Electrochemical Detection of Trichloroethylene with an Electrodeposited Pb-Modified Electrode

Article abstract of DOI:10.1149/1.1600466

The amperometric sensing of trichloroethylene (TCE) in the organic phase by using modified Pb electrode was described. The influence of pretreatment, electrodeposition current density, electrodeposition time, and electrodeposition temperature for preparat

Full text of DOI:10.1149/1.1600466

H214
Journal of The Electrochemical Society, 150 ͑9͒ H214-H219 ͑2003͒
0013-4651/2003/150͑9͒/H214/6/$7.00 © The Electrochemical Society, Inc.
Electrochemical Detection of Trichloroethylene with an
Electrodeposited Pb-Modified Electrode
,z
*
Min-Hua Chen and Tse-Chuan Chou
Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 701
The amperometric sensing of trichloroethylene ͑TCE͒ in the organic phase by using modified Pb electrode was described. The
influence of pretreatment, electrodeposition current density, electrodeposition time, and electrodeposition temperature for prepa-
ration of the working electrode were discussed. The optimal conditions for preparation of modified Pb electrode were obtained as
0.1 M HNO3 pretreatment, 20 mA/cm2 electrodeposition current density, and 30°C electrodeposition temperature. Additionally, the
optimal sensing conditions such as Ϫ2.10 V sensing potential ͓vs. Ag/Agϩ with 0.1 M tetrabutylammonium perchlorate in
acetonitrile ͑AN͒ solution͔, 155 rpm agitation rate, and at room temperature with 0.01 M tetrabutylammonium tetrafluoroborate
electrolyte concentration in AN solution were obtained in this system. Under the optimal sensing conditions, the results indicated
that the response time was 20 s ͑90% response time͒ and the correlation of sensing response current, id , and TCE concentration,
CL , is id ϭ 1.060 CL in the range from 100 to 700 ppm. Furthermore, the rate constant of TCE mass transfer, k, was also obtained
to be 7.217 ϫ 10Ϫ4/cm s.
© 2003 The Electrochemical Society. ͓DOI: 10.1149/1.1600466͔ All rights reserved.
Manuscript submitted December 9, 2002; revised manuscript received March 27, 2003. Available electronically July 30, 2003.
Trichloroethylene ͑TCE͒ is a generally used chemical material in
nium perchlorate ͑TBAP͒ are all GC grades of TCI ͑Japan͒. The
acetonitrile ͑AN͒ and TCE are high performance liquid chromatog-
raphy and gas chromatography grades of Tedia ͑USA͒ and Aldrich
͑USA͒, respectively. All aqueous solutions were prepared with
deionized ͑DI͒ water purified to a resistivity of a least 18.3 M⍀ cm
by a water purification system ͑Milli-RO 60, Japan͒. Additionally,
the surface structure of the prepared electrode was characterized
with scanning electron microscopy ͑SEM, FE-S 4200, Hitachi, Ja-
pan͒, X-ray diffraction ͑XRD, D/max 3. V, Rigaku, Japan͒, and elec-
tron spectroscopy for chemical analysis ͑ESCA, VG/ESCA 210,
U.K.͒.
industries such as electronic manufacturing,1 plastics production,2
textile process3 and organic synthesis.4 At the same time, TCE is
also an organic chemical that has been used in dry cleaning, for
metal degreasing, and as a solvent for oils and resins.5-7 Besides, the
waste solution is also a contaminant in the soil or groundwater
around environment.8 Furthermore, TCE was confirmed to be a car-
cinogenic material in many medical investigations.9-11 Therefore, to
detect and monitor TCE concentration in order to protect people
health is very important in the working environment.
At present, the monitoring of TCE relies principally on gas chro-
matography, which provides good sensitivity and selectivity.12 How-
ever, this method is not well suited for use as a simple, portable
measuring devices.13 Besides, other analytical techniques such as
electrochemical method,14,15 spectroscopic method,16,17 photocata-
lytic oxidation method,18,19 and biochemical method20-22 have been
applied to detect the concentration of TCE. However, all these meth-
ods had some disadvantages, for example, complicated operation,
high cost, poor stability, and long measurement time. Furthermore,
all these mentioned analytical instruments cannot, minimization in
production and in situ, apply in the field for automatic monitoring of
TCE concentration.
