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Studies on the system hydrochloric acid-water-tributyl phosphate-carbon tetrachloride
J. inorg,nucl.Chem.,1968,Vol.30, pp. 10l9 to 1023. PergamonPress. Printedin GreatBritain
STUDIES ON THE SYSTEM HYDROCHLORIC ACID-WATER-TRIBUTYL PHOSPHATECARBON TETRACHLORIDE* R. M I T A M U R A , 1. T O K U R A , S. N I S H I M U R A , t Y. K O N D O and N. C. LI Department of Metallurgy, Kyoto University, Kyoto, Japan and Department of Chemistry, Duquesne University, Pittsburgh, Pennsylvania (First received 12 June 1967; in revised form 11 A ugust 1967)
Abstract-The extraction of hydrochloric acid and water into dilute solutions of tributyl phosphate (TBP) in CC14 has been studied, and the nature and stability of the extracted species have been determined. For TBP concentrations up to 10 per cent by volume in CCI~ and for organic-phase hydrochloric acid concentration 0-01-0-04 M, the predominant species is TBP-H+.yH20 . . . C1-. The values of y are 0.2, 0.4 and 0.5 for 5, 7 and I 0 per cent by volume of TB P, respectively. The equilibrium constant of the reaction H+(aq.) + Cl-(aq.) + yH._,O(aq.) + TBP(org.) = TBP. H+.yH._,O... C1 (org.) at 25 ° has been determined to be: log K' =-4.65_+0.04, for TBP concentrations up to 10 per cent by volume. INTRODUCTION
SEVERAl_ studies have been conducted on the system hydrochloric acid-watertributyi phosphate (TBP)-organic diluent [ I-3]. However, no equilibrium constant for the formation of the complex formed in the organic diluent (assumed to be inert) has been reported. In view of the importance of TBP in solvent-extraction chemistry, we have investigated the nature and stability constant of the complex formed when hydrochloric acid and water are extracted into dilute solutions of TBP in CC14 from aqueous phase containing high concentrations of hydrochloric acid. By choosing CC14 as solvent, which has a low dielectric constant (D = 2,2) and using only low concentrations of TBP, the extracted complex would be expected to be an ion pair[4]. In fact, by applying Walden's equation, A7//60 = ~, to measurement of the conductivity, A, and viscosity, ~, of the organic phase containing hydrochloric acid at and above 5 M, Irving and Edgington[1] have shown that the degree of dissociation, ~, was always less than 0.1 per cent. In order to obtain true equilibrium constants, it is necessary to limit the concentration of TBP to a few tenths molar in an inert organic liquid, so that TBP may behave approximately ideally. Alcock et al.[5] have shown that the distribution of TBP between water and a diluent phase obeys the Nernst partition law for organic phase concentrations up to about 10 per cent. Whitney and Diamond[6] * Portion of this work was supported by the U.S. Atomic Energy Commission through Contract No. AT(30-1)-1922 with Duquesne University, paper No. NYO-1922-39. tOn leave from Kyoto University; presently at D. U. I. 2. 3. 4. 5.
H. Irving and D. N. Edgington, J. inorg, nucl. Chem. 10, 306 (1959), E. Foa, N. Rosintal and Y. Marcus, 3. inorg, nucl. Chem. 23, 109 ( 1961 ). Y. Marcus, Chem. Rev. 63, 139 (1962). T.J. Conocchioli, M. 1. Tocher and R. M. Diamond, J, phys. Chem. 69, I 106 (1965). K. Alcock. S. S. Grimley, T. V. Healy, J. Kennedy and H. A. C. McKay, Trans. Faraday Soc. 52, 39 (1956). 6. D. C. Whitney and R. M. Diamond, J. phys. Chem. 67,209 (1963). 1019
