- Email: [email protected]
Distribution of carboxylic acids between organic solvents and aqueous perchloric acid solution
J. inorg,nucl.Chem.,1970,Vot.32, pp. 987 to 995. PergamonPress. Printedin Great Britain
DISTRIBUTION OF CARBOXYLIC ACIDS BETWEEN ORGANIC SOLVENTS AND AQUEOUS PERCHLORIC ACID SOLUTION ISAO KOJIMA, MASASHI Y O S H I D A and M O T O H A R U T A N A K A Laboratory of Analytical Chemistry, Faculty of Science, Nagoya University, Chikusa-ku, Nagoya, Japan
(Received 28 April 1969) A b s t r a c t - T h e distribution of acetic, n-propionic, n-butyric, n-valeric, and n-caproic acids between various organic solvents and 0. l0 M (Na, H)CIO4 solution has been studied at 25°C. Organic solvents used include cyclohexanone, 2-ethylhexyl alcohol, isopropyl ether, nitrobenzene, 1,2-dichloroethane chloroform, benzene, carbon tetrachtoride, cyclohexane, n-hexane, toluene, chtorobenzene and isopropylbenzene. The distribution data have been explained in terms of the distribution of the monomer molecules and the dimerization of the monomer molecules in the organic phase for most of the organic solvents studied and in terms of only the distribution of the monomer molecules in the case of 2-ethylhexyl alcohol and cyclohexanone. The regular solution theory is useful to explain the solvent effect on the distribution of monomeric acids and the increment of distribution coefficient with increasing number of carbon atoms in carboxylic acids. INTRODUCTION
of carboxylic acids between various organic solvents and water has widely and repeatedly been studied by many authors [ 1-4]. From the data available, we can, in principle, estimate distribution coefficients and dimerization constants of carboxylic acids. However, previous findings by different authors are discouragingly scattered, and it can not be justified to draw any conclusion from these data. Thus it seems desirable to take up and to treat carefully this topic. Any trial does not seem successful to correlate distribution data with the dielectric constant of an organic solvent (E) [5] and with the water-organic phase interfacial surface tension[6]. Recently, many workers relied upon the regular solution theory to explain the effect of various organic solvents on solvent extraction. And they were successful in some cases [7-10]. T H E DISTRIBUTION
A. Seidell, Solubility o f Organic Compounds, 2nd Ed, Vol. I1. New York (1941). R. Collander, A cta chem. scand. 5, 774 ( 1951 ) and papers cited therein. T. Sekine, M. Isayama, S. Yamaguchi and H. Moriya, Bull. chem. Soc. Japan 40, 27 (1967). K. B. Sandell, Mh. Chem. 89, 36 (1958). C. P. Brown and A. R. Mathieson, J. phys. Chem. 58, 1934 (1957). A. Vignes,J. Chim. Phys. 57, 966 (1960). W. Kemula, H. Buchowski, R. Lewandowski, W. Pawlowski and J. Teperek, Bull. Acad. pol. Sci. Sdr. Sci. chim. 14, 395 (1966) and papers cited therein. 8. H. A. Mottola and H. Freiser, Talanta 13, 55 ( 1966); ibid. 14, 864 (1967). 9. T. Wakahayashi, Bull. chem. Soc. Japan 40, 2836 (1967) and papers cited therein. 10. A.M. Rozen, Solvent Extraction Chemistry p. 195. North-Holland, Amsterdam (1967). 987 1. 2. 3. 4. 5. 6. 7.
988
I. KOJIMA, M. Y O S H I D A and M. T A N A K A
As a basis of the extraction of metals with carboxylic acids [ 11-13], the present paper deals with the distribution of carboxylic acids between various organic solvents and an aqueous solution composed of 0.01 M HCIO4 and 0.09 M NaCIO4. The regular solution theory was applied to explain the effect of various organic solvents on the distribution coefficient of carboxylic acids and to explain quantitatively the increment of distribution coefficient of carboxylic acids for an added methylene group. EXPERIMENTAL
Reagents Chloroform, 2-ethylhexyl alcohol, isopropylbenzene, chlorobenzene, propionic acid, butyric acid and caproic acid were of the chemical pure grade and the other chemicals were of the G. R. grade. Isopropylbenzene, chlorobenzene and 2-ethylhexyl alcohol were purified by distilling twice and by collecting a fraction distilled at constant temperature and were used after saturation with water. The other solvents were purified by washing first with dilute sodium hydroxide solution, then with dilute hydrochloric acid solution and finally five times with distilled water. They were used without dehydration. The carboxylic acids were purified by distilling twice and by collecting a fraction distilled at constant temperature. Procedure All experiments were carried out in a room thermostatted at 25--- I°C. Phenolphthalein was used as an indicator in the acid-base titration. Stock solutions of acetic, propionic, and butyric acids were prepared by dilution of the acids with 0.01 M perchloric acid solution containing 0.09 M sodium perchlorate. Valeric and caproic acids were dissolved in organic solvents. The aqueous and organic phases were contacted in a separatory funnel, using a reciprocating shaker for overnight (200-250 strokes/minute). After the equilibration, both phases were allowed to stand for more than 6 hr for a complete phase separation. The concentration of carboxylic acid in the organic phase was directly determined by titration with sodium hydroxide solution under a nitrogen atmosphere. Perchloric acid and carboxylic acid in the aqueous phase was titrated and the total acid concentration was determined. The concentration of perchloric acid was estimated by means of Gran plot[14] of the data obtained from titration of the aqueous phase and then the concentration of carboxylic acid was calculated. The recovery of the acids from both phases was found to be quantitative. RESULTS AND DISCUSSION
The distribution of an acid between the aqueous and organic phases can be written as: H R ~ HR(o) Ka = [HR]o (1)* [HR]
*In practice, an acid will interact with organic solvents and/or water in the organic and aqueous phases. Then Ka can be rewritten as: [HR]o + [ H R . S.]o
Ka = [HR] + [HR(H~O)m] = K~(1 +/3,~[S]o")(1 +/3,n[HzO]'~) -~ = K~" Oqs)" a~-d2o)
where K~ refers to the distribution coefficient of non-solvated solute (HR) and a(s) and a(H,o) refer to the side reaction coefficient taking into account the interaction of solute with the organic solvents and/or water in the organic and aqueous phases, respectively, S denotes the organic solvents or water in the organic phase, and/3, and tim denote overall formation constants of H R • Sn and HR • ( H 2 0 ) m , -1 respectively. For a given solvent, the term a~) • a(~2o) can be regarded as constant. 11. M. Tanaka and T. Niinomi, J. inorg, nucl. Chem. 27, 431 (1965). 12. M. Tanaka, N. Nakasuka and S. Goto, Solvent Extraction Chemistry p. 154. North-Holland, Amsterdam (1967). 13. M. Tanaka, N. Nakasuka and S. Sasane (n~e Goto), J. inorg, nucl. Chem. 31,2591 (1969). 14. G. Gran,Analyst77, 661 (1952).
