was reported to exist between pH 3 and 5.5, accounting for the absorbance maximum at 515 nm; (c) the pK, values for oxalic acid are 1.23 and 4.19 for the first and second dissociations, respectively; therefore oxalate would exist largely as C20d2at pH 4.8; (d) the complexation of the uranyl ion is probably bidentate with respect to PAR- and oxalate; and (e) the PAR- regenerated in reaction 111 could then “re-enter” reaction I to explain the increased absorbance at 390 nm presumed to be due to an increase in the PAR concentration as a result of the oxalate interaction with the U02PAR+complex.
In conclusion, it is believed that a valuable new method for the determination of oxalate has been added to the analytical capability for anion analysis. The method should be especially helpful to researchers in the area of water resources research. RECEIVED for review July 14, 1971. Accepted September 19, 1971. This work was supported by the Office of Water Resources Research, Department of Interior, under Grant OWRR NO.A-014-MO.
Homogeneous Liquid-Liquid Extraction Method Extraction of Iron(ll1) Thenoyltrifluoroacetonate by Propylene Carbonate Katsuo Murata, Yu Yokoyama, and Shigero Ikeda
Department of Chemistry, Faculty of Science, Osaka University, Toyonaka, Osaka, Japan
A new homogeneous liquid-liquid extraction method by propylene carbonate was devised, which is characterized by immediate formation of the complex upon achieving a single homogeneous liquid phase at elevated temperature. Two distinct and separable phases appear upon cooling to room temperature. This method was applied to the extraction of iron(ll1) thenoyltrifluoroacetonate, which i s known to extract at a slow rate. Iron(ll1) was rapidly and completely extracted by using this proposed method, in contrast to the several hours required for complete extraction by conventional liquid-liquid extraction at room temperature. The nature of the mixed solvent, the chemical state of thenoyltrifluoroacetone (TTA) in some solvents, and the formation of iron(ll1) thenoyltrifluoroacetonate were also investigated in order to obtain information about the extraction behavior in comparison with the usual liquid-liquid extraction. THELIQUID-LIQUID EXTRACTION method has been extensively applied to studies of chemical equilibria, the separation of different elements, and the synthesis of inorganic compounds. Some problems do, however, remain in solvent extractione.g., the slow or incomplete extractions. The authors have devised a new homogeneous liquid-liquid extraction method and obtained satisfactory results in the extraction of molybdenum (VI) with a simple procedure ( I ) . This method is based on the high solubility of organic solvent in water at higher temperature and is characterized by immediate formation of the complex upon attaining a state of homogeneous solution consisting of water and the organic solvent during the procedure. At room temperature the two phases are present heterogeneously, but at the elevated temperature they change into a homogeneous solution, which separates into the two phases again upon cooling. During these sequential procedures, the species in the aqueous phase transfers into the organic phase-i.e., the extraction is achieved. This method of equilibration by achieving a homogeneous state is different from the common mechanical shaking method. Molecules of the organic solvent rather freely enter into the aqueous solution. Consequently, the water structure of the aqueous media and the environment of solute species will be (1) K. Murata and S. Ikeda, Bunseki Kagaku, 18, 1137 (1969).
