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Facilitated sodium transfer from aqueous electrolytes to resistive media
203
Chem., 334 (1992) 203-211 Elsevier Sequoia !%A., Lausanne
J. Electroanal.
JEC 02103
Facilitated sodium transfer from aqueous electrolytes resistive media Y. Shao and H.H. Girault Department
of Chemistry,
to
l
University of Edinburgh,
West Mains Road, Edinburgh EH9 3JJ (UK)
(Received 15 November 1991; in revised form 27 February 1992)
Abstract
The facilitated transfer of sodium by dibenzo-18-crown-6 from water to resistive media has been studied using micro liquid/liquid interfaces. The facilitated transfer wave can still be observed when the concentrations of the supporting electrolytes in the organic phase are decreased to micromolar levels. The results obtained have been analysed by the methodology proposed by Oldham (J. Electroanal. Chem., 2.50 (1988) 1) for the oxidation on microelectrodes of ferrocene in acetonitrile in the presence of low concentrations of supporting electrolytes.
INTRODUCTION
The formation of ion-pairs in the organic phase is always an important problem in the study of charge transfer reactions at liquid/liquid interfaces because supporting electrolytes are added at high concentrations to the organic phase to ensure adequate conductivity and to minimize the ohmic drop. This problem is particularly serious for organic solvents with low dielectric constants e.g., 1,2-dichloroethane (E = 10.36 at 25°C). In the case of ion transfer reactions, the high concentration of organic counterions is an impediment both because of bulk and interfacial ion pairing. Ion pairing in the bulk, for example, makes the determination of thermodynamic values such as the Gibbs energy of transfer more difficult to calculate. Interfacial ion pairing, on the other hand, results in specific adsorption [ll. In order to eliminate the effect of ion-pairing during charge transfer reactions, micro-liquid/liquid interfaces [2-51 present real advantages since the inherent low ohmic drop associated with spherical diffusion allows electrochemical measurements with low (or zero> concentrations of supporting electrolytes [6,7].
* Present address: Institut de Chimie Physique, E.P.F. Lausanne, CH 1015 Lausanne, Switzerland. 0022-0728/92/$05.00
0 1992 - Elsevier Sequoia S.A. All rights reserved
204
In the present work, sodium ion transfer, facilitated by dibenzo-18-crown-6 (DB18C6), across water/l,Zdichloroethane (1,2-DCE) micro-interfaces supported at the tip of micropipettes has been studied using small concentrations of organic supporting electrolytes. The facilitated transfer reaction studied is pseudo-first order, with the ionic concentration within the pipette in excess compared with that of the ionophore outside. The ion transfer occurs through a pick-up mechanism by the ionophore, very much like a reduction on a metallic electrode, the bare ionophore being the oxidised species, the complex being the reduced species [5]. Mass transfer of the reactants (i.e the bare ionophores) and of the products (i.e the complexed ions) occurs in a spherical/cylindrical manner, and, consequently, all the methodology developed for metallic microelectrodes can be applied. For this reason, the results obtained have been compared with the work of Bond et al. [61 on the oxidation of ferrocene in acetonitrile on microelectrodes, which has been rationalised by Oldham [7]. EXPERIMENTAL
TBATPBCl (tetrabutylammonium tetrakis[4-chlorophenyllborate), prepared by metathesis of TBACl and KTPBCI, was used as the organic supporting electrolyte. Dibenzo-18-crown-6 (DB18C6, Aldrich, 98%), NaCl (Fisons, AnalR) and 1,2-dichloroethane (1,2-DCE, BDH, AnalaR) were employed for the preparation of the solutions. Kwik-fill glass capillaries (Clark Electromedical Ltd, U.K.) and a vertical pipette puller Kopf 720 (U.S.A.) have been used for the fabrication of the micropipettes. The cell and the instrumentation used were described in refs. 3,4 and 8. RESULTS
AND DISCUSSION
Figure l(A) shows the potential window (x = 0) for the following system: Ag/AgCl/l M NaCl//x mM DB18C6 + 0.1 mM TBATPBCl/O.Ol
M TBACI/AgCl/Ag
(Cell I)
In the presence of 0.4 mM DB18C6, sodium transfer across the water/l,2-DCE interface facilitated by DB18C6 is observed clearly within the potential window (see Fig. l(B)) with a half-wave potential value of 285 mV referred to cell I. When the concentration of the supporting electrolyte in the organic phase is higher than 1 mM, plots of In[(Z, - Z)/Z] vs. A”4 have a slope which approaches the theoretical value of 39.6 V-’ corresponding to F/RT at 20°C. This proves that, within the range of pipette radii used (5 to 15 pm), the pseudo-first order ion transfer reaction appears to be reversible. However, when the concentration of the supporting electrolyte decreases, the slope of this type of plot decreases indicating the influence of increasing ZR drop and of migration mass transport. According to the theory which has been developed by Oldham [7] for voltammetry at metallic hemispherical microelectrodes in resistive media, the following
20.5
0
OA
02 03 0.4 0.5 0.6 0.7 lg+/v
Fig. 1. Potential window (A) and facilitated transfer of sodium (B) using Cell 1. (A) Approximate interface diameter = 10 pm. Scan rate u = 30 mV/s. (B) Approximate interface diameter = 30 pm. u = 100 mV/s.
