provide nondestructive analysis of micron size particles. The computer programs were originally written in FORTRAN and tested for about two years. On-line operation in BASIC has been applied successfully for about one year. The programs will be developed to include fluorescence correction for oxide systems and statistical evaluations. The results obtained so far confirm that a satisfactory technique has been developed for quantitative electron probe microanalysis of many different systems in minerals and metallurgy.
ACKNOWLEDGMENT The author wishes to thank the General Manager, B.H.P. Planning and Research, for permission to publish this The programming assistance of Miss C. Tillman and 1. R. Matthews is gratefully acknowledged. RECEIVED for review February 28, 1972. Accepted July 10, 1972.
Partition of Hexafluoroacetylacetone in the System: Water-Tri-nButyl Phosphate-Organic Solvent and the Extraction of Sodium by Various Fluorinated ,&Diketones and Tri-n-Butyl Phosphate Branko B . T o m a W and Jerome W . O’Laughlin* Ames Laboratory-USAEC and Department of Chemistry, Iowa State University, Ames, Iowa 50010 The partition of hexafluoroacetylacetone (HHFA) between tri-n-butyl phosphate (TBP) dissolved in a nonpolar organic solvent and waterwas studied. A second~ ~ log TBP order dependency in a plbt of log E a o a = vs. was established, and the infrared characterization of the organic phase indicated an interaction between HHFA and TBP which was associated with the increased extraction of water into the organic phase. The effect of the destruction of synergism which takes place in related extraction systems was discussed as a possible consequence of observed HHFA-TBP interaction. An increase of pH of the aqueous phase accounts for an additional increase in E’,RHFA and a parallel synergic extraction of sodium. This was found to be the case with a number of other fluorinated p-diketones, and the adducts formed seem to be thermally stable compounds. The possible implications of the observed extraction phenomena on some analytical procedures were emphasized.
A NUMNER OF PAPERS have appeared in the recent literature on the use of fluorinated /?-diketones for the extraction and gas chromatographic (GC) separation of metallic ions. A number of metallic ions form volatile and thermally stable metal chelate compounds with these fluorinated 0-diketones. Severs and coworkers have systematically studied a number of fluorinated /?-diketones and described successful procedures for extraction and the G C separation and determination of various metallic species ( I ) . However, the recovery of certain metallic ions was poor, and some isolated metal chelate compounds were thermally unstable ( 2 ) . This reduces the possibility of the application of G C for their analysis. Healy (3) and Irving and Edgington ( 4 ) , have shown that many metallic ions are efficiently extracted by
* USAEC Postdoctoral Fellow, on leave from “Rudjer Bobkovik” Institute, Zagreb, Yugoslavia. Present address, Department of Chemistry, University of Missouri, Columbia, Mo. 65201 (1) K. J. Eisentraut and R. E. Severs, J. Amer. Cliem. Soc., 87, 5254(1965). (2) E. W. Berg and J. J. C . Acosta, Anal. Chim. Acta, 40,101 (1968). (3) T. V. Healy, J . Ztiorg. Nucl. Clzern.,19, 314 (1961). (4) H. Irving and D. N. Edgington, ibid.,15,158 (1960).
