THE ANALYSIS OF FLUOROCARBONS: USE OF INFRARED SPECTROPHOTOMETRY FOR THE ANALYSIS OF SMALL SAMPLES'

Size: px

Start display at page:

Download "THE ANALYSIS OF FLUOROCARBONS: USE OF INFRARED SPECTROPHOTOMETRY FOR THE ANALYSIS OF SMALL SAMPLES'"

Edgar Bradley
6 years ago
Views:

1 THE ANALYSIS OF FLUOROCARBONS: USE OF INFRARED SPECTROPHOTOMETRY FOR THE ANALYSIS OF SMALL SAMPLES' ABSTRACT A method for the analysis of small quantities of mixtures of carbon tetrafluoride, hexafluoroethane, and fluoroform based on measurement of the intensity of infrared absorption bands is described. The relation between pressure and optical density for the main absorption bands of each gas was determined and used to calculate the composition of synthetic mistures. These results were compared with those obtained by mass spectrometric analysis. INTRODUCTION Analytical procedures for handling small quantities of fluorocarbons are, as yet, largely undeveloped, though the physical and chemical properties of these compounds are well known. In order to malce possible an examination of the reactions of trifluoromethyl radicals it was necessary to find an accurate and rapid method for the analysis of carbon tetrafluoride, hexafluoroethane, and fluoroform in quantities of 10 micromoles or less. A summary of the various methods for analyzing fluorine compounds is to be found in Volume I1 of fluorine Clzemistry edited by J. H. Simons (Academic Press Inc., 1954). Procedures based on differences in vapor pressure or vapor density were shown to be of little value for analyzing these mixtures, and the chemical methods, such as pyrolysis over white-hot silica, combustion in moist hydrogen, and sodium fusion, give only values for the average carbon and fluorine content. Conventional oxidation methods cannot be used because of the high resistance to oxidation of fluorocarbons. Mass-spectrometric analysis is one possible method since the mass spectra of most of the simple fluorine-containing compounds are known (2). However, a characteristic feature of the mass spectra of fluorocarbons is the almost complete absence of parent pealis and, for the straight-chain fluorocarbons, the preponclerance of the mass 69 peak (CF3+). This means that the analysis depends on the magnitude of the relatively minor pealcs which are frequently common to a large number of fluorocarbons. Carbon tetrafluoride is particularly difficult to estimate in this way because all of its peaks are shared by other fluorocarbons and it must be estimated from the residual 69 (CF3+) and 50 (CF2+) pealis. Nevertheless, the method has been used successfully during the course of this worlc for the estimation of mixtures of carbon tetrafluoride, hexafluoroethane, and fluoroform in the presence of carbon monoxide and carbon dioxide. The results of a few analyses of test samples will be presented in a later section. An examination of the infrared absorption spectra of carbon tetrafluoride, 'Manziscript received July 11, Contribzltio?~ from the Division of Pure Clzemistry, National Research Council, Ottawa, Canada. Issued as N.R.C. No National Research Council Postdoctorate Fellow,

2 AYSCOUGH: ANALYSIS OF FLUOROCARBONS 1567 hexafluoroethane, and fluoroform showed the presence of a number of strong well-defined bands in the region cm.? which might be used for analysis. It was shown experimentally that with the exception of carbon tetrafluoride these compounds obey Beer's law in this region and that for samples of 5-10 micromoles an accuracy of ~ 5 can % be obtained. The results of experiments designed to determine the relation between intensity and pressure for these compounds are presented in this paper, together with the analysis of some test samples. Although the spectra of higher fluorocarbons are more complex it is reasonable to suppose that this method can be extended to estimate mixtures of other fluorocarbons in the same way as it is applied to the estimation of complex mixtures of hydrocarbons. EXPERIMENTAL Apparatus Optical density measurements were made using a Perkin-Elmer Model 21 double beam recording spectrophotometer with a sodium chloride prism. The optical cell used in conjunction with this instrument was made of pyrex with polished sodium chloride windows fixed with Benolite cement. The cell had a volume of 155 ml. and was fitted with a stopcock and ground glass joint for admitting the samples of gas. Silicone grease was used on all joints to reduce absorption of the gas. No absorption of gas on the walls of the vessel was observed. The pressure of gas in the optical cell was calculated by measuring the pressure of the sample in a calibrated gas burette and then allowing the gas to expand into the cell. A pressure of 1 mm. in the cell is given by about 8 micromoles of gas. &Iaterials Samples of carbon tetrafluoride, hexafluoroethane, and fluoroform of unspecified purity were supplied by E.I. du Pont de Nemours and Co. Inc. to whom we are greatly indebted. The gases were purified by bulb to bulb distillation in the vacuum system, a small middle fraction being taken in each case for calibration purposes. The mass spectra and infrared absorption spectra of these samples were identical with those published previously (1, 2, 5-8) and they were therefore assumed to be pure. RESULTS AND DISCUSSION For the purpose of analysis the important spectral region is from 1000 to 1500 cm.-l. Since the spectra have already been published, they will not be reproduced here. The only strong line in the carbon tetrafluoride spectrum is that at 1284 cm.-', which is well separated from the strongest absorption bands of hexafluoroethane (1253 cm.-i) and fluoroform (1152 cm.-i), so that it is possible to use the intensity of these bands as a measure of the partial pressure of each gas in a mixture. Weaker bands in the hexafluoroethane spectrum (1117 and 1123 cm.-i) and in the fluoroform spectrum (1380 cm.-l) might be used under conditions in which the other bands are too intense. Measurements of peak optical density were made for each band, using the pure gases, and although some deviation from Beer's law is observed in the

