AbstractA simple, accurate and fast (180injectionsh−1) batch injection analysis (BIA) system with multiple-pulse amperometric detection has been developed for selective determination of ethanol in gasohol and fuel ethanol. A sample aliquot (100μL) was directly injected onto a gold electrode immersed in 0.5molL−1 NaOH solution (unique reagent). The proposed BIA method requires minimal sample manipulation and can be easily used for on-site analysis. The results obtained with the BIA method were compared to those obtained by gas-chromatography and similar results were obtained (at 95% of confidence level)

Marra, Mariana C.

Munoz, Rodrigo A.A.

Pereira, Polyana F.

Richter, Eduardo M.

Elsevier - Publisher Connector 

Fast batch injection analysis system for on-site determination of ethanol in gasohol and fuel ethanol 

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dTalanta 90 (2012) 99– 102
Contents lists available at SciVerse ScienceDirect
Talanta
j ourna l ho me  page: www.elsev ier .com/ locate / ta lanta
ast  batch  injection  analysis  system  for  on-site  determination  of  ethanol  in
asohol  and  fuel  ethanol
olyana  F.  Pereira, Mariana  C.  Marra,  Rodrigo  A.A.  Munoz,  Eduardo  M.  Richter ∗
nstituto de Química – Universidade Federal de Uberlândia, Av. João Naves de Ávila, 2121 Uberlândia, MG, Brazil
 r  t  i  c  l  e  i  n  f  o
rticle history:
eceived 17 November 2011
eceived in revised form
0 December 2011
ccepted 3 January 2012
vailable online 8 January 2012
a  b  s  t  r  a  c  t
A  simple,  accurate  and  fast (180  injections  h−1) batch  injection  analysis  (BIA)  system  with  multiple-pulse
amperometric  detection  has been  developed  for selective  determination  of  ethanol  in gasohol  and  fuel
ethanol.  A  sample  aliquot  (100  L)  was  directly  injected  onto  a gold  electrode  immersed  in 0.5 mol  L−1
NaOH  solution  (unique  reagent).  The  proposed  BIA method  requires  minimal  sample  manipulation  and
can be  easily  used  for on-site  analysis.  The  results  obtained  with  the  BIA  method  were  compared  to  those
obtained  by gas-chromatography  and  similar  results  were  obtained  (at  95%  of  conﬁdence  level).eywords:
atch injection analysis (BIA)
n-site analysis
uel ethanol
asohol
Published by Elsevier B.V. Open access under the Elsevier OA license.uel adulteration
. Introduction
Ethanol (bioethanol) has been an alternative energy source
idely produced and used in Brazil since 1975, when the Brazilian
overnment stimulated its production (fermentation of sugarcane)
nd its consumption as a fuel for automotive vehicles in order
o become more independent on petroleum-based fuels [1,2]. The
razilian fuel ethanol is used either in the hydrated form (94%, v/v
thanol) as a car fuel or in the anhydrous form, mixed with gasoline
nd then resulting in gasohol [1–3]. Ethanol acts as an antiknock
gent for gasoline and can be considered an environmental-friendly
ubstitute for alkyllead and manganese additives, highly toxic
nd environmental pollutants [3,4]. The National Brazilian Agency
ANP), responsible for the control of commercialization and of
roduction of biofuels and petroleum-based fuels, established the
pper limit of 25% (v/v) for ethanol in gasohol. Higher concen-
rations of ethanol can damage rubber seals and diaphragms of
asoline engines [3].  Besides the limited content of ethanol, gaso-
ol may  be adulterated by the addition of diesel oil, kerosene and
etrochemical rafﬁnates, due to the low cost of these solvents in
omparison to gasoline [5–7]. Methanol is not a common adulter-
nt found in gasohol but it is added in the Brazilian fuel ethanol,
nce again due to its low cost and to their similar physical-chemical
roperties [8,9]. Additionally, methanol is not detected by the ABNT
∗ Corresponding author. Tel.: +55 34 3239 4143x206; fax: +55 34 3239 4208.
E-mail address: emrichter@iqufu.ufu.br (E.M. Richter).
039-9140  Published by Elsevier B.V.
oi:10.1016/j.talanta.2012.01.004
Open access under the Elsevier OA license.(Associac¸ ão Brasileira de Normas Técnicas – Brazilian Association of
Technical Standards) analytical method, which is based on the vol-
ume  increase of the aqueous phase after the extraction with water
[10].
Gas chromatography (GC) is the analytical technique recom-
