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HUE UNIVERSITY
UNIVERSITY OF SCIENCES

NGUYEN THI HUE

STUDY ON DETERMINATION

OF TRACE CHROMIUM

BY ADSORPTIVE STRIPPING VOLTAMMETRY

SUMMARY OF

ANALYTICAL CHEMISTRY DOCTORAL

DISSERTATION

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HUE UNIVERSITY
UNIVERSITY OF SCIENCES

NGUYEN THI HUE

STUDY ON DETERMINATION

OF TRACE CHROMIUM

BY ADSORPTIVE STRIPPING VOLTAMMETRY

SPECIALTY: ANALYTICAL CHEMISTRY

CODE: 62 44 01 18

SUMMARY OF

ANALYTICAL CHEMISTRY DOCTORAL

DISSERTATION

SCIENTIFIC SUPERVISORS:
1. Assoc. Prof. Dr. NGUYEN VAN HOP
2. Prof. Dr. LE QUOC HUNG

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INTRODUCTION 
1. Rationale

Industrialization, modernization and environmental protection
in the current period require the analytical science to develop highly
selective and sensitive analytical methods to accurately determine
trace amounts of metals in complex samples. To complete this task,
many of determination methods have been developed, eg. atomic
absorption spectrometry, plasma emission spectrometry, capillary
gas chromatography, high performance liquid chromatography ... and
modern electrochemical analytical methods, typically differential
pulse polarization and stripping voltammetry methods.

Before, most studies on adsorptive stripping voltammetry
(AdSV) used electrodes either hanging mercury drop electrode
(HMDE) or static mercury drop electrode (SMDE), which give stable
and sensitive stripping signals. However, these electrodes are
expensive, difficult to be fabricated. Recently, easy-to-make
electrodes based on innert rotating disk electrode, such as mercury
film electrodes (MFEs), bismuth film electrode (BiFE) have been
increasingly used due to their low costs, and being environmentally
friendly in the case of BiFE. If successful, study using MFE, BiFE
for HMDE and SMDE substitution, when applied, will be a feasible
solution in our current situation.

Chromium is a relatively common element in nature;
chromium amount in the environment tends to be increased by
industrial activities such as electroplating, tanning, processing and
mining... In the environment, chromium exists in the form of Cr (III)
and Cr (VI), depending on the redox state of the water.

The nature and toxicity of chromium forms depend on its
oxidation state. Chromium is toxic only in Cr (VI) form. Cr (III) is
necessary for the body. Therefore, in environment monitoring,
quantifying the total chromium is not enough. It is necessary to
analyze the different forms of their existence. Therefore, the process
of analyzing chromium in environmental samples is a necessary and
urgent issue.

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pentaacetic acid (DTPA), base component is acetate buffer. To
implement this topic, it is necessary to address the following:

- Trace chromium determination by AdSV method using
HMDE, MFE and BiFE. Compare advantages and disadvantages of
electrodes and choose the best and most effective electrodes to apply
to the real samples.

- Develop a procedure to determine chromium in natural water
and in some other water sources in Thua Thien Hue Province. In
addition, a process is also needed to determine this element in solid
samples such as soil, sediment, etc., to quantify chromium in Huong
River and Cau Hai Lagoon sediments.

2. Research purposes

- Contribute to develop adsorptive stripping voltammetric
method.

- Develop a new type of electrode, which is easy to use and
applicable in practice.

- Develop a procedure for determination of trace chromium in
environmental samples (water, sediment).

3. New scientific and practical contributions

1. This is the first thesis in Vietnam, that systematically
study the types of electrodes using for chromium determination by
adsorptive stripping voltammetry.

2. For the first time in Vietnam, adsorptive stripping
voltammetry using bismuth film electrode was used to determine
trace chromium.

3. Successfully developed a new type of bismuth film
electrode: bismuth film in situ (in situ BiFE). Adsorptive stripping 
voltammetry using in situ BiFE gives better sensitivity than other 
electrodes (HMDE, MFE, ex situ BiFE). Limit of detection was 
low (0.1 ppb) with short analysis time.

With a simple fabrication, this electrode is suitable for not
well-equipped analytical laboratories. This could effectively contribute
to the monitoring and pollution control of chromium for water
resources planning and exploitating in Vietnam.

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4. Composition of the thesis

The thesis consists of 153 pages, 75 tables, 55 pictures,
including:

- Contents, List of Abbreviation, Table, Picture (11 pages)
- Introduction (4 pages)

- Chapter 1: Literature Review (31 pages)

- Chapter 2: Research Subjects - Methodology (5 pages)
- Chapter 3: Results and Discussion (86 pages)

- Chapter 4: Conclusion (2 pages)
- References (10 pages)

THESIS CONTENTS

Chapter 1. LITERATURE REVIEW

- Introduction to chromium, applications, sources of chromium
waste in the environment, the impact of chromium on humans and
the environment.

- Overview of adsorptive stripping voltammetry: principles,
complexing reagents, working electrodes used in adsorptive stripping
voltammetry, adsortive stripping voltammogram recording
techniques.

- Affecting factors needed to be considered when developing a
adsorptive stripping voltammetry determination procedure.

