Determination of Concentrations of Halogenated Compounds Dissolved in Various Liquids by Electron Capture Gas Chromatography Brian
K. Lamb and Fredrick H. Shair"
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, Calif. 9 7 725
Determination of halogenated compounds dissolved in liquids by electron capture gas chromatography is three to four times faster than usual volumetric-manometric solubility methods, requires no solvent degassing, and is capable of accuracies of approximately f 6 YO.Measured solubilities of sF6 in water over a range of temperatures were in good agreement with literature values. The solubility of SF6 in a homologous series of n-alcohols was also determined. The error in this case was within predicted limits, although larger than satisfactory. Means of improving the reproducibility are discussed. The technique is capable of measuring dissolved halogenated compounds at extremely low concentrations and is expected to form the basis for the development of a multiple liquid tracer system.
Knowledge regarding the concentration of gases dissolved in liquids is of importance to a wide variety of technical interests. In addition to determining the solubility which will occur under well-specified equilibrium conditions, there is often a need to measure the concentration of a gas or liquid tracer which is present in a liquid sample. In some cases, for example, tracer investigations, the concentration of the tracer in the liquid phase will be orders of magnitude lower than that which would exist in typical phase equilibria experiments. In an extensive review of gas solubility methods, Battino and Clever ( I ) have discussed the traditional volumetricmanometric methods used to determine the solubility of gases in liquids. The volumetric-manometric methods involve saturating a previously degassed solvent with the gas of interest under conditions where appropriate volumes, pressures, and temperatures may be accurately measured. Equilibrium between the gas and liquid phase is generally achieved by stirring a mixture of the two, bubbling the gas through the liquid, or circulating a film of the liquid through the gas. Careful degassing requires 3 to 5 hours followed by equilibrium times of 3 to 12 hours. These methods can yield gas solubility data thought to be accurate to better than 1%. The purpose of this work was to develop a sensitive technique accurate to approximately f 5 % , which would avoid solvent degassing and which would require much less time for analysis than the traditional methods, We report here such a technique which is based upon an analysis by means of electron capture gas chromatography. Consequently, this method can be used to determine the solubility of gases and liquids which have relatively high electron-capture cross-sections such as SFe, Freons, and other halogenated compounds. This technique offers a simple and useful method for gaining more information about the anomalous behavior of fluoro compounds in solution. In particular, the concentration of such gases as SFG,CC13F, and CH3Cl can easily be measured in various solvents, nonpolar and polar, including those possessing high vapor pressures a t the temperatures of interest.
The rapid determination of concentrations of halogenated compounds in air and dissolved in water, particularly sea water, appears very important in view of the current debate concerning stratospheric ozone decomposition by Freons and other halogenated compounds. Oceans have been suggested as possible natural sources and sinks for some of these compounds. The technique described here may be useful in determining the oceanic concentrations of halogenated compounds.
EXPERIMENTAL T o test t h e accuracy of t h e method, t h e solubility of SF6: in water from 10 t o 50 OC and in t h e straight-chain alcohols from methanol to heptanol a t 25.0 "C was determined. T h e SFs was obtained from Matheson, distilled water was taken from t h e building supply, and the alcohols, spectroquality, were purchased from Aldrich Chemical and Matheson, Coleman and Bell. Temperature was controlled to f 0 . 2 "C in a large Dewar flask with a Neslab Circulating Temperature Controller. Water saturation took place in 5.0-cm3 polyethylene disposable syringes attached t o a syringe carousel threaded on t h e shaft of a variable-speed electric stirrer. During saturation, t h e syringes a n d carousel were submersed in t h e bath and spun as rapidly as possible. Initially each syringe was flushed several times with SFs a n d t h e n filled with 4.0 cm3 of SFs. T h e capped syringes were placed in t h e temperature bath for approximately 15 min. For the same length of time, a glass screw-top vial containing 20 cm3 of distilled water was also placed in t h e bath. After both the gas and water were temperature equilibrated, 1.0 cm3 of water from t h e vial was quickly drawn into each syringe, and t h e syringe capped and clamped vertically t o the carousel. Normally, filling and clamping could be done in less than 60 s. During the water tests, three syringes were prepared and spun a t each temperature. T h e partial pressure of SF6 in t h e syringe was simply