Distribution of carboxylic acids
989
where HR denotes the acid molecule and the subscript o refers to the organic phase and the subscript referring to the aqueous phase is omitted for simplicity. The dimerization of the acid molecule in the organic phase can be written as: 2 HR~o~ ~ (HR)2.o
K2-- [(HR)2]o [HR]oZ •
(2)
The distribution ratio of the acid is given by: D=
concentration of the acid in the organic phase concentration of the acid in the aqueous phase
_ CHR,o _ [HR]o+ 2[(HR)2]o CHR,w [HRI + [R-I
(3)
Thus substituting Equations (1 and 2) into (3), Equation (4) is obtained. 2" K2[HR] 1 + I/(KnR[H+])
D = Ka+2Ka
(4)
where KHR denotes the formation constant of the monomer. W h e n - log [H +] < 2.5 dissociation of the acid is negligible in the aqueous phase and [HR] can be approximated as CnR.w. Then Equation (4) is rewritten as: D = Ka+
2Kd 2 •
K2CHR.w.
(5)
Under the present experimental condition, i.e. [H ~] = 10 -2 M, the distribution ratio is given by Equation (5). The results obtained were shown in Fig. 1. Comparing the plot of log D against log CHR,~, with a normalized function Y = log ( 1 + X), X = log x, we were able to estimate the dimerization constant and the distribution coefficient of carboxylic acids. In Table 1, the results obtained were sumTable I. Distribution coefficient, dimerization constant, molar volume and formation constant of carboxylic acids CH:jCOOH
Benzene 1.2-dichloroethane Nitrobenzene lsopropyl ether 2-ethyl hexyl alcohol
2-07 - 1.60 - 1.44 -0.76 -0.30
Chloroform Carbon tetrachloride Cyclohexane n-hexane Cyclohexanone lsopropylbenzene Chlorobenzene Toluene VHH log KnH
Ionic strength,
CzH~COOH
C,~HTCOOH
C4H~COOH
C~Hz~COOH
log Ka log K2 log Ka log K._, log Ka log K,2 log Ka log K2 log Ka log
Solvent
57.6 4-76
2.16 1.47 0.85 0.23
- 1.36 - 0-99 -0-86 - 0.09 0-30 -0.96 - 1-90 - 2-54 - 2.56 0.52 - 1-64 - [.53 -- 1.47 73.4 4.88
0.10 M ( N a , H ) C I O 4 : 2 5 + I ° C
2-21 - - 0 . 7 9 2.28 1-53 - - 0 . 3 9 1.45 0-97 - - 0 . 3 4 0'95 0-30 0.48 - - 0 " 3 7 0"86 1.94 3.14 3-71 3-94
--0.16 2.36 0"23 1.35 0'23 1'01 1-05 - - 0 . 2 0 1.36
0.31 2.45 0-82 1.19 0.77 0.96 1-48 - - 0 . 1 9
2-48 2.49 2.39 90-6 4.82
107 4.85
K..,
125 4.84
990
I. K O J I M A , M. Y O S H I D A and M. T A N A K A
marized together with the formation constant and the molar volume of carboxylic acids. Curves in Fig. 1 are calculated according to Equation (5) and the experimental results are all in good agreement with the calculated. When cyclohexanone was used as a solvent, perchloric acid was partly extracted in the organic phase and the per cent extraction of perchioric acid attained 41.3 per cent. Then the distribution ratio of carboxylic acid was calculated after correction for perchloric acid in the organic phase.
o.si o----oyc~o,o,o,.-~oo *
0.~
--O*-o-O o----o-o---o-o-~ 2 ~ ethy{hexyl olcohol
o-o----o---
G-'-'-
o
Isopropyl ether
/ / /
-0"5 chlOrofo*'m
fogD %0
'7,.0
nttrobe~z
-"'I
-I.5
/
-2.0
-21o
-,'.s
-,Io
-o'.s
L
' -2.0
I -I.5
logC~.w
I -I. 0
I -0.5
O
log C.m,*
Fig. 1. Distribution ratios of propionic acid as a function of acid concentration in the aqueous phase. The curves were the theoretical curves calculated with the constants given in Table 1.
