R. M. DIAMOND
1522
Vol. G l
THE SOLVENT EXTRACTION BEHAVIOR OF INORGANIC COMPOUNDS. IV. VARIATION OF THE DISTRIBUTION QUOTIENT WITH CHLORIDE, HYDROGEN ION HELD CONSTANT1 BY R. M. DIAMOND Contribution from the Department of Chemistry and the Laboratory for Nuclear Studies, Cornell University, Ithaca, N . Y. Received April 16, 1961
The variation in the distribution quotient for tracer indium(II1) distributing between the oxygenated solvents, diethyl, butyl ethyl, dibutyl and @,@'-dichlorodiethylether, methyl isobutyl and diisobutyl ketone, dibutyl phthalate, methyl beneoate, methyl salicylate, 2-ethylhexanol and decanol, and aqueous solutions of hydrochloric acid, in which the concentration of HCI was varied while keeping the total ionic strength constant with HC104or HN03, was studied. The results are compared with the behavior expected from expressions derived for D and d log D/d log [HCl]. It is shown that certain aspects of the variation in D are indeed indicated by the derived expressions, particularly a "common ion" effect when the ion InC14- is the principal organic phase indium-containing species and an increase in D when ion associations containing InC14- are possible in the organic phase. The remaining major effect is interpreted on the basis of a previous suggestion that the availability, or non-availability, of water in the aqueous phase for solvating ionic species is an important factor in the extraction process. These considerations are not restricted to the indium(II1) chloride system, but appear t o hold generally for those metal halides which extract into oxygenated organic solvents as the anions of strong acids.
Introduction
caused by changes in the nature of the phases in going from an aqueous solution of HC1 to one of HClO4 or HNOI. I n the organic phase there will be changes in the concentration of extracted acid, concn. of metal-containing species in organic phase D = and hence of hydrogen ion; in the aqueous phase concn. of metal-containing species in aqueous phase there will be changes in the water activity. With for the extraction of a metal halide into an organic some solvents these effects are large enough to cause solvent from a hydrohalic acid solution, and for the noticeable phase volume changes, and in extreme variation in D with changes in concentration of the cases, to cause complete miscibility of the two aqueous metal species, of the hydrogen halide and phases. of any other strong acid or halide salt also p r e ~ e n t . ~ The metal system studied in this work was inThat paper discussed the importance of three inter- dium(II1) chloride. Its use has two advantages related factors in determining the value and varia- over that of molybdenum(V1) used in the previous tion of D, namely, the composition of the average paper; the extracting species is known, HInC14, thus metal-containing species in each of the two phases, simplifying and making more concrete the pertinent the possibility of a "common ion" effect, and the equations and discussion, and secondly, the 49 day nature of the two phases. The first two factors half-life of In114"' makes it much more convenient were treated explicitly in the expressions, but it was to work with than 6.7 hour MoQ3". However, less mentioned that the third factor could be handled complete work also done with molybdenum(V1) and only qualitatively. The next paper,4 111, gave the with iron(II1) halide systems indicates that the efexperimental data for the variation of D with fects (and their causes) to be described for indiumchanges in molybdenum(V1) Concentration and (111) also occur with these two halides, and most verified the dependence of D on nietal concentra- likely also with gallium(III), gold(II1) and thaltion. As mentioned in that paper, such a study of lium(III), whose extraction behavior parallels in the dependence of D on metal concentration is a most respects that of indium(II1) and iron(III).6 most favorable test of the expression for D,as this For indium(II1) chloride extractions from hydrochange causes a minimum variation in the nature of chloric acid solutions containing either clod- or the two phases, that is, in the factor not explicitly NOa- (hereafter symbolized by Z-), the following taken into account. The concentration constants species must be considered involved in the expression for D are approximately Aqueous phase? I n + + + ,InCl++, InClz+, InC13, InCl4-, H + , constant, and the expression gives a good repre- c1-, zsentation of the experimental results. In the present paper, data will be presented on Organic phase: InC14-, H + , C1-, Z-, H+InC14-, H+Z-, (H+)zInCl4-,H+Z-InC14-, H+Cl-InC14-, H+(Cl-)z, the variation of D with changes in the concentra- H+Cl-, H +(Z-)z, H +Cl-Z-, ( H +)zCl-, ( H +)z( Cl-)n ( H +)z(Z-)~m, tion of the hydrohalic acid, keeping the ionic (H+)gCI-Z-, (H+)zInCl4-Z-, (H+)21nC14-C1', . . . . stdl strength constant with nitric or perchloric acid. larger ion associations7~8 (Case 3 of paper 11.) This situation is more com(5) R. H.Harber, W. E. Bennett, D. R. Bents, L. C. Bogar, R . J. plex, involving not only the variations in D given Dietz, Jr., A. S. Golden and J. W. Irvine, Jr., Abstracts of the 26th of the American Chemical Society, New York, N. Y., SepMeeting explicitly by the expression for D,but also those tember, 1954. An earlier paper,2hereafter called 11, has derived general expressions for the distribution quotient, D
(1) Work supported in part b y the U. 9. Atomic Energy Commission. (2) R. M. Diamond, THIS JOURNAL, 61, 69 (1957). (3) Although the expressions are derived for metal halide systems. they are more generally applicable, Le., to thiocyanate complex salt systems (R.Watkins and R. M. Diamond, to be published), and possibly to perchlorate and nitrate systems. (4) R. M. Diamond, THISJOURNAL, 61, 75 (1957).
