determinationofchromiuminnaturalwaterbyadsorptive...

Research ArticleDetermination of Chromium in Natural Water by AdsorptiveStripping Voltammetry Using In Situ Bismuth Film Electrode

Nguyen Thị Hue ,1 Nguyen Van Hop,1 Hoang Thai Long ,1 Nguyen Hai Phong ,1Tran Ha Uyen,1 Le Quoc Hung,2 and Nguyen Nhi Phuong1,3

1University of Sciences, Hue University, Hue 530000, Vietnam2Institute of Chemistry, Vietnam Academy of Science and Technology, Ha Noi 100000, Vietnam3Pham Van Dong University, Quảng Ngãi 570000, Vietnam

Correspondence should be addressed to Nguyen �ị Hue; [email protected] and Hoang �ai Long; [email protected]

Received 2 August 2019; Revised 7 January 2020; Accepted 10 April 2020; Published 14 May 2020

Academic Editor: Evelyn O. Talbott

Copyright © 2020 Nguyen �ị Hue et al. �is is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Development of adsorptive stripping voltammetry (AdSV) combined with in situ prepared bismuth film electrode (in situ BiFE)on glassy carbon disk surface using diethylenetriamine pentaacetic acid (DTPA) as a complexing agent and NO−3 as a catalyst todetermine the trace amount of chromium (VI) is demonstrated. According to this method, in the preconcentration step atEdep � −800mV, the bismuth film is coated on the surface of glassy carbon electrodes simultaneously with the adsorption ofcomplexes Cr(III)-DTPA. In addition to the influencing factors, the stripping voltammetry performance factors such as de-position potential, deposition time, equilibration time, cleaning potential, cleaning time, and technical parameters of differentialpulse and square wave voltammetries have been investigated, and the influence of Cr(III), Co(II), Ni(II), Ca(II), Fe(III), SO2−4 , Cl

−,and Triton X has also been investigated. �is method gained good repeatability with RSD

chromium traces in different objects, and achieves very lowLOD, size < 10−9–10−12M [10–20].

Another problem raised was the working electrode usedin the AdSV method. Currently, most studies on the AdSVmethod use hanging mercury drop electrodes (HMDEs)[21–34], or static mercury drop electrodes (SMDE) [19, 35]which are expensive and very difficult to fabricate. Researchon the use of mercury film electrode (MFE) and bismuth filmelectrode on glassy carbon disk surface (BiFE), which are lessexpensive and easier to fabricate, and environmentallyfriendly BiFE has been improved.

AdSV has been applied in combination with differenttypes of electrodes and stripping voltammetry signalingtechniques, and many authors have been successful indetermining the amount of traces and super chromiumtraces in complex objects. Most studies use HMDEelectrodes, some use MFE electrodes, and there are alsomany ex situ BiFE electrodes [10, 11, 14, 16, 17, 36] andgold film electrodes (AuFE) [13, 15, 37–39], and the latermodified electrodes [20, 36, 40–44] are applied more forchromium analysis in complex objects. �e complex li-gands used are DTPA [1, 21, 22, 25, 26, 28, 32, 35, 45],triethylenetetramine hexaacetic acid (TTHA)[19, 29, 31, 37], diphenylcarbazide (DPCB) [23, 29],pyrocatechol violet [24, 30], pyrogallol [34], rubeanicacid [33], neo TT [40], and quercetin [20], and thecommon base ingredients are CH3COONa, acetate buffer(CH3COOH/CH3COONa), and CH3COONa/NaNO3; allstudies analyzed chromium in river water, seawater,groundwater, tea water, and wastewater, and almost noworks have analyzed chromium in sediments by theadsorptive stripping voltammetry method and no authorhas used in situ BiFE to analyze chromium in environ-mental objects. Overall, studies have achieved very lowdetection limits from 10-9 to 10-10M (or from 0.05 ppb to0.005 ppb). Table 1 shows the summary of publishedstudies on chromium determination by the AdSVmethod.

When analyzing Cr(VI) by the AdSVmethod, Yong et al.[10] and Lin et al. [11] proposed the adsorption mechanismas follows.

�e first stage of the adsorptive stripping voltammetrymethod is the movement of Cr(VI) in the solution to theelectrode surface and is reduced by the following reaction.

Cr(VI) + 3e⟶Cr(III); E1/2 � −0.2V in comparisonwith 3M Ag/AgCl/KCl electrode; in the presence of DTPAions on the interface between the electrode membrane andthe solution, Cr(III) ions form complexes quickly andadsorbed onto the electrode surface. �e main nature of thiscomplex is unknown, and some authors [21, 46] said thatthis complex exists mainly as Cr(III)-DTPA2− and a little inCr(III)-HDTPA−, or Khan [47] suggested that the complexof Cr(III) with DTPA exists as [Cr(III) (H2O)HY]−, [Cr(III)-DTPA]2−, and so on.

After complex adsorption, potential scan was conductedfrom −0.9V to −1.35V [10], or from −0.8V to −1.4V [11]; asa result of this process, the Cr(III)-DTPA complex is re-duced to the Cr(II)-DTPA complex [10, 11]:

Cr(III) − DTPA + e⟶ Cr(II) − DTPA (1)

�e signal recorded with this reduction is at E1/2� −1.15V[10, 11] and at E1/2� −1.1V in comparison with 3MAg/AgCl/KCl electrode.

�e role of NO−3 ions is to oxidize Cr(II)-DTPA com-plexes into Cr(III)-DTPA complexes.

�anks to the oxidation of NO−3 , the peak is higher andthe sensitivity of the method is increased.

Cr(II) − DTPA⟶N O−3 Cr(III) − DTPA (2)

In order to contribute to the development of the AdSVmethod, we have conducted several studies to determinechromium by the AdSV method using mercury film elec-trodes, ex situ bismuth film electrode, and in situ bismuthfilm electrode. In this paper, we present the results ofchromium determination by the AdSV method using in situbismuth film electrode (in situ BiFE) in the presence ofDTPA as a complexing agent and ion NO−3 as a catalyst.Literature survey revealed that in situ BiFE has never beenused for chromium determination by the adsorptive strip-ping voltammetry.