altered remarkably by participation of the organic solvent molecules. This “unshielding” of the environment may change the extractability ; such a condition is not satisfied fully in the conventional extraction method. One of the most suitable organic solvents for the homogeneous liquid-liquid extraction is propylene carbonate (4-methyl-l,3-dioxolane-2-one), which has found recent use in solvent extractions (2, 3) and in electrochemical studies (4-6). It has a high dielectric constant (65 at 25 “C), low vapor pressure, and high boiling point. Especially noteworthy is the characteristic property of infinite solubility in water at temperatures higher than 73 “C. In the present paper, the method is applied for extracting ferric thenoyltrifluoroacetonate, which is known as a slow extraction system (7) (12 hours are required to establish equilibrium during extraction of this complex), and the behavior in the extraction is investigated. EXPERIMENTAL Reagent and Apparatus. Ferric perchlorate solution was prepared by first dissolving metal iron in hot perchloric acid, then evaporating almost to dryness with addition of several drops of hydrogen peroxide to prevent incomplete oxidation, and finally dissolving in dilute perchloric acid. Propylene carbonate (PrC03) was distilled under reduced pressure; bp 92 “C at 4.5 mm (8). 2-Thenoyltrifluoroacetone (TTA) was obtained from Wako Pure Chemical Inc. and used without further purification. The proton magnetic resonance measurements were made with a Varian T-60 or a Varian A-60 spectrometer operating
(2) B. G. Stephens and H. A. Suddeth, ANAL.CHEM.,39, 1478 (1967). (3) K.Murata and S. Ikeda, J. Inorg. Nucl. Chem., 32, 267 (1970). (4) R. E. Meredith and C. W. Tobias, J . Electrochem. SOC.,108, 286 (1961). (5) L. S. Marcoux, K. B. Prater, B. G . Prater, and R. N. Adams, ANAL.CHEM., 37, 1446 (1965). (6) R. F. Nelson and R. N. Adams, J. Electroanal. Chem., 13, 184 (1967). (7) Poskanzer and B. M. Foreman, Jr., J. Inorg. Nucl. Chem., 16, 323 (1961). (8) P. L. Kronick and R. M. Fuoss, J. Amer. Chem. SOC.,77, 6114 (1955). ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
805
0 1 ' ' 10
' ' ' 30 50
Temperature
"
'
c
70
v)
C
W c
, 'C
C
Figure 1. Influence of temperature on solubility of propylene carbonate
A
5.0
I
E
P
5.0
4.0
4.0
3.0
8 ,
n
2.0
I.o
ppm
Figure 3. PMR spectra of propylene carbonate
4
( A ) Pure propylene carbonate
(B) Propylene carbonate containing 0.111 mole water * Resonance line based on the water proton
3.0
0
0.2
0.4
0.6
0.8
1.0
Figure 2. Influence of propylene carbonate concentration on the chemical shift of the water proton at 60 MHz. The proton chemical shifts were referenced to dissolved TMS (tetramethylsilane) or DSS [sodium 3-(trimethylsily1)-propansulfonate] as the internal standard. Absorbance and absorption spectra were measured with a Hitachi 124 spectrophotometer and a Hitachi EPS-2 automatic recording spectrophotometer. A Nihonbunko IR-G infrared spectrophotometer was also used. Procedure. An eclual volume of warmed TTA propylene carbonate solution (80 "C) was added to 10 ml of 5 X 10-3M ferric perchlorate solution of pH 1.9 in a stoppered centrifuge tube on a water bath maintained at 80 "C. As reported (9, IO), the optimum pH for the extraction of iron(II1) with (9) Rene A. Bolomey and Leon Wish, J. Amer. Chem. SOC.,12, 4483 (1950). (10) A. K. De and S. M. Khopkar, Chem. Znd., 26,854 (1959). 806
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
thenoyltrifluoroacetone is around 2. Ferric perchlorate solution of pH 1.9 was used, for convenience, in the series of experiments. After making a uniform solution by shaking the mixture gently two or three times, the solution was cooled to room temperature and centrifuged to form two separate layers. The iron content of the aqueous solutions before and after extraction was determined by the iron(I1)-1 ,lophenanthroline method. RESULT AND DISCUSSIONS Solubility of Propylene Carbonate in Water. The solubility of propylene carbonate in water was determined by titrating 10 ml of water with propylene carbonate to the cloud point (2), while stirring with a stirrer at various temperatures. As Figure 1 shows, the solubility of propylene carbonate rises very sharply with temperature. At 73 "C,10 ml of propylene carbonate completely dissolves in 10 ml of water. The experimental procedure in the homogeneous liquid-liquid extraction was performed at 80 "C for convenience. Homogeneous Liquid-Liquid Extraction of Iron(II1) Thenoyltrifluoroacetonate by Propylene Carbonate. Iron(II1) thenoyltrifluoroacetonate is rapidly and completely extracted by using the homogeneous liquid-liquid extraction method. Complete extraction was observed for a 5- to 40-minute equilibration time in homogeneous state at 80 "C. It has been pointed out by some authors that certain kinds of metal complexes with thenoyltrifluoroacetone are extracted very
IO0
80
..-.--. * e /. * - C *-
-2 . - - . , 7
c
., 0
f
-c
60
I
0
c BC
W
8o
0 c 0 L c X
40
E 20 0
w
f $.
