19. M. K. Liu, D. T. Shioshita, and R. L. McDonald
2532
The standard error for the difference between the means, gir1-ir2,
iSW
Acknowledgment. We acknowledge the support of the National Institute of Health for this work.
References and Notes (1) G. E. Kistiakowsky, J. R. Ruhoff, H. A. Smith, and W. E. Vaughan, J.
Amer. Chem. SOC.,57, 876 (1935). Compu*ingthe standard score, Z = PI - ~ 2 / d =~ 1.14, ~ - ~ ~(2) G. B. Kistiakowsky, J. R. Ruhoff, H. A. Smith, and W. E. Vaughan, J. leads us to conclude that the difference is significant only Amer. Chem. Soc., 58, 137 (1936). (3) R. E. Williams, J. Amer. Chem. Soc., 64, 1395 (1942). a t the 0.25 level using a two-tailed test. If an alternating ef(4) T. Flirtcrofl, H. A. Skinner, and M. C. Whiting, Trans. Faraday Soc., 53, fect of about 0.23 kcallmol had existed, as inferred from 784 (1957). (5) H. A. Skinner and A. Snelson. Trans. Faraday Soc., SS,404 (1959). Kistiakowsky's data, we compute 2 = 3.59 which leads us (6) D. W. Rogers and E. Bretschneider, Mikrochim. Acta, 482 (1970). to a significnnce level of about 0.0004 using the same test. (7) D. W. Rogers and F. J. McLafferty, Tetrahedron, 27, 3765 (1971). We take tine large difference between these significance lev(8) Bolab Inc., Derry, N.H. 03038. (9) E. H. Sargent, Springfield, N.J. 07081. No. S-9055. els to indicate the absence of a measurable alternating ef(IO) Time constant 2.5 sec., Cole-Parmer Inst. Co., Chicago, Ill., 60684. fect among %!.nearterminal rnonoolefins from C5 to CZ0. Model 8436. (11) Microcord 44, Photovolt Corp., New York, N.Y. 10010. The least reliable values of AH are those of C5, because (12) Chemical Samples Co., Columbus, Ohio 43220. of its volatility and consequent handling difficulties and (13) Burdick and Jackson Labs., Muskegon, Mich. 49442. C ,, because I & has the least unsaturation per double bond, (14) Matheson Coleman and Bell, The Matheson Co., East Rutherford. N.J. (15)Shandon Scientific Co., Siwickley, Pa. 15143. placing the snaxinium demand on the sensitivity of the (16) H. F. Bartolo and F. D. Rossini, J. Phys. Chem., 84, 1685 (1960). method. When these two are deleted, the mean of the re(17) D. W. Rogers, P. M. Papadimetriou,and N. A. Siddiqui, Mikrochim. Acta, in press. maining sevei? odd olefins is -29.764 while that of the re(18) N. M. Downie and R. W. Heath, "Basic Statisrical Methods," Harper and maining even olefins is -29.766. These results lead, a forBrothers, New York. N.Y., 1959, p 117. tiori> to the rejection of an alternating effect. (19) Reference 18, pp 123-127.
plex Metal Acids. WIO. An lmpr II
f. Skioskita, and R. L. McDonald*
Deparimenf of Chemistry, University of Hawaii, Honolulu, Hawaii 96822 (Received September 20, 1973; RevisedManuscript Received August 2, 197.1)
1V1ewdata are presented for the distribution of HFeC14 between 8 M HC1 and bis(2-chloroethyl) ether (@@) in 1,2-dichloroethane or hexane or neat. A simple model based on only two organic phase species will explain these data and those for @@-benzenemixtures reported earlier. The hydrated proton is solvated with three molecules of @@; one @@ molecule is lost when the hydrated proton combines with FeC14- to form an ion pair. These species show surprising stability; they are unaffected by gross changes in the composition of the organic phase.
