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Ion transfer across the immiscible water—benzonitrile interface
291
J. Efectroamd.
Chem., 261 (1989) 297-300
Elsevier SequoiaS.A., Lausanne - Printed in The Netherlands
Ion transfer across the immiscible water-benzonitrile interface Hailemichael Alemu aud ‘lhx&os Departmeni
Solomon
of Chemistry, Addis Ababa University, P.O. Box II 76, Addis Ababa ~Ethiopia~
(Received 31 October 1988)
ABSTRACT Standard Gibbs energiesof ion transferacross the water-benzonitrik interfacehave been determined voIt~et~c~y using the tetr~h~yl~~~~ tetraphenylborateassumption.The results are compared with reported values obtained from solubilitydata.
INTRODUCTION
A number of electrochemical investigations have been carried out in benzonitrile. This solvent enables voltammetric investigations to be carried out over a wide potential range. It is a strongly polar aromatic solvent which dissolves many organic substances, anhydrous inorganic salts and organometallic compounds. It has a relatively high dielectric constant (25.2), a low miscibility with water (0.2% wt benzonitrile in water and 1% wt water in benzonitrile) and a density of 1.01 (at 15°C) [l]. It is not a strongly coordinating solvent as seen from its Gutman donor-acceptor number of 15 [2]. Recently, single ion Gibbs energies for transfer to benzonitrile derived from solubility data were reported [3]. In the present work we present the results of voltmeter studies of ion transfer across the water-be~o~~le interface. It should be recalled that Koczorowski et al. [4] studied the water/mixed organic solvents system using benzonitrile + nitrobenzene as the organic phase to observe the relative influence of the nature of the organic solvent and permittivity. However, ion transfers across the water-benzonitrile (BN) interface were not reported. Standard Gibbs energies of ion transfer have now been determined using the te~aphenyl~so~um tetraphenyl~rate (TPAsTPB) ass~ption, as was done for other immiscible systems such as water-acetophenone [S] and water-o-uitrotoluene [6]. It is believed that the present system adds usefully to the data available from the limited immiscible water-oil interfaces investigated so far [C-12]. 0022-0728,‘89/$03.50
0 1989 ElsevierSequoiaS.A.
298 EXPERIMENTAL
DC cyclic voltammetric studies were carried out on the cell Ag/AgCl/TBACl (W~/TBATPB (BN)/LiCl CW>/AgCl/Ag using a four-electrode potentiostat with IR compensation. The electrochemical cell, of a design similar to that used in ref. 13, had an interfacial area of 0.283 cm2. The supporting electrolyte used in benzonitrile was 10m2 M tetrabutylammonium tetraphenylborate (TBATPB; Fluka), although tetraphenylarsonium tetraphenylborate (TPAsTPB) was employed to fix A”d97 = 0. In the aqueous phase, 10m2 44 LiCl (BDH) was used as supporting electrolyte. The sodium salts of ClO;, IO; and picrate (BDH) and the chloride salts of TMA+, TEA+ and TBA+ (BDH) were investigated. RESULTS AND DISCUSSION
Figure 1 shows the dc cyclic volt~o~~ of the transfer of ClO; from water to benzonitrile using 10e2 M TBATPB in BN and 10v2 M LiCl in water as base electrolytes. The ion transfer was found to be reversible, as seen from the peak separation of 60 mV. This was also found to be the case for the other ions investigated, up to sweep rates of 100 mV/s. The dependences of Ii, vs. the square root of the sweep rate (u”‘) for a given concentration of four ions, as well as those of lr, vs. the ~ncentration of these ions at a given sweep rate, were found to be linear. Such dependences are shown in Fig. 2.
-60.00
Fig. 1. Cyclic voltammogram for the transfer of 1O-3 M CIO; across the water-benzonitrile interface ); supporting eIectroIytes IO-’ M TBATPB in BN and low2 iw Lit3 in water (- - -).
(-
299
80.00 60.00 40.00 20.00 i-
b
60
.oo-
40.00
-
20.00
-
0.001 0.00
4.00
12 .oo
0.00
I
104t/M
Fig. 2. (a) Peak current ( Zp) vs. square root of the sweep rate (u”‘) for 8 x 10m4 M Cloy , IO;, TMAC and Pi-. (b) Peak current (I,,) vs. concentrations of ClOi , IO;, TMA+ and Pi- at a sweep rate of 40 mV/s.
