THE JOURNAL OF
PHYSICAL CHEMISTRY (Registered in U.S. Patent Ofice)
(@I Copyright, 1963, by the American Chemical Society)
VOLUME67, NUMBE~R 2 ~~
FEBRUARY ~
21, 1963 ~~~
~~~
SYNERGISTIC EFFECT OF TRI-n-OCTYLAMINE QPI; THE SOLVENT EXTRACTION OF THORIUX BY THE~BYLTRIFLUQROACETO~El BY L. NEWMASAND P. KLOTZ Brookhaven National Laboratory, Upton, New York Received M a y 10, 1961 The synergistic effect of tri-n-octylamine (RBN)on the solvent extraction of thorium by thenoyltrifluoroacetone ( I I T ) has been shown to arise from the organic phase reaction ThTa RpKHCI = ThT4R3SHC1,where R3NHC1 represents the amine hydrochloride and T-, the enolate ion of HT. The formation constant for this reaction was measured in benzene a9 (2.6 4 0.4) X lo2. It has been demonstrated Chat the amine species, R3NHClHT and R3NHT do not give rise t o a synergLstic effect. It is concluded that the amine hydrochloride is attached to one of t h e thenoyltrifluoroacetone molecules on the thorium and not to the thorium itself. The synergistic efM , H + from 0.1 to 0.5 fect was studied over the ranges: H T from 0.05 t o 0.2 M,thorium from 1.0 to 25 X M , and C1- a t 0.1 and 2.0 M .
+
Introduction The term, a synergistic effect, is geneially reserved t o describe an unexpected cooperative action of discrete agencies such that the total effect is greater than the sum of the two effects taken independently. The term was first utilized in the field of solvent extraction by Blake, et a1.,2 t o describe their discovery of an enhancement of the extraction of uranium from aqueous solutions by mixtures of a dialkylphosphoric acid and certain neutral organophosphorus esters. Prior to the observations of the above vorkers, it was noted by Cunningham, Scargill, and Willis3 that the (addition of tri-n-butyl phosphate (TBP) increased the extraction of praseodymium and neodymium by solutions of thenoyltrifluoroacetone (TTA). Recently, a number of papers have appeared which deal with thle nature of the species that cause the synergistic effect in systems containing TTA and a phosphate ester.4r5 Most of the attention has been directed to studies of tlhe actinide elements but Healy has also investigated some rare earths and alkaline earths. It has been found that the phenomenon can be explained by an organic phase reaction whereby there is an addition of one or two molecules of a phosphate ester onto the neutral 'TTA complex of the metal. Only in the case of the tetravalent actinides has a replacement reaction been observed4 and even then, only for extractions from ni-
trate medium but not from chloride.6 Quite recently, a paper has appeared in 1Thich an attempt is made to classify various extraction systems containing mixtures of two extracting agents.6 For some time ~ O T V ,we ~ have been concerned with the synergistic effect exerted by tri-n-octylamine (TKOA) on the solvent extraction of thorium by TTA. The extent and nature of the various equilibria be1,ween the amine, TT'A, and hydrochloric acid have been evaluated.8 The subject of this investigation was to ascertain which amine species cause the synergistic effect, the nature of the species formed, and the values of the equilibrium constants involved. Experimental The source and purification of TKOA and TTA have been described8; thorin, l-(o-arsonophenylazo)-2-naphthol-3,6-disulfonic acid, was obtained from Eastman Organic Chemicals, Rochester, S . Y. All other chemicals were of standard rcagent quality. Benzene was used throughout as the organic diluent. Equal volumes of both phases were contacted, utilizing a reciprocating shaker, for a period of 2 hr. to obtain equilibrium. The studies were performed in an air conditioned room where the temperature was maintained a t 25'. The two phases were separated by centrifugation. n'hen the distributions were such that most of the thorium was in one phase, only the phase which had the least amount of thorium was analyzed. The concentration in the other phase was determined by difference. Both phases were analyzed when the amount of thorium in each phase was of comparable Concentration. Analyses of the aqueous phases mere accomplished by taking an appropriate size aliquot to dryness and destroying the residual organic with a nitric-perchloric acid mixture. The residue was redissolved in hydrochloric acid and the thorium concentration
(1) Research performed under t h e auspices of t h e United S t a t e s Atomic Energy Commission. (2) C. A. Blake, C. I'. Base, K. B. Brown, C. F Coleman, a n d J. C. White, P r o c Becond Intern. Conf Peaceful Uses At. E n e r g y , Geneva, 1968, 28, 289 (1959) (3) J. G. Cunningham. P. Scargill, a n d h[. H. Willis, A . E . R /C/M-215
(6) S. Siekieraki a n d ill. T a u b e , Sukleonzka, VI, 489 (1961). (7) Brookhaven Xational Laboratory Annual Report, J u l y , 1960, p. 117. (8) (a) L. Newman a n d P. Xlotz, J . Phgs. Chem., 65, 796 (1961): (b) submitted for publica tion.
