laboratory. Potassium iodide is stable in the pare potassium nitrate-sodium nitrate ciutectic. This is surprising in view of the observation that chromium(111) and manganese(I1) are oxidized to chromate and manganese dioxide h y the pure nitrate melt. Gruen has commented on the fact that, ‘‘nitrate melts n ould exhibit oxidizing properties with nitrate ions acting as oxide donors in the presence of strong oxide acceptors like U(IV), Np(IV), and Pu(II1)” (6). These species are oxidized to uranyl(II), neptunium(V), and plutonium(VI), the latter two presumably existing as oxygen-containing complex ions. It would appear t h a t strong oside-removing entities, Lux-Flood acids, must be present for nitrate to function as a n oxidant. I n the case of the iodide in a nitrate melt, the addition of sodium metaphosphate immediately causes violet iodine fumes to be given off. Although the stoichiometry has not been studied, the rcaction may he indicated as:
so;
+ PO;
II--,
PO;^
+
+ KO2
ride is bubbled through the melt, chlorine gas is given off, and the orange color of dichromate gives n a y to the purple of chromiuni(II1) in solution. Water is also a product of the reaction. i f sodium metaphosphate is d d e d t o a chloride melt containing dichromate, chlorine is likewise lib(Jr.ited, and a green precipitate, probably a chromic oxide, is formed. If small amounts of potassium iodide are dissolved in t h e KC1-LiCl eutectic and small amounts of potassium nitrate added, there is no apbarent reaction, However, the addition of sodium methaphosphate or metavanadate, neither of which oxidizes iodide in the pure chloride melt, causes iodine evolution, mixed with brown fumes of nitrogen oxides. Sodium metavanadate. honrever, will oxidize iodide in the chloride melt in the presence of a strongtlr acid such as metaphosphate. Although the stoichiometry has not been studird, the reaction may possibly be indicated as: Yo; 2 Po; I - = ’/?1 2 +
+
+
2 poi3 (9)
Similarly, a heavy metal ion capable of combining with oxide, such as cobalt ion, when added to the iodide containing nitrate melt, also causes slow evolution of iodine fumes In KC1-LiC1 eutectic melts, for rwmple. potassium dichromate appears stalde. If anhydrous hydrogen chlo-
TO-* ( i o )
On the other hand, such species as uranium(1V) and chromium(II1) are readily oxidized by nitrate in KC1LiCl melts in the absence of any additional acidic species. Oxygen-containing species, such as nitrate, dichromate, or permanganate, function only as oxidants in the presence of Lux-Flood acids. Where the
rc duc.t :i nt concerned [chromium(I I I), uranium(IT’), manganese(I1) ] can function as a Lux-Flood acid, no additional acid is needed; where the reductants cannot (chloride, bromide, iodide), additional acid is needed .4n analogy can be drawn between these relationships in fused salts and thow postulated by Duke as taking place ir, aqueous solution. Duke postu1ati.s that “an oxidation-reduction reaction is preceded by, or is simultaneous with, a generalized acid-base reaction’’ ( I ) . ACKNOWLEDGMENT
The aid of R. W. Schaefer in the mass spectral analysis is appreciated. LITERATURE CITED
(1) Duke, F. R., ANAL.C m v . 31, 527 (1959’). ( 2 ) Duke, F R., Iverson, hl. L., Ibid., 31, 1233 (1959). (3) Duke, F. R , Iverson, hl. L., J. Am. Chem. SOC.80, 5061 (19%) (4) Flood, H., Forland, T., Acta Chem. Scand. 1,592 (1917). ( 5 ) Gruen, D. XI,, Proc. 2nd U S . Coni. on Peaceful Uses of Atomic Energy 28, 112 (1958) ( 6 ) Laitinen, H. A,, Ferguson, Vir., Ostervoung, R. A., J . Electrochem. SOC. 104, 516 (1957). ( 7 ) Lux, H., 2. Elektrochem. 4 5 , 303 (1939).
RECEIVED for review September 9, 1959. Accepted December 17, 1959. Work supported in part by the U. 9. Atomic Energy Commission under Contract AT(30-3)-241.
