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Extraction of lead(II) with capric acid
J. inorg, natl. ( h e m . . 1970. Vol. 32, pp. 3667 to 3672.
Pergamon Press.
Printed in Great Britain
E X T R A C T I O N OF LEAD(II) WITH C A P R I C A C I D N O R I Y U K I N A K A S U K A , M A S A H I K O N A K A I and M O T O H A R U T A N A K A l_aboratory of Analytical Chemistry, Faculty of Science. Nagoya University, Chikusa-ku. Nagoya. Japan (Received 23 April 1970)
Abstract-Partition of lead was carried out at 25°C between water and benzene containing capric acid in excess, total concentration of lead and that of capric acid being less than 3 x 10 a M and 0,101.50 M respectively. Under the experimental conditions there exist two monomeric lead caprates with different compositions in the organic phase: PbR22HR and PbR24HR. Thus the extraction equili bria tbr lead can be written as follows: /x~z
Pb'-'++ 2 (HR)e.,, , ~
(PbR.,2HR),+ 2 H
Pb ~+ + 3 (HR),_, ,, ~
(PbR.,4HR),+ 2 H"
for which the extraction constants were determined as log K,,e = log ([PbR.,2HRJo[H +j2/IPb'e+)l(HR).eJ,,e) = - 7" 12 ± 0"05 log K,~ = log ([PbR,4H R),,[H +]e/[PbZ+][( HR).,],, a) = - 6"80 ± 0" 10. INTRODUCTION IN T H e e x t r a c t i o n o f m e t a l i o n s w i t h m o n o c a r b o x y l i c a c i d s , m o s t m e t a l s w e r e f o u n d to e x i s t as p o l y m e r i c as w e l l as m o n o m e r i c c a r b o x y l a t e s in t h e o r g a n i c p h a s e . C o p p e r ( l l ) c a p r a t e is e x t r a c t e d i n t o b e n z e n e c o n t a i n i n g c a p r i c a c i d in e x c e s s [1 ], t h e s t r u c t u r e o f t h e e x t r a c t e d s p e c i e s b e i n g w e l l k n o w n d i m e r i c s k e l e t o n as s e e n in c o p p e r ( I I ) a c e t a t e m o n o h y d r a t e in s o l i d s t a t e [2]. S i m i l a r r e s u l t s h a v e b e e n o b t a i n e d in t h e e x t r a c t i o n s o f c o p p e r ( l 1) w i t h v a r i o u s c a r b o x y l i c a c i d s [ 3 - 5 ] . D i m e r i c c o p p e r ( l l ) c a p r a t e w a s f o u n d to d i s s o c i a t e i n t o r n o n o m e r at v e r y low c o n c e n t r a t i o n s ( < 10 -~ M ) in t h e o r g a n i c p h a s e [61. F o r t h e e x t r a c t i o n o f n i c k e l a n d c o b a l t ( l l ) , t h e s i t u a t i o n is d i f f e r e n t f r o m t h e a b o v e c a s e s in t h e s e n s e t h a t m o n o m e r i c s p e c i e s N i R , , 4 H R a n d CoR._,4HR a r e e x t r a c t e d t o g e t h e r w i t h d i m e r i c s p e c i e s ( N i R e 2 H R ) . , a n d (CoR_,2HR)._, at millim o l a r l e v e l [7, 8]. I r o n ( I ! 1)[7] a n d a l u m i n i u m l 9 1 a r e e x t r a c t e d i n t o b e n z e n e p h a s e I. M. Tanaka and T. N iinomi, J. inore, natl. ('hem. 27.431 (1965 ). 2. J. N. van Niekerk and F. R. L. Schoening, Acre cO, smlloj,,r. 6. 227 (1953). 3. A. W. Fletcher and D. S. Flett. Solvent Extraction Chemist13, o1' Met¢tls (Edited by 1t. '\. ( . McKay. T. V. Healy. 