1020
R. M I T A M U R A et al.
have shown that there is a linear relationship between the water extracted by TBP to about I0 per cent TBP by volume in CC14, but that above 10 per cent a rapidly increasing amount of water is extracted. These experiments suggest that in the calculation of equilibrium constants, the concentration of TBP in CC14 should be below 10 per cent by volume if variations in the organic phase activity coefficients are going to be neglected. The results obtained for the extraction of hydrochloric acid and water in CC14 containing 5, 7 and I0 per cent by volume (corresponding to 0.183, 0.256 and 0.366 M) of TBP are reported in this paper. EXPERIMENTAL Reagents The CC14 and hydrochloric acid were reagent grade. TBP was purified by repeated washing with an equal volume of 4 per cent aqueous N a O H solution containing 4 per cent KMnO4, until a faint pink colour remained in the organic layer. The resulting organic layer was washed with water until it became clear and the washings neutral. The organic phase was then distilled to remove water and volatile impurities. Finally, the portion distilled at 160 ° at 20 mm Hg was collected. Distilled TBP was dehydrated with anhydrous magnesium sulphate and was kept in a dark brown bottle. All dilutions of TBP in CC14 were made on a volume per cent basis using volumetric glassware. Procedure Equal volumes of aqueous acid solution and TBP solution were shaken together for 15 rain. It was found that varying the shaking time from 10 min to 1 hr caused no difference in results, indicating that equilibrium had been reached. The layers were separated by centrifugation. The acidity of both the equilibrated aqueous and organic phases were determined by titration with standard alkali. The organic samples were backwashed into distilled water and the aqueous phase titrated. Water determination in the equilibrated organic phase was carried out by the Karl Fischer method. All experimental work was done at room temperature, 25 _+2 °. RESULTS AND DISCUSSION
Tuck and Diamond[7] have shown that the results of their studies on HCI extraction into TBP can be written as TB P.H20(org.) + H+(H20)h(aq.) + Cl-(aq.) = H20(aq.) + TBP'H+(H20)hCI-(org.) (1) with h = 4.1 -+-0.2 for organic-phase acid concentrations up to 1.5 M. They postulate that the chloride anion is unhydrated so that the water in the organic phase is associated with the proton. We have treated our data in accordance with the equation nTBP(org.) + H÷(aq.) + Cl-(aq.) + yH20(aq.) = n T B P ' H + . y H 2 0 . . . Cl-(org.). (2) In the absence of TBP, the solubility of water in CC1, is 0.013 M, decreases with increasing hydrochloric acid concentration in the equilibrated aqueous phase, until a constant value of 0.38 × 10 -2 M is obtained when the aqueous hydrochloric acid concentration increases beyond 0.05 M. Plots of organic-phase water, corrected for the water dissolved in CCI4 alone, vs. the organic-phase acid content (CCI4 diluent) at three different values of TBP are shown in Fig. 1. Whitney and Diamond[6] have shown that for the formation of H~O.TBP (org.) from TBP(org.) and H20(aq.) in dilute solutions in CC1, the value of Kfi,o is 0.15, where Kfi2o = (H20"TBP)/aH2o(TBP). (3) 7. D. G. Tuck and R. M. Diamond, J.phys. Chem. 65, 193 (1961).
Hydrochloric a c i d - w a t e r - t r i b u t y l p h o s p h a t e - c a r b o n tetrachloride
IO
NO
10%TBP 7% TBP
0 0
8
1021
A
5%rBP
f
X
0
2
0
I
I
0
I
2
E
4
1
I
I
I
6
8
10
I
I
I
f2
14
(HCl)org xlO = Fig. I. Plots of (H20)o~g. vs. (HCI)o=. for 5, 7 and ]0 per cent by volume of TBP in CC[4.
((H:O)o,~.= total M H20 -- M H20 dissolved by CC14.) In our experiments, log an20 is about --0.3 in the (HCl)org. range of 0.01-0.04 M, so that the value of K~12o.amo = 0.08. T h e total T B P concentration, T, is given by the equation T = (TBP) + n(nTBP.H+.yH.,O... CI-) + (H.,O.TBP) = ( T B P ) + n(HC1)org. + 0"08(TBP).
