NGHIÊN cứu PHÁT TRIỂN điện cực MÀNG VÀNG TRÊN nền CARBON PASTE CHO PHƢƠNG PHÁP VON AMPE hòa TAN ANOT xác ĐỊNH As(III) và as(v) TRONG nƣớc tự NHIÊN TT TIENG ANH

HUE UNIVERSITY UNIVERSITY OF SCIENCES LE THI KIM DUNG STUDY ON THE DEVELOPMENT OF GOLD FILM ELECTRODE ON THE SUBSTRATE OF CARBON PASTE FOR As(III) AND As(V) DETERMINATION IN NATURAL WATER BY USING ANODIC STRIPPING VOLTAMMETRY Major: ANALYTICAL CHEMISTRY Code: 9440118 PhD DISSERTATION ABSTRACT Academic supervisors: Assoc Prof Hoang Thai Long Dr Dang Van Khanh HUE, 2022 The dissertation is completed at: Department of Chemistry, University of Sciences, Hue University Academic supervisors: Assoc Prof Hoang Thai Long Dr Dang Van Khanh Reviewer 1: PGS.TS Tu Binh Minh - University of Science, Vietnam National University, Hanoi Reviewer 3: PGS.TS Pham Hong Phong - Institute of Chemistry, Vietnam Academy of Science and Technology Reviewer 3: PGS TS Duong Thi Tu Anh - University of Education, Thai Nguyen University The dissertation will be defended at Hue University’s dissertation defense committee on ……… 2022, at The dissertation can be found at: The National Library of Vietnam Library of University of Sciences, Hue University 1 INTRODUCTION Arsenic is an element that causes pollution, can penetrate and accumulate in human body and has been classified by the World Organization for Research on Cancer (IARC) as a group of carcinogenic compounds Arsenic exists in the environment in several oxidation states (-3, 0, +3 and +5), of which As(III) is the most toxic form, 60 times more toxic than As(V); Inorganic arsenic compounds are 100 times more toxic than organic arsenic Therefore, analysis of arsenic forms has an important role in assessing environmental pollution, studying metabolism and bioaccumulation, or deciding pollution control measures, etc Some analytical methods are often used to determine arsenic such as atomic absorption spectrometry, inductively coupled plasma atomic emission spectrometry, inductively coupled plasma mass spectrometry or modern electrochemical analytical methods, typically, stripping voltammetry (SV) The spectrometry has the advantage of low limit of detection (LOD), from 0.5 to 50 µg/L However, these methods require expensive, bulky equipment, high operating costs, and often require complicated sample preparation procedures, so they may not be suitable for on-site analysis purposes In contrast, SV has the same sensitivity and selectivity as spectrometry, but utilizes simple, easy-to-use, compact equipment, and has a short analysis time, especially when environmentally friendly modified electrodes can be developed, errors can be avoided due to sample transportation and storage before analysis Among SV, anodic stripping voltammetry (ASV) using a goldmodified electrode has been the most interested research trend for speciation of arsenic in water, especially the direct analysis of As(III) and As(V), with low LOD, high selectivity Carbon electrodes, such as glassy carbon electrodes, carbon paste electrodes (CPE), screen printing electrodes (SPE), boron doped diamond electrodes (BDDE) are usually used as substrate electrodes for modified electrode fabrication In particular, CPE has the advantage, because it can be easily modified, flexible for electrode design On the other hand, research results on CPE can be considered as a premise for the development of analytical methods on SPE, a new trend in electrochemical analysis One of the challenges when analyzing arsenic forms is how to directly analyze As(V) in water For many years, As(V) was generally considered to be electrochemically inactive form, its stripping signal was only received in highly acidic medium, at sufficiently negative enrichment potentials Due to unfavorable direct quantitative