1. L. Jenking and A. Naylor). p. 359. Macmillan (1966). 4. W.J. ltaffenden and G. J. Lawson, J. inor~, m,'f. Chem. 29. I 133 (1967). 5. I. Kojima, M. U chida and M. Tanaka, J. im)rj,,, m~(I. Chem. 32. 1333 (1970). 6. M. Tanaka, N. Nakasuka and S. Sasane (nee Goto), tTnpublished results. 7. M. Tanaka, N. Nakasuka and S. Goto. Solcent Extraction Chemistry (Edited by D. Dyrssen, J. -O. Litjenzin and J. Rydberg). p. 154. N orth-Holland, Amsterdam (1967). 8. M. Tanaka, N. Nakasuka and S. Sasane (nt~e Goto), J. inorg, natl. Chem. 31,2591 (1969). 9. M. Tanaka, N. Nakasuka and H. Yamada. ,/. inor,~,,, mwl. Chem. 32. 2791 (1970). 3667
3668
N. N A K A S U K A ,
M. N A K A I
and M. T A N A K A
exclusively as trimeric and hexameric species respectively even at lower concentrations, while indium caprate exists as monomeric, trimeric and hexameric forms in the organic phase [10]. Recently Schweitzer and Sanghvi have reported the extracted species of thulium(l I 1) with aliphatic monocarboxylic acids in 4-methyl-2-pentanone, TmR3HR, and in chloroform, TmR35 HR [ 11 ]. For the extraction of zinc with aliphatic monocarboxylic acids into benzene, chloroform and 4-methyl-2-pentanone the monomeric extracted species ZnR2HR has been proposed[12]. The extracted species T1R4HR has been reported to be responsible for the extraction of thallium(l) with hexanoic acid [ 13]. The present study was undertaken to investigate the extraction of lead from an aqueous solution into benzene containing capric acid. EQUILIBRIUM
TREATMENT
Distribution o f lead caprates When lead(I I) is extracted into organic phase in a form ofj-mer (PbR~xHR)j, the corresponding equilibrium is expressed as follows: j pb2+j( 1 +2)(HR)2,o K . . (PbR2xHR)~.o + 2 j H +
(1)
Kj~--- [(PbR2xHR)~]o[H+]2~[PbZ+]-~[(HR)2]o -jt~+xm.
(2)
where
HR denotes capric acid, the subscript o refers to the organic phase and Kjx is the relevant extraction constant. The total lead concentration in the organic phase can be equated as follows: J
X
Cpb,o---- ~, ~, j[(PbR2xHR)j]o j~l X=0. J
X
= ~, ~, jKjx(Cpb,w)~[(HR)2]j (1+x/2) [H+]-2~(a(pb))-J
(3)
j=l x=O
where Cpb,o and Cpb,w denote total lead concentrations in the organic and aqueous phases respectively, and a(pb) the side reaction coefficient taking into account the hydrolysis as well as the complexation of Pb ~+ in the aqueous phase, i.e. Cpb,w = a(pb) × [pb2+]. If particularj-mers alone are extracted, Equation (3) is rewritten as: X
Cpb,o = j(Cpb,w)J(a(pb))-~[H+] -2j ~, Kjx[(HR)2]Jo (1+x/2)
(4)
x=0 or
log Cpb,o = j (log Cpb,w-- log C t ( p b )
--
2 log [H + ] )
X
+ log ( x~=oKj~r[(HR)z,Jj ('÷~''2,) + log j. 10. 11. 12. 13.
M. G. G. G.
T a n a k a , N. N a k a s u k a and H. Y a m a d a , J. inorg, nucl. C h e m . 32, 2759 (1970). K. Schweitzer and S. M. Sanghvi, A nalytica Chim. A cta 47, 19 (1969). K. Schweitzer and F. C. Clifford, A nalytica Chim. A cta 45, 57 (1969). K. Schweitzer a n d R. H. Stevens, A nalytica Chim. A eta 45, 192 (1969).
