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Study of the Ph4SbBPh4 assumption for single ion free energies in some organic solvents
Thermochimica Acta, 140 (1989) 7-11 Elsevier Science Publishers B.V., Amsterdam
- Printed
in The Netherlands
STUDY OF THE Ph,SbBPh, ASSUMPTION FOR SINGLE ION FREE ENERGIES IN SOME ORGANIC SOLVENTS
ESAM A. GOMAA
*
Chemistry Department,
Faculty of Science, Mansoura University, Mansoura (Egypt)
(Received
1988)
24 February
ABSTRACT
A critical study has been carried out to verify the Ph,SbBPh, electrolyte assumption, in which the standard free energies of transfer of the electrolyte AG* from one solvent to another are divided equally between the cation and the anion. A discussion is presented on the basis of a model which treats the total solvation energy of an ion as a composite of electrostatic and non-electrostatic contributions. The neutral molecules Ph,Sn and Ph,C, are proved to be (from their sizes and structures) neutral analogues of the reference ions Ph$b+ and Ph,B-, respectively. The single free energies of the reference ions Ph$b+ and Ph,Bwere also estimated. On plotting the relationship between the free energies of transfer of Ph,SbBPh, against Gutmann’s acceptor and donor numbers, good agreement was obtained, indicating the similarity of the electrostatic charges on the two reference ions.
INTRODUCTION
The estimation of single ion free energies requires extra thermodynamic assumptions, for which a wide variety of methods have been introduced. The existing methods have been continuously discussed by a number of workers [l-6] and most extensively by Parker [2] and Popovych [3]. The methods [7-lo] that have been used most extensively for single ion thermodynamic determination are based on the assumption that the thermodynamic values for a large reference cation can be equal to those of a reference anion of molecular similarity. Different pairs of large reference ions have been suggested, i.e. TABPh,B (TAB, triisoamyl-n-butylammonium) [ll] and Ph,AsBPh, [12]. It has been proved by Kim [13,14] that Ph,AsBPh, is an asymmetric model. The aim of this work is to study the plausibility of Ph,SbBPh, electrolyte for single ion thermodynamic determination.
* Present
address:
0040-6031/89/$03.50
Teachers
Training
College, Salalah-P.O.19905,
0 1989 Elsevier Science Publishers
B.V.
Sultanate
of Oman.
8 EXPERIMENTAL
Tetraphenylantimony tetraphenylborate (Ph,SbBPh,) was prepared by adding tetraphenylantimony bromide (Ph,SbBr; Alfa Inorganic Chemicals) to sodium tetraphenylborate (NaBPh,; Merck) solutions in water, followed by filtration and recrystallization (several times) from acetonitrile. The Ph,Sn used (Alfa Inorganic Chemicals) was purified by repeated crystallization from chloroform. The solvents acetonitrile (MeCN) and dimethylacetamide (DMA) were obtained from Merck, whereas N,N-dimethylformamide (DMF), dimethylsulphoxide (DMSO), dioxane and ethanol (EtOH) were obtained from BDH. The solvents were used without further purification (a dehydrating agent was added for storage). Saturated solutions of Ph,Sn and Ph,SbBPh, in the organic solvents were prepared by shaking solutions (10 ml) in test tubes in a water thermostat (Assistent) for 1 week, followed by 2 days without shaking [15]. The saturated solutions (3-5 ml) were then evaporated to dryness in a small beaker under an IR lamp. These were dissolved in ethanol and their solubilities were determined spectroscopically (UV) using a Jobin Yvon JY 101 spectrophotometer and a calibration curve. In the case of water, saturated solutions of Ph,Sn and Ph,SbBPh, were prepared in 1 1 of water. Samples (250 ml) were shaken with 10 ml of chloroform in a separating funnel. The choroform layer was extracted, evaporated to dryness, dissolved in ethanol and the solubility of the tetraphenyl derivatives was determined spectrophotometrically.
