T. J. CONOCCHIOLI, M. I. TOCHER, AND R. M. DIAMOND
1106
The Extraction of Acids by Basic Organic Solvents. V.
Trioctyl
Phosphine Oxide-HC10, and Trioctyl Phosphine Oxide-HReO,’
by T. J. Conocchioli, M. I. Tocher, and R. M. Diamond Lawrence Radiation Laboratory, University of California, Berkeley, California
(Received J u l y 15, 1964)
The extraction of HC104 and HRe04 into dilute solutions of trioctyl phosphine oxide (TOPO) in CC14has been studied, and the nature of the extracting complex has been determined. It has been found that for TOPO concentrations 6, the only extracting species are the water complex, TOPO.Hz0, and the trisolvated hydronium ion, H30+.3TOPO, which is ion-paired with the C104-. For higher concentrations of acid in the organic phase, the predominant species appears to become the 1 :1 complex, TOPOH+. . . c104-. These results are compared with those for tributyl phosphate extraction and the model proposed earlier.
Solvent extraction systems composed of inorganic acid, water, and an organic base, either pure or diluted with an ‘(inert’’organic liquid, can be interpreted on the basis of a model which views the extraction as a competition among water, the extractant, and the anion for solvating the proton. The nature of this process is further dependent upon the dielectric constant and the possibly acidic or basic character of the diluent, if one is present. Two previous papers have described the extraction of aqueous HC104, HRe04, and HBr by solutions of tributyl phosphate in CC14.28a I n these systems the erttracted acid species was H 3 0 +-3TBP.yH2O. . . X-, where 0 _< y 3. That is, the TBP is not basic enough to take the proton alone out of the water phase, but extracts a partially hydrated hydronium ion; water is a stronger base than TBP. The extractant used in the present study was tri-noctyl phosphine oxide (TOPO), a stronger base than TBP. Thus it might be expected to compete better with water for the proton, and so permit less water in the extracted complex. The choice of TOPO was also influenced by the fact that it has only one basic site-the phosphoryl oxygen-as opposed to TBP which has three additional ester oxygens. A number of studies have been conducted on the extraction of mineral and complex acids by TOP0.4-9 A few have suggested the nature of the extracting adduct but most have neglected the presence of water in The Journal of Physical Chemistry
any such complex. This study will show that an investigation of the TOPO-H20 system and of the water content in the presence of acid extraction must be included to explain the extraction mechanism of strong acids satisfactorily. The acid chosen was HC104 because Clod- is a very weak base, and so does not enter the competition. Perchloric acid is also a good model for the strong complex metal acids such as HFeC14, HAuC14, HRe04, etc. In addition, Reo4- was used in tracer form because it also is a very weak base like clod- and has a convenient isotope, Re1%(til2 = 90 hr.), which made possible the extension of the studies to more dilute solutions. The (1) This work was supported by the U. S. Atomic Energy Commission, A.E.C. No. W7405eng-48.
(2) D. C. Whitney and R. M. Diamond, J . Phys. Chem., 67, 209 (1963). (3) D. C. Whitney and R. M. Diamond, ibid., 67, 2583 (1963). (4) J. C. White and W. J. Ross, “Separations by Solvent Extraction with TOPO,” National Academy of Sciences Report NASNS3102, 1961. ( 5 ) R. A. Zingaro and J. C. White, J . Inorg. Nucl. Chem., 12, 315 (1960).
(6) B. Martin, D. W. Ockenden, and J. K. Foreman, ibid., 21, 96 (1961). (7) W. J. Ross and J. C. White, “The Solvent Extraction of Iron with TOPO,” Oak Ridge National Laboratory Report ORNL-2382, 1957. (8) G. E. Boyd and Q. V. Larson, J . Phy8. Chem., 64, 988 (1960). (9) M. I. Tocher, D. C. Whitney, and R. M. Diamond, ibid., 68, 368 (1964).