Extraction of Thorium with Thenoyltrifluoroacetone Effect of Acetic Acid GERALD GOLDSTEIN, OSCAR MENIS, and D.
L. MANNING
Analyfical Chemisfry Division, Oak Ridge Nafional laboratory, Oak Ridge, Tenn.
b
The effect of acetic acid on the extraction of thorium with thenoyltrifluoroacetone in carbon tetrachloride and in hexone was studied. To account for the enhanced extraction of thorium in the presence of acetic acid, reactions are proposed which involve the formation of thoriumthenoyltrifluoroacetone-acetic acid addition compounds and a thoriumacetate complex. Approximate equilibrium constants for the proposed reactions were calculated. The selection of the organic solvent will depend upon its effect on several factors which influence the distribution of thorium. Among these are the miscibility of the phases, the solubility
400
ANALYTICAL CHEMISTRY
of the metal chelate in the organic phase, and the distribution of acetic acid and thenoyltrifluoroacetone between the organic and aqueous phases.
T
IIENOPLTRIFLUOROACETONE (TT-4) is an extremely useful chelating agent for the solvent extraction of numerous elements, and, in particular, for extraction of elements which are of interest in the atomic energy program, such as thorium, uranium, and zirconium. The TTA solvent extraction tecxhnique has also been utilized to determine the stability constants of some thorium complexes ( 2 , 9) from measuremeiits
of the decrease of the distribution ratio of thorium when complexing agents were added to the aqueous phase. Attempts to measure the stability constant of the thorium acetate complex by the addition of acetic acid to t h e aqueous phase failed because the distribution of thorium into the organic phase, instead of decreasing, became larger ( 2 ) . As this effect may be useful in extractions and separations, a detailed study was made of the effect of acetic acid on t h e evtrsction of thorium. Two organic solvents for the TTA, carbon tetrachloride and methyl isobutyl ketone (4-methyl-2-pentanoneJ hexone), were evaluated.
4
Figure 1 . Distribution of thorium into 0.1OM thenoyltrifluoroacetone
aqupous phase are considered, then [HT]: D=K1-[H+l:
5 5 M
If acetic acid had the effect of slightly decreasing the activity of the perchloric acid or increasing the activities of the TTA, a large increase in the distribution ratio of thorium would result. Any effect of acetic acid on the activity of perchloric acid or TTA, hoiyever, would result in a similar increase in the distribution of other elements besides thorium. Because it has been shown that acetic acid causes a decrease in the extraction of lanthanum ( 6 ) , it is not likely t h a t the activities of perchloric acid or TTA are substantially affected by acetic acid under these conditions. An increase in the activity of thorium perchlorate is also unlikely. T h e equilibrium constant in the previous equation
/'
0. I
KI = 02
34
06
18
10
I2
PH
EXPERIMENTAL
Reagents.
YsTh(CIO~)iY$T 'YThTi Y'HCIO(
TTA !vas purified by
LNO i.ecr.ystallisarions from hexane, After \\ Lich 0.1011/1sulutions in CC1, i.nd licsoirie were prepured b y direct
cr-free Th234tracer !vas pre*xired wcording to the procedure of Cowan (1 1. For this tipplication, the thoriuni SXI separated from 40 grams of CO2+-03)2.6 €120 and made up to :t mal volume of 25 ml. in a perchlorate :iiediuni. Procedure. Stock solutions were containing from 0 to 5 . 5 M cid and having p H values t o 0.9. T h e n i5 ml. of t h e +wK wlution were pre-equilibrated with 15 nil. of 0.lOM TTA for 2 hours a t 25" C', E q u d volumes of both phases (?ii nil.) were withdrawn, tine r h Z 3 *:raLer v a s added, and the phases $rere again equilibrated for 2 hours. Jiquots ui each phase were withdrawn, '-hc :activity of the Paza4dsilghter SI" 1hZ1*was determined by essentially the n e t h o d :>E Day and Stoughton (5'): ,ntd the 4stribution ratio of thorium vas determined. The final p H of the dtqucous phase was carefully measured. RESULTS AND DISCUSSION
T h e accuracy of the distribution measurements was monitored by obtaining & materials balance of the total activity in both the organic and aqueous phases in each series of experiment;. The coefilcieni variittwn of the t o t d ,.ctivity nevw exceeded 39''.
