Cyclic Voltammetry of Ion Transfer for Phenylpropanolamine Hydrochloride at Water|Nitrobenzene Interface.
Article
JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Cyclic Voltammetry of Ion Transfer for Phenylpropanolamine Hydrochloride at
Water|Nitrobenzene Interface
Irdhawati Irdhawati,a,b* Hirosuke Tatsumi,c Indra Noviandri,b
Buchari Bucharib and Slamet Ibrahimd
a
Department of Chemistry, Faculty of Mathematic and Natural Sciences, Udayana University,
Jl. By Pass Ngurah Rai Bali 80361, Indonesia
b
Department of Chemistry, Faculty of Mathematic and Natural Sciences, Bandung Institute of Technology,
Jl. Ganesha No. 10, Bandung 40132, Indonesia
c
Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1 Asahi Matsumoto 390-8621, Japan
d
School of Pharmacy, Bandung Institute of Technology, Jl. Ganesha No. 10, Bandung 40132, Indonesia
(Received November 12, 2010; Accepted March 10, 2011; Published Online August 19, 2011; DOI: 10.1002/jccs.201000580)
The transfer on phenylpropanolamine ion, PPAH+, has been studied at the Interface between Two Immiscible Solutions (ITIES). The polarizable potential range was determined by cyclic voltammetry at the interface between an aqueous solution of lithium chloride (LiCl) and a nitrobenzene (NB) solution of electrolyte tetrabutylammonium tetraphenylborate (TBATPB). The half-wave potential of ion transfer for
phenylpropanolamine accross the water|NB interface was found 465.3 mV. The peak separation, the diffusion coefficient, and the standard ion transfer potential of PPAH+ were observed to be 59.1 mV, 1.7 ´ 10-6
cm2/s, and 104.6 mV, respectively. The temperature of experiment was kept constantly at 25 ± 1 oC using
water flow thermostate.
Keywords: Phenylpropanolamine; Ion transfer; Diffusion coefficient; Liquid|liquid interface.
INTRODUCTION
Phenylpropanolamine hydrochloride [(±) norephedrine hydrochloride, (±)-2-amino-1-phenylpropan-1-ol hydrochloride, or PPAH+ is employed frequently as a bronchial decongestant, and administrated orally for the symptomatic relief of nasal congestion. It has been used for the
reduction of appetite and to monitor urinary incontinente.
It also finds its use as a bronchodilator and bronchial decongestant in asthma.1 The chemical structure is shown in
Fig. 1.
The electrifield interface is an important aspect of all
heterogeneous chemical system and in a variety of physicochemical phenomena. It is a vital feature of many biochemical systems as well as of other less complex system, for ex-
Fig. 1. Chemical structure of phenylpropanolamine
hydrochloride.
ample, colloids, gels, artificial membranes, and metal-electrolyte interfaces. Also, the electrochemical reaction at
ITIES represents an essential aspect of various practical applications in chemistry including electroanalysis, phasetransfer catalysis, ion extraction, electrocatalysis, and pursued experimentally the idea that liquid|liquid interface
could serve as a model for a half of a simplified biological
membrane. The similarity with a biological membrane surface has been noted by Du Bois-Reymond who then suggested that the surfaces of biological systems have properties similar to those of an electrode of a galvanic cell. The
success of these studies derives from several factors but especially from the smoothness, the unreactivity, and therefore the reproducibility of this interface.2-4
Reaction in which electrons are transferred from one
phase to another are of electrochemical nature, because a
charge particle, the electron, is transferred by an applied
electric field. However, it would not be reasonable to confine electrochemistry to electron transfer only. There is no
difference in principle when other charged species, i.e.
* Corresponding author. Tel: +62-361-701954 Ext. 235; Fax: +62-361-703137; E-mail: irdhawati@yahoo.com
40
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2012, 59, 40-45
JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Phenylpropanolamine at Water|Nitrobenzene Interface
ions, are transferred under the action of an electric field.
When an ion has a high chemical affinity toward a certain
phase, it will not cross the interface until the electrochemical potential are equal. This will create a potential difference the two phases, which counterbalances the chemical
affinity. It is also possible to force ions deliberately from
one phase into the other when a potential difference is applied across the interface. Imagine that two immiscible liquid phases are filled into a tube, so that they build up a common interface in the middle (Fig. 2).5
When each of the two liquids contains an electrolyte,
which is dissociated (this needs dipolar liquids), and two
inert metal electrodes are inserted into the two liquids, it is
possible to apply a potential difference across the liquid|
liquid interface. For exact measurements one will further
introduce into each liquid a reference electrode to control
the potential of each of the metal electrode separately.