THEEXTRACTION OF ACIDSBY BASICORGANIC SOLVENTS
solvent used was CCL, which has a low dielectric constant ( e = 2.2) and so cannot easily support a charge separation of even moderately sized ions. Thus the attraction between the proton and the anion in such a medium is increased over the value in water, and the extracting complex might be expected to be an ion pair. The method of studying the complex was that suggested by Hesford, et a1.,10-12in which the activity of one component of the system is varied while all others are kept constant, thereby showing the dependence of the extracting complex on that particular component. Because concentrations (as opposed to activities) are what are usually measured, one must use caution in drawing conclusions about the nature of the complex from such studies. This problem may be minimized by limiting the concentration of extractant to only a few tenths molar and choosing experimental conditions such that only a few per cent of the extractant are involved in the complex. Thus, the organic phase retains the properties of the diluent, and changes in concentration of the extractant, acid, and water in that phase will have only a slight effect on their activity coefficients. After the nature of the complex in dilute solution is established, studies may be extended to more concentrated solutions to see if, in fact, more complex interactions are occurring in the organic phase.
Experimental The TOPO used was Eastman White Label product. Purification by equilibration with mild base, washing with distilled water, and recrystallization gave a product with a behavior identical with that of the original product; therefore, the material was used as obtained. A stock solution of 0.1 M TOPO in CCl, (Baker and Adamson reagent grade) was prepared by weighing the desired amount of TOPO and making up to volume in a volumetric flask. All dilutions of TOPO were made in volumetric glassware from the stock solution. Solutions which had to be dried were stored over molecular sieves (Linde Co., 0.16-cm. diameter pellets), but could not be left for long periods due to the uptake of TOPO itself. At time of use, these solutions were filtered under mild vacuum in order to remove suspended particles of molecular sieves. The HClO, solutions were prepared by diluting G. F. Smith reagent grade HC10, (70-72%) with distilled water. Determinations of aqueous acid concentrations were made by standard analytical methods, i.e., titration with standard base to the red end point of phenol red (pH 7). Matheson Coleman and Bell stabilized and premixed single-solution Karl Fischer reagent was used in the water determinations. Xethanol used in the Karl Fischer analysis blank was Baker and Adam-
1107
son Electronic grade (50.1% H20). The pyridine used was Baker and Adamson reagent grade (O.lOj, HzO). The radioactive Re18604-tracer was prepared by irradiating KRe0, with neutrons in the Livermore pool-type reactor. The product was dissolved in distilled water to give an approximately low3M KRe0, solution which was further diluted before use. Procedure. Equal volumes of aqueous acid and of TOPO solutions in CC1, were equilibrated in glassstoppered bottles. Samples were normally shaken from 1 to 3 hr. with a wrist-action shaker. Some samples were shaken as much as 6 hr. with no detectable difference in extraction; previous work in this laboratory had indicated that equilibrium is attained in similar systems in less than 30 min. After equilibration, samples were transferred to centrifuge cones and centrifuged for 1 to 3 min. ; the organic phase was withdrawn with a pipet. The acid content of the organic phase was determined by a direct two-phase titration using 0.0100 or 0.1000 M NaOH, with stirring of the phases continued throughout the titration. Phenol red (pH 7 ) was used as the indicator, and determinations were made to its red end point. An indicator blank was run using water-saturated TOPO solutions; the amount of NaOH used for the blank was one-half the total titration for the smallest amount of organic-phase acid measured (1 X lop4M ) and vanishingly small for higher concentrations. The water content of the organic phase was determined by the Karl Fischer method, using a direct visual end point and a pyridine-methanol blank.13 The Karl Fischer reagent was standardized by titration of samples of methanol with known water content. A Beckman IR-5 double-beam recording spectrophotometer was also used to determine the water bound to the TOPO molecules by means of the peak heights of the symmetric and antisymmetric stretching modes of water. Dry CCl, or a dry TOPO solution in CCl, of the same concentration as the sample was used as a reference. Sample solutions and reference were contained in matched 2.0- or 0.2-mm. cells with CaFz windows or 0.5-mni. cells with AgCl windows. In the tracer studies, between 10 and 40 ~ 1of. KRe04 tracer was added to the mixture of aqueous acid and (10) E. Hesford, H. A. C. McKay, and D. Scargill, J . Inorg. Nucl. Chem.. 4 , 321 (1957). (11) E. Hesford and H. A . C. McKay, Trans. Faraday Soc., 54, 573 (1958). (12) E. Hesford, H. A. C. McKay, and E. E. Jackson, J . Inorg. Nucl. Chem., 9, 229 (1959).
