Anal. Chem. 1980, 52, 2073-2075
2073
Determination of Hydrocarbons in Anhydrous Hydrogen Fluoride by Gas Chromatography William P. Cottom and Dale E.
Stelr'
Racon Inc., Wichita, Kansas 67201
A method Is presented for the analysls of llght hydrocarbons In anhydrous hydrogen fluorlde at concentratlons ranglng from 1 to 1000 pg/g. Hydrocarbons are extracted into carbon tetrachloride and analyzed by gas chromatography. The resuits Indicate that as many as 15 organic compounds are present in commerdalty prepared anhydrous hydrogen fiuorkle. The detection llmlts for the constltuents studied are 0.5 pg/g. Samples spiked with propane, butane, pentane, and hexane yleld 103% recovery for the descrlbed procedure. Repllcate samples contalning less than 20 pg/g of hydrocarbons yleld a relative standard deviation of 7%.
Most methods used in analyzing anhydrous hydrogen fluoride for trace contaminants such as iron, fluosilicic acid, sulfur dioxide, and nonvolatile acids require an aqueous solution ( I ) . These methods for evaluating the quality of the aqueous solutions of hydrogen fluoride have been used successfully for many years and are accepted as the standard procedures (2). This practice has, however, prevented any determination of low molecular weight organics in the hydrogen fluoride, due to the highly exothermic reaction of anhydrous hydrogen fluoride and water. The uncertainty as to whether organics were present in anhydrous hydrogen fluoride provided the stimulus for the investigation. For organics to be present they had to occur as contamination in handling of the material or as compounds indigenous to the raw materials of fluorspar and sulfuric acid used to produce hydrogen fluoride. Hydrogen fluoride is used in such diverse industries as the aluminum, fluorocarbon, petroleum, uranium, and steel industries, as well as chemical manufacturing. These industries often receive bulk shipments of anhydrous hydrogen fluoride via rail tank cars. The large number of cars, with various unloading practices, increases the likelihood of different compounds contaminating the return cars. The possibility of organics attributed to indigenous compounds is limited to contaminants in the concentrated sulfuric acid or in the fluorspar ore used to produce anhydrous hydrogen fluoride. Dudorov and Agliulov noted and determined the organic impurities found in hydrogen chloride formed by the reaction of concentrated sulfuric acid and aqueous hydrogen chloride (3). Hydrocarbon impurities were detected, with the organics originating from either the concentrated sulfuric or aqueous hydrochloric. Studies conducted on fluorspar ore have also found organics, consisting primarily of alkanes, alkenes, and fluorinated alkanes, to be indigenous to the ore (4-7). The possibility of hydrocarbons, from whatever source, remaining in the hydrogen fluoride was supported by the high solubility of low molecular hydrocarbons in anhydrous hydrogen fluoride (8). A method to determine the level of organic impurities present in anhydrous hydrogen fluoride is necessary to the industries utilizing it to assess purity. The purpose of this paper is to present such an analytical method.
EXPERIMENTAL SECTION Apparatus. The custom-madeapparatus (Figure 1)consisted of an extraction column, filled with Teflon (trade name, E. I. du Pont de Nemours Co., Wilmington, DE) packing and attached 0003-2700/80/0352-2073$01 .OO/O
to the sample cylinder, enclosed in a brine reservoir. The flow of anhydrous hydrogen fluoride was regulated by a metering valve. Teflon and stainless steel were used on all components which came in contact with the anhydrous hydrogen fluoride, and Plexiglass was used for the support structure. Instrumentation. A microprocessor-controlled HewlettPackard Model 5830A gas chromatograph equipped with dualflame ionization detectors (FID) was used for the analysis. The column was 20 f t X in. stainless steel packed with 33% (w/w) di2-ethylhexyl sebacate on 80/100 mesh Chromosorb P(AWDMCS) with a flow rate of 30 mL/min of helium. A Finnegan Model 4000 GC/MS/data system utilizing a data base of approximately 25000 compounds was used for the mass spectra analysis. Reagents. Baker-Instra-AnalyzedCarbon tetrachloride, J. T. Baker Chemical Co. (Phillipsburg, NJ) was used without purification. Calibration Standards. Calibrated mixtures of normal alkanes, methane through hexane, and branched alkanes, butane through hexane, were obtained from Alltech Associates, Inc. (Arlington Heights, IL). Individual branched alkanes of hexane and heptane