Several papers Crom this laboratory2-j have dealt with the extraction of tIMX4 (M = Fe, In, or Au; X = C1 or Br) from fairly concentrated aqueous HX solutions into mixed organic phases of varying composition. We have used the law of mass action to interpret these data. This yields solvation numbers larger than can be accounted for by any reasonable model. Nonetheless, we continued these studies in the belief that as sufficient data became available, a better model of ion extraction would evolve. In this paper, we describe the distribution of HFeC14 between ca. 8 M HCl and solutions of bis(2-chloroethyl) ether (06)in i,2-dichloroethane (DCE) or hexane. The former solution cssentially obeys Raoult's law while the latter shows a large positive deviation. A simple model, based on only two organic phase metal species, hydrated -tFeC14[H+(@@)2Fel34"] ion pairs and hydrated H+(@@)3 (free ions), will explain all of these extraction data and those for m a t PB and /$?-benzene mixtures5 as well. The The Journal of P:iysicsi Chemistry, Vol 78, No. 25, 1974
free energies of the solvated ions vary linearly with the inverse of the organic phase bulk dielectric constant as pred i ~ t e d .This ~ , ~fact was omitted from our earlier interpretatiom2-j
Experimental Section Distribution Measurements. Eastman White Label bis(2chloroethyl) ether was purified by distillation under a reduced pressure of N2. The middle fraction was collected and diluted with spectroquality DCE or reagent grade hexane to the desired concentrations. These solutions were stored in the dark and used within 6 weeks of preparation. Reagent grade HCI was diluted with deionized water to 8.15 M as determined by titration. Approximately 0.1 M FeCI3 (in 8.15 M HCI) stock solutions were prepared from reagent grade FeC13 6H20; the Fe(Il1) concentration was determined spectrophotometrically.8 Other solutions of lesser Fe(II1) concentrations were made by successive
-
Solvation of Extracted Complex Metal Acids
2573
TABLE I: Dielectric C o n s t a n t s of Bis(2-chloroethyl) Ether-1,2-Dichloroethane Mixtures at 25 O
t i
'* t c
'-1 I
Mole fraction
of ether 0.00~ 0.07 0.14 0.22 0.31
Mole fraction of ether
D 10.3 11.4 12.2 13.3 14.3
D
0.40 0.50 0.61 0.73
15 .0 16.2 17.2
18.4
Literature value: 10.36. A. A. Maryott and E. R. Smith, Nut. Bur. Stand. (U.S.), Circ., No. 514 (1951).
I.
-k ": I
to aqueous phase total metal concentrations), us. the equilibrium aqueous phase metal concentration, C M, have the general shapes predicted for systems in which both H+MXd- ion pairs and dissociated H + MX4- are present in the organic phase.2-5 The E values for solutions containing 115 mol % @/3 in DCE were corrected for DCE extraction by subtracting from the observed E the product of the volume fraction of DCE times E for pure DCE at the same C M. As before, we write the distribution equilibrium
+
Figure 1. Plots of the vapor pressure over the solutions at 25' vs. mole fractions of bis(2-chloroethyl)ether in 1,2dichloroethane (0) or hexane (€3).
