T. KENJOAND R. M. DIAMOND
2454
pair concentration. The degree of dissociation a,however, is not necessarily a direct measure of the ion-pair concentration since solvent-separated ion pairs might exist in the equilibrium.’*
a20
[AB]
I
E . w
OJC
e’. IO6 Mol I I
-
Figure I . Corwlation of the extinction E/cm of the intermolecular charge-transfer absorption of 4-(4-nitrostyryl)-N-ethylquinoliniumiodide at 462 m p in CHCb at 25’ with the sum of the ion-pair concentration c’ = c ( 1 - a).
[A+. . , . .B-]
[A+]
+ [B-]
The visible C T band should arise only from the contact ion pair. 4b Since contact and solvent-separated ion pairs will be counted together as c’ = c ( l - a) (Table I) via the electrochemically obtained a, deviation from the straight line in the plot E us. c’ may be ascribed to the presence of solvent-separated ion pairs. I n the present case (Figure 1) we do not find such a deviation and the equilibrium constant for the ion pair-free ion equilibrium can be derived from absorption measurements a10ne.l~ I n CHCL with its low dipole moment charge-charge interaction is expected to be much larger than ion-solvent contact. Nevertheless, the procedure of the independently measured extinction and degree of dissociation could be a useful one for the observation of the often proposed solvent-separated ion pairs in solvents with higher dielectric constants. (14) S. Winstein, Experientia, Suppl,, 2, 137 (1955). (15) E. M.Kosower and J. C. Burbach, J. Amer. Chem. Sac., 78, 5838 (1956).
Hydration of Perchlorate, Tetraphenylboride, and Nitrate Ions in Some Organic Solvents1 by T. Kenjo and R. M. Diamond” Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 (Received February 83, 1978) Publication costs assisted by the Lawerence Berkeley Laboratory
The hydration of Clod-, B (CeH&-, and NOs-in various organic solvents has been studied by means of the extraction of the tetraalkylammonium salts. The dependence of the extraction on the aqueous salt concentration indicates that tetrapropylammonium perchlorate and tetrabutylammonium nitrate in nitrobenzene are essentially dissociated up to 10-2 M . But in solvents of lower dielectric constant, such as dichloroethane or 80% benzene-20% nitrobenzene, the salts associate to ion pairs and still larger ion aggregates. I n all these solvents about 0.3-0.4mol of water is coextracted with the tetraalkylammonium perchlorate. No water coextracts with tetraalkylammonium tetraphenylboride in nitrobenzene. With the nitrate salt, -1.4 mol of water is involved per Nos-.
Introduction The distribution of salts between water and an imhigh if Organic phase can be made either the cation or anion is large and hydrophobic enough. In particular, the extraction of anions can be studied by using large tetraalkylammonium cations. The Journal of Physical Chemistry. Val. 76,N o . 17, 1972
Earlier reports2 describe the organic phase complexing of F- and c1- in such salts by water-immiscible phenols (1) Work performed under the auspices of the U. S. Atomic Energy Commission. (2) (a) D. J. Turner, A. Beck, and R. M. Diamond, J. Phys. Chem., 72, 2831 (1968); (b) T.Kenjo, S. Brown, E. Held, and R. M. Diamond, ibid., 76, 1775 (1972).
HYDRATION OF ClOa-, BPhd-,
AND
NOs-
and alcohols. From the fact that definite complexes were indicated, we wondered if distribution of the salts alone and determination of the water coextracted would not yield information on the first-shell hydration of the anions. It would be advantageous in such a study if the extracted salts were dissociated into independent ions, for then if one ion could be shown to be anhydrous or to have a definite amount of water associated with it in a particular solvent, the determination of the total water extracted by a salt containing that ion yields the water bound to the coion. For this purpose a solvent of high dielectric constant was first chosen, namely nitrobenzene (e = 34.8 at 25°).3 Then, to see if ion association changes the hydration numbers found, lower-dielectric-constant solvents were also investigated. A number of recent studies have described the interactions of anions with protic solvents such as water, alcohol, and a m i n e ~ . ~ ~ ~Spectroscopic -"J evidence for nitrate-solvent interactions have been found for tetrabutylammonium nitrate-methanol in benzene14 for alkali metal nitrate-water in aqueous solution^,^ with alkylammonium nitrate salts in alcohol-methylene chloride or water-methylene chloride systems,6 and for aqueous and chloroform solutions of alkali metal and tetraphenylarsonium nitrates.' Transient perrhenatewater interactions have been suggested as the origin of the inhomogeneous electric-field gradient causing the quadrupole relaxation in aqueous solutions of ISaReO4 as observed in broad-line nmr spectroscopy.s Perrhenate-alkylammonium interactions are thought to be responsible for the appearance in the infrared spectrumof the perrhenate ion, in alkylammonium perrhenate SaltS, of splittings of the ~ ~ stretching - 0 vibrations at 850-1000 ~ m - l . ~However, nmrl0 and ir6 studies show that this interaction is one of the weakest between anions and the alkylammonium cations.