RESULTS
AND DISCUSSION
To determine the partial molar volume of Ph,SbBPh,, the density was measured with a digital oscillator densimeter (Heraeous-Paar DMA-50) in dimethylformamide (DMF). The molar volume of a solute in solution is evaluated by the relation [16] ~ _ M I (d,-dW03 ”
d
mdd,
(1)
where M is the molecular weight of the solute, m is its concentration and d and d, are the densities of water and the solution, respectively. The volume of Ph4Sb+ was estimated by subtraction of the volume of Ph,B- [17] from the volume of the salt electrolyte. The van der Waals’ volume of the cation (V,, Ph,Sb+) was estimated by subtracting the Ph,B- value given in ref. 17 from the van der Waals’ volume of Ph,SbBPh, calculated using the Bondi model [18]. From the experimental data and the calculated van der Waals’ volumes, the packing density Pd of each molecule is calculated by
(2)
9 TABLE 1 Van der Waals’ volume V,, molar volume &, packing density Pd and volume quotient f for tetraphenyl molecules
V,(cm3 mol-‘) r, (cm) & (cm3 mol- ‘) pd
Ph,C =
Ph,B-
186.7 4.2x10-’ 278.1 0.671
188.2 4.21 x lo-* 280.6 0.671
a
Ph,Sn
Ph,Sb+
204.32 4.59 x 1o-8 304.36 0.671
205.84 4.61 x 1O-8 307.38 0.669
fra = V,Ph,Sn/l/‘,Ph,C =1.0943 f,” = FwPh,Sb+/&Ph,B=1.0937 fr = cpvPh,Sn/&Ph,C =1.0944 fi = &Ph,Sb+/&Ph,B= 1.0954 a Ref. 17.
The calculated Pd values are given in Table 1; the average value is found to be 0.671 If: 0.02, which agrees well with the data of King [19] for large molecules. The van der Waals’ volumes and the partial molar volumes provide evidence for the similarity in molecular size of Ph,Sn and Ph,Sb+ and Ph,C and Ph,B-. The data given in Table 1 indicate that the reference cation (Ph,Sb+) and the reference anion (Ph,B-) do not have equal molecular volumes, although the difference is small. From the molal solubility values of Ph,SbBPh, in the solvents under consideration, the pK values and the free energies of transfer A”,G* from water to the organ;: solvents were calculated following eqns. (3) and (4) [15] and their values are listed in Table 2. p&r= -210gc+210gy* (3) A;G* = 2-303RT[pK,,,,, - p&r,,,] (4) C is the molal solubility, y+ is the activity coefficient [Ml, s denotes organic solvent and w denotes water molecules. By subtracting A”,G* values for TABLE 2 Free energies of transfer of Ph,SbBPh, scale at 25 o C)
from water to organic solvent (kcal mol-‘)
(molal
Solvents
Molar solubility
Molal solubility
log Y*
Ph,SbBPh, *G+ As,Ge
Ph,AsBPh, A”,G- a
MeCN DMF DMSO DMA Dioxane EtOH H,O
3.5761 x 1O-3 0.08146 0.03705 0.04756 1.6012~ 10m3 4.877 x 1O-5 2.9446~10-~
4.6031 x 1O-3 0.0863 0.03381 0.05077 1.5593 X 10e3 6.2116 x 1O-5 2.953Ox1O-9
-
6.6387 3.6772 4.4211 4.1585 7.7434 11.460 23.297
-
a Mean values in ref. 17.
0.0966 0.2840 0.1497 0.2300 0.0314 0.1307 0.0103
- 16.655 - 19.615 - 18.871 - 19.134 -15.549 - 11.830 ‘0
16.09 18.52 17.86 18.53 - 10.59 0
10 TABLE 3 Free energies of transfer from water to the organic solvents of the reference cations Ph,As+ ) and reference anion Ph,Bat 25O C (kcal mol-’ in molal scale) Solvents
AS,G*(Ph,B-)
ASwG*(Ph$b+)
MeCN DMF DMSO DMA EtOH
-
-
H2O
7.96 8.98 8.44 9.14 5.02 0
(Ph$b+,
AS,G*(Ph,As+) -
8.13 9.54 9.42 9.39 5.57 0
8.33 9.48 9.09 9.65 5.36 0
tetraphenylantimony tetraphenylborate from those of tetraphenylborate given in ref. 13, the corresponding values for Ph,Sb+ were evaluated. A”,G* values for the ions Ph,B-, Ph4Sb+ and Ph,As+ are shown in Table 3. The coefficient of AsWG*(Ph,Sbf)/AsWG*(Ph,As+) has a mean value of 1.006 and A”,G*(Ph,Sb+)/A”,G*(Ph,B-) has a mean value of 1.0671, indicating the plausibility of the Ph,SbBPh, assumption for the determination of single ion thermodynamics.
v
DMF
I
12
16
Donor (DN) or
I
I
I
I
I
2L
28
0
20
32
36
LO
acceptor IAN)
numbers
Fig. 1. Plots of free energies of transfer of Ph,SbBPh, the donor (a) and acceptor (b) numbers of solvents.