From the A’&J,, values, the half-wave potentials ( A~~1,2) were determined. These were then used to calculate the Azq” values using the diffusion coefficient of the ions in water calculated from the conductivity data given in ref. 14 and the diffusion coefficients of the ions in benzonitrile estimated from the ion conductivity data reported by Coetzee and Cunningham [15]. The ratio of the activity coefficients was assumed to be unity; ion association in the organic phase was not considered. It has been shown [la] that use of the ion-pair formation constant K, and the ion size parameter in the extended Debye-Htickel equation for the calculation of activity coefficients does not significantly affect the AG o values. The A:,cp“ values were then converted into AwG~,i and these are listed in Table 1. For comparison, the data of ref. 3 and the values of AIGc,i calculated from the theoretical equations given by Abraham and Liszi [17,18] for non-hydrated and fully hydrated ions in the organic phase are also given. It should be noted that the voltammetric experiments are done on mutually saturated solvents, whereas the solubility measurements are carried out
300
TABLE
1
Standard electrical potential differences, A’&“, and standard Gibbs energies of transfer of ions, AIG&, obtained experimentally and theoretically for the water-benzonitrile system Ion
A%“/V
TMA+ TEA+ TBA+ TPAs+ ClO,IO,piTPB-
0.072 - 0.042 -0.187 - 0.216 - 0.050 - 0.029 0.068 0.216
AIGC,, /kJ mol-
’
a
b
6.9 -4.1 - 18.0 - 20.8 4.8 2.8 -6.6 -20.8
2.5 -4.8 - 35.3 13.2 0.4 -35.3
c
d
18.6 8.1 - 17.4
0.8 -6.4 - 28.5
7.5 6.3 0.6
- 11.3 - 11.8 - 14.4
a From the present experimental results. b From the solubility data of ref. 3. ’ Calculated for non-hydrated ions in the organic phase (in mol 1-l scale) using the radius of chlorobenzene, r = 0.2769 nm, and the data for nitrobenzene (m = -3.2165, c =10.1899) in eqns. (6) and (7) of ref. 18. d Calculated for fully hydrated ions in the organic phase in mol 1-l scale.
in the pure dry solvents; hence the discrepancies observed between the two experimental results are not unexpected. The discrepancies are much larger than those observed in the nitrobenzene-water and 1,2-dichloroethane-water systems [8]. This may be due to the slightly higher mutual solubility of water and benzonitrile. However, as in previous immiscible systems [5,6,10], the voltammetrically obtained A:Gt,i values lie in between the theoretically estimated extremes. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
J.A. Riddick and W.B. Bunger, Organic Solvents, 3rd ed., Interscience Wiley, New York, 1970. K. Kadish and J. Anderson, Pure Appl. Chem., 59 (1987) 704. A.F. Danil de Namor and H. Berroa de Ponce, J. Chem. Sot., Faraday Trans. 1, 83 (1987) 1569. 2. Koczorowski, I. Paleska and G. Geblewicz, J. Electroanal Chem., 164 (1984) 201. T. Solomon, H. Alemu and B. Hundhammer, J. Electroanal. Chem., 169 (1984) 303. H. AIemu and T. Solomon, J. Electroanal. Chem., 237 (1987) 113. J. Koryta, P. Vanysek and M. Brezina, J. Electroanal. Chem., 75 (1977) 211. B. Hundhammer and T. Solomon, J. Electroanal. Chem., 157 (1983) 19. 2. Koczorowski, G. Geblewicz and I. PaIeska, J. Electroanal. Chem., 172 (1984) 327. T. Solomon, H. Alemu and B. Hundhammer, J. Electroanal. Chem., 169 (1984) 311. Z. Samec, D. HomoIka, V. Marecek and L. Kavan, J. Electroanal. Chem., 145 (1983) 213. S. Kihara, M. Suzuki, K. Maeda, K. Ogura and M. Matsui, J. Electroanal. Chem., 210 (1986) 147. G. Geblewicz and D.J. Schiffrin, J. Electroanal. Chem., 244 (1988) 27. R.A. Robinson and R.H. Stockes, Electrolyte Solutions, Butterworths Scientific Publications, London, 1959. J.F. Coetzee and G.P. Cunningham, J. Am. Chem. Sot., 87 (1965) 2529. A.F. Danil de Namor, T. Hill and E. Sigstad, J. Chem. Sot., Faraday Trans. 1, 79 (1983) 2713. M.H. Abraham and J. Liszi, J. Chem. Sot., Faraday Trans. 1, 74 (1978) 1604. M.H. Abraham and J. Lisxi, J. Inorg. Nucl. Chem., 43 (1981) 143.