Upon application of a voltage between the two working
electrodes, a current may flow. At the two metal electrodes
unknown faradaic reactions will occur (electron transfer
reaction). However, the overall current has also to cross the
liquid|liquid interface. Since the electrolyte solutions on
both side are ion conductors only, passage of current can
occur only when ions are tranferred from one liquid to the
other. The ion transfer at the interface is the rate determining process of the entire current flow. Indeed, it is possible
to record a cyclic voltammogram which shows current
peaks due to the transfer of, e.g., an anion from water to
nitrobenzene and back (Fig. 3). The mid point potential of
such cyclic voltammograms also has thermodynamic meaning.5
A voltammogram obtained for typical supporting
Fig. 2. Experimental arrangement for measuring the
transfer of ions between two immiscible liquid
electrolyte solutions.5
J. Chin. Chem. Soc. 2012, 59, 40-45
electrolyte is shown in Fig. 3 (A). The usual supporting
electrolytes are lithium chloride (LiCl) solution in the
aqueous phase and tetrabutylammonium tetraphenylborate
(TBATPB) in the nitrobenzene (NB) phase. Because LiCl
is a hydrophilic salt, Li+ and Cl- ions will remain confined
mostly in the aqueous phase. Similarly, TBATPB dissociates in NB, but its respective ions TBA+ and TPB- remain
almost exclusively in the NB phase. Thus, an interface between two ionically conductive, but immiscible phases, is
formed. Such an interface, as long as it does not charge carriers, can be polarized to a desired potential value and behaves as an ideally polarizable interface. The sign convention on ITIES is such that the aqueous phase is most positive at the right extreme of the voltammogram. The onset of
the currents limiting the supporting electrolyte working
range are due to the supporting electrolyte ions transport
into phases in which they are normally absent. The current
of the right extreme is caused by crossing of TPB- from NB
to water, and Li+ in the opposite direction.6
The left limit is caused by Cl- crossing from water
into NB and TBA+ cation crossing in the reverse direction.
In the middle of the potential window only a charging current corresponding to the double-layer region is observed.
This range of potentials in which no significant transport of
supporting electrolyte takes place is suitable for studies of
semihydrophobic ions, such as tetramethylammonium cation (TMA+) in the following sample. Curve B in Fig. 3 re-
Fig. 3. Voltammetry on ITIES. (A) Supporting electrolyte voltammogram, aqueous phase 0.1 M
LiCl, NB phase 0.1 M TBATPB. Scan rate 20
mV s -1 . (B) Transfer of TMA + (c = 4.7 ´ 10 -4
mol L -1 ) between water and NB, added to the
aqueous phase. The TMA+ transport responsible for the rise of the peaks is indicated.6
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.jccs.wiley-vch.de
41
Article
Irdhawati et al.
sult from addition of 0.47 mM of TMA+ in aqueous phase.
If the aqueous phase is made more positive by scanning to
the right, a transport of the TMA+ cation into NB will occur. Upon reversing the scan at the right extreme, the transport of TMA+ from NB back to water is observed. The transport across the interface is by all indications diffusion controlled and therefore the voltammetric curves and equations describing them are similar to those for transport of
oxidizable product away from the electrode. Therefore, the
voltammograms for both process have similar features, e.g.
a separation of the positive and negative peak potential by
58/n mV for a reversible process.6
Numerous methods have been developed for phenylpropanolamine analysis, including voltammetry,7 potentiometry,8,9 chromatography,10,11 electrophoresis,12 conductimetry,13 and spectrophotometry methods.14 Dvorak et al.
reported selective complexation of biogenic amines by
macrocyclic polyethers at a liquid|liquid interface, including phenylpropanolamine (norephedrine). Thermodynamics, transport and selectivity in the phase transfer formation
of complexes between b-phenylethylammonium ions in
water and dibenzo-18-crown-6 in NB were observed at
298 oK. The result for norephedrine analysis showed the
mid point potential and diffusion coefficient were obtained
125 mV and 8 ´ 106 cm2/s, respectively.7
In this work we employed a voltammetric technique
to investigate the mid point or half-wave potential, diffusion coefficient, and ion transfer of phenylpropanolamine
ion across the water|NB interface.