from water to organic solvents
against
11
On plotting the relationship between the free energies of transfer of Ph,SbBPh, against Gutmann’s acceptor (AN) and donor numbers (DN) [20,21] volcanic ( A ) and antivolcanic ( V ) relations were obtained (see Fig. 1). The lines (a) and (b) in Fig. 1 are symmetric, indicating that opposite relations proceed on the reference electrolyte, i.e. oxidation and reduction for DN and reduction and oxidation for AN at the sites of reference cation (Ph,Sb+) and reference anion (Ph,B-). That is to say there is a symmetry between the electrostatic free energies of the reference cation and anion.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
H.-M. Koepp, H. Wendt and H. StrehIow, Z. Elektrochem., 64 (1960) 483. A.J. Parker, Chem. Rev., 69 (1969) 1. 0. Popovych, Crit. Rev. Anal Chem., 1 (1970) 73. R.G. Bates, Determination of pH, Theory and Practice, Wiley, New York, 1973. I.M. Kolthoff, Pure Appl. Chem., 25 (1971) 305. K. Schawbe and Chr. Qeck, Bull. Sot. Chem. Belg., 433 (1974) 39. B.G. Cox, G.R. Hedwig, A.J. Parker and W.E. Wattes, Aust. J. Chem., 27 (1974) 477. I.M. Kolthoff and M.K. Chantooni, Jr., J. Phys. Chem., 76 (1972) 2024. 0. Popovych, A. Gibofsky and D.H. Beme, Anal. Chem., 44 (1972) 811. J.C. Rat, S. Villermaux and J.J. Delpuech, Bull. Sot. Chim. Fr., (1974) 815. R. Alexander and A.J. Parker, J. Am. Chem. Sot., 89 (1967) 5549. 0. Popovych, Anal Chem., 38 (1966) 558. J.I. Kim, Z. Phys. Chem., N.F., 113 (1978) 129. J.I. Kim and E.A. Gomaa, Bull. Sot. Chim. Belg., 90 (1981) 391. E.A. Gomaa, Thermochim. Acta, 80 (1984) 355. D.V. Jahagirdar and S.U. Pankanti, Indian J. Chem., 22A (1983) 195. J.I. Kim, J. Phys. Chem., 82 (1978) 191. A. Bondi, J. Phys. Chem., 68 (1964) 441. E.J. King, J. Phys. Chem., 73 (1969) 1220. V. Gutmann, A. Steinger and E. Wychera, Monatsh. Chem., 97 (1968) 460. V. Gutmann, The Donor-Acceptor Approach to Molecular Interactions, Plenum, New York, 1978.
- Printed
in The Netherlands
STUDY OF THE Ph,SbBPh, ASSUMPTION FOR SINGLE ION FREE ENERGIES IN SOME ORGANIC SOLVENTS
ESAM A. GOMAA
*
Chemistry Department,
Faculty of Science, Mansoura University, Mansoura (Egypt)
(Received
1988)
24 February
ABSTRACT
A critical study has been carried out to verify the Ph,SbBPh, electrolyte assumption, in which the standard free energies of transfer of the electrolyte AG* from one solvent to another are divided equally between the cation and the anion. A discussion is presented on the basis of a model which treats the total solvation energy of an ion as a composite of electrostatic and non-electrostatic contributions. The neutral molecules Ph,Sn and Ph,C, are proved to be (from their sizes and structures) neutral analogues of the reference ions Ph$b+ and Ph,B-, respectively. The single free energies of the reference ions Ph$b+ and Ph,Bwere also estimated. On plotting the relationship between the free energies of transfer of Ph,SbBPh, against Gutmann’s acceptor and donor numbers, good agreement was obtained, indicating the similarity of the electrostatic charges on the two reference ions.
INTRODUCTION
The estimation of single ion free energies requires extra thermodynamic assumptions, for which a wide variety of methods have been introduced. The existing methods have been continuously discussed by a number of workers [l-6] and most extensively by Parker [2] and Popovych [3]. The methods [7-lo] that have been used most extensively for single ion thermodynamic determination are based on the assumption that the thermodynamic values for a large reference cation can be equal to those of a reference anion of molecular similarity. Different pairs of large reference ions have been suggested, i.e. TABPh,B (TAB, triisoamyl-n-butylammonium) [ll] and Ph,AsBPh, [12]. It has been proved by Kim [13,14] that Ph,AsBPh, is an asymmetric model. The aim of this work is to study the plausibility of Ph,SbBPh, electrolyte for single ion thermodynamic determination.
* Present
address:
0040-6031/89/$03.50
Teachers
Training
College, Salalah-P.O.19905,
0 1989 Elsevier Science Publishers
B.V.
Sultanate
of Oman.