CC14 Solvent. T h e data obtained .!istribiitlon of thori!iin into CC,'li a r e presented in Zigure 1 as a :)iot of log D z's. pH of t h e best straight lines determined by t h e method of ieast square;?, where D = 2 [Th j organiciZ [Th] aqueous. I n the absence of acetic acid over t h e p H range studied, thorium exists as the uncomplexed ion, Thi4 (6j, and the overall equilibrium involved in the extraction of Th+4 can be represented b y Th,"
-+ 4 HTo * ThTdo + 4H:
(1)
There ET refers to TTA, and subscripts a anti o refer to the aqueous and organic phases. respectively. Then the equilibrium constant for this reaction is Ki
=z
'ThT~],[H+],4,/[Th+~].;RT]~
and DO = :ThTa],;[Th"'. =s ic,[HT]$/ [HAj: ( 2 ) From Equation 2 and the 2ata obtained, t h e average vaiue for K l was d c u l a t e d . Values for XI 2nd the other constants are shnn-n in Table I. presence of icetic cLridjthe on of thorium h t n t h e organic r h s e increased as the wetic w i d concentration was increased and the slope of the straight lines, d log D / d pH, decreased from 4 to 3. Several explanations for the greater distritmtion of thorium were evaluated. If the Activities of the species in the
KjDThTnKfIDhi
nliere K , is the formation constant of the ThT4 chelate, K , is the ionization constant of TTA, and DThTa and DHT are the distribution ratios between the organic and aqueous phases of ThT4 and TTX, respectively. Although all these terms probably vary slightly with changing ionic strength (Q), the variation would not be great enough to cause the large increase in the distribution of thorium which n as observed. For example, the distribution ratio of TTA, DHT, \vas determined by t h e spectrophotometric method of King and Reas (4) and n-as found to be 0.022 in the absence of acetic acid and 0.023 when t h e aqueous phase was 0.5.11 in acetic acid. Other possible explanations for the increased extractability of thorium in the presence of acetic acid are the formation of neutral thorium acetate ion association compounds, such as T h ( A ~ ) ~mixed o r species such as ThT&, nhich could also be extracted. I n the first case it should be possible to extract
Table I. Calculated Values of Equilibrium Constants H-, AI K1 K2 k-3 KC 0.10 1 0 . 9 0.31 132 0.81 0.16 10.5 0.30 139 0.81 1 0 . 1 0.28 148 0 . 7 6 0.25 0.36 9 . 9 0 . 2 7 161 0.73 0.63 9 . 4 0.26 165 0 . 6 7 1 0 . 2 0.28 147 0 . i 5
Coefficient
of
variation, 7o 6
8
VOL. 32, NO. 3, MARCH 1960
9 8
401
thorium from an acetate medium with CCI, without nny TTA; however, no thorium was extractcd from a 5.5M acetic acid medium whcn no TTA was present. If miscd species w r c formcd, the number of moles of T'QA associated with each molc of thorium would be less th:m 4. If thc thorium is cstracted from a .MYacetic acid medium a t a fixed pII, and the concentration of TTA in tlic organic ph:m is varicd, d log D/d log [IITj is equd to the molc ratio of TTA to thorium m d was found to bc 4. To account for the increased extraction of thorium, thc formation of a n addition compound of thc general formula ThT,. IIAc is postulated, In the prescnce of wetic acid the following reactions arc proposed. Th:' H A C . e T h h +'~ : €1: (3) K I = [ThhcC3][H+]/jTh"] [HAC] and ThAc:' 4 I-IT, " ,ThT4.IIAco 4-
+
+
5.