- Current trends in trace chromium determination:
spectrometry methods, modern electrochemical analysis methods.

- Noteworthy issues of trace analysis.

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Chapter 2. RESEARCH SUBJECTS AND METHODOLOGY 
2.1. Research subjects

 Study on the adsorptive stripping voltammetry characteristic of 
chromium.

 Study on the adsorptive stripping voltammetry determination of
trace Cr(VI) using HMDE and MFE;

 Study on the adsorptive stripping voltammetry determination of
trace Cr(VI) using BiFE:

+ Study on the determination of trace Cr(VI) by differential
pulse stripping voltammetry using ex situ BiFE;

+ Study on the determination of trace Cr(VI) by square wave
adsorptive stripping voltammetry using ex situ BiFE;

+ Study on the determination of trace Cr(VI) by differential
pulse stripping voltammetry using in situ BiFE;

+ Study on the determination of trace Cr(VI) by square wave

adsorptive stripping voltammetry using in situ BiFE;

 Compare advantages and disadvantages of concerned electrode
types to select suitable one and its fabrication for domestic laboratory
application.

 Develop determination procedure:
+ Develop determination procedure;

+ Verify the determination procedure by using certified reference
materials;

+ Analyze real samples (sample, preserve, pretreat and
analyze).

2.2. Methodology

- Adsorptive stripping voltammetry using HMDE, ex situ MFE, ex

situ and in situ BiFE;

- Evaluate influence factors by monovariant analysis;

- Evaluate method reliability through repeatability, sensitivity,
detection limits and linearity;

- Analyse and represent data by using MS. Excel 2010.
Working electrode preparation:

There are three types of electrode were used in this study:

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 MFE used in AdSV/ex situ MFE experiments was prepared as 
follows:

+ Disk electrode (glassy carbon, GC) cleanning: GC electrode was 
cleanned by polishing using fine Al2O3 powder (particle size 0.6

µm), rinsed with distilled water then with 1 M NaOH solution to
remove any residual Al2O3 particles on the electrode surface. Dip the

electrode into 1 M HCl solution to neutralize NaOH. Then wash the
electrode carefully with distilled water several times, wipe gently
with soft filter paper.

+ Ex-situ MFE preparing: mount the cleanned GC electrode into the 
electrochemical analyzer containing Hg(NO3)2 7.78×104 M solution,

apply a constant potential at -1000 mV (vs. Ag/AgCl reference
electrode) for 120 s, then carefully wash with double distilled water
and dry the electrode body (without touching the electrode surface).
 Ex situ and in situ BiFE used in AdSV/ BiFE ex situ and AdSV/ 
BiFE in situ experiments were prepared as follows:

+ GC disk electrode cleanning: same as the preparation of MFE. 
+ Ex situ BiFE preparing: mount the cleanned GC electrode into the 
electrochemical analyzer containing 0.1 M acetate buffer solution,
500 ppb Bi(III) and 4.2×106 M KBr; apply a constant potential at
-1200 mV for 120 s while stirring at 2000 rpm. After that, carefully
wash with double distilled water and dry the electrode body using
soft filter paper.

+ In situ BiFE preparing: In situ BiFE electrode was formed during 
analyte deposition step as follows: Dip the GC electrode into a
electrochemical cell containing reference, auxilỉay electrodes and
analytical solution (0.4 M acetate buffer, 0.4 M NaNO3, 0.4 × 103 M

DTPA, 600 ppb Bi (III), 5.0×106 M KBr and Cr (VI)); Rotate the
electrode at a constant speed and conduct electrodeposition step at
-800 mV (EAd) for a specified time (tad). During this step, Bi(III) is

reduced to metal binding on the GC surface, forming in situ BiFE. 
Chapter 3. RESULTS AND DISCUSSION

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were investigated with DP or SqW scanning techniques in a solution
containing acetate buffer, DTPA (complexing agents) and NaNO3.
3.1. Adsorptive stripping voltammetry characteristics of 
chromium

3.1.1. AdSV characteristics of chromium at HDME

To understand stripping voltammetry characteristics of chromium on
HMDE, cyclic voltammograms of Cr(VI) solution under various
experiment conditions were recorded and investigated. Potential was
scanned from -800 mV to -1600 mV, at a scan rate of 15 mV/s, and
stirring rate  = 2000 rpm.

(A): solution contains 10 ppm Cr(VI), 0.4 M acetate buffer. Without
preconcentration step.

(B): solution contains 10 ppm Cr(VI), 0.4 M acetate buffer, 0.4×103

M DTPA. Without preconcentration step.

(C): solution contains 10 ppm Cr(VI), 0.4 M acetate buffer, 0.4×103
M DTPA, 0.4 M NaNO3. Without preconcentration step.

(D): solution contains 0.4 M acetate buffer, 0.4×103 M DTPA, 0.4
M NaNO3. Without preconcentration step.

(E): solution contains 10 ppm Cr(VI), 0.4 M acetate buffer, 0.4×103
M DTPA, 0.4 M NaNO3.

(F): solution contains 10 ppm Cr(VI), 0.4 M acetate buffer, 0.4 M
NaNO3.