Sensors may be the most practical monitor setup because of their
convenience, accuracy, easy minimization, and automation-in situ
measurement. Several TCE sensors have been developed such as
fiber-optic-based TCE sensor,23 microbial TCE sensor,24 and flow
injection TCE sensor.25 However, these sensors may have some dis-
advantages also, for example, unstable, expensive and long response
time. Therefore, it is necessary to develop a new TCE sensor with
good performance, simple structure, and lower cost.
Preparation of working electrode.—The Pb foil (6 ϫ 1 ϫ 0.01
cm, Alfa Aesar, USA͒ attached to an alumina plate (5 ϫ 1.5
ϫ 0.1 cm, Lei Ke, Taiwan͒, then the sensing area 1 cm2 was con-
trolled by Teflon tape. The working electrode was ultrasonically
cleaned in 0.1 M HNO3 aqueous solution, then washing with DI
water thoroughly. After, the Pb foil substrate was immersed in elec-
trodeposited Pb aqueous solution containing 0.63 M Pb͑BF4)2 , 0.68
M HBF4 , 0.43 M H3BO3 , and 0.0006 M PEG 300. Then, the elec-
trodeposition of Pb for preparation of the working electrode was
carried out at 30 rpm agitation rate. Electrodeposition current den-
sity, time, and temperature were adjusted, respectively, to control the
electrodeposition process.
Sensing procedures.—All electrochemical measurements were
carried out in a conventional three-electrode electrochemical cell
with a 263A electrochemical analysis system ͑EG&G, USA͒. The
modified Pb electrode was used as working electrode. A platinum
plate and Ag/Agϩ ͑with 0.1 M TBAP in AN solution͒ were used as
auxiliary and reference electrodes, respectively. After, the sensing
potential was set in the limiting current region. Then, the steady
state amperometric signals were obtained, i.e., the background cur-
rent. Immediately, a desired concentration of TCE was added into
the sensing system, then the amperometric current of the working
electrode was also attained, i.e., the measured current. The response
current is equal to the measured current minus the background cur-
rent.
In this work, an electrodeposited Pb-modified electrode was used
to sense TCE because this configuration has been shown to give
good sensitivities.26 Furthermore, the characteristics of this devel-
oped TCE sensor such as amperometric response current and re-
sponse time were also explored.
Experimental
Chemicals and instrumentation.—The electrodeposition solution
components of Pb were purchased from several sources: lead͑II͒
tetrafluoroborate Pb͑BF ) , Alfa Aesar, USA͔, tetrafluoroboric acid
͓
4
2
Analysis and identification of products.—The GC-mass analysis
was performed on a Hewlett-Packard 5890 gas chromatograph ͑GC͒
connected to an ion trap detector, for separating the volatile material
and then into a VG 70–250S mass spectrometer. The GC was
equipped with a 30 m Rtx-5 capillary column. The column tempera-
ture was held at an initial temperature of 100°C for 2 min and then
increased at 6°C/min to 190°C where it was held for 20 min. High
(HBF4 , Rieded-de Haen, Germany͒, boric acid (H3BO3 , Alfa Ae-
sar, USA͒, and polyethylene glycol ͑PEG 300, Hayashi, Japan͒. The
tetrabutylammonium tetrafluoroborate ͑TBAT͒ and tetrabutylammo-
* Electrochemical Society Active Member.
z E-mail: tcchou@mail.ncku.edu.tw
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Journal of The Electrochemical Society, 150 ͑9͒ H214-H219 ͑2003͒
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Figure 1. The SEM morphologies of Pb surface at different pretreatments
for preparation of working electrode. ͑A͒ without treatment by 0.1 M HNO3
aqueous solution ͑B͒ with treatment by 0.1 M HNO3 aqueous solution. Pre-
treating conditions: 0.1 M pretreating HNO3 aqueous solution concentration
and 1 h pretreating time.