As shown in Fig. 2, the plot of log K2 against log Ka gives a straight line with a slope o f - 2. This suggests that an increase in the distribution coefficient, Ka, is correlated with a decrease in the dimerization constant, K2. The following relationship between the acid monomer in aqueous phase and the dimerized species in organic phase holds [ 15]:
[(HR)2]o [HR]2 -- K2 • Kd z = constant. The values obtained, K2 • Ka 2, are listed in Table 2. From these results and later discussion, we are able to estimate the distribution coefficient and the dimerization constant of higher aliphatic acids in organic solvents used in this study, because the increment of the distribution coefficient is constant for an added methylene group and the dimerization constant is nearly constant irrespective of carboxylic acids, i.e. the strength of hydrogen 15. D. D y r s s e n and L. D. Hay, Acta chem. scand. 14, 1091 (1960).
Distribution ofcarboxylic acids
99
4
x, \ \ \ \.
I
\ t%l
H
~c o
-I
-2
-3
I
-I
-2
I
0
2
3
log K d Fig, 2. Dimerization constant of acid in the organic phase as a function of the distribution coefficient. 1: cyclohexanone; 2: chloroform; 3: toluene; 4: chlorobenzene: 5: isopropylbenzene: 6: carbon tetrachloride; 7: cyclohexane; 8: n-hexane. A: acetic; B: propionic: C: butyric; D: valeric: E: caproic: F: enanthoic: G: caprylic; H: pelargonic: 1: capric. Broken lines with a slope o f - 2 are drawn for enanthoic, caprylic, pelargonic, and capric acids at same interval as for lower acids.
Table 2, Value of the product of the distribution coefficient and the dimerization constant (log K.ZK~)
Solvent
Solute
Benzene Dichloroethane Isopropyl-ether Nitrobenzene Mean
C H a C O O H C 2 H s C O O H C:~HrCOOH C4HgCOOH C ~ H . C O O H -----
1.98 1.73 1.75 2.03 1.87
- 0.51 -- 0.45 - 0-48 - 0-75 - 0.55
0.70 0.67 0.59 0.27 0.56
2.04 1-81 1-90 1-47 1.80
3.07 2.83 2.77 2.50 2.79
b o n d i n g o f c a r b o x y l i c acid seems to be a l m o s t i n d e p e n d e n t o f the n u m b e r o f c a r b o n atoms. I n a regular solution [ 16], the activity o f c o m p o n e n t 1, al, is e x p r e s s e d as: R T In a~ = R T In Xl + V1~,~2(8~ -- 8.) '~ 16. J. H. Hildebrand and R. L. Scott, (1964).
(7)
The Solubility ofNonelectrolytes, 3rd Edn. Dover. N e w York
992
|. KOJIMA, M. Y O S H I D A and M. T A N A K A
or
R T In a, = R T [In ¢1 +~o~(1 -- V1/V.) ] + Vies2(81 --Ss) 2
(8)
where the subscripts 1 and s denote the solute 1 and solvent, respectively, and x represents mole fraction, ~o volume fraction, V molar volume and ;5 solubility parameter. Equilibrium is attained at constant temperature and pressure when the chemical potentials of the solute in each phase are equal. Substituting suitable expression for the chemical potential and considering that the concentration of the component 1 is sufficiently small in both phases, we have: 2.30 log K?a V 1 6aq -- 8o~g = R T (Saq + 8o~g-- 281)
(9)
2"30 log Kd _ VI 6aq--8o~g R T (Saq + 8org-- 281)
(10)
RT 8org = 8o~g+Saq__8o~g (l/Vorg-- 1/Vaq).
(I 1)
where 8org is given by:
The distribution coefficient in terms of mole fraction, K], can be calculated from the distribution coefficient in terms of molarity, Kd, by means of the following equation: Vorg K°a = Ka Vaq"
(12)
According to Equations (9) and (10), the plot of 2.30 log K~/(aaq- aorg) against 8org or the plot of 2.30 log Kd/(Saq--8org) against 8org should yield a straight line with a theoretical slope of VHa/RT (see Table 3). As shown in Figs. 3 and 4, these plots yield a straight line with a slope of 0.124 in the case of propionic acid as a solute and benzene, n-hexane, dichloroethane, toluene and isopropylbenzene as solvents. This value is in agreement with the theoretical value of Vpr/RT = 0.124, calculated by using Vpr = 73.4 and R T = 592.5. As a value o f 8aq, 16.35 and 17.55 were used for Equations (9) and (10), respectively. For the other solvents, however, there was a large drift from the straight line. This deviation would be partly resulted from some interaction of carboxylic acid with organic solvents or with water in organic solvents [17], which would favour the distribution of acids in the organic phase. If this positive deviation from the line given by the regular solution theory can solely be attributable to the interaction of carboxylic acid with the organic solvent, the stability constants of the hydrogen bonded complex formed by carboxylic acid with isopropyl ether, cyclohexanone, 2-ethylhexyl alcohol and chloroform may be calculated. Assuming that simple HR • S complex prevail, the value offls = [HR • S]o[HR]o-I[S]0 -1 17. S. D. Christian, H. E. Affsprung and S. A. Taylor, J. phys. Chem. 67, 187 (1963).
