ESTIMATION OF IMPURITIES IN LIQUID CHLORINE By INFRARED ABSORPTION SPECTROPHOTONIETRYl

Transcription

1 ESTIMATION OF IMPURITIES IN LIQUID CHLORINE By INFRARED ABSORPTION SPECTROPHOTONIETRYl ABSTRACT Commercial chlorir~e may contain small amounts ol chlorinated organic impurities, some ol which have been estimated by infrared absorption spectrophotometry. Liquid chlorine has 110 infrared absorption spectrum up to 1180 cm-i. This has enabled long path lengths to be used to increase the sensitivity to low concentrations of organic compounds cm. tantalum-clad steel pressure cell fitted with calcium fluoride windows was used, and spectra were recorded on a single beam instrument fitted with a calciu~ll fluoride prism. Using extinction coefficients, estimations were made to a few parts per million on impurities such as chloroform, methylene chloride, phosgene, and lle?cachloro- benzene. The method was checked by known additions to pure liquid chlorine. The estimation of moist~lre in liquid chlorine is discussed. Commercial chlorine from the electrolysis of brine in either diaphragm or mercury cells contains small amounts of chlorinated organic impurities. Although such contaminants are present in only a few parts per nlillion, a knowledge of their concentration is of importance to the inanufacturer. The impurities appear to be predominantly C1 and C2 hydrocarbons such as carbon tetrachloride, chloroform, methylene chloride, and hexachloroethane with, in certain cases, more con~plex conlpounds such as hexachlorobenzene. The estimation of these impurities has been difficult, since the analytical methods available were time consuming and required several liters of sample. The most reliable method has been the distillation of approxin~ately 10 liters of liquid chlorine through a fractionating column as described by Hedgepath and Riggs (3) and modified by Matthews and Warren (4). However, distillation methods are at a disadvantage because the chemical nature of the impurity may change during distillation; chloroform for example may chlorinate further to carbon tetrachloride. This disadvantage is absent in a spectrometric method. Chlorine consists of the molecules C1$5-35, C , and the heterotope C The former, in common with other isotopically hon~ogeneous molecules, are transparent to infrared radiation. The molecular heterotope, however, would be expected to absorb, and since its concentration is normally 36.6y0 this absorption cannot be ignored (2). The fundamental band has been calculated to be at 554 cm.-l in the gas phase, with first and second overtones at 1100 cm.-l and 1638 cm.-i respectively. In liquid chlorine the absorption frequencies of these bands would be lowered by about one half of one per cent (1). The fundamental and the first overtone are beyond the range of calcium fluoride transn~ission. No band at around 1640 cm.-l corresponding to the second overtone has been detected. lnlanliscrifit received June 17, Contribrition from Canadian Industries Limited, Central Research Laboratory, A~cMasterville, Que. 956

2 PROSS: LIQUID CHLORINE 957 EXPERIMENTAL Wright and Smith employed a 1-in. tantalum-clad brass pressure cell with quartz windows for the estimation of moisture at 3700 cm.-' (7). This cell was limited b37 the cutoff of quartz at 2850 cni-i. For the following work a 5.25 cm. tantalum-clad steel cell was used with calcium fluoride windows which are transparent in the infrared as far as 1180 cm-i. The extreme transparency of liquid chlorine in the infrared has enabled the use of a path length which is approximately two thousand times greater than is usual for most organic liquids. This path length has caused the method to be sensitive to concentrations of absorbing substances of the order of a few parts per million. This cell was designed and made by the Perlcin-Elmer Corporation (5) - A section of the cell is shown in Fig. 1. It consists of a 1% in. diameter steel cylinder 2 in. long, clad with tantalum whicl~ is resistant to liquid chlorine FIG. 1. Section of the Perkin-Elmer 5 cm. tantalum-clad steel cell. A. Tantalum-clad steel cylinder. B. Calcium fluoride window. C. Rubber gasket in "Teflon" envelope. (A). The windows are t in. thick calciu~n fluoride (B). "Teflon" U-Section gaskets are used with neoprene O-ring inserts (C). The windows are secured by means of caps which are drawn against them. They are protected by i; in. diameter keys which prevent the caps from turning. The cell is designed for a pressure of 200 p.s.i. The vapor pressure of chlorine at 25OC. is 105 p.s.i. gauge. The cell was fitted to a stairiless steel reservoir and a valve system consisting of + in. stainless steel "Hoke" needle valves. This makes possible the filling