equation can also be employed to describe the shape of the steady-state wave when the concentration of the reactant is higher than that of the supporting electrolyte:
A{4 = AI4’ + (RT/F) In[ [ D,Z(4D,c,Z,,
+ D,c,Z)]/[4D&,Z,(
Id -Z)]]
(1) In the present system, the reactant is the free ionophore and the product the complexed ion. Thus, D,, D, are the diffusion coefficients of DB18C6 and the complexed ion in the organic phase respectively; ci and cq are the bulk concentrations of DB18C6 and of the supporting electrolyte in the organic phase; I,, is the steady state plateau current. If we write x = Z/Z,, and y = c,/c,, and taking into account that the ratio of the diffusion coefficients of the ionophore and the complexed ion is equal to D,/D, = 1.6 [5], then the above equation becomes: A$4=Al4’+(RT/F) ln[(1.6x+0.64x2/y)/(l-x)] (2)
206
-0.2
I
-200
I
-100
0
/
100
1
200
300
(A$ @ - AZ @) / mV Fig. 2. Plots of X vs. [A:4 III.
- A’&b’]. y = (1) 2.5, (2) 1, (3) 0.25, as in cell I, (4) 0.025, (5) 0.0025, as in cell
Figure 2 shows the predicted steady-state voltammograms for the facilitated sodium transfer at a microelectrode according to this equation for different ratios of y = cd/cl. It can be observed that as the supporting electrolyte concentration decreases the half-wave potential shifts, as if the rate of the transfer becomes slower (eqn. 2 assumes the reversibility of the charge transfer reaction). Figure 3 shows the plot ln[(Z, - Z)/Z] vs. A”,$ for the data of Fig. l(B) which is linear, with a slope equal to 30.5 V-‘. This value may be compared with the theoretical value of 33.8 V-t obtained by plotting In (x-’ - 1) vs. A”+. Figure 4 shows that, even if the concentration of supporting electrolyte is as low as 1 PM, an electrochemical response can clearly be observed for the system given in Cell II: Ag/AgCl/l
M NaC1//0.4
+ 1 /_LMTBATPBCl/O.Ol
mM DB18C6 M TBACl/AgCl/Ag
(Cell II)
Obviously, the influence of ZR drop is becoming significant. The corresponding plot of ln[(Z, - Z)/Z] vs. AI4, illustrated in Fig. 5, gives a straight line with a slope of 17.5 V-‘, indicating an increase in solution resistance, which may be compared with the theoretical value of 22.1 V-l. Figure 6 shows the sodium facilitated transfer in the absence of supporting electrolyte in the organic phase (see Cell III). Despite the fact that the signal is rather distorted, the facilitated transfer remains clearly visible. Ag/AgCl/l
M NaC1//0.4
mM DB18C6/0.01
M TBACl/AgCI/Ag
(Cell III)
207
Fig. 3.
In& - I)/11 vs. AI4
for the data of Fig. l(B).