I
1
I
4t
Figure 2. Thorium cornplex stability Constants as a function of acid strength
I
3
2 109
from which K , w*asevaluated. Although no independent evaluations of these cquilibrium constants are KI [ThT,.HAcj[H+]'/[T~AC+'] [HT]' avnilable, the cquilibrium constant for When the nqucous plinsc WIM 0,4M tho forinrrtioii of ThAciJ could bo 'in acetic acid, thc slope of the plot of verificti by other means. log D us. pH, d log D / d pH, was 4, The stability constants of the thorium indicating that the thorium in the complcxes with tri-, di-, and monoaqueous phase was essentially in an chloroacetic acids havc bcen calculated uncomplexed form. If the thorium in (S), and the ionization constants of all the aqueous phase was present as fuur acids are well known. In a series ThAc+S and the extrnction proceeded of acids such as this, a linear rclntionship as in Equation 4, d log D / d pH would be between the log of their ionization eon3. stants and the log of the dissociation Therefore, the species present in the constnnt of the first thorium complex orgnnic phme were ThT, and T h T t . should exit (7'). Accordingly, the &aHAC, and Th+4 was present in the bility constant of the thorium-acetnte aqueous phase. The distribution ratio complex was calculated, K2,'Ka = of thorium is then: 15970, and n plot (Figurc 2) was made D 5 [ThTJo [ThTd.HAc]o/[Th+']a of the pK. of the acids us. log K of the thorium complex stability constants. KI KzK; [HAC] (5) The calculatcd value of the thorium Dp( KI acetate stability constant (log K = from which the product, KtK8, cnn be 4.20) is in excellent ngreemcnt with the evaluated. anticipated value (log K = 4.18) from When the acetic acid concentration the best straight line of the data for the o f the aqueous phase was increased to chloroacctic acids. 3 . 5 M , d log D / d pH wm 3, indicating The calculated equilibrium constants that the thorium in the aqueous phnse must be considered approximate bewas present as the complex ion, ThAc cause of scvcral simplifications which Therefore were made in their calculntion. I n 0.4M acetic acid it WRS nssumcd that no ThAc+J was present in the aqueous phase, and in 3 , W acctic acid it was assumed that all thc thorium in the As K I nnd K I K Sarc known, K , and K, aqueous phase was present as ThAc+a. were calculated. The small amount of T h T + J which is The results of the cxtractiois from also known to be prcscnt in the aqueous the medium containing 5.5U acetic phase was not considered. In addition, acid, which show a further increase in the possible cffccts of vnriation in the the distribution of thorium while d ionic strength wcre not evaluated. log D/d pH way 3, suggest the formation However, the close agreement between of a second addition cornpound, ThT4.the theoretical and actual values of Kz 2BAc. indicatcs that the error due to thcse ThTd.HAc HAC S ThT4.2 HAC (7) simplifications is small. Kc = lThT4.2 HAC]/[ThTd.HAc][HAC] The concept of a thorium-TTA addiIn this c88e tion cornpound with acetic acid ia 3
JX'(4)
E
+ +
)
+J.
+
4
5
K
supportcd hy the work of Hok-Bernstrom (S), which s h o w that an addition compound is formed between the thorium salicylic acid chelntc and a n adtlitiond molc of undissociated snli= cylic acid. Hexone Solvent. T h e d a t a for the extraction of thorium with O.IOM T T A i n hexone a r e shown i n Figure 3. In gcncral, the extraction of thorium with T T A in hexonc i n the presence of acetic acid dcpcnds on the same reactions as the extraction with T T A in CCl,: forniation of a thoriumTTA-ncctic acid nddition compound, and fornintion of a thorium-acetate complcx. Thc organic solvent utilized, hon.evcr, has an cffcct on several factors which inff ucncc the distribution of thorium. Among thcse are the miscibility of the phases. thc activities of the specics in the aqueous phase, and the distrihution ratio of acetic acid and TTA. Because scvcral factors are involved which are not independent of one another nnd it is not certain that all the variable factors have been identified, it is not possible a t this time to describc adequately the extraction of thoFium with TTA in hexone by appropriate chcmicnl equilibria. However, compared to the data whcn CClr was thc solvent, the results of these experiments can be summarized 88 follows : In thc absence of acetic acid, tbe distribution of thorium into TTA in hexone is approximntcly the same as into TTA in CCI,. Values calculated for K , are 10.2 for extractions with TTA in CCId and 9,Q for extractions with TTA in hexone. In the presence of acetic acid, the distribution of thorium into TTA in hexone i n c m e , indicating that the thorium-TTA-acetic acid addition compound is formed. However, the dis-