RESULTS AND DISCUSSION
The cyclic voltammograms for ion transfer across the
interface between two immiscible electrolyte solution have
the shape of a peak known from the classical voltammetry
of a simple electron transfer at a planar electrode.16 Fig. 4
below shows the anodic and cathodic waves of PPAH+ and
TMA+ 0.4 mM at the 0.1 M LiCl (W)|0.1 M TBATPB (NB)
from each of five voltammograms taken at 200; 100; 50;
20; and 10 mV s-1.
The voltammograms are characterized by the anodic
and cathodic peak potentials, Epa and Epc respectively; the
peak potential difference, DEp, between the anodic and
cathodic peaks; the midpoint potential, Epm defined by (Epa
+ Epc)/2; and the peak current, Ip. From the Fig. 4, the values of Epa, Epc, and Epm were observed and shown in the Ta-
42
www.jccs.wiley-vch.de
ble 1.
The wave of the transfer of PPAH+ and TMA+ showed
the characteristic of the reversible reaction. The peak separation, DEp, at 0.4 mM was obtained to be 59.1 mV and
59.9 mV for PPAH+ and TMA+, respectively, closed to the
theoretical value of DEp = 59.9 mV/n, where n is the number of electrons involved in the redox couple. In this case,
n is clearly +1, so it can be confirmed that PPAH + and
TMA+ are monovalent cations. The pH of PPAH+ containing 0.1 M LiCl was 6.98, and the pKa value is 9.44. The
pH of solution was lower than pKa. It showed that PPAH+
ion were in protonated form. In addition, the reversible
half-wave potential being giving by E1/2rev for a reversible
redox couple, that can be easily found from an experimental voltammogram because it is equal to the mid point potential.17
Fig. 4. Cyclic voltammogram of PPAH+ and TMA+ 0.4
mM at W|NB interface.
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2012, 59, 40-45
JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Phenylpropanolamine at Water|Nitrobenzene Interface
Table 1. The observed values of anodic, cathodic, peak separation, and mid point potentials (mV) from the cyclic
voltammogram of PPAH+ and TMA+ in the different
scan rates
+
PPAH
TMA+
Epa
Epc
DEp
Epm
494.9
425.7
435.8
365.8
59.1
59.9
465.3
395.7
Plots of the square root of scan rate againts peak potentials gave straigh lines, and were found to be linear in
the scan rate range from 200 – 10 mV s-1. The anodic peak
potential increased and shifted to more positive potential
and the cathodic peak potential shifted to more negative
potential with increasing of scan rate. The plots are shown
in Fig. 5.
The peak current increases with larger scan rates.
Quantitative information regarding analyte concentration
can be obtained from the voltammogram using the Randles-
Sevcik equation, if the temperature is assumed to be 25
o 16,18
C,
ip = (2.69 ´ 105) n3/2 v1/2 D1/2 A C
where n is the number of electrons appearing in the half-reaction for the redox couple, v is the rate at which the potential is swept (V s-1), D is analyte diffusion coefficient (cm2
s-1), A is the electrode area (cm2), C is the analyte concentration (mM), directly proportional to the peak current. If
the analyte concentration is a known quantity, cyclic voltammetry can be used to measure the analyte diffusion coefficient. The diffusion coefficient measures of how fast
the analyte moves through the solution as a result of random collision with other molecules. This equation predicts
that the peak current shoud be proportional to the square
root of the scan rate. For the experimental result depicted in
Fig. 6, the electrode area, A, was 0.126 cm2 and the concentration, C, was 0.4 mM.
As expected, the plot of square root of scan rates versus peak current yields a straight line. The Randles-Sevcik
equation can be modified to give an expression for the
slope of this straight line as follows,
(slope) = (2.69 ´ 105) n3/2 A D1/2 C
where the constant is understood to have unit 2.69 ´ 105 C
mol-1 V-1/2.
For the result in Fig. 6, the slope are 17.854 and
62.051 for PPAH+ and TMA+, respectively. After careful
substitution and unit analysis, the diffusion coefficient, D,
Fig. 5. Plots of E pa , E pc , and E pm of the transfer of
PPAH + and TMA + at W|NB interface againts
v1/2 at C = 0.4 mM.
J. Chin. Chem. Soc. 2012, 59, 40-45
Fig. 6. Plots of square root of scan rate and peak current.
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.jccs.wiley-vch.de
43
Article
Irdhawati et al.
obtained a value equal to 2.1 ´ 10-5 cm2/s for TMA+, and
1.7 ´ 10-6 cm2/s for PPAH+. The value for the standard ion
transfer potential of TMA+ was taken from reference 19.