(4)
F r o m Equation (4), we obtain (H._,O.TBP) = 0.08[ T
-
-
n (HC1)org:]j.
<5)
T h e equilibrium constant of Equation (2) can be written K =
( n T B P ' H + ' y H , , O . . . CI-)T (TRP~ "" al]cl-a{~ao ~ - -
(6)
J 'FBI'
where parentheses denote the equilibrium concentrations. By assuming that the ratio of the dilute organic-phase activity coefficients ~//3'~BP, is constant, and that (n T B P . H + - y H z O . . . C1-) = (HCl)org., Equation (6) can be written log K' = log (HCl)org.- log a~icl-- n log (TBP) where K ' = yields
KT.~Bp T
(7)
' = ancvan,o. u Combination of Equation (4) and 7 and ancl
log K' = log
(HCl)org" --
log a~cl -- n log I_
~
J"
(8)
Table 1 lists the results obtained. T h e activities for hydrochloric acid water were calculated from Stokes and Robinson [8] and H a r n e d and Owen [9]. The results of Table 1 show that for T B P concentrations up to 10 per cent by 8. R. H. Stokes and R. A. Robinson, Electrolytic Solutions, 2nd Edn. Butterworth, L o n d o n (1959). 9. H. S. H a r n e d and B. B. O w e n , The Physical Chemistry of Electrolytic Solutions, 3rd Edn. Reinhold, N e w York (1958).
R. M I T A M U R A et al.
1022
Table 1. The equilibrium distribution of HCI between aqueous and CCI4 at three different total TBP concentrations and derived results. All concentrations, M (HCI)a~.
7.58 7.83 7.94 8.14 8.31 8.45 8.97
(HCl)org. X 102
-log K' (for n = 1)
(A) Total TBP concn., T, = 0.183 (5 per cent by volume) 0.82 3.46 4.76 1.17 3-57 4.70 1.37 3.60 4.66 1.53 3.67 4.68 1.99 3.75 4.64 2.59 3.81 4.56 3.91 4.02 4-55
(B) 7.01 7.21 7.43 7.57 7.85 8.43 8.55
(C) 6.73 6.80 6.82 7.03 7.37 7.69 7.91
log a~cl
T = 0.256 (7 per cent by volume) 0-86 3.28 1.03 3.34 1-18 3.43 1.56 3.48 2.28 3.60 3.50 3.84 4.35 3.86
4.71 4.69 4.71 4.64 4-57 4-61 4.52
T = 0.366 (10 p e r c e n t by volume) 0.94 3.18 1.00 3.21 1.16 3.22 1.51 3.31 2.33 3.48 2.94 3.60 3.27 3.69
4.73 4.73 4.68 4.64 4.61 4.62 4.67
volume in C C l 4 and for organic-phase hydrochloric acid concentration 0.01-0.04 M, the organic phase essentially retains the properties of the inert diluent. Only for n = 1, do the values o f - l o g K' remain constant. The predominant extracted species is therefore T B P . H + . y H 2 0 . . . CI-. In Fig. 1, the organic-phase water is the sum of the water in the complexes T B P . H ÷ . y H 2 0 . . . CI- and H20.TBP. The latter can be calculated from Equation (5), and hence the water in the species TBP.H÷.yH.,O... CI- alone can be evaluated. If Fig. 1 is now replotted, with the ordinate corresponding to water in T B P . H + . y H ~ O . . . CI-, straight lines are obtained in the range (HC1)org. = 0.01-0.04 M, with slopes equal to 0.2, 0.4 and 0.5 in 5, 7 and lOper cent TBP, respectively. The slopes are the values of y in TBP.H+.yH.,O... CI-. The increase in y upon increase in cencentration of TBP in CC14 is in line with the conclusion of Tocher et al. [ 10], that the extent of hydration of the hydronium ion in the organic phase depends upon the base strength of the extractant and upon its concentration in the organic phase. In Table 1, it is seen that in spite of the variation ofy (y = 0.2, 0.4 and 0.5 for 5, 7 and 10 per cent TBP, respectively), the average of K' remains constant up to 10 per cent TBP: log K' = ---4.65 ± 0.04. I 0. M. I. Tocher, D. C. Whitney and R. M. Diamond, J. phys. Chem. 68, 368 (1964).