(6) As C104- and NOa- do not complex indium(II1) as strongly as does C1-, any such complexes with the former ions in the aqueous phase are being neglected for simplicity. (7) Evidence for such ion associations in solvents of low dielectric constant are given in (a) R. M. Fuoss and C. A. Kraus, J . Am. Chem. Soc., 66, 2387 (1933); (b) R. J. Myers and D. E . Metzler, ibid., 72, 3772 (1950); (0) N. H. Nachtrieb and R. E . Fryxell, ibid.. 74, 897 (1952).
VARIATION OF DISTRIBUTION QUOTIENT
Nov., 1957
HfInCld-, H+Z- and H+Cl- represent ion pairs in the organic solvent and also true molecular species if they exist, i.e., HCI, (H+)J.nC14- . . H+Cl-InC14- stand for the indium-containing ion triplets possible in solvents of low dielectric constant ( i e . , ethers) and H+(C1-)2 . . . . (H+),Z- similarly represent other possible ion triplets. Of course, if sufficiently high concentrations of ions are possible in an organic phase of sufficiently low dielectric constant, still larger ion associations, such as ion quadruplets, quintuplets, etc., will be formed, and the number of possible species will become very large.' Although many of these ion associations may become important indium-containing species, for simplicity's sake only the ion quadruplets will be explicitly considered in the expression for D, that is, the ion quadruplets are to represent all ion associations larger than ion pairs. Then, leaving out charges on the ions
..
D=
(InCI4)o (In)
+ (HInCl4)o + (HCIHInC14)o + ( H Z H I n S ) , + (InC1) + (InCln) + (InC&) + ( I n c h ) (1)
By means of equilibria similar to those listed in 11,9this becomes
The term still involving an organic phase concentration, namely, (H)o,in the factor derived from (InCI&, can be replaced by using the relation among organic phase concentrations obtained from the principle of electroneutrali ty. But as mentioned in 11, (H)othen becomes a function of the concentration of every ionic species present in the organic phase, and so, formally, of all the odd-membered (charged) ion associations as well as of X-, 2-, H+, InC14-, etc. This would lead to an extremely complicated situation but for the physical fact that d log D = [HCl] d log D d l o g [HCI] (Cl) blog(C1) -
1523
Experimental Tracer and Reagents.-The radioactive indium tracer used, 49 day In114m,was obtained from Upion Carbide Nuclear Co., Oak Ridge, Tennessee. Count,ing way done with a well-type NaI scintillation counter whose pulses were selectively discriminated by a single channel analyzer. No radioactive impurity in the tracer could be detected during the course of the work. Reagent grade nitric, hydrochloric and perchloric acids were used without further treatment, as were reagent grade diethyl ether and C.P. butyl ethyl and dibutyl ethers, methyl isobutyl and diisobutyl ketones, dibutyl phthalate, methyl benzoate, methyl salicylate, decanol and practical grade 2-ethylhexanol. C.P. P,P'-dichlorodiethyl ether was distilled under reduced pressure and a colorless, twodegree cut taken. Procedure.-The procedure for the distribution experiments was the same as described previously in p:per III.4 The measurements were all performed at, 21 f 1 Duplicate trials showed that when D was in the range 0.001 to 100, reproducibility was usually 10% or better; difficult systems showed differences as large a8 25%, but these were rare. When D was less than 0.001, the accuracy was correspondingly less. The conductivity measurements were carried out using a Serfass Conductance Bridge (Model RCYI 15R1, A. H . Thomas Co., Philadelphia) and a cell with a constant of 0.17. Twenty-five ml. of organic solvent was equilibrated with 10 ml. of the 10 Jf aqueous solution, and then 20 ml. of the organic phase was pipetted into the cell. After the
.
