Northrup Co., for making available No. 124138 microelectrodes prior to their commercial release. LITERATURE CITED
(1) Bates, R. G., “Electrometric pH
Determinations,” Wiley, New York, 1054.
(2) Dixon, G. H., Wade, R. D., Science 127*338 (1958). (3) Jacohsen, C. F., Leonis, .J, Linderstrom-Lang, K., Ottesen, &I., Methods of Biochem. Anal. 4, 171 (1957). (4) Schwartz, M., Myers, T. C., ANAL. CHEW30, 1150 (1958). ( 5 ) Schwert, G. W., Neurath, H., Kaufman, S., Snoke, J. E., J . B i d . Chem. 172, 221 (1948).
RECEIVEDfor review July 21, 1058. Accepted November 10, 1058. Investigation supportei by a research grant (No. CY-3652) from the National Cancer Institute, National Institutes of Health, Public Health Service; and in part by an Armed Forces Special Weapons Project (contract No. Da-40-00;’-hID-725), administered through the OfFce of the Surgeon General, United States Army.
Reduction of Organic FIuoro Compounds with Lithium and n-Propylamine in Microdetermination of Fluorine R. DONALD STRAHM Redstone Arsenal Research Division, Rohm & Haas Co., Huntsville, Ala.
b Organic fluoro compounds are decomposed with lithium and n-propylamine. Fluoride, which is separated from the residue b y steam distillation from perchloric acid, is determined by a back-titration in Nessler tubes with thorium nitrate.
A
EXAh.IIN.4TION O f a 1lUlllber Of procedures proposed for the analysis of organic fluoro compouiids did not reveal a method generally acceptable for niicrodeterniination of fluorine. Difficulty was encountered particularly in the decoinposition of compounds of high fluorine content. Although simpler compounds could be decomposed satisfactorily in a Parr microbomb with sodium or potassium, low fluorine values \yere frequently obtained when more highly fluorinated compounds mere considered, even if they were heated more drastically. The low results are believed to be partially attributable to leakage, because it was difficult to seal the microbomb. The deconipositioii method of LIiller, Hunt, and ilIcBee (4) with sodium in liquid ammonia \vas used with some success; however, the hnzard and inconvenience of handling liquid ammonia under pressure in glass tubes reiider this method somewhat undesirable. 13enkeser aiid coworkers (1, 2 ) have 4’
Table
I.
studied lithium in low molecular weight amines and reported such reducing systems to be more effective toward the organic compounds investigated than sodium and ammonia. Their findings suggest the use of such a medium for the decomposition of organic fluoro compounds, I n addition to the possibility of introducing a superior reducing system, the undesirable features of handling liquid ammonia could be eliminated. Lithium in n-propylamine has been used for decomposing organic fluoro compounds with gratifying results. The amine, which boils at 49-50’ C., presents no pressure problem and can easily be distilled after decomposition of the sample. The analysis is completed by the distillation and titration method of Ma and Gwirtsman ( 3 ) with only minor modifications. Fluoride is steam-distilled as fluosilicic acid from a perchloric acid solution of the decomposition residue and is determiiied by a back-titration v i t h thorium nitrate in Nessler tubes. PROCEDURE
A 0.4- to 0.j-gram strip of clean lithium ribbon is cut in sniall strips into a 250-ml. iodine flask containing 20 nil. of n-propylamine. A saniple containing preferably about 1.5 mg. of fluorine is weighed and placed in the
flask. Solids are weighed into microcombustion bc ats, and nonvolatile liquids are weighed into small glassstoppered viak. Volatile liquids can be sealed in thin-ivalled glass ampoules, which are broken beneath tlie surface of the solvent. The stopper is t81ien placed in the fl:isk and secured with tape. The flask is clamped to a wrist-action shaker and allowed to shake moderately for 18 to 24 hours. The flask is then opened, and the solvent is removed by heating in a hot water bath in a fume hood. TT7l1e11 the residue appears dry, the bottle is permitted to cool, and a f e x milliliters of methanol is added t o decompose th: excess lithium. TT’hen reaction has :eased, 40 to 50 nil. of fluorine-free distilled water is added t o dissolve the salts, and tlie solution is transferred to a small polyethylene beaker. The solution is concentrated to about 35 1111. on a steam bath. Fluoride is separated aiid titrated by the method of Ma and Gwirstman ( 3 ) . I n disti ling the fluoride, steani is passed through the unit at a moderate rate from the start of the distillation, to assure uniform heating ant1 prevent bumping. Occasionally uiidesirable foaniiiig has been observed in the early stages of the distillation. Addition of a drop of iintifoani A, silicone defoaming agen;, with the sample eliminates any serious foaming. An aliquot of the distillate, chosen t o contain about 75 y of fluoride, is transferred to a polyethylene beaker, and enough distilll?cl water is added to give
Determination of Fluorine in Selected Organic Fluoro Compounds
Fluorine, yo Source B.P., C. nLo Calctl. Pound Eastman Kod:tk (redistilled) 114.2-114.4 1.4728 17.10 1 7 . 3 , l i . l 1,2-Dichloroliesafluorocyclo-l-penteIiePeninsular ChcmResearch (redistilled) 00.5 1.3678 16.54 46.1,46.1 3,5-Difluorobenzoic acide Synt’hesiz.edby C. 0. Parker, Rohm 6r: Haas Co. 120.2 (m,D,) , , , 24.04 2 3 . 8 . 2 4 .O Perfiuoropropionic acid 57.01 58.1;56.9 Peninsular ChcmResearch 95:5 - ‘ Perfluoro triethylamine Supplied by R. D. Dresdner, University of Florida 68.8 b 76.80 76.7,76.3 Diethyl perfluoroglutarate Synthesized by J. P.Freeman, Rohm & Haas Co. 1.3550 38.40 38.6,38.5 a 90.1yo purity by neutralization equivalent. * 7t$O less than 1.30, lower limit of refractometer. Compound o-Fluorotoluene