(4')
Extraction of lead(I I) with capric acid
3669
The concentration of capric acid C,n being kept constant, a plot of log Cpb,o vs. (log Cpb,w-- log a(pb)-- 2 log [H+]) should yield a straight line with a slope of j, from which the degree of polymerization is determined. When only a species (PbR2xHR)j is predominant in the organic phase, Equation (4') is simplified: log
Cpb,o
--j(Iog Cpb.,,,- log a¢pb)-- 2 log [H+]) = l o g j + l o g K~.,.+j(I + x ) log [(HR),,],,. A
(5)
According to this expression a plot of experimentally determinable values !og Ceb,o-j(Iog Cpb,w--log c~(pb)--2 log [H+]) against log [(HR)2]o provides us with a means to determine the number of capric acid incorporated in the extracted species. If the species (PbR~xHR)j exists together with (PbR2yHR)j in the organic phase, the expression for Cpb,o is rewritten as: log Cpb,o --j(log
Cpb,w - -
log t~(pb)-- 2 log [H+]) --j(1 + 2 ) log [(HR)2]o ---- log j + log Ksx + log (1 + K ~ K -1~x tr(HR~/21ol~(u/2-x/2)~1. (6)
The plot of the left-hand side of Equation (6) vs. log [(HR)2]o yields a curve which fits a normalyzed curve Y = l o g ( l + x ' ) vs. X = l o g x , where n = j ( y / 2 - x / 2 ) . Displacements of the curve from the origin along X-axis and Y-axis correspond to log K~.vKf'.~ and logj + log Kj~ respectively [ 14], RESULTS
AND
DISCUSSION
Determination of the degree of polymerization and the number of free capric acids involved in the extracted species Plot of log CPb,o against log (Cpb,~.- 2 log [H+]) at any constant total concentration of capric acid yielded a straight line with a slope of unity, as illustrated in Fig. 1: the degree of polymerizationj is 1, i.e. the extracted species is monomeric (cf. Equation (4)). Under the experimental conditions in this study (pH ~< 4-5), the hydrolysis of lead(li) ion in the aqueous phase was assumed to be negligible, that is c~vb) = 1 or [Pb 2+] = Cpb,w[15]. Plot of log D + 2 log [H +] vs. log [(HR)2]o is shown in Fig. 2, where D is the distribution ratio for lead (cf. Equation (5)). The slope, which is close to 2 at lower concentrations of capric acid, tends to increase gradually to 3 at higher ones. This fact implies that two species coexist in the organic phase and that ! + x]2 = 2 and 3, i.e. x = 2 and 4 respectively. To confirm the above conclusion and find the corresponding extraction constants, it is convenient to use the curve-fitting method. Substituting 1 f o r j and 2 for x in Equation (6), we obtain the following expression: logD+21og[H+]-21og[(HR)2],, = logK12+log(l+Kl~K~,~ tHR) (~/'~-')) • " "2,o "
(6')
14. F. J, C. Rossotti and H. Rossotti, The Determination of Stability Constants and Other Equilibrium Constants in Solution p. 87. McGraw-Hill,New York ( 1961). 15. A. Ringbom,Complexation in Analytical Chemistry p. 358. Interscience, New York (1963).
3670
N. NAKASUKA, M. NAKA! and M. TANAKA
"3"0
o
3
0..
