Solvent Extraction of Metal Ions with Trifluoroacetylacetone-lsobutyla mine W. G. SCRIBNER and ANTOINEllE M. KOTECKI Monsanto Research Corp., Dayton laboratory, Dayton, Ohio
b Nickel, cobalt, copper, zinc, cadmium, and palladium are rapidly extracted from aqueous media with chloroform containing tritluoroacetylacetone (TFA) and isobutylamine (IBA). Extractions are highly efficient, being generally greater than 99.9% at pH -9. Up to 0.5 mmole metal ion in 25 rrJ. of water containing 0 to 6 mmole acetic acid can be extracted b y an equal volume of chloroform, which i s 0.1M with respect to TFA and 0.4M with respect to isobutylamine. Although chloroform-TFA i s stable, chloroform-TFA-isobutylamine fresh solutions must be prepared and used within 2 hours. In the absence of an amine, nickel, zinc, cadmium, and cobalt are not efficiently extracted with chloroform-TFA, possibly because these metal trifluoroacetylacetonates also contain coordinated water. Addition of an amine to the system results in preferential formation of amine adducts, which are much more readily extracted with chloroform. Amine adduct formation was verified by infrared spectrophotometry and elemental analysis.
sorz
EXTRACTION of metal with trifluoroacetylacetone has been studied as a means to prepare organic solutiolis ut metals for gas liquid chromatographic analysis. Aluminum, gallium, and indium were extracted with benzene-trifluoroacetylacetone and determined by GLC (8). Similarly, copper, iron, and aluminum can be extracted with chloroformtrifluoroacetylacetone (11). Nickel and zinc, however, were extracted with poor efficiency by the latter solvent, presumably because of hydrate formation. Metal ions with a coordination number greater than twice the charge will, upon reaction with bidentate chelating agents, satisfy the coordination requirement via hydrate formation, solvate formation, or adduct formation. Hydrate formation generally is accompanied by poor solvent extraction efficiency, particularly with halohydrocarbon solvents, while solvate formation or adduct formation will permit or enhance extractions (1). Various
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ANALMICAL CHEMISTRT
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Beckman 39183 combination elec trode. Reagents. Trifluoroacetylacetone, purified as previously described (11), was used to prepare a 0.25M solution in chloroform. This was mixed in the proper ratio with a 0.67M solution of isobutylamine in chloroform to give the reagent-solvent : chloroform-trifluoroacetylacetone (0.1M) -is0 b u t ylamine(0.4M). The mixed reagent must be used within 2 hours. Approximately 0.05M solutions of copper, nickel, cobalt, zinc, and cadmium were prepared from reagent grade salts or high purity metals. The palladium(I1) solution, 0.01M, was prepared by adding the chloride to 10 ml. of 10% hydrochloric acid and diluting to 250 ml. Extraction Procedure. To 10.00 ml. of 0.05M metal ion contained in a 125-ml. separatory funnel, varying amounts of 2M acetic acid and water were added to give an aqueous phase volume of 25.0 ml. Then 25.0 ml. of premixed chloroform-trifluoroacetylacetone-isobutylamine were added. The mixture was shaken for 10 minutes, and the phases were allowed to separate. The equilibrium pH of the aqueous phase was measured. An aliquot of the aqueous phase was removed for determination of the amount of unextracted metal ion by titration with 0.05M or 0.005M (ethylenedinitrilo) tetraacetate or by spectro-
examples of adduct formation have been reviewed recently (10). To expand the scope of GLC analysis to divalent metal trifluoroacetylacetonates, a study of adduct formation to achieve extraction was undertaken. Preliminary results indicated that nickel could be extracted with trifluoroacetylacetone with n-butanol, isoamylalcohol, or methyl isobutyl ketone as solvents. Isobutylamine, t-butylamine, n-butylamine, and pyridine permitted the extraction to occur using chloroform, a preferred solvent for GLC applications. Isobutylamine, was selected for detailed study of extraction efficiency as a function of pH. Gas liquid chromatographic elution of isobutylamine adducts of divalent metal trifluoroacetylacetonates has been accomplished in a study in progress (6). EXPERIMENTAL