2. Experimental

2.1. Apparatus and Reagents. Square wave stripping vol-tammetric measurements were conducted on Electro-chemical Analyzer 797 VA Computrace (Metrohm,Switzerland) accompanied with three electrodes. �eseelectrodes were inserted into the 80ml capacity electro-chemical cell. �e working electrode was a glassy carbonrotating disk electrode with d� 2.8± 0.1mm; the referenceelectrode was a 3M Ag/AgCl/ KCl electrode and the aux-iliary electrode was a platinum wire. All measurements werecarried out at 25± 1°C.

DTPA (diethylenetriamine pentaacetic acid) is used as acomplexing agent for chromium. 50.10−3M DTPA solutionwas prepared by dissolving 4.916 g of DTPA in double-distilled water and then adding 25% aqueous ammoniasolution until the pH reaches 6.0.

0.2M Cr(VI) stock solution was prepared by weighing7.35 g of K2Cr2O7 (Merck, purity of 95–98%), dissolving,and making it up to 250ml with double-distilled water.Cr(VI) working solutions such as Cr(VI) 10−6M and Cr(VI)10−7M are diluted daily from this stock solution.

0.48.10−3M Bi(III) working solution was prepared from4.8 .10−3M Bi(III) (the type used for analyzing atomic ab-sorption spectrometry from Merck).

Acetate buffer (pH� 6) was prepared from NaCH3COO(Merck, purity of 95–98%) and CH3COOH (Merck, purity97%). 2.5M NaNO3 solution was prepared from NaNO3(Merck, purity of 95–98%).

Othermetal ionic solutions such as Co(II), Ni(II), Zn(II),Cr(III), Fe(III), and Ca(II) are from the corresponding1000mg/l stock solution (the type used for analyzing atomicabsorption spectrometry from Merck). Triton X-100working solution was prepared from Triton X-100 (Merck).

2 Journal of Environmental and Public Health

Tabl

e1:

Summaryof

publish

edworks

onstripp

ingvoltammetry

metho

dforchromium

determ

ination.

No.

Ligand

sBa

ckgrou

ndsolutio

nWorking

electrod

eMeasurement

techniqu

esLO

D(pbb

)Analytical

object

Determined

form

Timeof

publication

References

1DTP

ACH

3COONH

4,NaN

O3

(pH

�5.2)

HMDE

DP-AdS

V1.2

Fake

template

Total

chromium

1987

[21]

2Dipheny

lcarbazide

H2SO40.1M

GE

LSV

0.05

Fake

template

Cr(VI)

1988

[29]

3TT

HA

CH

3COONa,NaN

O3

(pH

�5.5)

HMDE

DP-CSV

1.04

Fake

template

Cr(VI)

1988

[29]

4TT

HA

CH

3COONa,NaN

O3

(pH

�5.5)

HMDE

SqW-A

V0.02

Fake

template

Cr(VI)

1988

[29]

5DTP

AKH

2PO

4,Na 2HPO

4(pH

�6–

7)HMDE

DP-CSV

0.01

Fake

template

Cr(VI)

1990

[32]

6Dipheny

lcarbazide

H2SO40.3M

HMDE

DP-AdS

V0.02

Groun

dwater

Cr(VI)

1992

[23]

7DTP

A2.5mM

Acetate

buffer(pH

5.2)

HMDE

SqW-A

dSV

0.05

Seaw

ater

Cr(VI),total

chromium

1992

[45]

8DTP

A0.01

MNaN

O30.5M,M

ES(m

orph

olinoethanesulfonic

acid)(pH

6.1)

HMDE

DP-AdS

V0.62

Riverwater,tap

water

Total

chromium

2000

[28]

9DTP

ANaN

O3,acetatebu

ffer(pH

5.7)

HMDE

DP-AdS

V

Cr(VI):

0.005

activ

ated

Cr(III):

0.52

Riverwater,

lake

water,

sewer

water

Cr(VI),

activ

ated

Cr(III),total

chromium

2001

[1]

10TT

HA

0.1M

(triethylenetetram

inhexaacetic

acid)

NaC

H3C

OO

1M,N

aNO35M

(pH

6.2–

6.5)

SMDE

SqW-A

dSV

0.52

Leaf

Cr(VI)

2003

[19]

11DTP

AAcetate

buffer(pH

�6),K

NO3

0.25

MHMDE

DP-AdS

V0.004

Wastewater

Cr(VI),total

chromium

2004

[22]

12Ru

beanic

acid

(dith

iooxam

ide)

Acetate

buffer(pH

6),K

NO3

0.20

MHMDE

DP-AdS

V1976

Riverwater,

seaw

ater,

sewage,vinegar

Cr(VI)

2012

[33]

13DTP

AAcetate

buffer(pH

6)NaN

O3

HMDE

AdS

V18.2

Hum

anurine

Cr(VI)

2005

[26]

14TT

HA

0.2M

NaC

H3C

OO0.1M,N

aNO35M

(pH

6.2)

HMDE

CV-A

dSV

0.3

Cr(VI)

1997

[31]

15DTP

ANaC

H3C

OO

0.01

M,N

aNO3

0.5M

(pH

8.5)

SMDE

156.10

−5

Riverwater,tap

water

Cr(VI)

1999

[35]

16DTP

A5mM

NaC

H3C

OO

0.15

M,N

aNO3

0.7M

(pH

6)HMDE

SqW-A

dSV

0.05

Cem

ent

Cr(VI)

2011

[25]

17Py

rogallo

lred

0.4M

acetatebu

ffer(pH

4.5)

HMDE

SqW-A

dSV

0.05

Seaw

ater

Cr(VI),

Cr(III),total

chromium

2012

[34]

18DTP

A0.05

MNaC

H3C

OO

0.2M

(pH

6.2)

HMDE

DP-AdS

V1.04

Electrop

latin

gwaste

water

Cr(VI)

2004

[27]

19Py

rocatechol

violet

Acetate

buffer

HMDE

DP-AdS

VCr(VI)

1997

[24]

20

HED

TA(N

-2-hydroxyethyl

ethylenediam

ine-N,N′,N″-triacetic

acid)andPC

V(pyrocatecho

lviolet)

0.1M

acetatebu

ffer(pH

6)KNO

32M

HMDE

DP-AdS

VCr(III),

Cr(VI)

2002

[30]

Journal of Environmental and Public Health 3

Tabl

e1:

Con

tinued.