(G): solution contains 90 ppb Cr(VI), 0.4 M acetate buffer; 0.4×103
M DTPA, 0.4 M NaNO3. Without preconcentration step.

(H): solution contains 100 ppb Cr(VI), 0.4 M acetate buffer;
0.4×103 M DTPA, 0.4 M NaNO3. Without preconcentration step.

Cyclic voltammograms shown in Figure 3.1 (in the thesis)
indicated that:

- Cyclic voltammograms in cases A, B, C and D showed that
stripping peaks of chromium appeared only when acetate buffer,
DTPA and NaNO3 are present together in the analytical solution.

- Cyclic voltammograms B and C indicated that stripping peaks of
chromium appeared only when analysis solution contained NaNO3.

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- Cyclic voltammograms C and E revealed that only Cr(III) newly
generated from the electrochemically reduction of Cr(VI) can react
with DTPA.

- Cyclic voltammograms C and E indicated that the complex
Cr(III)-DTPA may have been adsorbed on the surface of electrode.

3.1.2. AdSV characteristics of chromium at ex situ BiFE

From the investigation of cyclic voltammograms recorded using ex

situ BiFE, it is possible to confirm that:

- Chromium(VI) ion only shows electrochemical activity in the
analysis solution containing NaNO3.

- Zn(II), Co(II) and Cr(III) have no electrochemical activity in this
system.

- Among Zn(II), Co(II), Cr(III), and Cr(VI) only Cr(VI) own
electrochemical activity in this system (containing acetate buffer),
complexing agent DTPA, and NaNO3).

3.2. Study on determination of Cr(VI) by stripping voltammetry 
using HMDE and MFE

- Effect of background composition;

- Effect of mercury concentration on repeatability of the dissolved
signal on the ex situ MFE;

- Effect of rotating speed and differential pulse voltammetry
parameters (for DP-AdSV/MFE);

- Effect of adsorption potential, adsorption time, equilibrium time;
- Effect of DTPA concentration and NaNO3 concentration;

- Effect of dissolved oxygen
- Reliability of the method

3.3. Study on determination of Cr(VI) by adsorptive stripping 
voltammetry using BiFE

- Study on determination of Cr(VI) by differential pulse adsorptive
stripping voltammetry using ex situ BiFE (DP-AdSV/ ex situ BiFE). 
- Study on determination of Cr(VI) by square wave adsorptive
stripping voltammetry using ex situ BiFE (SqW-AdSV/ex situ BiFE). 
- Study on determined Cr(VI) by differential pulse adsorptive
stripping voltammetry using in situ BiFE (DP-AdSV/in situ BiFE). 
- Determination of Cr(VI) by square wave adsorptive stripping
voltammetry using in situ BiFE (SqW-AdSV/in situ BiFE)..

 Linearity, sensitivity, detection limits, and quantitative limits of

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Figure 3.1. (A) Linear regression line for the SqW-AdSV/in situ

BiFE method; (B) Stripping voltammograms of SqW-AdSV/in situ 
BiFE: baseline and 9 voltammograms obtained by sequently adding 
0.2 ppb Cr(VI) each time into the analysis solution ; (C) Ip and CCr 
(VI) relationship in DP-AdSV/in situ BiFE method; (D) linear

regression line for DP-AdSV/in situ BiFE method.

Linearity:

+ SqW-AdSV method: Ip and CCr(VI) had a good linear correlation in

CCr (VI) range from 0.3 to 1.8 ppb with a correlation coefficient R of

0.9994 (Fig. 3.1.A, Fig. 3.1.B);

30
40
50
60
70
80
90
100

0 0.4 0.8 1.2 1.6 2

IP

(

A)

(Cr(VI) (ppb)
Ip = 44.50 + 23.19 [Cr(VI)]
R = 0.9994

0
2
4
6
8
10
12

0 2 4 6 8 10 12 14 16 18 20 22

Ip

(µA

)

[Cr (VI)] ( ppb)

0
2
4
6
8
10
12

0 2 4 6 8 10 12 14

Ip(

µ

A

)

[Cr(VI)] ppb
Ip = 0.89 + 0.68 [Cr(VI)]
R = 0.9989

(A)

(B)

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+ SqW-AdSV method: Ip and CCr(VI) had a good linear correlation in

CCr (VI) range from 0.2 to 1.6 ppb with a correlation coefficient R of

0.9911 (P = 0.95);

+ DP-AdSV method: Ip and CCr(VI) had a good linear correlation in CCr
(VI) range from 5.2 to 20.8 ppb with a correlation coefficient R of

0.9995 (P = 0.95);

Sensitivity:

SqW-AdSV method achieved a sensitivity of 21.4 µA/ppb
(approximately 15.4 times higher than DP-AdSV, 1.389 µA/ppb).

LOD and LOQ:

+ SqW-AdSV (Ead = -800 mV, tad = 80 s): LOD = 0.2 ppb; LOQ =

0.6  0.8 pbb

+ DP-AdSV (Ead = -800 mV, tad = 120 s): LOD = 0.9 ppb; LOQ = 2.7

– 3.6 ppb.

