Synergic Extraction of Iron with Hexafluoroacetylacetoneand Tri-n-Butyl Phosphate Branko B. TomaiiE’ and Jerome W. O’Laughlin2 Ames Laboratory-USAfC and Department of Chemistry, Iowa State University, Ames, Iowa 50070
The synergic extraction of iron(ll) and iron(ll1) by the combined action of HHFA and TBP was studied for a broad range of concentrations of all reacting components. The pronounced enhancement of the distribution ratio of iron, as compared with the data for the simple chelate extraction system, was ascribed to the formation of the ternary complexes Fe(HFA)z(TBP)Z and Fe(HFA)3TBP, respectively. The over-all stability constant for the reaction “FAaq 2 TBPorg F-‘ Fe(HFA)2(TBP)Zorg 4- 2H’aq was estimated to be 13.5 mol/l.-2. The stability constant for the reaction in organic phase Fe(HFA)30rg TBPorg 2 Fe(HFA)3TBPorg, was estimated to be 1.92 X mol/l.-’. The destruction of synergism was observed in both iron(l1) and iron(Ill) extraction systems. This is discussed in terms of formation of the species HHFA.(H20)2-(TBP)2 at high concentrations of TBP in the organic phase. TGA and DTA measurements have shown that both iron complexes are volatile, but, at elevated temperature conditions, the iron(ll1) species is reduced. The gas chromatographic behavior of the ternary complexes may be understood as the result of thermal dissociation of the adducts, which is highly dependent on the excess TBP in the extract. The thermal stability of the adducts is discussed in terms of the coordination characteristics of the central cation in the ternary complexes.
+
+
+
The synergic extraction of various cations as ternary complexes using fluorinated /3-diketones and a neutral donor has been shown to be an effective method of obtaining volatile metallic species. When either tri-n-butylphosphate (TBP) or di-n-butylsulfoxide (DBSO) was used as a neutral donor, the extracted species was shown to be thermally stable and volatile. The ternary complexes had a definite chemical composition and were anhydrous. These features permitted the development of useful analytical procedures for the determination of uranium and thorium ( I ) and of rare earth mixture (2) by gas chromatogr aphy . A limited number of papers have been published on the formation and physicochemical characteristics of the ternary complexes of transition metals, and it seems that knowledge about such systems may make the development of new GC analytical procedures possible. The extraction behavior of transition metals with fluorinated pdiketones alone has been studied in some detail. Some of the chelates with transition metals were volatile, which USAEC Postdoctoral Fellow, on leave from “Rudjer Boskovic” Institute, Zagreb, Yugoslavia. * Present address, Department of Chemistry, University of Missouri, Columbia, Mo. 65201. Sieck, J . J . Richard, K. Iversen, and C . V . Banks, Ana/. Chem.. 43,913 (1971). ( 2 ) W . C. Butts and C. V . Banks, A n a / Chem.. 42, 133 (1970). (1) R. F.
made their analysis by GC possible ( 3 ) . In many cases, the extraction recovery was quite low and the chelates contained water in the coordination sphere of the metal which caused the chelates to be thermally unstable. Wang et al. ( 4 ) have shown that zinc can be efficiently extracted in a synergic system containing HHFA and TOPO, and Agget (5) described the synergic extraction of iron(II1) by the combined action of acetylacetone and TOPO, as an ion-paired species of type [Fe(Acac)z(TOPO)z]+[C104]-. These authors did not study the thermal characteristics of synergic complexes. This paper describes the extraction of iron(I1) and (111) with the synergic system of HHFA and TBP. The thermal characteristics of the ternary complexes formed are described. The possible application of the synergic extraction of transition metals as a simple and efficient method for preparing volatile complexes, and analysis by using GC methods are discussed.