Figure 2. ͑A͒ The current/potential curves at different concentration of TCE
by using an electrodeposited Pb modified electrode. Pretreating conditions:
same as Fig. 1. Electrodeposition conditions: 0.63 M Pb͑BF4)2 , 0.68 M
HBF4 , 0.43 M H3BO3 , and 0.0006 M poly͑ethylene glycol͒ 300 aqueous
solution, 20 mA/cm2 deposition current density, and 30 rpm agitation rate, at
30°C and 2 h. Sensing conditions: 0.01 M TBAT in AN solution, Ag/Agϩ
͑with 0.1 M TBAP in AN solution͒ as reference electrode, 155 rpm agitation
rate, at room temperature and 1 cm2 working electrode surface area. ͑B͒ The
current/potential curve of 500 ppm TCE reduction by using an electrodepos-
ited Pb modified electrode. Pretreating conditions: same as Fig. 1. Elec-
trodeposition conditions: same as Fig. 2A. Sensing conditions: same as
Fig. 2A.
purity helium ͑99.99%͒ was used as carrier gas at a flow rate of 36
mL/min. Electron impact mass spectroscopy was used at the ioniza-
tion energy of 70 eV.
Results and Discussion
Surface morphology of the electrode.—Comparison of the pre-
treatment time on the electrode surface morphologies is shown in
Fig. 1. In Fig. 1A, the electrode without treatment by a 0.1 M HNO3
aqueous solution, the smooth surface of electrode was obtained. On
the other hand, in the Fig. 1B, the Pb electrode with treatment by a
0.1 M HNO3 aqueous solution, the electrode surface showed large
roughness more than that without treatment by a 0.1 M HNO3 aque-
ous solution. These data clearly indicated that increasing the size of
the voids increased the amount of deposited Pb. Consequently, the 1
h pretreatment time by a 0.1 M HNO3 aqueous solution was chosen
as the pretreatment condition for preparation of the working
electrode.
Moreover, the limiting current region at the concentration of 500
ppm TCE was shown in Fig. 2B. In Fig. 2B, the response current is
equal to the measured current ͑500 ppm͒ minus the background
current ͑0 ppm TCE͒. The result showed that increasing the driving
force from Ϫ1.98 to Ϫ2.06 V ͑vs. Ag/Agϩ with 0.1 M TBAP in AN
solution͒ increased the current of reduction. The result showed that
the response current was controlled by both kinetic and mass-
transfer mechanisms in this potential range. Further increasing the
potential from Ϫ2.06 to Ϫ2.14 V ͑vs. Ag/Agϩ with 0.1 M TBAP in
AN solution͒, the current reached a constant, thus the reaction was
only controlled by mass transfer in that potential range. And then,
the current rapidly increased when the potential increased from
Ϫ2.14 to Ϫ2.28 V ͑vs. Ag/Agϩ with 0.1 M TBAP in AN solution͒;
the response current increased due to the decomposition of electro-
lyte. In Fig. 2B, the main reduction of TCE took place between the
potential from Ϫ2.06 to Ϫ2.14 V ͑vs. Ag/Agϩ with 0.1 M in AN
Determination of the amperometric potential window.—The
current/potential curves at different concentrations of TCE by using
electrodeposited Pb-modified electrode were shown in Fig. 2A. The
results show that increasing the concentration of TCE from 0 to 500
ppm, increased the measured current for TCE reduction. This is
understandable, because in the electrochemical reaction, the reduc-
tion current increased with the increase of the TCE concentration.
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Journal of The Electrochemical Society, 150 ͑9͒ H214-H219 ͑2003͒
Figure 3. Effect of pretreatment time on the sensitivity of TCE sensor. Pre-
treatment condition: 0.1 M HNO3 aqueous solution. Electrodeposition con-
ditions: same as Fig. 2A. Sensing conditions: 0.01 M TBAT in AN solution,
Ϫ2.10 V ͑vs. Ag/Agϩ with 0.1 M TBAP in AN solution͒, 155 rpm agitation
rate, at room temperature and 1 cm2 working electrode surface area.
solution͒. Because Ϫ2.10 V ͑vs. Ag/Agϩ with 0.1 TBAP in AN
solution͒ is within this constant current region, its sensing reaction is
mass-transfer controlled. Conclusively, the reasonable and suitable
applied potential for TCE sensing in this system was chosen to be
Ϫ2.10 V ͑vs. Ag/Agϩ with 0.1 M TBAP in AN solution͒.