8 EXPERIMENTAL
Tetraphenylantimony tetraphenylborate (Ph,SbBPh,) was prepared by adding tetraphenylantimony bromide (Ph,SbBr; Alfa Inorganic Chemicals) to sodium tetraphenylborate (NaBPh,; Merck) solutions in water, followed by filtration and recrystallization (several times) from acetonitrile. The Ph,Sn used (Alfa Inorganic Chemicals) was purified by repeated crystallization from chloroform. The solvents acetonitrile (MeCN) and dimethylacetamide (DMA) were obtained from Merck, whereas N,N-dimethylformamide (DMF), dimethylsulphoxide (DMSO), dioxane and ethanol (EtOH) were obtained from BDH. The solvents were used without further purification (a dehydrating agent was added for storage). Saturated solutions of Ph,Sn and Ph,SbBPh, in the organic solvents were prepared by shaking solutions (10 ml) in test tubes in a water thermostat (Assistent) for 1 week, followed by 2 days without shaking [15]. The saturated solutions (3-5 ml) were then evaporated to dryness in a small beaker under an IR lamp. These were dissolved in ethanol and their solubilities were determined spectroscopically (UV) using a Jobin Yvon JY 101 spectrophotometer and a calibration curve. In the case of water, saturated solutions of Ph,Sn and Ph,SbBPh, were prepared in 1 1 of water. Samples (250 ml) were shaken with 10 ml of chloroform in a separating funnel. The choroform layer was extracted, evaporated to dryness, dissolved in ethanol and the solubility of the tetraphenyl derivatives was determined spectrophotometrically.
RESULTS
AND DISCUSSION
To determine the partial molar volume of Ph,SbBPh,, the density was measured with a digital oscillator densimeter (Heraeous-Paar DMA-50) in dimethylformamide (DMF). The molar volume of a solute in solution is evaluated by the relation [16] ~ _ M I (d,-dW03 ”
d
mdd,
(1)
where M is the molecular weight of the solute, m is its concentration and d and d, are the densities of water and the solution, respectively. The volume of Ph4Sb+ was estimated by subtraction of the volume of Ph,B- [17] from the volume of the salt electrolyte. The van der Waals’ volume of the cation (V,, Ph,Sb+) was estimated by subtracting the Ph,B- value given in ref. 17 from the van der Waals’ volume of Ph,SbBPh, calculated using the Bondi model [18]. From the experimental data and the calculated van der Waals’ volumes, the packing density Pd of each molecule is calculated by
(2)
9 TABLE 1 Van der Waals’ volume V,, molar volume &, packing density Pd and volume quotient f for tetraphenyl molecules
V,(cm3 mol-‘) r, (cm) & (cm3 mol- ‘) pd
Ph,C =
Ph,B-
186.7 4.2x10-’ 278.1 0.671
188.2 4.21 x lo-* 280.6 0.671
a
Ph,Sn
Ph,Sb+
204.32 4.59 x 1o-8 304.36 0.671
205.84 4.61 x 1O-8 307.38 0.669
fra = V,Ph,Sn/l/‘,Ph,C =1.0943 f,” = FwPh,Sb+/&Ph,B=1.0937 fr = cpvPh,Sn/&Ph,C =1.0944 fi = &Ph,Sb+/&Ph,B= 1.0954 a Ref. 17.
The calculated Pd values are given in Table 1; the average value is found to be 0.671 If: 0.02, which agrees well with the data of King [19] for large molecules. The van der Waals’ volumes and the partial molar volumes provide evidence for the similarity in molecular size of Ph,Sn and Ph,Sb+ and Ph,C and Ph,B-. The data given in Table 1 indicate that the reference cation (Ph,Sb+) and the reference anion (Ph,B-) do not have equal molecular volumes, although the difference is small. From the molal solubility values of Ph,SbBPh, in the solvents under consideration, the pK values and the free energies of transfer A”,G* from water to the organ;: solvents were calculated following eqns. (3) and (4) [15] and their values are listed in Table 2. p&r= -210gc+210gy* (3) A;G* = 2-303RT[pK,,,,, - p&r,,,] (4) C is the molal solubility, y+ is the activity coefficient [Ml, s denotes organic solvent and w denotes water molecules. By subtracting A”,G* values for TABLE 2 Free energies of transfer of Ph,SbBPh, scale at 25 o C)
from water to organic solvent (kcal mol-‘)
(molal
Solvents
Molar solubility
Molal solubility
log Y*
Ph,SbBPh, *G+ As,Ge
Ph,AsBPh, A”,G- a
MeCN DMF DMSO DMA Dioxane EtOH H,O
3.5761 x 1O-3 0.08146 0.03705 0.04756 1.6012~ 10m3 4.877 x 1O-5 2.9446~10-~
4.6031 x 1O-3 0.0863 0.03381 0.05077 1.5593 X 10e3 6.2116 x 1O-5 2.953Ox1O-9
-
6.6387 3.6772 4.4211 4.1585 7.7434 11.460 23.297
-
a Mean values in ref. 17.