5
Phase contact time
10
,
IS
min
Figure 4. Extraction of iron(II1) with 2 X 10-2M TTA of propylene carbonate solution at various temperatures [Fe(III)] = 5 X 10-3M (1) 10.7 “ C , (2) 15.8 “C, (3) 20.3 “C, (4) 25.9 “ C , (5) 30.4 “C, (6) 37.0 “C
slowly. For example, 12 hours are required to establish equilibrium during extraction of iron(II1) thenoyltrifluoroacetonate from hydrochloric acid solutions with a 0.2M solution of the reagent in benzene (7). Akaza et al. reported the rather longer shaking time, 30 min, in the colorimetric determination of 2-thenoyltrifluoroacetone (11). Finston and Inoue showed the dependence of phase contact time on the concentration of TTA (0.01OM-0.50M) and succeeded in enhancing the extraction rate by addition of thiocyanate (12). The method proposed by the present authors leads to the rapid and complete extraction of metal chelate. The effectiveness of this method might result from higher reaction temperature and the disturbance of the water structure in the medium (the decrease of water activity), in which the aqueous species is “caught,” by intrusion of propylene carbonate molecules. Another characteristic is that the formation and the distribution of the extractable species are achieved in the single phase of the mixed solvent composed of water and propylene carbonate, which enables the extraction of iron(II1) from the aqueous phase very rapidly. Property of the Mixture of Water and Propylene Carbonate. The water structure must be changed somewhat by intrusion of propylene carbonate molecules, and vice versa. PMR spectra were examined in order to obtain information about the mixed solvent consisting of water and propylene carbonate. The results are shown in Figure 2. The water proton resonance shows that the upfield shift is proportional to the mole fraction of propylene carbonate. This indicates that “structure-breaking,” rather than the solvation, of water is predominant (13). The propylene carbonate proton resonance shows the small shift toward lower applied field as shown in Figure 3, thus implying solvation of propylene carbonate. Assuming that “structure-breaking” of water goes to completion at the extrapolated point [PrC03]/ [PrCO,]+[HtO] = 1, it is estimated that about 18% of water in the normal form is converted into a “structurebreaking” form in the mixture composed of equal volumes of water and propylene carbonate. Usua 1 Liquid-Liquid Extraction Method. Influence of Temperature and Shaking Time on the Extraction of Iron(II1) (11) I. Akaza, M. Kosaka, and T. Imamura, Bunseki Kaguku, 14, 825 (1965). (12) H . L. Finston and Y. Inoue, J . Inorg. Nucl. Chem., 29, 199 (1967). (13) J. F. Hinton and E. S.Amis, Chem. Rev.,67, 367 (1967).
”
I
I
20
IO
f
I
30
Shaking time
40
,
50
J
60
mln
Figure 5. Extraction of iron(II1) with 2 X 10+M TTA of benzene solution at various temperatures [Fe(III)] = 5 X lO+M (1) 27.2 “C, (2) 37.0 “C, (3) 46.7 “C, (4) 56.5 “C
Thenoyltrifluoroacetonate. In order to obtain some information about the extraction behavior of iron(II1) thenoyltrifluoroacetonate, it was desirable to investigate it by the usual liquid-liquid extraction method. As seen in the previous work (12), the higher the concentration of TTA and the longer the shaking time, the greater the per cent extraction of iron(II1) becomes. The complete extraction of iron(II1) was achieved after shaking for 1 hr with 0.5M TTA in benzene. However, this is not favorable for a routine analysis. The speed of chemical reactions in general increases with an increase in temperature. The temperature effect was not investigated in that extraction system. Figure 4 shows the influence of temperature and shaking time on the extraction of iron(II1) with 0.02M TTA in propylene carbonate. The temperature dependence is more remarkable than the dependence on shaking time. Thus an increase in temperature considerably promotes the extraction rate of iron(II1) thenoyltrifluoroacetonate with 0.02M TTA in propylene carbonate. At 37.0 OC the extraction equilibrium is achieved within 5 minutes; however, complete extraction is not attained. On the other hand, with 0.02MTTA in benzene solutions, the extraction equilibrium is not achieved within 1 hr, even at 56.5 “C, as shown in Figure 5 . Propylene carbonate solvent is also superior to benzene in the ordinary batch extraction of iron(II1) thenoyltrifluoroacetonate. Chemical State of Thenoyltrifluoroacetone in Water, Benzene, and Propylene Carbonate. As mentioned previously, a great difference was seen between the extraction by benzene solution and by propylene carbonate solution of TTA. This led to a study of the chemical state of TTA in these solvents. TTA exists in the three forms: keto, enol, and keto hydrate. In dilute acid solution ca. 1.6% of the TTA is in the enol form, the rest as the keto hydrate (14). It seems reasonable to assume that essentially all of the TTA in water is as the hydrate. From the spectrum of an aqueous solution, Zebroski (15) has concluded that the structure of the hydrate is the following form:
TTA
heto hydrate
form
(14) J. C. Reid and M. Calvin, J . Amer. Chem. SOC.,72, 2948 (1950). (15) E. Zebroski, Thesis, University of California, 1947. ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
807
10 n
2 "
w
5
5
15
10
,
time
RwctlOn
20
min
Figure 8. Formation of iron(1II)-TTA chelate in water at various temperatures [Fe(III)] = 4.5 X 10-aM, [TTA] = 5 X 10-4M,absorbance measured at 510 nm. (1) 13.4 "C, (2) 20.6 "C, (3) 25.7 "C, (4) 30.4 "C, (5) 37.0 "C
0
250
300
400
350 nm
Wavelength
Figure 6. Ultraviolet absorption spectra of TTA in some solvents (1) TTA in water, (2) TTA in benzene, (3) 'ITA in propylene car-
bonate
I 15
14
13
12
6
I
I1
IO
enolate form (18). The solvation of TTA is indirectly proved by the observation that the >C=O absorption band of propylene carbonate (at ca. 1800 cm-') in 0.5M TTA propylene carbonate solution shifts 15 cm-l toward lower wavenumber than pure propylene carbonate. The solvation of TTA by propylene carbonate can be also confirmed from PMR spectra. The protons of propylene carbonate (particularly methine and methylene groups) in TTA propylene carbonate solution show the small shift toward lower applied field. In order to observe directly the solvation of TTA, the OH proton of TTA was noted. The resonance lines of TTA have been previously assigned (16, 19). The OH proton resonance line appears at lowest applied field and is rather broad. The possible structures in the enol form of TTA (16) are the following intramolecular hydrogen-bonded one (TTAintrJ:
9
ppm
Figure 7. PMR lines of OH proton in TTA ( A ) 0.5M TTA in propylene carbonate ( B ) 0.5M 'ITA in benzene
In a benzene solution, 94.5% of the TTA is enol form (14). Yamazaki and Takeuchi (16) obtained similar results from the NMR measurement. Figure 6 shows the UV absorption spectra of TTA in water, in benzene, and in propylene carbonate. The spectrum in water corresponds to TTAketohydrate form, and that in benzene to the enol form. In carbon tetrachloride, TTA shows the same spectrum of the enol form as in benzene solution. The NMR measurement (16) is also representative of the enol form in carbon tetrachloride. In propylene carbonate the spectrum of TTA is different from those in benzene and carbon tetrachloride. The same UV absorption spectrum of TTA in tributylphosphate, corresponding to the enolate form (probably by hydrogen bonding onto the enolic form), was observed by Healy et al. (17). Thus, this spectrum is considered to be the enolate form. Suzuki et al. stated also that the peak at ca. 340 nm was attributed to enol form or most probably to (16) M. Yamazaki and T. Takeuchi, Kogyo Kagaku Zasshi, 72, 2223 (1969). (17) T. V. Healy, D. F. Peppard, and G. W. Mason, J . Znorg. Nucl. Chem., 24, 1429 (1962). 808
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
TTA
Intra
form
the dimer of TTA(TTAdimer), and the keto mother of TTA (TTAketomother); of which the intramolecular hydrogen bond breaks, and the intermolecular hydrogen bond forms with the solvent:
qfcyF' \
"......Solvent
TTA
&*IO
memr
f0ml
When chemical shift of the OH proton is considered in these three kinds of enol form TTA, the resonance lines of OH proton in TTAint,, and n A d i m e ,are observed at lower magnetic field owing to intramolecular hydrogen bond, and that of TTAk,t, mother is shifted to higher field owing to breaking of the intramolecular hydrogen bond. In polar solvent, the solvation is considered to stabilize TTAketomother form. TTA in propylene carbonate Indeed, as seen in Figure 7,0.5M solution shows the upfield shift of OH proton, which indicates solvation by propylene carbonate, compared with ben(18) N. Suzuki, K. Akiba, T. Kanno, and T. Wakabayashi, ibid., 30, 2521 (1968). (19) T. H. Siddall IV and W. E. Stewart, ibid., 31, 3557 (1969).
1.0
Oa4
t E
t
0.5
VI
9
.