Apparatus. A Burrell wrist action shaker was used for equilibration of aqueous and organic phases. Proton magnetic resonance spectra were obtained with a Varian Associates A-60 Spectrometer; infrared spectra were obtained by employing a Beckman IR4 spectrophotometer. A Beckman Expandomatic p H meter was used for pH measurements with a
100
80
20
0
2
4
6
8
10
PH
Figure 1. Solvent extraction of metal ions with chloroform-trifluoroacetylacetone (0.1 M)-isobutylamine (0.4M)
photometric analysis. Copper-PAN was used as the indicator for the titration of cobalt and nickel (4). Xylenol orange and Eriochrome Black T were used for zinc (7) and cadmium (9), and PAN was used for copper (2). Palladium was determined by adding concentrated hydrochloric acid to an aliquot of the aqueous phase and measuring the absorbance of chloropalladic acid at 475 mH (6). The solution must be boiled after the addition of hydrochloric acid to eliminate an interference caused by free trifluoroacetylacetone or isobutylamine. RESULTS AND DISCUSSION
Solvent Extraction. Results for the extractions of the various metal ions are presented in Figure 1. For nickel, cobalt, copper, zinc, and cadmium, highly efficient extractions occurred from equilibrium p H 8 t o 10. Up to 6 mmoles of acetic acid could be present initially and still permit effective extractions. Palladium(I1) was efficiently extracted over the pH range 5 to 10 in the presence of up to 20 mmoles of acetic acid. Copper was also extracted over this range, but the highest efficiency extractions occurred at p H 8-10. Extraction efficiencies of 99.9% or greater were observed in the optimum p H region as evidenced by the fact that less than 0.1 ml. of 0.005M EDTA was required to titrate the metal ion remaining in the aqueous phase. Titration blanks were 0.02 ml. A marked influence of reagent age on extraction efficiency was noted. I n the case of nickel, efficiency was constant a t 99.9% with reagent up to 100 minutes old, but dropped to 98yo a t 135 minutes and to 60% a t 155 minutes. Poor efficiency extractions were accompanied by a trace of precipitate a t the interface of the two phases. The reagent is rendered ineffective by a slow reaction in chloroform between trifluoroacetylacetone and isobutylamine. The reaction was studied by proton magnetic resonance. An equimolar solution of TFA and isobutylamine in deuterated chloroform was maintained a t 39OC. and rotated continuously while spectra were recorded. Several changes in the spectra were observed with increasing reaction time. I n general, the data are compatible with the removal of the enol proton from the trifluoroacetylacetone and the formation of a protonated amine group. Based on changes in isobutylamine peaks, the reaction is 50% complete in 330 minutes a t 39OC. The influence of EDTA on the extraction of the colored metal ions was qualitatively studied. EDTA prevented the extraction of copper, cobalt, and nickel a t p H 9-9.5. But if already extracted, copper could be quickly removed from the chloroform phase by
shaking with aqueous EDTA (pH 9.5). Under these conditions, nickel was not removed, and cobalt was partially removed. Cobalt and nickel were, how'ever, quickly removed a t p H 8.9 and 7.4, respectively. Characterization of Adducts. Residues obtained by evaporation of the chloroform phase were characterized by infrared spectra and by elemental analysis. From experiments where the equilibrium p H was about 9.5, the chloroform phase yielded isolates which were identified by elemental analysis as bis-isobutylamine adducts of nickel, cobalt, copper, and cadmium trifluoroacetylacetonates (Table I). Zinc, however, formed a mono-isobutylamine adduct and palladium formed a tetraamine compound. It is conceivable that with the palladium compound, the trifluoroacetylacetone is behaving as a monodentate ligand. Structures involving both monodentate and bidentate thenoyltrifluoroacetoneorganophosphorus adducts have been reported (3). The infrared spectra of the adducts were similar to the spectra for the parent chelates, with the addition in every case of absorptions principally a t 3220(N-H 3290(m) and 3120-3150(w) stretching vibrations), 2960(m) and 2900(w) (aliphatic C-H stretching vibrations), 1495(m) (aliphatic C-H deformation), and 1025-1030(m) cm.