H ydrochloric a c i d - w a t e r - t r i b u t y l p h o s p h a t e - c a r b o n tetrachloride
1023
The constancy of K' in Table 1 rules out the possibility that the electrostatic ion pair TBP.H+.yH._,O... C1- dissociates extensively to give TBP.H+.yH..,O (org.) and Cl-(org.) in CC14. This is because the equilibrium constant K" for the reaction TBP(org.) + H +(aq.) + Cl-(aq.) + yH=,O = TBP.H+-yH._,O(org.) + Cl-(org.) (9) would be given by the expression log K" -~ 2 log (HCl)org.- log a~icl- log (TBP). (10) Table 2 lists K", calculated from Equation (10) and the results of Table 1, and shows that the values of--log K" decrease monotonically with increase in (HCl)or~.. Table 2. Values of K" calculated from Equation (10) 5 per cent T B P (HCI),,rg. × 10z 0.82 1.17 1.37 1.53 1-99 2.59 3.91
7 per cent T B P
10 per cent T B P
- l o g K"
(HC1),,rg. × 10'-'
- l o g K"
(HCI)or,. × 10z
--log K"
6.85 6.63 6.52 6.50 6-45 6.16 5.98
0.86 1.03 1.18 1.56 2.28 3.50 4.35
6.79 6.68 6.65 6.45 6.27 6.08 5.89
0.94 1.00 1.16 1.51 2.33 2.94 3.27
6-77 6.74 6.63 6.47 6.25 6.17 6.17
Our conclusion that the extracted species is an ion pair agrees with that of Whitney and Diamond[11] who investigated the extraction of hydrobromic acid from aqueous solutions into dilute solutions of TBP in CCI4. They found that the principal extracted species over the range 1-10 per cent TBP and 3-8 M aq. H Br concn, was 3TBP.H30+.yH.,O... Br-, with 0.2 ~< y ~< 1.0. Although this calls for a 3TBP/H ÷ ratio, their data show that at large values of anBra~2o (> 103), the existance of a 1 : 1 complex of TBP and HBr is indicated. The values of aHc~.a~2o in our experiments range from 103.`' to 10a'°, and a I : 1 complex of TBP and HCI is obtained. It is of interest to note that Whitney and Diamond[11] state that most probably, the (hydrated) 3TBP.H30 + ion is the cationic species also in the dilute TBP extraction of HC1, but "experimental difficulties in proving this are considerable". Since HC1 extracts rather poorly, relative to HBr, it was necessary to use higher aqueous acid solution, in which region the tendency to form TBP/H + would be greater than 3TBP/H ÷. Whitney and Diamond[6] have found that so long as the TBP concentration in CC14 is < 0-1 M and the stoichiometric ratio TBP/H ÷ is > 3, the only extracting species are the hydrogen-bonded complex H._,O-TBP and the ion pair, 3TBPH:~O+-yH._,O... C 1 0 4 , with O ~< y ~< 1, whereas at TBP/H + = 1, the only species is T B P - H + . . . C I O 4 - . For both HBr and HC104 extraction into dilute TBP solution in CC14 therefore, there is evidence of the formation of TBP-H+.yH20 . . . X-. It is unfortunate that no quantitative equilibrium constant determinations have been made, so that we can compare the values with our present results using HCI. 11. D . C . W h i t n e y and R. M. D i a m o n d , J. phys. Chem. 67, 2583 (1963).