Experimental Section The desired form of tetraalkylammonium salt was obtained in the manner described in the previous paper.2b Tetraalkylammonium iodide (Eastman Organic Chemicals, White Label) was converted into the hydroxide form by shaking a suspension of the iodide in water with silver oxide (Baker and Adamson, reagent grade). The hydroxide obtained was neutralized with nitric or perchloric acid to the nitrate or perchlorate salt, respectively. Tetrabutylammonium tetraphenylboride was obtained as a precipitate by adding sodium tetraphenylboride to a tetrabutylammonium solution. The precipitate was Washed With Water until no bromide was C~bserved in the filtrate and then dried at 60". The benzene and dichloroethane used were J. T. Baker reagent grade. The nitrobenzene was Eastman Organic ChemReagents.
2455 icals, White Label. The sodium tetraphenylboride used was Mallinckrodt Chemical Works, reagent grade. Procedure. Various aqueous solutions of tetrabutylammonium nitrate were shaken with nitrobenzene for 2 hr. For the perchlorate salt, because of its low solubility in water, we started with the salt in the organic phase and shook with water for the same time. The volume ratio of organic to aqueous phase used was 1: 1. After shaking, the phases were centrifuged and separated. For dilute solutions the tetraalkylammonium cation concentration in the aqueous phase, and for more concentrated solutions the cation concentration in both phases, were determined spectrophotometrically using picrate;'l the procedure was essentially the same as in the previous work.2b After dilution of the sample of aqueous solution to the range to IM, a 5-ml aliquot was shaken with 5 ml of 0.010 M sodium picrate, 5 ml of saturated MgS04, and 5 ml of chloroform. For the organic phase determination, the solution was diluted to the same range of concentration with the organic solvent, and then an aliquot was shaken with the aqueous solution of picrate and MgSO4. Calibration curves for each solvent were prepared. The wavelengths used for nitrobenzene, dichloroethane, and 20y0 nitrobenzene-80yo benzene mixture were 434,383, and 426 mp, respectively. Water determinations in the organic phase were made by the Karl Fischer method using an electrometric end point. All experiments were done a t 25 f 2".
Results and Discussion As described in earlier papers, slope analysis is useful to determine the molecularity of the species in the organic phase. The equation for the distribution of a tetraalkylammonium salt into a solvent can be written R4N+ XmH2O
+
or R4N+
+
R4N+.. .X-.mHzO(org)
(la)
+ X-.mH20(org)
(lb)
+ X- + mHzO R4N+(org)
(3) A. A. Maryott and E. R. Smith, Nat. Bur. Stand. U.S., Circ., No. 14, 18 (1951). (4) J. Bufalini and K. H. Stern, J . Amer. Chem. Soc., 83, 4362 (1961). (5) D. E. Irish and A. R. Davis, Can. J . Chent., 46, 943 (1968). (6) Yu. G. Frolov, V. V. Sergievskii, and A. I. Sergievskaya, Zh. Neorg. Khim., 13, 1909 (1968) [Russ. J . Inorg. Chem., 13,994 (1968) 1. (7) A. R. Davis, J. W. Macklin, and R. A. Plane, J . Chem. Phys., 50, 1478 (1969). (8) R . A. Dwek, Z. Luz, and M. Shporer, J . Phys. Chem., 74, 2232 (1970). (9) K. A. Bol'shakov, N. M. Sinitsyn, K. I. Petrov, V. F. Travkin, and M. V. Rubtsov, Zh. Neorg. Khim., 13, 3082 (1968) [Russ. J . Iorg, Chem,, 13, 1589 (1968)
(10) W. E. Keder and A. S. Wilson, Nucl. Sci.
