315
V O L U M E 22, NO. 2, F E B R U A R Y 1 9 5 0 Grams of Cl/gal. where .I
=
tCKNOWLEUGMENT
134.22(0 - E ) B
milliliters of silver nitrate bromide B = normality of silver nitrate C = milliliters of silver nitrate bromide in the blank D = milliliters of silver nitrate chloride E = milliliters of silver nitrate chloride in the blank T 7 = milliliters of sample used =
v
requirpd to titrate the required t o titrate the
The authors aish to acknowledge the assistance of C. T. Leacock and s. R. Henderson in many determinations, and the kindness of the Carbide and Carbon Chemicals Corporation in supplying samples of ethers.
rcqrnred to titrate the rcquired to titratc the
RESULTS
Indications of the accuracy and precision of analyses obtainable by the disodium biphenyl method are given in Tables I to V. As shown in Tables I V and Ti, the preferred combination of disodium biphenyl, ethylene glycol dimethyl ether, and ammonium persulfate gives results within *2% of the actual bromine and chlorine contents. Applicability of Method Apart from Gasoline Analysis. To test the applicability of the method to the determination of organic halogen compounds other than ethylene halides, a few duplicate analyses mere made on benzene solutions containing approximately 1% halo compound, with the good accuracy shown in T a b k TI.
LITERATURE CITED
Agruss, 11.S., Ayers, G. W., Jr., and Schindler, H., ISD. ENQ. CHEM., AXAL.ED.,13,69 (1941). Am. SOC. Testing Materials, “Tentative Method of Test for Sulfur in Petroleum Products by the Lamp-Gravimetric Method,” D 90-4iT. Benton, F. L., and Hamill, W.H., A s a ~CHEM., . 20, 269 (1048). Chabloy, E., Ana. chim., 1, 469 (1914). Parr, S. W., J . Am. Chem. SOC.,30, 764 (1908). Scott, N. D., Walker. J. F.,and Hansley, V. L., I b i d . , 58, 2443 (1936). Stepsnoir, A , , Ber., 39, 4033 (1906). Thompson, J. J., and Oakdale, U. O., J . Am. Chem. Soc., 52,1195 (1930). Wear, G. E. C., and Quirain, E. It., ANAL. CHEM.,21, 721 (1949). n’irth, C., and Stross, hZ. J., ISD. E s a . CHEM.,ANAL.ED.,5 , 85 (1033). R E C E I V ESeptember D 26, 1949. Presented before the Division of Analytical and Micro Chemistry a t the 116th Meeting of the AXERICANC H E ~ C I L SOCIETY, Atlantic City, S . .J.
Determination of Fluorine (and Carbon) in Fluorinated Hydrocarbons High Temperature Combustion Technique OSCAR I . \ I l L S E K , Socony-Vucuurn Laboratories, Paulsboro,
\.
J.
Fluorine in high13 fluorinated organic conipouiids is ordinarily determined by tedious methods, whereas carbon, in the presence of fluorine, cannot easily be determined by the conventional combustion procedure. A method is described whereby the fluorinated material is burned in an atmosphere of oxygen and water vapor, utiliding a platinum combustion tube. The fluorine is liberated as hydrogen fluoride, which is estimated by allcalimetric titration; the carbon is simultaneousl~converted to carbon dioxide and is absorbed in Ascarite in the conventional manner. Partiall? or completely fluorinated compounds, in either solid or liquid state. may be anal?zed by the method in about 1.5 hour+. The accuracy is comparable to that obtained b? more length) procedures.