conditions, As(V) often has to be chemically reduced first to As(III) by suitable reducing agents such as Na2SO3, SO2, hydrazine + HBr, KI (ascorbic acid) in solution heating conditions Then, formed As(III) is determined by SV If As(V) is determined directly in sample without going through reduction step to As(III) before analysis (referred to as direct analysis), it not only shortens analysis time but also reduces risk of environmental pollution because it does not need to use much chemicals for reduction Direct analysis of As(V) also avoids sample contamination and loss of arsenic due to formation of volatile compounds, such as AsH3, during the reduction process To contribute to solving the above problems, we choose to carry out the dissertation with the title: “STUDY ON THE DEVELOPMENT OF GOLD FILM ELECTRODE ON THE SUBSTRATE OF CARBON PASTE FOR As(III) AND As(V) DETERMINATION IN NATURAL WATER BY USING ANODIC STRIPPING VOLTAMMETRY” in order to find the appropriate conditions to develop a procedure for direct determination of As(III) and As(V) in natural water using the lowcost stripping voltammetry, suitable with the current conditions of laboratories in Vietnam Objectives of the study Developing electrodes capable of determining trace As(III) and As(V) in natural water for stripping voltammetry in accordance with the conditions of laboratories in Vietnam, to serve the needs of monitoring arsenic in the process of exploiting and treating water for domestic use Subjects of the study - Gold film electrodes (AuFE) on the substrate of boron doped diamond (BDD), carbon paste (CP) - Anodic stripping voltammetry; - As(III) and As(V) in natural water The contributions of the dissertation: - Development of ex-in situ gold film electrode on the substrate of carbon paste to determine inorganic As(III) in aqueous solution containing HCl and ascorbic acid by using differential pulse anodic stripping voltammetry - Development of ex-in situ gold film electrode on the substrate of carbon paste to determine inorganic As(III) and As(V) in natural water by using differential pulse anodic stripping voltammetry in aqueous solution containing Na2SO3, Mn(II) Chapter OVERVIEW 1.1 Introduction to arsenic 1.2 Methods for trace arsenic in natural water analysis 1.3 General conclusions Chapter RESEARCH CONTENTS AND METHODS 2.1 Research contents i) Investigation of electroactive surface area of electrodes, the stripping voltammetry characteristic of arsenic on AuFE/BDD, AuFE/CP electrodes in HCl medium; AuFE/CP in Na2SO3 medium by using cyclic voltammetry technique ii) Determination of As(III) by using differential pulsed anodic stripping voltammetry (DP-ASV) on AuFE/BDD and AuFE/CP electrodes in HCl medium: Investigation of the effect of experimental conditions (or factors), and the interference of matrix components; Evaluation of the method reliability iii) Determination of As(V) by DP-ASV on AuFE/CP electrode in Na2SO3 medium: Investigation of the effect of experimental conditions (or factors), and the interference of matrix components; Evaluation of the method reliability iv) Determination of As(III) and As(V) in a mixture by using DP-ASV on AuFE/CP electrode in Na2SO3 medium: Investigation of the effect of Au(III) and experimental conditions (or factors); Evaluation of the method reliability v) Comparison of DP-ASV methods for the direct quantification of As(III) and As(V) vi) Development of a procedure for direct determination of As(III), As(V) by using DP-ASV with AuFE/CP electrode and practical application 2.2 Research methods - Methods of preparing working electrode - Methods of preparation, preservation and treatment of samples - Differential pulse anodic stripping voltammetry and cyclic voltammetry 5 - Methods for investigation of the effect of