mended by the American Society for Testing and Materials (ASTM
D 4815-03) for the determination of volatile ethers and alco-
hols (including ethanol) in gasoline [11]. This technique can also
be applied for the analysis of fuel ethanol [9].  Data from dis-
tillation curve proﬁles [5],  from Fourier transform near infrared
spectroscopy [7–9] and from gas chromatography–mass spectrom-
etry [6] were associated to chemometric data treatment in order
identify adulteration of gasoline and fuel ethanol. A simple spec-
trophotometric method was described for the determination of
ethanol in gasohol after aqueous extraction [12]. Electrochemical
detectors have been proposed for the determination of ethanol in
gasoline using contactless impedance [13,14] and amperometry
[3]. The amperometric sensor for ethanol was  based on its elec-
trocatalytic oxidation at copper electrodes in alkaline media [3,15].
However, the sensor was  not free of interference from methanol
which was  also oxidized at the electrode [3].
Gold electrodes have been described to present electrocatalytic
properties toward oxidation of ethanol and other short-chain alco-
hols in alkaline media [16–20].  However, the oxidation process is
accompanied by passivation of the electrode surface due to the
adsorption of oxidation products and, consequently, the current
signal does not follow a linear behavior with increasing concentra-
tions of ethanol [19]. In order to obtain a constant clean and reactive
1 alanta 90 (2012) 99– 102
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c00 P.F. Pereira et al. / T
old surface and to avoid electrode passivation, a triple-pulse
mperometric detection technique has been proposed for accurate
thanol determination [19]. Similarly, multiple-pulse amperomet-
ic (MPA) detection was required for the accurate determination of
ert-butylhydroquinone (phenolic antioxidant) in biodiesel sam-
les at a bare glassy-carbon electrode without any electrode
assivation [21]. MPA  was  also applied to introduce an internal
tandard [22] in ﬂow injection analysis (FIA) and for simultaneous
eterminations in FIA [23–27] and batch injection analysis (BIA)
ystems [28]. Batch injection analysis (BIA) is a non-ﬂow injec-
ion technique introduced by Wang and Taha [29]. This approach
enders several desirable characteristics, such as the need of small
ample volumes (typically 1–100 L) and the possibility of high
peed and easy to use analysis. Also, it can be easily used in on-site
nalysis, besides its low cost [29–31].
In this work, we propose the determination of ethanol in gasohol
nd fuel ethanol samples using BIA with MPA  detection at a gold
lectrode under basic media. This method is free of interference
rom methanol as well as from other solvents that can be used as
dulterant in gasohol and fuel ethanol.
. Experimental
.1. Reagents and samples
All solutions were prepared with deionized water (Millipore
irect-Q3) with a resistivity no less than 18 M-cm. All reagents
ere of analytical grade and used without further puriﬁcation.
odium hydroxide was obtained from Dinâmica (Diadema, Brazil)
nd ethanol and methanol from Synth (Diadema, Brazil). Fuel
thanol samples were analyzed after appropriate dilution in
.5 mol  L−1 NaOH. Gasohol samples could be analyzed after sim-
le manual liquid/liquid extraction after manual agitation (10 mL
f each—1:1 water/sample) in a closed small ﬂask and subsequent
ilution of an aliquot from the aqueous solution containing ethanol
n 0.5 mol  L−1 NaOH. The extraction procedure is similar to the one
sed by the Brazilian Association of Technical Standards for the
etermination of anhydrous ethanol content in gasohol samples
10].
.2. Instruments and apparatus
Electrochemical measurements were performed with a three-
lectrode BIA cell system employing a -Autolab Type III
otentiostat (Metrohm Autolab B.V.). A mini Ag/AgCl/KClsat [32],
latinum and gold (∅  = 3 mm;  Metrohm) were employed as refer-
nce, auxiliary and working electrodes, respectively.
BIA measurements were carried out using a cylindrical acrylic
ell (∅I = 7 cm;  height = 5 cm;  volume≈200 mL)  similar to that pre-
iously described [33]. Fig. 1 presents a schematic diagram of the
atch injection cell used in the study.
A micro DC-motor (3–24 V; used in hair dryer or electric toys)
as used for solution stirring. Only, a Teﬂon rod was  adapted on