 Linearity, sensitivity, detection limits, and quantitative limits of

the AdSV/in situ BiFE method

+ DP-AdSV method: Ip and CCr(VI) had a good linear correlation in CCr
(VI) range from 2 to 12 ppb with a correlation coefficient R of 0.9989;
Sensitivity:

SqW-AdSV method achieved a sensitivity of 23.0 µA/ppb
(approximately 34 times higher than DP-AdSV, 0.682 µA/ppb). (Fig.
3.1.C, Fig. 3.1.D);

LOD and LOQ:

+ SqW-AdSV (Edep = -800 mV, tdep = 160 s): LOD = 0.1 ppb; LOQ =

0.3 pbb

+ DP-AdSV (Edep = -800 mV, tdep = 50 s): LOD = 0.6 ppb; LOQ = 2 ppb.

SqW-AdSV method achieved a narrower linear range than the
DP-AdSV method, but it got higher sensitivity than the DP-AdSV
(due to having lower LOD and higher slope of the standard curve). It
can be said that with LOD as above, DP-AdSV and SqW-AdSV
methods can be used with in situ BiFE electrodes to determine Cr 
(VI) trace.

3.4. Evaluation of studied adsorptive stripping voltammetry 
methods for chromium determination

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different measurement modes were evaluated. Some comments were
deduced from this evaluation:

 pH of analysis solutions were in the range of 5.8  6.2 (except pH of
solutions for DP- AdSV/ex situ MFE experiments were from 5.0 to 5.2). 
 Appropriate concentration of complexing agent (CDTPA) was of 4×103

M for all experiments (except for DP-AdSV/HMDE experiments of
0.8×103 M).

 Appropriate concentration of NaNO3 in DP-AdSV/HMDE

experiments were of 1 M, in DP-AdSV/MFE experiments were of 0,5
M, and of 0.4 M for all the others.

 Suitable adsorption potential (EAd) for DP-AdSV/HMDE experiments

was of -1100 mV, for DP-AdSV/MFE of -1000 mV, and of 800 mV
for all the others.

 Suitable adsorption times (tAd) depended on each specific method.

 With a low detection limit (0.3 ppb), MFE can be used to analyze
Cr(VI) in environment samples. HMDE had a higher detection limit (1.2
ppb) and some difficulty in using. The disadvantage of both electrodes is
that they use toxic metal mercury so the use of them is limited.

- Two SqW-AdSV/in situ BiFE and SqW-AdSV /ex situ BiFE methods 
gained lowest detection limits (0.1 ppb and 0.2 ppb, respetively); in
addition, bismuth is a non-toxic element, environmentally friendly.
Determination procedures are relatively simple, so these two methods
are chosen for the real sample analysis. However, to be sure about the
applicability of these two Cr (VI) methods, it is necessary to control
their quality before applying them to reality.

3.5. Application and procedure development

Experimental results show that the use of ex situ BiFE and in situ 
BiFE to determine trace chromium by SqW-AdSV method is the most
feasible.

3.5.1. Determination of chromium in natural water by SqW-AdSV 
method using BiFE

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determining the Cr(VI) (), the total content of Cr(VI) + (III) (), from
which Cr (III) is the difference of () and ().

Results from the previous sections show that the square wave
adsorptive voltammetry with ex situ BiFE and in situ BiFE can be

applied to determine the trace amount of Cr(VI) with LOD of 0.2 ppb
and 0.1 ppb, respectively. With these LOD, SqW-AdSV method using

ex situ BiFE and in situ BiFE can directly determine the trace amount of

Cr(VI) in natural water, without preconcentration step, which is the
main advantageous of the SqW-AdSV/BiFE method.

To answer the question of whether these methods can be applied
to analyze trace amount of chromium in natural water samples, we have
conducted experiments to verify the accuracy and trueness (through
analyze CRM sample) and applied to analyze some samples of natural
water. Based on the experiments mentioned, a determination procedure
for Cr(VI) and total chromium (Cr (VI + III)) in water were proposed
using SqW-AdSV/in situ BiFE.

3.5.1.1. Quality control through standard sample analysis

In order to confirm the practical applicability of the
SqW-AdSV method to analyze trace chromium using BiFE, the quality of
the analytical procedure needs to be controlled by evaluating the
accuracy and repeatability of the result of standard samples analysis.

a. Surface water samples

Select Surface water Certified Material Reference (SPS-SW1
Batch 122) to evaluate the accuracy of the method. The actual value
of the chromium content of the sample is 2.00 ± 0.02 ppb (95%
confidence boundary ε = ± 0.02 ppb). Analysis of standard SPS-SW1
surface water (CRM) samples by the SqW-AdSV using two types of

electrodes ex situ BiFE and in situ BiFE with the appropriate 
experimental conditions in Table 3.5.1. Analyse repeatedly 3 times.
The volume of the solution to be charged to the electrolyser is 2 mL,
and the volume of solution in the electrolyser is 10 mL.