No.

Ligand

sBa

ckgrou

ndsolutio

nWorking

electrod

eMeasurement

techniqu

esLO

D(pbb

)Analytical

object

Determined

form

Timeof

publication

References

21Cup

ferron

0.01

MPIPE

S0.2M

(pH

�7)

BiFE

exsitu

SqW-A

dSV

0.1

Labo

ratory

water,

cigarette

s,soil

sample

Total

chromium

2004

[14]

22DTP

A5mM

Acetate

0.1M

(pH

�6.0)

BiFE

exsitu

SqW-A

dSV

0.015

Riverwater

Total

chromium

2005

[11]

23DTP

A5mM

NaO

Ac0.1M

(pH

�6.0)

BiFE

exsitu

SqW-A

dSV

0.015

Bloo

dsample

Total

chromium

2006

[10]

24DTP

A5mM

Acetate

0.1M

(pH

�6.0)

BiFE

exsitu

SqW-A

dSV

0.017

(CrV

I );0.022(C

rtotal)

Riverwater

Cr(VI);

Cr(III)

2010

[16]

25PA

R(4-(2-pyridylazo)resorcinol)

CH

3COOH–C

H3C

OONa,

triso

dium

citrate

BiFE

exsitu

SqW-A

dSV

0.01

Tapwater,lake

water,soil

sample

Total

chromium

2013

[17]

26DTP

ACH

3COONH

4,NaN

O3

(pH

�5.2)

Bifilm

wrapp

edsin

glewalled

carbon

nano

tubes

DP-AdS

V0.12.10−

3Fake

template

Total

chromium

2013

[18]

27DTP

A0.01

M0.1M

acetatebu

ffer(pH

�6.0)

Hg(A

g)FE

DP-AdS

V0.004

Natural

water,

drinking

water

Cr(VI)

2006

[12]

28TT

HA

CH

3COONa,NaN

O3

(pH

�5.5)

Goldfilm

mod

ified

carbon

compo

site

electrod

eDP-CSV

4.0

Fake

template

Cr(VI)

2014

[37]

29PE

T(4pyridine-ethanethiol)

NaF

0.15

M(pH

�4.5)

PET/nano

-Au/Pt-

RDelectrod

eDP-AdS

V0.001

Seaw

ater

Cr(VI)

2015

[13]

30DTP

AKH

2PO

4,Na 2HPO

4(pH

�6–

7)

(Flower-like

self-

assemblyof

gold

nano

particles)

AuN

Ps/G

CE

DP–

CSV

0.001

Fake

template

Cr(VI)

2012

[38]

31HCl(pH

�2)

AuN

Ps/nano-TiC/

GCE

DP-AdS

V2.08

Seaw

ater

Cr(VI)

2015

[15]

32HClO

40.06

MAuN

Ps/SPC

EASV

0.002

Tapwater,

seaw

ater

Cr(VI)

2015

[39]

33QH2(quercetin)

Acetate

buffer(pH

�6),K

NO3

0.7M

QH2/MWCNT-

SPCE(quercetin/

multiw

alledcarbon

nano

tubesscreen-

printedcarbon

electrod

e)

DP-AdS

V15.9

Drink

ingwater

Cr(VI)

2013

[20]

34-

Acetate

buffer(pH

�5)

μNPs/G

CE

DP-SV

0.01

Electrop

latin

gwaste

water

Cr(III)

2015

[44]

35DTP

A0.1M

Acetate

buffer(pH

6),K

NO3

0.25

MμNPs/BiFE

DP-AdS

V0.12.10−

3Fake

template

Cr(VI)

2011

[36]


Tabl

e1:

Con

tinued.

No.

Ligand

sBa

ckgrou

ndsolutio

nWorking

electrod

eMeasurement

techniqu

esLO

D(pbb

)Analytical

object

Determined

form

Timeof

publication

References

36Po

lyviny

lbutyral/SPE

s+4.7%

DTP

AH

2SO4(pH

1)SP

Es(screen-

printedelectrod

e)CV-A

dSV

52.0

Fake

template

Cr(VI)

2014

[42]

372,5,8,11,14-Pentaaza-15,16,29-

phenanthrolin

ophane

(NeoTT

)1,6-Dichloro-hexane

(DCH),

LiCl1

0mM,H

Cl1

mM

Liqu

id/liqu

idinterface

SqW-A

dSV

250.0

Fake

template

Cr(VI)

2005

[40]

38

Silver:A

gClO

40.1mM,b

riton

robinson

(pH

2)gold:

HAuC

lO40.1mM,H

2SO4

0.5mM.

Carbo

nscreen-

printedelectrod

e(C

SPEs)

DPV

Silver:4

4.2

gold:2

0.8

Fake

template

Cr(VI)

2008

[43]

39Septon

ex10

−6 M

(1-

pentadecyltrim

ethylamon

ium

brom

ide)

HCl0

.25M,N

aCl0

.1M

(pH<2)

Carbo

npaste

DP-CSV

2.6

Tea

CrO

2− 42004

[41]