0
-4"( I
4"0
5"0 log Cr~,= - 2 log[H*]
610
Fig. 1. Determination of the degree of polymerization of lead(ll) caprate. See Equation (4'). As illustrated in Fig. 3, the plot o f log D + 2 log [H +] - 2 log [(HR)2]o vs. log [(HR)z]o fits well to the normalized curve Y = log (1 + x ) , X = logx. Then it follows that y/2 - 1 = 1, or that y = 4. In the calculation of [(HR)z]o, though [(HR)z]o is approximately equal to 0.5 C~n, the concentration o f m o n o m e r i c capric acid (HR)o in the organic phase was corrected for by taking into consideration the dimerization constant of capric acid [8]. T h e compositions for the extracted species are, therefore, written as P b R 2 2 H R and P b R 2 4 H R and corresponding extraction equilibria are described as follows: KI2
Pb 2+ + 2 (HR)2,o ~
KI4
Pb 2 + 3 (HR)2,o ~
(PbR~2HR)o + 2H +
(7)
(PbR24HR)o + 2H +
(8)
with logarithmic extraction constants log K12 = -- 7.12 ± 0.05 and log K 1 4 +0.10. F o r the equilibrium b e t w e e n (PbR22HR)o and (PbR24HR)o, we have
(PbR22HR)o + (HR)~.o ~
ks
(PbR24HR)o, Ks = K14/K12
with log Kf = 0-32 --_+0-05.
=
--
6.80
(9)
Extraction of lead( 1I) with capric acid
-7,0
3671
@a
-i- -8.0 0 +
O
oi
0
-9"0
-I 0"0 I
I
- 1.0
0"0
log [(HR)2] 0 Fig. 2. Determination of the number of capric acid incorporated in the monomeric caprate. Solid lines are asymptotes. In this figure D is identical with C~,b.,,/('~,h.,,. See Equation (5).
The lack of polymeric species in the organic phase is an interesting feature of the extraction of lead(|I) caprate. This might be associated with the poor solubility of the caprate in benzene, especially at lower capric acid concentrations. In view of the fact that the solubility of the lead(i 1) caprate is low and increases with capric acid concentration, the following equilibrium might be responsible for the formation of PbRz4HR in the organic phase: (PbR,,2HR • 2H20)o+ (HR)2., ~
(PbR24HR),+ 2H20.
It is unfortunate in this connection that, owing to the poor solubility of the lead(l I) caprate, Karl Fisher method does not allow to determine definitively the number of water molecules bound to the extracted species.
3672
N. N A K A S U K A , M. N A K A I and M. T A N A K A o
~'~-6.0 "ltTI 0 I
.z _? oJ +
C3 o
-7.0
i
i
0.o
- I.O
log [(HR}z] o
Fig, 3. Determination of the number of capric acid incorporated in the monomeric caprate. Solid line represents a normalized curve Y = log (1 + x) vs, X = log x, and dotted lines represent its asymptotes. Plot is an average of experimental values for a particular CnR. See Equation (6). EXPERIMENTAL
Reagent Lead(ll) perchlorate was prepared by the reaction of perchloric acid and G.R. lead(ll) nitrate which had been recrystallized three times from water. Lead perchlorate was dissolved in dilute aqueous perchloric acid solution and standardized compleximetrically with xylenol orange as an indicator. PA R, 4-(2-pyridylazo) resorcinol (Dojindo Chem. Co., Kumamoto, Japan) was recrystallized twice from aqueous methanol. All other reagents used were prepared according to the methods described elsewhere [8]. Apparatus Mechanical shaking for partition was performed in a thermostat, Taiyo Incubator M-1, Taiyokagaku-Kogyo Co., Tokyo. Radiometer priM 4d pH-meter was used for the pH determination at a constant temperature, 25.0_+ 0.2°C. The temperature was kept constant in a Sharp TEB-10 thermostat, Hayakawa Electric Co., Osaka.
Experimental procedure 15 ml each of the aqueous and organic phases were shaken in a 50 ml centrifuge tube immerged in a bath thermostatted at 25.0--0-2°C. Ionic strength of the aqueous phase was kept 0.10 M in sodium perchlorate, pH determination was performed by dipping the glass electrode and salt bridge with calomel electrode directly into the aqueous phase without removing the upper organic phase. The liquid junction potential was corrected for by the method described elsewhere [10]. The concentration of lead was determined compleximetrically in the same way as for the stock solution. Lower concentration of lead (< 10-4 M) was determined photometrically with PAR, 4-(2pyrizylazo) resorcinol.
