Table 11. Results for the Determination of Fluorine in Organic Compounds Fluorine Sample Theoretical Experimental pFluorobenzoic acid 13.56 13.7, 13.2, 1 3 . 1 Trifluoroacetanilide 30.14 29.8, 29.5 Perfluorooctanoic acid 68.8 67.7 Decafluorobiphenyl 56.8 56.0
z
sion of replicate extractions. From these data a detection limit of 5 pg may be expected: Absorbance minus blank (5-cm. cell) for 150 pg F-/5 ml aliquots 0.619 0.572 0.595 Mean = 0.591 Std dev = 0.014 Re1 std dev = 2 . 4 z
0.594 0.588 0.577
0.601 0.585 0.587
Determination of Fluorine in Organic Compounds. This extraction-spectrophotometric method was applied to several organic compounds as listed in Table 11. The results indicate an approximate 99% recovery of fluoride from the oxygen bomb combustion of organic compounds possessing different carbon-fluoride structures. The relative error of the determination appears to be no larger than the 2.4x relative standard deviation of the extraction step. This method may be applied to any organic compound which can be decomposed in a manner that does not introduce interfering ions. The total analysis can be carried out rapidly and accurately with good sensitivity. The determination of fluoride in inorganic materials by this technique should be attempted only if interfering substances are known to be absent, Perhaps if a similar extraction approach can be developed utilizing the strongest fluoride-complexing cations, the door would be open to a rapid and simple determination of fluoride ion in inorganic samples.
RECEIVED for review August 11, 1967. Accepted October 27, 1967.
Separation of Anionic and Cationic Metal Chelates by Thin-Layer Chromatography Judith L. Swain and James L. Sudmeier Department of Chemistry, Uniuersity of California, Los Angela, Calif. 90024 IN METAL CHELATE synthesis, a frequent problem is the determination of the extent of reaction and of the number of isomers formed. For example, in synthesis of aminocarboxylate chelates of platinum(I1) and cobalt(III), mixtures of isomers are often formed, not only of the cis-trans variety, but also those involving mixed donor groups. These chelates are sometimes anionic, and although there have been thin-layer chromatography (TLC) methods reported in the literature for cationic (1, 2) and neutrally charged chelates (3), no TLC method for anionic chelates has appeared. It has been suggested (1) that the separation of cationic chelates on silica gel is based on the weakly cation-exchanging properties of silica gel, owing to the presence of =Si-0groups. The objective of the present work was to develop a TLC method for anionic metal chelates. Among the advantages of TLC are its speed, versatility, high resolution, and ability to separate minute quantities of material (4). Although ionexchange methods are known for anionic chelates, they require larger quantities of material, and sometimes result in irreversible adsorption on the resin, particularly for anions of the higher charge types. An unexpected dividend of the present work is that methods were found which work equally well for both anionic and cationic chelates. In fact, the methods reported herein yield results which are equal to any previously reported cation method, and offer the advantage of greater generality. (1) L. F. Druding and R. B. Hagel, ANAL.CHEM., 38,478 (1966). (2) H. Seiler, C. Biebricher, and H. Erlenmeyer, H e h . Chim. Acra., 46, 2636 (1963). (3) G. B. Kauffman and B. W. Benson, Inorg. Chem., 6, 41 1 (1967). (4) E. Stahl, “Thin-Layer Chromatography”, Academic Press, New York, 1965.