2.2. -e Working Electrode and Adsorptive Stripping Vol-tammetric Procedure. In this method, the working electrodeis bismuth film electrode created in in situ on glassy carbonrotating disk (in situ BiFE) and it is formed during thedeposition process in the following way: �e glassy carbondisk electrode was inserted into the electrochemical cellcontaining the reference electrode, platinum auxiliaryelectrode, and analysis solution (0.4.10−3M DTPA, 28.8.10−5M Bi(III), 5.0.10−6M KBr, 0.4M NaNO3, 0.4M acetatebuffer solution, and Cr(VI)).�e glassy carbon electrode wasrotated with constant speed and deposition at −800mV wasobserved (deposition voltage, Edep) at a definite time (de-position time, tdep); in the process, Bi(III) is reduced to Bi0which adheres to the surface of glassy carbon plate formingin situ BiFE; at the same time, Cr(VI) is reduced to Cr(III),and then new Cr(III) forms complexes with DTPA in thesolution layer close to the electrode surface and Cr(III)-DTPA complex adsorbed onto the surface of in situ BiFE, sochromium is enriched on the surface of the in situ BiFE [10].At the end of this period, the electrode stops rotating for30–60 seconds (equilibration time, tequal). Subsequently, thepotential scan was carried out in a negative potential di-rection from −800mV to −1450mV, and at the same time,the stripping voltammogram was recorded using a certainstripping voltammetry technique, differential pulse ad-sorptive stripping voltammetry (DP-AdSV), or square waveadsorptive stripping voltammetry (SqW-AdSV). During thisperiod, Cr(III) in the Cr(III)-DTPA complexes is reduced toCr(II) forming Cr(II)-DTPA complexes and generating thestripping peak current of chromium (Ip) [10]. If NO−3 is notpresent in solution, Ip will be very small, NO−3 present in thesolution will oxidize Cr(II)-DTPA to Cr(III)-DTPA andthen Cr(III)-DTPA is electrochemically reduced to Cr(II)-DTPA, and the repeated cycle increases the height of Ip [10].In other words, the NO−3 ion acts as a catalyst. After dis-solving, the electrode was cleaned by electrolysis at +400mVfor 30 seconds to dissolve Bio and other metals that may bepresent into the solution. Ip is proportional to the con-centration of Cr(VI) in the solution.

In all experiments, for Cr(VI) with trace, the firstmeasurement result must be discarded because it is unstable.�e stripping voltammogram was recorded 3 times (n� 3),and the peak current (Ip) and peak potential (Ep) values areaveraged from three repetitions.

�e glassy carbon electrodes were cleaned by polishingthe surface with fine Al2O3 powder (particle size 0.6 μm) andthen washed with distilled water and then with 1MNaOH toremove all Al2O3 particles on the glassy carbon surface, andthen the electrodes were dipped into 1M HCl solution andfinally washed with distilled water and the electrodes weredried with soft filter paper.

3. Results and Discussion

3.1. Differential Pulse Adsorptive Stripping Voltammetry (DP-AdSV) Using In Situ BiFE. In order to select the appropriateconditions for themethod, the experimental conditions werefixed as shown in Table S1. A univariate method is applied toexamine the effect of factors. �e magnitude of the stripping

peak current (Ip) and the relative standard deviation of the Ip(RSD) are used for selecting the appropriate test conditions.

3.1.1. Effect of Acetate Buffer Concentration. Acetate bufferis chosen to stabilize the pH of the solution. Acetate bufferis one of the factors that strongly influence the complex ofCr(III) and DTPA [10]. �e complex formation betweenCr(III) and DTPA usually occurs at pH� 6 [10, 11]. Atchromium concentration CCr(VI) � 3.8.10−8M, ligand con-centration CDTPA � 0.4.10−3M, and bismuth concentrationCBi(III) � 24.10−5M, the survey results of the effect of acetatebuffer concentration (CAc) in the range of 0.1M–0.6M(pH � 6) showed that CAc � 0.4M was appropriate. Withthis condition, the peak current is 31.36 μA and the re-peatability is relatively good (RSD� 1.6% with n� 3)(Figure 1(a)).

3.1.2. Effect of Bi(III) Concentration and DTPAConcentration. Previous studies with ex situ BiFE electrodeshave suggested that the presence of KBr in the solutionincreases the bismuth’s sustainability on the glassy disksurface and at the same time improves the conductivity ofthe solution [48]. In the acetate buffer with CAc � 0.4M andthe presence of KBr at a concentration of 5.0.10−6M,CDTPA � 0.4.10−3M, and CCr(VI) � 3.8.10−8M, a Bi(III) con-centration of 28.8.10−5M is appropriate. At those concen-trations, the peak current is 37.6 μA and the repeatability isgood (RSD� 1.7% with n� 3) (Figure 1(b)). �e surveyresults of the effect of DTPA concentrations in the range of0.1 to 1.0.10−3M to Cr peak current show that DTPAconcentration of 0.4.10−3M was appropriate (Figure 1(f)).

3.1.3. Effect of NaNO3 Concentration. In the presence ofNO−3 , peak current of chromium (Ip) was enhanced sig-nificantly. Many authors argue that NO−3 ions act as oxi-dizing agents that convert Cr(II)-DTPA complexes toCr(III)-DTPA complexes and thus increase the concentra-tion of Cr(III) on the electrode surface, and this leads to anincrease in Ip [10, 11, 21]. At CAc � 0.4M, CKBr � 5.0.10−6M,CDTPA � 0.4.10−3M, CBi(III) � 28.8 .10−5M, andCCr(VI) � 3.8.10−8M, Ip increased when the NaNO3 con-centration increased from 0.1M to 0.4M (Figures 1(e) and2(c)). However, when the NaNO3 concentration is greaterthan 0.4M, it increases the baseline and may contaminatethe analysis solution. When the NaNO3 concentration isequal to 0.4M, the peak current is 17.6 µA and the re-peatability is quite good (RSD� 3.7% with n� 2). NaNO3concentration value of 0.4M was selected for furtherinvestigation.

3.1.4. Effect of Deposition Potential (Edep) and Deposition Time(tdep). When CAc � 0.4M, CKBr � 5.0.10−6M, CCr(VI) � 2 ppb,CDTPA � 0.4.10−3M, and CBi(III) � 28.8.10−5M, the surveyresults of the effect of the deposition potential in the rangefrom −700mV to −1000mV are shown in Figure 1(c) andthe stripping voltammetry is shown in Figure 2(a).


From this result, it shows that when Edep is equal to−800mV, the peak current is 42.1 µA and the repeatability isgood (RSD� 1.3% with n� 3). Edep of −800mV was selectedfor further studies.

With the above conditions and when Edep is equal to−800mV, Ip is almost unchanged when the deposition time(tdep) is greater than 80 s, which means that it tends to reach

saturation (Figure 1(d)). tdep of 50 s is selected for the nextexperiment (the stripping voltammetry at tdep � 50 s isshown in Figure 2(b)).