0.0966 0.2840 0.1497 0.2300 0.0314 0.1307 0.0103
- 16.655 - 19.615 - 18.871 - 19.134 -15.549 - 11.830 ‘0
16.09 18.52 17.86 18.53 - 10.59 0
10 TABLE 3 Free energies of transfer from water to the organic solvents of the reference cations Ph,As+ ) and reference anion Ph,Bat 25O C (kcal mol-’ in molal scale) Solvents
AS,G*(Ph,B-)
ASwG*(Ph$b+)
MeCN DMF DMSO DMA EtOH
-
-
H2O
7.96 8.98 8.44 9.14 5.02 0
(Ph$b+,
AS,G*(Ph,As+) -
8.13 9.54 9.42 9.39 5.57 0
8.33 9.48 9.09 9.65 5.36 0
tetraphenylantimony tetraphenylborate from those of tetraphenylborate given in ref. 13, the corresponding values for Ph,Sb+ were evaluated. A”,G* values for the ions Ph,B-, Ph4Sb+ and Ph,As+ are shown in Table 3. The coefficient of AsWG*(Ph,Sbf)/AsWG*(Ph,As+) has a mean value of 1.006 and A”,G*(Ph,Sb+)/A”,G*(Ph,B-) has a mean value of 1.0671, indicating the plausibility of the Ph,SbBPh, assumption for the determination of single ion thermodynamics.
v
DMF
I
12
16
Donor (DN) or
I
I
I
I
I
2L
28
0
20
32
36
LO
acceptor IAN)
numbers
Fig. 1. Plots of free energies of transfer of Ph,SbBPh, the donor (a) and acceptor (b) numbers of solvents.
from water to organic solvents
against
11
On plotting the relationship between the free energies of transfer of Ph,SbBPh, against Gutmann’s acceptor (AN) and donor numbers (DN) [20,21] volcanic ( A ) and antivolcanic ( V ) relations were obtained (see Fig. 1). The lines (a) and (b) in Fig. 1 are symmetric, indicating that opposite relations proceed on the reference electrolyte, i.e. oxidation and reduction for DN and reduction and oxidation for AN at the sites of reference cation (Ph,Sb+) and reference anion (Ph,B-). That is to say there is a symmetry between the electrostatic free energies of the reference cation and anion.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
H.-M. Koepp, H. Wendt and H. StrehIow, Z. Elektrochem., 64 (1960) 483. A.J. Parker, Chem. Rev., 69 (1969) 1. 0. Popovych, Crit. Rev. Anal Chem., 1 (1970) 73. R.G. Bates, Determination of pH, Theory and Practice, Wiley, New York, 1973. I.M. Kolthoff, Pure Appl. Chem., 25 (1971) 305. K. Schawbe and Chr. Qeck, Bull. Sot. Chem. Belg., 433 (1974) 39. B.G. Cox, G.R. Hedwig, A.J. Parker and W.E. Wattes, Aust. J. Chem., 27 (1974) 477. I.M. Kolthoff and M.K. Chantooni, Jr., J. Phys. Chem., 76 (1972) 2024. 0. Popovych, A. Gibofsky and D.H. Beme, Anal. Chem., 44 (1972) 811. J.C. Rat, S. Villermaux and J.J. Delpuech, Bull. Sot. Chim. Fr., (1974) 815. R. Alexander and A.J. Parker, J. Am. Chem. Sot., 89 (1967) 5549. 0. Popovych, Anal Chem., 38 (1966) 558. J.I. Kim, Z. Phys. Chem., N.F., 113 (1978) 129. J.I. Kim and E.A. Gomaa, Bull. Sot. Chim. Belg., 90 (1981) 391. E.A. Gomaa, Thermochim. Acta, 80 (1984) 355. D.V. Jahagirdar and S.U. Pankanti, Indian J. Chem., 22A (1983) 195. J.I. Kim, J. Phys. Chem., 82 (1978) 191. A. Bondi, J. Phys. Chem., 68 (1964) 441. E.J. King, J. Phys. Chem., 73 (1969) 1220. V. Gutmann, A. Steinger and E. Wychera, Monatsh. Chem., 97 (1968) 460. V. Gutmann, The Donor-Acceptor Approach to Molecular Interactions, Plenum, New York, 1978.