-l (C-N stretching vibration). The nickel, cadmium, cobalt, zinc, and palladium compounds could be dried a t 50°C. and 125 mm. for 1 hour without loss of isobutylamine. The copper compound, however, readily lost isobutylamine under these conditions and formed copper trifluoroacetylacetonate. This conversion occurs slowly with a sample open to the atmosphere a t room temperature also. A marked change in the nature of the extracted species was observed with decreasing equilibrium pH. For the colored metal ions this was evident by a pronounced change in the color of the chloroform layers. For example, nickel was blue-green in p H regions of highly efficient extractions, but progressed from yellow-green to mint green in regions of partial extraction. Cobalt was rust colored a t p H 10, yellow-orange a t pH 9, yellow-brown a t p H 8 and red-pink a t p H 6. Copper was Kelly green a t p H 9-10 and blue-gray a t pH 6. Results of an elemental analysis of a cobalt isolate a t p H 7.4 showed a considerably reduced nitrogen content which could not be made to fit any stoichiometric amine adduct or mixed hydrate amine. This change was also substantiated by infrared spectra, which showed decreased intensity of the 1495, 1025, etc., cm.-' absorptions. These compositional changes are probably a reflection of the increasing
Table I. Elemental Analyses of Divalent Metal Trifluoroacetylacetonatelsobytylamine Adducts
Per cent
EXCu(TFA)Z.PIBA C H Ni( TFA)2.2IBA
~~
Co(TFA)z *2IBA Zn(TFAh ,-.IBA Cd(TFA)2 .2IBA
pected 41.90 5.86 5.43 42.30 ~. 5.92 5.48 42.28 5.91 5.48 37.81 4.31 3.15 38.27 5.35 4.96 44.29 7.43 7.95
H N
C H N Pd(TFA)z*4IBA C H N
Found 41.74 5.70 5.53 42.41 5.95 5.57 42.00 5.92 5.55 37.71 4.21 3.18 38.09 5.10 4.94 44.21 7.13 7.53
competition of the hydrogen ion with the metal chelate for the amine. Use of a weaker base permits extractions to occur a t lower p H values. Nickel, for example, was effectively extracted a t pH 4.3 with chloroform containing trifluoroacetylacetone and pyridine. Although the isolated zinc compound was established as a mono-amine adduct, it is possible that the extracted species is a bis-amine adduct with the second amine molecule being rather weakly bonded. For this system it was considered of interest to determine if nuclear magnetic resonance techniques could be employed to determine if the extracted species differed from the isolated species. A proton magnetic resonance spectrum of 0.07M zinc trifluoroacetylacetonate mono-isobutylamine in deuterated chloroform was obtained. Then isobutylamine was added in four progressive stages to yield free amine concentrations of 0.03, 0.05, 0.09, and 0.12.M. NMR spectra were obtained after each addition. Although there were no major spectral changes detected as a consequence of the addition of isobutylamine, slight displacements of 2 to 3 C.P.S. were observed for the resonances characterizing the methine and methyl protons of the trifluoroacetylacetone. Because the chemical shift for residual chloroform in deuterated chloroform remained constant a t 439 C.P.S. during the amine addition, the change in bulk susceptibility which might account for these shifts is assumed negligible. Also, these displacements are greater than the repeatability of the instrument, zt0.5 C.P.S. The data therefore suggest a very slight increase in shielding of the methine and methyl protons of the trifluoroacetyIacetone ligand. The methine appears to be influenced greater than the methyl. This shielding sugVOL 37, NO. 1 1 , OCTOBER 1965
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gests that there is some weak association between the zinc trifluoroacetylacetonate mono-isobutylamine and added isobutylamine, which may be either hydrogen bonding between the methine proton of the TFA or bonding of the donor molecule with the central metal atom of the chelate. ACKNOWLEDGMENT
The authors gratefully acknowledge the NMR and other analyses by John V. Pustinger and the Monsanto Research Gorp. Instrumental Analysis Group.
LITERATURE CITED
(1) Alimarin, I. P., Zolotov, Y. A,. Talanfa 9,891 (1962). (2) Cheng. K. L.. Brav. R. H.. ANAL.
CHEM.i 7 . 7 8 2 11’955).” ‘ (3) Ferraro,’ J., Peppard, D., Nucl. Sci. Eng. 16,389 (1963). (4) Flaachka, H., Abdine, H., ChemistAnalyst 45,58 (1956). (5) Gere, D. R., Aerospace Research Laboratories, Wright-Patterson Air Force Base, Ohio, private communication, June 1965. (6) Kolthoff, I. M., Elving, P. J., “Treatise on Analytical Chemistry,” Part 11, Vol. 8 , p. 472, Interscience, New York, 1963. (7) Korbl, J., Pribil, R., Chemist-Analyst 45, 102 (1956).