experimental conditions on arsenic stripping voltammetric signals: One-factor-ata-time optimization - Methods of evaluating the reliability of the analytical methods - Statistical analysis method 2.3 Equipment, tools and chemicals Chapter RESULTS AND DISCUSSION 3.1 Investigation of stripping voltammetry characteristics by using cyclic voltammetry 3.1.1 Electroactive surface area of electrode Results of the determination of electroactive surface area of the electrode showed that, compared to BDDE, CPE has nearly times larger electroactive surface area Modifying BDDE, CPE by ex-situ gold film on the electrode surface can increase SV signals; AuFE/CP yields better stripping signals than AuFE/BDD 3.1.2 Stripping voltammetry characteristics of arsenic i) In HCl M medium Experimental results showed that AuFE/BDD, AuFE/CP can only be used to determine As(III) at a differential anodic stripping potential scan from -200 mV to +400 mV in a solution containing HCl, Au(III), ascorbic acid (AA) The stripping signal of arsenic on AuFE/CP is much higher than that on AuFE/BDD ii) In Na2SO3 0.05 M medium When the CPE electrode was modified with gold film, the arsenic stripping signal was obtained from As(III) solution containing Na2SO3 0.05 M However, with the case of As(V) solution, the stripping signal appeared only in the presence of Mn(II) 6 3.2 As(III) determination with boron doped diamond electrode and carbon paste electrode modified with gold film in HCl medium 3.2.1 Effect of supporting electrolytes on the arsenic stripping peak current 3.2.1.1 Effect of HCl concentration HCl concentrations (CHCl) selected for the next experiment were of 0.7 M with AuFE/BDD and M with AuFE/CP Figure 3.7 Influence of CHCl on Ip of arsenic at AuFE/BDD and AuFE/CP 3.2.1.2 Effect of Au(III) concentration If the analyte solution did not contain Au(III), the obtained Ip of arsenic was low and not repeatable Adding a small amount of Au(III) (CAu–in) to solution increased Ip and its repeability For analytical cost saving, CAu-in of mg/L (for AuFE/BDD) and mg/L (for AuFE/CP) were selected for next experiments Figure 3.8 Influence of CAu-in on Ip of arsenic at AuFE/BDD and AuFE/CP 7 3.2.1.3 Effect of ascorbic acid concentration To obtain high Ip of arsenic with good repeatability, AA concentrations (CAA) of 0.3 mM (for AuFE/BDD) and 0.5 mM (for AuFE/CP) were selected to carry out further survey experiments Figure 3.9 Influence of CAA on Ip of arsenic at AuFE/BDD and AuFE/CP 3.2.2 Effect of differential pulse voltammetry parameters Experiment results showed that the appropriate values of pulse amplitude (∆Epulse) and potential scan rate () were of 50 mV and 40 mV/s, respectively 3.2.3 Effect of electrode rotation speed Rotation speed of ω = 2000 rpm was selected to carry out further experiments for both types of investigated electrodes 3.2.4 Effect of deposition potential and deposition time Deposition potential (Edep) Effect of deposition potential was investigated in the range from -300 mV to -50 mV When the deposition potential is set at a negative potential more than -300 mV, Ip of arsenic begins to decline This might be because at this deposition potential, H2 gas bubbles begin to appear covering part of the working electrode surface Edep = -200 mV was chosen for the follow-up studies of both AuFE/BDD and AuFE/CP, in order to obtain high Ip of arsenic and good repeatability 8 Deposition time (tdep) Experiment results showed that, in order to achieve high arsenic stripping response with not too long deposition time, a tdep of 90 s was chosen to carry out further experiments 3.2.4 Effect of cleaning potential and cleaning time In order to improve repeatability, but not prolong analysis process and reduce sensitivity, a Eclean of +600 mV and a tclean of 10 s