B
ECAUSE of the exceptional activity of fluorine and hydrogen
fluoride, the determination of fluorine frequently presents a problem not, encountered in the case of the other halogens. In the analysis of organic compounds, the problem is further complicated by the necessit,y for first decomposing the material so as to convert the fluorine into the ionic form. Highly fluorinated compounds are especially difficult’ to decompose in this manner, because they contain a large number of the relatively stable carbon-fluorine bonds. JVith the increased interest in polyfluorinated materials of the type described in the hlanhattan Project and recent literature ( 2 ) ,the development of a reasonablj, rapid and accurate analytical method has become extremely desirable. Several methods have been employed for the determination of fluorine and other halogens in highly fluorinated compounds. Miller and his co-workers (6)modified the Vaughn and Sieuwland method ( 8 ) , decomposing the material by sodium in liquid ammonia and precipitating the fluoride as lead chloiofluoride. Excellent results were reported, but the reagents used are objection-
a h k to handle and the procedure is somewhat lengthy. Kimball and Tufts (5’)designed a nickel bomb for the decomposition of fluorocarbons by the action of metallic potassium a t an elevated temperature. This procedure is also lengthy, because the fluorine must be isolated b> distillation ah fluosilicic acid before final determination. A combustion method for the simultaueous determination of carbon and fluorine was developed by Teston and RlcKenna ( 7 ) . However, the procedure was not directly applicable in the presence of hydrogen, and the analysis of a single completely fluorinated compound yielded low fluorine results. Bockemuller ( 1 ) proposed a method involving combustion in a platinum tube which he applied to the analysis of compounds containing one or two atoms of fluorine in the molecule. More recently Schumb and Radimer (6) applied a conibustion technique to the determination of fluorine in highly fluorinated volatile compounds. The method reported herein employs the same principle. EIowever, whereas the Schumb and Radimer method is designed for gases and volatile liquids, the proposed procedure may be applicd
ANALYTICAL CHEMISTRY
316 to any compound boiling higher than about 60" C. I t has thc furthur advantage that carbon may be determined simultaneously. Elements other than carbon, oxygen, hydrogen, and fluorine interfere, however, and require modifieation of the basic procedure. The method is based on the principle that a t high temperatures the combustion of fluorocarbons in a mixture of oxygen and water vapor results in a quantitative decomposition to yield carbon dioxide and hydrogen fluoride. The combustion is carried out in a tube made of platinum, which effectively resists the action of hrtli ogscn fluoride. The hydrogen fluoride is absorbed in water, \I hile the carbon dioxide largely passes through the aqueous solution and is absorbed in Ascarite. Any dissolved carbon dioxide is swept out and recovered prior to the final estimation of thi, fluorine, xhich is by simple alkalimetric titration of the hydrofluoric acid. APPARATUS
The assembled apparatus is shown in Figure 1.
Figure 1.
Combustion Train Assembly
The gas saturator, B, consists of a series of three gas-washing bottles containing water and through which the oxygen stream may be dii.c,ctedby means of the 3-way stopcock, A . The oxygen is first purified by passing through Ascarite. The vaporizer, D, is used only when volatile samples are analyzed. I t consists of a cylindrical flask of approximately 100ml. capacity, fitted with a %hole neoprene stopper into one hole of which the vial containing the sample is inserted (Figure 2). By means of t h e &way stopcock, C, the vaporizer may be bypassed when nonvolatile materials are analyzed. The combustion tube, F , is a platinum tube 100 cm. long, 1.25 cm. in inside diameter, and 0.4 to 0.5 mm. in wall thickness. The inlet end is fitted with a L$14/20 hollow platinum plug, G , and a side arm, E , to which a section of 6-mm. soft-glass tubing is fused. The outlet fits into a 5-mm. inside diameter platinum delivery tube, J , by means of a T 7/25 joint. The combustion tube contains a 10-cm. (4inch) roll of 50-mesh platinum gauze and is supported in the furnace by a '/rinch McDanel high-temperature combustion tube, I . The platinum tube used by Miller et al. for the determination of hydrogen in hydrohalocarbons (4)should be sui table. The furnace, H,is a Burrell Model A-13 furnace, fitted with Lvheels and mounted on a track so that it may be easily moved. This permits different sections of the tube to be heated. The furnace is equipped with t a p transformer, voltmeter, and pyrometer, and is capable of continuous operation at temperatures upwards of 1300' C. The absorption train, K , L, M , N, is comprised of an unetched 250-ml. heat-resistant Erlenmeyer flask containing approximately 125 ml. of cool, freshly boiled distilled water, a Fleming gaswashing bottle containing concentrated sulfuric acid, followed by a U-tube containing magnesium perchlorate (Anhydrone), and a tared U-tube containing Ascarite and prepared in the conventional manner. It was experimentally established that with the concentrations of hydrogen fluoride involved, no loss occurs through either incomplete absorption in the single absorber or rcaction with the glass. Figure 2 shows the vaporizer in detail. The vial is approximately 7.5 cm. (3 inches) long and the bulb is blown to a volume of approximately 0.2 ml. The tip of the capillary is drawn or ground to an external diameter of 0.5 mm. SAMPLE