3 9.58 CANADIAN JOLJRNAL OF CIIEMISTRY. I.OL. 32 of the cell with liquid chlorine directly from an inverted commercial cylinder without cooling. Alternatively, by placing the reservoir in a Dewar flask paclced with dl->i ice, liquid chlorine could be introduced from a flaslc as shown in the line diagram Fig. 2. As a precaution_ary measure the nitrogen inlet tube at A was held in place by hand so that it could be instantly released should excessive back pressure develop because some part of the system was insuffic- FIG. 2. Diagram to show method of filling reservoir. iently cooled. After the reservoir had been filled, the dry ice packing around the inlet tube was removed. Valves B and C were then closed, and the reservoir was removed from the Dewar. Before the reservoir was inverted, the systein was brought to room temperature with the help of lukewarm water. This avoided possible fracture of the windows which might have resulted if cold chlorine were allowed to enter the cell. When filled, the reservoir and cell were supported in front of the entrance slit of the spectrometer. Spectra were recorded on a Perkin-Elmer Model 12C single beam instrument using a calciunl fluoride prism. The increased resolution afforded by a calcii~m fluoride over a rock-salt prism is an advantage in this work. The specific extinction coefficient E, is given by the Lambert-Beer relationshi~:

4 where c = concentration in gm. per ml., I = path length in cm., I0 = intensity of incident radiation, I = intensity of absorbed radiation. In this work band intensities are a function of the experimental conditions. Apparent specific coefficients Eva have therefore been quoted, following the example set by Ramsay (6). The incident and absorbed radiation intensities were taken as proportional to the background and band peaks respectively. Finite slit width and scatter corrections were dispensed with. Band intensities thus measured gave estimations of impurities which were accurate to between 10% and 20%. Formula (1) was rearranged and parameters were substituted to give concentrations in parts per million from a constant multiplied by the optical density. The method was tested by addition of known quantities to liquid chlorine. The liquid chlorine ~ised in these experiments was purified by distillation using a column packed with "Fenske" rings having a performance equivalent to approximately eight theoretical plates and a reflux to product ratio of 2: 1. A spectrum was first recorded to ensure the absence of impurities, after which the chlorine as distilled back into the flask surrounded by dry-ice. Volatile compounds such as chloroform and methylene chloride were added from a weighing pipette, and solids were added as such with subsequent agitation until dissolved. Recording time for the spectrum over the range 4000 cm.-l to 1180 cm.-l was approximately 30 min. Table I summarizes the analytical results obtained with this technique. The frequencies of the analytical bands are those observed for the compounds TABLE I SUMMARY OF ANALYTICAL RESULTS Concentration in p.p.m. Cornpound v7 EvB cm.-' Added Found in liquid chlorine as solvent. Further analytical bands of interest, with their frequencies and apparent extinction coefficients, are given in Table 11. For concentrations that may be encountered in practice no suitable bands are ANALYTICAL TABLE I1 ABSORPTION BANDS AND APPARENT EXTINCTION COEFFICIENTS v, Compound cm.-i Eva CHzClCOCl COCI? f f 110 H?O 1640 lolof 20

5 960 CANADIAN JOURNAL OF CHEMISTRY. VOL. 32 present in the range 400 cm.-' to 1180 cm.-' for carbon tetrachloride or l~exachl~roetha~e. Analysis for these c~mpounds will clepencl on tinding a suitable cell window material that will transmit further into the infrared. Analytical estimations by this method are under investigation. The absorption band at 1640 cm.-l has an advantage over the 3700 cnl.-' band used by IVright and Smith as there is no correction required for carbon dioxide. Any interference in the 1640 cm.-' region would be from amide-carbonyl and C=C unsaturation; and neither would be expected in liq~lid chlorine. The vaporizing of sufficient liquid chlorine into a gas cell as a means of correction for carbon dioxide is open to criticism as carbon dioxide boils off first resulting in a false value for this correction. ACKNOWLEDGMENTS ' The constant help and advice of F. W. Nlatthews of this laboratory is gratefully acknowledged, as is also the distillation work carried out by G. G. Warren, also of this laboratory. The Perkin-Elmer Corporation is thanked for the loan of the cell. REFERENCES 1. BERNSTEIN, H. J. Private communication. 2. GIAUQUE, W. F. and OVERSTREET, R. J. Am. Chem. Soc. 64: HEDGEPATH, L. L. and RIGGS, W. S. J. Am. Water Worlcs Assoc. 30: MATTHEWS, F. W. and WARREN, G. G, Can. J. Technol. 32: The Perkin-Elmer Corporation. Norwalk, Connecticut, U.S.A. 6. RAMSAY, D. A. J. Am. Chem. Soc. 74: WRIGHT, E. R. and SMITH, R. A. American Chemical Society Meeting, Buffalo, N.Y. March