STUDIES ON THE SYSTEM HYDROCHLORIC ACID-WATER-TRIBUTYL PHOSPHATECARBON TETRACHLORIDE* R. M I T A M U R A , 1. T O K U R A , S. N I S H I M U R A , t Y. K O N D O and N. C. LI Department of Metallurgy, Kyoto University, Kyoto, Japan and Department of Chemistry, Duquesne University, Pittsburgh, Pennsylvania (First received 12 June 1967; in revised form 11 A ugust 1967)
Abstract-The extraction of hydrochloric acid and water into dilute solutions of tributyl phosphate (TBP) in CC14 has been studied, and the nature and stability of the extracted species have been determined. For TBP concentrations up to 10 per cent by volume in CCI~ and for organic-phase hydrochloric acid concentration 0-01-0-04 M, the predominant species is TBP-H+.yH20 . . . C1-. The values of y are 0.2, 0.4 and 0.5 for 5, 7 and I 0 per cent by volume of TB P, respectively. The equilibrium constant of the reaction H+(aq.) + Cl-(aq.) + yH._,O(aq.) + TBP(org.) = TBP. H+.yH._,O... C1 (org.) at 25 ° has been determined to be: log K' =-4.65_+0.04, for TBP concentrations up to 10 per cent by volume. INTRODUCTION
SEVERAl_ studies have been conducted on the system hydrochloric acid-watertributyi phosphate (TBP)-organic diluent [ I-3]. However, no equilibrium constant for the formation of the complex formed in the organic diluent (assumed to be inert) has been reported. In view of the importance of TBP in solvent-extraction chemistry, we have investigated the nature and stability constant of the complex formed when hydrochloric acid and water are extracted into dilute solutions of TBP in CC14 from aqueous phase containing high concentrations of hydrochloric acid. By choosing CC14 as solvent, which has a low dielectric constant (D = 2,2) and using only low concentrations of TBP, the extracted complex would be expected to be an ion pair[4]. In fact, by applying Walden's equation, A7//60 = ~, to measurement of the conductivity, A, and viscosity, ~, of the organic phase containing hydrochloric acid at and above 5 M, Irving and Edgington[1] have shown that the degree of dissociation, ~, was always less than 0.1 per cent. In order to obtain true equilibrium constants, it is necessary to limit the concentration of TBP to a few tenths molar in an inert organic liquid, so that TBP may behave approximately ideally. Alcock et al.[5] have shown that the distribution of TBP between water and a diluent phase obeys the Nernst partition law for organic phase concentrations up to about 10 per cent. Whitney and Diamond[6] * Portion of this work was supported by the U.S. Atomic Energy Commission through Contract No. AT(30-1)-1922 with Duquesne University, paper No. NYO-1922-39. tOn leave from Kyoto University; presently at D. U. I. 2. 3. 4. 5.
H. Irving and D. N. Edgington, J. inorg, nucl. Chem. 10, 306 (1959), E. Foa, N. Rosintal and Y. Marcus, 3. inorg, nucl. Chem. 23, 109 ( 1961 ). Y. Marcus, Chem. Rev. 63, 139 (1962). T.J. Conocchioli, M. 1. Tocher and R. M. Diamond, J, phys. Chem. 69, I 106 (1965). K. Alcock. S. S. Grimley, T. V. Healy, J. Kennedy and H. A. C. McKay, Trans. Faraday Soc. 52, 39 (1956). 6. D. C. Whitney and R. M. Diamond, J. phys. Chem. 67,209 (1963). 1019