VOL. 31, NO. 4, APRIL 1959
615
a total volume of 70 ml. The p H is adjusted to 3.0 with a p H meter and dilute hydrochloric acid. The blank is prepared by treating 70 ml. of distilled water in the same manner as the sample aliquot. The p H of the solutions a t the end of the titration should be nearlj- 3.2. DISCUSSION
The results of analyses of several known fluorine compounds are presented in Table I. The maximum difference between experimental and calculated fluorine values is less than 2% relative. Four or five repeat titrations were made on aliquots of each sample distilled, and the average value for the fluoride titer was used in calculating the percentage of fluorine prescnt. After a n analyst has gained experience with the titration, the volumes of standard fluoride solution required for duplicate titrations should
not differ by more than 0.05 ml. Blanks are low and have run consistently about 1 y of fluorine. No interference is encountered from substances commonly present in samples, such as nitrogen and other halogens. Samples were customari y weighed out in the afternoon and allowed to decompose overnight, so that the reduction took place over a t 1c:tst an 1s-hour period. No attempt was made to determine the minimum time ix:quired for decomposition. Lithium in n-Iiropylamine is generally more satisfactory for reducing organic fluoro compounc 6 than sodium in liquid ammonia. The lithium and amine is a more powerful reducing system and can be handled more conveniently and safely. The m i n e is superior to ammonia as a solvent for the metal and for samples. In the analysis of a wide variety of mate:ials, the addition of a co-solvent has not been necessary.
While n-propylamine was chosen because of its boiling point (49-50' C.), other low molecular weight amines could probably be used snt,isfactorily in its place. ACKNOWLEDGMENT
The author gratefully acknowledges the helpful suggestions and advice of Keith S. kIcCallum, under whose direction the work was carried out. LITERATURE CITED
(1) Benkeser, R. A., Robinson, R. E., Sauve, D. M., Thomas, 0.H., J . Am. Chem. SOC.,77, 3230 (1955). ( 2 ) Benkeser, R. A,, Schroll, G., Sauve, D. M., Zbid., 77, 3378 (1955). (3) Ma, T. S., Gwirtsman, J., ANAL.CHEiII. 29, 140 (1957). (4) ?filler, J. F.,Hunt, H., McBee, E. T., Ibzd., 19, 148 (1947).
RECEIVED for review June 26, 1958. Ac. cepted October 10, 1958.
Rapid Microdetermination lof Fluorine in Organic Compounds R. N. ROGERS and S. K. YASUDA University o f California, 10s Alamos Scienfific laboratory, l c x Alarnos, In a rapid and accurate method for the microdetermination of ,,fluorine in
organic compounds the Schoniger combustion technique is used for initial decomposition of the sample, followed by an improved ferric salicylate colorimetric analysis. Samples ranging from 0.4 to 20 mg. were successfully analyzed in 10 to 20 minutes. Accuracy and precision are adequate for determination of empirical formulas.
A
sImm method for the determination of fluorine in organic compounds has been sought by many investigators; however, few procedures in the literature can truly be called routine. No method has been found which would enable many laboratory technicians to determine fluorine with acceptable accuracy and precision. The Schoniger combustion technique (2), involving combustion of a sample in a flask filled with oxygen a t atmospheric pressure, greatly simplified the decomposition of organic compounds for the determination of chloride, bromine, and iodine. Schoniger later (5), giving a n improved method for the determination of bromine and adding a procedure for sulfur analysis, cited application to fluorine analysis.
616
ANALYTICAL CHEMISTRY
N. M.
The Schonigei combustion technique eliminates two of the more difficult problems of organic fluorine analysis: decomposition i f the sample, and separation of the ionic fluoride from the decomposition medium. Schoniger's answer to the third problem, determination of the fluoriqe after decomposition of the sample, wis not, however, applicable to routine malysis. A colorimetric procedure should be the most simple for determining fluoride ion. On the basis of accuracy, the method of Rickird, Ball, and Harris (1) was chosen foi, further development. PRELIMII.IARY STUDIES
Application oj the Schoniger combustion to the analysis of fluorine compounds indic rtted certain requirements for quan itative results. The combustion flask and stopper must be completely free o ' boron and aluminum, eliminating the common types of glass from considerat .on. Vitreous silica was used exclusiwly in this apparatus. A stable flame frcnt must be established before any sarnlde is volatilized into the flame; then. quantitative results can be obtained o 7 even volatile samples with complete safcty. Rickard, Ball, :ind Harris's procedure (1) was somewl-nt difficult to apply
PH
Figure 1. Sensitivity of ferric salicylate colorimetric method as a function of final pH routinely because of the critical p H control required. Maximum absorbance of the colorimetric reagent occurred a t pH 3.1, but this pH does not correspond to the range of maximum sensitivity to fluoride ion. Figure 1 shows the dependence of sensitivity to fluoride upon pH, a change of 1 pH unit changes 2. the sensitivity by a factor of The steep slope of the sensitivity curve from pH 2.25 to 3.5 makes it obvious why critical p H control was necessary in the original method. I n
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