measurement, the organic phase was re-equilibrated with the aqueous phase to which a measured amount of water had been added. The measurement was then repeated on the organic phase equilibrated with the aqueous phase of lower concentration. These measurements were for orientation purposes only, that is, to give relative magnitudes, and were not intended as precision measurements.
Results and Discussion Since the experimental data are most easily presented in plots of log D us. log [HCl], it is convenient to consider the logarithmic derivative of equation 3 for D = P/Q. Proceeding again as in I1
[HCl] blog D = -(Cl) dP (Cl) d& -_-_ _ _ - (Cl) bP ---+--( Z ) d l o -g ( Z ) P d(C1) P d(Z) Q d(C1)
(Cl) d& Q d(Z)
when the ion InC14- itself is important in the or- where the total logarithmic derivative with respect ganic phase, the ion triplets and larger associations to the aqueous HC1 concentration is made up of are unimportant, and vice versa. Thus, when the partial derivatives with respect to the aqueous confirst term in the numerator of D is important, the centrations of C1- and Z-, subject to the restricmajor ionic species possible in the organic phase tions [HCI] = (Cl) and (Cl) (Z) = (H) = conbesides InC14- are only H f , C1- and Z-, and (H)o stant. depends mainly on the concentrations of these ions. (H)/(H)o then reduces to ~ c Y H c I ( H ) / [ c Y H c ~ ( C ~(9)) The necessary equations are: ~ H C I ( H ) ( C=~ ) (H)o(Cl)o, = (H)o(InClr)o, ~ H Z ( H ) ( Z = ) (H)o(Z)O, SHInCIr(H)o ~ H Z ( Z )f a H I n C l r (InC14)I. The term in a H I n C l r aaIncir(H)(InClr) ( I n c h ) @= (HInCldo, SaclHrnclr(H)o(Cl)o(H)a(Incl~)o= (HClHInClr)o, (InCI4) in the denominator may also be omitted in sHzHrnci4(H)o(Z)e(H)o(InClr)o = (HZHInClrJo. this study because the InCI4- is present in only ( 1 0 ) Actually, even with the indium concentration -10-5 M , as in tracer amounts, lo and then this work, aHr.cir(InCl)r may not be negligible compared t o aHci(C1)
+
+
i=O ( 8 ) I n the present study, the total indium concentration was low,
M , EO there was little possibility of ion associations containing more t h a n one InCl4- ion; such species, i . e . , H+(InClr-)z, are therefore not included.
with certain solvents, cf. ref. 4. But sinoe the indium concentration is approximately constant, no qualitative error is caused by omitting the term aHInC14(InCl~)when considering the variation of D with the type of changes of conditions of this study.