3.1.5. Effect of Rotating Rate of Electrode (ω) and Equili-bration Time (tequal). By increasing the rotation speed of the

IpRSD

0

5

10

15

20

25

30

35I p

(μA

)

0.25 0.5 0.750CAc (M)

0

5

10

15

20

25

30

RSD

(%)

(a)

0

10

20

30

40

I p (μ

A)

148 248 34848CBi(III) (10–4M)

0

2

4

6

8

RSD

(%)

IpRSD

(b)

IpRSD

34

36

38

40

42

44

I p (μ

A)

–1.0 –0.9 –0.8 –0.7 –0.6–1.1Edep (V)

0

1

2

3

4

5

RSD

(%)

(c)

IpRSD

0

10

20

30

40

I p (μ

A)

40 50 60 70 80 90 100 11030tdep (s)

0

2

4

6

8

10

12

RSD

(%)

(d)

IpRSD

0

5

10

15

20

I p (μ

A)

0.1 0.2 0.3 0.4 0.50CNaNO3 (M)

0

2

4

6

8

10

12

RSD

(%)

(e)

IpRSD

0

2

4

6

8

I p (μ

A)

0.2 0.4 0.6 0.8 1 1.20CDTPA (10–3M)

0

1

2

3

4

5

RSD

(%)

(f )

Figure 1: Effect of acetate buffer concentration (a), Bi(III) concentration (b), deposition potential (c), deposition time (d), NaNO3concentration (e), and DTPA concentration (f) on the Ip of chromium in DP-AdSV using in situ BiFE. Experimental conditions are asmentioned in Sections 3.1.1 to 3.1.4, and other conditions are as shown in Table S1.


electrode to a specified value, it will increase the masstransfer and the efficiency of the enrichment will be better.�e survey results of the rotating rate of the electrode in therange from 800 rpm to 2400 rpm showed that ω of 2000 rpmwas appropriate. At the end of the enrichment phase, theelectrode should not be rotated for a specified period of timeto keep the solution quiet and the electrode surface is sta-bilized (this time is also called equilibration time, symbol-ized as tequal). Ip survey results according to tequal showed thattequal of 50 s was appropriate.

3.1.6. Effect of Cleaning Potential (Eclean) and Cleaning Time(tclean). �e cleaning of the surface of the carbon glassy discelectrode at the end of each stripping voltammetry is es-sential, as it will create repeating electrode surfaces for

subsequent measurements. In terms of experimental con-ditions as in Section 3.1.3, the survey results of the influenceof Eclean in the range of 200mV to 500mV and tclean in therange of 60 s to 120 s showed that Eclean is equal to 300mVand tclean is equal to 110 s which are appropriate.

3.2. Square Wave Adsorptive Stripping Voltammetry (SqW-AdSV) Using In Situ BiFE. Some authors argue that, inaddition to differential pulse stripping voltammetry tech-niques, square wave stripping voltammetry can be used torecord the signal (at this time, the method is called squarewave adsorptive stripping voltammetry (SqW-AdSV)) andalso allow the determination of very sensitive chromium.Based on the experimental conditions initially fixed asshown in Table S1, the effects of the factors were investigated

Cr

–70.0u

–80.0u

–90.0u

–100u

–110u

–120uI (

A)

–1100 –1300 –1400–1200–900 –1000–800E (mV)

(a)

Cr

–60.0u

–70.0u

–80.0u

–90.0u

I (A

)

–900 –1000 –1100 –1200 –1300 –1400–800E (mV)

(b)

Cr

0.4 M

0.3 M

0.2 M

0.1 M

Baseline–30.0u

–40.0u

–50.0u

–60.0u

–70.0u

I (A

)

–900 –1000 –1100 –1200 –1300 –1400–800E (mV)

(c)

Figure 2: �e stripping voltammetry DP-AdSV using in situ BiFE of chromium is recorded for (a) Edep � −800mV and (b) tdep � 50 s.(c) NaNO3 concentrations are changed: the bottom line is the baseline, followed by concentration of NaNO3 increasing from 0.1 to 0.4M.Other experimental conditions are as shown in Figure 1.


in a similar way to the DP-AdSV method, and we obtainedthe appropriate conditions for SqW-AdSV (using in situBiFE) to determine Cr(VI) as shown in Table S2.

3.3. Interferences. Interferences for the determination ofCr(VI) consist of metallic cations that have the strippingpeak current near the stripping peak current of chromiumand anions that can form complexes or make conjugateswith the forms of chromium and Bi(III) which can beadsorbed onto the surface of the in situ BiFE, and surfactantscan be adsorbed onto the working surface of the electrode.

�e influence of interferences can be estimated by rel-ative error values of stripping peak current (RE). Considerthat RE for Ip was equal to RE for C (because Ip � kC). RE forIp (or C) was accepted when it was equal to ½ Horwitzfunction RSD (RE Ip(Cr)≤½ RSDHorwitz �½.2(1−0.5lgC) � 32%with C� 0.2 ppb). RE was calculated as follows:

RE Ip(Cr)(%) �Ip(Cr) − Ip(Cr)

0

Ip(Cr)0∗100, (3)

where RE is the relative error values of stripping peakcurrent, Ip (Cr)0 is the stripping peak current without addinginterferences and Ip(Cr) is the stripping peak current withadding interferences.

3.3.1. Interference Studies. When the chromium(III) con-centration is about 100 times higher than the chromium(VI)concentration, the Ip does not change significantly(RE< 18%). In fact, rarely encountered CCr(III) case is 300times higher than CCr(VI), so it can be assumed that Cr(III)does not affect the Cr(VI) determination. �is investigationagain confirms that Cr(III) does not affect the determinationof Cr(VI) (Table S3 and Figure S3).

In the acetate buffer (pH� 5–6), Zn(II), Co(II),and Ni(II) can affect the determination of Cr(VI) because ithas a stripping peak current close to the stripping peakcurrent of Cr(VI). Ep (Zn)≈−1040÷−1050mV, Ep (Co)≈−1290÷−1300mV, Ep (Ni)≈−1080÷−1100mV, and Ep(Cr)≈−1180÷−1240mV.