1020
R. M I T A M U R A et al.
have shown that there is a linear relationship between the water extracted by TBP to about I0 per cent TBP by volume in CC14, but that above 10 per cent a rapidly increasing amount of water is extracted. These experiments suggest that in the calculation of equilibrium constants, the concentration of TBP in CC14 should be below 10 per cent by volume if variations in the organic phase activity coefficients are going to be neglected. The results obtained for the extraction of hydrochloric acid and water in CC14 containing 5, 7 and I0 per cent by volume (corresponding to 0.183, 0.256 and 0.366 M) of TBP are reported in this paper. EXPERIMENTAL Reagents The CC14 and hydrochloric acid were reagent grade. TBP was purified by repeated washing with an equal volume of 4 per cent aqueous N a O H solution containing 4 per cent KMnO4, until a faint pink colour remained in the organic layer. The resulting organic layer was washed with water until it became clear and the washings neutral. The organic phase was then distilled to remove water and volatile impurities. Finally, the portion distilled at 160 ° at 20 mm Hg was collected. Distilled TBP was dehydrated with anhydrous magnesium sulphate and was kept in a dark brown bottle. All dilutions of TBP in CC14 were made on a volume per cent basis using volumetric glassware. Procedure Equal volumes of aqueous acid solution and TBP solution were shaken together for 15 rain. It was found that varying the shaking time from 10 min to 1 hr caused no difference in results, indicating that equilibrium had been reached. The layers were separated by centrifugation. The acidity of both the equilibrated aqueous and organic phases were determined by titration with standard alkali. The organic samples were backwashed into distilled water and the aqueous phase titrated. Water determination in the equilibrated organic phase was carried out by the Karl Fischer method. All experimental work was done at room temperature, 25 _+2 °. RESULTS AND DISCUSSION
Tuck and Diamond[7] have shown that the results of their studies on HCI extraction into TBP can be written as TB P.H20(org.) + H+(H20)h(aq.) + Cl-(aq.) = H20(aq.) + TBP'H+(H20)hCI-(org.) (1) with h = 4.1 -+-0.2 for organic-phase acid concentrations up to 1.5 M. They postulate that the chloride anion is unhydrated so that the water in the organic phase is associated with the proton. We have treated our data in accordance with the equation nTBP(org.) + H÷(aq.) + Cl-(aq.) + yH20(aq.) = n T B P ' H + . y H 2 0 . . . Cl-(org.). (2) In the absence of TBP, the solubility of water in CC1, is 0.013 M, decreases with increasing hydrochloric acid concentration in the equilibrated aqueous phase, until a constant value of 0.38 × 10 -2 M is obtained when the aqueous hydrochloric acid concentration increases beyond 0.05 M. Plots of organic-phase water, corrected for the water dissolved in CCI4 alone, vs. the organic-phase acid content (CCI4 diluent) at three different values of TBP are shown in Fig. 1. Whitney and Diamond[6] have shown that for the formation of H~O.TBP (org.) from TBP(org.) and H20(aq.) in dilute solutions in CC1, the value of Kfi,o is 0.15, where Kfi2o = (H20"TBP)/aH2o(TBP). (3) 7. D. G. Tuck and R. M. Diamond, J.phys. Chem. 65, 193 (1961).
Hydrochloric a c i d - w a t e r - t r i b u t y l p h o s p h a t e - c a r b o n tetrachloride
IO
NO
10%TBP 7% TBP
0 0
8
1021
A
5%rBP
f
X
0
2
0
I
I
0
I
2
E
4
1
I
I
I
6
8
10
I
I
I
f2
14
(HCl)org xlO = Fig. I. Plots of (H20)o~g. vs. (HCI)o=. for 5, 7 and ]0 per cent by volume of TBP in CC[4.
((H:O)o,~.= total M H20 -- M H20 dissolved by CC14.) In our experiments, log an20 is about --0.3 in the (HCl)org. range of 0.01-0.04 M, so that the value of K~12o.amo = 0.08. T h e total T B P concentration, T, is given by the equation T = (TBP) + n(nTBP.H+.yH.,O... CI-) + (H.,O.TBP) = ( T B P ) + n(HC1)org. + 0"08(TBP).