wherefo is the fraction of the total indium in the organic phase present in the form of the species indicated by the subscript, and similarly, f is that fraction in the aqueous phase. The fa's and f ' s are themselves functions of (H), (In), (Cl), (Z), etc. Equations 3 and 4 explicitly give three ways in which D may change with [HCl], the hydrogen ion concentration being held constant with HZ. One way is due to any difference in the chloride content of the average metal-containing species in the aqueous and in the organic phases. Whichever species has the larger chloride content naturally will be favored with increasing (Cl). Since only InC1,- or ion associations containing InC14-, such as H+InC14-, (H+)21nC14-,H+InC14-Z-, etc., exist to any appreciable extent in the organic phase, but lower chloro-complexes can and do exist in the aqueous phase, the smaller the chloride concentration becomes, the lower the value of D, considering only this effect. For example, if only H+InC14-can extract, D will be zero in pure HZ, but with increasing [HCl], increasing amounts of Inc14- will be formed and will extract as H+InCI4-, and D will increase. A plot of log D vs. log [HCl] would start with a slope of +4,11 and as the average indium-containing species in the aqueous phase changes from In+++ to InCl++ to InClz+to Inc13 to InC14-, the slope of the log plot would decrease, becoming zero when all the indium in the aqueous phase is in the same form, InC14-, as that in the organic phase. l 2 When the nature of the solvent is such that the extracting species H+InC14- can dissociate appreciably into (solvated) H + and InC14-, the "conimon ion" effect described in I1 can also occur. For, although the hydrogen ion concentration in the aqueous phase is kept approximately constant, the hydrogen ion concentration in the organic phase may not be constant as the aqueous solution changes from a certain concentration of HCl to the same concentration of HC104 or HN03. The latter two acids are more soluble in the organic phase than HC1, and HCIO, is more highly dissociated, so that they produce more H + in the organic phase than does HCI. This is indicated by the data given in Table I on the conductivities of organic phases equilibrated with aqueous HC1, HNOI and HClO4 solutions of the concentrations of interest. With all the solvents tried, and at 10, 5 and 2.5 M initial aqueous acid, perchloric and nitric acid containing solvents conduct better than those containing hydrochloric acid, and usually by orders of magnitude, that is HZ >> CYHC1. The higher concentration of
H + present in the organic phases in contact with HC104 or "03 represses the extraction of the Inch-, as can be seen very simply by considering the equilibrium expression aHInClr(H+)(InC14-) = (H+)0(1nCl~-)~. The left-hand (aqueous) side is constant under the condition of constant aqueous ionic strength (assuming a ~ l ~ ~ ~ ( Iremains n C 1 ~constant, ) see later) and so an increase in (H+)o,as occurs with increasing aqueous HClO4 or H N 0 3 concentration, requires a corresponding decrease in (InC14-)o. This effect appears explicitly in the term for InC14- in eqs. 2, 3 and 4. Thus, in ionizing solvents where InC14- is a principal species in the organic phase, and where HZ is more soluble and highly dissociated than HCl, a major term in eq. 4 is +1/2 foInCl,-aHZ(C1)/[(UHC1(C1) L Y H Z ( ~ ) ] . This term contributes zero in pure HZ (when (Cl) = 0), but reaches a maximum of ' / z~ O I ~ C ~ , - ~ H Z / ~ H Cwhen ~ [HCI] has increased to its maximum value, Le., to the total ionic strength. I n solvents of moderate , "comdielectric constant, where E HZ >> ~ H C I this mon ion" effect should be observable super-imposed on that of the previous paragraph, and should yield a plot of log D which decreases markedly with the initial addition of HZ. Thus, according to these two effects, D should decrease with decreasing HC1 a t a constant total aqueous ionic strength. If H+InC14- is the principal indium-containing species in the organic phase and InC14- that in the aqueous phase, the decrease in D will start with a gentle slope, while if InC14-is the principal organic phase species, the slope will fall with the steep initial value of '/z aHz/CrHcl characteristic of the repression of the InC14- extraction by the H f from the HClO, or HNOI in the organic phase. I n solvents of low dielectric constant, such as the aliphatic ethers, the common ion effect is negligible due to the association of InC1,- to H+InC14-. Furthermore, if the concentration of ionic material in the organic phase is increased, still larger ion associations may form, such as ion triplets, quadruplets, etc. Such ion clusters correspond to new types of metal-containing species, or in a sense to polymer formation, in the organic phase, and so might lead to an increase in D. Actually, just such a situation occurs in the extraction of iron(II1) and gallium(111) chlorides into diethyl and diisopropyl ethers when the organic phase metal ion concentration is increased above about M . Ion clusters containing more than one MC14- ion then become important in the organic phase and D becomes a function of the metal concentration, increasing with in(11) Actually, t h e slope probably would start with a value less than creasing metal c o n ~ e n t r a t i o i i . ~Similarly, ~~~ the 4-4, as. at t h e very s m d l values of D involved, t h e possible extraction of formation of such metal-containing ion clusters can a small proportion of InCla and of cationic indium species m a y not be occur even when the metal ion concentration is held negligible. (12) If higher chloro complexes can occur in t h e aqueous pliase (no constant at a low value if the concentration in the evidence for sucti exists with Fe(III), and n o t definitively for I n f I I I ) ) , organic phase of some other ionic compound, such h u t do not extract, (they most likely would n o t extract, for reasons as H+C1O4- or H+HN03-, is increased. Then the to be discussed in a later paper) the value of the slope would become ion associations may contain mixed anions, ie., negative, that is, t h e plot of log D VS. log HC1 would show a maximum.