(1954). (4) H.Irving a n d D. N. Edgington, J . Inorg. Xucl. Chem., 16, 158 (1960); 20, 314 (1961), 20, 321 (1961); 21, 169 (1961). ( 5 ) T. V. Healy, zbzd., 19, 311 (1961); 19, 328 (1961).
205
L. NEWMAN AXD P. KLOTZ DAT.4 FOR THE IXTERPRETATION O F THE SYNERGISTIC
c -
Concn. moles/l.
-
~
H+
HT
Thorium x 104
0.10 .IO .10 .50
0.05 .10 .20 .10 .20 .10
5 0 5 0 5 0 6.0 5.0 25.0
.50 .50
-Fractional RaNHCl
0.56 .39 ,25 40 .26 .40
TABLE I EFFECT OBTAIKED FROM SOLUTIONS >lAIXTAISED WITH LITHIUM CHLORIDE
distribution of the amineRsNHClHT RaNHT
0 43 .59 72 .60 .i3 .60
determined spectrophotometrically with thorin.9 The procedure was modified so that the dye concentration was 0.02% and the acid concentration 0.5 M . The analyses of the organic phases were performed by stripping with an equal volume of 3 M hydrochloric acid and then treating an appropriate size aliquot of the strip as above. When the ionic strength was maintained a t two with lithium chloride, the size of the aqueous aliquot was limited by the effect of chloride ion on the analysis of thorium by thorin. Consequently, the usual procedure was t o extract the thorium remaining in the aqueous phase into a 0.5 iM TTA solution and then t o extract with 3 M hydrochloric acid. This solution was then analyzed as described above. Each distribution series was performed by contacting organic phases containing a constant TTA concentration and a varying amine concentration with aqueous phases containing a particular composition of thorium, hydrochloric acid, and lithium chloride. Each series consisted of fifteen organic solutions in which the M. amine concentration was varied from 0.04 to 4.0 X In order to keep the free TTA concentration constant, the HT = amount of TTA consumed by the reactions RaN RINHT and R3S H+ C1H T = R3NHClHT was calculated from the measured equilibrium constants and the conditions of chloride and hydrogen ion concentration of a particular series. In order to make this calculation, the competitive reaction R3N H+ C1- = R3NHCl must also be considered. The appropriate compensation was made by preparing an organic stock solution containing 4.0 X 10-2 M amine plus the calculated analytical concentration of TTA t o provide the desired equilibrium concentration of free TTA. I n order to bring this solution t o its equilibrium condition, it was pretreated by a procedure which consisted of three 10-min. equal volume contacts with aqueous solutions containing the hydrochloric acid and lithium chloride concentrations corresponding to the particular series. The various levels of amine were obtained by mixing aliquots of this stock solution with an organic solution containing TTA a t the desired free concentration. Thus, the free TTA concentration was conveniently kept constant obviating an adjustment for each of the amine levels. These solutions were always used on the day of preparation. The various levels of TTA, thorium, hydrochloric acid, and lithium chloride studied are outlined in Tables I and 11. General.-The over-all equilibrium constant for the extraction of thorium from a non-complexing medium can be written as
+
+
+
+
+
Vol. 67
+
where H T stands for TA, and T- for the enolate ion. Unless otherwise indicated, in this and all subsequent equations, the charged species are in the aqueous phase and the uncharged species in the benzene phase. If the ionic strength of the aqueous phase is maintained a t two, then a value of 22 is obtained for this equilibrium constant.10 When the ionic strength is maintained with lithium chloride, it is necessary to account for the effect of complexation by the chloride ion. Under these conditions the free thorium ion concentration is
(9) P. F. Thomason, h i . A. Perry, a n d K. RI. Rgerly, Anal. Chern., 21, 1239 (1949). (10) I,. Newrnan a n d P. Klotz, unpublished results.