The results obtained in table 3.5.2 show that:

- The SqW-AdSV method using ex situ BiFE has good 
repeatability (RSD = 7% (n = 3) ≤ ½ RSDH (RSDH = 2 (1 - 0.5 lgC), for

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- Both methods have good accuracy due to the determined Cr
content within the 95% confidence interval of the CRM sample.
 - The SqW-AdSV/ in situ BiFE method has better accuracy and 
repeatability than the SqW-AdSV / ex situ BiFE method.

b. For sea water samples

Analysis of the standard seawater CRM coded NASS 6 by the
SqW-AdSV method using two types of ex situ BiFE and in situ BiFE 
electrodes with the appropriate experimental conditions in Table
3.5.1.

Because the concentration of Cr in NASS 6 seawater was too
small to be directly analyzed, so only NASS 6 standard specimens
were used as the matrix for analysis and validity. The actual value of
the Cr content in the NASS 6 sample is 0.116 ± 0.008 ppb (the 95%
confidence bound ε = ± 0.008 ppb). NAAS 6 standard specimens
were added with standard Cr(VI) to attain 3 levels of 2 ppb, 6 ppb
and 10 ppb, and then analyzed with standard added samples to
determine recovery.

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Table 3.5.1. The appropriate experimental conditions for the SqW-AdSV / ex situ BiFE and 
the SqW-AdSV / in situ BiFE for determination of Cr (VI).

N0 Parameter (unit of measure) Symbol SqW-AdSV/ex situ

BiFE

SqW-AdSV/ in situ 
BiFE
1 DTPA Concentration (10-3M) C

DTPA 0.40 0.40

2 Concentration of Acetate buffer

(pH=6) (M) CĐệm Axetat 0.40 0.40

3 NaNO3Concentration (M) CNaNO3 0.40 0.40

4 KBr Concentration (mM) CKBr 0.004 0.005

5 Bi (III) Concentration (ppb) CBi(III) 500 600

6 Cleaning Potential (mV) Eclean - 200 300

7 Cleaning Time (s) tclean 60 100

8 Rotating speed of working electrode

(vòng/phút)  2000 2000

9 Deposition potential (mV) Edep -800 -800

10 Deposition time (s) tdep 250 200

11 Equilibration time (s) tequal 50 50

12 Potential Sweep Range (mV) Erange -800 ÷ -1450 -800 ÷ -1450

Technical parameters
Amplitude (mV)
Voltage step (mV)
 Sweep rate (mV/s)
 Frequency (Hz)

E
Ustep

v
f

30
6
210

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Table 3.5.2. Correctness of the two methods SqW-AdSV/ ex situ 
BiFE and SqW-AdSV / in situ BiFE for determination chromium in

surface water

Informations

ex situ BiFE in situ BiFE

[Cr(VI)](ppb) CCr

(ppb)

[Cr(VI)](ppb) CCr

(ppb)

Experiment

1 0.37 1.85 0.38 1.90

2 0.42 2.11 0.40 2.00

3 0.38 1.90 0.40 2.00

Average  S
(ppb)

1.95  0.13 1.97  0.08
Cr content in the

CRM sample
(ppb)

2.00  0.02

(Hay CCr = 1.98 ÷ 2.02 ppb)

RSD(%), n = 3 7 4
(a)[Cr (VI)] is the concentration of Cr(VI) in the electrolyte minus the blank. 
White sample has [Cr(VI)] = 0.034 ppb; CCr is the Cr content in the sample 
(calculated by the formula: CCr = [Cr(VI)] .V2 / V1). V1: volume of solution 
taken into the electrolyser (V1 = 2 ml), V2: volume of solution in the 
electrolyser (V2 = 10 mL), S is the standard deviation. Exerimential 
conditionnal: As in Table 3.5.1.

The above results show that both in situ BiFE and ex situ BiFE 
electrodes can be used to determine chromium in surface water, sea
water ... by the SqW-AdSV method. However, when coupled with
surveys of linearity, sensitivity, LOD, accuracy, etc., the use of in

situ BiFE is better because in situ BiFE gives higher sensitivity and

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3.5.1.2. Analyze the actual samples

For the purpose of testing the possibility of applying the
SqW-AdSV / in situ BiFE method for the analysis of chromium in water 
environment, well water, tap water, lagoon water and seawater in some
different áreas in Thua Thien Hue province was taken for analysis.

Take and preserve samples

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Table 3.5.3. Cr content in lagoon water samples, tap water, 
Well water, salt water (a)

N0 Sample

type

Sample
symbol

Chromium concentration
(C mean ± ) ppb,

n = 3, P = 0.95
CCr(VI + III) CCr(VI)

Water
sample
of Cau
Hai
lagoon

M1 13.8  0.2 1.0  0.1

2 M2 19.0  1.0 1.0  0.2

3 M3 7.3  0.4 1.5  0.2

4 M4 26.1  5.8 1.6  0.3

5 M5 14.1  0.8 1.3  0.7

6 M6 1.0  0.1 0.7  0.2

7 M7 11.1  4.4 0.8  0.3

Tap
water

PTN 20.0  2.3

9 GĐ 19.2  3.3

10 GĐ1 18.1  0.8

Well
water

G1 28.6  1.0

12 G1’ 24.6  4.0

13 G2 22.3  4.2

14 G2’ 12.5  4.8

15 G3 6.4  0.6

16 G3’ 14,4  0,5

17 G4 13.6  3.3

18 G4’ 21.2  4.4

19 Salt 
water

B1 1.3  0.3 1.01  0.2

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Figure 3.2. Diagram of CCr(VI+III) analysis procedure in water sample by SqW-AdSV/ in situ BiFE method.