106
various fluorinated &diketones when they are a part of a synergic system and used in conjunction with a neutral donor. Li, Wang, and Walker (5) applied the hexafluoroacetylacetone (“FA)-trioctylphosphine oxide(TOP0) synergic system for the extraction of zinc and observed a remarkable synergic effect. They also found the evidence of destruction of synergism in the same system and attributed this to the acid decomposition of the ternary complex, which was followed by the formation of an adduct of the form HHFA-2H20-2TOP0. Banks and coworkers ( 6 , 7) have studied the thermal properties of various ternary complexes which were isolated from synergic extraction systems in which various fluorinated /?-diketones were used as the chelating agents and with neutral donors such as tri-n-butyl phosphate (TBP) and dimethyl sulfoxide (DMSO). Thermogravimetric analyses indicated that these ternary complexes are volatile and thermally stable; this made possible their G C characterization and determination (6, 7). The present authors in a study of the synergic extraction of iron (111) found that after the TBP-”FA ratio exceeds certain value, destruction of synergism commences (8). There are no published data on the partition of HHFA itself into an organic phase which contains TBP as a neutral donor. This process would undoubtedly influence the extraction of metallic species in this synergic system. In this paper, an attempt has been made to elucidate this particular process. EXPERIMENTAL Reagent grade chemicals were used in all experiments. TBP (Fisher Scientific Co.) was purified by a procedure described by Irving and Edgington (9). Cyclohexane (Matheson Coleman and Bell) was additionally purified by distillation. 1,1,1,5,5,5-Hexafluoro-2,4-pentanedione (HHFA), Columbia Organic Chemicals Co., Inc., and 1,1,1,2,2,3,3-heptafluoro-7,( 5 ) N. C . Li, S. M. Wang, and W. R. Walker, ibid., 27,2263 (1965). (6) W. C. Butts and C . V. Banks, ANAL.CHEM.,42, 133 (1970)
(7) R. F. Sieck, J. J. Richard, K. Iversen, and C. V. Banks, ibid., 43, 913 (1971). (8) B. Tomazic and J. W. O’Laughlin, unpublished data, 1972. (9) H. Irving and D. N. Edgington, J. Ztiorg. Niicl. Cliem., 10, 306 (1959).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
7-dimethyl-4,6-octanedione (HFOD) Peninsular Chem-research, Inc., were additionally purified by distillation. 1,1,1,2,2,6,6,7,7,7-Decafl~ioro-3,5-heptanedione (HFHD) and 1,l ,I 2,2,6,6,7,7,8,8,8,dodecafluoro-3,5-octanedione (HDODEFOD) were prepared at the Ames Laboratory by John Richard or Charles Burgett. The desired concentrations of HHFA in water, and TBP in organic solvent were prepared by weighing the corresponding extractants and diluting to the appropriate volume. The partition of HHFA was studied in systems containing equal volumes (5 ml) of organic and aqueous phase after shaking 3 hours on a Burrel, Wrist-Action, shaker. The pH of the aqueous phase containing 0.1M in HHFA was adjusted by addition of a 1 M NaOH-O.1M HHFA solution. The equilibrium was attained rather slowly and a 1- or 2-minute stirring period was necessary after each addition of base. The pH of aqueous phases was checked again after the extraction equilibria were attained. Ultraviolet spectra were recorded by using a Cary 14 spectrophotometer and infrared measurements were performed on a Beckman IR-7 infrared spectrophotometer. All pH measurements of the aqueous phases were performed by using a Beckman, Model G, pH meter. The sodium content in organic phases was determined after sodium was completely removed from the organic phases by shaking with 1M hydrochloric acid, and the sodium was determined by atomic absorption spectrometry using a Perkin-Elmer 303 Atomic Absorption Spectrophotometer. Sodium distribution ratios in different fluorinated P-diketone-TBP extraction systems were determined by labeling the aqueous phase with 22Na(Amersham, Searle) radioisotope. After equilibration, the distribution ratios were determined by measuring activity of the aqueous and organic phases, which were separated by centrifuging. RESULTS HHFA is very soluble in many organic solvents, but unlike many organic extractants, it is quite soluble in water as well. The distribution ratio, E",, of HHFA in the system: cyclohexane-water is very low, i.e., 6.8 X for pH 53.88 (10). However, the presence of TBP in organic phase enhances the distribution ratio of HHFA. The infrared spectra of organic phases containing increasing TBP concentrations in cyclohexane were obtained after equilibration with 0.1M aqueous solutions of HHFA. If pure cyclohexane was used as organic phase, no HHFA could be detected. However, as the TBP concentration was increased, characteristic bands due to HHFA of increasing intensity were recorded which indicates an increased partition into the organic phase. A marked difference was noticed in the shape of the infrared spectrum of HHFA when extracted from an aqueous phase, as compared to the spectrum of an anhydrous HHFA-TBPcyclohexane mixture. In the case of anhydrous mixtures the resulting spectrum is merely a composite of the spectra of anhydrous HHFA and TBP and there is no evidence of any HHFA-TBP interaction (Figure 1). The addition of a small amount of water (1%) to the mixtures, followed by overnight shaking, causes a significant change, and the resulting spectrum was identical to the spectrum of HHFA extracted from aqueous solutions into cyclohexane containing TBP. Characteristic bands at 1138, 1168, 1198, and (10) J. W. Mitchell, "Synergic Solvent Extraction and Thermal Studies of Fluorinated Beta-Diketone-organophosphorous Adduct Complexes Lanthanide and Related Elements." USAEC Rep. IS-T-353 (1970). available from National Technical Information Service, US. Department of Commerce, Springfield, Va. 22151.