quantitative dilutions of the stock solutions with 8.15 M HCI. Radioactive iron-59 chloride in aqueous HCl was purchased from International Chemical and Nuclear Corp. and diluted with 8.15 M WCl before use. The distribution experiments were done in triplicate a t . ~ reproducibility between 25O as described ~ l s e w h e r e The triplicates was almost always better than &lo%. Vapor Pressure Measurements. Each solution was shaken with 8.15 M HC1 before its vapor pressure was measured. The isoteniscope consisted of a IT tube containing mercury, a flask containing the sample, and a stopcock to isolate t,he sample side of the IJ tube from the vacuum pump. Air was removed by the freeze-pump-thaw technique. The entire device was placed in a glass jar containing water at 25 f 0.1' and the difference in mercury levels was measured with a cathetometer. The apparatus was left in the water bath for at least 30 min before any measurements were made. After this, readings were taken a t 5-min intervals for 30 min and averaged. Dielectric Constant Measurements. Each PP-DCE mixture was shaken with 8.15 M HCl before its dielectric constant was measured. The heterodyne beat method9 was used to measure the change in capacitance of a parallel plate capacitor in air and in the liquid mixture a t 25 f 0.1'. Carefully dried Mallinckrodt Nanograde benzene was used to calibrate the apparatus. Results The vapor pressures of both PP-DCE and @hexane solutions a t 25' are shown in Figure l. Since the solutions were equilibrated with 8.15 M HCl, all of the vapor pressures are too large by about 1.4 cm, the vapor pressure of 8 M HCI.'oa However, we are more interested in the shapes of the curves. Clearly BB-DCE can be considered ideal for our purposes, but the activity coefficients of the PB-hexane solutions are significantly different from unity. Calculation using our data (after subtracting 1.4 cm from each point; 38over the solution is negligible) prothe vapor pressure of 1 duced activity coefficients identical within experimental error to those reported by Neckel and Volk" for the dry solutions at 30'. The latter were used calculate &3 activities. ?'he dielectric constants of the BB-DCE mixtures (equilibrated with 8.15 M WC1) are tabulated in Table I. Log-log plots of the distribution ratio, E (= ratio of organic phase
H'
- FeC1,- + nap
K'
= [H'
+
FeC!,-]
*
r7BO
(1)
where the bar represents the organic phase, and the ion pair dissociation
The proton is of course hydrated, probably with four water molecules.2 For intermediated values of C M ,extracted HCl makes a negligible contribution to the ionic strength of the organic p h a ~ e , ~thus - 5 E can be writtens E =
&/EM)[ p/31("-p) +
~ * ( I t f p i 3 / n / C ~ ~ ) 1 '(3) 2
g+ is mean ionic activity coefficient of H + and F e q in the organic phase; the Debye-Huckel limiting law predicts it to be near unity a t the ionic strengths of interest here. lfip] represents the activity of /3/3 and the distribution constant, K , although proportional to K , depends also on the aqueous phase HCl concentration2 which is constant. As predicted by eq 3, plots of E us. C M-'D for a fixed organic phase are good straight lines. Values of K [/3if]n and K M ( / ~ @ ] Pobtained from such plots are given in Tables IT and 111. Discussion In earlier papers,2-5we chose a simple law of mass action approach to explain data similar to those in Tables I1 and 111. This extreme approach assumes that K' (thus K ) and K ,M are truly constant even though the composition of the organic phase changes drastically. The slopes of plots of log K [SI" or log a ~ [ S ]us. p log [SI are taken as the solvation numbers n and p . These plots are shown in Figures 2 and 3. Values of n so obtained range from 3 in the dilute @DCE mixture to near infinity in the Po-hexane mixtures; p is ca. I .5 in the former and infinite in the latter. Another extreme approach that has had some success,6'12 at least for neat solvents, is to assume that K' and K M are described by expressions based on the assumptions of the Born charging e q ~ a t i o nLe., ; ~ that n = p = 0 and that log K' and log K M vary linearly with the inverse of the organic phase dielectric constant, I J D . Figures 4 and 5 , where we have included also data for show plots to test this hypothesis. ( D values for @@-hexanemixtures were estimated from data for /3/3-octane.'oh) The Journal of Physical Chemistry. Vol. 78 No 25. 1974
0.K. K. Liu, 0.T. Shioshita, and R. L. McDonald
2574
TABLE [I: Apparent Distribution and Ion Pair Dissociation C o n s t a n t s for HFeClr Extracted from 8.15 M HCll by Bis(2-chloroethyl) E t h e r in 1 , 2 -Dichloroet h a n e Mole fraction of P P
K [PP I” 1.02 x 2.23 x 5.33 x 1.35 x 3.46 X 6.25 x 2.27 x 2.46: x 5.86 x 1.30 x 2.35 x 4.11 x 6.85 X
0.03 0.04 0.05 0.0;’ 0.09 0.11 0 , 15 0.25 0.30 0.35 0.40 0.45 0.50
1c-7 10-7 10-7 10-6
10-G 10-6
10-4 10-4 10-3 10-3 10-3
-2,
RM [PPI” 3 . 0 3 x 10-5 5 . 5 7 x 10-6 8.90 X 9 . 9 7 x 10-5 1 . 7 0 x 10-4 1 . 8 0 x 10-4 3.41 X 8 . 9 0 x 10-4 1.31 x 10-3 1.66 x 10-3 1 . 8 7 x 10-3 1.97 x 10-3 2.14 x 10-3
TABLE TII: Apparent Distribution and Ion Pair Dissociation C o n s t a n t s f o r HFeCI, Extracted from 8.15 M HCI by Bis(2-chloroethyl) E t h e r in Hexane
LOC
-i -1.0 1 -
i o
-1.9.