Effect of pretreatment time with 0.1 M HNO3.—Effect of pre-
treatment time with 0.1 M HNO3 aqueous solution on the sensitivity
is shown in Fig. 3. The result shows that increasing the pretreatment
time with 0.1 M HNO3 aqueous solution from 0 to 60 min, increased
the sensitivity from 0.28 to 1.06 ␮A/cm2 ppm. Apparently, the un-
treated Pb foil by 0.1 M HNO3 aqueous solution exhibited bad per-
formance for TCE sensing. It can be explained that the void fraction
and coarseness increased with pretreatment time in the 0.1 M HNO3
aqueous solution. Consequently, the optimal pretreating condition is
60 min pretreatment time.
Figure 5. The SEM morphologies for preparation of working electrode at
different electrodeposition current densities: ͑A͒ 5 mA/cm2, ͑B͒ 10 mA/cm2,
and ͑C͒ 20 mA/cm2. Pretreating conditions: same as Fig. 1. Electrodeposition
conditions: same as Fig. 4.
Effect of electrodeposition current density.—Effect of elec-
trodeposition current density on the sensitivity for sensing of TCE is
shown in Fig. 4. The result shows that increasing the electrodeposi-
tion current density from 0 ͑i.e., original Pb foil͒ to 20 mA/cm2 for
preparing the modified Pb electrode, decreases the sensing sensitiv-
ity from 1.813 to 1.060 mA/cm2 ppm. The surface morphologies of
modified Pb electrode at different electrodeposition current densities
for preparing the modified Pb electrode are shown in Fig. 5. These
data clearly suggest that the working electrode surface was not elec-
troplated completely under 5 and 10 mA/cm2 electrodeposition cur-
rent densities, respectively, which induced the different sensitivity
Figure 4. Effect of electrodeposition current density for preparing the work-
ing electrode on the sensitivity of TCE sensor. Pretreating conditions: same
as Fig. 1. Electrodeposition conditions: 0.63 M Pb͑BF4)2 , 0.68 M HBF4 ,
0.43 M H3BO3 , and 0.0006 M PEG 300 aqueous solution, 30 rpm agitation
rate, at 30°C and 2 h. Sensing conditions: same as Fig. 3.
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Journal of The Electrochemical Society, 150 ͑9͒ H214-H219 ͑2003͒
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Figure 6. Amperometric response of TCE by using a modified Pb electrode
prepared at different electrodeposition current densities: ͑A͒ 0 mA/cm2, ͑B͒ 5
mA/cm2, ͑C͒ 10 mA/cm2, and ͑D͒ 20 mA/cm2. Pretreating conditions: same
as Fig. 1. Electrodeposition conditions: same as Fig. 3. Sensing conditions:
same as Fig. 4.
TCE by using these electrodes. However, the unstable and nonlinear
response current was apparently found during the sensing reaction
when the electrodeposition current density for preparing the working
electrode was smaller than 20 mA/cm2 as shown in Fig. 6. It may
have been due to the formation of unstable material with negatively
charged metal particles on the Pb electrode surface27 which induced
to the failure of the sensing reaction. Additionally, the XRD patterns
and ESCA spectra of naked and electrodeposited Pb electrodes are
shown in Fig. 7 and Table I, respectively. Comparison of the data of
Fig. 7A and B clearly suggest that the crystal structure was trans-
formed from a face centered cubic structure28 to a tetragonal struc-
ture between the general Pb metal and the electrodeposited Pb. Ad-
ditionally, comparison of Tables IA and B show that the B, F
elements were only indwelled on the surface of electrodeposited
Pb-modified electrode. Comparison of Fig. 7B and Table IB sug-
gests that surface derivatives of electrodeposited Pb-modified elec-
trode can be confirmed as BF3 and PbF2 . Therefore, the BF3 and
PbF2 were doped into the Pb foil electrode surface which had a
positive effect on the sensitivity of TCE by using this electrode.
Consequently, the 20 mA/cm2 electrodeposition current density for
preparing the working electrode was chosen for further development
of the TCE sensor.
Figure 7. ͑A͒ The XRD patterns for preparation of working electrode: ͑A͒
Pb foil electrode and ͑B͒ modified Pb electrode. Pretreating conditions: same
as Fig. 1. Electrodeposition conditions: same as Fig. 2A. ͑B͒ The XRD
patterns for preparation of working electrode: ͑A͒ Pb foil electrode and ͑B͒
modified Pb electrode. Pretreating conditions: same as Fig. 1. Electrodepo-
sition conditions: same as Fig. 2A.