Pergamon Press.
Printed in Great Britain
E X T R A C T I O N OF LEAD(II) WITH C A P R I C A C I D N O R I Y U K I N A K A S U K A , M A S A H I K O N A K A I and M O T O H A R U T A N A K A l_aboratory of Analytical Chemistry, Faculty of Science. Nagoya University, Chikusa-ku. Nagoya. Japan (Received 23 April 1970)
Abstract-Partition of lead was carried out at 25°C between water and benzene containing capric acid in excess, total concentration of lead and that of capric acid being less than 3 x 10 a M and 0,101.50 M respectively. Under the experimental conditions there exist two monomeric lead caprates with different compositions in the organic phase: PbR22HR and PbR24HR. Thus the extraction equili bria tbr lead can be written as follows: /x~z
Pb'-'++ 2 (HR)e.,, , ~
(PbR.,2HR),+ 2 H
Pb ~+ + 3 (HR),_, ,, ~
(PbR.,4HR),+ 2 H"
for which the extraction constants were determined as log K,,e = log ([PbR.,2HRJo[H +j2/IPb'e+)l(HR).eJ,,e) = - 7" 12 ± 0"05 log K,~ = log ([PbR,4H R),,[H +]e/[PbZ+][( HR).,],, a) = - 6"80 ± 0" 10. INTRODUCTION IN T H e e x t r a c t i o n o f m e t a l i o n s w i t h m o n o c a r b o x y l i c a c i d s , m o s t m e t a l s w e r e f o u n d to e x i s t as p o l y m e r i c as w e l l as m o n o m e r i c c a r b o x y l a t e s in t h e o r g a n i c p h a s e . C o p p e r ( l l ) c a p r a t e is e x t r a c t e d i n t o b e n z e n e c o n t a i n i n g c a p r i c a c i d in e x c e s s [1 ], t h e s t r u c t u r e o f t h e e x t r a c t e d s p e c i e s b e i n g w e l l k n o w n d i m e r i c s k e l e t o n as s e e n in c o p p e r ( I I ) a c e t a t e m o n o h y d r a t e in s o l i d s t a t e [2]. S i m i l a r r e s u l t s h a v e b e e n o b t a i n e d in t h e e x t r a c t i o n s o f c o p p e r ( l 1) w i t h v a r i o u s c a r b o x y l i c a c i d s [ 3 - 5 ] . D i m e r i c c o p p e r ( l l ) c a p r a t e w a s f o u n d to d i s s o c i a t e i n t o r n o n o m e r at v e r y low c o n c e n t r a t i o n s ( < 10 -~ M ) in t h e o r g a n i c p h a s e [61. F o r t h e e x t r a c t i o n o f n i c k e l a n d c o b a l t ( l l ) , t h e s i t u a t i o n is d i f f e r e n t f r o m t h e a b o v e c a s e s in t h e s e n s e t h a t m o n o m e r i c s p e c i e s N i R , , 4 H R a n d CoR._,4HR a r e e x t r a c t e d t o g e t h e r w i t h d i m e r i c s p e c i e s ( N i R e 2 H R ) . , a n d (CoR_,2HR)._, at millim o l a r l e v e l [7, 8]. I r o n ( I ! 1)[7] a n d a l u m i n i u m l 9 1 a r e e x t r a c t e d i n t o b e n z e n e p h a s e I. M. Tanaka and T. N iinomi, J. inore, natl. ('hem. 27.431 (1965 ). 2. J. N. van Niekerk and F. R. L. Schoening, Acre cO, smlloj,,r. 6. 227 (1953). 3. A. W. Fletcher and D. S. Flett. Solvent Extraction Chemist13, o1' Met¢tls (Edited by 1t. '\. ( . McKay. T. V. Healy. 1. L. Jenking and A. Naylor). p. 359. Macmillan (1966). 