I
74
L~~Ipa
Figure 3. Log-log plots of the apparent ion pair dissociation constant of HFeCI4 vs. the activity of bis(2-chloroethyl) ether in 1,2dichloroethane (0),hexane (e), or neat (e).
___x--___
Mole fraction of pcr
[PP I“ 1.29 x 10-7 1 . 0 0 x 10-5 7.69 x 10-5 8 . 9 6 x 10-4 9.13 x 10-3 2 . 5 X lo-’
0.40 0.53 0.60 0.70 0.80 1.00
-._
R M
[PPI”
2.47 x 10-8 2.59 x 9.72 X 3.83 x 10-4 2.20 x 10-3 8 . 2 x 10-3
,
-2-
r
LOCKbq I
Figure 4. Plots of the apparent distribution constant of HFeCI4 (log scale) vs. the inverse organic phase dielectric constant for bis(2chloroethyl) ether in 1,2-dichloroethane (03, hexane (e), benzene (0), or neat (e).
-4‘
,/
-I
e
dd.--_
I
I
i
I
- 4
4 9.
0
La*-&]
Figure 2. Log-4og plots of the apparent distribution constant of HFeCI4 vs the activity of bis(2-chloroethyl) ether in 1.2-dichloroethane (O), hexane (e), or neat (e).
Both of these extreme approaches are unsatisfying, the first because it often yields solvation numbers too large to fit any reasonable m o d e P 5 and the second because the K (or EM)data do not fall on a single line as predicted by the Born charging e q u a t i o n . 5 ~ ~ ~ lmproved models of ion solvation have been suggested which allow for specific interactions between an ion and its nearest neighbor solvent molecules, but treat the solvent as a continuum beyond the first shell.l* To apply this approach we assume that the apparent value n = 3, which obis nearly tains at low (36 concentrations in DCE where constant, is the “true” solvation number of the dissociated M + FelCld-, and that this true n is independent of pp
+
The Journal of Physical Chemistry, Vol. 78, No. 25, 1974
-4
t
t
-1U L .os
ia
I7
21
‘/b Figure 5. Plots of the apparent ion pair dissociation constant of HFeCI4 (log scale) vs. the inverse organic phase dielectric constant for bis(2-chloroethyl) ether in 1,2dichloroethane (O), hexane (e), benzene (0), or neat (e).
concentration and diluent. That is, we break 1 into two steps: an aqueous equilibrium,
2575
Solvation of Extracted Complex Metal Acids
-- 1 -2
.os
.01
I -.1--_L.-i
.ir
.21
Figwe 7. Plot of the ion pair dissociation constant of HFeCI, (log scale) vs. the inverse organic phase dielectric constant for bis(2chloroethyl) ether in 1,2-dichloroethane (0). hexane (0).benzene
(01o , r neat W.
Figure 6. Plot of the distribution constant of HFeCI, (log scale) vs.
the inverse organic phase dielectric constant for bis(2chloroethyl) or neat ether in 1,2-dichloroethane (0),hexane (e),benzene (0).
(e).