Digestion water sample (add 0.5 mL of concentrated HCl and
0.25 mL concentrated H2O2 into 500 mL sample, boil water at

90oC until volume is 50 mL)

Cr(VI) analysis by SqW-AdSV/ in situ BiFE: 
 10 ml of analytical solution include: Cr (VI), 0.4 M
acetate buffer, 0.4 M NaNO3, 0.4 mM DTPA, 600 ppb Bi,

5.10-6 M KBr.

 Adsorptive concentrating: -800 mV, 160 s, 2000 rpm,
rest time: 50 s.

 Stripping: Square wave mode, cathode sweep:
-800 to -1450 mV (E = 30 mV; Ustep= 6 mV;

f = 35 Hz, stirring off);
 Cleaning : +400 mV, 30 s, stirring off ;
  Recording: Ipeak;

 Determining: using standard addition method.

C

Cr(VI + Cr(III)

White sample: (Add 0.5 mL of 
concentrated HCl, 0.25 mL H2O2 in 
500 mL distilled water, boil water at

90oC until volume is 50 mL)

Take a definite volume

Grind the surface of the GC electrode

with Al2O3 powder (particle size 0.6 
μm), rinse with 1M NaOH, 1M HCl, 
then distilled water, dry with soft filter

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3.5.2. Determination of chromium in solid samples by the 
SqW-AdSV method using BiFE

3.5.2.1. Quality control of the analytical method by standard sample 
analysis

Selection of acid mixture for sample digestion and electrode type 
To select the appropriate mixture of acid for sample digestion,
digest certified reference material sample (CRM) NCS DC87101
(GBW 07418) (freshwater sediment samples, a product of China
National Analysis Center, containing 93  5 ppm Cr) using three
different acid mixtures:

- HH1: 10 mL 33% HNO3 + 3 mL 30% H2O2 [30, 55];

- HH2: 3 mL 37% HCl + 1 mL 65% HNO3 [31,55];

- HH3: 3 mL 37% HCl + 1 mL 65% HNO3 + 1 mL 40% HF

Then determine chromium in the CRM sample (replicate 3
times, n = 3) by using AdSV/in situ BiFE and 
SqW-AdSV/in situ BiFE with appropriate experimental conditions 
shown in Table 3.5.

The results in Table 3.5.4 show that:

 Repeatability: It is thought that, when analyzing within a
laboratory, the method achieves good repeatability if RSD ≤ ½ RSDH

(RSDH is calculated from Horwizt equation, RSDH = 2(1  0.5lgC)).

Thus, for C = 93 ppm, if RSD ≤ 4%, then the method is considered to
have good repeatability (C = 93 ppm, RSDH = 8%). Table 3.7.6

shows that:

+ SqW-AdSV/ex situ BiFE method: with HH2 and HH3 acid 
mixtures for sample digestion, achieved a good repeatability with
RSD ≤ 4% (n = 3);

+ SqW-AdSV/in situ BiFE method: with all digestion acid 
mixtures (HH1, or HH2, or HH3) the method had poor repeatability
with RSD ranging from 7 to 14%.

 Accuracy: The analytical method achieves a good level of accuracy
when the average result is within the 95% confidence interval of the
value reported in the CRM sample certificate, which is in the range
of 88 to 98 ppm. The results in Table 3.5.4 show that:

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with the use of t-test: The t-values for HH1 and HH3 are 0.99 and
2.60, respectively, less than t (p = 0.05; f = n - 1 = 2) = 4.3; In other words,

the Cr content determined using the method with sample digestion
mixture HH1 or HH3 and Cr content in CRM samples are the same
with significant level P > 0.05.

+ SqW-AdSV/in situ BiFE method, with all three digestion 
mixtures of HH1, or HH2, or HH3, the method had low accuracy,
determined Cr concentration was outside the 95% confidence
interval published in the cetification.

Comparing the three digestion mixtures and two working
electrode types (ex situ BiFE and in situ BiFE), it can be argued that 
the SqW-AdSV/ex situ BiFE method with the HH3 digestion mixture 
gave the best results: Best repeatability with RSD = 2% (n = 3) and
best accuracy. The reason why the SqW-AdSV/in situ BiFE method 
got low repeatability and low accuracy, in our opinion, might be due
to the fact that during the electrolytic enrichment at -800 mV, the
metal bismuth membrane was formed together with other metals
available in the analytical solution (eg, Zn, Cd, Cu, Pb ...), and thus
the surface of in situ BiFE electrode was contaminated and not to be 
repeated in successive measurements; This lead to reduced efficiency
of the enrichment process and lessend the repeatability of the
measurements. Therefore, the SqW-AdSV/ex situ BiFE method, with 
wet digestion technique using HH3 acid mixture, was selected for
analysis of real samples .