. .... ..
!
'
i
0
2
.
.%.I. '
200
10
20
Shaking lime
30
40
50
'
".
300
500
400
60
mln
Figure 9. Difference of extractability between the case using propylene carbonate and that using benzene [Fe(III)] = 5 X lO+M, [TTAI = 5 X 10-4M Absorbance of the organic phase was measured at 510 nm. (A) extraction with propylene carbonate ( B ) extraction with benzene
zene solution. Therefore, in benzene and carbon tetrachloride, the enol form of TTA is expected to be TTAintra o r TTAdi,,,; while, in propylene carbonate, to be TTAM, mother. Provided that ionization of TTA is required before the reaction between iron(II1) and TTA enolate ion, mother type is considered to be more favorable for extracting than TTA,,t,, or TTAd,mer. Formation and Extraction Behavior of Iron(II1) Thenoyltrifluoroacetonate. Since the extraction is generally considered to proceed through the following steps : distribution of organic reagent (in this case, TTA) into aqueous phase, ionization of TTA, reaction of iron(II1) with TTA enolate ion, and distribution of the chelate into organic solvent; it is also important to investigate the formation of iron(II1) thenoyltrifluoroacetonate in water. Taft and Cook (20) carried out the kinetic and equilibrium studies of iron(II1) thenoyltrifluoroacetonate by spectrophotometric means. They reported that the rate determining step was not the ionization of TTA, but was the reaction of iron(II1) with TTA enolate ion. We observed the effect of temperature of the formation of iron(II1) thenoyltrifluoroacetonate in aqueous solution as shown in Figure 8. At 13.4 "C,the chemical reaction is slow and the equilibrium is not attained within 20 min, while at 37.0 "C the reaction goes to completion within 5 min. The character of Figure 8 is similar to Figure 4 and not to Figure 5. As apparent from Figure 8, iron(II1) thenoyltrifluoroacetonate formation is complete in aqueous solution at 37.0 OC; but the extraction with 0.02M TTA in benzene solution is not achieved at 37.0 "C even after 60 minutes. This indicates qualitatively that the rate-determining step in the slow extraction with benzene solution is the transfer process of extracted species (the formation of the extractable species), and in the case of the extraction with TTA propylene carbonate solution the rate-determining step is the formation of the complex in the aqueous solution. This is also confirmed from the fact that when benzene or propylene carbonate containing no TTA is shaken with the aqueous iron(II1)-TTA complex solution previously prepared, propylene carbonate extracts the complex rapidly, whereas benzene extracts very slowly. These results are shown in Figure 9. When the concentration of TTA becomes (20) Robert W. Taft, Jr., and E. H. Cook; J. Amer. Chem. Sac., 81, 46 (1959).
600
nrn
Wavelength
Figure 10. Absorption spectra of TTA and iron(II1)-TTA complex in water [Fe(III)] = 5 x 10-4M,[TTA] = 1 X lO-*M * TTA, -iron(IIIbTTA complex 9
2 00
300 Wavelength
400
SO0
600
n rn
Figure 11. Absorption spectra of the organic phases extracted from the aqueous solution [Fe(III)l = 5 X 10-4M, [TTA] = 1 X lO-4M) by propylene carbonate (solid line) and benzene (dotted line) (The spectrum of benzene phase was obtained after 6-hr shaking because of the slow extraction)
1 X lO-4M, the absorption spectrum of TTA is observed over the whole region. In Figure 10, the spectra of TTA and iron(II1)-TTA complex in the aqueous solution are shown. In addition to citing the result of previous workers (21) that the formation constants for the first complex of P-diketones generally exceed those for the second complex by factors of about ten, Taft and Cook (20) mentioned that it is possible to regulate the point of equilibrium so as to stop the reaction at the first step. Iron(II1) in excess stops the successive chelation of iron(II1)-TTA at the first step, i.e., the excess of iron(II1) stabilizes the first complex of iron(II1) thenoyltrifluoroacetonate. Indeed, we observed the formation of the insoluble iron(II1)-TTA complex (the third complex) under the conditions of [Fe] = 3.13 X 10-4M and [TTAI = 5 X 10-4M at 25.7 "C, but did not, under the conditions of [Fe] > 5 X lO-4Mand [TTA] = 5 X lO-4M. Consequently, the spectrum of iron(II1)-TTA complex in Figure 10 is considered to be that of the first complex because of the presence of excess iron(II1). Shoulders in the spectrum of TTA appear at 335, 380, and 510 nm, attributed to the chelation between TTA and iron(II1). The intensity (absorptivity) of both maxima at 265 nm and 290 nm in the spectrum of TTA decreases with formation of iron(II1)-TTA complex. In (21) N. C . Fernelius, B. E. Bryant, and L. G. Van Ultert, un-
published compilation of formation constants. ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