J. Efectroamd.
Chem., 261 (1989) 297-300
Elsevier SequoiaS.A., Lausanne - Printed in The Netherlands
Ion transfer across the immiscible water-benzonitrile interface Hailemichael Alemu aud ‘lhx&os Departmeni
Solomon
of Chemistry, Addis Ababa University, P.O. Box II 76, Addis Ababa ~Ethiopia~
(Received 31 October 1988)
ABSTRACT Standard Gibbs energiesof ion transferacross the water-benzonitrik interfacehave been determined voIt~et~c~y using the tetr~h~yl~~~~ tetraphenylborateassumption.The results are compared with reported values obtained from solubilitydata.
INTRODUCTION
A number of electrochemical investigations have been carried out in benzonitrile. This solvent enables voltammetric investigations to be carried out over a wide potential range. It is a strongly polar aromatic solvent which dissolves many organic substances, anhydrous inorganic salts and organometallic compounds. It has a relatively high dielectric constant (25.2), a low miscibility with water (0.2% wt benzonitrile in water and 1% wt water in benzonitrile) and a density of 1.01 (at 15°C) [l]. It is not a strongly coordinating solvent as seen from its Gutman donor-acceptor number of 15 [2]. Recently, single ion Gibbs energies for transfer to benzonitrile derived from solubility data were reported [3]. In the present work we present the results of voltmeter studies of ion transfer across the water-be~o~~le interface. It should be recalled that Koczorowski et al. [4] studied the water/mixed organic solvents system using benzonitrile + nitrobenzene as the organic phase to observe the relative influence of the nature of the organic solvent and permittivity. However, ion transfers across the water-benzonitrile (BN) interface were not reported. Standard Gibbs energies of ion transfer have now been determined using the te~aphenyl~so~um tetraphenyl~rate (TPAsTPB) ass~ption, as was done for other immiscible systems such as water-acetophenone [S] and water-o-uitrotoluene [6]. It is believed that the present system adds usefully to the data available from the limited immiscible water-oil interfaces investigated so far [C-12]. 0022-0728,‘89/$03.50
0 1989 ElsevierSequoiaS.A.
298 EXPERIMENTAL
DC cyclic voltammetric studies were carried out on the cell Ag/AgCl/TBACl (W~/TBATPB (BN)/LiCl CW>/AgCl/Ag using a four-electrode potentiostat with IR compensation. The electrochemical cell, of a design similar to that used in ref. 13, had an interfacial area of 0.283 cm2. The supporting electrolyte used in benzonitrile was 10m2 M tetrabutylammonium tetraphenylborate (TBATPB; Fluka), although tetraphenylarsonium tetraphenylborate (TPAsTPB) was employed to fix A”d97 = 0. In the aqueous phase, 10m2 44 LiCl (BDH) was used as supporting electrolyte. The sodium salts of ClO;, IO; and picrate (BDH) and the chloride salts of TMA+, TEA+ and TBA+ (BDH) were investigated. RESULTS AND DISCUSSION
Figure 1 shows the dc cyclic volt~o~~ of the transfer of ClO; from water to benzonitrile using 10e2 M TBATPB in BN and 10v2 M LiCl in water as base electrolytes. The ion transfer was found to be reversible, as seen from the peak separation of 60 mV. This was also found to be the case for the other ions investigated, up to sweep rates of 100 mV/s. The dependences of Ii, vs. the square root of the sweep rate (u”‘) for a given concentration of four ions, as well as those of lr, vs. the ~ncentration of these ions at a given sweep rate, were found to be linear. Such dependences are shown in Fig. 2.
-60.00
Fig. 1. Cyclic voltammogram for the transfer of 1O-3 M CIO; across the water-benzonitrile interface ); supporting eIectroIytes IO-’ M TBATPB in BN and low2 iw Lit3 in water (- - -).
(-
299
80.00 60.00 40.00 20.00 i-
b
60
.oo-
40.00
-
20.00
-
0.001 0.00
4.00
12 .oo
0.00
I
104t/M
Fig. 2. (a) Peak current ( Zp) vs. square root of the sweep rate (u”‘) for 8 x 10m4 M Cloy , IO;, TMAC and Pi-. (b) Peak current (I,,) vs. concentrations of ClOi , IO;, TMA+ and Pi- at a sweep rate of 40 mV/s.