EXPERIMENTAL Materials and Procedures. Reagent grade chemicals were used in all experiments. Tri-n-butyl phosphate (TBP) (Fisher Scientific Co.) was purified by the procedure given by Irving and Edgington (6). 1,1,1,5,5,5-Hexafluoro-2,4-pentanedione (HHFA) (Columbia Organic Chemicals Co.,) and cyclohexane (Matheson Coleman and Bell) were additionally purified by distillation, and checked by infrared spectral characterization. Aqueous solutions of HHFA were prepared by weighing pure HHFA and dissolving in de-ionized water. The p H of the aqueous phases was adjusted by addition of 1M NaOH which already contained HHFA of desired concentration, so that by addition of NaOH, the total HHFA concentration was kept constant. The aqueous phases of extraction systems were initially made 0.1M in acetic acid to prevent the hydrolytic precipitation of iron(II1). It was noticed that the equilibrium pH after the addition of base was attained slowly; it was therefore necessary to stir the solution up to five minutes before reading equilibrium pH. Organic phases were prepared by weighing T B P and dissolving it in cyclohexane. 59Fe radioactive tracer (Amersham, Searle 3-30 mCi/mg Fe) was used for the determination of the distribution of iron(II1) between the organic and the aqueous phase. Equal volumes of organic and aqueous phases were contacted; the solution of iron was added afterward. The extraction systems were shaken by using a Burrell Wrist Action shaker. Because it was established t h a t a t least three hours of shaking was necessary to reach the equilibrium in extraction or back extraction experiments, an overnight shaking period was used to ensure that the equilibrium was attained in all extraction systems. The phases were separated by centrifugation, and aliquots were taken for counting the activity of phases. Precautions were taken to avoid the contamination of the phases. The distribution ratios of iron(II1) were calculated by dividing the activity of the organic phase by t h a t of the aqueous phase, both of which were corrected for background activity. The distribution of Fe(I1) in the synergic extraction systems was determined spectrophotometrically by measuring the absorption of the intense violet-colored organic phase a t 555 mk. This makes it possible to determine the per cent extraction of iron(I1) from which values (3) K . J. Eisentraut and R. E. Sievers, J. Amer. Chem. Soc., 87, 5254 (1965). (4) M. S. Wang. W. R. Walker, and N. C. Li, J. Inorg. Nuci. Chem., 28, 875 (1966). ( 5 ) J . Aggett, J . Inorg. Nuci. Chem., 32, 2767 (1970). (6) H. Irving and D. N. Edgington, J. Inorg. Nuci. Chem., 10, 306 (1 959).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8 , JULY 1973
1519
analyzer coupled with a DuPont 900 Differential Analyzer with recorder. The heating rate was 10 "C/min. Radiochemical measurements were carried out using a Nuclear Chicago Single Channel Analyzer, Model 27352, Spectrometer coupled to a Nuclear Chicago Decade Scaler, Model 27104. A 3-in. X 3-in. KaI(T1) scintillation crystal (Harshaw Chemical Co.) was used as a detector. The Beckman Model GC 4 was used for gas chromatographic characterization of extracted ternary complexes of iron. The Ushaped GC column was 4 feet long. The stationary phase was SE-30 (5%), adsorbed on Chromosorb W(100/120 mesh).
RESULTS AND DISCUSSION Fe(II1)-"FA-TBP System. The synergic extrac-
I0
2
[
PERCHLORATE NO TEP
t I d3
1
0 0 5 M TBP
2
3
4 PH
5
6
7
Figure 1. Extraction isotherms for chelate and synergic extraction of iron(ll1) trace, 0.1M H H F A
tion system Fe(II1)-HHFA-TBP has many common characteristics with the simpler extraction system Fe(II1)HHFA recently described by Scribner ( 7 ) .In both extraction systems, equilibrium is slowly attained and, as noticed in this work, is accompanied by a considerable increase in the acidity of the aqueous phase. The acidity of aqueous phases reported in this paper refers to the values measured after the extraction equilibrium was attained. In Figure 1, the data for the extraction of tracer iron(II1) with 0.1M HHFA are presented. The presence of 0.05M T B P in the organic phase accounts for a pronounced synergic effect. Both extraction isotherms have very similar forms and the parallel spacing between them indicates an average synergic enhancement of a factor of 30 under the given experimental conditions. The extraction maximums occurred at pH 4.5 regardless of whether TBP was present in the organic phase or not. The composition of the synergic species extracted in this system can be expressed as Fe(HFA),X,(OH),(TBP),(HZO)~, where m n p = 3 is required for electrical neutrality of the extracted species. Stary has shown that the extracted species in related systems as a rule do not contain the hydroxide ligand and the term p can probably be neglected (8). Recently Agett has shown that the extraction of iron(II1) in the system iron(II1)-HACACT O P 0 is influenced by the concentration of perclilorate in the aqueous phase ( 5 ) . He claimed that the pronounced synergic effect in this system is due to the formation of the extractable ionic pair [Fe(ACAC)22TOPO]+C104]-. The position of the synergic maximum for this ion pair extraction was found at much lower pH values than for the maximum in the iron(II1)-HACAC system alone which indicated two different extraction mechanisms. For the system iron(111)-HHFA-TBP, the nature of the anion did not affect the extraction of iron as shown in Figure 1 where the aqueous phase was 0.1M in sodium chloride, acetate, or perchlorate. This suggests that the extraction can be described by the over-all reaction.