The standard for ion transfer potential of PPAH+ was
determined by,
Scheme I Electrochemical cell for measurement of
ion transfer potential of PPAH+
Ag
AgCl
0.1 M
TBACl
0.1 M
TBATPB
0.1 M LiCl
´ mM
PPAH+
w
NB
w
0.1 M
LiCl
AgCl
Ag
+
D wo j0 PPAH+ = D wo j0 TMA+ - E rev
1/ 2 TMA
+
rev
+ E 1/ 2 PPAH
By taking the average of mid point potentials were 465.3
mV and 395.7 mV for PPAH+ and TMA+, respectively, the
standard ion transfer potential of PPAH + at W|NB was
found to be 104.6 mV.
EXPERIMENTAL
Chemicals
Phenylpropanolamine hydrochloride, crystalline powder, was obtained from Indonesian Medicine and Food
Testing Centre, Jakarta. Tetra-n-butylammonium bromide
(TBABr, white-nearly white, crystalline powder, 98%) and
tetramethylammonium chloride (TMACl, white-nearly
white, crystalline powder, 98%) were purchased from
Wako as special grade chemicals. Tetra-n-butylammonium
chloride (TBACl, solid, crystal coagulation, extra pure reagent, 95%) and lithium chloride anhydrous (LiCl, solid,
crystal aggregated, guaranted reagent, 99%) were purchased from Nacalai Tesque Inc., Kyoto, Japan. Kalibor/
sodium tetraphenylborate (NaTPB, white crystalline powder, 99.5%) was purchased from Dojindo. Tetrabutylammonium tetraphenylborate (TBATPB) was precipitated
from aqueous solution of TBABr and NaTPB, repeated
crystallization by acetone as described elsewhere.15 Nitrobenzene from Merck and distilled water were used for preparing solutions in organic and water phases, respectively.
All of the chemicals were used as received without further
purification.
Electrochemical Measurements
The ion transfer of phenylpropanolamine ion at the
interface between water and nitrobenzene solutions investigated by cyclic voltammetry. As suppoting electrolytes
in voltammetric measurement, 0.1 M LiCl and 0.1 M
TBATPB were added in water and NB, respectively. The
electrochemical cell can be represented by the following
scheme:
44
www.jccs.wiley-vch.de
where x = 0.1, 0.2, 0.3, 0.4, or 0.5 mM. Voltammetric measurements for the ion transfer across water and NB interface were performed in a three-electrode potentiostat
(BASi Epsilon_EC Ver. 1.60.70_XP) with IR compensation, at temperature 25 ± 1 oC. The potential difference was
applied between two Ag/AgCl electrodes (working and reference electrodes) and the current between working and
counter (Pt wire) electrodes was recorded.
ACKNOWLEDGMENT
We gratefully acknowledge to the Directorate General of Higher Education, Ministry of National Education,
Republic of Indonesia for the Grant in Sandwich-Like
Programme (2009) in Department of Chemistry, Faculty of
Science, Shinshu University, Matsumoto, Japan.
REFERENCES
1. Ashutosh, K. Medicinal Chemistry, 3rd ed.; New Age International: New Delhi, 2005; Chapter 12, p 316.
2. Girault, H. H. J.; Schiffrin, D. J. In Electroanalytical
Chemisty: Electrochemistry at Liquid|Liquid Interface;
Bard, A. J., Eds.; Marcel Dekker Inc.: New York, 1989; Vol.
15, p 71.
3. Samec, Z. Pure Appl. Chem. 2004, 76, 2147-2180.
4. Koryta, J.; Vanysek, P.; Brezina, M. J. Electroanal. Chem.
1976, 67, 263.
5. Scholz, F. In Electroanalytical Chemistry; Scholz, F.; Eds.;
Springer: Heidelberg, 2010, pp 27-29.
6. Vanysek, P. Electrochim. Acta 1995, 40(18), 2841.
7. Dvorak, O.; Marecek, V.; Samec, Z. J. Electroanal. Chem.
1991, 300, 407.
8. Issa, Y. M.; Khalil, M. M.; Zayed, S. I. M.; Hussein, A. Arabian J. Chem. 2009, 2, 41.
9. Ganjali, M. R.; Hariri, M.; Riahi, S.; Norouzi, P.; Javaheri,
M. Intern. J. Electrochem. Sci. 2009, 4(2), 295.
10. Abbasi, K.; Bhanger, M. I.; Khuhawar, M. Y. J. Chem. Soc.
Pakistan 2008, 30(5), 692.