Effect of electrodeposition time.—The sensitivity of the sensing
of TCE was investigated as a function of electrodeposition time for
preparing the modified Pb electrode as shown in Fig. 8. The result
indicated that increasing the electrodeposition time from 1 to 2 h
deceased the sensing sensitivity of TCE from 1.910 to 1.060
␮A/cm2 ppm. Then, the sensitivity increased slightly from 1.060 to
1.280 ␮A/cm2 ppm when the electrodeposition time increased from
2 to 4 h. However, an unstable and nonlinear response current was
obtained also, when the electrodeposition time for preparing the
working electrode was 1 h. Additionally, Fig. 8 clearly suggests that
the Pb film thickness and deposition time are insignificant factors on
the sensitivity of the TCE sensor. Comparing the energy consump-
tion and small enlargement effect of sensitivity, the 2 h electrodepo-
sition time was chosen to prepare the working electrode.
Table I. „A… ESCA spectra for Pb foil electrode. Pretreating con-
ditions: same as Fig. 1. „B… ESCA spectra for electrodeposited
Pb-modified electrode. Pretreating conditions: same as Fig. 1.
Electrodeposition conditions: same as Fig. 2A.
A
Binding energy
Area
Component
͑eV͒
͑%͒
O ͑1s͒
C ͑1s͒
Pb ͑4f7͒
532.07
284.98
138.05
29.20
50.70
20.20
B
Binding energy
Area
͑%͒
Effect of electrodeposition temperature.—The effect of tempera-
ture for preparing the electrode on the sensitivity of the TCE sensor
was shown in Fig. 9. The result shows that increasing the elec-
trodeposition temperature slightly increased the sensitivity. For ex-
ample, increasing the electrodeposition temperature for preparing
the working electrode from 25 to 40°C slightly increased the sensi-
tivity of TCE sensing from 0.854 to 1.210 ␮A/cm2 ppm. The active
Component
͑eV͒
F ͑1s͒
684.42
532.62
285.02
193.02
139.42
27.80
23.60
15.40
10.40
22.90
O ͑1s͒
C ͑1s͒
B ͑1s͒
Pb ͑4f7͒
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Journal of The Electrochemical Society, 150 ͑9͒ H214-H219 ͑2003͒
Figure 8. Effect of electrodeposition time for preparing the working elec-
trode on the sensitivity of TCE sensor. Pretreating conditions: same as Fig. 1.
Electrodeposition conditions: 0.63 M Pb͑BF4)2 , 0.68 M HBF4 , 0.43 M
H3BO3 , and 0.0006 M poly͑ethylene glycol͒ 300 aqueous solution, 20
mA/cm2 deposition current density, 30 rpm agitation rate and 30°C. Sensing
conditions: same as Fig. 3.
Figure 10. Amperometric response of electrodeposited Pb-modified elec-
trode to the addition of TCE ͑A͒ signal concentration increasing TCE con-
centration after each addition, ͑B͒ different concentration increasing TCE
concentration after each addition. Pretreating conditions: same as Fig. 1.
Electrodeposition conditions: same as Fig. 2A. Sensing conditions: same as
Fig. 3.
surface area increased slightly by SEM morphology with increasing
temperature results in an insignificant increase of sensitivity. How-
ever, the electrodeposition temperature for preparing the electrode
showed a positive effect on the sensitivity. However, in this system,
the temperature was over 40°C which allowed the formation of hy-
drofluoric acid as shown in Eq. 129
shows that one 100 ppm concentration of TCE was injected into the
electrochemical cell for each sensing step, and that the response
current increased rapidly and reached a steady state. Additionally,
curve B was obtained by different injection concentration of each
sensing step. Comparison of the curves A and B in Fig. 10 shows
that a similar amperometric response current curve and identical
sensitivity were obtained. These data clearly show that TCE sensing
has good reliability on the sensitivity and reproducibility. Addition-
ally, response currents are plotted against the TCE concentration
resulting in a straight line in the range from 100 to 700 ppm of TCE
concentrations as shown in Fig. 11. The equation of linear fitting is
4HF ϩ H3BO3 ꢀ HBF4 ϩ 3H2O
͓1͔
In order to prevent the vent of HF gas, the 30°C electrodeposition
temperature was chosen as temperature for preparing the electrode.
i ϭ 1.060 TCE ϩ 9.167, where the slope of the straight line is
͓
͔
Amperometric measurement of TCE.—Under previously selected
optimal conditions for preparation of working electrode, the typical
amperometric response curves at single and different TCE addition
concentration for detection of TCE by using an electrodeposited
Pb-modified electrode are shown in Fig. 10. Curve A of Fig. 10
1.060 ␮A/cm2 ppm which is the sensing sensitivity and the R2 is
over 0.998.