Elzg.,
17, 287 (1963).
(11) R. R. Grinstead and J. C. Davis, Annual Summary Progress Report, 1966-67, from the Dow Chemical Go. to the Office of Saline Water, p 66; modified from K. Gustavii and G. Schill, Acta Pharm. Suecica, 3 , 2 4 1 (1966); ibid., 3,259 (1966); ibid., 4,233 (1967).
The Journal of Physical Chemistry, Vol. 76, No. 17,1978
2456
T. KENJOAND R. M. DIAMOND
depending on whether the species in the organic phase is associated to an ion pair or is dissociated. The corresponding equilibrium constants are
where parentheses denote activities, brackets indicate concentrations, and y is an activity coefficient, A log-log plot of the (organic phase activity)"% us. the aqueous concentration should be a straight line whose slope is 2 or 1, depending on whether case a or b is involved. It has been shown in a different type of study12that c104- in nitrobenzene is essentially anhydrous, so tetrapropylammonium perchlorate was the first salt studied. Figure 1 shows a log-log plot of the organic phase salt concentration in nitrobenzene us. the equilibrium aqueous concentration for tetrapropylammonium perchlorate. But even with a dielectric constant of 35, the decrease in the activity coefficients from unity at the higher organic phase salt concentrations becomes significant. So they were calculated by a DebyeHuckel type expression13 (using a distance of closest approach, a, of 6 8). Activity coefficients in the aqueous phase were estimated by means of Poirier's expression^,'^ and, as can be seen in Figure 1, the corM fall on the rected points (filled triangles) up to same straight line of unit slope extended from the more dilute solutions where the corrections are negligible. Since the salt is surely dissociated in the dilute aqueous phase, the observed slope of unity shows that it is also dissociated in the organic phase over that range of concentration. Table I shows the results of determining, by Karl Fischer titrations, the water coextracted by three concentrations of tetrapropylammonium perchlorate in nitrobenzene. Approximately 0.3-0.4 mol of water appears per mole of extracted salt. This value is obtained by subtracting the water distributing into the nitrobenzene alone (calculated as the volume fraction of nitrobenzene times the solubility of water in nitrobenzene, 0.16-0.18 M , depending on the temperature) from the total amount of water determined for the organic phase. It is not clear whether the -0.4 mol of water found is (a) associated with the cation, (b) associated with the anion, or (c) just an increase in water dissolved in the nitrobenzene solvent because the presence of the dissolved salt changes the properties of the solvent. To help understand this, a still larger cation, The Journal of Physical Chemistry, Vol. Y6,No. l Y , 10Y2
Figure 1. The tetrapropylammonium perchlorate concentration in nitrobenzene us. the aqueous tetrapropylammonium perchlorate concentration, 0 ; organic phase concentration corrected by Debye-Huckel type $activity coefficient with a distance of closest approach of 6 A, 0; corrected to aqueous phase activityb calculated from Poirier's expressions with a = 2.5 A, A.
Table I : Water Coextracted with Tetraalkylammonium Salts in Nitrobenzene Molarity of snlt(org) Salt
Tetrapropylammonium perchlorate Tetrahexylammonium perchlorate Tetrabutylammonium tetraphenylboride Tetrabutylammonium nitrate
x
10'
13.9 2.66 1.41 20.3 8.13 4.07 1.63 10.0 3.98 1.99 20.4 9.11 4.12 1.52
Molarity of wnter(org)a x 10'
Ratio of water to salt molarity
5.2 1.0 0.5 7.9 2.7 1.5 0.6 0.8 0.2 0.1 29.6 12.8 5.8 1.9
0.37 0.38 0.4 0.39 0.33 0.37 0.4 0.08 0.05 0.05 1.45 1.41 1.41 1.3
a The water extracted by the diluent alone (diluent volume fraction x water solubility in diluent) has been subtracted.
the tetrahexylammonium ion, was used, and the results are also shown in Table I. They are the same, -0.4 mol of waterlmole of salt. Since we might expect that doubling the size of the cation would change (re(12) J. J. Bucher and R. M. Diamond, to be published. (13) R. A . Robinson and R. H. Stokes, "Electrolyte Solutions," Butterworths, London, 1959,2nd ed, Chapter 9. (14) J. C. Poirier, J . Chem. Phvs., 21, 965 (1953).