(8) Morie, G. P., Sweet, T. R., ANAL. CHEM.37, in press. (9) Schwarzenbach, G., Flaschka, H.,
“Komplexone. Ftration mit Hilfe von Komplexonen. . , Firma B. Siegfried, Zofingen, Switzerland, 1953. (10) Schweitzer, G. K., Van Willis, W., “Advance in Analytical Chemistry and Instrumentation,” C. N. Reilley, ed., Interscience, New York, 1965 (in press). (11) Scribner, W. G., Treat, W. J., Weis, J. D., Moshior, R. W., ANAL. CHEM.37, 1136 (1965). RECEIVEDfor review June 28, 1965. Accepted August 11, 1965. Division of Analytical Chemistry, 149th Meeting, ACS, Detroit, Mich., April 1965.
Determination of Diethylene Glycol in Polyethylene Terephthalate J. R. KIRBY, A. J. BALDWIN, and R. H. HEIDNER Chemsfrand Research Center, Inc., Research Triangle Park, Box 73’1, Durham, N. C.
b A chemical method is described for determining the diethylene glycol content of a polyester polymer or fiber. Saponification with alcoholic potassium hydroxide is employed to decompose the polymer quantitatively into ethylene glycol, diethylene glycol, and dipotassium terephthalate. Following neutralization, and removal of precipitated potassium chloride and dipotassium terephthalate, the ethylene glycol is oxidized to formaldehyde with sodium metaperiodate. Interfering ionic species are removed by ion exchange. Subsequently, the volatile formaldehyde. and ethanol are removed prior to reaction of the diethylene glycol with a known excess of potassium dichromate. The residual dichromate is then determined by redox titrimetry with ferrous ammonium sulfate. Results expressed as mole per cent diethylene glycol terephthalate give directly the number of ester units that contain a diethylene glycol group per 100 repeating ester units in the polymer. Samples covering the range up to 25 mole per cent have been analyzed successfully by this method. At the 7 mole per cent level, the standard deviation is *0.07 mole per cent. As little as 1 mg. of diethylene glycol derived from a 1 gram sample of polyester can be detected.
P
OLYESTER FIBERS formed from essentially pure polyethylene terephthalate are noted for their excellent physical and thermal properties, good
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ANALYTICAL CHEMISTRY
color, and light stability. The presence of ether linkages such as result from the incorporation of diethylene glycol units into the polymer chain, however, is claimed to be seriously detrimental to fiber properties. Several references ( 1 , 4, 5 ) are noted, particularly in the patent literature pertaining to the adverse effects of copolymerized diethylene glycol on polyethylene terephthalate: polymer melting point is depressed, and fiber properties reflect a loss in light, thermal, and hydrolytic stability. Wash-and-wear properties may be seriously impaired, and accelerated dye fading may be anticipated. I n view of these asserted undesirable side effects, it became a matter of interest to be able to measure the ether content of polyethylene terephthalate. The polymerization of terephthalic acid and ethylene glycol to form polyethylene terephthalate can be written: n
Two methods have been reported in the literature for the determination of diethylene glycol copolymerized in polyethylene terephthalate. In both methods, measurement of diethylene glycol is made after it is freed from the polymer. I n the method of Janssen and coworkers (4),the polymer is transesterified under pressure with an excess of ethanol and the liberated diethylene glycol is measured by gas chromatography. This technique has two distinct disadvantages. Elaborate and expensive equipment is required and the time of analysis is long, some transesterifications requiring as much as 16 hours. In the procedure used by Mifune and Ishida ( 5 ) , the polymer is saponified in aqueous Ba(OH)2 to precipitate barium terephthalate, and the ethylene glycol is oxidized to formaldehyde with periodic acid. After removal of formaldehyde by distillation and reduction of periodate to iodate, diethylene glycol is oxidized with dichromate. The quantity of dichromate consumed is determined colorimetrically and is related to the amount of diethylene glycol present. This approach is based on the earlier work of Francis (3) who devised a procedure for measuring diethylene or dipropylene glycol in the presence of monoglycols. The method reported here, although similar in principle to the method of Mifune and Ishida, differs significantly
HOOC-DCOOH + n HO(CH2),0H -,
H O [ - O C -OCO
9
Ether linkages in the polymer can arise from the incorporation of glycolic ethers, such as diethylene glycol, HO(CH2)2-O-(GH2)20H, into the polymer chain in a manner analogous to the incorporation of ethylene glycol. Diethylene glycol results primarily from an undesirable side reaction, the dehydration of ethylene glycol, in the polymerization mixture :