I
A'
1300
I
I
I
1200
1100
I
1000
FREQUENCY CM-I
Figure 1. Infrared spectra of -.- TBP, -HHFA-TBP anhydrous 1 :2 mixture, "FA-TBP 1 : 2 extract and - ' . Na-HHFATBP extract in 1100-1300 cm-' region. 0.1M TBP in cyclohexane
----
1274 cm-1 are observed which are not seen in the spectrum of the anhydrous mixture. These differences are indicative of "FA-TBP interaction, probably due to the influence of water, which is expected to enter the organic phase. At high TBPiHHFA ratios in either the wet or extracted systems, the 1295 cm-' band is present, but with a decrease of the above ratio the band intensity decreases and finally vanishes. A simultaneous broadening of absorption maximum at 1275 cm-' is also observed. An attempt to determine the amount of HHFA in the organic phase by measuring absorption at 273 nm (11) failed, apparently because of the presence of water in organic phase. In the aqueous phase, there is not any significant absorption due to HHFA at pH 1 3 . 5 . A spectrophotometric titration showed that the absorbance at 300 nm increased sharply between pH 3.5-5.0, because of the HFA- anion [PKHHFA= 4.6 ( ] I ) ] . At pH 5.5, the absorbance becomes constant. A series of HHFA solutions were prepared with pH 6.0 using 0.1M sodium acetate buffer, and a linear calibration curve was obtained for the lop5-5 X 10-4M concentration range of "FA. The HHFA concentration in the aqueous phase was determined using the calculated regression line : HHFA
=
Absorbance
x
(1,150 i 0.01) X 10-4
+ (2.04 f o 47) x
10-6
(1)
The error across the entire calibration curve was estimated to be less than 1.7 %. (11) R. L. Belford, A. E. Martell, and M. Calvin, J. Zuorg. Nud. Chem., 2, 11 (1956).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
107
Id'
10-1
M TBP
Figure 2. Dependence of the distribution ratio of HHFA on the molar concentration of TBP in the organic phase. Solvents: 0 carbon tetrachloride, cyclohexane
c !621
I
13-1
I
I
I
i l l 1 1
,,
100''
I
10-1
I
I
I l l 1 1
1 IO0
[TWO
Figure 3. Concentrations of extracted H 2 0 and HHFA in organic phases containing varying concentrations of TBP Extraction of HHFA as a Function of TBP Concentration. The extraction of HHFA from 0.1M aqueous solutions was studied as a function of the concentration of TBP in cyclohexane or carbon tetrachloride. The amount of HHFA left in the aqueous phase was determined by the absorption of HFA- at 300 nm. Plots of the log distribution ratio, log eon"^* cs. log TBP were linear with a slope of two for both solvents (Figure 2), which indicates that for the TBP concentration range from 0.05-0.5M the species of the stochiometry 108
"FA. (TBP)?, is extracted. The partition of HHFA into cyclohexane phase is greater than the carbon tetrachloride phase ~ for 3 X 10-IMTBP of 0.95 and 0.35, with log E o a " ~values respectively. This shows that HHFA is extracted into cyclohexane roughly three times more efficiently than into carbon tetrachloride. Extraction of Water into an Organic Phase Containing TBP. Data on the extraction of water as a function of the TBP concentration are shown in Figure 3. There is a nearly linear increase in water concentration in the organic phase with the TBP concentration. When the aqueous phase is 0.1M in HHFA, much higher amounts of water ([HzO]cot)are extracted. The difference in concentration
