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 an 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 an 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 the sensitivity by a factor of 2. 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
-
the present work thc rragrnt has hren Table I. Fluorine Deterrnincition in Organic Compounds modified for use a1; pH 1.9 to 2.1. ~~o~ ~ n..s . , . Thr hnffer capacity "f t,hc rr:ler"t,. NO. of %F specified, limits pH excursions to a Compound Formula Detns. Thwrv Caled.0 Found rn m-.... negligible value; homver, within the p,p'-DifliiorodiphenyI CnH,F, 9 19.98 19.88 19.75 *0.32 *0.11 specified range, larger pH excursions C,H,FaN,O, 4 19.86 19.86 20.00 5 ~ 0 . 5 9~ t 0 . 3 0 can be tolerated before arrnracy and Teflon (W3F2-L 4 i 3 . 1 9 71.66 +1.34 5 ~ 0 . 6 7 precision are seriously affected. AcHeptafluorohutylamine hyceptable accuracy and precision can drochloride CIH~CIF~N 5 56.40 56.40 56.81 *2.95 1 1 . 3 2 Trifluoroethyltosylate CoHoFaOnS 4 22.44 21.85 21.26 zt0.93 + 0 . 4 i be obtained with samples containing N-Trifluoroethylphthalimide CloHsFINOz 5 24.89 24.23 24.86 +0.26 1 0 . 1 2 between 50 and 100 y of fluoride. Heptafluorobutylhydroramic chloride CqHClhNO 3 53.73 53.73 5 4 . 1 1 +0.76 +0.44 Thenoyltrifluoroacetone C~IWIO~S 4 25.08 24.41 23.77 +0.69 f 0 . 3 5 REAGENTS AND APPARATUS %
~
~~~
~
~
I
Sodium fluoride solution, rontaining 40 y of fluoride per ml. COLORIMETRIC RXAGENT. Dissolve 13.515 grams of ferric chloride hexahydrate in 50 ml. of distilled or, preferahly, demineralized water; then add 10 grams of salicylic acid and 25 ml. of concentrated bydrochlorie acid. Dilritr to 10 liters and agitate periodically until a homogeneous solution is ohtainrd. The final pH of the reagent should he approximately 1.55. SPECIAL 4PPARATUS. A 300-ml. quartz Schoniger combustion flask, romplete !3-ith plat,inum gauze holder.
~~
~
~
Exon 4OOa ... 3 21.90' 26.54 h 0 . 3 8 1 0 . 2 2 N,N',S"-tris(2,2,2-trifluoroethyl)trimethylenetriamine CsHnF.K, 2 51.35 . . . 50.67 +O. 19 +0.13 a Calculated on basis of purity as determined hy carbon, hydrogen and, where applicable, nitrogen analytical data. Plaatics Division, Firestone Tire and Rubber Co. Composition not disclosed.
1)etermined hy tube furnace combustion and thorium nitrate titration.
Figure 2. Lucite dry box for transferring hygroscopic rompler
ANALYTICAL PROCEDURE
Weigh nonvolatile solids (0.4 to 20 mg.) by differrnce and place directly on a piece of Whatman KO. 42 filter paper, cut as specified by Schoniger ( 2 ) . Wrap the sample with special care to ensure a uniform thickness of paper on each sidc of the sample. Burn as specified by SchRniger, using 5 to 10 ml. of watcr as absorber. Handle extremely refractory samples in the same way as volatile compounds, for more efficient combustion, Keigh liquids or volatile solids into a KO.5 gelatin capsule containing finely powdered sucrose (-10 mg.). Transfer hygroscopic, sensitive, artive, or reactive samples in a small, easily construrted, Lucite dry box (Figure 2) that is being flushed with dry nitrogen. Wrap the combined sample and gelatin capsule in filter paper and burn, while rotating the flask at an approximate 45" angle from the vertical, Although the colorimetric reagent can he used without damage as the absorption liquid during combustion under normal conditions, it cannot he used in the presence of elements other than carbon, hydrogen, oxygen, and fluorine. On completion of comhustion, condensation of the products is hastened by rooling the flask in ice water for several minutes Nith periodic vigorous shaking. Absence of haze above the liquid indicates complete absorption of the combustion products. It may he necessary to take aliquots of the absorption solution to he within the best range of the colorimetric procedure, although sample weights can normally he adjusted to rliminate this rxtra mnniprllntion.
Transfer the sample to a 100-ml. volumetric flask; then add 15 ml. of the ferries alicylate rragent and 2 ml. of 0.25A' hydrochloric acid, and adjust to the mark with water. Use blank, prepared under identical conditions, as the spectrophotometric referenre. Recanse thc sample solution has a lower absorbance than the blank, set the fluoride-containing solution to zero absorbance at 530 mp, and compare the reference solution against it. Prepare a calibration curve, using the standard sodium fluoride solution, and read the concentration of the unknown solution from the curve. The order of addition of rcagents is not critical, hut a few determinations of the final pH of the solution should be made. If the pH of the referenre solution, for any given hatch of reagent, is outside the range 1.9 to 2.1, the amount of 0.25.V hydrochloric acid added is adjusted. Once the proper volume of acid is determined for any hatch of reagent, the remainder of the reagent can be expended without further reference to pH. DISCUSSION A N D RESULTS
As no standard organic fluorine compounds were available. a 99.5% pure (calculated from carbon and hydrogen analytical data) sample of p,p'-difluorodiphenyl was used as thc primary standard for fluorine determination. A number of othcr compounds of reasonably well known composition have since been analyzed (Table I). Accuracy acceptable for the determination of an empirical
formula was obtained in all cases; however, no claims for the ultimate in accuracy are made. The reasonably good results obtained in the determination of fluorine in sulfur compounds are somewhat surprising, in view of results of other investigators ( I ) . I n some cases, a slight carbon residue remained after combustion; however, this had no detectable effert on the analytical result. The thenoyltrifluoroacetone left a slight residue after rvery comhustion. A much higher temperature than is easily attainable in tube furnaces or oxygen bomhs is reached in the Schoniger comhustion, owing to the use of loa-pressure oxygen. KO difficulty was encountered wit,h eompounds containing a -CFa group, as in tube furnace combustions and sodium and/or potassium fusion methods. ACKNOWLEDGMENT
The authors express their gratitude to M. J . Naranjo for carbon, hydrogen, and nitrogen data. LITERATURE CITED
R. R., Ball, F. L., Harris, W. IT., ANAL. CHEM. 23, 919-21
(I) Rickard, /,or.,, \I.lylj.
(2) Schhniger, W., Mikroehim. Acta 1955, 123. (3) [bid., 1956, 869.
RECEIVEDfor review August 15, 1958. Accepted November 19, 1958. Work performed under the auspices of the U. S. Atomic Energy Commission. VOL. 31,
NO. 4,
APRIL 1959
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