he motor shaft. Therefore, a common battery could be used as
 power supply of the micro DC-motor and the proposed stirring
rrangement could be easily employed in outside analysis, as pro-
osed in this work. The stirring rate could be easily changed by
arying the battery voltage. All studies were performed at a con-
tant stirring rate of 280 ± 10 rpm (with the application of 4.5 V).
any micro DC-motors commercially available can be used for this
urpose.Injections solutions were performed with a motorized elec-
ronic micropipette (Eppendorf Multipette® stream) with a
onstant distance from the working electrode to Multipette®
ombitip® (≈2 mm),  as recommended in a previous work [30].Fig. 1. Schematic diagram of the batch injection cell.
Results for the ethanol determination in gasohol and fuel
ethanol were compared to those obtained by using gas
chromatography (GC). A Shimadzu GC-2014 chromatographer
with a ﬂame ionization detector (FID) and carbowax column
(30 m × 0.25 mm × 0.25 m)  was used.
3. Results and discussion
Fig. 2 shows cyclic voltammograms obtained at a gold electrode
in 0.5 mol  L−1 NaOH before (—) and after addition of ethanol (- - -)
or methanol (· · ··  · ·).
Under this condition, only ethanol is oxidized to the correspond-
ing aldehyde at about 0.18 V, which is in agreement with previous
report [19]. These results clearly demonstrate that this system (gold
electrode in alkaline medium) can be used for selective determina-
tion of ethanol in the presence of methanol.
Fig. 3 presents the results obtained for successive injections
(n = 10) of a solution containing 0.1% (a) and 0.5% (b) of ethanol
using the BIA system with amperometric detection.
When the conventional amperometry (Fig. 3A) was employed
(constant potential at 0.18 V vs Ag/AgCl), the response decreases
run-to-run for a solution containing 0.1% of ethanol (a) and
decreases dramatically for a solution containing 0.5% of ethanol
(b). As previously described [19], the ethanol oxidation product
(the corresponding aldehyde) strongly adsorbs on the gold sur-
face and its accumulation leads to the loss of electrode activity.
P.F. Pereira et al. / Talanta 90 (2012) 99– 102 101
Fig. 2. Cyclic voltammograms of gold electrode in NaOH 0.5 mol  L−1 before (—) and
after  addition of 0.3% (v/v) of ethanol (- - -) or methanol (· · ··  · ·). Scan rate = 50 mV s−1.
Step  potential = 5 mV.
Fig. 3. Comparison between the results obtained by BIA with (A) conventional
amperometric and (B) pulsed amperometric detection for successive injections
o
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s
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t
(f  solutions containing (a) 0.1 and (b) 0.5% of ethanol. Applied potentials: (A)
nly 0.18 V; (B) +0.18 V/100 ms,  +0.70 V/300 ms  and −0.25 V/300 ms;  electrolyte:
.5 mol L−1 NaOH; ﬂow rate: 4 mL  min−1; injection volume: 100 L.
owever, if pulsed amperometry is used (Fig. 3B), a second poten-
ial pulse (0.7 V) can also periodically be applied and the adsorbed
pecies are oxidized to carbon dioxide, simultaneously with the
ormation of gold oxide. In addition, a third potential pulse (nega-
ive potential region) can also be applied to reduce the gold oxide
cleaning procedure). Then, further experiments were carried outFig. 4. Comparison between the results obtained by BIA with pulsed amperometric
detection with and without the solution stirring. Injection solution: 0.5% ethanol;
other conditions see Fig. 3.
employing pulsed amperometry with the application of three
sequential potential pulses using gold as the working electrode
and NaOH solution as the electrolyte: (+0.18 V/100 ms)  oxidation
of ethanol and adsorption of the oxidation product at the electrode
surface; (+0.70 V/300 ms)  oxidation of the adsorbed species (clean-
ing procedure) and formation of gold oxide; (−0.25 V/300 ms)
removal of the oxide layer (reactivation procedure). After the
implementation of the triple-pulse waveform, the system started
to show high reproducibility on peak currents (Fig. 3B), with an
RSD value of 1.6% and 0.6% for solutions containing 0.1 and 0.5% of
ethanol, respectively.
BIA experiments can be carried out with and without solution
stirring. However, if pulsed amperometry detection and gold work-
ing electrode in alkaline medium are used, the peak current is