In this case, the common ion between the organic is still TBA+ because the salt TBACI must be aqueous phase and the wet 1,Zdichloroethane (K,) of TBACl in both phases can be calculated AG$,“,“,
= -RT
In K, = AG$&z!+
phase and the reference solution partitioned slightly between the phase. The partition coefficient from the following equations.
AGtG”=
29.3 k.I mol-’
(3)
using AG$$x!= - 21.7 kJ mol-’ and AG~~~“=51.0 kJ mol-‘. However, it should be stressed that the partition coefficient for TBACI is very small (KP = 7.3 X 10P6) and therefore the concentration of TBACl in 1,ZDCE is in the nanomolar range. The potential difference between the organic phase and the aqueous reference solution is therefore a distribution potential given by: Wk4c,
= [ AGo+“?‘t ,CI
AGt&z!]/2F
= 377 mV
In the case of Cell I, the Galvani potential potential A;E by: AIE = A~I$ - [A‘;;&&,++
(RT/E)
(4)
A‘t;4 is related to the measured cell
In(u&+,+/a!&,+)]
+ constant
(5)
where the constant refers to the difference in chloride concentration between the two aqueous solutions. In the case of Cell III, the Galvani potential is related to the cell potential by A”E = A”4 - A~&,,,,
+ constant
(6) Figure 7 illustrates the change in the reference electrode potential with the concentration of the organic salt. The effect of the different concentrations of
I=02nAt
1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 @p/V
Fig. 4. Potential window (A) and facilitated transfer of sodium (B) using Cell II. (A) Approximate diameter = 12 pm, u = 30 mV/s. (B) Approximate diameter = 20 pm, u = 40 mV/s.
TBATPBCl on the activity coefficients can be calculated by using a combination of the Debye-Hiickel equation at dilute concentration and eqn. (5) (see eqn. (7)). At low concentration of TBATPBCl, eqn. (5) becomes invalid as the TBACl salt is extracted from water to oil and the potential is governed by the distribution potential of TBACl. A;( Aw041’2)exp= A; E:/’ - A: Ei/’ + (RT/F) + (JWF)
ln[ c+L,++/c$i*+]
ln[ rX,+/r%L+]
= Aw,E;‘2 - Aw,E;‘2 + 59 log[ c$;*+/c+;*+] + 628[(~‘,~)l’~ - (poS1)1’2]mV where p” is the ionic strength of the solution.
(7)
209
0.0 -0.5 H I: _
v
-1.0
\
Ai s
\r
-15 -2.0 -2.5 -3.0
\
I
03
04
0.5
0.6
AoUdV
Fig. 5. In&
- 1)/I] vs. A’$$ for the data of Fig. 4(B).
It is interesting to note that the potential windows in the systems studied and illustrated in Figs. l(A), 4(A) and 6(A) increase when the concentration of the supporting electrolyte in 1,Zdichloroethane is decreased, illustrating that the window is limited by the transfer of TBA+ and Na+ as the negative and positive ends (water versus oil), respectively. Table 1 compares experimental results with the predictions of eqn. (2). A shift of the half-wave potential calculated from ln[
TABLE 1 The relationship between the ratio cq/c, C4/Cl
2.5 0.25 0.025 0.0025
and the steady state wave characteristics. c, = 0.4 mM
G#&&
AWE’/2 0 exp
AW’d+‘2)theor
A(A;~“2)exp
S,,
s theor
/mv
/mV
/mV
/mV
/v-’
/v-l
10.7 27.1 68.3 124.6
224 285 380 425
16 41 56
15.5 40.3 46.4
36.0 30.5 22.0 17.5
38.3 33.8 25.7 22.1
a Calculated from Fig. 7.
a
210
(A)
0
0.1
0.2
0.3 $pV
0.4
0.5
0.6
0.7
0.8
Fig. 6. Potential window (A) and facilitated transfer of sodium (B) using Cell III. (A) Approximate diameter = 10 CL,u = 10 mV/s. (B) Approximate diameter = 25 pm, u = 50 mV/s.
the theory was developed for microhemispheres and that the micro-liquid/liquid interfaces used have a behaviour which is comprised between that of a microdisc and that of a microhemisphere [9]. Owing to the reasonable agreement between the experimental data and Oldham’s theory, we can conclude that the micropipette technique, with low concentrations of supporting electrolyte in the organic phase, opens a new route to circumvent the effect of ion-pair formation in the organic phase, especially for low dielectric constant solvents. In conclusion, it is clear that ionic enrichment of the organic phase upon transfer of sodium increases the conductivity of the organic phase in the vicinity of the interface, thus decreasing the ZR drop [ll].