(4)
F r o m Equation (4), we obtain (H._,O.TBP) = 0.08[ T
-
-
n (HC1)org:]j.
<5)
T h e equilibrium constant of Equation (2) can be written K =
( n T B P ' H + ' y H , , O . . . CI-)T (TRP~ "" al]cl-a{~ao ~ - -
(6)
J 'FBI'
where parentheses denote the equilibrium concentrations. By assuming that the ratio of the dilute organic-phase activity coefficients ~//3'~BP, is constant, and that (n T B P . H + - y H z O . . . C1-) = (HCl)org., Equation (6) can be written log K' = log (HCl)org.- log a~icl-- n log (TBP) where K ' = yields
KT.~Bp T
(7)
' = ancvan,o. u Combination of Equation (4) and 7 and ancl
log K' = log
(HCl)org" --
log a~cl -- n log I_
~
J"
(8)
Table 1 lists the results obtained. T h e activities for hydrochloric acid water were calculated from Stokes and Robinson [8] and H a r n e d and Owen [9]. The results of Table 1 show that for T B P concentrations up to 10 per cent by 8. R. H. Stokes and R. A. Robinson, Electrolytic Solutions, 2nd Edn. Butterworth, L o n d o n (1959). 9. H. S. H a r n e d and B. B. O w e n , The Physical Chemistry of Electrolytic Solutions, 3rd Edn. Reinhold, N e w York (1958).
R. M I T A M U R A et al.
1022
Table 1. The equilibrium distribution of HCI between aqueous and CCI4 at three different total TBP concentrations and derived results. All concentrations, M (HCI)a~.
7.58 7.83 7.94 8.14 8.31 8.45 8.97
(HCl)org. X 102
-log K' (for n = 1)
(A) Total TBP concn., T, = 0.183 (5 per cent by volume) 0.82 3.46 4.76 1.17 3-57 4.70 1.37 3.60 4.66 1.53 3.67 4.68 1.99 3.75 4.64 2.59 3.81 4.56 3.91 4.02 4-55
(B) 7.01 7.21 7.43 7.57 7.85 8.43 8.55
(C) 6.73 6.80 6.82 7.03 7.37 7.69 7.91
log a~cl
T = 0.256 (7 per cent by volume) 0-86 3.28 1.03 3.34 1-18 3.43 1.56 3.48 2.28 3.60 3.50 3.84 4.35 3.86
4.71 4.69 4.71 4.64 4-57 4-61 4.52
T = 0.366 (10 p e r c e n t by volume) 0.94 3.18 1.00 3.21 1.16 3.22 1.51 3.31 2.33 3.48 2.94 3.60 3.27 3.69
4.73 4.73 4.68 4.64 4.61 4.62 4.67
volume in C C l 4 and for organic-phase hydrochloric acid concentration 0.01-0.04 M, the organic phase essentially retains the properties of the inert diluent. Only for n = 1, do the values o f - l o g K' remain constant. The predominant extracted species is therefore T B P . H + . y H 2 0 . . . CI-. In Fig. 1, the organic-phase water is the sum of the water in the complexes T B P . H ÷ . y H 2 0 . . . CI- and H20.TBP. The latter can be calculated from Equation (5), and hence the water in the species TBP.H÷.yH.,O... CI- alone can be evaluated. If Fig. 1 is now replotted, with the ordinate corresponding to water in T B P . H + . y H ~ O . . . CI-, straight lines are obtained in the range (HC1)org. = 0.01-0.04 M, with slopes equal to 0.2, 0.4 and 0.5 in 5, 7 and lOper cent TBP, respectively. The slopes are the values of y in TBP.H+.yH.,O... CI-. The increase in y upon increase in cencentration of TBP in CC14 is in line with the conclusion of Tocher et al. [ 10], that the extent of hydration of the hydronium ion in the organic phase depends upon the base strength of the extractant and upon its concentration in the organic phase. In Table 1, it is seen that in spite of the variation ofy (y = 0.2, 0.4 and 0.5 for 5, 7 and 10 per cent TBP, respectively), the average of K' remains constant up to 10 per cent TBP: log K' = ---4.65 ± 0.04. I 0. M. I. Tocher, D. C. Whitney and R. M. Diamond, J. phys. Chem. 68, 368 (1964).
