Rapid Extraction-Spectrophotometric Determination of Fluorine in Organic Materials William E. Dah1 Central Research Department, Monsanto Co., St. Louis, Mo. 63166
SEVERAL REVIEWS on the determination of fluorine have described the increasing importance of this element (1-3). In spite of the variety of approaches and methods, there is general agreement that this determination is difficult. Frequently, the interaction between fluoride ions and various metal-dye complexes is utilized. Measurement is made of the bleaching of metal-dye complexes as more stable metalfluorides are formed. Alternatively, fluoride is titrated by the metal ion with the dye present as an indicator. These and other methods suffer from the interference of diverse species capable of reacting with the fluoride, dye, or metal ion. When other compensations cannot be made for these interferences, the fluoride must be separated by a tedious and time-consuming distillation. Nondestructive measurement of fluorine in organic compounds has been achieved by fast neutron activation analysis (4). However, activation of impurities in an unknown matrix can cause serious error. Nuclear magnetic resonance for the accurate determination of fluorine in organic compounds requires 1 M solutions of the sample in a solvent such as CClr (5). The use of electrodes for sensing fluoride ion activity also shows promise (6). The first positive color reaction of the fluoride ion was introduced in 1959 (7). A blue complex was formed when Ce(II1) was added to a solution of alizarin complexone and fluoride ion. This sensitive method is not affected by the common anions with the exception of a moderate phosphate interference. Many metals, however, prevent the formation of the complex. Although solvent extraction may give promise of simple and selective fluoride analysis, few methods rely on this technique. Tetraphenylstibonium fluoride has been extracted into CC14 and determined by weighing after evaporating the solvent (8). Bromide, chloride, and metals which strongly complex fluoride interfered. A radiotracer method involved tantalum-1 82 extraction as a fluoride complex from 6.5Msulfuric acid solution into 2,6-dimethyl-4-heptanone (9). This technique offered a good tolerance to anions with phosphate a possible exception. The cations, Zr(IV), Fe(III), Al(III), Ca(I1) and UOz+*, interfered.
(1) P. J. Elving, C. A. Horton, and H. H. Willard, “Fluorine Chemistry,” J H. Simons, Ed., Vol. 11, Academic Press, New York, 1954, Chap. 111, pp. 51-211. (2) C. A. Horton, in “Advances in Analytical Chemistry and Instrumentation,” C. N. Reilly, Ed., Interscience, New York, London, 1960. (3) C. A. Horton, in “Treatise on Analytical Chemistry,” I. M. Kolthoff and P. J. Elving, Eds., Part 11, Section A, Vol. 7, Interscience, New York, 1961, pp. 207-334. (4) Robert Blackburn, ANAL.CHEM., 36, 669 (1964). (5) P. J. Paulsen and W. D. Cooke, Ibid.,36, 1713 (1964). (6) M. S. Frant and J. W. Ross, Jr., Science, 154, 1553 (1966). (7) Ronald Belcher, M. A. Leonard, and T. S . West, J. Chem. SOC., 1959, p. 3577. (8) K. D. Moffett, J. R. Simmler, and H. A. Potratz, ANAL. CHEM., 28, 1356 (1956). (9) F. L. Moore, Ibid.,35, 1032 (1963).