the difference between the atmospheric pressure in the room measured on a mercury barometer and t h e vapor pressure of water a t the saturation temperature. T h e low solubility of SFs in t h e small amount of solvent present caused less t h a n a 1%pressure drop in the syringe due t o dissolution of the gas, while allowing equilibrium to be reached within very short periods of time. T h e syringes were spun 40 inin t o ensure saturation, although preliminary tests indicated equilibrium was reached in 20 min. After spinning, each syringe was allowed t o rest in t h e bath a t least 10 min before being analyzed. Allowing t h e syringes t o rest 3 h before analysis did not change t h e results. For the alcohol tests, 20-cm3 syringes containing 1 cm3 of solvent were used t o maintain a negligible pressure drop. Six differe n t alcohols were simultaneously saturated with SFs by spinning for 90 min. Each syringe was allowed to rest a t least 30 min prior t o analysis. Although t h e alcohol results were not as reproducible as t h e water tests, no consistent correlation between resting time and results was found, giving no indication of a problem with dispersed gas bubbles in t h e solvent. Although SF6: is extremely insoluble in water, t h e sensitivity of t h e electron capture detector t o SF6 required t h a t SFs-saturated water be diluted a t least three orders of magnitude prior t o injection into t h e chromatograph. I t is this property t h a t gives the method its important flexibility for analyzing compounds under both equilibrated and extremely nonequilibrated conditions. Dilution was accomplished by means of a well-mixed exponential dilution system similar t o t h a t used by Drivas e t al. (2, 3). A 6 X 6 X 6 inch Lucite cube was built with a magnetically driven fan positioned inside. A slow nitrogen flow rate of 120 cm3/minute assured ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
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I sF6
2An
O2
IAin
O
TIME
Figure 1. Typical sF.5 chromatogram: [SF6] = 9.63 X mol SF6/cm3. Sample taken during dilution of 50 OC H 2 0 solubility sammol fraction ple, X,,, = 2.28 X
good mixing. If the system were well mixed, the dilution would follow
c = Coe-(s/")t where q is t h e nitrogen flow rate through the cube, V is the cube volume, and t is t h e time since flow began; ( q / V ) t is simply t h e number of resident air changes N in the cube since flow began. T h e measured sample concentration a t N is given by C, and t h e unknown initial concentration in the cube is given by Co. A graph of In C vs. N should yield a straight line with slope of -1. T h e intercept of t h e graph gives t h e initial Concentration in the cube. For each water test, 1.0 pl of saturated water was transferred to the cube with a 5.0-pl SGE syringe. T h e cube was sealed, and the contents were mixed for 20 min prior to starting the nitrogen flow. The saturated water injected into t h e cube evaporated in less than 5 min, thus liberating dissolved SF6. Flow through the cube was measured with a Precision Scientific Wet-Test Meter. One dilution run consisted of five cube concentration measurements taken over an hour period. After a complete analysis, t h e cube was rapidly flushed with nitrogen in preparation for the next dilution run. Cube concentrations were measured using a Loenco Model 70 Gas Chromatograph equipped with a n 8-port Valco gas sampling valve fitted with a matched pair of l-cm3 sample loops. T h e concentric electron capture detector contained a 200-mCi tritium foil, which under pulsed voltage operating conditions produced a A. An 8-ft X ?$-in. stainless steel colstanding current of 3 X umn packed with 80-100 mesh Porapak Q was fitted t o a Carle 6port switching valve t o allow solvent venting. T h e column eluted 0 2 in 25 s, SF6 in 70 s, and water and t h e alcohols sometime after 105 s. A typical chromatogram is shown in Figure 1. T h e column venting is an important part of this technique since t h e radioactive foil becomes rapidly contaminated and performance deteriorates if solvents are allowed t o pass through t h e detector. T h e carrier gas was prepurified nitrogen run a t 110 ml/min. An injection port on the chromatograph allowed direct injection of solvent blanks. Samples containing low levels of dissolved fluorochemicals, as in tracer experiments, can also be injected directly. T h e injection port, column, and detector temperatures were respectively 160, 60, and 120 O C . T h e SF6 peak was integrated with a Spectra-Physics Autolab System I digital integrator. T o avoid absorption problems, calibrations were begun by injecting 1.0 p1 of pure SF6 into t h e dilution cube and measuring peak area with respect t o the number of air changes. A graph of In peak area vs. number of air changes produces a slope of -1.00, T h e initial concentration divided by the peak area intercept yields a calibration factor, typically 1000 p V - ~ e c / l O - ' mol ~ SF6, t h a t is programmed into the integrator for the analyses. T h e detection range mol SF6, and t h e range is linear from is from 5 X lo-'* t o 5 X 5X mol t o t h e detection limit. All solubility measurements were made in the linear range. T h e system was modified for the alcohol tests by fitting t h e GC sample valve with a 15-pl sample loop, which was calibrated during a GC calibration with respect to a l-cm3 loop. Also, only 0.5 pl of saturated solvent was transferred t o t h e cube from t h e saturation syringe. These changes were necessary because of the much greater solubility of SF6 in hydrocarbons. In all cases, the solvents evaporated in the cube in less than 10 min. 474