Distribution of carboxylic acids
993
Table 3. Relation of solubility parameter of organic solvents and the distribution coefficient of propionic acid Solvent
8org
Benzene 1,2-dichloroethane Nitrobenzene lsopropylether 2-ethylhexyl alcohol Chloroform C a r b o n tetrachloride Cyclohexane n-hexane Cyclohexanone Isopropylbenzene Chlorobenzene Toluene
9.15 9-9 10.0 7.0 9.5 9-3 8-6 8-20 7.30 9.9 8.65 9.5 8.90
8~rg Iog K~
Vo~g
6.02 6.58 6.40 4.28 5.94 6.20 5.60 5.26 4.51 6.35 5.43 6-13 5.73
89 79 103 140 156 81 97 109 132 103 140 102 107
- 0.67 - 0.35 --0.11 0.80 1-23 -0-31 - 1.17 - 1,76 -1.70 1.27 -0.75 -0.78 -0.70
0.5 0 12
0 13
0.~ 0 II
0.1
~
_o~ o .
90
8 -O'l~
~
6
-0"3
4
/ r
z
-0'5 -
o I
J
I
I
7
8
9
IO
org
Fig. 3. Correlation of distribution coefficients of propionic acid with solubility parameter of solvents (Equation 9). 1: n-hexane; 2: cyclohexane; 3: carbon tetrachloride; 4: chlorobenzene; 5: isopropylbenzene; 6: toluene; 7: benzene; 8: chloroform; 9: nitrobenzene; 10: 1,2-dichloroethane; 11: isopropyl ether; 12: 2-ethylhexyl alcohol: 13: cyclohexanone.
are logfls = 2.69, 1.69, 1.57, and 0.26 for isopropyl ether, 2-ethylhexyl alcohol, cyclohexanone, and chloroform, respectively, and the ability of solvents to interact with carboxylic acid increases in the order: chloroform < cyclohexanone < 2-ethylhexyl alcohol < isopropyl ether.
994
I. KOJIMA, M. YOSH1DA and M. T A N A K A 0.3
o 13 0 12 0 II -O'i
0
I,O
8 9
o'
"~ g-0.~
/y
2 0
-0.~ 4
I
i
5
6
L
~'org Fig. 4. Correlation of distribution coefficients of propionic acid with solubility parameter of solvents (Equation 10). Numbers are the same as in Fig. 3.
It is known that the distribution coefficient of organic molecules between the aqueous and organic phases increases by a factor of 3-4 for each methylene group added to the molecule I18-23]. Combining Equations (9) and (l 2), we have log K~ = log Ka + log (Vorg/Vaq) VHR
-- 2"30R T [(Saq -- 8HR)2 -- (8org-- 8nR)2]-
(13)
From Equation (13), it is evident that a plot of log Ka against VHa gives a straight line if the solubility parameter of carboxylic acids is constant. In fact, solubility parameters of compounds having the same functional group do not change appreciably with increasing molecular weight. Therefore log Ka depends mostly on the difference of molar volume of organic molecules. Then the increment of the distribution coefficient of carboxylic acid for an added methylene group was estimated by using Equation (14) for a given solvent. A log Ka =
A VHR 2 " 3 0 R T [(~aq - - ~HR) 2 - - ( ~ o r g - ~HR) 2]
(14)
where A log Ka and A VHR are the difference of distribution coeffÉcient and the 18. 19. 20. 21. 22. 23.
B. B. Wroth and E. E. Reid, J. Am. chem. Soc. 38, 2316 ( 1916). A. Frumkin, Z. phys. Chem. 116, 501 (1925). D. Dyrssen, Proc. 7th Int. Conf. Co-ord. Chem. Paper No. 753, Stockholm (1962). N. S. Saha, A. Bhattacharjee, N. G. Basak and A. Lahili, J. chem. Engng Data 8, 405 (1963). D. Dyrssen, S. Ekberg and D. H. Liem, Acta chem. scand. 18, 135 (1964). D. Dyrssen and Dj. Petkovic,J. inorg, nucl. Chem. 27, 1381 (1965).
995
Distribution of carboxylic acids
molar volume of carboxylic acids, respectively. Then a plot of log Ka against the number of carbon atoms in acids and against the difference of molar volume of acids by selecting acetic acid as a reference acid ( V , ~ - V.OAC) is shown in Fig. 5, this plot yielded a straight line. From Fig. 5, Equation (14) and Table 1, it is evident
J
I
log Kd
q,/
, ~ . ~o~c
/
0
number of carbon otorns 4 5 6
-2 3 I
o
,
I
i I
210
t
4~0 crn~ VHR- VNo,=c
I
60
II
80
Fig. 5. Correlation of the distribution coefficient of carboxylic acids with the number of carbon atoms in carboxylic acid and with the molar volume of acids.
that the increment of log Ka for each methylene group is nearly constant irrespective of the solvent, since the increment of molar volume of solute for an added methylene group is constant and its value is 16-17. Then 6HR calculated from the slope of lines in Fig. 5 was 10-5__+0.4. This value does not agree with the previous finding by Kemula et al.[7], who related the distribution coefficients of formic, acetic, propionic, benzoic, and salicylic acids with the solubility parameter of solvent and estimated the solubility parameters of these solutes. They used 23-4 as 6aq and obtained 17.26 as 6 for formic and acetic acids and 16.47 for propionic acid. However, using 17.55 as 6aq, we got 10.6_+ 0.5 for 6nR from their data. This value compares favourably with ours. And from the present results, A log Ka for each methylene group is about 0.56 for benzene, nitrobenzene, isopropyl ether and 2-ethylhexyl alcohol and 0.60 for 1,2-dichloroethane. From the data available[l, 2, 21-23], it can be safely concluded on the sound theoretical basis that the increment of log Ka for an added methylene group is nearly constant irrespective of solvents and of solute, i.e. A log Ka/a CH,, = 0.560-60 (see Equation (14)). Acknowledgement-The financial support given by the Ministry of Education (Japan) is gratefully acknowledged.