apparently corresponds to the amount of water brought to the organic phase together with extracted HHFA. The amount of HHFA extracted is given in Figure 3 as [HHFA],. From the very similar shapes of water uptake curves in cyclohexane and carbon tetrachloride systems, one can suspect that the process of water interaction with components of organic phase, namely HHFA and TBP, is the same in both extraction systems. The ratios [H20Itot: [HHFA], and : [HHFA], for cyclohexane are 1.86 + 0.25 and [H~OIHHFA 1.21 i. 0.1, and for carbon tetrachloride 1.58 i 0.17 and 1.04 + 0.06, respectively. Dependency of HHFA Extraction on Acidity of Aqueous Phase. Initially, it was anticipated that with an increase in the pH of the aqueous phase which would result in the dissociation of HHFA, a decrease in the partition of HHFA into organic phase would occw. However, the opposite effect was observed. In the pH range 1-3, the fraction of HHFA left in aqueous phase is constant at a given TBP concentration of organic phase. With a further increase in pH, a gradual decrease of HHFA in aqueous phase is observed (Figure 4). This was accompanied by the gradual change in the infrared spectrum of the organic phase. (The 1138 cm-1 and 1190 cm-l bands decrease in intensity, and the 1168 cm-' band decreases and eventually vanishes.) Additional bands were observed which suggests that HHFA is extracted in some other form, possibly as the ion pair [Na+][HFA-1. To check this assumption, organic phases were shaken with 0.2M hydrochloric acid solution to back-extract sodium, if present in organic phase. The results of analysis for sodium stripped from organic phase are also presented in Figure 4 and show that the sodium content in the organic phase does coincide with the observed decrease of the HHFA concentration in aqueous phase. The infrared spectrum of the organic phase after stripping the sodium corresponded to that for HHFA extracted from water. The most pronounced increase of organic phase sodium content takes place between pH 4-5 which is very close to the pK, for HHFA. Because the synergic extraction of sodium is important in the use of the fluorinated p-diketone-neutral donor systems as a method of forming volatile metal chelates of various cations for their determination by GC, the extraction behavior of sodium was studied with several other @-diketones. The systems, (0.1M fluorinated /3-diketone-O.1M TBPcyclohexane),-(0,lM Na+)aq, were studied as a function of pH. The pH of aqueous phases was adjusted by addition of sodium hydroxide and sodium chloride to a sodium acetate buffer, and the sodium concentration thus held constant at 0.1M. Sodium is not extracted in systems containing acetylacetone and trifluoroacetylacetone over the pH range
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
I
0 4
v Lb
I
3
Io-2
i
I
I
I
,
‘ 1
4 A-
Q 0
0 0 . 2 M TBP
0.05M TBP 0-0-
0.1 M Na’ 0.1 M “FATOT
P
/”
0 HDODEFOD 0 HFHO
IP
4
,d j
2
A HFOD [NalTOT=O.l M
VBP] =0.1M
4
6
8
IO
12
PH
0
I
2
3
4
5
6
Figure 5. Dependence of the distribution ratio of sodium on pH of the aqueous phase. Organic phase, 0.1M fluorinated P-diketone 0.1M TBP-c yclohexane. Aqueous phase initially 10-3Min Fe(II1) (+)or Eu(II1) ( X )
7
PH
Figure 4. Equilibrium concentrations of Na,,, and HHFA,, cs. pH of the aqueous phase after extraction with TBP-cyclohexane phase
DISCUSSION