were also obtained with a minimum purity of 99.5% from the same source. Procedure. (Caution: Anhydrous hydrogen fluoride is extremely hazardous and should only be handled in a hood while wearing proper protective clothing.) The anhydrous hydrogen fluoride was collected in a tared, evacuated 10-mL sampling cylinder and attached to the apparatus. A known amount (30.0 g) of CC14in the extraction column was chilled to -15 O C by the addition of dry ice to the brine solution. The contents of the cylinder were then slowly purged through the CC14, and the cylinder was flushed with 100 mL of dry nitrogen. A portion (ca. 10 mL) of the CC4,containing the hydrocarbon impurities, was sealed in a chilled vial and stored at -15 "C. A 5.0-pL sample of the chilled extract was then injected into the gas chromatograph isothermally at 90 "Cand the resulting chromatogram integrated during the 30-min analysis. Qualitative Study. The retention times of the hydrocarbon impurities were compared to the retention times of calibration standards analyzed under identical conditions. Solvents commonly used in the laboratory and reagent blanks of CC14were analyzed to ensure they would not interfere with the hydrocarbon determination. CCll extracts from selected samples were submitted to an independent laboratory for a mass spectra analysis. Quantitative Study. A portion of a previously analyzed sample was spiked with the saturated normal homologues of methane through hexane to determine the range of organics which could be extracted. Another portion of the original sample was spiked at concentrations of 10-1350 wg/g with a mixture of four hydrocarbons to determine the accuracy of the method over a wide range. Replicate samples taken from an anhydrous hydrogen fluoride tank car were analyzed so as to determine the precision of the method described.
RESULTS AND DISCUSSION As many as 15 organic compounds were identified in bulk shipments of anhydrous hydrogen fluoride. With the exception of ethyl fluoride, all organic compounds were alkanes, ranging from methane through hexane. Identification based on retention times, though subject to possible interferences caused by coelution, proved to be reliable on the basis of the results of the mass spectra determinations. Mass spectra analyses confiied constituents which accounted for over 90% of the organics found using the described procedure. 0 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 13,NOVEMBER 1980 -A
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Figure 1. Extraction apparatus consisting of: (A) lid, Plexiglass; (B) column, TFE, 11 mm i.d. by 500 mm long; (C) brine cylinder, Plexiglass, 4 in. 0.d.; (D) packing, TFE, approximately 5 mm squares; (E) connector, in. MNPT to 3/a in. hose; (F) leg supports, Plexighss, 2 in. 0.d.; TFE, 'I4 (G) drain tube, SS, ' / 4 in. 0.d.; (H) male connector, SS, '/, in. MNPT to I/., in. tubing; (I) three-way stopcock, TFE, 1/4 in. FNPT; (J) nipple, TFE, ' / 4 in. MNPT; (K) metering valve, TFE, ' I , in. FNPT; (L) male in. MNPT to '/4 in. tubing; (M) tubing, SS,'/4 In. 0.d.; connector, SS,'I4 (N) bulkhead union, SS,' / 4 in. tubing; (0)female connector, SS, '/e in. FNPT to 'I4 in. tubing; (P) valve, SS, in. MNPT to ' I 8in. MNPT; (Q) sample cylinder, SS, 10 mL, 'la in. FNPT; (R) tube nut, SS, '/, in. with rubber septum.
Table I. Recovery Data for C1 Through C 6 Alkanes hydrocarbon
amt alkane spike, p g
amt alkane recovered, p g
methane ethane propane n-butane n-pentane n-hexane total, C3-C6
64 119 175 23 1 287 34 2 1035
7 42 177 287 321 277 1062
%
recovery 10.9
35.3 101
124 112 81.0 103
Table 11. Hydrocarbons Detected in Anhydrous Hydrogen Fluoride compound methane ethane ethyl fluoride propan$: isobutane n-butane 2,2-dimethylpropane isopentane n-pentane branched hexane 2,2-dimethylbutane 2-methylpentane; 2,3-dimethylbutane 3-methylpentane n-hexane 2,3-dimethylpentane
detection limits, p g / g 5.9 1.15
0.45 0.46 0.53 0.32 0.28 0.52 0.40 0.52 0.56 0.51 0.48
Although methane and ethane were detected in some shipments of anhydrous hydrogen fluoride, the results from the sample spiked with the homologues methane through hexane indicated that the method was not totally quantitative
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
Table IV. Precision for Analysis of Hydrocarbons in Anhydrous Hydrogen Fluoride for Tank Car NLXa amt
date
found, p g / g
date
8-10-79
14.05
8-21-79
8-12-79
16.40
8-22-79
x= 15.49, SD
=
Table VI. Hydrocarbon Level of Samples Taken from Tank Cars
amt found, p g / g 15.86 15.65
1.01, RSD = 6.5%.