were selected for further experiments 3.2.6 Effect of interference species - AuFE/BDD: 500 mg/L Ca2+; 2.0 mg/L Fe2+; 500 µg/L As(V) not interfere with the determination of arsenic Ip Fe3+ ( 1.0 mg/L); Cu2+ ( 30 g/L); SO42- ( 300 mg/L); Triton X-100 ( 5.0 g/L) significantly increase the Ip - AuFE/CP: 500 mg/L Ca2+; 500 mg/L SO42-; 500 µg/L As(V) not interfere with the determination of arsenic Ip Fe2+ ( 1.5 mg/L); Fe3+ ( 2.5 mg/L); Cu2+ ( 150 g/L); Triton X-100 ( 2.5 g/L) significantly change the Ip value 3.2.7 Verifying the method The stripping currents of arsenic were recorded from a solution containing µg/L As(III) with 15 replications The results show that DP-ASV method has a good repeatability when using AuFE/BDD (RSD = 3.1 %) and AuFE/CP (RSD = 2.2 %) Table 3.17 Sensitivity, LOD, and LOQ of As(III) determination by using DP-ASV on AuFE/BDD and AuFE/CP a ± εa b ± εb Sy LOD LOQ Electrode -1 (µA) (µA/µg.L ) (µA) (µg/L) (µg/L) 0.44 AuFE/BDD 0.008 ± 0.033 0.226 ± 0.010 0.010 0.13 0.39 AuFE/CP 0.13 ± 0.04 0.284 ± 0.005 0.011 0.12 AuFE/BDD 150 μg/L AuFE/CP (A) µg/L 40 (B) 3000 μg/L µg/L 500 AuFE/BDD (C) AuFE/CP (D) Ip(μA) Ip(A) 400 20 300 200 100 0 50 100 CAs(III)(µg/L) 150 1000 2000 CAs(III) (g/L) 3000 Figure 3.19 Differential pulse voltammograms and stripping peak currents (Ip) as a function of CAs(III) at AuFE/BDD (A, C) AuFE/CP (B, D) The linear range of Ip–CAs(III) and linear regression equations determined by the least squares method with the tested electrodes are as follows: AuFE/BDD: Linear range: 0.480 g/L Ip = (0.245 ± 0.002).CAs(III) (R2 = 0.9999; p < 0.05) AuFE/CP: Linear range: 0,41000 g/L Ip = (0.245 ± 0.020).CAs(III) + (3.5 ± 10.2), R2 = 0,9980; p < 0,05 (values after the “±” signs are 95% confidence limit) The results of trueness verification by using standard addition method to determine recovery showed that the results of As(III) analysis by DP-ASV with AuFE/BDD, AuFE/CP are reliable (within the allowable limits recommended by AOAC) 10 3.2.8 Procedure for As(III) determination by using DP-ASV on AuFE/CP in HCl medium Pretreatment Water sample + 1.0 mL concentrated HCl/L sample Filter with 0.45 µm fiberglass filter paper AuFE/CP fabrication CPE: Graphite powder + Paraffin oil (7:3, w:w) Create Au ex situ film: Electrolyze CPE at -100 mV, 1000 mg/L Au(III), 120 s, without rotating the electrode Electrode cleaning: +500 mV, 30 s, rinse with distilled water twice Analysis of As(III) by DP-ASV Analytical solution: 10.0 mL contains V mL of sample; 1.0 M HCl; 0.5 mM ascorbic acid; mg/L Au(III) Record stripping voltammograms (replicate times, omit first recording): + Deposition step: Edep = -0.2 V, tdep = 90 s, ω = 2000 rpm, tres = 15 s + Stripping step: Scan potential from -200 to +400 mV, ν = 40 mV/s, DP technique (∆Epulse = 50 mV, tpulse = 40 ms, Estep = mV; tstep = 150 ms) + Cleaning electrode: Eclean = +600 mV, tclean = 10 s Quantification: based on Ip by standard addition method (34 standard additions) Contamination control with blank samples: 1.0 mL concentrated HCl + L double distilled water Purging time (N2): 180 s Figure 3.20 Procedure for As(III) determination by DP-ASV on AuFE/CP in HCl medium 3.3 As(V) determination with AuFE/CP in Na2SO3 medium 3.3.1 Effects of supporting electrolytes on the arsenic stripping response 3.3.1.1 Effect of Na2SO3 concentrations When Na2SO3 is used as supporting electrolyte in addition to supporting the electrochemical processes occurring on the electrode, it also helps to remove dissolved oxygen Experimental results showed that, at the of 0.05 M, the high stripping peak currents are obtained with good repeatability, therefore this concentration of Na2SO3 was selected for next experiments 11 Ip RSD RSD (%, n = 5) 2.0 IP (µA) 1.5 1.0 0.5 -0.05 Figure 3.21 Influence of 0.05 