from 2-12. However, the presence of other fluorinated diketones causes a very pronounced extraction of sodium as shown in Figure 5. Apparently “FA, HFHD, and HDODEFOD extract sodium at essentially the same pH, H H F A being the poorest extractant of the three and HDODEFOD the best. The sodium extraction with H F O D is significant but takes place a t much higher pH values. All the extraction isotherms have in common a n approximately unit slope, and a flat portion after the extraction maximum is reached. This suggests an essentially identical mechanism for the extraction of sodium. (The presence of 0.001M Fe(II1) and Eu(II1) in extraction systems did not change the sodium distribution ratio a t given p H value of aqueous phase, i.e., sodium and Fe(II1) or Eu(II1) extraction are two parallel independent processes.) Sodium extraction into the organic phase is accompanied with a pronounced change in the infrared spectra of organic phases. Sodium extracts when compared with the same organic phases not containing sodium, present a complex spectra containing bands due to TBP, the fluorinated $-diketone, and some additional bands, the most significant of which is a new sharp band a t 16501680 cm-’. In order to check the thermal stability of the sodium extracts, the organic phases were evaporated a t 200 “C and heated 5 minutes. The oily light brown residues were diluted with cyclohexane, and infrared spectra were recorded. These spectra did not differ significantly from fresh extracts. Prolonged heating at 250 “C for 30 minutes changed the color of residues to a dark brown, probably due to the partial decomposition. Infrared spectra of residues diluted with cyclohexane show essentially only TBP lines, while those characterizing the sodium complex were decreased in intensity or absent. It is important to note that the TBP doublet appears in heated extracts, which indicates that the sodium complex undergoes decomposition under prolonged heating.
The P-0 stretching frequency which was shown to be a doublet in TBP due to the presence of two rotational isomers in solution (12) is shifted toward higher frequencies with a decrease in the concentrations of TBP (13). Therefore, it is not surprising that this doublet appears a t 1295 and 1278 cm-1 in dilute solutions of TBP as compared with the frequency of 1280 cm-I reported for this band in pure TBP (14). There appears to be a small shift in the P-0 stretching frequency, which may be associated with the formation of a weak T B P ’ H 2 0complex (15, 16), but since the extraction of water into 0.1M TBP-cyclohexane is very low (cf. Figure 3), and the TBP-H?O interaction is weak, this effect is hardly noticeable for the wet 0.1MTBP solutions. Apparently the extraction of H H F A into the organic phase is the process which is responsible for the appreciable increase of water content in the organic phase and the shift of 20 wave numbers is in good accordance with the literature ( 1 6 ) . The increase of the HHFAiTBP ratio in extracts or in the wet HHFA-TBP mixtures is accompanied by the appearance of a new sharp line at 1190 cm-I. Since this 1190 cm-I band does not appear in either wet TBP or wet H H F A solutions, this could be assigned t o a TBP-HHFA interaction which probably (12) F. S. Mortimer, Spectrochim. Acta, 9,270 (1957). (13) D. M. Petkovic, “Proceedings International Conference on Solvent Extraction Chemistry, Gothenburg 1966,” North Holland Publishing Company, Amsterdam, Netherlands, 1967, p 305.
