Determination of 2,4-dinitrophenol (DNP) by voltammetry using a hanging mercury drop electrode (HMDE)

JOURNAL OF SCIENCE OF HNUE Interdisciplinary Science, 2014, Vol. 59, No. 5, pp. 17-23 This paper is available online at DETERMINATION OF 2,4-DINITROPHENOL (DNP) BY VOLTAMMETRY USING A HANGINGMERCURY 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 DNPwas 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. 1. Introduction 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 17 T.D.Luong, T.V.Tai, N.H.Nam, H.V.Quy, T.V.Chung and N.X.Thanh (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. Content 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 SwissMetrohom 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. 18 Determination of 2,4-dinitrophenol (DNP) by voltammetry using a hanging mercury drop... 2.2. 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− 19 T.D.Luong, T.V.Tai, N.H.Nam, H.V.Quy, T.V.Chung and N.X.Thanh * The influence of pH on peak current and peak potential The pH of the electrolyte NH4Cl (0.1 M) containing (DNP) 1.6×10−6 Mwith 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 pH 3.0 5.0 7.0 9.0 DNP Ipic (nA) 132 95 92.8 128 -Ep(V) 0.318 0.603 0.603 0.69 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, 20 Determination of 2,4-dinitrophenol (DNP) by voltammetry using a hanging mercury drop... stirred, 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 Time (s) 0 10 20 30 60 90 120 Ipic (nA) 111 165 269 388 568 641 692 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. 21 T.D.Luong, T.V.Tai, N.H.Nam, H.V.Quy, T.V.Chung and N.X.Thanh Table 3. Influence of the DNP concentration on the current Parameters Electrolyte pH Acc.potential Acc.time O2 removal time Scan rate Values 0.1 M NH4Cl 3 -0.5V 60s 60s 40 mV/s 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 a = 10.1.266 414 = 0.03057(mg/L) 3. 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