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:
1. Assoc. Prof. Hoang Thai Long
2. Dr. Dang Van Khanh

HUE, 2022


The dissertation is completed at:
Department of Chemistry, University of Sciences, Hue
University.

Academic supervisors:
1. Assoc. Prof. Hoang Thai Long
2. 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:
1. The National Library of Vietnam
2. 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 1 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


2
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


3
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 1. OVERVIEW
1.1. Introduction to arsenic
1.2. Methods for trace arsenic in natural water analysis
1.3. General conclusions


4
Chapter 2. 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 3. 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 2 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 1 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 1 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 2 mg/L (for AuFE/BDD)
and 5 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)
do 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)
do 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 5 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


9
AuFE/BDD
150 μg/L

AuFE/CP

(A)

1 µg/L

40

(B)

3000 μg/L

1 µg/L

500

AuFE/BDD
(C)

AuFE/CP
(D)

Ip(μA)

Ip(A)

400

20

300
200

100
0

0

0

50
100
CAs(III)(µg/L)

150

0

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) và
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; 5 mg/L
Au(III).
Record stripping voltammograms (replicate 3
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 = 6 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 + 1 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

8
RSD (%, n = 5)

2.0

6

IP (µA)

1.5

4
1.0
0.5
-0.05

Figure 3.21. Influence of

2
0
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

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+
( 5 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+ do 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) 5 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
8
6

4
2
0

(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

1 µg/L

5.00u

0

10

20

30

40

As

10.0u

50

CAs(V) (µg/L)

-1.00

-0.75

-0.50

-0.25

0

U (V)

Figure 3.30. Stripping peak currents (Ip) as a function of CAs(V) from
1 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) + 5 µg/L As(III); Curve (3): 0.05 M Na2SO3 + 0.2
mg/L Mn(II) + 5 µg/L As(III) + 5 µg/L As(V); (B): Curve (1): 0.05
M Na2SO3 + 5 µg/L As(III); Curve (2): 0.05 M Na2SO3 + 5 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 3 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 = 3 mg/L was selected for next experiments.

Ip(µA)

3.4.3. Effect of deposition time (tdep)
5
4
3
2
1
0
0

50

100

tdep(s)

150

200

Figure 3.34. Effect of tdep on the Ip of arsenic with 5 µg/L As(V)
solution containing 3 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 do 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 5 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

0 µg/L

0 µg/L

Ip (µA)

30

(C)

20
Ip-As(III)
Ip-As(V)

10
0
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 4 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
1
NN01
20.5 ± 7.4
4.2 ± 2.5
2
NN02
25.1 ± 7.1
30.2 ± 3.6
3
NN03
15.1 ± 3.4
3.8 ± 0.4
4
NN04
2.8 ± 1.2
1.8 ± 1.1
5
NN05
Well water
45.0
±
1.4
51.7
± 6.2
6
NN06
7.4 ± 3.6

1.6 ± 0.4
7
NN07
7.9 ± 2.3
1.7 ± 1.0
8
MC1
2.9 ± 1.0
4.3 ± 0.6
9
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
1. 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).
2. 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.
3. 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 3 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.
4. 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
1. 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.
2. 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.
3. 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.
4. 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.
5. 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.