Reaction mechanism.—Figure 12 shows that the peak for gas
chromatography-mass spectroscopy of the main product locates at
60 of molecular weight. Hence, the product of electrolysis was
acetylene chloride. Furthermore, after electrolysis, an AgNO3
Figure 9. Effect of electrodeposition temperature for preparing the working
electrode on the sensitivity of TCE sensor. Pretreating conditions: same as
Fig. 1. Electrodeposition conditions: 0.63 M Pb͑BF4)2 , 0.68 M HBF4 , 0.43
M H3BO3 , and 0.0006 M poly͑ethylene glycol͒ 300 aqueous solution, 20
mA/cm2 deposition current density, 30 rpm agitation rate, and 2 h. Sensing
conditions: same as Fig. 3.
Figure 11. Typical calibration plot in sensing TCE by using modified Pb
electrode. Pretreating conditions: same as Fig. 1. Electrodeposition condi-
tions: same as Fig. 2A. Sensing conditions: same as Fig. 3.
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Journal of The Electrochemical Society, 150 ͑9͒ H214-H219 ͑2003͒
Conclusions
H219
Electrodeposition of Pb has been shown to be a suitable metal
catalyst as sensing electrode for amperometric TCE sensor. The
SEM morphologies of prepared working electrode show that the
substrate surface has the larger roughness by pretreating electrode
with 0.1 M HNO3 aqueous solution. Additionally, the XRD pattern
also showed the modified Pb electrode containing some derivatives
after the electrodeposition which enhanced the stability of the TCE
sensor. The optimal conditions for preparation working electrode
were obtained as 0.1 M HNO3 pretreatment, 20 mA/cm2 elec-
trodeposition current density, and 30°C electrodeposition tempera-
ture. Under the optimal sensing conditions, a linear response, i
ϭ 1.060 TCE ϩ 9.167, was obtained under TCE concentration
͓
͔
range from 100 to 700 ppm. Additionally, k is 7.217 10Ϫ4/cm s.
Therefore, the electrochemical TCE sensor by using modified Pb
electrode showed promising features for commercial application.
Figure 12. GC-Mass spectroscopy of electrolysis product. Pretreating con-
ditions: same as Fig. 1. Electrodeposition conditions: same as Fig. 2A. Elec-
trolysis conditions: 0.01 M TBAT in AN solution, electrodeposited Pb-
modified electrode as cathode, Pt plate as anode, Ag/Agϩ ͑with 0.1 M TBAP
in AN solution͒ as reference electrode, 155 rpm agitation rate, and at room
temperature.
Acknowledgments
The support of the Ministry of Education of the Republic of
China ͑EX-91-E-FA09-5-4͒ and National Cheng Kung University
are gratefully acknowledged.
National Cheng Kung University assisted in meeting the publication
costs of this article.
aqueous solution was added to the catholyte and the white AgCl was
found. It can be confirmed that chlorine ion presents in the
catholyte. Moreover, the pH of catholyte decreased from 7.0 to 1.0
during the electrolysis. All results show that hydrogen ion is gener-
ated in the catholyte after electrolysis. The acetylene chloride not
only has stronger acidity than TCE but also dissociated to acetylene
chloride anion in the solution. The electrochemical reduction mecha-
nism for TCE by using electrodeposited Pb-modified electrode as
working electrode is proposed as shown in Eq. 2
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͓3͔
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where Z is the number of electron transfer, F is the Faraday constant,
A is the sensing area of electrode, and k is the rate constant of mass
transfer control. Equation 3 implies a linear relationship between
TCE concentration and cathodic current. The relationship of id and
CL at the desired sensing conditions was obtained from the data of
Fig. 11 resulting in Eq. 4
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id ϭ 1.060CL
͓4͔
Comparing Eq. 3 and 4 and substituting the number of electron
transfer, Z, which is 2,26 and A, which is 1.0 cm2, k can be obtained
to be 7.217 ϫ 10Ϫ4/cm s.
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