418
ANALYTICAL CHEMISTRY
EXPERIMENTAL
Chemicals. Ethylenediaminetetraacetic acid (EDTA), reagent grade, was obtained from the J. T. Baker Chemical Co., trans-l,2-cyclohexanediaminetetraaceticacid (CyDTA) was obtained from Geigy Chemical Co., and ethylenediamineN,N’-diacetic acid (EDDA) was obtained from K and K Rare and Fine Chemicals, Inc. Aquo-(ethylenediaminetriacetatoacetic acid)-rhodium(II1) [Rh(III)EDTA-l] (3, dichloro - (tetrahydrogen- ethylenediamine- tetraacetate) - platnum(I1) 5-hydrate [Pt(II)CI,H,EDTA] (6), potassium (cyclohexanediarninetetraacetat0)-cobaltate(II1) 3-hydrate [Co(III)CyDTA-’1 (7), cis- and trans-dinitrobis(ethy1enediamine) cobalt(II1) nitrate [cis- and rrans-Co(III)(en)2(NO2)+] (8), trans - ethylenediamine - N,N’ - diacetato - (ethylenediamine)-cobalt(II1) nitrate [Co(III)EDDA(en)+] (9), transethylenediamine - N,N‘ - diacetato - (N,N’ - dimethylethylenediamine)-cobalt(II1) nitrate [Co(III)EDDA(dmen)+] (IO), trans - ethylenediamine - N,N’ - diacetato - (N,N‘ - diethylethylenediamine)-cobalt(II1) nitrate [Co(III)EDDA(deen)+] (IO), and tris(ethylenediamine)cobalt(III) chloride [Co(III)(en)3C13] (11) were prepared as described in the literature cited. Chromatography. Precoated silica gel plates [Merck (Darmstadt) F-2541 and an Eastman sandwich-type develop(5) F. P. Dwyer and F. L. Garvan, J. Am. Chem. SOC.,82, 4823 (1960). (6) D. H. Busch and J. C. Bailar, Ibid., 78, 716 (1956). (7) F. P. Dwyer and F. L. Garvan, Ibid., 83, 2610 (1961). (8) H. F. Holtzclaw, D. P. Sheetz, and B. D. McCarty, Znorg. Syn., 4, 176 (1953). (9) J. I. Legg and D. W. Cooke, Znorg. Chem., 4, 1576 (1965). (10) J. L. Sudrneier and G. Occupati, University of California, Los Angeles, unpublished data, 1967. (11) J. P. Work, Znorg. Syn., 2, 221 (1946).
ing chamber were used in this work. Iodine proved to be the most effective visualization agent. The usual capillary spotting techniques were employed, with water as the solvent, adding dilute NaOH to facilitate solution of some compounds. After oven-drying at 100" C for several minutes to evaporate the spotting solvent, the plates were developed by the ascending technique until the solvent attained a height of 7-8 cm. (requires -1 hr). The plates were then ovendried at 100" C for several minutes, and visualized in an iodine chamber. R, values were measured from the approximate centers of gravity of the spots, and are estimated accurate to =k0.05. RESULTS AND DISCUSSION Table I lists the developing solvents which showed enough promise in the present work to merit reporting. Some of these solvent mixtures are commonly used for separation of amino acids, and thus have the desirable property of separating unreacted aminocarboxylate ligands with ease. The solvents are divided into alkaline and acidic groups. The general formula for the alkaline solvents is 70 ml alcohol, 30 ml concentrated ammonium hydroxide, and sometimes 2 ml glacial acetic acid. The general formula for the acidic solvents is 70 ml alcohol, 20 ml glacial acetic acid, and 20 ml water. Table I also gives the R1 values of what is intended as a representative, though by no means inclusive selection of chelating agents and anionic and cationic metal chelates. The chelates are kinetically inert, and poor results are obtained for labile chelates using the methods described herein. For example, the EDTA chelates of Cu(I1) and CO(I1) apparently undergo dissociation during development on silica