4. W.J. ltaffenden and G. J. Lawson, J. inor~, m,'f. Chem. 29. I 133 (1967). 5. I. Kojima, M. U chida and M. Tanaka, J. im)rj,,, m~(I. Chem. 32. 1333 (1970). 6. M. Tanaka, N. Nakasuka and S. Sasane (nee Goto), tTnpublished results. 7. M. Tanaka, N. Nakasuka and S. Goto. Solcent Extraction Chemistry (Edited by D. Dyrssen, J. -O. Litjenzin and J. Rydberg). p. 154. N orth-Holland, Amsterdam (1967). 8. M. Tanaka, N. Nakasuka and S. Sasane (nt~e Goto), J. inorg, natl. Chem. 31,2591 (1969). 9. M. Tanaka, N. Nakasuka and H. Yamada. ,/. inor,~,,, mwl. Chem. 32. 2791 (1970). 3667
3668
N. N A K A S U K A ,
M. N A K A I
and M. T A N A K A
exclusively as trimeric and hexameric species respectively even at lower concentrations, while indium caprate exists as monomeric, trimeric and hexameric forms in the organic phase [10]. Recently Schweitzer and Sanghvi have reported the extracted species of thulium(l I 1) with aliphatic monocarboxylic acids in 4-methyl-2-pentanone, TmR3HR, and in chloroform, TmR35 HR [ 11 ]. For the extraction of zinc with aliphatic monocarboxylic acids into benzene, chloroform and 4-methyl-2-pentanone the monomeric extracted species ZnR2HR has been proposed[12]. The extracted species T1R4HR has been reported to be responsible for the extraction of thallium(l) with hexanoic acid [ 13]. The present study was undertaken to investigate the extraction of lead from an aqueous solution into benzene containing capric acid. EQUILIBRIUM
TREATMENT
Distribution o f lead caprates When lead(I I) is extracted into organic phase in a form ofj-mer (PbR~xHR)j, the corresponding equilibrium is expressed as follows: j pb2+j( 1 +2)(HR)2,o K . . (PbR2xHR)~.o + 2 j H +
(1)
Kj~--- [(PbR2xHR)~]o[H+]2~[PbZ+]-~[(HR)2]o -jt~+xm.
(2)
where
HR denotes capric acid, the subscript o refers to the organic phase and Kjx is the relevant extraction constant. The total lead concentration in the organic phase can be equated as follows: J
X
Cpb,o---- ~, ~, j[(PbR2xHR)j]o j~l X=0. J
X
= ~, ~, jKjx(Cpb,w)~[(HR)2]j (1+x/2) [H+]-2~(a(pb))-J
(3)
j=l x=O
where Cpb,o and Cpb,w denote total lead concentrations in the organic and aqueous phases respectively, and a(pb) the side reaction coefficient taking into account the hydrolysis as well as the complexation of Pb ~+ in the aqueous phase, i.e. Cpb,w = a(pb) × [pb2+]. If particularj-mers alone are extracted, Equation (3) is rewritten as: X
Cpb,o = j(Cpb,w)J(a(pb))-~[H+] -2j ~, Kjx[(HR)2]Jo (1+x/2)
(4)
x=0 or
log Cpb,o = j (log Cpb,w-- log C t ( p b )
--
2 log [H + ] )
X
+ log ( x~=oKj~r[(HR)z,Jj ('÷~''2,) + log j. 10. 11. 12. 13.
M. G. G. G.
T a n a k a , N. N a k a s u k a and H. Y a m a d a , J. inorg, nucl. C h e m . 32, 2759 (1970). K. Schweitzer and S. M. Sanghvi, A nalytica Chim. A cta 47, 19 (1969). K. Schweitzer and F. C. Clifford, A nalytica Chim. A cta 45, 57 (1969). K. Schweitzer a n d R. H. Stevens, A nalytica Chim. A eta 45, 192 (1969).