0.15 0.25 CNa2SO3(M) 0.35 on Ip of arsenic at AuFE/CP 2.0 1.8 1.6 1.4 1.2 1.0 5.0 4.0 0.1 0.2 CMn (mg/L) 3.0 Ip 2.0 RSD 1.0 0.3 RSD (%, n = 4) IP (µA) 3.3.1.2 Effect of Mn(II) concentrations Figure 3.22 Influence of CMn on Ip of arsenic at AuFE/CP The results showed that in the absence of Mn(II), no stripping peak of arsenic appears on the voltammograms In the presence of Mn(II), Ip of arsenic appears and gradually increases when increasing CMn(II) Therefore, CMn(II) = 0.2 mg/L was chosen for next experiments 3.3.2 Effect of differential pulse voltammetry parameters on Ip Survey experiments showed that the appropriate pulse amplitude (∆Epulse) and potential scanning rate () are 50 mV and 50 mV/s, respectively 3.3.3 Effect of electrode rotation speed When increasing electrode rotation speed (ω), the Ip of arsenic increases However, increasing ω higher than 2000 rpm, the electrode begins to shake strongly, which can lead to the risk of deformation of the gold film on working electrode surface Therefore, ω = 2000 rpm was selected to carry out further experiments 12 3.3.4 Effect of deposition potential and deposition time Deposition potential (Edep) Experimental results showed that, at deposition potentials of more positive than –1000 mV, no stripping peaks of manganese and arsenic appeared on the voltammograms At the Edep of –1200 mV, high stripping peak currents of arsenic and manganese were obtained with good repeatability, thus this deposition potential was chosen for further studies 7.0 Ip (μA) 5.5 As Mn 4.0 2.5 1.0 -0.5 -600 -1000 -1400 -1800 Edep (mV) Figure 3.26 Effect of Edep on Ip of arsenic and manganese at AuFE/CP Deposition time (tdep) The Ip only linearly increased with the deposition times when tdep < 120 s This is also in accordance with experiment results from some previous studies It might due to the saturation of electrode surface sites by As or Mn Therefore, tdep = 90 s was chosen for further experiments 3.3.5 Effect of cleaning potential and cleaning time In order to improve the repeatability, not prolong analysis process and reduce sensitivity, Eclean = +300 mV and tclean = 10 s were selected for further survey experiments 3.3.6 Effect of interference species Experiment results showed that: 13 - Fe3+ ( 0.9 mg/L), Fe2+ ( 0.4 mg/L), Cu2+ ( 120 g/L), Pb2+ ( g/L), Zn2+ ( 30 g/L) significantly change Ip values - Other ions such as 1000 mg/L Cl–, 1000 mg/L SO42–, 500 mg/L Ca2+, 1000 mg/L Mg2+ not have a significant effect on the arsenic Ip - Triton X-100 ( 15 g/L), 1.10–5 M Trilon B significantly reduce or extinguish stripping peak of arsenic 3.3.7 Verifying the method Record the Ip of solution containing As(V) µg/L with 20 replications The results showed that the DP-ASV has good repeatability with AuFE/CP (RSD = 5.5%) With the deposition time of 90 s, LOD, LOQ and sensitivity of As(V) determination method in Na2SO3 medium were 0.13 µg/L, 0.42 µg/L and 0.359 µA/µg.L–1, respectively 12 10 (B) (A) 15.0u 40 μg/L I (A) Ip (µA) Au-CPE +0.3V10s-1.2V90s 50mVs AsV+Mn LR Na2SO30.05M+Mn0.2pm+AsV 20.0u µg/L 5.00u 10 20 30 40 As 10.0u 50 CAs(V) (µg/L) -1.00 -0.75 -0.50 -0.25 U (V) Figure 3.30 Stripping peak currents (Ip) as a function of CAs(V) from µg/L to 40 µg/L (A) and the corresponding DP-ASV curves (B) Figure 3.30 shows that, Ip increases linearly with the increase of As(V) concentrations in the range of 0.425 g/L with linear regression equation: Ip= (0.31 ± 0.05).CAs(V) + (0.8 ± 0.8); R2 = 0.9924; p < 0.05 (values after “±” signs are 95% confidence limit) 14 3.4 Analysis of As(III) and As(V) mixtures with AuFE/CP in Na2SO3 medium 3.4.1 Suitable conditions to determine As(V) Experiment results showed that As(V) does not give a stripping voltammetry response in Na2SO3 medium Stripping peaks of As(V) only appear from