THE JOURNAL OF
PHYSICAL CHEMISTRY (Registered in U. S. P a t e n t Office)
(0Copyright, 1961, by the .4nierican
Chemical Society)
FEBRUARY 24, 1961
VOLUME65
NUMBER 2
THE PRIMARY SOLVATION OF THE PROTON IS THE SOLVEiVT EXTRACTION OF STROYG ACIDS' BY D. G. TUCKA N D Department of Chemistry, University of Nottingham, lVottingham, England
RICHARDAI. DIAMOND Lawrence Radiation Laboratory, University of California, Berkeley, California Received October 16, 1959
Evidence is reported for the presence of the trihydrated hydronium ion, H30(H,0)3+, in extracts from aqueous solutions into basic organic solvents of the strong acids HC104, HBr and HC1. In the organic phase, this species is further solvated by (hydrogen-bonded) coordination to the basic organic molecules, and so the degree of extraction depends partly upon the and CC13COyH,the proton cocoordinating ability of the solvent molecules. With the someffhat weaker acids, "03 ordinates preferentially in the low-dielectric-constant organic phase with the more basic (charged) anion than with water. The molecule formed is solvated by water and (or) organic solvent molecules, depending upon the solvent basicity. For solvents such as the ethers and ketones, the resulting species is HN03.H20(sol), and with the more basic tributyl phosphate, an anhydrous species, HNOJ.TBP(sol), results.
Introduction Since the early years of the ionic theory, at'tempts have been made to measure t'he number of water molecules associated with an ion in aqueous solution. Vario.us properties of solvated ions have been studied witlh this end in view, but the measurements do not give concordant results because they are concerned with different aspects of solvation. Much of the progress made in recent years can be ascribed to the differentiation now made between primary, i.e., fir;st-shell or coordinate, and secondary solution.?-5 A review by Bockris discusses the various experimental techniques that have been employedJ6and St'okes and Robinson give a more recent survey. Z b A great deal of attention has been directed to the hydration of t'he hydrogen ion. Because of its very small size, the bare proton has a much higher (1) This work was )supported i n part b y the U. S. Atomic Energy Commission. The early part was perfornied at the Laboratory of Nuclear Studies and the Chemistry Department, Cornell University, l t h a c a , Ki.w T o r k . (2) (a) N. 13jerriini Z . anorg. Chem., 109, 275 (1920); (b) R. A . Robinson and R . TI. Stokes, "Electrolyte Solutions," Butternortiis Scientific Publications London. 195.5. (3) J. D. Bernall a n i R. I€. Fowler, J . Chem. Piiys., 1, 215 (1933). (4) D. I). Eley ani1 11. G . Evans, Trans. F a r a d a y SOC., 34, 1093 (1938). ( 5 ) J. P. H u n t and II. Taube, J . Chem. P h y s . , 18, 757 (1960). (6) J. O ' M . Bockris, Quorl. Reus., 3 , 173 (194'J).
charge density than any other ion. This charge density makes the existence of the free proton in any solvat'ing medium very improbable and, in particular, explains the strong binding of the proton to a water molecule to form the hydronium ion, E&+. There is already evidence from X-ray,' nuclear magnetic resonance,*infraredgand Raman'O studies for the existence of this species in the solid state, and it must certainly exist as a more highly hydrated species in aqueous solution. An accumulation of indirect evidence now available suggest's that in not too concentrat'ed aqueous solution the H30+group behaves as an ion that can be further solvated by a "primary" shell of three water molecules to give as a predominant species the symmetrical structure shown in Fig. la. Because of the unique charge distribution of the H30+ion, with approximately of t'he positive charge concentrated on each proton, the hydrogen bonding of the three water molecules is much stronger than that between water molecules in bulk mater. This results in much stronger coordinate solvation (7) F. S.Lee and G. B. Carpenter, TIrI8 JOUBNAI., 63,279 (1959). ( 8 ) (a) R. E. Richards and ,J. A . S. Smith. T r a m . Faraday Soc., 47, 1261 (1951); (b) Y. Jiakiuclii, H. Sliono, €1. Kornatsu and Ihe Eolvent molecule is b l o ~ k e d . ~ ~ J ~ ~ ~ ~ with a ratio of E[zO:HC104of 3.9 zt 0.3. The results of the present work, then, suggest Similarly the st,udies on HC1 and HBr extraction that the strong Rcids extract from aqueous soluinto T B P yield (corrected) values of HzO:acid of tions of less than about 7 M acid into organic sol4.1 rt 0.2 and 4.5 i 0.2, respectively, for oreanic vents of moderate basichy (ethers, ketones, esters, phase acid concentrations up to 1.5 M. At higher but not the amines) with the hydronium ion reacid concentrations the plots of AV us. acid concen- taining a primary hydrat,ion shell of t'hree water tration yield lower values; the sta,rt of the non- molecules. The solvent molecules coordinate to linear portions of the plots for T B P come progres- this shell of water, and are not involved in direct sively at a. lower aqueous and organic phase acid coordination to the proton, that is, in displacement concentration for HC1, HBr and HC104.35 It of the primary hydration shell.40 But as the aqueshould be not,ed that the work of Baldwin, Higgins ous acid Concentration is increased, t'here will be a and S01dano~~ on the extraction of HC1, HBr and decreasing amount of water available t'o Polvate H I int,o T B P also can be similarly corrected for the (37) For example, one may compare t h e limiting equivalent conwater already present in the T B P and leads to a ductivities of Br-, I - and C 1 0 ~ - ,which decrease in t h a t order, with the hydration value for the acids of approximately four. limiting equivalent conductivities of t h e alkali or alkaline earth ions, or They find, by Karl Fischer tit,ration of the organic even of F - and C1-. T h e values of the cations a n d small anions inphase, water to acid ratios of 4.1 and 4.6 for HC1 crease with increasing atomic weight, t h a t is, in t h e inverse order of ionic radii. This indicates t h a t they are hydrated, and t h a t t h e and HBr, respectively, in comparison with the their degree of hydration, and hence effective hydrated radius, decreases in values 4.1 and 4.5 found by the volume-change the order L i + > . . . > Cs+, B e + + > . . . > B a + + , F-> C1-. B u t for method. This again demonstrates t'he agreement the larger anions, Br-, I- and C104-, and t h e larger cations, NRZea+, of the two independent types of water measure- NEtd+, NPra+, NBus+, the effective aqueous radii go in t h e same order a s t h e ionic radii, suggesting t h a t these large ions d o not drag a shell of ment. water molecules with them, t h a t is. they d o not have a primary hydraThese result's indicate that when the strong tion shell. Or, one may consider t h e activity coefficient. calculations of acids HC104, HBr and HCl extract into the typical Glueckanf (ref. 14) which indicate little primary hydration of t h e basic organic solvents DBS, DIPK and TBP, 4.0 larger anions.
+
+ +
t h e organic phase water solubility decreases with a n increase in t h e aqueous phase acidity. both because more solvent molecules are involved in secondary solvation of the proton and because the aqueous phase water aotivity decreases. For these reasons we have increasingly over-corrected for water solubility in t h e organic phase. (33) K. Alcock, S. S Grimley, T. V. Healey, J. Kennedy and €1. A. C. McKay, Trans. Faraday SOC., S a , 39 (1956). (34) R . L. Moore, "The Mechanism of Extraction of Uranium b y Tributyl Phosphate," I7.S.A.E.C. Report AECD-3196 (n.d.). (35) Such a curvature in the volume-change plots could arise because of a n increase in so1ubi:ity of t h e organic solvent in t h e aqueous phase with increasing acid concentration. B u t i t was shown t h a t t h e solubility of DHS in 851 €IC101 is still quite small and essentially t h e same a s in pure water. (36) W. €I. Baldwin, C . E. Hipgins a n d B. A. Soldano, THISJOGRN.AL, 68,
118 (1959).
(38) Such a n assumption of a trihydrated hydronium ion, HaO+(HrO)s, a s t h e extracting cation in strong acid extraction systems involving moderately basic solvents such a s ethers, ketones, esters, etc.. b u t not amines, has been used t o explain why t h e acids extract a n order of magnitude or more better t h a n t h e corresponding lithium salts, which have comparable activities in aqueous solution (ref. 11). (39) I. Nelidow and R. 11. Diamond, THISJOURSAL, 69, 710 (1955). (40) Organic molecules do not. in general, replace the primary-shell water molecules because t h e € I t 0 molecule is small a n d possesses a relatively high dipole moment. T h e ion-water interactions are stronger t h a n t h e corresponding ion-solvent interactions. and t h e water molecules can pack around t h e hydronium ion in a primary solvation shell much more easily t h a n can t h e bulkier organic molecules. These considerations d o not apply. however, t o the w r y basir amine extractant systems, nor t o any h u t the very strong acids. (See t h e discussion later in this paper for HNO3 and CC13COzIT.)