3 1568 CANADIAN JOURNAL OF CHEMISTRY. VOL. 33 case of carbon tetrafluoride and fluoroform this is not serious enough to affect the validity of the method. All measurements were made with comparable slit widths with no matching cell in the control beam of the spectrophotometer. The base lines were obtained with the cell evacuated and all measurements of optical density were made from these reference lines. Tables and Figs. 1-3 indicate the results of these experiments. Sample TABLE I OPTICAL DENSITY OF CARBON TETRAFLUORIDE Sample Pressure Optical density, (mm TABLE I1 OPTICAL DENSITY OF HEXAFLUOROETHANE Optical density Pressure (mm cm.-l 1123 cm.-l 1117 cm.-l Sample TABLE 111 OPTICAL DEXSITY OF FLUOROFORM Optical density Pressure (mm.) 1380 cm.-l 1152 cm.-i

4 AYSCOUGH: ANALYSIS OF FLUOROCARBONS PRESSURE (rnrn.1 FIG. I. Optical density of carbon tetrafluoride. PRESSURE (rnm.1 FIG. 2. Optical density of hesnfluoroethane.

5 1570 CANADIAN JOURNAL OF CHEMISTRY. VOL. 33 PRESSURE Imm.1 FIG. 3. Optical density of fluoroform. Spectra of Mixtures In order to check the accuracy of analyses using these absoi-ption bands a number of mixtures were prepared. The absorption spectrum of each was observed under the same conditions as were used for the samples of pure materials, and from the peak heights the partial pressure of each was calculated and compared with the true values. There is vei-y little overlap of absorption bands and it is necessary to apply a correction only in the case of the 1152 cnl.-' band of fluoroform in the presence of hexafluoroethane. This correction becomes significant when C2F6/CFBH is of the order of 5: 1 or greater. The results of analyses of these test mixtures are shown in Table 117. It will be seen that there is good agreement between the values obtained in this way and those obtained by synthesis. In some cases the mixtures were also analyzed mass spectrometrically. The results of these analyses are added to the table for comparison. Since the time this method of analysis was established, it has been used on numerous occasions in connection with a series of investigations of the reactions of trifluoromethyl radicals (3, 4). The mixtures have consisted, in general, of hexafluoroethane and fluoroform in the presence of small amounts of methane, ethane, and hexafluoroacetone. Small quantities of impurities such as these are of no importance since they do not absorb in the same region as the fluorocarbons. The only serious limitation so far observed is that resulting from pressure broadening of the fluoroform band in the presence of large amounts of hydrogen-containing material. Abnormally high results are obtained in this

6 Sample AYSCOUGH: ANALYSIS OF FLUOROCARBONS 1571 TABLE IV OPTICAL DENSITY OF MIXTURES Volume (ml.) Volume (ml.) Volume (ml.) by synthesis by infrared absn. by mass spectrometer CF4 CzFs CFJH CF4 C2Fs CFJH CF4 CzFs CFZH M M way, and to overcome this difficulty new calibration curves have to be plotted for each total pressure. This is tedious, but under these conditions the massspectrometric method was equally unsatisfactory. The authoris greatly indebted to Dr. R. N. Jones for the use of the infrared spectrophotometer and to Mr. R. Lauzon for its operation. Thanks are also due to Miss F. Gauthier for the mass-spectrometric analyses. REFERENCES 1. American Petroleum Institute Research Project 44, Catalog of infrared spectral data. 2. American Petrole~un Institute Research Project 44, Catalog of mass spectrometric data. 3. AYSCOUGH, P. B. J. Chem. Phys. In press AYSCOUGH, P. B., POLANYI, J. C., and STEACIE, E. W. R. Can. J. Chem. 33: BARCELO, J. J. Rescarch Natl. Bur. Standards, 44: NIELSEN, J. R., RICHARDS, C. M., and ~\~~C~URRY, H. L. J. Chem. Phys. 16: PLYLER, E. I<. and BENEDICT, W. S. J. Research Natl. Bur. Standards, 47: WOLTZ, P. J. H. and NIELSEN, A. H. J. Chem. Phys. 20:

GO-CATALYSIS IN FRIEDEL-CRAFTS REACTIONS

GO-CATALYSIS IN FRIEDEL-CRAFTS REACTIONS VI. POLYMERIZATION OF 2-BUTENE BY BORON FLUORIDE - METHANOL'. H. R. ALL COCK^ AND A. M. EASTHAM Division of Applied Chemistry, National Research Council, Ottawa,