(13) J. Mitchell, Jr., and D. M.Smith, “Aquametry.” Interscience Publishers, Inc., New York, N. Y., 1948, p. 235.
Volume 69, Number 4
April 1966
1108
-
T. J. CONOCCHIOLI, M. I. TOCHER, AND R. h/I. DIAMOND
organic extractant before shaking. After shaking, centrifugation, and separation, y counting was done on 2-ml. aliquots of each phase with a well-type n’a(T1)I scintillation crystal and a single-channel analyzer. The total number of counts/minute in both phases (corrected for background) was equated to the quantity of macro HClO, originally taken. The fraction of the total counts found in each phase times the original quantity of HC1O4 taken gave the amount of acid in that phase, under the assumption that HC104 and HRe0, behave similarly (but see later). All experimental work was done a t room temperature, 23 f 2O, with no detectable changes in extraction over this range.
Results Investigation was first made of the TOPO-HzO system. Solutions of 0.002 to 0.5 M TOPO in cc14 were equilibrated with water, and the water content in the organic phase was determined by the Karl Fischer method. These values must then be corrected for the amount of water dissolved in CC14 alone, namely, the solubility of HzOin CCl, (0.010 M ) times the volume fraction of CC1, in the solution. However, the lowest concentration of TOPO-bound water was best determined from infrared analyses which can distinguish the two kinds of water. The fundamental vibrationa stretching modes of unbound water molecules, the antisymmetric and symmetric stretches, are a t 2.70 and 2.76 p, respectively, with the shorter wave length peak showing the greater absorbance. The positions and relative magnitudes of these peaks are nearly the same for gaseous HzO and for HzO dissolved in CC14,indicating that there is little hydrogen bonding present in the latter c a ~ e . 1 But, ~ when water is hydrogen bonded through its OH groups, the ratio of peak heights changes, and the absorption moves to longer wave lenths. For the water extracted into dilute TOPO solutions ( 5 0 . 1 M ) the two stretching modes appear a t 2.71 and 2.97 p , respectively, and the 2.97 p peak shows the greater absorbance. These changes indicate the hydrogen bonding of water to the TOPO and a corresponding weakening of the 0-H bond involved. Spectra were taken of water-saturated TOPO solutions ranging in concentration from 0.001 to 0.5 M TOPO. Calibration curves relating the absorption peak height of the 2.71 and 2.97 p peaks to the water content were normalized to the Karl Fischer titration values in the region around 0.06 M TOPO; here, the water concentrations were sufficiently high so that the titrations were subject to relatively small experimental error. A plot of log organic-phase water us. log total TOPO, determined by either peak, gave results in The Journal of Physical Chemistry
I
lo-’
I , !
11
io-’
10-2
M
,
1
1
, I 1
100
ToPototal
Figure 1. Variation of water content of organic phase (CC1, diluent) with TOPO concentration, corrected for solubility of HzO in CCl,: 0 Karl Fischer; 0 2.75 p peak; and A 3.0 p peak.
good agreement with the corrected Karl Fischer values for solutions as high in concentration as 0.1 M TOPO, as shown in Figure 1. At this point the curves of the two peaks diverged; that of the 2.97 p peak rose faster than that of the 2.71 p peak and approximated the Karl Fischer values. The next series of experiments had to do with the HClO, extraction itself. Concentrations of acid from 0.01 to 11 M HC104 were equilibrated with dilute solutions of TOPO in CCl, ranging from 0.01 to 0.1 M , and the organic-phase acid content was determined at each external (aqueous) acid concentration. Figure 2 shows a plot of log organic-phase acid concentrations us. log equilibrium aqueous acid activities. It should be noted that the experimental organic-phase acid concentrations must be corrected for the amount of acid extracted by CC1, without TOPO present; however, the extraction of HC104 into CC14was found to be negligible over the range of HClOd concentrations used. The Reo4- tracer extractions from macroconcentrations of HC104were used to extend the range of aqueous acid concentrations investigated toward more dilute solutions, namely down to 0.001 M HClO,. A log-log plot of the organic phase acid concentration (as calculated from the Reo4- tracer distribution) us. the equilibrium aqueous HClO4 activity is given in Figure 3. (14) G . C. Pimentel and A. L. McClennan, “The Hydrogen Bond,” W. H. Freeman and Co., San Francisco, Calif., 1960.