+ +
I
I
I
I
10
20
30
40
I
50
I
60
70
VH
Figure 2. Effect of the initial iron(l1l) concentration on the synergic extractionof iron(lll), 0.1M H H F A the distribution ratios were calculated. The reliability of this method was confirmed by the determination of the concentration of iron(I1) left in the aqueous phase by the 1,lO-phenanthroline spectrophotometric method, Both methods gave essentially identical results. The spectrophotometric method was used to determine F a F ~ , ~ rather I , than the radiometric method because of the possible oxidation of iron(I1). The extraction of iron(II1) would lead to erroneous values for E O a F e (. ~A~solution of hydroxylamine hydrochloride was added ( u p to 1%) to the extraction systems. Apparatus. Spectrophotometric measurements were performed using a Cary Model 14 Recording Spectrophotometer. Infrared spectra were recorded using a Beckman IR-7 infrared spectrophotometer. The cyclohexane extracts were placed in KBr cells and recorded against cyclohexane as a blank. All p H measurements of aqueous phases were performed using a Beckman Model G, pH Meter. TGA and DTA characterizations of cyclohexane-free extracts were performed using a DuPont 950 Thermogravimetric 1520
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, J U L Y 1973
Fe3+
+ 3HHFA + nTBP =+
Fe(HFAMTBP),
+ 3Hf
(1)
An increase in the concentration of iron(II1) does not significantly change the distribution ratio in the system Fe(II1)-HHFA, providing that the pH and initial HHFA concentrations are kept constant. Therefore, the extraction isotherms in the whole concentration range up to 3 X lO-3M in iron(II1) are practically identical. The synergic extraction of iron(III), unlike the simpler binary system, is dependent on the initial concentration of iron(II1). The set of extraction isotherms obtained in the synergic system iron(III)-O.lM "FA-O.05M TBP, varying the initial concentration of iron(II1) from tracer to 4 X lO-3M is shown in Figure 2 . The dependence of the distri(7) W. Scribner. "Solvent Extraction of Metal Fiuoroacetylacetonates." ARL 68-0193 (1 968). (8) J. Stary, "The Solvent Extraction of Metal Chelates," Pergarnon Press, Oxford, 1964.
I l l 1 I
I
I
I I I I I I I
Fe (Ill) TRACE
1
tc
A
d 3.0
/i
0
c K e z 0
t I OO
L -
165
10-3
D e (11 111 IN IT l P L Figure 3. Dependence of Eaa,Fe(IIIIon of Fe(lll),0.1M H H F A
Ilj2
the initial concentration
I,;:/
I
, v I02
Figure 5. ( I l l ) as a
IO'
I
llllj I00
Extraction isotherms for synergic extraction of ironfunction of the H H F A concentration at constant p H
value
PH
Figure 4. Extraction isotherms ( I l l ) as afunction of p H
for the synergic extraction of iron-
bution ratio of the metallic species on its initial concentration of the metal ion is commonly explained as due to the formation of polymeric species, in the organic phase (9). By the intersection of the p H profiles from Figure 2 a t chosen p H values, sets of E O a ( ~ ereferring ( I I I !tolconstant p H values were estimated. These values were plotted us. initial concentration of iron(III), and the results are presented in Figure 3. Theije lines show an approximately linear increase of distribution ratio of iron(II1) with concentration of iron(II1) and this dependence can be empirically presented in the form log Eoa(FeiIII,,= 1/3 log Fe(III)inc + K, where K is a constant which depends solely on the acidity of the aqueous phase. The slope of 0.33 is not consistent with even dimer formation in the organic phase (9) Y. Marcus and A . S. Kertes "Ion ,Exchange and Solvent Extraction of Metal Complexes." Wiley-Interscience, New York, N.Y., 1969.