0.01 .02 .03 .00 .Ol 00
.
AT B N IONIC STRENGTH OF T W O
Intercept
1 . 4 X lo-' 1.9 2 . 2 x 10' 5 . 6 X lop3 6 8 x 4 . 9 x 10-3
2.3 X 1.9 x 1i x 6.4 X 4.8 6.0 X
10' 102 103 10-l
3.8 3.2 3 9 3.7 3.3 3.9
IO-'
where (Th)&is the total concentration of thorium in the aqueous phase after equilibration. The equilibrium constants are defined as
(3) where n takes on the values 0 t o N and consequently Ko,cl is defined as unity. The synergistic effect of TXOS on the solvent extraction of thorium by TTA can be caused by any or all of the amine species which are formed when a benzene solution containing both TNOA and TTS is contacted with an aqueous phase containing hydrochloric acid. If only T S O A is present in the benzene phase then upon contact an amine hydrochloride is formed which remains soluble in the organic phase. The extent of this reaction can be expressed by the equilibrium constant
Kci
=
(RaSHCl) (R&) (H+)(Cl-)
where R3X represents TSOA. The value measured for Kcl is (1.3 =t 0.3) X 104.8a If TTA is also present in the organic phase, then an acid-base reaction occurs with the amine. This reaction can be expressed by the equilibrium constant
(R3n"T)
K - (R3N)(HT) The value measured for KT is (1.4 =t0.1) x
103,ss An addition
reaction also occurs which can be expressed as
K c ~ ,= T
(R3NHClHT) (RaS) (H+)(Cl-) (HT)
(6)
The value that was measured for KCLT is (2.1 0.4) X 105.8b It is of primary interest to ascertain which of these species contributes to the synergistic effect. The initial experiments were designed to limit the number of species and t o ascertain whether the synergistic effect can be explained by the changes in concentration of any of the species present. Finally, experiments were designed permitting the formation of additional amine species and evaluating their contribution t o the synergistic effect. To simplify the interpretation, the study was conducted under conditions where there is negligible aqueous complexation of thorium by the TTA.10 Accordingly, the study of the synergistic effect was limited to TTA concentrations 5 0.2 ill and t o acid concentrations 2 0.1 M .
Results Evaluation of The Species RsNHCl and R3NHC1HT. -When the acid Concentration is at least 0.1 M , the amount of TNOA which is present as free amine is negligible, less than 0.02% of the total. If the concentration of chloride ion in the aqueous phase is set a t two molar, then the fractional amount of RZHNT can be kept to less than 3y0 of the total amine present. Consequently, under these conditions, the only species that need be considered are R3SHC1 and R3NHC1HT. Table I summarizes the conditions and the fractional distribution of the amine for a series of six experiments. It was found that there was a general increase in extrac-
SYSERGISTIC EFFECTOF TRI-V,-OCTYLAMINE ON SOLVEXTEXTRACTIOK
Peh., 1963
207
tion with increasing amine concentration. For each of the six series, a plot was made of the distribution coefficient, D , ve. the total amine concentration, (R&, where the distribution coefficient is defined as
D = - -(Th), (Th),
- 2
thorium species in organic phase -
Z: thorium species in aqueous phase
(7) I n each case a straight line relationship was obtained. a typical plot is shown in Fig. 1. It was observed that the increase in the distribution coefficient is not due to the extraction of thorium by TKOA. Furthermore, the extraction of thorium by trioctylamine has been shown to be negligible under conditions even more favorable for extraction than employed in this study." Consequently, the enhanced extraction can only occur by the formation of an organic species containing thorium, TTA, and TSOA. Since, in each experiment, the free TTA concentration was maintained constant, the first power dependency of D on ( R 3 N )indicates ~ that the species which causes the synergistic effect contains one amine per molecule. Considering that TTA extracts thorium as ThTd, and there are two different amine species available for reaction, it would appear that there are two possible products which can form in the organic phase by means of the reactions ThT4
+ R3SHC1 = ThTIR3KHC1
(Sa)
and ThT,
+ R&HClHT
=
ThT&&HClHT
TOTAL CONCENTRATION OF AMINE ( m o l e s / l i t a r ).
