Microdetermination of Fluorine in Organic Compounds T. S. M A and J. GWIRTSMAN' Department of Chemistry, Brooklyn College, Brooklyn
b A simple procedure for microdetermination of fluorine in organic comThe organic pounds is described. fluoro compound i s decomposed b y sodium fusion in a Parr microbomb. The product is taken up with water, and fluorine i s separated os fluosilicic acid by steam distillation from perchloric acid in a compact distilling apparatus, followed b y titration with thorium nitrate, using the colorimetric or photaelectric end point. Nitrogen, sulfur, phosphorus, and other halogens do not interfere.
F
LUORINE DETERMINlTIOX has
drawn the attention of the organic microanalyst only recently, as is evidenced by the fact that laboratory inanuals on quantitative organic microanalysis (9, 16) published in 1951 do not contain a method for fluorine. determination. However, with the rapid increase in the number of new synthetic organic fluoro compounds, there has been a general demand for fluorine determiuation in the microanalytical laboratory. Numerous publications have appeared on macro and semimicro methods for determining fluorine in organic compounds (5, 6, 7), but few of these methods are adaptable to a micro scale. I n the first micromethod of relatively general application, credited to Clark (4,the fluoro compound is decomposed in the presence of platinum at 900" C. in a quartz combustion tube and the hydrofluoric acid formed is titrated with 0.01N sodium hydroxide. For highly fluorinated substances, Freier and coworkers (8) have found it necessary to modify Clark's method by introducing moisture into the combustion train and raising the furnace temperature to 1150" C. The other halogens and sulfur, if present, may be removed by suitable reagents, hut nitrogen will interfere with the titration. Roth (14) favors decomposition by heating n-ith calcium hydroxide in a sealed tube, followed by distillation of fluosilicic acid and titration with thorium nitrate. Belcher, Caldas, and Clark (2) recommend fusion with metallic sodium or potassium a t 600" C. for an hour in a nickel bomb of special design, followed by titration of the resulting fluoride solution without dis1 Present address, Boyce Thompson Institute for Plant Research, Yonkers, N. Y.
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ANALYTICAL CHEMISTRY
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tillation. Samples containing nitrogen or sulfur require special treatment, because sulfide and cyanide ions interfere with the determination. I n the authors' experience, calcium hydroxide is not an efficient reagent for the microanalysis of organic fluoro compounds. Dirscherl and Zacherl (5) have reported that calcium oxide is not suitable for ashing of fluoroacetate. The authors have found that the Parr microbomb with the incorporation of a copper gasket serves very well as a reaction chamber for the conversion of organically bound fluorine to the fluoride ion. The sample may be oxidized by means of sodium peroxide, but a more convenient and simple m y is to use the reduction reaction by heating with sodium. After dissolution of the sodium fusion product in water, the mixture is transferred into an electrically heated distilling apparfitus, and fluosilicic acid is steamdistilled from a perchloric acid solution a t 135'+ =t2" C. Fluorine in the distillate i s determined by titration of the fluoride ion with thorium nitrate in the presence of sodium alizarin sulfonate a t p H 3.0. The final step is preferahly carried out in a photoelectric filter photometer developed by Mavrodineann and Gwirtsman ( I S ) , though it may be accomplished by means of Nessler tubes using the visual back-titration technique. The new method is applicable to samples containing nitrogen, sulfur, phosphorus, and the other halogens.
consists of three concentric tubes: I n
IT, and ET2 the vapors are condensed, and in ET, the cooling wstcr circuiates. The ground-glass joint, J,, serves as the opening for the introduction of the sodium fluoride solution as well as the seat of a thermometer (150" C.) during distillation with the mercury bulb immersed in the liquid, L. As the steam distillation from a perchloric acid solution requires a temperature of 135" C., an additional heating system is provided by the electric heating jacket, H . This is made from nickel wire, W (resistance, 2.120 ohms per foot), 600 cm. long. The actual length of the resistance spiral is 420 cm., the rest being used for terminals (3 X 30 em. bent and twisted a t each end). A glass cylinder, 48 mm. in diameter, is covered with aluminum foil on which a sheet of asbestos is affixed. The Kichrome spiral is then wound on the cyl-
APPARATUS
Parr microbomb, 2.5-ml. capacity, Series 2300, No. AlMB, from Parr Instrument Co., Moline, Ill. Copper gaskets are cut from a sheet of coppe