and more likely reflects a change in the fraction of iron( I n ) present in various forms in the aqueous phase as a function of the initial iron(II1) concentration. The experimental results for the synergic system a t tracer iron(II1) concentration, as a function of the initial concentration of HHFA and the p H of the aqueous phase, are shown in Figures 4 and 5 for the TBP concentration of 0.05M. The pH profiles, which were obtained by changing the initial concentration of HHFA in aqueous phase, show a pronounced curvature in the whole p H range investigated and with slopes not exceeding the value of 1.5. By intersecting the p H profiles a t chosen p H values, the set of E",,F,,II~ , , was re-estimated and plotted as a function of initial concentration of HHFA in the aqueous phase. In this way, a number of HHFA profiles were obtained as shown in Figure 5 with slopes ranging from 1.0 to 1.4. The fraction of iron(II1) present in the aqueous phase in any one form is the function of the p H and aqueous HHFA concentration; the concentration of aqueous HHFA is the function of pH and TBP. Consequently the fraction of TBP present as the free base in the organic phase depends on the p H and initial HHFA concentration (10). Obviously, the simple slope analysis in such a complicated system is precluded. Present data on the extraction of HHFA and the nature of the interaction of HHFA with T B P are not sufficiently exact to justify a more complete analysis of the above system. Over a limited concentration range, the unit slope of log-log plots of the distribution ratio cs. the TBP concentration does suggest the species in the organic phase is Fe(HFA)sTBP. By a further increase in the TBP concentration in organic phase, the destruction of the synergic effect is emphasized (Figure 6). The destruction of synergism a t higher TBP concentrations in similar systems has been reported, and possible explanations given by Healy et al. (11) and Wang et al. ( 4 ) . Both authors realized that water is extracted into the organic phase. Wang gave a plausible explanation of the destruction of (10) B. 8.T o m a X a n d J. W. O'Laughlin, A n a / . Chem., 43, 106 (1973). (11) T. V . Healy. D. F. Peppard, and G. W . Mason, J . Inorg. Nucl. Chem.. 2 4 , 1429 (1962).
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 8, JULY 1973
1521
101
-
= I2
.?
-
.6
-
.5
-
00
w
100
e5 a
W
z u a
p
z 0
2 -a
.4-
l 0 n m
I-
a
10-1
+
.3 -
2 c I
10-2
1 I I11111
I
1
I
I 1 1 1 1 1 1
10-3
10-4
I
I I IIIII
10-2
I
I
I
.2
-
.I
-
I I,
100
10-1
M TBP
Figure 6. Synergic extraction of iron(ll1). Effect of the concentration of TBP on synergism and destruction of synergism; 10-3M iron(lll),0.1M H H F A
I
300
I
400
500
600
WAVELENGTH (mp)
Figure 8. Spectra of iron(I I I ) chelate and iron ( I 1 I ) adduct
--t
HHFA
SLOPE 12 101
00
w
:-
g
-
Z
F -
5 loo?0
2
s
i
10-1:
--/
10-2- 1 1 1 1 1 1 1 1 10-3
10-2 M TBP
10-1
IO0
Figure 7. Synergic extraction of Fe(l1). Effect of concentration of T B P on synergism and destruction of synergism; 10-3M iron(II), 0.1M H H F A
Table I. Formation Constant for t h e Reaction: Fe(HFA)sOrg f TBP0rg Fe(HFA)3TBPOrg I r o n j i l l ) concn
P1
1.1 x 10-4 5.0 x 10-4
1.92x 10-4 1.91 x 10-4
n
6 10
S‘
0.52 x 10-4 0.50 x 10-4