Asample of not morc than 40 to 60 mg. is taken. Volatile liquids are introduced into the weighed vial by warming the bulb
gently, dipping the tip into the liquid, and allowing the sample to be drawn in as the bulb cools. After filling, the vial is maintained in an upright position and any excess sample is carefully expelled from the capillary portion by touching the latte momentarily with a warm rod. Solids and high-boiling liquids-Le., those showing no measurable evaporation from an open container after 2 or 3 minutes on the balance pan-are weighed in a platinum boat. PROCEDURE
Heat the furnace to a t,mperature of 1250" to 1275" C. and pass the water-saturated oxygen through the system a t the rate of 30 ml. per minute. Bfter several minutes connect the absorption train, adding 0.20 ml. of standard 0.1 S alkali and a few drops of phenolphthalein to the water in flask K . Gradually introduce the sample into the combustion zone according to either of the two methods given below, depending upon whether the material is volatile or not. A. Volatile Materials. Adjust the position of the furnace so that it heats the center section of the combustion tube. Insert the vial into the 3-hole stopper, making a gas-tight seal. Gradually vaporize the sample into the oxygen stream by allowing a small droplet (4 to 5 mg.) to drop into the mixing chamber every 3 to 4 minutes. This is best done by touching the bulb momentarily with the hand or viith a warm rod, so that the expanding vapor forces out the droplet. Very low-boiling liquids may volatilize from the tip of the capillary without forming a drop. I n this case, control the rate of addition so that about 45 minutes are required to evaporate the sample completely. It is essential that this rate not be exceeded, for combustion may be incomplete or, in extreme cases, reaction with explosive violence may occur. The volatilization of materials boiling below about 60' C. is difficult to control and such materials cannot generally be analyzed by the described technique. B. Nonvolatile Materials. Place the furnace so that it heats the oortion of the combustion tube nearest the outlet end. WraD the length of tubing extended from the furnace (about 35 to 4b cm.) with a layer of gauze and saturate the latter with water. Remove the plug and place the boat in the combustion tube a t a distance of approximately 15 cm. from the open end. Immediately replace the plug and allow the oxygen to pass through for several minutes, then gradually remove the cooling wick. After 10 minutes move the furnace toward the sample a t the rate of 1 cm. every 2 to 3 minutes until, by the decolorization of the phenolphthalein, it is evident that decompositiofi has begun. (The precautions against too rapid decomposition as indicated under ( A ) should also be followed in this instance.) Maintain the tube in this position for 20 minutes, then resume the controlled movement of the furnace until the sample is subjected to its full heat. Heat the portion of the tube between the side arm and the furnace to redness with a gas burner to ensure volatilization of any last traces of sample.
6 MU. TUBING
0 . 5 ~ ~ . CAPILLARY TUBING
Figure 2.
Vaporizer for Volatile Samples
Place the furnace so that the outlet end of the tube is again heated, and adjust stopcock A to permit dry oxygen to pass through the system. After 5 minutes disconnect the absorption train from the combustion tube and replace the stopper containing the platinum delivery tube by one fitted with a heat-resistant fritted-glass gas dispersion tube. Flush the acid solution for 15 minutes with a stream of oxygen or purified air to recover any dissolved carbon dioxide. Finally disconnect the U-tube, S, and weigh. The gain in weight represents the carbon dioxide formed by the combustion.
V O L U M E 22, NO. 2, F E B R U A R Y 1 9 5 0 Table I.
Determination of Fluorine and Carbon Theoretical, % Fluoiine Carbon
11aterial 0-1 luorotoluenea p,p'-Difluorodiphenyl Benzotrifluoride
317
h
C
Trifluoroinetli~ltienzoicaciillj
n-Perfluoroheptaned
17. 26 19.97
... ...
39.01
...
29.98
i 8 33
...
...
Perfluoronaphthalaned 74.01 25.99 Perfluorornethylnaphthalaned 74.20 25.80 Perfluoro-tert-butylnaphthalaneci 74.60 25.40 Perfluorornethylcvclohe\aned 73 98 21,02 a Eastman Kodak S o . 2967. b E a s t m a n Kodak S o . 3281. Hooker Electrochemical Co. d Material prepared in Socony-1-acuurii Laboratoriez
Found, % '
17.2
19.9 19.