PH
Figure 3. Distribution of thorium into 0.1 OM thenoyltrifluoroacetone in hexone
tribution of thorium into TTA, in the presence of a given concentration of acetic acid, when hexone is used as the solvent, is not as great as the distribution when CC1, was used as the solvent. When hexone is used as the solvent, as the concentration of acetic acid in the aqueous phase is increased up to 2.8M, d log D/d p H decreases from 4 to 3, indicating that a thorium-acetate complex is formed in the aqueous phase. When the acetic acid concentration is further increased to 3.6X, d log D / d p H w s found to be 2.3, a value lower than can be accounted for by the formation of a thorium acetate complex. On the other hand. for the extraction of thorium Fith T T A in CCI4, a d log D / d p H value oC,3 was found when the aqueous phase was either 3.5111 or 5 . 3 4 in acetic acid. i n a n experiment in which the p H and acetic acid concentrations of the aqueous phase n-ere held constant and the concentration of TTA in hexone varied from 0.02 to O . l O J f , d log D/d !og [HT] was 2.7, as compared to 4 found under similar conditions when CC14 was used as the solvent. No thorium was extracted in the absence of TTA .
The effect of the organic solvent on the ext'raction of thorium is probably due to several factors. While CCla and water are essentially immiscible, hexone does distribute into the aqueous phase. The concentrat.ion of hexone in the aqueous phase was determined spectrophoton~etricallyand depends on both hydrogen ion and acetic acid concentration (Figure 4 ) . The miscibility of hexone must certainly affect the sctivities of ail the species in the
Figure 4. solutions
Equilibrium concentration of hexone in aqueous
aqueous phase. as well as the distribution ratios of TTS and the thoriumT T A chelate. I n addition, acetic acid does not distribute into CC14 to a n y great extent; however, from 20 to 30% of the acetic acid present distributes into hexone, as shown in Figure 5 . Acetic acid concentrations, in the presence of perchloric acid. were determined by conductometric titrations with standard NaOH. The distribution ratio of T T A between aqueous acetic acid solutions and both solvents was measured by the ultraviolet spectrophotometric method (4). The distrihution ratio from hexone (Figure G) depends on hoth the hydrogen ion and acetic acid concentrations. This variation in the distribution of T T A , DHT,can be expected to affect the distribution of thorium. When the distribution ratio of metal is measured as a function of p H a t constant reagent and acetic acid concentration, if DHT varies with p H . then
=
.(%)"
(10)
where I< is the product oi the constant terms and the other terms have their usual significance. Then
1-(
d log D / d pH = n
+ n (11)
as DHT is a linear function of [H+],
DHT= A[H+]
+B
dlogD/dpH =n+n(dlog(A[H+]
4-
B)/dpH) (12) =
n ( B / A [ H + ]+ R ;
(13)
Therefore, when DEITis a function of [H+], d log D'd pH is not equal to n but is less than n. depending on the values of il and B. The increased distribution of T T A into the aqueous phase in the presence of acetic acid, therefore, is probably a contributing factor in causing the values of d log D / d p H vihich were found in the ?horium distribution experiments to be low. For the extraction of thorium with acetylacetone, Rydberg (8) concluded t h a t ail four cornplr~es-ThAa+~, ThAa2+2, ThAa3+, and ThAar, There Aa refers to the acetylacetonate anionwere present in the aqueous solution. TJ'nder these conditions it can be shown that both the values of d logD/d p H and d log D Id log [HAa] will be less than 4. lIoreover, the distribution of thorium into acetylacetone in hexone is subqtantially different than into acetylacetone in CHCls, and this difference was attributed to the formation of a complex b e t w r n ThAa, and hexone molecules. Although the reactions of TTA. are probably very similar t o the reactions of acetylacetone and it is well known t h a t small amounts of Th(T)+s are present in t h e aqueous solutions SOL. 32, NO. 3, MARCH 1960
403
I
O€
0
0.I
1
I
I
I
rWQ0
D:
I
b!