analyte solutions containing Na2SO3 and Mn(II) The suitable experimental conditions for As(V) quantification by DP-ASV with AuFE/CP were determined in section 3.3 3.4.2 Effect of in situ Au concentration Figure 3.32 DP-ASV curves of arsenic with AuFE/CP (A): Curve (1): 0.05 M Na2SO3 + 0.2 mg/L Mn(II); Curve (2): 0.05 M Na2SO3 + 0.2 mg/L Mn(II) + µg/L As(III); Curve (3): 0.05 M Na2SO3 + 0.2 mg/L Mn(II) + µg/L As(III) + µg/L As(V); (B): Curve (1): 0.05 M Na2SO3 + µg/L As(III); Curve (2): 0.05 M Na2SO3 + µg/L As(III) + 0.2 mg/L Mn(II) At the suitable conditions, the Ip recorded from As(III) solutions is higher than the Ip obtained from As(V) solutions having the same concentrations (Figure 3.32A), moreover, the Ip of arsenic is attenuated when Mn(II) is added to analyte solutions To overcome the difference of arsenic stripping peak currents in the presence and absence of Mn(II), a small amount of Au(III) was added to analyte solution containing As(III) At the concentration of Au(III) (CAu-in) being mg/L, the mean Ip values recorded in the presence and 15 absence of 0.2 mg/L Mn(II) are not statistically significant (p = 0.48) Therefore, CAu-in = mg/L was selected for next experiments Ip(µA) 3.4.3 Effect of deposition time (tdep) 0 50 100 tdep(s) 150 200 Figure 3.34 Effect of tdep on the Ip of arsenic with µg/L As(V) solution containing mg/L Au(III) at AuFE/CP For the purpose of shortening analysis time, the tdep of 90 s was selected to carry out further experiments 3.4.4 Effect of Cl and HCO3 ions Cl– ions not significantly affect arsenic stripping peak, even at a concentration of 42 g/L However, HCO3– ions should not be added to the concentration levels higher than 5.10-3 M when neutralizing the analyte samples 3.4.5 The reliability of the method Record Ip of the solution containing µg/L As(III) or 40 µg/L As(V) with 20 replications Experiment results showed that DP-ASV methods for As(III) and As(V) determinations using AuFE/CP have a good repeatability with the RSD < ½RSDH Table 3.39 Sensitivity, LOD, and LOQ of As(III), As(V) determinations by using DP-ASV on AuFE/CP in analyte solutions As(III) As(V) containing Au(III) a ± εa b ± εb Sy (µA) (µA/µg.L-1) (µA) -0.08± 0.05 0.478 ± 0.015 0.015 -0.08± 0.08 0.358 ± 0.024 0.024 LOD (µg/L) 0.09 0.20 LOQ (µg/L) 0.31 0.67 16 Linear range and linear regression equations: As(III): Linear range: 0.370 g/L Ip = (1.8 ± 1.4) + (0.36 ± 0.03).CAs(III) (R2 = 0.9917; p < 0.05) As(V): Linear range: 0.770 g/L Ip = (1.5 ± 1.3) + (0.28 ± 0.03).CAs(V) (R2 = 0.9913; p < 0.05) (values after “±” signs are 95% confidence limit) (B) As(V) (A) As(III) 100 μg/L 100 μg/L µg/L µg/L Ip (µA) 30 (C) 20 Ip-As(III) Ip-As(V) 10 0 20 40 60 80 100 CAs (µg/L) Figure 3.37 Differential pulse voltammograms (A, B) and stripping peak currents (Ip) as a function of CAs(III), CAs(V) in the analyte solution containing Au( III) (C) 17 3.4.6 Procedure for direct determinations of As(III) and As(V) by using DP-ASV on AuFE/CP in Na2SO3 medium Figure 3.38 Procedure for direct determinations of As(III) and As(V) in natural water samples by DP-ASV on AuFE/CP 3.5 Comparison of DP-ASV methods for the determination of arsenic The comparison of arsenic determination results indicated that the DP-ASV method using AuFE/CP in Na2SO3 medium has some outstanding advantages: 18 - Simple working electrode fabricating technique, low cost, environmentally friendly; - Good repeatability; low LOD, wide linear range; - Capable of As(III) and As(V) direct quantifications; - Relatively simple determination procedures; Therefore, this method will be chosen to analyze real samples according to the procedure described in Figure 3.38 3.6 Practical applications 3.6.1 Sample preparation Samples of well water, tap water in some areas in Thanh Tri (Hanoi