synergism caused by the acid catalysed hydrolysis of the synergic species Zn(HFA)2.2 TOPO, accompanied by the subsequent formation of the extractable species HHFA.2H20.(TOP0)2. The present authors have shown that the extraction of water and HHFA into the organic phase containing TBP is more pronounced a t higher p H values because sodium extraction takes place simultaneously. This extraction of HHFA results in a decrease in the free HHFA concentration in the aqueous phase available to complex with iron(II1) or other metals. We would emphasize the possibility of destruction of synergism as a consequence of the reaction: 1522
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
+ 2H2O + 2TBP
HHFA.2H20.2TBP (2)
which was shown to be responsible for the extraction of HHFA into the organic phase containing T B P (IO). This conclusion is in agreement with Marcus who assumed that “at high neutral ester or alcohol concentration, the antagonistic effect is due to competition between water and alcohol and the metal diketonate for the neutral ligand molecules in the system” (9). Spectral Observations on the Iron(II1)-HHFA-TBP System. When the organic and aqueous phases are first brought into contact in the simple binary system, a yellow color appears in the organic phase but, depending on the iron(II1) concentration, the color of the organic phase changes to orange or red during the course of the equilibration process. The visible spectrum of the organic phase after equilibration when the iron(II1) concentration was 1.1 x 10-4M is shown in Figure 7 . The organic phase of the iron(II1) synergic system has a much brighter yellow color; when the organic phase was separated from the aqueous phase and allowed to stand overnight, the color changed to dark yellow or brown. When the organic phase was again equilibrated with the aqueous phase, the bright yellow color again appeared. The darker yellow or brown colored species is apparently due to the loss of T B P from the ternary adduct. The driving force causing such a reaction may involve the slow formation of polymeric species in the organic phase. The return of the yellow color on reequilibration of the organic phase with the aqueous phase is apparently the result of the iron(II1) repartitioning between the two phases. The yellow color attributed to the ternary complex could also be regenerated by adding an excess of TBP to the organic phase as shown in Figure 7 where the TBP/Fe ratio was increased to 200. The molar absorption of the ternary adduct a t 340 nm was estimated a t 7680 cm-I mol-1 and that of the binary complex alone as 2450 cm-1 mol-1. These values were used to estimate values for the constant, PI,
(3)
Table II. Relative Ratio of Fe(lll)/Fe(ll) Adducts Present in Organic Phase after Back-Extraction with Aqueous Solutions of HHFA; pH = 3.3, Feorg = 10-3M TBP: Fe ratio in organic phase
0.1
0.05
0.03 Fe( I I l ) / F e ( l I ) ratio
> 20 2.1 0.82
>20 0.68 0.61
0.01
I TBP 2 Fe(II)ADDUCT
0
> 20 0.41 0.16
0.2 (TBP)2 in the Region 4000-650 c m - ' a Fe(HFA)3 670 msb 747 m s b 8 1 4 ssb
... ... ... 11 1 4 ssb 1 1 4 8 ss 1175 s s b
Fe(HFA)3TBP 670 rns 748 ws 8 0 0 ss 8 7 4 ws 9 1 2 ws 1040 ss
... 1158 ss
...
... 1 2 2 4 ssb
1217ss
1258 sbb
1265 sb (1290 sh) 1358 ss 1478 sb
... 1355 ws 1430-1470 sbb 1540 ws 1565 ws 161 8 ssb 1643 rnsb 1744 wb (1793 ss)
...
Fe(HFA)2(TBP)1 6 7 0 rns 745 ws 794 ss 875 ws 912 ws 1040 ss
... 1 1 5 3 ss
... 1205 ss
... 1265 sb
...
... ...
1358 ws 1 4 8 0 rnb 1 5 2 2 rns 1 5 5 0 rns
... 1655 ss
1 6 3 5 ss
...
... , . .
