stripping voltammetry
with a hanging mercury drop electrode was studied The experimental conditions
such as pH, supporting electrolyte, accumulation potential and accumulation time
were optimized In these conditions, the sharp peak of DNP appeared at -0.348 V
The peak current of DNP was proportional to the concentration of DNP in the range
of 0.03 - 0.75 mg/L The limit of detection of DNP in water sample was 0.00917
mg/L and its limit of quantity was 0.03057 mg/L
Keywords:Dinitrophenol, hanging mercury drop electrode, stripping voltammetry
The 2,4-dinitrophenol (DNP) C6H4N2O5, as with other nitro aromatic compounds,
is considered to be a serious environmental contaminant and highly toxic to humans, animals and plants [1] Because of the toxicity of DNP and its ubiquitous existence in the environment, it pollutes the environment and affects the up-growth of animals and human health It causes vomiting, skin and eye irritation and headache with continuous exposure leading to liver damage and anemia For these reasons, a rapid, sensitive and selective analytical method to detect DNP is significance in environmental pollution treatment A number of methods such as spectrophotometry [2, 3], differential pulse voltammetry (DPV), cyclic voltammetry (CV) [4], reversed-phase liquid chromatography
Received April 4, 2014 Accepted May 15, 2014.
Contact Tran Van Chung, e-mail address: tranchunghhvl@gmail.com
Trang 2(RPLC) [5], liquid chromatography-Mass spectrum-mass spectrum (LC-MS-MS) [10] and gas chromatography (GC) [5] have been used to determine DNP levels This study investigated an anodic stripping Voltammetric method using HMDE It also proposed
a new, simple sensitive and inexpensive way to determine the concentration of DNP in wastewater This method may be used to determine DNP presence and treatment with atmospheric oxygen activated by zero-valent iron (ZVI) and ethylenediamine tetraacetic acid (EDTA)
2.1 Experiment
* Chemicals
A stock solution of 100 mg/L DNP was prepared in distilled water using a crystalline solid with the purity of 99.5% obtained from a company in Shanghai, China All other reagents were of analytic grade
* Apparatus
The experiments were performed using a Swiss Metrohom electrochemical analyzer Model 757 with three electrodes The working electrode was a hanging mercury drop electrode (HMDE) An Ag/AgCl electrode with saturated KCl and a counter electrode and
a platinum wire were used as reference and counter electrode, respectively In addition, a pH-meter (METTLER, TOLEDO) was used to determine the solution pH
* Experimental procedure
10 mL of buffer solution with the concrete pH and 10 mL of DNP-sample were transferred into an electrochemical cell The solution (20 mL) in the cell, prior to electrochemical measurements, was deaerated by bubbling with extra pure nitrogen for
15 min The electrochemical measurements were carried out as follows:
First, the electrolyte with DNP was preelectrolysed and concentrated onto HNDE
in the permanent electric potential, stirring continuously
Second, after a period of between 30 and 60 seconds, the DNP concentrated onto the electrode (HMDE) dissolves again, scanning with a scan rate of 40 mV/s, from a negative electric potential to more positive electric potential This method allowed a peak current (anodic stripping of DNP) versus the changed electric potential
All the experiments related to the influence on the current peak were carried out using this procedure
Trang 32.2 Results and discussion
* The choice of supporting electrolyte
Different supporting electrolyte solutions of Na2CO3 and NaHCO3, CH3COONa and CH3COOH, NH4Cl and NH4OH, NH4Cl were applied to determine DNP in the sample The experimental data proved that there was a sharp oxidized peak of the adsorptively reduced DNP on HMDE in NH4Cl (0.1 M) This supporting electrolyte was selected for this study
* Voltammetric behavior of DNP on HMDE
In the solution of NH4Cl (0.1 M), pH = 3.0, with a DNP concentration of 1.6×10−6
M, the sharp oxidized peak of DNP was produced at -0.348 V, after being preelectrolyzed with sample stirred, at HMDE for 20s at the electric potential of -0.8 V and scanning rate
of 40 mV/s, as shown in Figure 1
Figure 1 Anodic stripping Voltammogram of DNP (1.6 ×10−6 M)
in the supporting electrolyte of NH4Cl (0.1M)
During the preconcentration of DNP on HMDE, the reductive process of the nitro group of DNP (noted DNP (NO2) has been suggested by [8, 10], as follows:
DNP(NO2) + 2e−+ 2H+→ DNP(NO)ads.+ H2O
DNP(NO)ads.+ 2e−+ 2H+ → DNPNHOHads.
