DISTRIBUTION OF SINQLE ANIONB BETWEEN AMMONIUM SALTSAND LiCl
375
Distribution of Single Anions between Carbon Tetrachloride Solutions of High Molecular Weight Tertiary Ammonium Salts and Aqueous Lithium Chloride Solutions
by G. Scibona, R. A. Nathan, A. S. Kertes, and J. W. Irvine, Jr. Laboratory for Nuclear Science and Departmat of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, and Centro Studi Nucleari, Casacck, Rome, Italy (Received July 6, 1966)
The distribution of Reo4- ions between cc14solutions of tridodecylammonium salts and aqueous solutions of lithium salts has been studied as a function of the ammonium salt and perrhenate ion concentrations. The chloride, bromide, nitrate, and perchlorate ammonium salts have been used. Also, the distribution of Br- ion between a CCI, solution of tridodecylammonium chloride and aqueous LiCl solution has been studied as a function of the ammonium salt and of the bromide ion concentrations. The deviation of the distribution data of some of these systems from a simple mass law model has been interpreted in terms of the formation of the mixed dimer [(RX)(RY)] with X- = C1-, Br-, NOa-, and Y- = Reo4-. The (Re04*-C104)and (Br*-CI) systems are apparently following a simple mass action law, but in the case of the (Re04*-C104) system, this effect may not be real.
It is frequently assumed that the following simple anion-exchange process occurs when an organic solution of an alkylammonium salt, is equilibrated with an aqueous solution of electrolyte containing the anion Y -
m,
RX + Y
- z
RY + x-
(1)
However, it is known that the alkylammonium salts tend to aggregate in inert solvents of low dielectric constant, so eq. 1 cannot represent all such systems. In order to take into account every polymerization process in the organic phase, the general expression for the distribution coefficient can be derived
D
= Ki,x,y(RX)(X)-'P'
A
+
B
C1 C1 b K 4.X,YKa,b (RX)a+b (Y) - (XI-bF-
(2)
where J , A , and B indicate the maximum number of units interacting to form polymers, K ~ , x , is Y the equi8 librium constant of the j-mer formation
jm
(m),,
Ka,bis the equilibrium constant of the mixed (a
+
+ b)-
mer formation uRX b R T [(RX),(RY)~], (Y) is the equilibrium aqueous concentration of the anion Y-, (X) is the equilibrium aqueous concentration of the is the equilibrium monomer concentraanion X-, (E) tion of the salt RX in the organic phase, and F is the ratio of the square of the mean activity coefficients, Y'*MX(MY)/Y~*MY(MX), with Y*MX(MY) and YIMY(MX) the mean activity coefficients of the salts MX and MY in the presence of MY and MX, respectively. In eq. 2 the first term is the expression of the mass exist in action law if only monomers (TX) and solution. The second term accounts for the homopolymers (=),The . third term takes into consideration the formation of the heteropolymers [(=),(=)a]. Equation 2 accounts for the deviation of the distribution coefficient from a simple mass action law by considering solute-solute interactions in terms of monomer a n-mer equilibria. If the solvent is inert, the solute-solvent interactions can be neglected. In this work eq. 2 has been used to interpret the distribution of the bromide ion and the perrhenate ion
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Volume 70, Number 2 February 1966
G. SCIBONA, R. NATHAN, A. KERTES,AND J. IRVINE, JR.
376
between aqueous lithium salt solutions and carbon tetrachloride solutions of tridodecylammonium salts.