City); Quang Ninh (Quang Binh Province); and Hue City (Thua Thien Hue Province) were taken for analysis 3.6.2 Quality control of analytical procedure 3.6.2.1 Repeatability 3.6.2.1.1 Repeatability To verify the repeatability of the analytical procedure, well water samples named NN03, NN04, NN06 were randomly selected for arsenic determinations DP-ASV method for quantifying As(III), As(V), or total As (after sample is UV decomposed) with AuFE/CP using this procedure allows to gain a good repeatability (RSD < ½RSDH) 3.6.2.1.2 Reproducibility Reproducibility of analytical procedure was evaluated by daily quantifying As(V) of well water sample named H2 with 25 µg/L, 50 µg/L, 90 µg/L As(V) spikes for days AuFE/CP was freshly fabricated at each measurement Using this analytical procedure for the As(V) determination with AuFE/CP allows to get good reproducibility (RSD < ½RSDH) 19 3.6.2.2 Accuracy 3.6.2.2.1 Certified reference material analysis (CRM) The determined arsenic concentrations of seawater CRM sample named CASS-6 were within the certified concentration range of 1,04 ± 0,10 μg/L (CAs ± U) Thus, DP-ASV method using AuFE/CP can be applied to analyze arsenic in even seawater samples with good trueness Table 3.44 The arsenic analysis results of CASS-6 certified reference material 1.15 0.96 0.99 CAs(V) (μg/L, test results) 9.9 RSD (%, n = 3) 22.5 ½ RSDH (%, at 1.04 μg/L) 1.03 ± 0.25 CAs ± ε (μg/L, test results) 1.04 ± 0.10 CAs ± U (μg/L, certified values) ε: confidence limit (P = 0,95; n = 3); U: uncertainty 3.6.2.2.2 Spiked sample analysis Selected real samples for spiked sample analysis were tap water (sample H1) and well water (sample H2) that was used for domestic use in Hue City The obtained spike test recoveries for the determination of both As(III) and As(V) in the mixture are within allowable limits according to AOAC guidelines 3.6.2.2.3 Analysis of real samples by standard method Conduct analysis to determine arsenic in seven well water samples from Hanoi and two well water samples from Quang Binh by both methods: (1) DP-ASV with AuFE/CP; and (2) GF-AAS (ISO 15586:2003) Paired-t-test was used to compare the average results of arsenic determination (As(III+V)) by DP-ASV using AuFE/CP, and by GFAAS (standard method) Results showed that DP-ASV using AuFE/CP and GF-AAS methods for arsenic determination are not 20 statistically different, with tcalc = 0.11 < t (p = 0.05; f = 8) = 2.31 and p = 0.91 > 0.10 This shows that the DP-ASV method achieves good accuracy when compared with GF-AAS method 3.6.3 Analysis results of As(III), As(V) of natural water samples Table 3.48 Analysis results of As(III), and As(V) concentrations in samples from Thua Thien Hue, Quang Binh, Hanoi by DP-ASV with AuFE/CP Concentrations (μg/L) Sample (Mean ± ε, n = 3, P = 0.95) No Samples Code As(III) As(V) 7.4 ± 3.0 2.6 ± 1.4 NN01 20.5 ± 7.4 4.2 ± 2.5 NN02 25.1 ± 7.1 30.2 ± 3.6 NN03 15.1 ± 3.4 3.8 ± 0.4 NN04 2.8 ± 1.2 1.8 ± 1.1 NN05 Well water 45.0 ± 1.4 51.7 ± 6.2 NN06 7.4 ± 3.6 1.6 ± 0.4 NN07 7.9 ± 2.3 1.7 ± 1.0 MC1 2.9 ± 1.0 4.3 ± 0.6 MC2 10 < LOD 0.2 ± 0.2 H2 0.20 ± 0.04 0.2 ± 0.1 11 Tap water H1 Although water sample analysis was conducted to evaluate the analytical method, the obtained data also indicate that some groundwater samples taken from the arsenic contaminated area in Hanoi (NN01, NN02, NN04) have rather high arsenic concentrations and should not be used for domestic needs, and even for agricultural irrigation purposes (NN03, NN06) CONCLUSIONS To develop a procedure for determination of trace amounts of As(III) and As(V) in natural water by stripping voltammetry, from the obtained experiment results, we come to the following main conclusions: 21 Trace As(III) in natural water can be accurately determined with good repeatability, and low detection