(1 793 ss) ... ... 1995 w b ... ... 261 2 ss 2612 ss ... 2710 ws 2702 ws ... 2995 sb 2990 sb ... 31 70 w b 31 70 w b ... 3460 w b 3460 w b a KBr cells, cyclohexane diluent. Extracts of ternary complexes have an excess of TBP to eliminate the presence of the binary complex. b A s signed in ( 7 5 ) . ss, strong sharp, ms. medium sharp; b, broad; sh, shoulder.
change of the iron(II1) adduct in the apparatus for the determination of the melting point; in the interval from 100-140 "C, the brown compound gradually turned violet. Additional evidence for this conversion was obtained from the DTA measurements. As shown in Figure 10, a flat 1524
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
0 0.8
0.01
0.55
broad exothermic peak appeared a t -150 "C for the Fe(HFA)3TBP sample. This apparently corresponds to the reduction of the iron(II1) compound. The second broad peak corresponds to the evaporation of the reduced adduct and the excess TBP, which may be concluded since a t the same temperature, endothermic peaks were obtained for the Fe(HFA)2TBP2 and TBP samples. The peak of TBP alone is much sharper, and DTA cannot resolve processes of evaporation of TBP and synergic species. The results indicate that the formation constant of the iron(I1) adduct is rather low, and one might expect that a t elevated temperatures, i. e., GC conditions, a partial decomposition of the complex might occur. The thermogram for Fe(HFA)sTBP shows a small tailing indicating that some decomposition products which formed from the reduction of the iron(II1) adduct are not volatile. The amount of residue is dependent on the ratio of TBP:iron(III) in the extract, as shown in Table 111. Apparently the excess T B P in the extract improves the volatility of the whole mixture. This may be due to a thermal instability of the iron(I1) adduct formed after the iron(II1) adduct was reduced; the formation of the former compound requires an additional molecule of TB,P and if it is not available, a partial decomposition results. Infrared Spectra of Iron Binary and Ternary Complexes. The infrared absorption bands of Fe(HFA)3, Fe(HFA)3TBP, and Fe(HFA)2(TBP)2 are outlined in Table IV. The absorption data of Fe(HFA)3 are in close agreement with the data published by Morris e t ai.,(15). It is important to note the presence of a sharp band a t 1793 cm-1, present in freshly prepared cyclohexane extracts. This band is indicative of free C=O stretch in anhydrous HHFA ( 7 ) . However, it is not likely that free HHFA is present in the organic phase, because its distribution ratio in the cyclohexane-water system is very low (10).The observed C=O stretch could be explained by assuming that one of the HFA- ligands acts as a monodentate ligand and the sixth coordination of iron(II1) in chelate compound is satisfied by the coordination of a water molecule. The existence of mixed adduct compounds of such type was confirmed in many cases (9) and, apparently, the stability of such compounds is rather low. Prolonged heating of the extract of the iron(II1) chelate resulted in a gradual decrease and finally complete disappearance of the 1793 cm-1 band, the rest of the spectrum being unaffected. This indicates the closure of the ring structure which presumably takes place after the water molecule is released from chelate structure. The freshly prepared synergic extract of iron(II1) shows the same 1793 cm-1 band. Healy suggested that the chelate ring in several synergic species may remain open to allow the donor molecule t o enter and satisfy the coordination requirements of the central atom ( 1 6 ) . Present in(15) M. L. Morris, R. W. Moshier. and R. E. Sievers. lnorg. Chem.. 2, 411 (1963). (16) T. V . Healy, Nucl. Scl. Eng.. 16, 413 (1963).