From the A’&J,, values, the half-wave potentials ( A~~1,2) were determined. These were then used to calculate the Azq” values using the diffusion coefficient of the ions in water calculated from the conductivity data given in ref. 14 and the diffusion coefficients of the ions in benzonitrile estimated from the ion conductivity data reported by Coetzee and Cunningham [15]. The ratio of the activity coefficients was assumed to be unity; ion association in the organic phase was not considered. It has been shown [la] that use of the ion-pair formation constant K, and the ion size parameter in the extended Debye-Htickel equation for the calculation of activity coefficients does not significantly affect the AG o values. The A:,cp“ values were then converted into AwG~,i and these are listed in Table 1. For comparison, the data of ref. 3 and the values of AIGc,i calculated from the theoretical equations given by Abraham and Liszi [17,18] for non-hydrated and fully hydrated ions in the organic phase are also given. It should be noted that the voltammetric experiments are done on mutually saturated solvents, whereas the solubility measurements are carried out
300
TABLE
1
Standard electrical potential differences, A’&“, and standard Gibbs energies of transfer of ions, AIG&, obtained experimentally and theoretically for the water-benzonitrile system Ion
A%“/V
TMA+ TEA+ TBA+ TPAs+ ClO,IO,piTPB-
0.072 - 0.042 -0.187 - 0.216 - 0.050 - 0.029 0.068 0.216
AIGC,, /kJ mol-
’
a
b
6.9 -4.1 - 18.0 - 20.8 4.8 2.8 -6.6 -20.8
2.5 -4.8 - 35.3 13.2 0.4 -35.3
c
d
18.6 8.1 - 17.4
0.8 -6.4 - 28.5
7.5 6.3 0.6
- 11.3 - 11.8 - 14.4
a From the present experimental results. b From the solubility data of ref. 3. ’ Calculated for non-hydrated ions in the organic phase (in mol 1-l scale) using the radius of chlorobenzene, r = 0.2769 nm, and the data for nitrobenzene (m = -3.2165, c =10.1899) in eqns. (6) and (7) of ref. 18. d Calculated for fully hydrated ions in the organic phase in mol 1-l scale.
in the pure dry solvents; hence the discrepancies observed between the two experimental results are not unexpected. The discrepancies are much larger than those observed in the nitrobenzene-water and 1,2-dichloroethane-water systems [8]. This may be due to the slightly higher mutual solubility of water and benzonitrile. However, as in previous immiscible systems [5,6,10], the voltammetrically obtained A:Gt,i values lie in between the theoretically estimated extremes. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
J.A. Riddick and W.B. Bunger, Organic Solvents, 3rd ed., Interscience Wiley, New York, 1970. K. Kadish and J. Anderson, Pure Appl. Chem., 59 (1987) 704. A.F. Danil de Namor and H. Berroa de Ponce, J. Chem. Sot., Faraday Trans. 1, 83 (1987) 1569. 2. Koczorowski, I. Paleska and G. Geblewicz, J. Electroanal Chem., 164 (1984) 201. T. Solomon, H. Alemu and B. Hundhammer, J. Electroanal. Chem., 169 (1984) 303. H. AIemu and T. Solomon, J. Electroanal. Chem., 237 (1987) 113. J. Koryta, P. Vanysek and M. Brezina, J. Electroanal. Chem., 75 (1977) 211. B. Hundhammer and T. Solomon, J. Electroanal. Chem., 157 (1983) 19. 2. Koczorowski, G. Geblewicz and I. PaIeska, J. Electroanal. Chem., 172 (1984) 327. T. Solomon, H. Alemu and B. Hundhammer, J. Electroanal. Chem., 169 (1984) 311. Z. Samec, D. HomoIka, V. Marecek and L. Kavan, J. Electroanal. Chem., 145 (1983) 213. S. Kihara, M. Suzuki, K. Maeda, K. Ogura and M. Matsui, J. Electroanal. Chem., 210 (1986) 147. G. Geblewicz and D.J. Schiffrin, J. Electroanal. Chem., 244 (1988) 27. R.A. Robinson and R.H. Stockes, Electrolyte Solutions, Butterworths Scientific Publications, London, 1959. J.F. Coetzee and G.P. Cunningham, J. Am. Chem. Sot., 87 (1965) 2529. A.F. Danil de Namor, T. Hill and E. Sigstad, J. Chem. Sot., Faraday Trans. 1, 79 (1983) 2713. M.H. Abraham and J. Liszi, J. Chem. Sot., Faraday Trans. 1, 74 (1978) 1604. M.H. Abraham and J. Lisxi, J. Inorg. Nucl. Chem., 43 (1981) 143.