211
log a
Fig. 7. Plot of A”,E vs. log uTBAC..
REFERENCES 1 Y. Cheng, V. Cunnane, D. Schiffrin, L. Murtomaki and K. Kontturi, J. Chem. Sot. Faraday Trans., 87 (19911 107. 2 R.M. Wightman and D.O. Wipl, in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 15, Marcel Dekker, New York, 1986, pp. 267-344. 3 G. Taylor and H.H. Girault, J. Electroanal. Chem., 208 (1986) 179. 4 J.A. Campbell, A.A. Stewart and H.H. Girault, J. Chem. Sot. Faraday Trans. I, 85 (1989) 843. 5 Y. Shao, M.D. Osborne and H.H. Girault, J. Electroanal. Chem., 318 (1991) 101. 6 A.M. Bond, K.B. Oldham and C.G. Zoski, Anal. Chim. Acta, 216 (1989) 177. 7 K.B. Oldham, J. Electroanal. Chem., 250 (1988) 1. 8 A.A. Stewart, Y. Shao, CM. Pereira and H.H. Girault, J. Electroanal. Chem., 305 (1991) 135. 9 A.A. Stewart, G. Taylor, J. Mcaleer and H.H. Girault, J. Electroanal. Chem., 296 (1990) 491. 10 R. Weissberger (Ed.), Techniques of Organic Chemistry, Vol. 7, 2nd edn. Interscience, New York, 1955. 11 K.B. Oldham, in MI. Montenegro, M.A. Queiros and J.L. Daschbach (Eds.), Microelectrodes: Theory and Applications, NATO ASI Series, E 197, Kluwer Academic Publishers, 1991, p. 83.
Chem., 334 (1992) 203-211 Elsevier Sequoia !%A., Lausanne
J. Electroanal.
JEC 02103
Facilitated sodium transfer from aqueous electrolytes resistive media Y. Shao and H.H. Girault Department
of Chemistry,
to
l
University of Edinburgh,
West Mains Road, Edinburgh EH9 3JJ (UK)
(Received 15 November 1991; in revised form 27 February 1992)
Abstract
The facilitated transfer of sodium by dibenzo-18-crown-6 from water to resistive media has been studied using micro liquid/liquid interfaces. The facilitated transfer wave can still be observed when the concentrations of the supporting electrolytes in the organic phase are decreased to micromolar levels. The results obtained have been analysed by the methodology proposed by Oldham (J. Electroanal. Chem., 2.50 (1988) 1) for the oxidation on microelectrodes of ferrocene in acetonitrile in the presence of low concentrations of supporting electrolytes.
INTRODUCTION
The formation of ion-pairs in the organic phase is always an important problem in the study of charge transfer reactions at liquid/liquid interfaces because supporting electrolytes are added at high concentrations to the organic phase to ensure adequate conductivity and to minimize the ohmic drop. This problem is particularly serious for organic solvents with low dielectric constants e.g., 1,2-dichloroethane (E = 10.36 at 25°C). In the case of ion transfer reactions, the high concentration of organic counterions is an impediment both because of bulk and interfacial ion pairing. Ion pairing in the bulk, for example, makes the determination of thermodynamic values such as the Gibbs energy of transfer more difficult to calculate. Interfacial ion pairing, on the other hand, results in specific adsorption [ll. In order to eliminate the effect of ion-pairing during charge transfer reactions, micro-liquid/liquid interfaces [2-51 present real advantages since the inherent low ohmic drop associated with spherical diffusion allows electrochemical measurements with low (or zero> concentrations of supporting electrolytes [6,7].