From the experimental results at approximately 3.810−9M (0.2 ppb) chromium concentration, 120 s depositiontime, and the suitable conditions as in Table 2, we have seenthat Zn has not influenced the determination of chromiumwhen Zn(II) concentration is 800 times larger than Cr(VI)concentration (REIp(Cr)≤ 16%) (Table 2), and in conse-quence, we can determine chromium in the natural waterwith attendance of Zn(II) because ordinarily Zn(II) con-centration is 500 times smaller than Cr(VI) concentration innatural water (Table 2). Co and Ni have not influenced thedetermination of chromium when Co(II) and Ni(II) con-centrations are 90 times larger than Cr(VI) concentration(RE Ip(Cr) � 1.7–8.0% for Co(II) and 3.9–19.4% for Ni(II)(Table 2).

In natural water, Fe(III) and Ca(II) usually exist at highconcentrations of mM; in seawater, Ca(II) exists at quitehigh concentration, about 10−2÷10−3M, so it is necessary toexamine the effect of Fe(III) and Ca(II) on the stripping peak

current of Cr(VI).�e results in Table S4 show that when theconcentration of Fe(III) and Ca(II) increases to 36.10−6Mand 50.10−6M, respectively, meaning that the concentrationsof Fe(III) and Ca(II) are about 10,000 times greater than theconcentration of Cr(VI), the determination of Cr(VI) is notaffected. RE is less than 17%.

In natural water, Cl− and SO2−4 ions have significantconcentrations (mM and larger), and they can form com-plexes with metals present in the study solution and maytherefore affect the determination of Cr(VI). To investigatethe effects of Cl− and SO2−4 , a series of experiments wereconducted with Cl− concentrations ranging from 0 to281.7.10−3M and SO2−4 concentrations ranging from 0 to10.4.10−3M. �e results in Table S5 show that Cl− does notaffect the determination of Cr(VI) when CCl− is in the rangeof 0 to 14.09. 10−3M.When CCl− is greater than 28.17.10−3M(nearly equivalent to Cl− concentration in brackish water),Cl− affects the determination of Cr(VI) with RE> 32%.�erefore, when analyzing Cr(VI) in samples with highCl−concentration, it is necessary to take Cl− removal methodfrom the sample. SO2−4 did not affect the determination ofCr(VI) by SqW-AdSV/in situ BiFE method with RE< 27%.

In the adsorption stripping voltammetry method, thesurfactant can be adsorptive on the surface of the workingelectrode, and this can affect the adsorption process of themetallic complexes on the working electrode. Triton X-100(polyethylene glycol) is a typical nonionic surfactant andusually is used in order to observe the influence of thesurfactant on the adsorption stripping voltammetry method.�e effects of Triton X-100 are investigated at concentrationsbetween 0 and 93. 10−9M, and the results in Table S6 showthat, when increasing the concentration of Triton X-100 to25 times higher than the concentration of Cr(VI), it still didnot affect the determination of Cr(VI) with RE< 8%.

In fact, the concentration of natural surfactants is rarelygreater than 77.10−9M, and therefore, it can be assumed thatthey do not affect the Cr(VI) determination. �us, whendetermining Cr(VI) by the adsorption stripping voltam-metry method, it is not necessary to remove the surfactantfrom the analytical solution.

In some cases, natural water and wastewater contain manyorganic substances including surfactants. It is necessary totreat the sample to exclude organic substances before analysisusing UV irradiation and decomposition in acid mixture.

3.4. Evaluation of Reliability of DP-AdSV and SqW-AdSVMethods

3.4.1. Repeatability. Repeat recording of 7 stripping vol-tammetry lines (n� 7) on the same in situ BiFE according tothe DP-AdSV or SqW-AdSV method in Figure 3 shows thatIp in both approaches has good repeatability with RSD< 4%(n� 9) and RSD< 3% (n� 7), respectively, for DP-AdSV andSqW-AdSV. �e stripping peak of chromium (Ep) is neg-ligible, only about 20mV toward the positive side.

3.4.2. Linear Range and Detection Limits. �e linear rangeand LOD of the two methods SqW-AdSV and DP-AdSV


were investigated with the appropriate experimental con-ditions as shown in Table S2 and the stripping voltammetryspecifications as shown in Table S1, and the following resultswere obtained:

(1) -e Linear Range. For the SqW-AdSV method, Ip andCCr(VI) have a good linear correlation in the rangeCCr(VI) � 0.3÷1.8 ppb with a correlation coefficient (R) of 0.9994 (linear regression equation is shown in Figure 4(a), andthe stripping voltammetry is shown in Figure 4(b)).

For the DP-AdSV method, there is a good linear cor-relation in the range CCr(VI) � 2÷12 ppb with R� 0.9989(linear regression equation is shown in Figure 4(d)).

(2) Sensitivity.�e SqW-AdSVmethod achieved a sensitivity(23 µA/ppb) of about 34 times higher than the DP-AdSV(0.682 µA/ppb) method.

(3) Detection Limits and Quantitative Limits. For SqW-AdSV (when Edep � −800mV and tdep � 160 s):

LOD� 0.1 ppb; LOQ� 0.3 ppb. For DP-AdSV (whenEdep � −800mV and tdep � 50 s): LOD� 0.6 ppb;LOQ� 2 ppb.

�us, the SqW-AdSVmethod achieves a narrower linearrange than the DP-AdSV method, but it achieves highersensitivity than the DP-AdSVmethod (due to its lower LODand greater slope linearity). It can be said that with LOD asabove, DP-AdSV and SqW-AdSV methods can be used within situ BiFE to determine the trace amount of Cr(VI).

3.5. Determination of Chromium in Natural Water by theSqW-AdSV Using In Situ BiFE. In natural water samples,chromium usually exists in both Cr(VI) and Cr(III)forms. As investigated, the SqW-AdSV/in situ BiFEmethod identifies Cr(VI) and also determines the totalCr(VI) + Cr(III) if during the decomposition of thesample, an additional oxidizer is added to oxidize Cr(III)to Cr(VI). �us, we can determine chromium in indi-vidual forms by determining Cr(VI) (∗) and total

Table 2: Influence of Zn(II), Co(II), and Ni(II) concentrations on peak current.