HYDRATION OF C104-, BPh4-,
AND
NO*-
2457
duce) the amount of water associated with it, this result suggests that possibility a is not likely. Indeed, the use of a larger anion, the tetraphenylboride ion, which coextracts essentially zero water under these conditions (Table I), indicates that possibility b is the correct one. That is, essentially an independent and anhydrous tetraalkylammonium cation is present in these (dilute) nitrobenzene salt solutions, and the -0.4 mol of water with the perchlorate salt is probably bound to c104-. For such large, poorly solvated anions, we might expect that the process of associating to ion pairs or t o ion quadrupoles or to still higher aggregates would not change the water uptake much, possibly reducing it still further. We can test this by employing lowerdielectric-constant solvents where the extracted species is indeed aggregated. So the water uptake for tetraalkylammonium perchlorate in 20 vol yo nitrobenzene80 vol % benzene and in dichloroethane was studied. The extraction curves, Figures 2 and 3, show that in these solvents, particularly the former, a t concentrations above M the salt has aggregated to ion pairs and then to ion quadrupoles and beyond. But the water coextracted is still 0.3-0.4 mol/mol of salt, as listed in Table 11, and again changing the size of the cation had little effect. In fact, even going to pure benzene as solvent, where the solution properties would surely be most changed by the addition of the salt, made little difference in the water uptake per mole of salt; the slight decrease observed may be related to the much lower water solubility in benzene compared to nitrobenzene or dichloroethane. The results indicate that only one out of three to
t
'
I
I
I l l l l l l
IO-^
I
I
I
I
io-51
, , (,,,,
,
,
, , ,,J 10-2
1
Figure 3. The tetraalkylammonium perchlorate concentration in dichloroethane us. the aqueous tetraalkylammonium perchlorate concentration: tetrabutylammonium perchlorate, 0; corrected by Debye-Huckel activity coefficient, A ; tetrapropylammonium perchlorate, 0 ; corrected by Debye-Huckel activity coefficient, A. The straight line segments are drawn with unit slope, indicating a dissociated species.
Table I1 : Water Coextracted with Tetraalkylammonium Perchlorate in Organic Solvents
Solvent
Cation
Dichloroethane
Tetrapropylammonium Tetrabutylammonium Tetrahexylammonium
20% Nitrobenzene-80% benzene
I l l 1
Figure 2. The tetraalkylammonium perchlorate concentration in 20% nitrobenzene-80% benzene vs. the aqueous tetraalkylammonium perchlorate concentration: tetrabutylammonium perchlorate, 0 ; tetrapentylammonium perchlorate, 0 . The straight line segment is drawn with a slope of 2, indicating association to ion pairs.