DISTRIBUTION OF CARBOXYLIC ACIDS BETWEEN ORGANIC SOLVENTS AND AQUEOUS PERCHLORIC ACID SOLUTION ISAO KOJIMA, MASASHI Y O S H I D A and M O T O H A R U T A N A K A Laboratory of Analytical Chemistry, Faculty of Science, Nagoya University, Chikusa-ku, Nagoya, Japan
(Received 28 April 1969) A b s t r a c t - T h e distribution of acetic, n-propionic, n-butyric, n-valeric, and n-caproic acids between various organic solvents and 0. l0 M (Na, H)CIO4 solution has been studied at 25°C. Organic solvents used include cyclohexanone, 2-ethylhexyl alcohol, isopropyl ether, nitrobenzene, 1,2-dichloroethane chloroform, benzene, carbon tetrachtoride, cyclohexane, n-hexane, toluene, chtorobenzene and isopropylbenzene. The distribution data have been explained in terms of the distribution of the monomer molecules and the dimerization of the monomer molecules in the organic phase for most of the organic solvents studied and in terms of only the distribution of the monomer molecules in the case of 2-ethylhexyl alcohol and cyclohexanone. The regular solution theory is useful to explain the solvent effect on the distribution of monomeric acids and the increment of distribution coefficient with increasing number of carbon atoms in carboxylic acids. INTRODUCTION
of carboxylic acids between various organic solvents and water has widely and repeatedly been studied by many authors [ 1-4]. From the data available, we can, in principle, estimate distribution coefficients and dimerization constants of carboxylic acids. However, previous findings by different authors are discouragingly scattered, and it can not be justified to draw any conclusion from these data. Thus it seems desirable to take up and to treat carefully this topic. Any trial does not seem successful to correlate distribution data with the dielectric constant of an organic solvent (E) [5] and with the water-organic phase interfacial surface tension[6]. Recently, many workers relied upon the regular solution theory to explain the effect of various organic solvents on solvent extraction. And they were successful in some cases [7-10]. T H E DISTRIBUTION
A. Seidell, Solubility o f Organic Compounds, 2nd Ed, Vol. I1. New York (1941). R. Collander, A cta chem. scand. 5, 774 ( 1951 ) and papers cited therein. T. Sekine, M. Isayama, S. Yamaguchi and H. Moriya, Bull. chem. Soc. Japan 40, 27 (1967). K. B. Sandell, Mh. Chem. 89, 36 (1958). C. P. Brown and A. R. Mathieson, J. phys. Chem. 58, 1934 (1957). A. Vignes,J. Chim. Phys. 57, 966 (1960). W. Kemula, H. Buchowski, R. Lewandowski, W. Pawlowski and J. Teperek, Bull. Acad. pol. Sci. Sdr. Sci. chim. 14, 395 (1966) and papers cited therein. 8. H. A. Mottola and H. Freiser, Talanta 13, 55 ( 1966); ibid. 14, 864 (1967). 9. T. Wakahayashi, Bull. chem. Soc. Japan 40, 2836 (1967) and papers cited therein. 10. A.M. Rozen, Solvent Extraction Chemistry p. 195. North-Holland, Amsterdam (1967). 987 1. 2. 3. 4. 5. 6. 7.
988
I. KOJIMA, M. Y O S H I D A and M. T A N A K A
As a basis of the extraction of metals with carboxylic acids [ 11-13], the present paper deals with the distribution of carboxylic acids between various organic solvents and an aqueous solution composed of 0.01 M HCIO4 and 0.09 M NaCIO4. The regular solution theory was applied to explain the effect of various organic solvents on the distribution coefficient of carboxylic acids and to explain quantitatively the increment of distribution coefficient of carboxylic acids for an added methylene group. EXPERIMENTAL
Reagents Chloroform, 2-ethylhexyl alcohol, isopropylbenzene, chlorobenzene, propionic acid, butyric acid and caproic acid were of the chemical pure grade and the other chemicals were of the G. R. grade. Isopropylbenzene, chlorobenzene and 2-ethylhexyl alcohol were purified by distilling twice and by collecting a fraction distilled at constant temperature and were used after saturation with water. The other solvents were purified by washing first with dilute sodium hydroxide solution, then with dilute hydrochloric acid solution and finally five times with distilled water. They were used without dehydration. The carboxylic acids were purified by distilling twice and by collecting a fraction distilled at constant temperature. Procedure All experiments were carried out in a room thermostatted at 25--- I°C. Phenolphthalein was used as an indicator in the acid-base titration. Stock solutions of acetic, propionic, and butyric acids were prepared by dilution of the acids with 0.01 M perchloric acid solution containing 0.09 M sodium perchlorate. Valeric and caproic acids were dissolved in organic solvents. The aqueous and organic phases were contacted in a separatory funnel, using a reciprocating shaker for overnight (200-250 strokes/minute). After the equilibration, both phases were allowed to stand for more than 6 hr for a complete phase separation. The concentration of carboxylic acid in the organic phase was directly determined by titration with sodium hydroxide solution under a nitrogen atmosphere. Perchloric acid and carboxylic acid in the aqueous phase was titrated and the total acid concentration was determined. The concentration of perchloric acid was estimated by means of Gran plot[14] of the data obtained from titration of the aqueous phase and then the concentration of carboxylic acid was calculated. The recovery of the acids from both phases was found to be quantitative. RESULTS AND DISCUSSION
The distribution of an acid between the aqueous and organic phases can be written as: H R ~ HR(o) Ka = [HR]o (1)* [HR]
*In practice, an acid will interact with organic solvents and/or water in the organic and aqueous phases. Then Ka can be rewritten as: [HR]o + [ H R . S.]o
Ka = [HR] + [HR(H~O)m] = K~(1 +/3,~[S]o")(1 +/3,n[HzO]'~) -~ = K~" Oqs)" a~-d2o)
where K~ refers to the distribution coefficient of non-solvated solute (HR) and a(s) and a(H,o) refer to the side reaction coefficient taking into account the interaction of solute with the organic solvents and/or water in the organic and aqueous phases, respectively, S denotes the organic solvents or water in the organic phase, and/3, and tim denote overall formation constants of H R • Sn and HR • ( H 2 0 ) m , -1 respectively. For a given solvent, the term a~) • a(~2o) can be regarded as constant. 11. M. Tanaka and T. Niinomi, J. inorg, nucl. Chem. 27, 431 (1965). 12. M. Tanaka, N. Nakasuka and S. Goto, Solvent Extraction Chemistry p. 154. North-Holland, Amsterdam (1967). 13. M. Tanaka, N. Nakasuka and S. Sasane (n~e Goto), J. inorg, nucl. Chem. 31,2591 (1969). 14. G. Gran,Analyst77, 661 (1952).