809
order to further study the extraction behavior with benzene and propylene carbonate, these solvents were equilibrated with aqueous solutions of the “first” complex ([Fe] = 5 x lO-W, [TTA] = 1 X lO-4M). Propylene carbonate immediately extracts this complex, which is considered to be the first complex since it shows the same spectrum as the aqueous phase; benzene does not extract rapidly as shown in Figure 11 (the extraction is appreciable after 6-hr shaking, but the spectrum of the organic phase is somewhat different from the aqueous one). In other words, propylene carbonate can extract iron(II1)-TTA complex after the step corresponding to the formation of the first charged complex, but benzene can not extract until the subsequent chelation is achieved. Considering the result and the fact that the third complex of iron(II1)-TTA prepared is not soluble in water but is very soluble in benzene, it can be expected that iron(II1) is not extracted by benzene until the aquocomplex of ferric ion is completely substituted by TTA chelating agent. The very slow rate in the benzene extraction is attributed to the long time required in the stepwise substitution of ferric aquocomplex by TTA. On the other hand the extraction rate of iron(II1) is greater in the case of propylene carbonate solvent, because iron(II1) can be extracted at the step at which the substitution reaction is incomplete (as the first complex of iron(II1)-TTA chelate). Stephens and Suddeth (Z), who studied the extractability of ion association chelate systems into propylene carbonate, reported that the Fe(phen)32+ and Fe(TPTZ)P complexes are completely extracted in the presence of perchlorate. Thus, in our extraction system from perchloric acid medium, it is considered that the first complex of iron(II1)-TTA chelate associated with perchlorate is extracted into propylene carbonate as the ion association chelate. CONCLUSION
Homogeneous liquid-liquid extraction method was applied to the extraction of ferric thenoyltrifluoroacetonate. Iron (111) was extracted rapidly and completely. The efficiency arises from using propylene carbonate solvent and operating at the higher extraction temperature. The use of propylene carbonate solvent because of its low vapor pressure permits the extraction at higher temperature, and its
810
0
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4 , APRIL 1972
solubility in water at higher temperature gives rise to the homogeneous liquid-liquid extraction. This homogeneous state has some favorable conditions for extracting. The medium is a uniform one consisting of water and the organic solvent. The activity of water is lowered by the presence of propylene carbonate. This is shown from PMR measurement of water proton. Therefore the hydration sphere of iron(II1) is affected somewhat (becomes small or broken), and the substitution reaction of iron(II1) aquocomplex with TTA tends to proceed. Since it is known that the stoichiometric distribution coefficient (total TTA in benzene/total TTA in aqueous) is 40 (7), the concentration of TTA in the aqueous phase equilibrated with a given concentration of TTA benzene solution is very low, that of the organic phase. On the other hand, in the homogeneous liquid-liquid extraction, the concentration of TTA in the homogeneous solution is about that in the propylene carbonate phase (in the case of V,,, = VaQ). In the latter case the chelation of iron(II1) is carried out in the higher TTA concentration (ca. 20 times that of the former case). The chemical state of TTA in propylene carbonate is somewhat different. It interacts with propylene carbonate to produce TTAket, mother form, which is a more reactive form for iron(II1) than TTAint,,. When iron(II1) can transfer into propylene carbonate phase containing TTA, the iron(II1)-TTA complex forms instantly. This condition can be achieved easily by the homogeneous liquid-liquid extraction method. Thus, the homogeneous liquid-liquid extraction method yields favorable conditions for the extraction. These effects act additively, and good results are obtained. The effectiveness of the homogeneous liquid-liquid extraction is attributed just to the higher reaction temperature. The higher temperature yields the following results : the constitution of the mixed solvent (water and propylene carbonate), the easy mass transfer and interaction between iron(II1) and TTA because of the appearance of a single phase, and the completion of the successive chelation of iron(II1) with TTA. It is considered that these affect both the synergic and the rapid and complete extraction of iron (111). RECEIVED for review June 21, 1971. Accepted November 4, 1971.