(14) D. F. Peppard and J. R. Ferraro, J . I m v g . W i l d . Chem., 15, 365(1960). (15) K. Nukada, K. Naito, and U. Maeda, Bid/. CIiem. SOC.Jup., 33, 894 (1960). (16) P. J. Kinney and M. Smutz, “An Infrared Study of the Tributyl Phosphate-Nitric Acid-Water Solvent Extraction System” USAEC Rep., IS-728(1963), available from National Technical Information Service, U. S. Department of Commerce, Springfield, Va. 22151.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
109
takes place through a hydrogen bridging mechanism involving water. The parallel increase of intensity of this line with increase of HHFAjTBP ratio, if indicative of the presence of a new complex, supports this assumption. Because HHFA is extracted into the organic phase as a hydrate, the 1790 cm-l C=O stretch is absent. In the 16201650 cm-’ region, a broad weak band is observed which was previously assigned as the OH bending frequency (16) and is indicative of the small amount of water in the organic phase. It is clear from Figure 3, that the extraction of HHFA into an organic phase containing TBP is accompanied by a parallel extraction of water. The infrared spectra of HHFA extracts indicate that presence of water is responsible for the “FA-TBP interaction. The second-order dependence of TBP on extraction of HHFA in carbon tetrachloride and cyclohexane supports the overall reaction: HHFA
+ 2 TBP + nHzO F! HHFA.(HzO),.(TBP)z
(3)
The role of water can be understood from the presentation given by Wang et al. (17) who claimed the existence of the moiety, as directly responsible for the destruction of synergism in the Zn-HHFA-TOP0 system. HO
.OH
Analogously with this presentation, it might be expected that water molecules present in organic phase act as hydrogen bridges between HHFA and TBP molecules. The first-order dependency of TBP on extraction of water into organic phase is in accordance with formation of weak complex TBP’H20, but the total water content which is apparently a combined function of the HHFA and TBP concentrations closely parallels the HHFA concentration in organic phase. The value for HnO-”FA ratio (1.86) in the case of cyclohexane is in reasonably good agreement with the possibility of this species “FA. (Hz0)2.TBPz. As presented in Figure 4, the partition of HHFA between organic and aqueous phase is practically constant in the range pH 1-3, providing the concentration of TBP in organic phase is held constant. With a further increase in pH, more HHFA enters organic phase, and this is accompanied by the extraction of sodium into the organic phase. As illustrated (17) S. M. Wang, W. R. Walker, and N. C. Li, J. Znorg. Nucl. Cliem., 28,875 (1966).
110
in Figure 5 , sodium extraction into the organic phase is quite common with a number of fluorinated P-diketones, and the unit dependency on the pH of the aqueous phase indicates a 1:1 interaction with chelating agent. The extraction process takes place because of the presence of TBP, and therefore it is clear that we have evidence of synergic extraction of sodium in these systems. Healy has reported data on the synergic extraction of alkaline metal species using HTTA and a number of neutral donors (18), and gave evidence that extractable species are of the form M(TTA). SZ. The extraction of sodium with these fully fluorinated /?-diketones is considerably greater than with acetylacetone or HTFA. Wang et al. (17) rationalized similar phenomena by taking the view that as more electronegative groups were substituted on the ligand, the interaction between the ligand and the metal ion decreased, and the interaction between the metal and other donor groups increased. This resulted in a more stable ternary complex. It is also possible to view the extraction of sodium as an ion-exchange reaction involving sodium and a proton on the postulated “FA. 2 H 2 0 2TBP moiety. The actual mechanism of the extraction cannot, of course, be determined from equilibrium data. Any appreciable stability of a species such as “FA. 2H20.2TBP in the organic phase would have as a consequence a decrease in free HHFA in the organic phase. The finding that the phenomenon of “the destruction of synergism” is connected with the water content in the organic phase (19) may be due to the stability of a species involving water, TBP, and HHFA. The presence of such a species might cause the partial destruction of synergism in the extraction of metallic ions. In numerous reported synergic extraction studies, the pH of aqueous phases was adjusted by using sodium acetate buffer, and the sodium concentration was kept constant to maintain a constant ionic strength. It should be noted that the organic phases probably contained appreciable amounts of extracted sodium, as shown in the present study. In most cases the presence of sodium is not a serious disadvantage, but in some instances it might interfere with analytical determination of other metallic species or add an additional complication in the interpretation of infrared spectra of a particular extract. e
RECEIVED for review September 20, 1971. Accepted June 29. 1972. (18) T. V. Healy, “Proceedings International Conference on Sol-
vent Extraction Chemistry, Gothenburg 1966,” North Holland Publishing Company, Amsterdam, Netherlands, 1967, p 119. (19) T. V. Healy, D. F. Peppard, and G. W. Mason, J . Znorg. N z d Cliem., 24,1429 (1962).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, J A N U A R Y 1973