+
+
L
Nov., 1957
vARI.4TIOK O F
1525
DISTRIBUTION QUOTIENT
TABLE I SPECIFIC CONDUCTIVITIES OF SOLVENT PHASES (OHMS-' Solvent ( 2 5
Id.)
p,p'-Dichlorodiethyl ether 10 ml. additional H20 in aqueous phase +20 ml. additional HtO Methyl amyl ketone +10 ml. H 2 0 +20 ml. H20 Diisobutyl ketone + l o ml. H 2 0 +10 ml. HtO +10 ml. H20 2-Ethylhexanol +10 ml. H 2 0 +20 ml. H 2 0 Dibutyl phthalate +10 ml. H20 Methyl benzoate +IO ml. HtO +20 ml. H 2 0 Methyl salicylate $10 ml. HnO Dibutyl ether +10 ml. H 2 0 Diethyl ether $10 ml. H 2 0 Decanol +10 ml. H20 +20 ml. HtO
+
Methyl isobutyl ketone +10 ml. H20 +20 ml. H20
10
144
HCI
3.0 0.30 > LYHCI,arid eq. 3 and 4 reduce to
4
+ 4 - i = O ifrncli
(4a)
We should then expect to observe a marked decrease in D with the initial substitution of HCI04 or H N 0 3 for HCl, i.e., the occurrence of the common ion effect, factor 2 above. Methyl isobutyl ketone, Fig. 1, shows just this effect. The repression of the extraction of InC14- by the larger hydrogen ion concentration produced in the organic phase by HC104 or HNO, leads to a 100-fold decrease in D in going from 2.0 M HCl to 1.5 M HC1-0.5 M HC104 or from 6.0 M HCI to 4.6 M HCI-1.4 M HC104.15 Furthermore, the value of log D for the (15) If the indium were carrier-free, that is, less than 10-6 M , this decrease in D (and those described below) would be still larger. See ref. 10.
I
I
* log
I
[HCI],
I
Fig. 4.-Log D us. log [HCl] with dibutyl phthalate, hydrogen ion concentration held constant with "01, , or HC104, - - - - - -, a t 6 hf and 10 M ( D X 10-l).
HCl-HC104 solutions lies below that for the HC1HE08 mixtures as would be expected from the effect of the greater hydrogen ion concentration in the organic phase with the former mixtures. (That is, LYHCIO, > F H N O ~ . See Table I.) However, at low concentrations of HC1, the HC1-HN03 curves continue to decrease with decreasing [HCl] while the HC1-HCI04 curves begin to rise again, crossing the HC1-HNO, curves. The author would like to suggest that this difference in behavior is due to the fourth factor described above, in particular to the effect of the lower water activity in solutions of HC1-HC104 over that in HCl-HIV03 mixtures of similar HC1 and total acid concentration. For the lower water activity in HCl-HClO, solutions means less hydration of the H 3 0 + and InC14- ions, especially the former, and forces more of them into the methyl isobutyl ketone to solvate there, thus increasing the value of D. It would be expected that any increase in D due to a decrease in mater activity would be more prominent at a higher ionic strength, as the change in water activity in going from, say, 10 M HC1 to 10 M HClOd is much larger than that with 2 M solutions. This expectation apparently is true if one considers the data taken with diisobutyl ketone, Fig. 2. With decreasing [HCl] the 2 M HCl-HC104 curve starts increasing after n~65-folddecrease in D, the 6 M curve starts increa,sing after a 12-fold decrease in D, and the 10 M curve after only a 4-fold decrease in D. That a similar behavior is not observed with the 2 and 6 M H.C1-HC104 methyl isobutyl ketone
R. M. DIAMOKD
1528 I
0---0--
I \
\
x lo-' ' r\
A -
I
I
\
Vol. 61
i
log [HCI].