11. Harsono, T.; Yuwono, M.; Indarayanto, G. J. AOAC Intern.
2005, 88(4), 1093.
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2012, 59, 40-45
JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Phenylpropanolamine at Water|Nitrobenzene Interface
12. Azhagvuel, S.; Sekar, S. J. Pharm. Biomed. Anal. 2007,
43(3), 873.
13. Issa, Y. M.; Youssef, A. P. A.; Mutair, A. A. Farmaco 2005,
60(6-7), 541.
14. Khuhawar, M. Y.; Rind, F. M. A.; Rajper, A. J. Food Drug
Anal. 2005, 13(4), 388.
15. Kakutani, T.; Osakai, T.; Senda, M. Bull. Chem. Soc. Jpn.
1983, 56, 991.
16. Samec, Z. In Electrochemistry for Environmental Protec-
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17.
18.
19.
20.
tion; Rao, D. B.; Eds.; Discovery Publishing House: New
Delhi, 2001; No. 2, p 33.
Liu, B.; Mirkin, M. V. J. Anal. Chem. 2001, 1, 670A.
Wang, Anal. Electrochem. 2nd ed.; Wiley-VCH: New York,
2001; Chapter 2, 28.
Osakai, T. Rev. Polarogh. 2006, 52(1), 3-12.
Sasakura, H.; Kubota, S.; Onishi, J.; Ozaki, S.; Kano, K.;
Shirai, O. Electrochemistry 2008, 76(8), 597.
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.jccs.wiley-vch.de
45
JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Cyclic Voltammetry of Ion Transfer for Phenylpropanolamine Hydrochloride at
Water|Nitrobenzene Interface
Irdhawati Irdhawati,a,b* Hirosuke Tatsumi,c Indra Noviandri,b
Buchari Bucharib and Slamet Ibrahimd
a
Department of Chemistry, Faculty of Mathematic and Natural Sciences, Udayana University,
Jl. By Pass Ngurah Rai Bali 80361, Indonesia
b
Department of Chemistry, Faculty of Mathematic and Natural Sciences, Bandung Institute of Technology,
Jl. Ganesha No. 10, Bandung 40132, Indonesia
c
Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1 Asahi Matsumoto 390-8621, Japan
d
School of Pharmacy, Bandung Institute of Technology, Jl. Ganesha No. 10, Bandung 40132, Indonesia
(Received November 12, 2010; Accepted March 10, 2011; Published Online August 19, 2011; DOI: 10.1002/jccs.201000580)
The transfer on phenylpropanolamine ion, PPAH+, has been studied at the Interface between Two Immiscible Solutions (ITIES). The polarizable potential range was determined by cyclic voltammetry at the interface between an aqueous solution of lithium chloride (LiCl) and a nitrobenzene (NB) solution of electrolyte tetrabutylammonium tetraphenylborate (TBATPB). The half-wave potential of ion transfer for
phenylpropanolamine accross the water|NB interface was found 465.3 mV. The peak separation, the diffusion coefficient, and the standard ion transfer potential of PPAH+ were observed to be 59.1 mV, 1.7 ´ 10-6
cm2/s, and 104.6 mV, respectively. The temperature of experiment was kept constantly at 25 ± 1 oC using
water flow thermostate.
Keywords: Phenylpropanolamine; Ion transfer; Diffusion coefficient; Liquid|liquid interface.
INTRODUCTION
Phenylpropanolamine hydrochloride [(±) norephedrine hydrochloride, (±)-2-amino-1-phenylpropan-1-ol hydrochloride, or PPAH+ is employed frequently as a bronchial decongestant, and administrated orally for the symptomatic relief of nasal congestion. It has been used for the
reduction of appetite and to monitor urinary incontinente.
It also finds its use as a bronchodilator and bronchial decongestant in asthma.1 The chemical structure is shown in
Fig. 1.