gel, leaving the metal ions trailing behind the EDTA. Preliminary experiments indicate, however, that such labile chelates might successfully be separated on anion exchange cellulose (e.g. Bio-Rad Cellex D, TLC grade) using horizontal elution with 0.1M NaC104. Table I shows that separations are achieved for chelates of widely varying charge types, and that cis- and trans- isomers of the same compound, for example, are easily separated. Some of the trends in Table I are as follows: for the chelating agents, anionic chelates, and most cationic chelates, the alkaline solvent systems produce significantly larger R, values than the acidic solvent systems and thus yield better separations, in general; the addition of 2 ml glacial acetic acid to alkaline solvents (e.g. compare E and F) lowers the R, values slightly for chelating agents, and causes a striking increase in the R, values of the cationic chelates (some of which have R, = 0 in the absence of acetic acid); and increasing the dielectric constant of the alcohol portion invariably increases the R, values of all chelating agents and metal chelates. SUMMARY
The best general purpose solvent system for use on silica gel plates is 70 ml 95% ethanol, 30 ml concentrated (30%) aqueous ammonium hydroxide, and 2 ml glacial acetic acid. This solvent is expected to separate most free chelating agents and kinetically inert anionic and cationic metal chelates (provided that sufficient solubility in this solvent is attained). The R, values can be tailored to meet specific needs by varying the dielectric constant of the 70 ml alcohol portion-i.e., using butanol, propanol, ethanol, methanol, or binary mixtures
Table I. Summary of Solvent and R, Data for Selected Chelating Agents and Metal Chelates
Solvent" A B
C D E
Compound EDDA trans-Co(III)EDDA(en)+1 EDTA CyDTA Co(II1)CyDTA-l Rh(II1)EDTA-1 Pt(1I)ClzHdEDTA EDTA CyDTA EDDA R(1I)ClzHIEDTA Co(II1)CyDTA-1 cis-Co(II1) (en)z(NOz)r+l
RI
0.16 0.21 0.15
0.19 0.41 0.48 0.15 0.30 0.30 0.24 0.23 0.73 0.50
0.63
rrans-Co(III)(en)z(NOz)z+ I
F
G
H I
trans-Co(III)EDDA(en)+l trans-Co(III)EDDA(dmen)+1 trans-Co(III)EDDA(deen) C0(11I)(en)~+~ EDTA CyDTA EDDA Rh(II1)EDTA-1 R(II)ClzH4EDTA Co(1II)CyDTA-1 cis-Co(III)(en)z(NO&+1 rrans-Co(III~en)z(NOz)z +1 trans-Co(III)EDDA(en)+l trans-Co(III)EDDA(dmen)+ I trans-Co(III)EDDA(deen) Co( EDDA Co(1II)CyDTA-1 CyDTA Co(1II)CyDTA-1 EDTA CyDTA EDDA Co(II1)CyDTA-1 cis-Co(III)(en)2(NOz)z +1
0.44
0.57 0.78 0.18 0.33 0.43 0.44
0.77 0.47 0.69 -0 NO
0.40 0.09 0.11 NO
0.56
0.68 0.18 0.18 0.15
0.15 0.13 0.57
0.29 0.35 truns-Co(III)EDDA(en)+l 0.31 trans-Co(III)EDDA(dmen)+ I 0.31 trans-Co(III)EDDA(den) 0.42 0.30 Co(III)(en),+a Alkaline solvents: A, propanol-NH3-HOAc (70: 30 :2); B, propanol-NH, (70 :30); C, propanol-ethanol-NHa-HOAc (30 : 30:30:2); D, propanol-ethanol-NH3 (30:30:40); E, ethanolNH3-HOAc (70:30:2); F, ethanol-NHt (70:30); G, methanolethanol-NH,-HOAc (30: 30: 40: 2). Acidic solvents: H, butanol-HOAc-HtO (80:20:20); I, ethanolHOAc-HZO (70:20:20). Concentrated (30z) aqueous ammonium hydroxide, glacial acetic acid, and 9 5 z ethanol used throughout. trans-C~(III)(en)~(NO~)~ +1
thereof-with the resulting Rl values being directly proportional to the dielectric constant. Research supported by the National Institutes of Health Grant No. 1-R01-AM10889-01. RECEIVED for review August 14, 1967. Accepted November 1, 1967.
VOL 40, NO. 2, FEBRUARY 1968
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