3.5.2.2. Analysis of real samples

To verify the applicability of the SqW-AdSV/ex situ BiFE 
method for the determination of chromium in sediment samples, Cau
Hai Lagoon and Huong River sediment samples were collected for
analysis.

The results of one-way ANOVA, for Cr concentrations in
sediment samples, in Table 3.5.5 show that:

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Table 3.5.4. Determination results of chromium in CRM samples using AdSV/ex situ BiFE and

SqW-AdSV/in situ BiFE methods with different sample digestion mixtures (*). 
Results

HH1 HH2 HH3

ex situ

BiFE

in situ

BiFE

ex situ

BiFE

in situ

BiFE

ex situ

BiFE

in situ

BiFE

CCr

(ppm)

Run 1 82 69 82 77 96 61,3

Run 2 92 79 76 81 94 74,7

Run 3 94 79 76 69 98 81,3

TB  S (n = 3) 89  6 76  5 78  4 76  6 96  2 72  10

RSD (%), n = 3 7 7 4 8 2 14

Cr concentrations in
CRM sample (ppm)

93  5

(95% confidence interval = 88 - 98 ppm)

(*) The volumes of sample solution taken for analysis (V2) using ex situ BiFE and in situ BiFE were 10

μL and 15 μL, respectively; TB and S are mean and standard deviation. Experiment conditions: As in
Table 3.5.1.

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Table 3.5.5. Chromium concentrations in Cau Hai Lagoon and

Huong River sediment samples (a).

Cau Hai Lagoon sediments Huong River sediments 
STT Samples TB   (ppm

dried weight) STT Samples

TB   (ppm 
dried weight)

1 CH1 20.9  0.4 1 SH1 0.1 ± 0.01

2 CH2 5.6  2.0 2 SH2 0.01 ± 0.02

3 CH3 9.7  0.9 3 SH3 2.5 ± 0.1

4 CH4 12.2  5.0 4 SH4 2.1 ± 0.2

5 CH5 12.6  0.8 5 SH5 1.1 ± 0.1

6 CH6 13.4  6.1 6 SH6 3.9 ± 0.1

7 CH7 10.8  1.9 7 SH7 0.8 ± 0.1

8 SH8 3.2 ± 0.2

9 SH9 3.5 ± 0.2

10 SH10 4.5 ± 0.5

11 SH11 0.74 ± 0.03

(a)CH

1: Near Tu Hien sea mouth; CH2: Randomly sampled; CH3: In the 
cetral area of lagoon; CH4: in the lagoon between the mouth of the Dai 
Giang River and the mouth of the Truoi River; CH5: Contiguous area

between Thuy Tu lagoon and Cau Hai lagoon; CH6: Randomly sampled;

CH7: Randomly sampled. (Cau Hai samples were taken in 2/2015); SH1: 
Upstream of Thao Long dam; SH2, SH3: Upstream and downstream of 
Bao Ving town, respectively; SH4, SH5: Two branches of Huong River

surrounding Hen Islet; SH6, SH7: Upstream and downstream of Gia Vien

Islet; SH8, SH9: Upstream and downstream of Gia Vien Water supply 
station inlet; SH10: Huu Trach branch; SH11: Ta Trach branch. (Huong 
River sediment samples were taken in 2/2014); : Confidence interval at a 
significant level P= 0,95. Experiment conditions: As in Table 3.5.1. 
 Chromium concentrations of sediment sampling sites in Cau Hai
lagoon were different with statistical significance level p <0.05
(Fcalculated = 37.46 > F(p = 0.05, 6,14) = 2.85); Compare the difference

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identical (≡) (p > 0.05) with an average value of 12.3 ppm;
However, the content of Cr in CH1, CH2, CH3 samples and above

samples (CH4 - CH7) were different (≠) with p < 0.05;

- Chromium concentrations of sediment sampling sites in Huong
river were also different with statistical significance level p <0.05
(Fcalculated = 220.20 > F(p = 0.05, 10.22) = 2.29); In the same way,

compare the difference between the two average concentrations of
chromium in Table 3.5.5 (after sorting mean values of chromium
concentration in ascending order) with the minimum significant
deviation (LSD) of 0.33 ppm, found that the Cr content in samples
SH1 ≡ SH2;SH5 ≡ SH7 ≡ SH11; SH8 ≡ SH9 (with p > 0.05); Cr content

in samples SH3 ≠ SH4 ≠ SH5 ≠ SH6 ≠ SH8 ≠ SH10 and were different

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Figure 3.5. Diagram of chromium analysis procedure in sediment sample by 
SqW-AdSV/ ex situ BiFE method.

Decompose sediment samples : Put 0.2500 g of sediment into teflon
digestion bomb, add 3 ml 37% HCl, 1ml 65% HNO3 and 1 ml 40% HF.

Digest the sample in the oven at 110oC for 12 hours. Transfer the sample

into a teflon cup, dry at 95°C. Dissolve the residue with 5 mL of 2 M
HNO3, filter through a 0.45 μm glass fiber filter, rinse, fill up to 50 mL

with distilled water.

Cr(VI) analysis by SqW-AdSV / ex situ BiFE:

 10 mL analysis solution included: Cr(VI), 0.4 M acetate
buffer, 0.4 M NaNO3, 0.4 mM DTPA.