-i P I
Fig. 5.-Log D us. log [HCI] with methyl b,enzoate, 0-0, and with methyl salicylate, A-A, hydrogen ion concentration held constant with "03, , or HCIOI, ------ , a t 2 Af, 6 A f (D X 10-l) and 10 AP (D X loF2). ~
plots of log D us. log [HCI] is most likely due to the much greater mutual solubility of the two phases in this system a t 6 dl acid. The study of P,p'-dichlorodiethyl ether, which has a still higher dielectric constant, 21.2 at 2Oo,l4and a lower mutual solubility with the aqueous acids than methyl isobutyl ketone, supports the arguments of the preceding paragraph (Fig. 3). Again the initial substitution of HC10, or HNOI for HCl causes a marked decrease in D, and again the greater effect occurs with HCl-HC104 mixtures. Again the decrease in D with increasing [HN03] is monotonic, but with increasing [HC104] a minimum value of the distribution quotient occurs and then the value of D increases. As suggested, this increase due to the lowered water activity is more marked the higher the ionic strength. With 2 M HC1-HC104 solutions the minimum in D occiirs a t about 50% HClO, and after a decrease in D of 80-fold, with 6 dd HC1-HC104 solution the minimum occurs at about 15% HClO4 and after a 33-fold d(?crease in D, and with 10 M HC1-HCIO, solutions t;he minimum oc-
A
,a
I
I
log [HCI],
I
Fig. 6.-Log D us. log [HCI] with diethyl ether, 0-0, and with dibutyl ether, A-A, hydrogen ion concentration held constant with "03, ___ or HC1O4, ------, at 2 ill, 6 M ( D X l O - I ) , and 10 AT (D X 10-I). Log D us. log [HCl] with dibutyl ether, hydrogen ion concentration held constant with "Os, M-m, or HCIO,, E---., a t 12.6 m ( = 10 Al HC1).
curs a t about 8% HC1O4 and after -9-fold decrease in D. Similar behavior might also be expected with the extraction of indium into the esters dibutyl phthalate, methyl benzoate and methyl salicylate. These solvents have dielectric constants of 9.41 (30"), and so again, 6.59 (20"),6.44 (30"), re~pectively,'~ with the initial substitution of HClO4 or "01 for HCI, the common ion effect should repress the extraction of InC14-, and the "dehydrating" effect of increasing [HC104] should lead to an increase in D for the HCL-HC104 mixtures as the water activity decreases. Figures 4 and 5 show the results, and verify this expectation. In all cases replacement of HC1 by HC104 or HNOI results in an initial marked decrease in the value of D. With HNOa the decrease is monotonic with decreasing [HCl]. But with HC104 a minimum D is reached after which the value of D increases, passes through a maximum,
.
Nov., 1957
VARIATION OF DISTRIBUTION QUOTIENT
and then finally decreases again as the proportion of indium in the form InC14- in the aqueous phase decreases to zero. Again, the magnitude of the h i tial decrease in D to the minimum decreases as the ionic strength increases, that is, as the water activity decreases. With dibutyl phthalate and 6 and 10 Ill acids, the minimum occurs after a 4- and 2-fold decrease, respectively, in D, with methyl benzoate and 2, 6 and 10 AI acids after a 60-, 7.5- and 1.5-fold decrease in D , and with methyl salicylate and 6 and 10 M acids after a 10- and 5.6-fold decrease, respectively, in D. It might also be noted that the magnitude of D with methyl benzoate is from 10 to 100 times larger than that with methyl salicylate. This is an illustration of the fact that the solvent molecule must contain an oxygen (or nitrogen or sulfur) capable of coordinating with the extracting metal-containing species. The internally hydrogen-bonded methyl salicylate cannot so coordinate as well as the otherwise somewhat similar methyl benzoate, resulting in a lower extraction efficiency even though it has the higher dielectric constant. When extraction into butyl ethyl and dibutyl ethers is considered, it must