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The use of triple complexes in photometric analysis was reviewed recently (IO). These complexes, formed when a third component enters into the composition of the coordination sphere, have been applied to the determination of tantalum ( I I , 1 2 ) . Triphenylmethane dyes (methyl violet and malachite green) interacting with fluoride complexes of tantalum have been selectively extracted from aqueous solutions into benzene. In this study, an extraction-spectrophotometric method is described for fluoride determination which utilizes the tantalum-fluoride-malachite green complex. The method is not greatly influenced by diverse anions. However, the presence of cations which form stronger fluoride complexes than tantalum will cause results to be low; thus the method is applicable principally to organic compounds. EXPERIMENTAL
Apparatus. A Parr No. 1101 double valve oxygen bomb with associated oxygen filling equipment was used to combust the organic samples. Absorbance readings on the extracts were made with a Beckman Model B spectrophotometer using 5-cm cells. Reagents. 0 . 2 x TANTALUM SOLUTION.Fuse 0.2442 gram of purified Taz06 with 2.5 grams of sodium pyrosulfate at 800-50” C. Dissolve the cooled fusion cake in 100 ml of distilled water containing 4 ml of concentrated H2S04and 20 grams of tartaric acid (to keep the tantalum salt from hydrolyzing). Stir and gently boil the solution until it is clear, adding more water if necessary. Filter and dilute slowly with rapid stirring to 1 liter with distilled water. All chemicals used in this study were reagent grade unless otherwise specified. COMPOSITE TANTALUM-MALCHITE GREENREAGENT.Dissolve 0.12 gram of malachite green oxalate, about 1.2 grams of sodium tartrate . 2Hz0, and about 0.066 gram of sodium fluoride in 100 ml of the 0.2% tantalum solution. The necessary amounts of sodium tartrate (to adjust the pH) and sodium fluoride (to compensate for traces of strong fluoride complexers present in the other reagents) should be determined experimentally for each fresh liter of 0.2% tantalum solution. A reagent mixture containing no added sodium fluoride would not be sensitive to low levels of fluoride in the sample solutions. The correct amount to add is determined by increasing the sodium fluoride content in single-run mixtures of malachite green, sodium tartrate, and tantalum solutions until the extractable color increases markedly with further addition, From this point, sensitivity will continue to increase with further addition (the reagent blank will also increase, of course). Since the reagents are stable for at least 2 weeks when mixed and their order of addition is not critical, a composite reagent is used. The sensitivity remains constant, but blank and standard absorbances decrease slowly with time, Thus, it is advisable to standardize with each use. (10) A. K. Babko, Pure Appl. Chem., 10, 557 (1965). (11) N. S. Polvetkov, L. I. Kononenko, and R S . Laver, J. Anal. Chem. U.S.S.R.13,449 (1958). (12) Yachiyo Kakita and Hidehiro Goto, ANAL.CHEM., 34, 618 (1962).
Procedure. Weigh 0.1 to 1 gram of sample into the sample capsule of an oxygen bomb. Solid samples should be thoroughly moistened with methanol. Place the capsule in the bomb along with 10 ml of 5 % sodium tartrate solution. Afix the fuse, seal, and charge the bomb to 30-atm pressure following the standard directions (13). Immerse the bomb in a cooling bath, fire it, and allow it to cool. Shake the bomb for 2 minutes, slowly and carefully release the pressure, and dilute the contents to 1 liter (or a volume such that the final concentration is 10-40 pg F/ml) using polyethylene volumetric ware. Adjust the pH of the sample solution to 6 d= 1 with pH paper. With a polyethylene pipet, transfer a 5-ml aliquot of the sample solution into a 4-oz polyethylene bottle fitted with a leak-proof cap. Add 5 ml of the composite tantalummalachite green reagent and 25 ml of benzene. Extract by shaking the mixture for 2 minutes (an automatic shaker was used in this study). Allow the phases to separate, transfer the organic extract to a 5-cm cuvet, and read the absorbance at 630 mp against a benzene reference. Prepare a calibration curve by following the extraction procedure with standard NaF solutions. Determine the fluoride content of the sample by referring to the calibration curve or by closely bracketing the sample level with standards. RESULTS AND DISCUSSION