Fig. 1.-Typical data demonstrating the first power dependence of the distribution coefficient with total amine concentration: circles, data for the third experiment in Table I; squares, data for the fourth experiment in Table 11.
In the experiments which were performed, the concentration of thorium was lorn enough so that the amount extracted did not significantly alter the amine concentrations. Consequently, the total amine concentration can be expressed by (R3N)t
=
(R3n"Cl)
+ (R3NHClHT)
(14)
The term R3iYHC1HT can be eliminated from this expression by substitution of eq. 6 and 4, whereupon we obtain
(8b)
The equilibrium constants for these reactions are In the same manner RsNHCIHT can be eliminated from eq. 12; upon substitution of eq. 15 we can eliminate RdSHC1 and obtain and
D KTh,CI,T
=
(ThT4RSNHClHT) (ThT4)(R31;HClHT)
(10)
Under these conditions the distribution coefficient becomes
+ (ThT,R&HClHT) D=--(ThT4) + (ThTdR3NHC1) N 0
(11)
Substitution of eq. 1, 9, and 10 into 11 we can obtain
D
=
I
+ I[KT~,cI(R~"C~)-t-
K T ~ , c I , T ( R ~ H C ~1 H(12) T)
where the intercept, I , is defined as
The intercept corresponds to the extraction of thorium from chloride solutions due solely to TTA. (11) F. L. Moore, Anal. Chem., 30, 908 (1958).
=
I
+ h'i(R8N)t
(16;
where the slope, 81,is defined as
Equation 16 predicts the linear behavior which was observed in the plots of D us. (RsN)$. The intercepts and slopes of these plots are summarized in Table I. From the parameters of a given experiment and the known equilibrium constants, a value can be calculated for a term, S1[Kc1-tKc1,T (HT)]/I. By transposing in eq. 17, it can be seen that such a term should still be a function of the TTA concentration if the species ThT4R3NHCfHT can exist. In order to test this hypothesis, values €or the above term were calculated and are summarized in Table I. Activities for (HT') calculated from the expression of King and ReasI2 were employed in the calculation of this term. It is obvious that the value is independent of the level of TTA concentration. Consequently, the term KCl,T. K T ~ , c ~ , T ( H must T) be negligible; thus it was concluded that one of the postulated species, ThT4R3NHClHT, is insignificant. It nom remains to ascertain if the synergistic effect (12) E. L. King and W. H. Reas,
J. A m . Chem. Soc., 75,
1804 (1951).
L. KEWMAN AND P. KLOTZ
208
I
I
I
0
I
1
0. IO 0.15 ACTIVITY OF TTA (moles/ I iter).
0.05
i
1’01. 67
chloride which permitted an appreciable fraction of the amine to exist as R3NHT. The distribution experiments were performed in the identical manner employed for the previous evaluation. A summary of the conditions which were employed is presented in Table 11. Plots of D vs. (R3N)tyielded a straight line relationship. A typical plot has been included in Fig. 1. Since the first power dependence of D on (RsN)tis maintained, the only additional reaction which can take place under these conditions is ThT,
0.20
+ R3XHT = ThT4R3XHT
(20)
The equilibrium constant for this reaction is
Fig. 2.-Plot demonstrating the formation of ThTlRaNHCl from data taken a t an ionic strength of two maintained with lithium chloride: open circles, thorium 5 X 1 O - l M ; filled M. circles, thorium 25 X
can be explained by assuming that it is due solely to the formation of ThT4R3NHC1. If this is the case, then eq. 17 can be reduced and rearranged to
By including this possibility, an expression can be derived which is identical with that of eq. 16 with the exception that the slope, Xz,becomes
xz = x
+
This equation predicts that a plot of the intercepts divided by the slopes, which were obtained from the us. the activity of the TTA should plots of D us. (R”, yield a straight line. If a straight line is obtained, the value of the equilibrium constant, KTh.C1, can be obtained frm this intercept. Utilizing this value, the ratio of the equilibrium constants, &l,T/KCl, can be calculated from the slope. This ratio yields the value for the equilibrium constant for the formation of R3NHClHT from the organic phase reaction between R3NHC1 and HT