The electrifield interface is an important aspect of all
heterogeneous chemical system and in a variety of physicochemical phenomena. It is a vital feature of many biochemical systems as well as of other less complex system, for ex-
Fig. 1. Chemical structure of phenylpropanolamine
hydrochloride.
ample, colloids, gels, artificial membranes, and metal-electrolyte interfaces. Also, the electrochemical reaction at
ITIES represents an essential aspect of various practical applications in chemistry including electroanalysis, phasetransfer catalysis, ion extraction, electrocatalysis, and pursued experimentally the idea that liquid|liquid interface
could serve as a model for a half of a simplified biological
membrane. The similarity with a biological membrane surface has been noted by Du Bois-Reymond who then suggested that the surfaces of biological systems have properties similar to those of an electrode of a galvanic cell. The
success of these studies derives from several factors but especially from the smoothness, the unreactivity, and therefore the reproducibility of this interface.2-4
Reaction in which electrons are transferred from one
phase to another are of electrochemical nature, because a
charge particle, the electron, is transferred by an applied
electric field. However, it would not be reasonable to confine electrochemistry to electron transfer only. There is no
difference in principle when other charged species, i.e.
* Corresponding author. Tel: +62-361-701954 Ext. 235; Fax: +62-361-703137; E-mail: irdhawati@yahoo.com
40
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2012, 59, 40-45
JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Phenylpropanolamine at Water|Nitrobenzene Interface
ions, are transferred under the action of an electric field.
When an ion has a high chemical affinity toward a certain
phase, it will not cross the interface until the electrochemical potential are equal. This will create a potential difference the two phases, which counterbalances the chemical
affinity. It is also possible to force ions deliberately from
one phase into the other when a potential difference is applied across the interface. Imagine that two immiscible liquid phases are filled into a tube, so that they build up a common interface in the middle (Fig. 2).5
When each of the two liquids contains an electrolyte,
which is dissociated (this needs dipolar liquids), and two
inert metal electrodes are inserted into the two liquids, it is
possible to apply a potential difference across the liquid|
liquid interface. For exact measurements one will further
introduce into each liquid a reference electrode to control
the potential of each of the metal electrode separately.
Upon application of a voltage between the two working
electrodes, a current may flow. At the two metal electrodes
unknown faradaic reactions will occur (electron transfer
reaction). However, the overall current has also to cross the
liquid|liquid interface. Since the electrolyte solutions on
both side are ion conductors only, passage of current can
occur only when ions are tranferred from one liquid to the
other. The ion transfer at the interface is the rate determining process of the entire current flow. Indeed, it is possible
to record a cyclic voltammogram which shows current
peaks due to the transfer of, e.g., an anion from water to
nitrobenzene and back (Fig. 3). The mid point potential of
such cyclic voltammograms also has thermodynamic meaning.5
A voltammogram obtained for typical supporting
Fig. 2. Experimental arrangement for measuring the
transfer of ions between two immiscible liquid
electrolyte solutions.5
J. Chin. Chem. Soc. 2012, 59, 40-45
electrolyte is shown in Fig. 3 (A). The usual supporting
electrolytes are lithium chloride (LiCl) solution in the
aqueous phase and tetrabutylammonium tetraphenylborate
(TBATPB) in the nitrobenzene (NB) phase. Because LiCl
is a hydrophilic salt, Li+ and Cl- ions will remain confined
mostly in the aqueous phase. Similarly, TBATPB dissociates in NB, but its respective ions TBA+ and TPB- remain
almost exclusively in the NB phase. Thus, an interface between two ionically conductive, but immiscible phases, is
formed. Such an interface, as long as it does not charge carriers, can be polarized to a desired potential value and behaves as an ideally polarizable interface. The sign convention on ITIES is such that the aqueous phase is most positive at the right extreme of the voltammogram. The onset of
the currents limiting the supporting electrolyte working
range are due to the supporting electrolyte ions transport
into phases in which they are normally absent. The current
of the right extreme is caused by crossing of TPB- from NB
to water, and Li+ in the opposite direction.6
The left limit is caused by Cl- crossing from water
into NB and TBA+ cation crossing in the reverse direction.
In the middle of the potential window only a charging current corresponding to the double-layer region is observed.
This range of potentials in which no significant transport of
supporting electrolyte takes place is suitable for studies of
semihydrophobic ions, such as tetramethylammonium cation (TMA+) in the following sample. Curve B in Fig. 3 re-
Fig. 3. Voltammetry on ITIES. (A) Supporting electrolyte voltammogram, aqueous phase 0.1 M
LiCl, NB phase 0.1 M TBATPB. Scan rate 20
mV s -1 . (B) Transfer of TMA + (c = 4.7 ´ 10 -4
mol L -1 ) between water and NB, added to the
aqueous phase. The TMA+ transport responsible for the rise of the peaks is indicated.6
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.jccs.wiley-vch.de
41
Article
Irdhawati et al.
sult from addition of 0.47 mM of TMA+ in aqueous phase.