Distribution of carboxylic acids
989
where HR denotes the acid molecule and the subscript o refers to the organic phase and the subscript referring to the aqueous phase is omitted for simplicity. The dimerization of the acid molecule in the organic phase can be written as: 2 HR~o~ ~ (HR)2.o
K2-- [(HR)2]o [HR]oZ •
(2)
The distribution ratio of the acid is given by: D=
concentration of the acid in the organic phase concentration of the acid in the aqueous phase
_ CHR,o _ [HR]o+ 2[(HR)2]o CHR,w [HRI + [R-I
(3)
Thus substituting Equations (1 and 2) into (3), Equation (4) is obtained. 2" K2[HR] 1 + I/(KnR[H+])
D = Ka+2Ka
(4)
where KHR denotes the formation constant of the monomer. W h e n - log [H +] < 2.5 dissociation of the acid is negligible in the aqueous phase and [HR] can be approximated as CnR.w. Then Equation (4) is rewritten as: D = Ka+
2Kd 2 •
K2CHR.w.
(5)
Under the present experimental condition, i.e. [H ~] = 10 -2 M, the distribution ratio is given by Equation (5). The results obtained were shown in Fig. 1. Comparing the plot of log D against log CHR,~, with a normalized function Y = log ( 1 + X), X = log x, we were able to estimate the dimerization constant and the distribution coefficient of carboxylic acids. In Table 1, the results obtained were sumTable I. Distribution coefficient, dimerization constant, molar volume and formation constant of carboxylic acids CH:jCOOH
Benzene 1.2-dichloroethane Nitrobenzene lsopropyl ether 2-ethyl hexyl alcohol
2-07 - 1.60 - 1.44 -0.76 -0.30
Chloroform Carbon tetrachloride Cyclohexane n-hexane Cyclohexanone lsopropylbenzene Chlorobenzene Toluene VHH log KnH
Ionic strength,
CzH~COOH
C,~HTCOOH
C4H~COOH
C~Hz~COOH
log Ka log K2 log Ka log K._, log Ka log K,2 log Ka log K2 log Ka log
Solvent
57.6 4-76
2.16 1.47 0.85 0.23
- 1.36 - 0-99 -0-86 - 0.09 0-30 -0.96 - 1-90 - 2-54 - 2.56 0.52 - 1-64 - [.53 -- 1.47 73.4 4.88
0.10 M ( N a , H ) C I O 4 : 2 5 + I ° C
2-21 - - 0 . 7 9 2.28 1-53 - - 0 . 3 9 1.45 0-97 - - 0 . 3 4 0'95 0-30 0.48 - - 0 " 3 7 0"86 1.94 3.14 3-71 3-94
--0.16 2.36 0"23 1.35 0'23 1'01 1-05 - - 0 . 2 0 1.36
0.31 2.45 0-82 1.19 0.77 0.96 1-48 - - 0 . 1 9
2-48 2.49 2.39 90-6 4.82
107 4.85
K..,
125 4.84
990
I. K O J I M A , M. Y O S H I D A and M. T A N A K A
marized together with the formation constant and the molar volume of carboxylic acids. Curves in Fig. 1 are calculated according to Equation (5) and the experimental results are all in good agreement with the calculated. When cyclohexanone was used as a solvent, perchloric acid was partly extracted in the organic phase and the per cent extraction of perchioric acid attained 41.3 per cent. Then the distribution ratio of carboxylic acid was calculated after correction for perchloric acid in the organic phase.
o.si o----oyc~o,o,o,.-~oo *
0.~
--O*-o-O o----o-o---o-o-~ 2 ~ ethy{hexyl olcohol
o-o----o---
G-'-'-
o
Isopropyl ether
/ / /
-0"5 chlOrofo*'m
fogD %0
'7,.0
nttrobe~z
-"'I
-I.5
/
-2.0
-21o
-,'.s
-,Io
-o'.s
L
' -2.0
I -I.5
logC~.w
I -I. 0
I -0.5
O
log C.m,*
Fig. 1. Distribution ratios of propionic acid as a function of acid concentration in the aqueous phase. The curves were the theoretical curves calculated with the constants given in Table 1.