THEEXTRACTION OF ACIDSBY BASICORGANIC SOLVENTS
IO"
10c
+,o I
El 10-3
H
I
-I
1109
amounts of H + and H 2 0 in the organic phase were measured, the latter by the Karl Fischer method. The water values, however, include the water dissolved in CC14 alone, which quantity may be calculated as the product of the solubility of pure water in CCI,, the aqueous water activity, and the volume fraction of CCl,. The amount of water extracted minus this CC14water is shown in Figure 4 plotted us. the organic phase acid concentration. Infrared spectra were also recorded for 0.1 M TOPO solutions equilibrated with various concentrations of aqueous acid. Because a dry TOPO solution was used as a reference, the water absorption peaks (after correction for H20in CCl, alone) gave an experimental determination of the amount of water bonded to TOPO, but not in the acid complex.
j d :
0.04 I
"HCIO,
Figure 2. Variation of organic-phase acid content (CCl, diluent) with aqueous HClO, activity for total TOPO concentrations of C1 0.1 M ; A 0.05 M ; and 0 0.01 M .
2 0.02
I
I I I
10-1E
I
0
I
I
I
I
I
1
I
I
I
I
I
Discussion TOPO-H20. It is assumed that the TOPO-water system is maintained independently of any TOPO-acid system so that TOPO molecules bonded to water are not readily available for acid extraction. Therefore, the initial investigation was of the extraction of pure water by TOPO (eq. 1, where (0) indicates the organic nTOPO(o, 10-6
*-;I
J
l 10-2
I
I
100 IO*
I
10.
I
IO*
1
108
OHCIO,
Figure 3. Variation of organic-phase acid content (measured by HReO, tracer, CCl, diluent) with aqueous HClO, activity for total TOPO concentrations of 0 0.1 M and 0 0.01 M .
Determinations were also made of the amount of H 2 0 extracting with HC104. Solutions of 0.05 and 0.1 M TOPO were equilibrated with aqueous solutions of acid ranging from 0.01 to 11 M in HCIO4and the
+ HzO
=
HzO.nTOPO(o1
(1)
phase). The corresponding equilibrium constant is given in eq. 2 where parentheses signify activities;
KH%O =
(HzO*nTOPO) - [H~~.~TOPO]Y~I,~.~TOPO (TOPO)"(HzO) [TOP0]"YTOPO~(H~O) (2)
brackets, concentrations; y, activity roefficients; and where [TOPO] always means the equilibrium TOPO concentration. Initial or total concentration will be specifically indicated. Although very little is known about activity coefficients in organic solutions, the assumption was made Volume 89,Number 4
April 1966
T. J. CONOCCHIOLI, 31. I. TOCHER, .4ND R. hf. DIAMOND
1110
that the ratio of activity coefficients Y H ~ O . ~ T O P O / Y T O P O " was constant ; thus, organic-phase concentrations were used instead of thermodynamic activities. This does not seem unreasonable as the solutions of TOPO were kept dilute so that the properties of the CCI, diluent were essentially retained. As will be seen later, the results of the extraction studies substantiate this assumption. Thus, eq. 2 may be rewritten K ' H ~ o=
[H~OTLTOPO] [TOPO1" (HzO)
(3)
Taking logarithms and rearranging yields log [HzO.nTOPO] = n log [TOPO]
+
+
log K'H~o log (HzO) (3%) Assuming that the water activity is approximately equal to 1, since the solubility of TOPO in water is negligibly small, a plot of log [HzO.nTOPO] US. log [TOPO] will determine n, the number of TOPO molecules per water molecule. The equilibrium TOPO concentration may be calculated as [TOPO] = [TOPOItot,l - n[HzO.nTOPO] in which [TOPO]tot,l is the initial TOPO concentration and n[HzO.nTOPO]is n times the water content in the organic phase (exclusive of water just dissolved in the CC1,). It can be seen that the log-log plot of organic-phase water us. log initial TOPO concentration is linear from 0.001 to 0.1 M TOPO