Experimental Section Maferials. Except for the perchlorate, the tridodecylammonium salts mere all prepared as purified crystalline solids The chloride and bromide salts were prepared by shaking a slight excess of the concentrated aqueous hydrohalic acid with a petroleum ether (bp 35-65’) solution of the amine. The salts were precipitated from the petroleum ether cooling to 0’. The salts were recrystallized a t least three times from petroleum ether. The tridodecylammonium nitrate was prepared in a similar manner, but the solvent was a 1 : l acetonepetroleum ether mixture, and the salt was crystallized by cooling to -78’. The filtration apparatus was precooled to Dry Ice temperature before the salt was filtered out’. The salt was recrystallized five times from the acetone-petroleum ether mixture a t 0’. The purity of the salts was checked by chemical analysis, infrared spectra, and melting point determination. Tridodecylammonium perchlorate was prepared by equilibration of a 0.01 M solution of the free amine in CCl, with an excess of 0.1 M aqueous perchloric acid for 1 hr. The CC1, solution was washed once with water and used without isolating the solid salt. Potentiometric titration in methanol showed that the CCl4 solution contained neither free amine nor free acid. The tridodecylaniine was Eastman White Label, the acids were Du Pont reagent grade, and the carbon tetrachloride and petroleum ether were Fisher reagent grade. The Brh2and Re186tracers were prepared by (n,y) reactions in the XIIT reactor from NH4Brand Re metal. The bronitde solution was prepared by dissolving the salt in water. The rhenium metal was oxidized to Reo4- with aqua regia, evaporated to dryness, and taken up in water. Procedure. The studies of the dependence of the distribution coefficient on the ammonium salt concentration were carried out by equilibrating a solution of an ammonium salt in CC1, with an aqueous solution of lithium chloride containing the tracer anion. The range of concentration of the ammonium salt was M ,the tracer anion was less than 10-4 to M , and the supporting electrolyte was 2 AI. In order to keep the hydrolysis of the ammonium salt below the lithium chloride solution was adjusted to pH 2 with HCL. The dependence of the distribution coefficient on (Reo,-) and (Br-) was measured by varying the con-
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The Journal of Physical Chemistry
centration of these species in aqueous phase over the range to M with the ammonium salt concentration M and supporting electrolyte 2 M . Activity Coeficient. Values of the activity coefficients for the supporting electrolyte were taken from the data of Robinson and Stokes.’ The activity coefficients of the bromide and perrhenate salts in the presence of much larger concentration of the supporting electrolyte were calculated from the Brprnsted equalog tion2: log Y*MY(MX) = ‘/z(log Y ~ M Y ( M Y ) Y*MX(MX)). In this equation Y*~,IY(MY) is the activity coefficient of LiBr or LiReOd a t 2 M , the concentration of the supporting electrolyte. Since the values of activity coefficient for the perrhenate salt are not available in the literature, we have approximated these values with the activity coefficients of the corresponding perchlorate salt. The introduction of this approximation in the calculation of F affects only the absolute value of the equilibrium constants.
+
Results and Discussion The distribution of (Y) between a CCl, solution of
m,
a tridodecylammonium salt, and an aqueous LiX solution has been determined for the following systems: Y- = Reo4-, X- = C1-, Br-, Clod-; Y- = Reo4-, X- = KO3- (as the sodium salt) ; and Y- = Br-, X- = C1-. For convenience in referring to these systems, the symbols for tracer will be starred and coupled with the symbol for the niacroscopic anion, e.g., (Re04*-C1). The results of these measurements are reported in Tables I-V in terms of distribution coefficient
D = (Y)ord(Y)w (3) The experimental D values in Tables I-V, column 2 , show that D is dependent on and independent of (Y) (Table VI). The use of D’ = DF is more convenient for us since it eliminates F in all of the terms of eq. 2 with (F) at the first power. Values of D‘ are reported in Tables 1-111, column 3. Rots of log D’ us. log ( E ) for the systems (Re04*-C1) (Figure l), (Re04*-X03), and (Re04*-Br) give straight lines with slopes >l. For the systems (Re04*-C104) (Figure 2 ) and (Br*-C1) the plots give straight lines with slopes of 1. According to eq. 2, a plot of log D os. log (E) will give a straight line with slope of 1 if only monomeric species are involved. It will give a straight line with
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(1) R. A. Robinson and R. H. Stokes, “Electrolyte Solutions,” 2nd ed, Academic Press Inc., New York, N. T.,1959. (2) H. S. Harned and B. B. Owen. “The Phvsical Chemistrv of Electrolytic Solutions,” 3rd ed, Reinhold Pubiishing Corp., New York, N. Y., 1959.
377
DISTRIBUTION OF SINGLEANIONSBETWEEN AMMONIUM SALTSAND LiCl
Table I: System (Re04*-Cl)
(El, 1x 2x 6X 1X 2x 6X 1x
10-4 10-4 lO-'3
10-3 10-2
3.7 x 9.3 x 4.8 X 9.5 X 2.5 x 1.6 4.3
10-3 10-3 lo-* 10-1
Q
3 . 0 x 10-3 7 . 4 x 10-3 3.8 X 4.6 X 2 . 0 x 10-l 1.28 3.44
55 69.5 128 152 200 416 620
1.0 7.7 1.2 9.8 7.3 6.0 6.2
I
M
D
1 x 10-4 2 x 10-4 6X 1 x 10-3 2X 6X 1X
4.0 x 1.1 x 4.9 X 9.4 x 2.6 X 1.1 X 2.2 X
Q
DF
10-4 10-3 10-3
lo-' lo-'
lo-'
4.1 x 9.0 x 4.0 X 7.7 x 2.2 x 9.2 x 1.8X
10-4 10-4 10-3 lo-' lo-' lo-'
t
x x
104 104 X lo4 X lo4 X 10' X lo4 X lo4
Table 11: System (ReOd*-NOa)
(RNOa)
I
K'
DF
D
M
lo
K'
7.0 7.3 11.5 15 18 25 29
5.0 x 4.0 x 8.5 X 8.5 x 4.7 x 3.1 x 2.2 X
103 103
0'
6'
"V t
lo3
miI
103
lo3 lo3 lo3
Figure 1. Logarithmic plot of D'us. ( E l ) for the system (ReOr*-C1). Aqueous phase: 2 M LiCl and 0.02 M Hf.