* Present address: Institut de Chimie Physique, E.P.F. Lausanne, CH 1015 Lausanne, Switzerland. 0022-0728/92/$05.00
0 1992 - Elsevier Sequoia S.A. All rights reserved
204
In the present work, sodium ion transfer, facilitated by dibenzo-18-crown-6 (DB18C6), across water/l,Zdichloroethane (1,2-DCE) micro-interfaces supported at the tip of micropipettes has been studied using small concentrations of organic supporting electrolytes. The facilitated transfer reaction studied is pseudo-first order, with the ionic concentration within the pipette in excess compared with that of the ionophore outside. The ion transfer occurs through a pick-up mechanism by the ionophore, very much like a reduction on a metallic electrode, the bare ionophore being the oxidised species, the complex being the reduced species [5]. Mass transfer of the reactants (i.e the bare ionophores) and of the products (i.e the complexed ions) occurs in a spherical/cylindrical manner, and, consequently, all the methodology developed for metallic microelectrodes can be applied. For this reason, the results obtained have been compared with the work of Bond et al. [61 on the oxidation of ferrocene in acetonitrile on microelectrodes, which has been rationalised by Oldham [7]. EXPERIMENTAL
TBATPBCl (tetrabutylammonium tetrakis[4-chlorophenyllborate), prepared by metathesis of TBACl and KTPBCI, was used as the organic supporting electrolyte. Dibenzo-18-crown-6 (DB18C6, Aldrich, 98%), NaCl (Fisons, AnalR) and 1,2-dichloroethane (1,2-DCE, BDH, AnalaR) were employed for the preparation of the solutions. Kwik-fill glass capillaries (Clark Electromedical Ltd, U.K.) and a vertical pipette puller Kopf 720 (U.S.A.) have been used for the fabrication of the micropipettes. The cell and the instrumentation used were described in refs. 3,4 and 8. RESULTS
AND DISCUSSION
Figure l(A) shows the potential window (x = 0) for the following system: Ag/AgCl/l M NaCl//x mM DB18C6 + 0.1 mM TBATPBCl/O.Ol
M TBACI/AgCl/Ag
(Cell I)
In the presence of 0.4 mM DB18C6, sodium transfer across the water/l,2-DCE interface facilitated by DB18C6 is observed clearly within the potential window (see Fig. l(B)) with a half-wave potential value of 285 mV referred to cell I. When the concentration of the supporting electrolyte in the organic phase is higher than 1 mM, plots of In[(Z, - Z)/Z] vs. A”4 have a slope which approaches the theoretical value of 39.6 V-’ corresponding to F/RT at 20°C. This proves that, within the range of pipette radii used (5 to 15 pm), the pseudo-first order ion transfer reaction appears to be reversible. However, when the concentration of the supporting electrolyte decreases, the slope of this type of plot decreases indicating the influence of increasing ZR drop and of migration mass transport. According to the theory which has been developed by Oldham [7] for voltammetry at metallic hemispherical microelectrodes in resistive media, the following
20.5
0
OA
02 03 0.4 0.5 0.6 0.7 lg+/v
Fig. 1. Potential window (A) and facilitated transfer of sodium (B) using Cell 1. (A) Approximate interface diameter = 10 pm. Scan rate u = 30 mV/s. (B) Approximate interface diameter = 30 pm. u = 100 mV/s.
equation can also be employed to describe the shape of the steady-state wave when the concentration of the reactant is higher than that of the supporting electrolyte:
A{4 = AI4’ + (RT/F) In[ [ D,Z(4D,c,Z,,
+ D,c,Z)]/[4D&,Z,(
Id -Z)]]
(1) In the present system, the reactant is the free ionophore and the product the complexed ion. Thus, D,, D, are the diffusion coefficients of DB18C6 and the complexed ion in the organic phase respectively; ci and cq are the bulk concentrations of DB18C6 and of the supporting electrolyte in the organic phase; I,, is the steady state plateau current. If we write x = Z/Z,, and y = c,/c,, and taking into account that the ratio of the diffusion coefficients of the ionophore and the complexed ion is equal to D,/D, = 1.6 [5], then the above equation becomes: A$4=Al4’+(RT/F) ln[(1.6x+0.64x2/y)/(l-x)] (2)
206
-0.2
I
-200
I
-100
0
/
100
1
200
300
(A$ @ - AZ @) / mV Fig. 2. Plots of X vs. [A:4 III.