0
0.23
’’
0
0.56 1.1 I 2.10 4.06
0.4
I)
3
0.2
0
Figure 5. Distribution ratio of acetic acid as a function of the acetic acid concentration of the aqueous phase Organic phase, hexone
after od,r:ictioii with TTA in bcnzcnc, thc d a h prcscntcd hcrc: lire not suificicrit to support siniilnr conclu~ionsfor the cstr:rction of t!ioriiini \\it11 TTA in Iicsonc. CONCLUSION
Generally, for t h e nnalyticnl separation of t u o clcnicnts Iiich form cxtraetable coin~rouiicl~ T\ ith thc s:inw reagent, tkc p1-I of thc iujiicous p h is ntljustcd ~ ~ so that onc of tlic t’l(mciits is coinl)lctciy e.rtrocted nhile thc other is not estracted. In ninny instances the extrnction clmxctcriqtics of sonic pairs of clcmciits arc so siniilnr that it is not possible to s q w i t c t h m qunntit:itivcly hy siinpiy controlling thc pH nt which the cstrnction is carried out. In some of thcse cases a coniplcsing or ~nasking ngent can be added which forms n &table, noncstrnctable comaound with one of the components and prewnts its extraction. but docs riot prcvcnt ntraction of thc second componeni,. The work reported here demonstrates that ncctic acid, which acts as a complcxing ugcnt in thc extraction of lanthanum with TTA, incrcascs the extraction of thorium, probnbly through the forinstion of thorium-TTA-acetic acid addition compounds. This incresse in the extraction of thorium in the prcsence of acetic acid combined with the decreased ehraction of other elements has been extremely useful in dcveloping methods e
ANAlYnCAL CHEMISTRY
0
I
I
02
0.4
I
0.6
CHq. !
f
0.a
I 1.0
Figure 6. Distribution ratio of thtlioyltrifluoroacetone between aqueous acetic ncid solutions and hexone
for scpnrnting thcsc elements in the annlysis of thorium oxide. As the ~ formiltion of addition e o ~ n p o u n t ihns also been rcportcd in the estraction of the uranium-salicylic acid ctielntc, it is possiblc tli:~t other elements can also form metal chclntc addition conipounds of various types (5). The sclcction of tlic organic solvent is an important fnctor in perrorming nnnlyticnl qcparations and n.ill drpend t o n lnrgc cstcnt upon the specific application under study. AIthough in most previous work involving TTA cstrnctions benzene was utilized as the solvrnt, CCll n:is tested bccnuse it is not fln~nninblc, is convcnicnt in cstraction work because it is heavier thnn wntcr, nnd is nvailnble in reagent grade quality. Hcsone wns tested bccausc it is a very uscfui medium for the flame phof o~netricdctcrniinstion of numcrous c l m c n t s following their extraction (6). The equiiibriuni constant, Ki, is 25.4 for benzene (Z), 10.2 for CClr, and 9.9 for hcsonc. For the extraction of microgram qunntities of thorium from perchloric acid incriiurn, therefore, the highcst efficiency is obtained when bcnzene is used as the solvcnt. For extractions from nn acetate medium, CC1, was preferable t o hcxonc. Several other factors must also be considered, such as the solur bility of the metal chelate in the orgmic
solvent and tlic cffcct of ncctic acid: w t n t c or otlicr buffer systems on the miscibility of tlic phnscs. ACKNOWLEDGMENT
Tht nutliors acknowlctlgc t k nssistnncc of H. P. Housc in the prcpnration of thiP rcport. LITERATURE CITED
(1) (;o\v:in G. A., “Prepnrat iim of Ciirrier-
1:rce UX: (TIL**‘) Tracer,” U. S. Atomic Encrgy Comm, LA-1721 (195G). ( 2 ) I h y , R. h., Jr., Stoughton, IL W.,J . ,.t?i). Chetn. SOC. 72, 5662 (1950). (3) Iiok-Bernstrom, E., A d a Cheiit. S c a d . 10. 174 il9iiG’i. ( 4 ) King, E. L., Reas, W. H., J . Am. Clicvi. Soe. 73, 1804 (1951). ( 5 ) Kmus, I