 Adsorptive concentrating: -800 mV, 250 s, 2000 rpm, rest
time: 50 s;

Stripping: cathodic squarewave scanning mode:
-800 to -1450 mV; E = 30 mV; Ustep = 6 mV;

f = 35 Hz, stirring off;

 Cleanning: -200 mV, 30 s, stirring off; 
 Recording: Ipeak;

 Determining: using standard addition method.

C

Cr(VI + III)

White samples: Put 0.2500 g of

distilled water into teflon digestion

bomb, add 3 ml 37% HCl, 1ml 65%
HNO3 and 1 ml 40% HF, digest the

sample in the oven at 110 ° C for 12
hours.

Take a definite volume

Fabrication of ex situ BiFE:

Clean the GC electrode, dip into an
electrolyte containing 0.1 M acetate
buffer, 500 ppb Bi(III), 4.2.10-6 M KBr,

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CONCLUSION

For the purpose of studying the process of determining trace
chromium in the environmental object by adsorptive stripping
voltammetry using Bismuth film electrode. From practice studies,
we can come to the following main conclusions:

1. Based on the study of different working electrodes (HMDE, ex

situ MFE, ex situ BiFE and in situ BiFE) and two techniques to

record stripping signals (pulsed differential voltammetry/ DP and
square - wave voltammetry/ SqW), ex-situ BiFE and in situ BiFE - 
two types of electrodes friendly to the environment- were selected
for square-wave adsorptive stripping voltammetry method
(SqW-AdSV). The determination of Cr(VI) got high sensitivity
(detection limits: 0.1 - 0.3 ppb) similar to the ex-situ MFE and 
better than the HMDE.

2. It was confirmed that the presence of interfere metals (which have
stripping peaks near the stripping peak of chromium, as Zn(II),
Co(II), Ni(II)) in the analysis solution do not affect the determination
of Cr (VI) by the SqW-AdSV and DP-AdSV using ex situ BiFE and

in situ BiFE. In particular, the presence of Cr (III) in solution does

not affect the analysis of Cr (VI). The presence of other metals and
high concentrations of anions, such as Fe(III), Ca(II), Cl, SO42-,

PO43- and Triton X-100 surfactants also does not affect the Cr(VI)

determination. This allows the use of SqW-AdSV method with ex

situ BiFE or in situ BiFE for determination chromium species in

water: Cr(VI) (high toxicity) and Cr(III) (very low toxicity).

3. It has been confirmed for the first time that it is possible to use an

in situ BiFE (simple electrode model created in the analysis solution)

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4. Two trace chromium determination methods have been developed:
 Analysis procedure of Cr(VI) and total Cr (Cr(VI+III) in natural
water by SqW-AdSV using in situ BiFE;

 Analysis procedure of total Cr in environmental solid samples
(sediment /soil samples) by SqW-AdSV using ex situ BiFE.

These two methods have been evaluated for accuracy and
repeatability according to international regulations when developing
an analytical method.

5. Successfully applied the developed methods to determine
chromium in water samples (03 tap water samples, 08 well water

samples, 07 lagoon water samples and 02 saline / brackish water
samples) and sediment samples (11 samples of Huong river sediment
and 07 samples of Cau Hai sediment in Tam Giang - Cau Hai
Lagoon) in Thua Thien Hue province. With the results obtained, it
was initially allowed to confirm that the chromium content of the
samples was very low compared to the current National Technical
Standards for Water and Sediment Quality.

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LIST OF THE SCIENTIFIC WORKS OF THE AUTHOR 
RELATED TO

DISSERTATION

1. Nguyen Thi Hue, Nguyen Van Hop, Hoang Thai Long, 
Nguyen Hai Phong, Le Quoc Hung (2015), “Influence of Cr(III)

and several various factors on determination of Cr(VI) by 
adsorptive stripping voltammetry method, using bismuth film 
electrode”, Conference Proceeding, The Analytica Vietnam

Conference, HCM City, pp. 32-40.

2. Nguyễn Thị Huệ, Nguyễn Văn Hợp, Hoàng Thái Long, 
Nguyễn Hải Phong, Trần Hà Uyên, Lê Quốc Hùng (2015), “Phát

triển điện cực màng bismut in situ để xác đinh lượng vết crom 
(VI)bằng phương pháp von - ampe hòa tan hấp phụ”, Vietnam

Journal of Science and Technology, vol. 53 (1B), pp. 403-411.
3. Nguyễn Thị Huệ, Nguyễn Văn Hợp, Hoàng Thái Long, 
Nguyễn Hải Phong, Lưu Thị Hương, Lê Quốc Hùng (2016),
“Xác định crom trong trầm tích bằng phương pháp von - ampe

hòa tan hấp phụ”, Journal of Analytical Sciences, vol. 21,

number 4, pp. 7-17.

4. Nguyễn Thị Huệ, Nguyễn Văn Hợp, Hoàng Thái Long, 
Nguyễn Hải Phong, Lê Quốc Hùng (2016), “Xác định crom

trong nước bằng phương pháp von - ampe hòa tan hấp phụ”,

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