limit by DP-ASV using AuFE/BDD or AuFE/CP in medium containing HCl, Au(III), acid ascorbic Compared with using of AuFE/BDD electrode, the DPASV method for As(III) determination using the AuFE/CP electrode has a much wider linear range (0.41000 µg/L g/L compared to 0,480 µg/L µg/L) In the analyte solutions containing Na2SO3, the stripping von-ampere signals of As(III) can be measured on the AuFE/CP electrode with good sensitivity, repeatability, but Ip of As(V) cannot be recorded under the same conditions When the analyte solution contains Na2SO3 and Mn(II), the stripping peak currents of both As(III) and As(V) can be measured This is the basis for successfully establishing a procedure for direct determination of As(III), and As(V) in a mixture A procedure for direct determination of As(III) and As(V) in natural water has been established by DP-ASV method using AuFE/CP electrode in medium containing 0.05 M Na2SO3, 0.2 mg/L Mn(II), and mg/L Au The evaluation results showed that, this proposed analytical procedure can be applied to directly determine traces of As(III) and As(V) in water samples with good repeatability, accuracy, low detection limit (As(III): 0.09 µg/L; and As(V): 0.20 µg/L), and a wide linear range up to 70 µg/L The developed procedure has been successfully applied to determine As(III) and As(V) in some natural water samples taken from Hanoi, Quang Binh and Thua Thien Hue The analysis results of these samples by using DP-ASV were not significantly different from the results obtained with GF-AAS Therefore, it is possible to use DP-ASV with AuFE/CP to determine As(III) and As(V) traces in natural water samples 22 LIST OF PUBLICATIONS Le Thi Kim Dung, Hoang Thai Long, Dang Van Khanh (2018) Factors influencing anodic stripping voltammetry signals of As(III) at Au-coated boron-dropped diamond electrode, Hue University Journal of Science: Natural Science, Vol 127, No 1B, pp 49–57 Le Thi Kim Dung, Hoang Thai Long, Huynh The Minh Quoc, Dang Van Khanh (2019) Determination of As(III) by differential pulse anodic stripping voltammetry with Au-coated carbon paste electrode, Journal of Analytical Sciences: Chemistry, Physics and Biology, Vol 24, No 3, pp 46–52 Le Thi Kim Dung, Dang Van Khanh, Hoang Thai Long (2019) Determination of arsenic(V) by differential pulse anodic stripping voltammetry using manganese-coated, gold film carbon paste electrode, Journal of Analytical Sciences: Chemistry, Physics and Biology, Vol 24, No 4, pp 57–62 Le Thi Kim Dung, Dang Van Khanh, Hoang Thai Long (2021) The effect of interferent ions and the reliability of differential pulse anodic stripping voltammetry using aucoated boron doped diamond electrode for arsenite determination, Journal of Science and Technology, University of Sciences, Hue University, Issues in Chemistry - Biology - Earth Sciences, Vol 18, No 2, pp 1–12 Hoang Thai Long, Le Thi Kim Dung (2021) Glassy carbon electrode modified with ex-in situ gold film – A simple and effective working electrode for As(III) determination by using differential pulse anodic stripping voltammetry, Hue University Journal of Science: Natural Science, Vol 130, No 1D DOI: 10.26459/hueunijns.v130i1D.6459  ... water, especially the direct analysis of As(III) and As(V), with low LOD, high selectivity Carbon electrodes, such as glassy carbon electrodes, carbon paste electrodes (CPE), screen printing electrodes... mg/L Mn(II) + µg/L As(III); Curve (3): 0.05 M Na2SO3 + 0.2 mg/L Mn(II) + µg/L As(III) + µg/L As(V); (B): Curve (1): 0.05 M Na2SO3 + µg/L As(III); Curve (2): 0.05 M Na2SO3 + µg/L As(III) + 0.2 mg/L... electrodes (AuFE) on the substrate of boron doped diamond (BDD), carbon paste (CP) - Anodic stripping voltammetry; - As(III) and As(V) in natural water The contributions of the dissertation: -