- A’&b’]. y = (1) 2.5, (2) 1, (3) 0.25, as in cell I, (4) 0.025, (5) 0.0025, as in cell
Figure 2 shows the predicted steady-state voltammograms for the facilitated sodium transfer at a microelectrode according to this equation for different ratios of y = cd/cl. It can be observed that as the supporting electrolyte concentration decreases the half-wave potential shifts, as if the rate of the transfer becomes slower (eqn. 2 assumes the reversibility of the charge transfer reaction). Figure 3 shows the plot ln[(Z, - Z)/Z] vs. A”,$ for the data of Fig. l(B) which is linear, with a slope equal to 30.5 V-‘. This value may be compared with the theoretical value of 33.8 V-t obtained by plotting In (x-’ - 1) vs. A”+. Figure 4 shows that, even if the concentration of supporting electrolyte is as low as 1 PM, an electrochemical response can clearly be observed for the system given in Cell II: Ag/AgCl/l
M NaC1//0.4
+ 1 /_LMTBATPBCl/O.Ol
mM DB18C6 M TBACl/AgCl/Ag
(Cell II)
Obviously, the influence of ZR drop is becoming significant. The corresponding plot of ln[(Z, - Z)/Z] vs. AI4, illustrated in Fig. 5, gives a straight line with a slope of 17.5 V-‘, indicating an increase in solution resistance, which may be compared with the theoretical value of 22.1 V-l. Figure 6 shows the sodium facilitated transfer in the absence of supporting electrolyte in the organic phase (see Cell III). Despite the fact that the signal is rather distorted, the facilitated transfer remains clearly visible. Ag/AgCl/l
M NaC1//0.4
mM DB18C6/0.01
M TBACl/AgCI/Ag
(Cell III)
207
Fig. 3.
In& - I)/11 vs. AI4
for the data of Fig. l(B).
In this case, the common ion between the organic is still TBA+ because the salt TBACI must be aqueous phase and the wet 1,Zdichloroethane (K,) of TBACl in both phases can be calculated AG$,“,“,
= -RT
In K, = AG$&z!+
phase and the reference solution partitioned slightly between the phase. The partition coefficient from the following equations.
AGtG”=
29.3 k.I mol-’
(3)
using AG$$x!= - 21.7 kJ mol-’ and AG~~~“=51.0 kJ mol-‘. However, it should be stressed that the partition coefficient for TBACI is very small (KP = 7.3 X 10P6) and therefore the concentration of TBACl in 1,ZDCE is in the nanomolar range. The potential difference between the organic phase and the aqueous reference solution is therefore a distribution potential given by: Wk4c,
= [ AGo+“?‘t ,CI
AGt&z!]/2F
= 377 mV
In the case of Cell I, the Galvani potential potential A;E by: AIE = A~I$ - [A‘;;&&,++
(RT/E)
(4)
A‘t;4 is related to the measured cell
In(u&+,+/a!&,+)]
+ constant
(5)
where the constant refers to the difference in chloride concentration between the two aqueous solutions. In the case of Cell III, the Galvani potential is related to the cell potential by A”E = A”4 - A~&,,,,
+ constant
(6) Figure 7 illustrates the change in the reference electrode potential with the concentration of the organic salt. The effect of the different concentrations of
I=02nAt
1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 @p/V
Fig. 4. Potential window (A) and facilitated transfer of sodium (B) using Cell II. (A) Approximate diameter = 12 pm, u = 30 mV/s. (B) Approximate diameter = 20 pm, u = 40 mV/s.
TBATPBCl on the activity coefficients can be calculated by using a combination of the Debye-Hiickel equation at dilute concentration and eqn. (5) (see eqn. (7)). At low concentration of TBATPBCl, eqn. (5) becomes invalid as the TBACl salt is extracted from water to oil and the potential is governed by the distribution potential of TBACl. A;( Aw041’2)exp= A; E:/’ - A: Ei/’ + (RT/F) + (JWF)
ln[ c+L,++/c$i*+]
ln[ rX,+/r%L+]
= Aw,E;‘2 - Aw,E;‘2 + 59 log[ c$;*+/c+;*+] + 628[(~‘,~)l’~ - (poS1)1’2]mV where p” is the ionic strength of the solution.
