EXTRACTION OF ACIDSBY BASICORGANIC SOLVESTS
Feb., 1963
DATAFOR -Concn., HT
0 05 .05 .10 .10
.IO
THE
TPI'TERPRETATIOR
TABLE I1 SYNERGISTIC EFFECT OBTAINEDFROM 0.11 M HYDROCHLORJC ACID SOLUTIONS S2 [Kci(H+)(Cl-)+KT(IIT) I
moles/l.Thorium
x
OF THE
209
-Fractional RsNHCl
104
0.41 .41 .26 .26 .26
1 0 50 1.0 5.0 25 0
distribution of the amineR3SHClHT RaNHT
0.31 .31 .39 .39 .39
investigated. It is apparent from the data in Table I1 that a 25-fold change in thorium concentration does not influence the magnitude or the nature of the symergistic effect. Discussion The acid concentration required to prevent hydrolysis of the thorium and aqueous complexation of TTA also prevented the formation of an appreciable fraction of R3X. However, the synergism cannot be attributed to a large effect due to a small amount of R3N. If this were the case, then the formation constant for ThT4R3?;HC1 mould have been a function of changes in the relative amount of R&. The formtion constant was evaluated under varying concentrations of hydrogen ion, chloride ion, and TTA, all of which affect the relative amount of RsN. Since the formation constant was found to be independent of the concentrations of thesevariables, then it must be concluded that the low concentration of R3X cannot contribute significantly to the synergistic effect. The possibility of a replacement of a molecule of TTA upon the addition of R8SHC1 is also ruled out on the basis that the formation constant was not a function of the changes in these same variables. The question of how the amine hydrocliloride is bound to a molecule, ThT,, wlhich is cobrdinately saturated, is difficult to answer. This problem is further complicated by the fact that of the three species, R3KHC1, R&HClHT, and R&HT only the amine hydrochloride exerted a synergistic effect.
+ K c I , Tx( H10-4 +)(C~-)WI"I
Slope
x
Intercept
5 0 3.7 43
0.28 .28 .35 .35 .35
10-2
9.5 5 5 5s 60 31
58
26
4.6 3.6 4.6 4.5 4.5
h possible suggestion is that a chelate bond of one of the TTA groups opens up and becomes monodentate, allowing an R3SHC1 molecule to attach directly to the thorium. Healy has suggested this possibility as an explanation for the synergistic effect observed in the extraction of the tetravalent actinides by TTA and TBP.14 This suggestion would appear somewhat unlikely since the monodentate character thus imposed on the TTA would result in a decrease in the stability of the molecule which probably could not be compensated for by the addition of the amine hydrochloride. Furthermore, this does not answer the question why both R3P\;HC1HT and R3SHT did not exert a synergistic effect. An alternate suggestion is that the amine hydrochloride attaches directly to one of the TTA molecules on the thorium. This suggestion is supported by the existence of the species, R3KHC1HT, which forms upon the reaction of amine hydrochloride and TTA. The attachment of the species, R&HClHT or R31\"T, to one of the TTA molecules on the thorium would imply that they can react with an additional molecule on TTA. Such reactions were not observed, and consequently they would not be expected to exert a synergistic effect on the extraction of thorium by TTA. On the contrary, an antisynergistic effect can occur due to these species when the amine concentration is sufficiently high to decrease the TTA concentratior~l5 (14) Private communication. ( 1 5 ) L. Kewman and P. Klotz, unpublished data.
THE EXTRACTION OF ACIDS BY BASIC ORGANIC SOLVENTS. I. TRIBUTYL PHOSPHATE-HC10, AND TRIBUTYL PHOSPHATE-HRe04 * BY DAVIDC. 'WHITTEY
AND
RICHARD XI. DIAMOSD
Lawrence Radiation Laboratory, University of California, Berkeley, California Received J u n e 9, 1962 The extraction of HC104 into dilute solutions of tributyl phosphate (TBP) in CCl, has been studied, and the extracting species and extraction mechanism have been determined. It has been found that so long as the T B P concentration is 3, the only extracting species are the molecular adduct TBP. HsO and the solvated hydronium ion 3TBP.H80+.yHzO. . . Clod-, an ion pair, where 0 6 6 1. For 1 < T B P / H + < 3 there are several possible species, whereas a t TBP/H+ = 1, the only species present in the organic phase is the salt TBPH + . , .CIOa-,. These results are interpreted in terms of a proposed general model for such strong acid-basic solvent extraction systems.
Introduction
HC104,
"03,
HZS04, and the hydrogen halides. but
.
*
This work was supported b y the
U. S. Btomic Energy Commission.
phat&, ketones, and ethers.
-
-
210
DAVIDC. WHITNEYa m RICHARD M. DIAMOSD
Vol. G i
With any such system as described above, the extraction involves a competition among the anion, the water, and the basic organic extractant for the proton. In this paper the acids chosen for study were HC10, and HReO,, as with these strong acids the anion is too weak a base to enter successfully into the competition and only the water and the organic extractant need to be considered. The extractant used in this first study was tributyl phosphate (TBP) ; later reports will deal with both more and less basic compounds. The aim in the series of investigations is to determine the nature and composition of the extracted species and see how these vary with the basicity and steric availability of the organic coordinating group, the water activity, and the base strength of the acid anion. A considerable amount of work already has been published on the HC104-TBP and HRe04-TBP extraction ~ystems.~-g Where these data and those of our work overlap, there is agreement. But much of the earlier work is either incomplete for the present purpose or deals only with extraction into either pure or concentrated TBP solutions. As pointed out by Hesford, et ul.,10-12 one must be very cautious in drawing conclusions from calculations involving organic phase concentrations (as opposed to activities) in such solutions, as they are non-ideal mixtures of (solvated) ions, water, and TBP. One way to avoid this problem is to limit the concentration of the extractant to less than a few tenths molar in some inert organic liquid, and to choose the experimental conditions such that only a few per cent of the extractant molecules are involved in the extracted complex. Thus, the organic phase essentially retains the properties of the inert diluent, and changing the coilcentration of the extractant, acid, or water in that phase will have only a slight effect on the activity coefficients of these species. However, the resulting variation in the extraction then will yield the dependence of the extracting species on the particular component varied (all others held constant), and hence its part in the complex. After establishing in this manner the nature of the extracting species in the dilute extractant solutions, it is possible to extend the studies to the more concentrated solutions by making use of spectroscopic methods. This is the approach used throughout this series of investiga tions. Experimental Methods
Smith constant-boiling HC104,73.6%, and was used as purchased. The HReOb was prepared by dissolving 50 g. of KRe04 (Varlacoid Corp., New York, S . Y.) in 6 1. of distilled water and passing this solution through 100 ml. of Dowex AG 50 X 12, 50-100 mesh (Bio-Rad Laboratories, Richmond, Gal.) which had been thoroughly washed with 3 M HClOd and distilled water. The effluent was evaporated down to about 100 ml. and the HReOa solutions were made by accurate dilution of this 1.74 M HReOl with distilled water. Analysis showed