Cation Zn(II) Co (II) Ni (II)No. CZn(II)(nM) Ip(Cr)(μA) RE Ip(Cr)(%) CCo(II)(nM) Ip(Cr)(μA) REIp(Cr)(%) CNi(II)(nM) Ip(Cr)(μA) RE Ip(Cr)(%)1 0 96.4 0 0 76.5 0 0 49.7 02 770 96.3 0.1 84 82.4 7.6 84 53.2 3.93 1540 94.6 1.9 168 82.5 7.9 168 58.3 10.44 2310 87.9 8.7 252 81.7 6.8 252 61.5 14.65 3080 80.9 16.0 336 77.8 1.7 336 62.4 19.4Conditions: CCr(VI) � 3.8.10−9M� 0.2 ppb; CBi(III) � 28.8.10−5M; tad � 120 s; Eclean � 400mV; tclean � 100 s; Ustep � 6mV; v � 210mV/s; ∆E� 30mV; f� 35Hz.;ω � 2000 rpm; CDTPA � 0.4mM; CAc � 0.4M; CNaNO3 � 0.4M; Ead � −800mV.

Cr

–80.0u

–100u

–120u

–140u

–160u

–180u

I (A

)

–900 –1000 –1100 –1200 –1300 –1400–800E (mV)

(a)

Cr

–40.0u

–50.0u

–60.0u

I (A

)

–900 –1000 –1100 –1200 –1300 –1400–800E (mV)

(b)

Figure 3:�e stripping voltammetry lines were repeated: (a) according to the SqW-AdSVmethod (n� 7) with CCr(VI) � 1 ppb; (b) accordingto the DP-AdSV method (n� 9) with CCr(VI) � 2 ppb. Other experimental conditions are as shown in Table S2.


Cr(VI) + (III) (∗∗). It follows that the Cr(III) content isthe difference of (∗∗) and (∗).

Based on the above results, it is possible to apply theSqW-AdSV/in situ BiFE to determine the trace of Cr(VI)

with LOD≈ 0.1 ppb. With that LOD, the SqW-AdSV/in situBiFE can directly determine the amount of Cr(VI) in naturalwater, without the stage of getting rich, and this is a greatadvantage of the SqW-AdSV/BiFE method.

Ip = 44,50 + 23,19 [Cr(VI)]R = 0,9994

30

40

50

60

70

80

90

100I p

(μA

)

0.4 0.8 1.2 1.6 20CCr(VI) (ppb)

(a)

1.8 ppb1.6 ppb1.4 ppb1.2 ppb1.0 ppb0.8 ppb0.6 ppb0.4 ppb0.2 ppb

nen

Cr

–50.0u

–75.0u

–100u

–125u

–150u

–175u

I (A

)

–900 –1000 –1100 –1200 –1300 –1400–800E (mv)

(b)

0

2

4

6

8

10

12

I P (μ

A)

2 4 6 8 10 12 14 16 18 20 220CCr(VI) (ppb)

(c)

Ip = 0,89 + 0,68 [Cr(VI)]R = 0,9989

0

2

4

6

8

10

12

I p (μ

A)

2 4 6 8 10 12 140CCr(VI) (ppb)

(d)

Figure 4: (a) Linear regression line for the SqW-AdSV method; (b) stripping voltammetry of SqW-AdSV method: the bottom line is thebaseline, followed by nine additional standard lines, each adding 0.2 ppb; (c) relation between Ip andCCr(VI) when examining the linear rangeof the DP-AdSV method; (d) linear regression line for the DP-AdSV method. Experimental conditions are as shown in Table S2.

Table 3: Appropriate experimental conditions for the SqW-AdSV/in situ BiFE for the determination of Cr(VI).

No. Parameter (unit of measure) Symbol SqW-AdSV/in situ BiE1 DTPA concentration (M) CDTPA 0.4. 10−3

2 Concentration of acetate buffer (pH� 6) (M) CAc 0.403 NaNO3 concentration (M) CNaNO3 0.404 KBr concentration (M) CKBr 5.10−6

5 Bi(III) concentration (M) CBi(III) 28.8. 10−5

6 Cleaning potential (mV) Eclean 3007 Cleaning time (s) tclean 1008 Rotating speed of working electrode (rpm) (ω) 2000 20009 Deposition potential (mV) Edep -80010 Deposition time (s) tdep 20011 Equilibration time (s) tequal 5012 Potential sweep range (mV) Erange −800÷−1450

13

Technical parameters• Amplitude (mV) ∆E 30

• Voltage step (mV) Ustep 6• Sweep rate (mV/s) v 210

• Frequency (Hz) f 35


In order to answer the question of whether the analyticalmethod used to analyze the amount of chromium in naturalwater samples can be applied, we have conducted experi-ments to verify the correctness (through the CertifiedReference Material (CRM)) and an analysis of some naturalwater samples. On the basis of the experiments mentionedabove, the analysis process of Cr(VI) and total Cr(III, VI) innatural water by the SqW-AdSV method was proposed.

3.5.1. Quality Control of Analytical Methods through Stan-dard Sample Analysis. In order to confirm the practicalapplicability of the SqW-AdSV method to analyze chro-mium traces using BiFE electrodes, it is necessary to controlthe analytical method quality by evaluating the accuracy andrepeatability when analyzing standard samples.

(1) For Surface Water Samples. Surface water CertifiedMaterial Reference (SPS-SW1 Batch 122) was selected toevaluate the accuracy of the method. �e actual value of thechromium content of the sample is 2.00± 0.02 ppb (95%confidence boundary e�± 0.02 ppb). Analysis of standardSPS-SW1 surface water (CRM) samples by the SqW-AdSVusing in situ BiFE with the appropriate experimental con-ditions is shown in Table 3. �e analysis was repeated 3times. �e volume of the solution to be charged to theelectrolyser is 2mL, and the volume of solution in theelectrolyser is 10mL.

�e results in Table 4 show that the SqW-AdSV/in situBiFE has good repeatability (the standard deviation is 4% for

the repetition of 3, and it is less than half the standarddeviation based on the Horwitz function (RSDH � 2(1−0,5lgC);when the chromium concentration is 2 ppb, the standarddeviation of the Horwitz equation is 41%) [49] and themethod has good accuracy because the chromium content iswithin the 95% confidence interval of the CRM sample.

(2) For Seawater Samples. Analysis of the standard seawaterCRM coded NASS 6 by the SqW-AdSV/in situ BiFE with theappropriate experimental conditions as shown in Table 3.