If the aqueous phase is made more positive by scanning to
the right, a transport of the TMA+ cation into NB will occur. Upon reversing the scan at the right extreme, the transport of TMA+ from NB back to water is observed. The transport across the interface is by all indications diffusion controlled and therefore the voltammetric curves and equations describing them are similar to those for transport of
oxidizable product away from the electrode. Therefore, the
voltammograms for both process have similar features, e.g.
a separation of the positive and negative peak potential by
58/n mV for a reversible process.6
Numerous methods have been developed for phenylpropanolamine analysis, including voltammetry,7 potentiometry,8,9 chromatography,10,11 electrophoresis,12 conductimetry,13 and spectrophotometry methods.14 Dvorak et al.
reported selective complexation of biogenic amines by
macrocyclic polyethers at a liquid|liquid interface, including phenylpropanolamine (norephedrine). Thermodynamics, transport and selectivity in the phase transfer formation
of complexes between b-phenylethylammonium ions in
water and dibenzo-18-crown-6 in NB were observed at
298 oK. The result for norephedrine analysis showed the
mid point potential and diffusion coefficient were obtained
125 mV and 8 ´ 106 cm2/s, respectively.7
In this work we employed a voltammetric technique
to investigate the mid point or half-wave potential, diffusion coefficient, and ion transfer of phenylpropanolamine
ion across the water|NB interface.
RESULTS AND DISCUSSION
The cyclic voltammograms for ion transfer across the
interface between two immiscible electrolyte solution have
the shape of a peak known from the classical voltammetry
of a simple electron transfer at a planar electrode.16 Fig. 4
below shows the anodic and cathodic waves of PPAH+ and
TMA+ 0.4 mM at the 0.1 M LiCl (W)|0.1 M TBATPB (NB)
from each of five voltammograms taken at 200; 100; 50;
20; and 10 mV s-1.
The voltammograms are characterized by the anodic
and cathodic peak potentials, Epa and Epc respectively; the
peak potential difference, DEp, between the anodic and
cathodic peaks; the midpoint potential, Epm defined by (Epa
+ Epc)/2; and the peak current, Ip. From the Fig. 4, the values of Epa, Epc, and Epm were observed and shown in the Ta-
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ble 1.
The wave of the transfer of PPAH+ and TMA+ showed
the characteristic of the reversible reaction. The peak separation, DEp, at 0.4 mM was obtained to be 59.1 mV and
59.9 mV for PPAH+ and TMA+, respectively, closed to the
theoretical value of DEp = 59.9 mV/n, where n is the number of electrons involved in the redox couple. In this case,
n is clearly +1, so it can be confirmed that PPAH + and
TMA+ are monovalent cations. The pH of PPAH+ containing 0.1 M LiCl was 6.98, and the pKa value is 9.44. The
pH of solution was lower than pKa. It showed that PPAH+
ion were in protonated form. In addition, the reversible
half-wave potential being giving by E1/2rev for a reversible
redox couple, that can be easily found from an experimental voltammogram because it is equal to the mid point potential.17
Fig. 4. Cyclic voltammogram of PPAH+ and TMA+ 0.4
mM at W|NB interface.
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2012, 59, 40-45
JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Phenylpropanolamine at Water|Nitrobenzene Interface
Table 1. The observed values of anodic, cathodic, peak separation, and mid point potentials (mV) from the cyclic
voltammogram of PPAH+ and TMA+ in the different
scan rates
+
PPAH
TMA+
Epa
Epc
DEp
Epm
494.9
425.7
435.8
365.8
59.1
59.9
465.3
395.7
Plots of the square root of scan rate againts peak potentials gave straigh lines, and were found to be linear in
the scan rate range from 200 – 10 mV s-1. The anodic peak
potential increased and shifted to more positive potential
and the cathodic peak potential shifted to more negative
potential with increasing of scan rate. The plots are shown
in Fig. 5.
The peak current increases with larger scan rates.
Quantitative information regarding analyte concentration
can be obtained from the voltammogram using the Randles-
Sevcik equation, if the temperature is assumed to be 25
o 16,18
C,
ip = (2.69 ´ 105) n3/2 v1/2 D1/2 A C
where n is the number of electrons appearing in the half-reaction for the redox couple, v is the rate at which the potential is swept (V s-1), D is analyte diffusion coefficient (cm2
s-1), A is the electrode area (cm2), C is the analyte concentration (mM), directly proportional to the peak current. If
the analyte concentration is a known quantity, cyclic voltammetry can be used to measure the analyte diffusion coefficient. The diffusion coefficient measures of how fast
the analyte moves through the solution as a result of random collision with other molecules. This equation predicts
that the peak current shoud be proportional to the square
root of the scan rate. For the experimental result depicted in
Fig. 6, the electrode area, A, was 0.126 cm2 and the concentration, C, was 0.4 mM.