, [CJO'i
Tetrabutylammonium Tetrapentylammonium
10-2
[cw]
, , , (,,#,
10-5
Benzene
Tetrahexylammonium
Molarity Molarity of saltof water-
of water
(ord X 108
(ow)" X 102
to salt molarity
5.30 2.06 1.00 0.47 20.6 10.3 5.09 7.57 3.78 1.89 0.76 18.3 8.98 4.31 1.58 7.89 3.93 1.96 9.44 3.78 1.89 0.76
2.2 0.9 0.4 0.2 6.7 3.3 1.7 2.7 1.3 0.7 0.4 5.1 2.9 1.5 0.5 2.4 1.3 0.6 1.85 0.79 0.40 0.17
0.42 0.4 0.4 0.4 0.33 0.32 0.33 0.36 0.34 0.4 0.5 0.28 0.32 0.35 0.3 0.30 0.33 0.3 0.20 0.21 0.21 0.22
Rstio
The water dissolved by the volume fraction of solvent alone has been subtracted. The Journal of Phyaical Chemistry, Vol. 75, No. 17, 1979
2458 five C104- ions carry a water molecule in these organic solvents. What can be said about their first-shell hydration number in an aqueous solution? For such poorly hydrated ions as Clod-, it may not be meaningful to even speak of such a quantity in the aqueous phase, as the water molecules may switch rapidly between orientation towards the ion and towards the surrounding water structure, viith no fixed coordination number around the ion. This would correspond to the “thawed” region of Frank and Wen,15 or to the range of zero (negative) hydration of Sanioilov,16or to the “structurebreaking” of G ~ r n e y . 1 ~ However, it does seem clear that there is no water associated with the tetraalkylammonium cations. So it should be possible to study the xyster carried into nitrobenzene by the distribution of other dissociated tetraalkylammonium salts and ascribe all the water found to the anions. Figure 4 shows a log-log plot of the organic phase concentration of tetrabutylammonium nitrate us. the aqueous phase value. Activity coefficient corrections are made for the most concentrated organic solutions, again using the same DebyeHuckel expression, and for the concentratcd aqueous phase by use of Poirier’s expressionsJcalculated with a distance of closest approach of 2.5 A so as to approximate the experimental values for tetrabutylammonium bromidel8 for molalities between 0.1 and 1.0). A straight line of unit slope results up to 10+ I l l , indicating dissociation of the salt in nitrobenzene for that range of concentration, and a considerable proportion still dissociated even at 10-1 M . Table I lists values for the water coextracted with the Koa-; of the order of 1.4 mol of water is involved per xo3-. Since we only determine the stoichiometric water uptake, the average value for the anion, we cannot translate this result uniquely into a description of the anion hydrates involved. For example, there may be a mixture of iY03-.H20 and x03-.2H20 extracting, or all possible mixtures of these t\To plus anhydrous ;?jo3- and the species NO3-.3Hz0 with a completely hydrated first shell, so as to yield the observed hydration number, -1.4. Even if only the two species, S 0 3 - - H z 0 and S 0 4 - . 2 H 2 0 exist, we do not know whether the dihydrate has two water molecules hydrogen-bonded to two oxygens of the nitrate anion, giving it a coordination number of 2, or whether the second water is hydrogen-bonded to the first leaving the nitrate anion with a coordination number of 1. The observed hydration of YO3- can be compared mith the alcoholation number found in an extraction study of tetrabutylammonium nitrate into 1,2-dichloroethane with decano1;lg only one alcohol complexed the S O 3 - for alcohol concenlrations from 0.08 to 0.5 M . But in that study it vias not possible to determine the water uptake to better than 1 + 1 water molecule, and if there is a water molecule involved, again we do not know whether the water and alcohol molecules are The Journal of Physical Chemistry, Vol. 7 6 , N o . 17, 1072
T. KENJOAND R. n/r. DIAMOND
Figure 4. The tetrabutylammonium nitrate concentration in nitrobenzene us. the aqueous ammonium nitrate concentration, 0 ; corrected to organic phase a!tivity using a Debye-Huckel activity coefficient with a = 6 A, 0; corrected to an aqueous phase activity reproducing the experimental activity coefficients of tetrabutylammonium bromide, A.
hydrogen-bonded to two nitrate oxygens, or whether the water is a bridge between the alcohol and the anion, leaving the latter monocoordinated. In that study, it was also necessary to determine the amount of water extracted with uncomplexed xO3- into the 1,2-dichloroethane diluent alone. The ratio of waterlnitrate found there in two determinations was 1.25, in agreement with the present results in nitrobenzene. For such anions that carry into the organic phase more than 1 mol of water per ion, we believe that the hydration number observed in the organic phase may be only a lower limit to the first-shell hydration number in water. For in the aqueous phase, the first-shell waters receive additional solvation from the water molecules farther out; but in the organic phase, they exchange this (hydrogen-bonded) solvation for the (usually) poorer solvation of the surrounding organic diluent. The poorer environment there would thus tend to reduce the first-shell hydration number over that in water itself. The procedure described in this paper for determining the water carried by an anion is certainly simpler than the use of trioctylphosphine oxide in nitrobenzene or in 1,2-dichloroethane to extract the corresponding (15) H. S. Frank and W.-Y. Wen, Discuss. Faraday Soc., 24, 133 (1957). (16) 0. Ya. Samoilov, ibid., 24, 141 (1957). (17) R. IT’, Gurney, “Ionic Processes in Solution,” McGraw-Hill, New York, N. Y., 1953, Chapter 16. (18) S. Lindenbaum and G. E. Boyd, J . Phys. Chem., 6 8 , 911 (1964). (19) D. J. Turner and R. M. Diamond, J . Imrg. Nucl. Chem., 30, 3039 (1968).