As shown in Fig. 2, the plot of log K2 against log Ka gives a straight line with a slope o f - 2. This suggests that an increase in the distribution coefficient, Ka, is correlated with a decrease in the dimerization constant, K2. The following relationship between the acid monomer in aqueous phase and the dimerized species in organic phase holds [ 15]:
[(HR)2]o [HR]2 -- K2 • Kd z = constant. The values obtained, K2 • Ka 2, are listed in Table 2. From these results and later discussion, we are able to estimate the distribution coefficient and the dimerization constant of higher aliphatic acids in organic solvents used in this study, because the increment of the distribution coefficient is constant for an added methylene group and the dimerization constant is nearly constant irrespective of carboxylic acids, i.e. the strength of hydrogen 15. D. D y r s s e n and L. D. Hay, Acta chem. scand. 14, 1091 (1960).
Distribution ofcarboxylic acids
99
4
x, \ \ \ \.
I
\ t%l
H
~c o
-I
-2
-3
I
-I
-2
I
0
2
3
log K d Fig, 2. Dimerization constant of acid in the organic phase as a function of the distribution coefficient. 1: cyclohexanone; 2: chloroform; 3: toluene; 4: chlorobenzene: 5: isopropylbenzene: 6: carbon tetrachloride; 7: cyclohexane; 8: n-hexane. A: acetic; B: propionic: C: butyric; D: valeric: E: caproic: F: enanthoic: G: caprylic; H: pelargonic: 1: capric. Broken lines with a slope o f - 2 are drawn for enanthoic, caprylic, pelargonic, and capric acids at same interval as for lower acids.
Table 2, Value of the product of the distribution coefficient and the dimerization constant (log K.ZK~)
Solvent
Solute
Benzene Dichloroethane Isopropyl-ether Nitrobenzene Mean
C H a C O O H C 2 H s C O O H C:~HrCOOH C4HgCOOH C ~ H . C O O H -----
1.98 1.73 1.75 2.03 1.87
- 0.51 -- 0.45 - 0-48 - 0-75 - 0.55
0.70 0.67 0.59 0.27 0.56
2.04 1-81 1-90 1-47 1.80
3.07 2.83 2.77 2.50 2.79
b o n d i n g o f c a r b o x y l i c acid seems to be a l m o s t i n d e p e n d e n t o f the n u m b e r o f c a r b o n atoms. I n a regular solution [ 16], the activity o f c o m p o n e n t 1, al, is e x p r e s s e d as: R T In a~ = R T In Xl + V1~,~2(8~ -- 8.) '~ 16. J. H. Hildebrand and R. L. Scott, (1964).
(7)
The Solubility ofNonelectrolytes, 3rd Edn. Dover. N e w York
992
|. KOJIMA, M. Y O S H I D A and M. T A N A K A
or
R T In a, = R T [In ¢1 +~o~(1 -- V1/V.) ] + Vies2(81 --Ss) 2
(8)
where the subscripts 1 and s denote the solute 1 and solvent, respectively, and x represents mole fraction, ~o volume fraction, V molar volume and ;5 solubility parameter. Equilibrium is attained at constant temperature and pressure when the chemical potentials of the solute in each phase are equal. Substituting suitable expression for the chemical potential and considering that the concentration of the component 1 is sufficiently small in both phases, we have: 2.30 log K?a V 1 6aq -- 8o~g = R T (Saq + 8o~g-- 281)
(9)
2"30 log Kd _ VI 6aq--8o~g R T (Saq + 8org-- 281)
(10)
RT 8org = 8o~g+Saq__8o~g (l/Vorg-- 1/Vaq).
(I 1)
where 8org is given by:
The distribution coefficient in terms of mole fraction, K], can be calculated from the distribution coefficient in terms of molarity, Kd, by means of the following equation: Vorg K°a = Ka Vaq"
(12)
According to Equations (9) and (10), the plot of 2.30 log K~/(aaq- aorg) against 8org or the plot of 2.30 log Kd/(Saq--8org) against 8org should yield a straight line with a theoretical slope of VHa/RT (see Table 3). As shown in Figs. 3 and 4, these plots yield a straight line with a slope of 0.124 in the case of propionic acid as a solute and benzene, n-hexane, dichloroethane, toluene and isopropylbenzene as solvents. This value is in agreement with the theoretical value of Vpr/RT = 0.124, calculated by using Vpr = 73.4 and R T = 592.5. As a value o f 8aq, 16.35 and 17.55 were used for Equations (9) and (10), respectively. For the other solvents, however, there was a large drift from the straight line. This deviation would be partly resulted from some interaction of carboxylic acid with organic solvents or with water in organic solvents [17], which would favour the distribution of acids in the organic phase. If this positive deviation from the line given by the regular solution theory can solely be attributable to the interaction of carboxylic acid with the organic solvent, the stability constants of the hydrogen bonded complex formed by carboxylic acid with isopropyl ether, cyclohexanone, 2-ethylhexyl alcohol and chloroform may be calculated. Assuming that simple HR • S complex prevail, the value offls = [HR • S]o[HR]o-I[S]0 -1 17. S. D. Christian, H. E. Affsprung and S. A. Taylor, J. phys. Chem. 67, 187 (1963).