H ydrochloric a c i d - w a t e r - t r i b u t y l p h o s p h a t e - c a r b o n tetrachloride
1023
The constancy of K' in Table 1 rules out the possibility that the electrostatic ion pair TBP.H+.yH._,O... C1- dissociates extensively to give TBP.H+.yH..,O (org.) and Cl-(org.) in CC14. This is because the equilibrium constant K" for the reaction TBP(org.) + H +(aq.) + Cl-(aq.) + yH=,O = TBP.H+-yH._,O(org.) + Cl-(org.) (9) would be given by the expression log K" -~ 2 log (HCl)org.- log a~icl- log (TBP). (10) Table 2 lists K", calculated from Equation (10) and the results of Table 1, and shows that the values of--log K" decrease monotonically with increase in (HCl)or~.. Table 2. Values of K" calculated from Equation (10) 5 per cent T B P (HCI),,rg. × 10z 0.82 1.17 1.37 1.53 1-99 2.59 3.91
7 per cent T B P
10 per cent T B P
- l o g K"
(HC1),,rg. × 10'-'
- l o g K"
(HCI)or,. × 10z
--log K"
6.85 6.63 6.52 6.50 6-45 6.16 5.98
0.86 1.03 1.18 1.56 2.28 3.50 4.35
6.79 6.68 6.65 6.45 6.27 6.08 5.89
0.94 1.00 1.16 1.51 2.33 2.94 3.27
6-77 6.74 6.63 6.47 6.25 6.17 6.17
Our conclusion that the extracted species is an ion pair agrees with that of Whitney and Diamond[11] who investigated the extraction of hydrobromic acid from aqueous solutions into dilute solutions of TBP in CCI4. They found that the principal extracted species over the range 1-10 per cent TBP and 3-8 M aq. H Br concn, was 3TBP.H30+.yH.,O... Br-, with 0.2 ~< y ~< 1.0. Although this calls for a 3TBP/H ÷ ratio, their data show that at large values of anBra~2o (> 103), the existance of a 1 : 1 complex of TBP and HBr is indicated. The values of aHc~.a~2o in our experiments range from 103.`' to 10a'°, and a I : 1 complex of TBP and HCI is obtained. It is of interest to note that Whitney and Diamond[11] state that most probably, the (hydrated) 3TBP.H30 + ion is the cationic species also in the dilute TBP extraction of HC1, but "experimental difficulties in proving this are considerable". Since HC1 extracts rather poorly, relative to HBr, it was necessary to use higher aqueous acid solution, in which region the tendency to form TBP/H + would be greater than 3TBP/H ÷. Whitney and Diamond[6] have found that so long as the TBP concentration in CC14 is < 0-1 M and the stoichiometric ratio TBP/H ÷ is > 3, the only extracting species are the hydrogen-bonded complex H._,O-TBP and the ion pair, 3TBPH:~O+-yH._,O... C 1 0 4 , with O ~< y ~< 1, whereas at TBP/H + = 1, the only species is T B P - H + . . . C I O 4 - . For both HBr and HC104 extraction into dilute TBP solution in CC14 therefore, there is evidence of the formation of TBP-H+.yH20 . . . X-. It is unfortunate that no quantitative equilibrium constant determinations have been made, so that we can compare the values with our present results using HCI. 11. D . C . W h i t n e y and R. M. D i a m o n d , J. phys. Chem. 67, 2583 (1963).