(4')
Extraction of lead(I I) with capric acid
3669
The concentration of capric acid C,n being kept constant, a plot of log Cpb,o vs. (log Cpb,w-- log a(pb)-- 2 log [H+]) should yield a straight line with a slope of j, from which the degree of polymerization is determined. When only a species (PbR2xHR)j is predominant in the organic phase, Equation (4') is simplified: log
Cpb,o
--j(Iog Cpb.,,,- log a¢pb)-- 2 log [H+]) = l o g j + l o g K~.,.+j(I + x ) log [(HR),,],,. A
(5)
According to this expression a plot of experimentally determinable values !og Ceb,o-j(Iog Cpb,w--log c~(pb)--2 log [H+]) against log [(HR)2]o provides us with a means to determine the number of capric acid incorporated in the extracted species. If the species (PbR~xHR)j exists together with (PbR2yHR)j in the organic phase, the expression for Cpb,o is rewritten as: log Cpb,o --j(log
Cpb,w - -
log t~(pb)-- 2 log [H+]) --j(1 + 2 ) log [(HR)2]o ---- log j + log Ksx + log (1 + K ~ K -1~x tr(HR~/21ol~(u/2-x/2)~1. (6)
The plot of the left-hand side of Equation (6) vs. log [(HR)2]o yields a curve which fits a normalyzed curve Y = l o g ( l + x ' ) vs. X = l o g x , where n = j ( y / 2 - x / 2 ) . Displacements of the curve from the origin along X-axis and Y-axis correspond to log K~.vKf'.~ and logj + log Kj~ respectively [ 14], RESULTS
AND
DISCUSSION
Determination of the degree of polymerization and the number of free capric acids involved in the extracted species Plot of log CPb,o against log (Cpb,~.- 2 log [H+]) at any constant total concentration of capric acid yielded a straight line with a slope of unity, as illustrated in Fig. 1: the degree of polymerizationj is 1, i.e. the extracted species is monomeric (cf. Equation (4)). Under the experimental conditions in this study (pH ~< 4-5), the hydrolysis of lead(li) ion in the aqueous phase was assumed to be negligible, that is c~vb) = 1 or [Pb 2+] = Cpb,w[15]. Plot of log D + 2 log [H +] vs. log [(HR)2]o is shown in Fig. 2, where D is the distribution ratio for lead (cf. Equation (5)). The slope, which is close to 2 at lower concentrations of capric acid, tends to increase gradually to 3 at higher ones. This fact implies that two species coexist in the organic phase and that ! + x]2 = 2 and 3, i.e. x = 2 and 4 respectively. To confirm the above conclusion and find the corresponding extraction constants, it is convenient to use the curve-fitting method. Substituting 1 f o r j and 2 for x in Equation (6), we obtain the following expression: logD+21og[H+]-21og[(HR)2],, = logK12+log(l+Kl~K~,~ tHR) (~/'~-')) • " "2,o "
(6')
14. F. J, C. Rossotti and H. Rossotti, The Determination of Stability Constants and Other Equilibrium Constants in Solution p. 87. McGraw-Hill,New York ( 1961). 15. A. Ringbom,Complexation in Analytical Chemistry p. 358. Interscience, New York (1963).
3670
N. NAKASUKA, M. NAKA! and M. TANAKA
"3"0
o
3
0..