Because the concentration of chromium in NASS 6seawater was too small to be directly analyzed, only NASS 6standard sample was used as the matrix for analysis andvalidity. �e actual value of the chromium content in theNASS 6 sample is 0.116± 0.008 ppb (the 95% confidencebound e�± 0.008 ppb). NAAS 6 standard sample was addedwith standard Cr(VI) solution to attain 3 levels of 2 ppb,6 ppb, and 10 ppb and then analyzed with the standardadded samples to determine recovery.

�e results showed that the SqW-AdSV/in situ BiFE forchromium analysis in seawater samples has a good accuracy(recoverability from 94 to 109%). According to the AOAC(American Association of Analytical Chemists) when ana-lyzing the levels of 1.0 to 10 ppb, achieving a recovery rate of80 to 110% is acceptable [50]. �erefore, it is possible to usethis method to analyze chromium in seawater samples(Table 5).

�e results of the linear range, sensitivity, limit ofdetection, and accuracy showed that it is possible to use

Table 4: Accuracy of the SqW-AdSV/in situ BiFE for the determination of chromium in surface water.

Information [Cr(VI)](ppb) CCr (ppb)

Experiment1 0.38 1.902 0.40 2.003 0.40 2.00

Average± S (ppb) 1.97± 0.08Cr content in the CRM sample (ppb) 2.00± 0.02 (CCr � 1.98÷ 2.02 ppb)RSD (%), n� 3 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 � 2ml), V2: volume of solution in the electrolyser(V2 �10mL), S is the standard deviation. Experimental conditions are as shown in Table 3.

Table 5: Determination of the accuracy of the SqW-AdSV/in situ BiFE on the NASS 6a.

[NAAS6]

�e content of chromium in thesample (ppb) ×1

Chromium standard added(ppb)×o

�e content of chromium in standard addedsamples (ppb) ×2

Recovery(%)

2 ppb 0.116 1.8841.915 962.043 1021.941 97

Average± S 1.966± 0.054

6 ppb 0.116 5.8845.958 996.258 1045,655 94

Average± S 5.957± 0.213

10 ppb 0.116 9.88410.687 10710.125 10110.887 109

Average± S 10.566± 0.279aRecovery� (x2−x1).·100/x0; S is the standard deviation; experimental conditions are as shown in Table 3.


SqW-AdSV/in situ BiFE to determine chromium in surfacewater and seawater.

3.5.2. Real Sample Analysis. For the purpose of testing thepossibility of applying the SqW-AdSV/in situ BiFE method forthe analysis of chromium in water environment, well water, tapwater, lagoon water, and seawater in some different areas in�ua �ien Hue province were taken for analysis.

Water samples were taken in clean PET bottles and acidifiedwith concentrated HCl (500μl HCl/500ml of sample). Sampleswere filtered through 0.45μm porous fiberglass filter paper andanalyzed immediately after filtration.

Collected and stored samples were analyzed directly(after filtration through a 0.45 μm porous fiberglass filterpaper) to determine the Cr(VI) content by the SqW-AdSV/in situ BiFE; the total chromium content of the sample wasdetermined after the decomposition of the sample by themethod of (a) ((a): add 50 μl of concentrated HCl, 25 μl ofH2O2 35% to 50ml of sample in the Teflon cup, boil for 90minutes, let it cool, and adjust up to 25ml) [51].

Samples were analyzed by SqW-AdSV/in situ BiFEaccording to the process shown in Figure S2. �e results ofactual sample analysis are presented in Table 6.

4. Conclusion

Using in situ BiFE electrodes with DTPA complexing ligands inacetate buffer solution pH 6 with the presence of KBr and NO−3ion, DP-AdSV and SqW-AdSV methods can determinechromium(VI) concentrations of 0.3 ppb and 2.0 ppb, respec-tively. �e proposed method has been successfully applied forchromium analysis in some natural water samples such aslagoon water, well water, tap water, and saltwater in some areas

of �ua�ien Hue province, Vietnam. �is CCr(VI+III). analysisprocedure in water sample by SqW-AdSV/in situ BiFE methodwas satisfactorily applied for the determination of chromium inreal water such as tap water, river water, and well water samplesin all countries of the world.�e determination of chromium inthe above real water sample could be carried out within 60min.

Data Availability

�e data used to support the findings of this study areavailable from the corresponding author upon request.

Conflicts of Interest

�e authors declare that they have no conflicts of interest.

Supplementary Materials

Table S1: for the DP-AdSV and SqW-AdSV using in situBiFE. Table S2: suitable experimental conditions for DP-AdSV and SqW-AdSV methods using in situ BiFE. Table S3:influence of chromium(III). Figure S1: SqW-AdSV/BiFE insitu stripping voltammograms of chromium(VI) when ex-amining the effects of chromium(III). Table S4: influence ofFe(III) and Ca(II). Table S5: influence of Cl− and SO42−.Table S6: influence of Triton X-100. Figure S2: diagram ofCCr(VI + III) analysis procedure in water sample by SqW-AdSV/in situ BiFE method. . (Supplementary Materials)

References

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Table 6: Chromium content in lagoon water samples, tap water, well water, and saltwater.

No Sample type Sample symbolChromium concentration

(Cmean± ε) ppb, n� 3, P� 0.95CCr(VI + III) CCr(VI)

1

Water sample of Cau Hai Lagoon

M1 13.8± 0.2 1.0± 0.12 M2 19.0± 1.0 1.0± 0.23 M3 7.3± 0.4 1.5± 0.24 M4 26.1± 5.8 1.6± 0.35 M5 14.1± 0.8 1.3± 0.76 M6 1.0± 0.1 0.7± 0.27 M7 11.1± 4.4 0.8± 0.38

Tap waterPTN 20.0± 2.3

9 GÐ 19.2± 3.310 GÐ1 18.1± 0.811

Well water

G1 28.6± 1.012 G1′ 24.6± 4.013 G2 22.3± 4.214 G2′ 12.5± 4.815 G3 6.4± 0.616 G3′ 14.4± 0.517 G4 13.6± 3.318 G4′ 21.2± 4.419 Saline water B1 1.3± 0.3 1.0± 0.220 B2 16.1± 1.3 12.1± 1.4


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