be remembered that these solvents have low dielectric constants, 3.06 a t 25" for the latter,14 so that ion pairs and higher ion associations will predominate in the organic phase. I n contrast with the previous cases already described, factor 2, the common ion effect, will now be relatively unimportant, but factor 3, the association of InC14- into ion clusters mill be important. As already mentioned, such association will occur more readily with H+C1O4- than with H+Cl-,l6 so that there mill be an increase in That D with an initial increase in HC104 or "Os. is, eq. 3 and 4 become essentially
4
leading to an increase in log D with the substitution of HC1 by HZ. Again," however, the fourth factor, the greater dehydrating action of HC104 compared to comparable concentrations of "Os, will differentiate between the two acids, forcing the indium into the ether phase more with HC104 than with " 0 3 , and ,so leading to a larger D in the former case. (lG) However, such association with H + C I - may well be the explanation f o r the extra HCI found by ti. ICato and R. Ishii, Sci. P a p e r s Insl. Phus. Chem. Research (Tokyo), 36, 82 (19391, and Nekrosoy and Ousyankina. J . Gen. Chem. (U.S.S.R.), 11, No. 8, 573 ( 1 9 4 I J , see 6. A. 8 8 , 72GG (1941), associated with HFeClr extracted into diethyl ether f r o m high concentrations of aqueous HCI, and by N . H. Nachtrieb and J. G . Conway. J . A m . Chem. Soc., 70,3547 (19481, associated with the HFeC14 extracted into diisopropyl ether from high conoentrations of aqueoua HCI. For even a t these high HCI concentrations there is no such co-extraction of excess HC1 with HFeClr into p8'-dicblorodiethyl ether, J. dxelrod and E. H. Swift, ibid., 62, 33 (1940). The relatively high dielectric constant of this solvent precludes any appreciahle amount of t h e nccessary multiple ion association such as H+CI-H+FeClr-, eto.
I
I
'log
1529
I
[HCI].
I
I
Fig, 'i.-Log D us. log [HCI] with butyl ethyl ether, hydrogen ion concentration held constant with "03, , or HCI04, ------,at 0 A i and 10 N (D X 10-l). two ether phnRes with HCI, . . . , . indicates region of HCIOl solutions.
These expectations are verified in the results of the dibutyl ether extractions shown in Fig. 6. As the hydrochloric acid i n the 10 M HC1 solution is initially replaced with increasing amounts of HC104 or HN03, the distribution quotient, D, increases. With "03 a maximum is reached at about 8 AI HCI-2 M HX03,and then D decreases. With HC104, the rise in the value of log D is much more prolonged; D reaches a value of 4.5 at 9.5 Ill HC104-0.5 A l HCI, and the maximum value is still larger and a t a still lower HCl concentration. But eventually the value of D begins to decrease when factor 1, the effect, of the increasing proportion of non-extractable indium species in the aqueous phase (InC13, InCla+, InCl++, In+++),overcomes the combined effect of the ionic associations in the organic phase, factor 3, and the dehydrating effect of the HC104in the aqueous phase, factor 4. The experiments in this study were done keeping the initial ionic strength constant at 2, 6 or 10 M , and this condition exaggerates the dehydrating effect of HC104, as 10 JI HC1O4 is 17 m, 10 d4 "03 is about 15 m, and 10 M HC1 is oiily 12.6 m. However, keeping the ionic strength constant at 10 m does not change the nature of the results significaiitly, as is also shown in Fig. 6.
1530
R. M. DIAMOND I
I
1
t\
-I \
\
\ '\
\
a 0
-0
-2
-
c --0-&.-0
O M
I
I
log [HCI]. Fig. 8.-Log D us. log [HCI] with 2-ethylhexanol, hydrogen ion concentration held constant with HYO,, , 01 HClO,, ------, at 2 M , 6 M and 10 A4.
d
-
9,
0
I
log [HCI].
I
Fig. 9.-Log D us. log [HCI] with decanol, hydrogen ion concentration held constant with "01, , or HC104, - - _ - - -, a t 2 M , 6 A I and 10 M .