Table V I . Data on the GC Peak Height Resulting after Injecting 0.01M Fe(ll)-O.1M TBP-Cyclohexane Extract, and Corresponding Standard Deviations Calculated from Ten Independent Measurements 1 .o 0.462 0.032
Volume, pI G C peak height, arbitrary units Standard deviation, S'
1.5 0.996 0.036
frared data support the hypothesis of "one and dissociation" of ligand in the binary as well as in the ternary comr'lex of iron(1II). However, if the concentration of TBP in Lie organic phase is rather high, the gradual decrease of the 1793 cm-I band takes place. The excess of T B P may interact with synergic species a t the free carbonyl site, possibly through a hydrogen bridging mechanism, which accounts for the disappearance of the carbonyl band. In this case, the formation of a species of the type:
2.0 2.22 0.188
4.0 5.92 0.168
3.0 3.968 0.123
2.5 2.98 0.122
6
--2
5.0 6.54 0.309
U
5
2 3
t
a
4
a k K
m
a 3
5 I-
O-P(OBU)~
I
;2
,OH,
0
0-C-CH-C
I CF',
1 'OH"'
"O-P(OBu)s
CF; probably occurs. The infrared spectrum of the iron(I1) ternary complex does not show a band a t 1793 cm-1, indicating that there is no free carbonyl group in the extracted species. This indicates that the coordination requirements of iron(I1) are satisfied with bidentate ligation of two HFA- anions along with two T B P molecules, which is in agreement with extraction data. The analyses of water content in cyclohexane extract of iron(II1) che!ate and iron(II1) adducts are presented in Table V. For comparison, the results of water content in the organic phases not containing iron(II1) after equilibration with the aqueous phases are listed in the same Table. The considerably high ratios found for the chelate extraction indicate the possibility that the binary complex is extracted in the form of hydrate which supports the infrared evidence that one of the ligands acts as a monodentate ligand. When T B P was added to cyclohexane chelate extract, the ternary complex was formed immediately, as observed by the change of visible spectra. The infrared spectrum of this mixture shows that the 1793 cm-1 band was not affected by formation of the adduct, which supports the idea that the formation of the ternary complex in the organic phase proceeds through a TBP-Hz0 exchange reaction in the binary complex. The bands 1635 cm-1 and 1655 cm-l characterizing iron(I1) and iron(II1) adduct, respectively, can be assigned to metal chelated carbonyl absorptions. The formation of adduct compounds actually results in the shift of the C=C stretch found and assigned for the binary complexes Fe(HFA)z and Fe(HFA)3 which both show a doublet a t 1613 and 1641 cm-1. As Belford et al. pointed out (17), the increase of frequency of the carbonyl band indicates a lower stability of metal chelate resulting from increased fluorine substitution in the molecule. If this rule can be applied in this case t o compare the stability of iron(I1) and iron(II1) adducts interacting with the same ligand, this indicates that the C-0-Fe interaction is stronger in the case of iron(I1) adduct. The original P=O stretch from T B P is affected because of the interaction with iron because it is decreased and shifted toward the lower frequencies. Presumably it makes the composite band (17) R L Belford A E Martel and M Calvin, J lnorg Nuci Chern 2, 11 (1956)
Y
a n
w V
I
W
0 I1
1;44
0
@r
I
2
3
4
INJECTION SIZE. p!
Figure 11. Relationship of the G C peak height of the iron(ll) adduct on injection size and TBP iron(l I ) ratio in the extract. 1O-'M iron ( I I ) adduct, cyclohexane solvent. Column temperature, 180 " C
with one of the CF3 bands which characterize iron binary complexes, as shown by Morris et a / (15). Katzin assigned the bands in the 1183-1237 cm-1 region to the PO-Fe(II1) interaction when iron(II1) was extracted with TBP from chloride or nitrate media (18). Probably the bands which were registered a t 1205 and 1217 cm-I in iron(I1) and iron(II1) adducts correspond to the P-0-Fe stretch in adduct compounds. Because of the bigger shift from normal T B P phosphoryl frequency, one could conclude that iron-donor interaction is stronger in the iron(11) adduct. This is quite possible, because steric effects in the iron(II1) adduct might result in a decrease in the interaction between the iron and the donor molecule. Gas Chromatographic Behavior of Iron Adducts. The GC behavior of the iron(I1) adduct presents an additional proof that the stability of the complex is low and dependent on the excess of TBP in the extract. The GC peak of the volatile iron(I1) adduct has an average retention time of 2.58 min under the given experimental conditions, and because the peak for T B P exhibits a pronounced tailing, a complete base-line separation between TBP and adduct was not achieved. The dependency of the adduct peak height on the injection size and the TBP:iron(II) ratio in the extract is presented in three-dimensional form in Fig(18) L I Katzin, J lnorg Nuci Chem 20, 300 (1961) A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 8, J U L Y 1973
1525