Table V. Composite Determination of Hydrocarbons in Anhydrous Hydrogen Fluoride
no. of
no. of
concn, rg/g
samples
concn, p g / g
samples
0.00-1.99
168 89
5.00-9.99 10.00+
36
2.00-4.99
compound methane ethane ethyl fluoride
0.14 0.28 1.99
propane is0 bu tane
1.76 1.35 0.55 0.84 0.65
n-butane 2,2-dimethylpropane isopentane n-pentane branched hexane 2,2-dimethylbutane 2-methylpentane; 2,3-dimethylbutane 3-methylpentane n-hexane 2,3-dimethylpentane
0.91
0.22 0.24 0.08 0.10
0.02 , A.
U.UI
mean % 1.5 3.1
21.8 19.2 14.8 6.0 9.2 7.1 10.0 2.4 2.6 0.9 1.1 0.1 n-, u. 1
for methane and ethane (Table I). Approximately 10% of the methane and 35% of the ethane remained in the carbon tetrachloride, with the balance leaving the extraction column. The low recovery was due primarily to the low solubilities of these low boiling compounds in carbon tetrachloride a t the temperatures used to extract the material. The portion of methane and ethane lost due to the nitrogen purge was minimized by not exceeding 100 mL of nitrogen. The loss was not critical within the scope of this method, although the method could be modified to include a vapor trap after the extraction column if it was necessary for a particular application. The recovery of homologues propane through hexane was 103%, well within normal experimental error. On the basis of the recovery data and calibration factors of the various constituents identified, limits of detection were established (Table 11). Limits were not established for ethyl fluoride or the branched hexane since pure compounds were not available in house. Results from the sample spiked with the mixture of pure hydrocarbons revealed that recovery levels ranged from 99.7% at the 10 pg/g concentration to 80.6% at the 1350 pg/g concentration, with an overall average recovery of 89.5% for all concentrations (Table 111). Thus a wide range of organic concentrations could be extracted without any modification.
26
Analyses of four samples from a shipment of anhydrous hydrogen fluoride indicated the precision of the method was well within acceptable values, especially when it is considered that the errors associated with samding as well as the errors associated with the procedure were included in the test (Table IV). The method has been used on over 300 samples taken from bulk shipments from various producers in North America. A composite average of each of the constituents and its contribution to the total amount of organics indicated that ethyl fluoride, propane, and isobutane accounted for over 55% of total organic compounds (Table V). Also, it was found that a majority of samples analyzed contained a level of hydrocarbons less than 10 pg/g, which was in the most accurate range of the procedure (Table VI). However, nearly 20% of the samples contained more than 5 pg/g hydrocarbons, indicating a need for users concerned with the level of contamination to routinely analyze bulk shipments of anhydrous hydrogen fluoride. The proposed method is accurate over a wide range of concentration levels and can provide useful information concerning the organic composition of anhydrous hydrogen fluoride. Producers and users should find the information to be extremely useful in monitoring the quality of anhydrous hydrogen fluoride. -
mean concn, d g
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I
ACKNOWLEDGMENT The authors thank the Research and Development Laboratories of Vulcan Materials Co. for their mass spectra determinations.
LITERATURE CITED (1) “Hy&ofluorlc Acid”; Allied Chemical Corp.: New York, 1978; pp 36-44. (2) Anno. Book ASTM Stand. 1978, Part 40, Section E271, 782-773. (3) Dudorov. V. Ya.; Aglluiov. N. Kb. Zb. Anal. Kbim. 1970, 25 (2).
162-165. (4) Kranz, Reimer Org. Geocbm. 1962. 521-533.
(5) (6) (7) (8)
Kranz, Reimec Trans. Inst. Min. Mefall., Sect. B lS68, 77, 826-836. Krariz, Reimer Natufw/senscheflen 1966, 53 (23). 593-800. Toway. J. C.; Yajima, J. Minerel. Despostfa 1967, 2 (4). 288-290. Simons, J. H. “Fluuine Chemistry”;Academic Press: New York, 1950; Vol. 21, pp 836-840.
RECEIVED for review June 23,1980. Accepted August 7,1980. Portions of this paper were presented at the 1980 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 10-14, 1980.