Distribution of carboxylic acids
993
Table 3. Relation of solubility parameter of organic solvents and the distribution coefficient of propionic acid Solvent
8org
Benzene 1,2-dichloroethane Nitrobenzene lsopropylether 2-ethylhexyl alcohol Chloroform C a r b o n tetrachloride Cyclohexane n-hexane Cyclohexanone Isopropylbenzene Chlorobenzene Toluene
9.15 9-9 10.0 7.0 9.5 9-3 8-6 8-20 7.30 9.9 8.65 9.5 8.90
8~rg Iog K~
Vo~g
6.02 6.58 6.40 4.28 5.94 6.20 5.60 5.26 4.51 6.35 5.43 6-13 5.73
89 79 103 140 156 81 97 109 132 103 140 102 107
- 0.67 - 0.35 --0.11 0.80 1-23 -0-31 - 1.17 - 1,76 -1.70 1.27 -0.75 -0.78 -0.70
0.5 0 12
0 13
0.~ 0 II
0.1
~
_o~ o .
90
8 -O'l~
~
6
-0"3
4
/ r
z
-0'5 -
o I
J
I
I
7
8
9
IO
org
Fig. 3. Correlation of distribution coefficients of propionic acid with solubility parameter of solvents (Equation 9). 1: n-hexane; 2: cyclohexane; 3: carbon tetrachloride; 4: chlorobenzene; 5: isopropylbenzene; 6: toluene; 7: benzene; 8: chloroform; 9: nitrobenzene; 10: 1,2-dichloroethane; 11: isopropyl ether; 12: 2-ethylhexyl alcohol: 13: cyclohexanone.
are logfls = 2.69, 1.69, 1.57, and 0.26 for isopropyl ether, 2-ethylhexyl alcohol, cyclohexanone, and chloroform, respectively, and the ability of solvents to interact with carboxylic acid increases in the order: chloroform < cyclohexanone < 2-ethylhexyl alcohol < isopropyl ether.
994
I. KOJIMA, M. YOSH1DA and M. T A N A K A 0.3
o 13 0 12 0 II -O'i
0
I,O
8 9
o'
"~ g-0.~
/y
2 0
-0.~ 4
I
i
5
6
L
~'org Fig. 4. Correlation of distribution coefficients of propionic acid with solubility parameter of solvents (Equation 10). Numbers are the same as in Fig. 3.
It is known that the distribution coefficient of organic molecules between the aqueous and organic phases increases by a factor of 3-4 for each methylene group added to the molecule I18-23]. Combining Equations (9) and (l 2), we have log K~ = log Ka + log (Vorg/Vaq) VHR
-- 2"30R T [(Saq -- 8HR)2 -- (8org-- 8nR)2]-
(13)
From Equation (13), it is evident that a plot of log Ka against VHa gives a straight line if the solubility parameter of carboxylic acids is constant. In fact, solubility parameters of compounds having the same functional group do not change appreciably with increasing molecular weight. Therefore log Ka depends mostly on the difference of molar volume of organic molecules. Then the increment of the distribution coefficient of carboxylic acid for an added methylene group was estimated by using Equation (14) for a given solvent. A log Ka =
A VHR 2 " 3 0 R T [(~aq - - ~HR) 2 - - ( ~ o r g - ~HR) 2]
(14)
where A log Ka and A VHR are the difference of distribution coeffÉcient and the 18. 19. 20. 21. 22. 23.
B. B. Wroth and E. E. Reid, J. Am. chem. Soc. 38, 2316 ( 1916). A. Frumkin, Z. phys. Chem. 116, 501 (1925). D. Dyrssen, Proc. 7th Int. Conf. Co-ord. Chem. Paper No. 753, Stockholm (1962). N. S. Saha, A. Bhattacharjee, N. G. Basak and A. Lahili, J. chem. Engng Data 8, 405 (1963). D. Dyrssen, S. Ekberg and D. H. Liem, Acta chem. scand. 18, 135 (1964). D. Dyrssen and Dj. Petkovic,J. inorg, nucl. Chem. 27, 1381 (1965).
995
Distribution of carboxylic acids
molar volume of carboxylic acids, respectively. Then a plot of log Ka against the number of carbon atoms in acids and against the difference of molar volume of acids by selecting acetic acid as a reference acid ( V , ~ - V.OAC) is shown in Fig. 5, this plot yielded a straight line. From Fig. 5, Equation (14) and Table 1, it is evident
J
I
log Kd
q,/
, ~ . ~o~c
/
0
number of carbon otorns 4 5 6
-2 3 I
o
,
I
i I
210
t
4~0 crn~ VHR- VNo,=c
I
60
II
80
Fig. 5. Correlation of the distribution coefficient of carboxylic acids with the number of carbon atoms in carboxylic acid and with the molar volume of acids.
that the increment of log Ka for each methylene group is nearly constant irrespective of the solvent, since the increment of molar volume of solute for an added methylene group is constant and its value is 16-17. Then 6HR calculated from the slope of lines in Fig. 5 was 10-5__+0.4. This value does not agree with the previous finding by Kemula et al.[7], who related the distribution coefficients of formic, acetic, propionic, benzoic, and salicylic acids with the solubility parameter of solvent and estimated the solubility parameters of these solutes. They used 23-4 as 6aq and obtained 17.26 as 6 for formic and acetic acids and 16.47 for propionic acid. However, using 17.55 as 6aq, we got 10.6_+ 0.5 for 6nR from their data. This value compares favourably with ours. And from the present results, A log Ka for each methylene group is about 0.56 for benzene, nitrobenzene, isopropyl ether and 2-ethylhexyl alcohol and 0.60 for 1,2-dichloroethane. From the data available[l, 2, 21-23], it can be safely concluded on the sound theoretical basis that the increment of log Ka for an added methylene group is nearly constant irrespective of solvents and of solute, i.e. A log Ka/a CH,, = 0.560-60 (see Equation (14)). Acknowledgement-The financial support given by the Ministry of Education (Japan) is gratefully acknowledged.