K c-~ , T (RINHClHT) Kcl
(R3SHC1) (HT)
(19)
A comparison can then be made of the value obtained in this manner with that obtained directly from the measured equilibrium constants. The plot of intercept divided by the slope vs. the activity of the TTA yields a straight line (Fig. 2 ) . The method of least squares yielded a value of (2,s jl 0.4) X loz for K T ~ , c ~The . value obtained for KCI,T/KCIis 1.5 f 5 , which compares very favorably with the value of 16 5 which is calculated from the measured equilibrium constants. The assumption that the species responsible for the synergistic effect is ThT4R3KHC1,under the conditions of these measurements, is validated because a straight line plot is obtained and there is good agreement between the value of this constant with an experimentally measured value. All the experiments but one were performed with an 114. The initial thorium concentration of 5 x one experiment which was performed with a thorium concentration of 25 x 10-4 M yielded results which mere not significantly different (see Fig. 2 ) . Consequently, the synergistic effect is independent of changes in thorium concentration. Evaluation of the Species R3NHT.-The previous experiments were performed under conditions which limited the presence of the amine species, RENHT. A series of experiments was performed without lithium
*
+
1
KTh,CIKC1(Hf)(GI-) KTh,TKT [Kci(H’)(Cl-) KT(HT) KcI,T(H+)(C~-)(HT) (22)
+
In the derivation of this expression, it was of course unnecessary to postulate the existence of the species ThT,R3NHClHT, but it was necessary to include the concentration of R3NHT in the expression for (R3N),. By transposing eq. 22 a term, Sz [Ka(H+)(Cl-) KT(HT) Kc~,T(H+) (Cl-) (HT)]/I, is obtained which should still be a function of the TTA concentration if the species ThT4R3KHT can exist. This hypothesis was tested by calculating this term from the parameters of a given experiment and the known equilibrium constants. In order to calculate the term, it was necessary to employ activities for (H+), (Cl-), and (HT). The activities were calculated from the usual
+
+
source^.^^,^^
It was possible to investigate two levels of TTA, 0.05 and 0.10 M . For values greater than this, the distribution constant was so great that it could not be measured accurately. For lower values, the free TTA concentration could not be maintained constant because of the extent to which it reacted with the amine. The above term was calculated from the data obtained a t these two levels of TTA. A summary of these values is presented in Table 11. As can be seen, they are independent of the TTA concentration and therefore demonstrate that the formation of the species ThT4R3SHTdoes not take place. As a result, the term KTh,TKT(HT) can be eliminated from eq 22. Consequently, the synergistic effect under these conditions is due solely to the formation of ThT4R3NHC1. From the data in Table I1 and the appropriate constants, a value was calculated for K T h , C l as 4.4 X lo2. Considering that so many experimentally measured constants are employed in this calculation, it can be considered that this value is in good agreement with the value of 2.6 x IO2 which was measured under conditions with fewer competing equilibria. The effect of thorium concentration was further (13) H. S. Harned and B. B. Owen, “The Physical Chemistry of Electrolytic Solutions,” Second Edition, Reinhold Publ. Gorp., New York, N. Y., 1950.
EXTRACTION OF ACIDSBY BASICORGANIC SOLVESTS
Feb., 1963
DATAFOR -Concn., HT
0 05 .05 .10 .10
.IO
THE
TPI'TERPRETATIOR
OF THE
209
TABLE I1 SYNERGISTIC EFFECT OBTAINEDFROM 0.11 M HYDROCHLORJC ACID SOLUTIONS S2 [Kci(H+)(Cl-)+KT(IIT) I
moles/l.Thorium x 104
-Fractional RsNHCl
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.
5 0 3.7 43
0.28 .28 .35 .35 .35
Slope 10-2
+ K c I , Tx( H10-4 +)(C~-)WI"I
9.5 5 5 5s 60 31
4.6 3.6 4.6 4.5 4.5
x
Intercept
58
26
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 t h e
U. S.
Btomic Energy Commission.
phat&, ketones, and ethers.
-
-