Table I11 : System (ReO**-Br)
(RBr), M
D
DF
Q
K'
1 x 10-4 2 x 10-4 6X 1 X 10-3
3 . 4 x 10-4 7 . 4 x 10-4 3.2 X 5 . 8 X 10-3
3 . 0 x 10-4 6 . 5 x 10-4 2.8 X 6 . 0 X 10-3
6.0 6.5 9.3 12.0
1 x 104 7 . 5 x 103 7 . 1 X lo3 7 . 0 X lo3
-
D 1x 2 x 6 X 1x 2 x 6 x 1x
10-4 10-4 10-3 10-3 10-3 10-2
9.6 x 2.2 x 5.2 x 9.3 x 1.8 x 5.5 x 9.2 x
10-l 10-4 10-4 10-4 10-3 10-3 10-3
(4-) for the system Figure 2, Logarithmic plot of L)' vs. R Aqueous phase: 2 M LiC104 and 0.02 M H+. (ReO,*-ClO.).
Table V : System (Br*-Cl) (RBr), M
a slope >1 or a curve whose derivative, d log D/d log (E), is a function of (TX) and has avalue > 1 when polymeric species are present. The assumption is made in the following discussion of eq. 2 that the concentrais negligible in the tion of polymeric species, (E)*, CC14 solution. Although no measurements in this solvent are available, osmometric measurements have
1x 2 x 6X 1x 2x 6 X 1x
10-4 10-4 10-3 10-3 10-8 10-2
D
7.3 x 1.3 X 1.4 x 5.5 x 1.0 x 1.5 X 6.4 x
Volume 70,Number d
10-4 10-3 10-3 10-2 10-2
February 1966
378
G. SCIBONA, R. NATHAN, A. KERTES,AND J. IRVINE, JR.
Table VI: Distribution Coefficients, D,as Function of the Aqueous Reo&and Br- Concentrations (RX = 10-8M; X = 2; t = 25') n
(Y), M
e
(Br*-CI)
1 x 10-4 2 x 10-4 6 X low4 1 x 10-3 2 x 10-8 6X 1 x 10-2
(ReO+Cl)
5.0 X 5.0 X 5.1 X 5 . 5 x 10-8 5 . 0 X lo-' 5 . 1 X lo-'
1.0 x 1.1 x 1.0 x 9.5 x 1.0 x
(ReOd*-Br)
6.6 x 7.0 x 6.7 x 6.8 x 7.05 x
10-1 10-1 10-1
10-2 10-1
been made for benzene solutions of tridodecylammonium chloride and nitrate.s In the nitrate case, where polymerization was greatest, it was less than 20% M. at concentration of the salt According to eq. 2 the lack of dependence of D on (Y-), shown in Table VI, along with the slope >1 for the systems (ReO4*-C1), (ReOa*-NOa), and (Re04*-Br), indicates the presence of a mixed dimer [ ( E ) (=) ] in these systems. For the further treatment of the data, it is convenient to introduce a new function, Q, obtained by multiplying D of eq. 2 by (X)(RX)-'(F)
d
3
Q
=
Kl,X,Y
+ c j ~ ~ l , x , Y K , , Y ( R x ) j - ' ( Y ) ~x- l
10-8
I
,
I
I 1 1 1 1 1 1 1
I
l(x)- ( b - 1) (q- (a- 1)
lim Q = K1,x,Y
I , , , ,
lo-'
x 10-4
10-
9.1
lo-' 10-8 10-8
9 . 3 x 10-4
,
I
I
I
I
I 4 , I I I I
I t , , , ,
d'
I
I
I
,
I
800
1 1 1
1
1
1
4
1
6
16'
(ax)
(4)
(5)
(EXr-tO
Since only the mixed dimer is expected from previous considerations, j = 1, a = b = 1, and eq. 2 and 4 reduce to
Ki,x,y(RX)(X)-'(F)-'
I
(ReO4*-C103
(El)
It follows then from eq. 4 that
=
1.0 x 1.0 x 9.3 x 9.4 x 9.8 x
10-8 10-8 10-8
2
(Fx)d-bl(y)b-
D
I
(ReOd*-NOI)
+
Figure 3. Q plotted against for the systems: (Re04*-Br), bottom curve; (ReO4*-NOa), middle curve; (ReOl*-Cl), top curve. The left scale in unit increments refers to the system (ReO&*-ClOd).