As expected, variations in D similar to those found with dibutyl ether occur with extractions into butyl ethyl ether from 10 and 6 i ll acid solutions, but with an added complication, Fig. 7. As with dibutyl ether, when HN03 is substituted for HC1, the value of D initially increases, attains a maximum and then decreases, and when HClO4 replaces HCl, there occurs a still larger initial increase in the
VOl. 63
value of D. But with butyl ethyl ether at 9 M HCI-1 M HC104 and a t 2.4 M HC1-3.6 M HC104, for the 10 and 6 M solutions, respectively, the organic phase separates into two phases. As still more HC104 is added, the original upper (light) ether phase disappears leaving only the new, heavier one. This latter phase contains much more indium than the former one, resulting in a large discontinuous increase in the value of D. This phenomenon of the ether phase splitting into two phases, one relatively concentrated in the metal-containing acids, has previously been observed in the iron(II1) chloride-HC1-H20-diisopropyl ether s y ~ t e m , ' ~ , ~ , ' ~ and has not yet been satisfactorily explained. Figure 6 shows the results for the extraction into diethyl ether from 6 and 2 h1 acid solutions. The 6 Ad curves are similar to those of butyl ethyl ether, except that with the smaller, more soluble diethyl ether molecules miscibility occurs with the HC1-HClO, mixtures, rather than a splitting in two of the organic phase. At the 2 M ionic strength apparently too little acid extracts to allow the formation of appreciable amounts of ion triplets and larger clusters. In fact the organic phase must be dilute enough for some ion pair dissociation18; the decrease in D with the initial replacement of HC1 by HC104 or H N 0 3 indicates that a significant proportion of the indium in the ether phase exists as InC14- and is displaced from the ether by the common ion effect. But the smallness of the drop in the value of D with HC1O4shows that the proportion i8 much smaller than in the case of the solvents with higher dielectric constants such as methyl isobutyl and diisobutyl ketone and &p'-dichlorodiethyl ether. Still a different type of behavior might be expected with 2-ethylhexanol and decanol. The dielectric constants of these alcohols are high enough, 8.1 at 20" for the decanol,14so that ion clusters and their effects should be negligible, as with the ketones. But also the common ion effect might be unimportant due to the increase in solubility and dissociation of HC1 relative to HCIOl and HxO3 with the alcohols as compared to the other solvents. That is, although the mineral acids are generally more soluble in alcohols than in other oxygenated solvents of comparable molecular weight, the alcohols are poor differentiating solvents with respect to acid strengths. In particular, HC1, though still weaker, comes closer to HC10, and HNO, in solubility and in acid strength, i.e., CYHCl 5 CYHZ,with alcohols. Then only the stepwise formation of the indium complexes in the aqueous phase and the effect of the changing water activity should have much influence on D. Equations 3 and 4 become approximately
F (Inch) + L Y ~ ~ ~ ~ c I ~ ~ ~ ~ I ~ c I ~ ( H ) ( I LYHcl LYHInClp
D
(3c)
4
(InCI,) i=O (17) (a) R. W.Dodson, G. J. Forney and E. H. Swift, J. B m . Chem. A. H. Laurence, Ph.D. Dissertation, Rensselaer Polytecbnic Institute, 1952. (18) D. E. Clialkley and R. ,J. P. Williams, J . Chem. Soc., 1920
Soc., 68, 2573 (1936); (b)
(1955).
Nov., 1957
TRANSPORT PHENOMEN-4 I N THE
CRYOLITE-ALUMINA SYSTEM
1531
concentrations while holding the metal and hydrogen ion concentrations constant (this paper), and varying the hydrogen ion concentration while holdThe experimental rcsults for 2-ethylhexanol and ing the metal and halide concentrations constant decanol are shown in Figs. 8 and 9. The plots of log (next paper), The sources of variations in D inD us. log [HCl] for the 2 M acids are reasonably cluded explicitly in the derived expressions are flat as expected, with the small differentiation be- changes in D due to differences in the composition tween HC1-HC104 and HC1-HNO, mixtures due of the average metal-containing species in the two t o the small difference in water activities. With G phases and changes due to common ion effects on and 10 M acids, this differentiation would be ionic species (usually in the organic phase). Among expected to increase, and it does, but an initial the first type are the stepwise formation of comdecrease in D characteristic of a small common plex ions of the metal in the aqueous phase, polyion effect also occurs. Apparently a t these higher mer formation in the aqueous phase and ion assoionic strengths the lowered water activity increases ciation in the organic phase. Among the second CYHZ more rapidly than CYHCl, so that instead of type are the repression of the extraction of the CYHCl < OCHZ, the condition ffHC1