Selection of Conditions. TANTALUM CONCENTRATION. The presence of excess tantalum is required for efficient extraction in this system. A sensitivity increase of about 15-fold was obtained on increasing the tantalum concentration from 0.005 to 0.2%. The use of a higher concentration than 0.2% should result in a higher partition ratio for the extraction of fluoride ion, and suitable experimentation is recommended if greater sensitivity is desired. The partition ratio for extractable fluoride was determined by multiple extractions to be slightly greater than 1 when 0.275 tantalum is used. The sensitivity achieved with one extraction is more than adequate for the applications described herein. The dye concentraMALACHITE GREENCONCENTRATION. tion was optimized experimentally at 0 and 40 pg/ml fluoride levels using 0 . 2 % tantalum solution. Sensitivity increased as a linear function of dye concentration until reaching a level of 0.12 malachite green in the tantalum-malachite green mixture. At higher dye levels, only the blank increased. OPTIMUMEXTRACTION PH. With 0.12% dye in 0.2% tantalum solution, extractions were carried out at several acidities. Sensitivity increased with increasing pH until a plateau was reached at about 1.7. The plateau extended from 1.7 to beyond 2, but slowly increasing blank levels indicated that a pH of 1.75 should be optimum. The acidity of the composite tantalum-malachite green reagent was adjusted experimentally so that this desired pH was obtained when reagent and sample solutions were mixed. CHOICE OF SOLVENT.Various solvents have been evaluated for this extraction and benzene has proved the most satisfactory. In general, the more polar solvents yield excessively high blanks because they will extract the uncomplexed dye. The nonpolar aliphatic solvents do not extract any color, Previously reported characteristics of solvents for this extraction were confirmed in this study (12). Interferences. The results of a study of diverse ion effects on the extraction of the tantalum-fluoride-malachite green complex are summarized in Table I. The listed ions were added to fluoride solutions at the molar ratios shown. The presence of significant amounts of some of these ions in (13) Parr Instrument Co., “Oxygen Bomb Calorimetry and Combustion Methods,” Technical Manual No. 130 (1960).
Table I. Effects of Diverse Ions on Fluoride Determinations Effect, Diverse ion relative Molar Ratio Diverse ion added Fluoride ion’ error C1-, Br25 0 1I-, Ag+
NO,-
1 1
1 5 25 5
25 5
Ca+*,MgC2 Na+, K+ Cr+3 Fe+3 Ag+,
V+6
B+~(BOJ-~) Sn+2 Thi4 Ti+‘
Zr+‘ ~ 1 + 3
Si+4(Si03-2)
+200
+3 +I8
+90 -2
- 10 -4
25
-20
5
0
25
0
5 5 5
- 35 -80 - 80 - 50
2 2 0.3 0.3 0.3 0.1 0.1
-60
- 50 - 50 -60
- 25 - 35
the reagents is accounted for by fluoride addition to the composite reagent and by the reagent blank. Hence, the values given represent interference levels of diverse ions in samples. The only two ions found to produce positive errors are nitrate and iodide. This effect is the result of the slight solubility in benzene of the dye salts of these anions. The iodide interference was easily removed by adding a slight excess of Ag+ to the composite reagent. The formation of unextracted complexes between fluoride and diverse ions is probably the cause of the negative interferences shown. Fluoride complex stability constant data summarized on pages 225-31 of (3) correlate well with the observed interferences. These appear to fall into three classes. The largest interferences are due to the cations which form fluoride complexes markedly more stable than the tantalum fluoride complex (Si+4, Al+3, Zr+4,Th+4,Ti+4). Moderate interferences are due to metal-fluoride complexes of about the same strength as that of tantalum (B+3,Sn+2,V5,Fe+3,Crf3). Cations which form weaker fluoride complexes than tantalum show no interference (Ca+*, Mg+2, Na+, K+, Cu+, Ag+). The interference of Si dictated the use of an oxygen bomb for sample decomposition since other methods bring the sample into contact with glass surfaces during and after combustion. It was found, for example, that considerable Si was picked up in the course of a Shoniger oxygen flask combustion. Fortunately, the high pressure oxygen bomb has been proved as a satisfactory means of decomposing organic compounds for subsequent fluorine analysis (14). It is of importance that most of the heteroatoms commonly found in organic compounds do not interfere significantly at levels up to fivefold molar excess. Sensitivity and Precision. Under the defined conditions a standard curve will result which has a slight curvature as may be deduced from the following data: absorbance minus blank (5-cm cell) of 0.180,0.372,0.591 and 0.850for 50,100, 150, and 200 pg fluoride/ml, respectively. Shown below is the preci(14)J. J. Bailey and D. G. Gehring, ANAL.CHEM., 33, 1760 (1961). VOL 40, NO. 2, FEBRUARY 1968
0
417
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.
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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).