are reported in Tables 1-111, column 5 , and are, of course, fairly constant as expected for the presence of The deviation observed for the dimer (Re04*-NOa) at concentrations of ammonium salt higher than loea M may be due to the presence of homopolymers (RNoa) (. In the (Re04*-C104) system a simple linear relationship was obtained in plotting log D us. log (RClOJ
[(=)(m)].
K'(RX)2(X)-1(F)--l (6) and
Q
= K1,X.Y
+K ' ( m
Table VI1
(7)
(m) (m)
with K' = Kl,x,d&. The dependence of Q on for the systems whose plot of log D us. log has a slope > 1 is shown in Figure 3. Values of Q are reported in Tables I-IV, column 4. Using eq. 5 the values of K1,X,Y have been calculated and are reported in Table VII. Using eq. 7 with the values of K1,X,Yfrom Table VII, the values of K' are calculated for each concentration. These values
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The Journal of Physical Chmistrv
System
K'lW
KI,X,Y
KI,~
(ReOk*-Cl) (ReOl*-Br) (ReOl*-NOa) (ReO4*-CIO4) (Br *-CI)
7 . 6 x 104 7 . 2 X lo8 5 . 3 x 108
54 5.0 6.5 1.9 10
1 . 4 X loa 1 . 4 X loa 9 . 2 X loz
-~ (3) G. Scibona, S. Basol, P. Danesi, and F. Orlandini, submitted to J . Inorg. Nucl. Chem.
DISTRIBUTION OF SINGLE ANIONSBETWEEN AMMONIUM SALTSAND LiCl
(Figure 2). This line had a slope of 1. It has been determined4that the RClO, in benzene solution polymerizes extensively, and consequently we expect that similar polymerization occurs in the CC14 solution. Thus, the stoichiometric RClO, concentration, from which this straight line was derived, is not a good approximation for the monomeric (RC104). This means that the straight line with slope 1 is accidental and that the proper treatment of the (Re04*-C104)system must be deferred until the degree of polymerization of RC1O4 in CC14has been determined. The distribution data for the (Br*-C1) system indicate simple monomer distribution with no evidence for mixed dimer formation. This agrees with the observation of constancy of the ratio of the activity coefficients of RC1 and RBr in the organic phase for the distribution of Br- between aqueous solutions of alkali chlorides and a 0.1 M toluene solution of trin-octylamine ~hloride.~ In Table VI1 are reported the average values of K’ and K1,1 and the values of K 1 , X , Y for the (Reo4*Cl), (Re04*-Br) , and (Reo4*-NO3) systems. Values of K 1 , X , Y of the other systems are also reported. The K l , x , y have been calculated by limit operation from
379
Figure 3 using eq. 4. The K1,l values have been calculated by using the relation K’ = K 1 , X , Y K 1 , l . When the values of K 1 , X , Y are known, it should be possible to calculate the equilibrium constant for a given reaction from those of two other reactions. Thus, for example, in the three systems (Br*-Cl), (ReO4*-C1), and (Reo4*-Br), the relationships K1 ,CI , ~ e 0 4 = K1 ,c 1 , B r K 1 ,Br,ReOc K1,cl,Br
=
K I , C I , R ~ O ~,Re04 /KI,B~
(8) (9)
should be valid. The following K 1 , X , Y can be calculated: K1,C1,C104 = 29; K I , B ~ , C= I O27; ~ K I , N O ~ , C=I O 3.5; ~ Ki,CI,NOa = 8.3; K ~ , B ~ , = N o0.77. ~ These K 1 , X , Y values show the following amine selectivity order: Reo4- > c104- > Br- > NO3- > C1-. This sequence is the same as is found for anion-exchange resinse6 ~~
(4) (a) J. J. Bucher and R. M. Diamond, J. Phys. Chem., 69, 1505 (1965); (b) P. Danesi, F. Orlandini, and G. Scibona, unpublished results. (5) S.Lindenbaum and G. E. Boyd, J. Phys. Chem., 66, 1383 (1962). (6) B. Chu, D. C. Whitney, and R. M. Diamond, J . Inorg. Nucl. Chem., 24, 1405 (1962).
Volume 70,Number 8 February 1066