R'
-
Ki
Feel4- -1 3&'3 - [H'
+
FeCl4']-3flfl
(la)
followed by a transfer step K2'
IH' i FcC14-].3PP -- [H' t Feel,-] *3PP
(lb)
We further assume that K 1' is in fact constant but log K2' varies linearly with l/D ; K' is of course K 1'K 2'. If this is valid, plots of log ( K [@p]nl[p@]a] us. 1/D should yield a single line for all diluents so long as the aqueous phase is constant. Figure 6 shows vividly that the data conform to our expectations. We use a similar approach on the R M [ s p I P values. At low @pconcentrations in DCE, the apparent p is near unity (see Figure 6). Figure 7, a plot of log {RM[@s]p/[@ us.@l/D ] ] shows that p = 1 will fit all of the solutions. Note that n and p are insensitive to both ether concentration and diluent. Apparently there is a single value for the number of solvent molecules associated with the ions (or ion pairs) and these solvated species have surprising stability. The solvation number 3 has been reported before for acids in a variety of solvents, most of them stronger bases than &3. The structure of the hydrated proton in these solutions, probably Hy04+,2is not known. Nonetheless, there is much evidence to suggest that it possesses three sites to hydrogen bond to a weakly basic donor solvent.15 It is reasonable to postulate that the ionic species present in our organic phases are Hy04+(/3@)3, FeCld-, and IH5104+(@/3)2FeC14-]. is a poor donor; apparently FeC14can compete successfully for one hydrogen bonding site in
the ion pair. Far-infrared spectra have shown that FeC14hydrogen bonds to R:iNH+ in benzene.I6 The constancy of the cation solvation number despite significant changes in the composition of the solvent mixtures deserves further discussion. Not only does it emphasize the usefulness of thinking chemically in terms of definite solvated species in solution, it also suggests that information concerning these species obtained from dilute solution studies can be extrapolated even to neat solvents. I t is hoped that additional studies of this type in the future will test the generality of this hypothesis.
References and Notes (1)(a) Work supported in part by the Air Force Office of Scientific Research under Grant No. AFOSR-68-1387.(b) NSF Undergraduate Research Participant.
(2)R. L. Erickson and R. L. McDonald. J. A m r . Chem. Soc., 88, 2099 (1966). (3)D. A. Meyers and R. L. McDonald, J. Amer. Chem. Soc.,89, 486 (1967). (4)S.L. Law and R. L. McDonald. J. Phys. Chem., 72, 1617 (1968). (5)R. L. McDonald and T. H. Hufen, J. Phys. Chem., 74, 1926 (1970). (6)J. T. D e n i m and J. B. Ramsey. J. A m . Chem. Soc., 77,2615 (1955). (7)M. Born, Z. Phys., 1.45 (1920). (8)D. A. Skoog and D. M. West, "Fundamentals of Analytical Chemistry," Hok, Rinehart and Winston. New York, N.Y., 1969,pp 689-90. (9)D. P. Shoemaker and C. W. Garland, "Experiments in Physical Chemistry," m a w - H i . New York. N.Y., 1962,pp 280-283. (IO)(a) J. Timmermans, "The Physico-chemical Constants of Binary Systems In Concentrated Solutions." Vol. 4,Interscience, New York, N.Y., 1959,p 442:(b) Bid.. V d . 1. p 399. (11) A. Neckel and H. Volk. Monatsh. Chem., 88,925 (1957). (12)G.R. Haugen and H. L. Friedman, J. Phys. Chem., 72,4549 (1968). (13)D. Feakens. "PhysicoChemical Processes in Mixed Aquews SOL vents, F. Franks, Ed., American Elsevier. New Yark. N.Y., 1987,pp 71-89. (14)A. D. Buckingham, Discuss. Faraday Soc..24, 150 (1957). (15)D. C. Whitney and R. M. Diamond, J. Phys. Chem., 67,209(1963). (16)R. A. Work, 111. and R. L. McDonald, J. horg. Nucl. Chem., 34, 3123 (1972).
The Journal of Physical Chemistry, Vo! 78. No 25, 1974