0
-4"( I
4"0
5"0 log Cr~,= - 2 log[H*]
610
Fig. 1. Determination of the degree of polymerization of lead(ll) caprate. See Equation (4'). As illustrated in Fig. 3, the plot o f log D + 2 log [H +] - 2 log [(HR)2]o vs. log [(HR)z]o fits well to the normalized curve Y = log (1 + x ) , X = logx. Then it follows that y/2 - 1 = 1, or that y = 4. In the calculation of [(HR)z]o, though [(HR)z]o is approximately equal to 0.5 C~n, the concentration o f m o n o m e r i c capric acid (HR)o in the organic phase was corrected for by taking into consideration the dimerization constant of capric acid [8]. T h e compositions for the extracted species are, therefore, written as P b R 2 2 H R and P b R 2 4 H R and corresponding extraction equilibria are described as follows: KI2
Pb 2+ + 2 (HR)2,o ~
KI4
Pb 2 + 3 (HR)2,o ~
(PbR~2HR)o + 2H +
(7)
(PbR24HR)o + 2H +
(8)
with logarithmic extraction constants log K12 = -- 7.12 ± 0.05 and log K 1 4 +0.10. F o r the equilibrium b e t w e e n (PbR22HR)o and (PbR24HR)o, we have
(PbR22HR)o + (HR)~.o ~
ks
(PbR24HR)o, Ks = K14/K12
with log Kf = 0-32 --_+0-05.
=
--
6.80
(9)
Extraction of lead( 1I) with capric acid
-7,0
3671
@a
-i- -8.0 0 +
O
oi
0
-9"0
-I 0"0 I
I
- 1.0
0"0
log [(HR)2] 0 Fig. 2. Determination of the number of capric acid incorporated in the monomeric caprate. Solid lines are asymptotes. In this figure D is identical with C~,b.,,/('~,h.,,. See Equation (5).
The lack of polymeric species in the organic phase is an interesting feature of the extraction of lead(|I) caprate. This might be associated with the poor solubility of the caprate in benzene, especially at lower capric acid concentrations. In view of the fact that the solubility of the lead(i 1) caprate is low and increases with capric acid concentration, the following equilibrium might be responsible for the formation of PbRz4HR in the organic phase: (PbR,,2HR • 2H20)o+ (HR)2., ~
(PbR24HR),+ 2H20.
It is unfortunate in this connection that, owing to the poor solubility of the lead(l I) caprate, Karl Fisher method does not allow to determine definitively the number of water molecules bound to the extracted species.
3672
N. N A K A S U K A , M. N A K A I and M. T A N A K A o
~'~-6.0 "ltTI 0 I
.z _? oJ +
C3 o
-7.0
i
i
0.o
- I.O
log [(HR}z] o
Fig, 3. Determination of the number of capric acid incorporated in the monomeric caprate. Solid line represents a normalized curve Y = log (1 + x) vs, X = log x, and dotted lines represent its asymptotes. Plot is an average of experimental values for a particular CnR. See Equation (6). EXPERIMENTAL
Reagent Lead(ll) perchlorate was prepared by the reaction of perchloric acid and G.R. lead(ll) nitrate which had been recrystallized three times from water. Lead perchlorate was dissolved in dilute aqueous perchloric acid solution and standardized compleximetrically with xylenol orange as an indicator. PA R, 4-(2-pyridylazo) resorcinol (Dojindo Chem. Co., Kumamoto, Japan) was recrystallized twice from aqueous methanol. All other reagents used were prepared according to the methods described elsewhere [8]. Apparatus Mechanical shaking for partition was performed in a thermostat, Taiyo Incubator M-1, Taiyokagaku-Kogyo Co., Tokyo. Radiometer priM 4d pH-meter was used for the pH determination at a constant temperature, 25.0_+ 0.2°C. The temperature was kept constant in a Sharp TEB-10 thermostat, Hayakawa Electric Co., Osaka.
Experimental procedure 15 ml each of the aqueous and organic phases were shaken in a 50 ml centrifuge tube immerged in a bath thermostatted at 25.0--0-2°C. Ionic strength of the aqueous phase was kept 0.10 M in sodium perchlorate, pH determination was performed by dipping the glass electrode and salt bridge with calomel electrode directly into the aqueous phase without removing the upper organic phase. The liquid junction potential was corrected for by the method described elsewhere [10]. The concentration of lead was determined compleximetrically in the same way as for the stock solution. Lower concentration of lead (< 10-4 M) was determined photometrically with PAR, 4-(2pyrizylazo) resorcinol.