with a slope of 1.0. Correction to equilibrium TOPO yields a value of n = 1.0 and K'HzO = 0.56, which means that a t TOPO concentrations below 0.1 M in CCL, 36% of the total TOPO is bonded to HzO, and the water-TOP0 complex contains only one TOPO. This is analogous to the earlier studied TBP-CCl, systemj2and the larger va,lue of K'H,o is an indication of the greater basicity of TOPO over TBP. Above 0.1 M TOPO, the curve rises more steeply than a slope of unity. This rise indicates either the introduction of a new TOPO-H20 complex or the breakdown of the assuiuptions made for dilute solutions. The linear portion of the curve, however, supports the assumption of a constant activity coefficient ratio for TOPO species in the organic phase in the region of concentrations in CCI4 below 0.1 M . ?'OPO-Hz0-HC104 ( H R e 0 4 ) . The reaction for the extraction of dilute acids by dilute solutions of TOPO may be written as eq. 4a, b, or c, depending upon whe-
HX.nTOPO.yHzO(o, (4a) The Journal of Physical Chemistry
ther the extracting species is (a) molecular, (b) ionic but associated, or (c) ionic and dissociated. The assuinption is made that the anion in eq. 4b and 4c is not hydrated or complexed by the extractant in the organic phase, and C10,- and Reo4- appear to satisfy this requirement. The equilibrium constants for eq. 4a, 4b, and 4c, respectively, are given by eq. 5a, 5b, and 5c. KHX
=
(HX .nTOPO.yHzO) (TOPO)"(HzO)'((H +X-) [HX.nTOPO.yHzO]
(54
[TOPO]"~TOP~"(HZO)'(H+X-) (H+*nTOPO.yHzO...X-) KHX= (TOPO)" (HzO)'(H +X-) [H+.nTOPO.yHzO. . . X-17 -~
[ToPo]"~ToFo"(H~O)'(H+X-) KHX=
(5b)
(H +.nTOPO.yHzO)(X-) (TOPO)"(HzO)' (H +X-)
It is again assumed that the ratio y / y ~ o p o "or y+y-/ is a constant, as the TOPO solutions are in a dilute, near-ideal region. Substituting [H+](o)mto represent the extracted acid in the organic phase, with m = 1 for cases (a) and (b) and m = 2 for case (c), leads to the sinipler general expression of eq. 6. Thus, if the YTOPO"
equilibrium TOPO concentration and water activity are kept constant, the slope of the plot of log [H+Ico) us. log (H+X-) will yield l/m. Figures 2 and 3 show that below an aqueous acid activity of unity for 0.01 M TOPO and of about for 0.1 M TOPO a log--log plot of [H+](o)us. [TOPOItotaldoes give a slope of about unity. Experimentally, however, as the amount of extracted acid increases, the amount of TOPO tied up in the complex also increases and so [TOPO] decreases. Therefore, the raw data must be corrected to a fixed equilibrium TOPO concentration, to satisfy the requirement of eq. 6. Equation 7 may be used (as will be
THEEXTRACTION OF ACIDSBY BASICORGANIC SOLVENTS
1111
justified below) where [H+]’(o)is the corrected organicphase concentration corresponding to a fixed equilibrium TOPO concentration, [TOPO]‘, taken to be the initial concentration; and the unprimed values are the experimental equilibrium concentrations, ie., [TOPO] is the actual concentration of unbonded TOPO. The latter does not include TOPO.HZ0 or TOPO bound to the proton; thus, these corrections must be evaluated. The concentration of TOPO used in the acid complex will be n[H+](o,with n = 3 as shown below, so that [TOPO]
=
[TOPO]tot,l - [TOPO.HzO] - 3[H+](o)
(8) Substitution of the value of TOPO.HzO from eq. 3 yields eq. 9. The use of eq. 7 and 9 permits thedata
plotted in Figure 2 to be corrected to a constant equilibrium TOPO concentration, and these corrected data = 1 are obare shown in Figure 5 . Slopes of 1”, tained, so m = 1 and the species is either molecular or ion-paired. This conclusion agrees with conductivity measurements made by Hesford and ,IlcI