SEL~CTIVITY IN HETEROVALENT ANIONEXCHANGE acid.lZ By the latter method a value for the hydration number of KO3- in nitrobenzene of -2 was obtained, but there was uncertainty introduced by the presence of binitrate ion produced from the aqueous nitric acid solution. This problem of binitrate ion formation cannot arise in the present study of neutral salt solutions, so that the value of 1.3-1.4 mol of water per
2459 NOa-is the better one. The distribution of other alkylammonium salts, particularly the halides, into nitrobenzene and into 1,Zdichloroethane will be similarly studied to see what regularities do occur in the hydration of these anions in the organic solvents.
Acknowledgment. The authors would like to thank Mr. J. Bucher for helpful discussions.
Selectivity in Heterovalent Anion Exchange. Ion Pairing us. Ion Hydration1 by J. Bucher, R. M. Diamond,* Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720
and B. Chu Department of Chemistry, State University of New York, Stony Brook, New York
(Received February 23, 1972)
Publication costs assisted by the Lawerenee Berkeley Laboratory
The values of the selectivity coefficients, K B I Afor , heterovalent exchange with a strong-base resin and radiotracer Re04-, CrOd2-, and W04*- vs. macro C1-, and radiotracer Cr(CN)P, Co(CN)Ba-, and Fe(CN)04- vs. macro CN-, have been determined. These results show that in such systems, contrary to early ideas on the nature of resin selectivity, the direction of the exchange is determined by the superior hydration of the ions in the dilute external aqueous phase over that in the resin phase, and not by ion pairing in the latter phase.
or
Introduction For the ion-exchange reaction
KB~A =
SA
-
TB
where the superscript bar indicates the resin phase, it has become customary to write the equilibrium constant
and to assume the same standard state for both p h a ~ e s . ~Then ,~
(3)
If, as is usually the case, the standard state chosen is the hypothetical state of unit activity with the properties of the infinitely dilute solution, the ratio Y A / Y B of the external-phase activity coefficients can be made as close to unity as is desired by decreasing the concentration of that phase. Thus eq 3 becomes (4)
The equilibrium quotient or selectivity constant, KBIA,depends only on the resin phase activity coefficients. Partly as a result of this type of derivation, early researchers often formulated ion-exchange resin selectivity as a function mainly of resin proper tie^.^-'^ (1) Work performed under the auspices of the U. S. Atomic Energy Commission. (2) (a) K . A. Kraus and F. Nelson in “The Structure of Electrolyte Solutions,” W.J. Hamer, Ed., Wiley, New York, N. Y., 1959, p 340; (b) B. Chu, Thesis, Cornel1 University, Ithaca, N . Y., 1959. (3) Y. Marcus and A. S. Kertes, “Ion Exchange and Solvent Extraction of Metal Complexes,” Wiley-Interscience, New York, N. Y., 1969, p 278. (4) G. E . Boyd, J. Schubert, and A. W.Adamson, J . Amer. Chem. Soc., 69, 2818 (1947). (5) H . P. Gregor, ibid., 73, 642 (1951). (6) T. R. E. Kressman and J. A. Kitchener, J . Chem. Soc., 1190 (1949). (7) K . W. Pepper and D. Reichenberg, Z . Elektrochem., 57, 183 (1953). (8) J. L. Pauley, J . Amer. Chem. Soc., 76, 1422 (1954). (9) H. P. Gregor, J. Belle, and R.A. Marcus, ibid., 77, 2713 (1955). (10) F. E . Harris and 5. A. Rice, J . Chem. Phys., 24, 1258 (1956).
The Journal of Physical Chemistry, Vol. 7 6 , N o . 17, 1978