D. G. TCCKAXD RICHARD 11.DIAMOSI)
198
each ion, and even the primary hydration shell of the extracted hydronium ion will be disturbed. The plots of solvent volume change us. acid extracted all show an initially linear region where the hydronium ion extracts with three water molecules. Eventually, with increasing acid concentration, they all begin to curve toward lower hydration numbers as the scarcity of "free" water interferes with even the primary hydration shell. For example, the deviation from linearity shown in Fig. 2 for HC1O4 into DBS becomes significant above -15 mmoles HC1O4 extracted per 5 ml. of solvent; the equilibrium aqueous phase is approximately 6.3 M in perchloric acid, arising from an initial aqueous solution of 7 M HC104. Such an initial solution corresponds to 50% HC1O4, or a water to acid ratio of about 5.5. The fact that with T B P extractions the acid concentration a t which the volume change plots begin to deviate from a straight line is progressively lower in the order HC1, HBr, HC104, indicates the correctness of this explanation, as this is the order of decreasing water activity for aqueous solutions of these acids a t a fixed concentration. A very different situation exists in the extraction of acids weaker than HC1, HBr, HC1O4, HFeC14 (and other metal complex acids) into organic solvents of moderate basicity. Acids as weak as nitric and trichloroacetic do not show the extraction of a tri-hydrated hydronium ion. It seems that the hydration number of nitric acid extracted into organic solvents is seldom above unity, and in only one case does a value near four appear. (For pentaether,41 the hydration number found by direct titration is 6.6, but the solubility of water in this solvent is high, and the possibility of inter- and intra-molecular interactions of the basic groups rather confuses the issue.) For example, in extraction into TBP, the acid is present as "03. TBP,27.33 and the process can be represented as
T'ol. 05
Whether the hydration number is 0 or 1or something between, indicating a mixture, depends upon the basic strength of the solvent molecule, that is, upon its ability to compete with HZOfor hydrogenbonded coordination to the HN03molecule. If the solvent is not too basic, a water molecule will hydrogen-bond to the HNO, molecule and will in turn be similarly bonded t o by an organic molecule. This behavior has been ob)wvxl Ivith extraction into ethers, polyethers and ketones.2935 46 However, a more basic solvent such as T B P can compete favorably with water for coordination to the HNOR molecule, and so leads to the anhydrous species, TBP.HN03. Evidence that the low hydration numbers of HXOa are really due to its weakness as an acid and consequent extraction as a molecule, that is, evidence for the importance of acid strength iii determining the extent of proton hydration in the solvent phase, is elearly shown by the extractive behavior of trichloroacetic acid ( p K = 0.T).j7 The results of the wdume-change measurements (Fig. 5 and 6) are remarkably similar t o those for nitric acid. For DBS the slope of 9.95 mmoles/ nil. corresponds to a solution in which the CC13. COOH : HSO ratio is 1:0.9. For TBP, the analogy with HK03 is even closer ITp to a TBP:CC13COOH ratio of approximately one (18.1 mmoles CC13COOHextracted), the reciprocal of the volume change is 12.5 mmoles/ml. Taking the extrapolated density of CC13C02H as l.A2,48one finds that the volume change foi- the process TBP.H20(org)
+ CCI,COOH(aq) + TBPCCLCOOH(org)
+ H20(aq)
is (101-18) ml./mole, corresponding t o 12.1 mmoles/ml. The excellence of the agreement with the experimental result is perhaps rather fortuitous, because the density of liquid CC13COOH is an extrapolated value. However, there seems to be little doubt possible about the conclusion. Above a TBP.HN03 4 H20(aq) TBP.H20 HNOs(aq) CC13C02H:TBPratio of 1 0 in the organic phase, Thus, iii extract solutions of HN03 in a basic the phosphoryl group is completely saturated, and solvent, the proton is essentially different in its the volume-change slope changes to approximately chemical bonding and environment from its state 7.9 mmoles/ml. Here again then, there is rather in similar extract solutions of strong acids such as less than one molecule of H 2 0per acid in the region HC104, HI, HBr and HC1. We believe that the in which extractioii is due to the basic oxygen reason lies in the comparatively low ionization of the P-0-C bond; nitric acid hehaves in preconstant of nitric acid (Ki= 23)42relative t o those cisely the same way.2i It seems, then, that as far of HC1, HBr and HC104, for which values of pK = as hydration in the organic phase of solvent extrac-7 and higher have been e ~ t i m a t e d .4~4 , That is, tion systems, the weaker atroiig acids, nitric and the small primary hydration numbers of HK03 trichloroacetic, have very similar behavior and this in organic solrents (0 1) are due to the ability is also borne out by results on the extraction of of X03- to compete successfully with HZO for the these acids into diisoproppl proton in a low dielectric constant medium. This Acknowledgment.-We wish to thank R. P. results in the extraction of the "03 molecule rather than of the hydrated hydronium ion Bell, F.R.S., for helpful discussions of this problem.
+
-
(41) E . Glueckauf a n d H. .4. C . hfoKay, unpublished results, quoted b y H. A. C. hfcKay, J . Inorg. Nuclear Chem., 4 , 375 (1957). (42) 0 Redlich a n d G. C. Hood, Dzsc Faradaw Soc., 24, 87 (1957). (43) J C . LfcCoubrey, Trans. Faraday Soc , 6 1 , 743 (1955). (44) R. P. Bell, "Acids a n d Bases " hfethuen and C o , London, 1952, TI. 59.
(45) J. Sutton, "Distnbutlon of Nitric Acid Between Water and Report A E R E C/R 438 Yo\ ember, 1949. (46) D. G . T u c k , unpublished work (47) R . P Bell ref. 44 p. 63. (48) J. Iiendnll and E . Bmckelel. J . A m C h e m Soc , 43, 1826 (1921).