ure 11. As expected, the increase of injection size of the 10-2 iron(I1) adduct results in an almost linear increase in peak height. The data obtained by GC analysis for the injection of from 1 to 5 pl of a solution which was 0.01M in iron(I1) and 0.1M T B P in cyclohexane, are given in Table VI. The standard deviation for each point was obtained from 10 independent measurements. However, two calibration curves which were obtained by injecting extracts having TBP:iron(II) ratios of 5 : l and l O : l , respectively, differ considerably in slope; the lower the excess of TBP in the extract, the lower the slope. Apparently, the GC peak height is very sensitive to the TBP:Fe(II) ratios and a 1 : l ratio, no peak was obtained, just a pronounced tailing after the TBP peak. By increasing the ratio TBP: iron(11), the peak height corresponding to the iron(II) adduct progressively increases. The apparent reason for this is thermal dissociation:
Fe(HFA)2(TBP)2 == Fe(HFA)*
+ 2TBP
(7)
which is partially supressed by using a higher excess of TBP in extracts. This assumption was checked by sealing extracts containing different excesses of TBP in glass tubes and heating them to 200 "C. The color of the extracts changed from violet to dark red, the color of the chelate, Fe(HFA)2. The color change was more pronounced in extracts which contained lower excesses of TBP. After cooling, the color of the extract changed back to violet, and the visible and infrared spectra show again the presence of the iron(I1) adduct; however, the concentration of adduct had decreased. The decrease was more pronounced a t lower TBP:iron(II) ratios in the extracts. The TGA data have shown that the iron(II1) adduct is volatile as well, however, its GC behavior seems to be even more complicated. The iron(1II) adduct undergoes
thermal reduction, and as a result of this, as well as of a number of other nonpredictable reactions in the gas phase, the GC peaks were not reproducible. However, it is noteworthy that the retention time was the same as for the iron(I1) adduct, which confirms the assumption that under the GC conditions, the iron(II1) adduct undergoes thermal reduction. The rare earth adducts which were obtained in the same extraction system show a remarkable thermal stability, which makes possible their successful GC determination and separation even when no excess TBP was employed ( 2 ) .This may be explained as the consequence of stereochemical factors, as the average ionic radius for trivalent rare earth ions equals 1.05 A, compared with 0.76 and 0.64 A for iron(I1) and iron(II1) ions. A larger radius might explain the higher thermal stability of rare earth adducts. It seems possible that the adducts of other transition metals formed in the same extraction system could show a similar thermal instability as a consequence of steric crowding of the bulky ligands and donor molecules which surround the small central cation. The GC method, which appears to be a very promising method for the determination of low quantities of metallic species, necessarily reflects the coordination properties of analyzed compounds, and our present knowledge shows that the choice of appropriate ligands and donors can improve the thermal and GC characteristics of ternary complexes. Therefore, further studies of related systems, which will give new information on the coordination and thermal properties of ternary complexes, seem to merit attention.
ACKNOWLEDGMENT Purification of reagents by J. Richard was appreciated. Received for review November 4, 1971. Accepted January 24, 1973.
NOTES
Potentiometric Determination of Boron in Aluminum Oxide-Boron Carbide Using an Ion Specific Electrode H. E. Wilde Nuclear Laboratories, Combustion Division, Combustion Engineering. Inc., Windsor, Conn. 06095
The determination of boron in materials has traditionally depended upon the successful separation of boric acid from interfering elements. The techniques for this separation are usually long and tedious ( 1 ) . Recently, advances in liquid ion exchange membrane electrodes have made it possible to potentiometrically determine boron as tetrafluoroborate in agricultural samples. This technique of boron determination compares favorably in accuracy and sensitivity with the older technique and, in addition, it is much more convenient and less time consuming (2, 3). In particular, the potentiometric technique is free of the in(1) H. Blurnentha1,Anai. Chem., 23,992 (1951). (2) R. M . Carlson and J. L. Paul, Anal. Chem., 40, 1292 (1968). (3) R . M. Carlson and J. L. Paul, SoilSci. 108, No. 4 (1969).
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
terferences that plague the other methods. We have recently extended this technique t o boron determination in crystalline solids. This article discusses the procedure and results for boron determination in aluminum oxide-boron carbide (A1203 BIG) matrices, which are important materials as burnable poisons (consumable neutron absorbers) in nuclear reactors. One major difference exists between boron determination in A1203. B4C and the previously described determination (2, 3) involving agricultural samples. Whereas interfering elements in the agricultural or water samples form soluble salts and can be separated from tetrafluoroborate by using boron specific resins, similar separation is not feasible when the samples are crystalline solids which form insoluble salts that interfere physically with the ac-