As expected, the plot of square root of scan rates versus peak current yields a straight line. The Randles-Sevcik
equation can be modified to give an expression for the
slope of this straight line as follows,
(slope) = (2.69 ´ 105) n3/2 A D1/2 C
where the constant is understood to have unit 2.69 ´ 105 C
mol-1 V-1/2.
For the result in Fig. 6, the slope are 17.854 and
62.051 for PPAH+ and TMA+, respectively. After careful
substitution and unit analysis, the diffusion coefficient, D,
Fig. 5. Plots of E pa , E pc , and E pm of the transfer of
PPAH + and TMA + at W|NB interface againts
v1/2 at C = 0.4 mM.
J. Chin. Chem. Soc. 2012, 59, 40-45
Fig. 6. Plots of square root of scan rate and peak current.
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.jccs.wiley-vch.de
43
Article
Irdhawati et al.
obtained a value equal to 2.1 ´ 10-5 cm2/s for TMA+, and
1.7 ´ 10-6 cm2/s for PPAH+. The value for the standard ion
transfer potential of TMA+ was taken from reference 19.
The standard for ion transfer potential of PPAH+ was
determined by,
Scheme I Electrochemical cell for measurement of
ion transfer potential of PPAH+
Ag
AgCl
0.1 M
TBACl
0.1 M
TBATPB
0.1 M LiCl
´ mM
PPAH+
w
NB
w
0.1 M
LiCl
AgCl
Ag
+
D wo j0 PPAH+ = D wo j0 TMA+ - E rev
1/ 2 TMA
+
rev
+ E 1/ 2 PPAH
By taking the average of mid point potentials were 465.3
mV and 395.7 mV for PPAH+ and TMA+, respectively, the
standard ion transfer potential of PPAH + at W|NB was
found to be 104.6 mV.
EXPERIMENTAL
Chemicals
Phenylpropanolamine hydrochloride, crystalline powder, was obtained from Indonesian Medicine and Food
Testing Centre, Jakarta. Tetra-n-butylammonium bromide
(TBABr, white-nearly white, crystalline powder, 98%) and
tetramethylammonium chloride (TMACl, white-nearly
white, crystalline powder, 98%) were purchased from
Wako as special grade chemicals. Tetra-n-butylammonium
chloride (TBACl, solid, crystal coagulation, extra pure reagent, 95%) and lithium chloride anhydrous (LiCl, solid,
crystal aggregated, guaranted reagent, 99%) were purchased from Nacalai Tesque Inc., Kyoto, Japan. Kalibor/
sodium tetraphenylborate (NaTPB, white crystalline powder, 99.5%) was purchased from Dojindo. Tetrabutylammonium tetraphenylborate (TBATPB) was precipitated
from aqueous solution of TBABr and NaTPB, repeated
crystallization by acetone as described elsewhere.15 Nitrobenzene from Merck and distilled water were used for preparing solutions in organic and water phases, respectively.
All of the chemicals were used as received without further
purification.
Electrochemical Measurements
The ion transfer of phenylpropanolamine ion at the
interface between water and nitrobenzene solutions investigated by cyclic voltammetry. As suppoting electrolytes
in voltammetric measurement, 0.1 M LiCl and 0.1 M
TBATPB were added in water and NB, respectively. The
electrochemical cell can be represented by the following
scheme:
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where x = 0.1, 0.2, 0.3, 0.4, or 0.5 mM. Voltammetric measurements for the ion transfer across water and NB interface were performed in a three-electrode potentiostat
(BASi Epsilon_EC Ver. 1.60.70_XP) with IR compensation, at temperature 25 ± 1 oC. The potential difference was
applied between two Ag/AgCl electrodes (working and reference electrodes) and the current between working and
counter (Pt wire) electrodes was recorded.
ACKNOWLEDGMENT
We gratefully acknowledge to the Directorate General of Higher Education, Ministry of National Education,
Republic of Indonesia for the Grant in Sandwich-Like
Programme (2009) in Department of Chemistry, Faculty of
Science, Shinshu University, Matsumoto, Japan.
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