The appearance of the peak current at -0.348 V may be due to the oxidation of RNPHOHads. to RNOads.:
DNPNHOHads.→ DNPNO + 2H++ 2e−
Trang 4* The influence of pH on peak current and peak potential
The pH of the electrolyte NH4Cl (0.1 M) containing (DNP) 1.6×10−6M with a pH that changed from 2 to 10 (by adding the diluted HCl and NH4Cl solutions) was prepared
to study its influence on the current peak and peak potential The experimental conditions were an accumulation time at HMDE of 20 s, an accumulation of potential of -0.5 V and
a scanning rate of 40 mV/s The experimental data are presented in Table 1
Table 1 Influence of pH on current and potential
Figure 2 The influence of pH on current and potential
The experimental data in Table 1 shows that the voltammeric peak potential for DNP became more negative with increasing pH This suggested that the proton concentration affects the rate of electrochemical reaction as presented above A pH of
3 was selected for this study This phenomenon is consistent with other work [8] The influence of pH on the peak current is given in Figure 3
Figure 3 The influence of pH on the current
* Influence of accumulation potential
The accumulation potentials of DNP on HMDE have been implemented at -0.5; -0.4 and -0.3 V The experimental conditions consisted of an accumulation of sample,
Trang 5stirred, at HMDE for 20 seconds in NH4Cl 0.1 M, with a scanning rate of 40 mV/s, DNP concentration: 0.3 mg/L The highest current peak obtained in this case corresponded to the potential of -0.5 V which was selected for this study (Figure 4)
Figure 4 Influence of the accumulation potential on current
* Influence of the accumulation time
With a sample consisting of DNP (0.3 mg/L) in the supporting electrolyte NH4Cl (0.1 M), pH = 3, preconcentration of DNP on HMDE at electric potencial of -0.5 V, the accumulation time from 0 to 120 seconds were determined The experimental data are presented in Table 2
Table 2 The influence of accumulated time on current
The obtained results have shown that the accumulation time to be selected is from
60 to 90 seconds
* Effect of the scan rate
The peak current of DNP increased with an increase in scan rate At a low scan rate (25 - 125 mV/s) and peak current (Ipic), the square root of the scan rate (ν) exhibited well the linear relation The linear relation was Ipic = 3.56 + 7.49ν1/2 with a correlation coefient of 0.989 By the experimental data, the scan rate of 40 mV/s should be selected for this work The peak potential was observed to shift to a more negative potential with
an increase in scan rate and, with the obtained linear relation, this may confirm that the reduction in DNP current was influenced by an adsorption
* Influence of DNP concentration
The experiment with concentrations of DNP from 0.03 to 0.75 mg/L was carried out in the conditions listed in Table 3
Trang 6Table 3 Influence of the DNP concentration on the current
Parameters Electrolyte pH Acc.potential Acc.time O2removal
time
Scan rate Values 0.1 M
The voltammogram reading corresponding to the DNP concentration and its standard plot are presented in Figure 5
Figure 5 Voltammogram and standard plot of DNP
According to the experimental data, the values of the limit of detection and quantity are as follows:
LODDNP= 3.Sy
a =
3.1.266
414 = 0.00917(mg/L)
LOQDNP= 10.Sy
10.1.266
414 = 0.03057(mg/L)
This study showed that the proposed anodic stripping using the votammetric method could be a simple way to determine DNP in water samples Its main advantages over other methods were its speed and lack of need for pretreatment The parameters influencing the current peak of the DNP were studied
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