considerably inﬂuenced by the solution with and without stirring
which can be clearly observed in the results presented in Fig. 4.
Under solution stirring, the peak current quickly returns to
baseline (fast current decrease) and one injection can be car-
ried out every 20 s (180 injections h−1). However, if the solution is
maintained without stirring, the current slowly decreases and one
injection can be carried out each 120 s (30 injections h−1). Prob-
ably, this occurs because the periodical cleaning and activation
procedure are most effective if the solution is stirred constantly.
In this case, the products generated during the cleaning step are
quickly removed from the working electrode surface. It is impor-
tant to emphasize that the stirring procedure proposed in this work
(using a micro DC-motor) can be easily used in outside analysis. The
precision of the BIA system is slightly better under stirred solution
than under static solution (RSD = 0.4 and 1.4%, respectively; n = 5).
Fig. 5 presents the amperogram obtained for injections of 100 L
of standard solutions (in triplicate) containing increasing concen-
trations of ethanol (a–e: 0.1–0.5%) and fuel ethanol (A–C) and
gasohol (D–F) samples adequately diluted. The sample E is gasoline
adulterated with methanol.
The calibration curve showed good linearity in the investigated
concentration range with the following calibration equation:
I (A) = −3.632 + 96.964 c (%) r = 0.998
The limits of detection and quantiﬁcation for ethanol were found
to be 1.1 ×10−3 and 3.63 × 10−3%, respectively. The sampling fre-
quency was calculated at approximately 180 and 30 injections h−1with and without stirring, respectively. The proposed BIA method
was used to determine ethanol in three fuel ethanol samples, one
gasohol sample (containing 20–25% ethanol, according to Brazil-
ian legislation) and two  gasoline samples after the addition of
102 P.F. Pereira et al. / Talanta
Fig. 5. BIA-MPA amperogram obtained by the injections of ﬁve ethanol standard
solutions (a–e: 0.1–0.5%), fuel ethanol (A–C) and gasohol (D–F) samples diluted
appropriately in 0.5 mol  L−1 NaOH; other conditions see Fig. 3.
Table 1
Comparison of results obtained for ethanol determination using BIA versus GC meth-
ods (n = 3).
Samples Added BIA (%) GC (%)
Fuel ethanol – 89 ± 1 91 ± 2
Fuel  ethanol 20% H2O 65 ± 2 59 ± 10
Fuel  ethanol 20% MetOH 67 ± 1 69 ± 2
Gasohol – 22 ± 1 23 ± 6
Gasoline 30% MetOH <LD <LD
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[Gasoline 15% EtOH + 15% MetOH 16 ± 2 13 ± 3
etOH: methanol; EtOH: ethanol.
ethanol or ethanol. The results obtained through the proposed
IA method were also compared to those obtained by the GC
ethod. Table 1 shows the results for the analysis of these samples
ith their respective standard deviations (n = 3).
No signiﬁcant differences between the two methods (BIA
nd GC) were observed, which indicates the absence of sys-
ematic errors. The statistical t-test at 95% conﬁdence intervals
hows reasonable agreement between the results for ethanol
oncentrations obtained by the proposed BIA and GC methods
tcalculated < ttable = 2.78; n = 3).
. Conclusions
In this work, we report for the ﬁrst time the determination
f ethanol by BIA with multiple pulse amperometric detection
sing gold as working electrode and 0.5 mol  L−1 NaOH as unique
eagent. The proposed BIA method is simple, accurate, rapid
180 injections h−1) and can be easily used in outside (on-site)
nalysis. The sample manipulation requires only a simply dilution
fuel ethanol) or a manual extraction and further dilution (gasohol)
hich can also be easily performed outside of the laboratory. The
roposed electrochemical method detects only ethanol, what can
[
[
[
[ 90 (2012) 99– 102
be considered an advantage in relation to previous published stud-
ies [3,13,14] and the ABNT NBR 13992:1997 method [10] in which
methanol (more toxic alcohol) is considered as a potential chemical
interfering molecule.
Acknowledgements
The authors are grateful to CNPq (476269/2010-5 and
305227/2010-6), FAPEMIG (CEX-APQ-02276-09) and CAPES for
ﬁnancial support.
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Fast batch injection analysis system for on-site determination of ethanol in gasohol and fuel ethanol

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