(7)
209
0.0 -0.5 H I: _
v
-1.0
\
Ai s
\r
-15 -2.0 -2.5 -3.0
\
I
03
04
0.5
0.6
AoUdV
Fig. 5. In&
- 1)/I] vs. A’$$ for the data of Fig. 4(B).
It is interesting to note that the potential windows in the systems studied and illustrated in Figs. l(A), 4(A) and 6(A) increase when the concentration of the supporting electrolyte in 1,Zdichloroethane is decreased, illustrating that the window is limited by the transfer of TBA+ and Na+ as the negative and positive ends (water versus oil), respectively. Table 1 compares experimental results with the predictions of eqn. (2). A shift of the half-wave potential calculated from ln[
TABLE 1 The relationship between the ratio cq/c, C4/Cl
2.5 0.25 0.025 0.0025
and the steady state wave characteristics. c, = 0.4 mM
G#&&
AWE’/2 0 exp
AW’d+‘2)theor
A(A;~“2)exp
S,,
s theor
/mv
/mV
/mV
/mV
/v-’
/v-l
10.7 27.1 68.3 124.6
224 285 380 425
16 41 56
15.5 40.3 46.4
36.0 30.5 22.0 17.5
38.3 33.8 25.7 22.1
a Calculated from Fig. 7.
a
210
(A)
0
0.1
0.2
0.3 $pV
0.4
0.5
0.6
0.7
0.8
Fig. 6. Potential window (A) and facilitated transfer of sodium (B) using Cell III. (A) Approximate diameter = 10 CL,u = 10 mV/s. (B) Approximate diameter = 25 pm, u = 50 mV/s.
the theory was developed for microhemispheres and that the micro-liquid/liquid interfaces used have a behaviour which is comprised between that of a microdisc and that of a microhemisphere [9]. Owing to the reasonable agreement between the experimental data and Oldham’s theory, we can conclude that the micropipette technique, with low concentrations of supporting electrolyte in the organic phase, opens a new route to circumvent the effect of ion-pair formation in the organic phase, especially for low dielectric constant solvents. In conclusion, it is clear that ionic enrichment of the organic phase upon transfer of sodium increases the conductivity of the organic phase in the vicinity of the interface, thus decreasing the ZR drop [ll].
211
log a
Fig. 7. Plot of A”,E vs. log uTBAC..
REFERENCES 1 Y. Cheng, V. Cunnane, D. Schiffrin, L. Murtomaki and K. Kontturi, J. Chem. Sot. Faraday Trans., 87 (19911 107. 2 R.M. Wightman and D.O. Wipl, in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 15, Marcel Dekker, New York, 1986, pp. 267-344. 3 G. Taylor and H.H. Girault, J. Electroanal. Chem., 208 (1986) 179. 4 J.A. Campbell, A.A. Stewart and H.H. Girault, J. Chem. Sot. Faraday Trans. I, 85 (1989) 843. 5 Y. Shao, M.D. Osborne and H.H. Girault, J. Electroanal. Chem., 318 (1991) 101. 6 A.M. Bond, K.B. Oldham and C.G. Zoski, Anal. Chim. Acta, 216 (1989) 177. 7 K.B. Oldham, J. Electroanal. Chem., 250 (1988) 1. 8 A.A. Stewart, Y. Shao, CM. Pereira and H.H. Girault, J. Electroanal. Chem., 305 (1991) 135. 9 A.A. Stewart, G. Taylor, J. Mcaleer and H.H. Girault, J. Electroanal. Chem., 296 (1990) 491. 10 R. Weissberger (Ed.), Techniques of Organic Chemistry, Vol. 7, 2nd edn. Interscience, New York, 1955. 11 K.B. Oldham, in MI. Montenegro, M.A. Queiros and J.L. Daschbach (Eds.), Microelectrodes: Theory and Applications, NATO ASI Series, E 197, Kluwer Academic Publishers, 1991, p. 83.