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RESULTS AND CONCLUSIONS A least squares computer fit In C vs. N for each dilution gave the initial cube concentration and, hence, the moles of SF6 per moles of water in 1.0 ~1 or, more simply, the mole fraction. Only dilutions which gave a slope within f0.05 of -1.00 were accepted for calculating the results. In only two or three instances were dilutions not acceptable. The mole fraction of SF6 in water plotted vs. temperature is shown in Figure 2. The data of Ashton et al. ( 4 ) , Morrison and Johnstone ( 5 ) ,and Friedman ( 6 ) are also presented. The work of Ashton and co-workers offers the best reference range of data and also the most accurate results, with an experimental error of 1%.Our results agree favorably with those of the Ashton group, being in perfect agreement a t 50 "C and 7% high a t 10 "C. The average standard deviation of our results over the temperature range was 6%. The Ashton coworkers noted that their results were higher than those of Morrison and Johnstone and Friedman, with discrepancies being largest a t lower temperatures. This difference was credited to more complete saturation in the Ashton work. Similarly, we note that our results are slightly higher a t lower temperatures than any of the referenced papers. It is possible that our method represents the most complete saturation of the reported techniques. It is interesting to realize that our data consisting of 18 separate analyses were taken during a period of four working days, while a single solubility analysis from the Ashton group volumetricmanometric method required 12 hours for saturation to occur. The solubility of SF6 in n-alcohols from C1 to C7 plotted vs. solvent skeleton length is given in Figure 3. Data for several other solvents taken from the gas solubility data review by Wilhelm and Battino are also given ( 7 ) . Chain lengths and molecular sizes were calculated from bond length and bond angle data taken from ref. 8. The solubility of SF6 increases in the alcohols with increasing chain length, which corresponds to increasing nonpolar character. In addition, the mole fraction of SF6 levels off with hexanol and heptanol, mirroring the behavior of heptane and octane. The SFs molecular size, as indicated in Figure 3, is within a factor of two of that for most of the solvent molecules for which measurements have been reported. The maximum uncertainty, calculated in the standard manner as given by Bevington ( 9 ) , for the measurement of the mole fraction of SFs in water was f 1 2 % . The data were easily within that limit, with an average standard deviation of f 6 % . The predicted uncertainty for measuring the mole fraction of SFs in the alcohols was f 2 2 % , while the experimental average deviation was f20%. With water as solvent, the major source of experimental error as predicted by the calculations was the measurement of the very small volumes of saturated water injected into the dilution cube. I t accounted for 10 out of the 12% predicted uncertainty. With alcohols as solvents, the major sources of error were the measurements of the small volumes injected into the cube and the even smaller volumes injected from the cube into the chromatograph. Together, these measurements accounted for 19 out of the 22% predicted uncertainty. Without these small volume uncertainties, the technique might be capable of solubility measurements of about 2 to 3%. Adsorption of SF6 on the Lucite cube or on the stainless steel GC column is not considered a significant source of error. During typical atmospheric SFs tracer calibrations in this laboratory using the dilution method ( I O ) , linear SFs calibration curves are routinely measured down to 1 part SFs in 1 O I 2 parts air. Work in this laboratory on suitable Freon sampling techniques has shown that some Freons which are adsorbed by Lucite are not absorbed by Tedlar
t
i
E;L
0 Q LL iL
2 t
It
i i
Figure 2. Solubility of sF6 in water: ( 0 )this work; (0)Ashton e t al. (1968); (0)Morrison and Johnstone (1955); (A) Friedman (1954) I
0
(11). In these instances, a Tedlar-lined dilution cube would permit utilization of the solubility technique. Hester et al. ( 2 2 ) and Donohoe (11) have reported the use of stainless steel columns for the separation of Freons with no adsorption observed. T o achieve reproducibility of the procedure close to the predicted limit, f 3 % , the uncertainty associated with small volumes must be eliminated. Use of an internal standard present in the solvent in accurately known concentrations and detectable by electron capture would allow peak area comparisons between the standard and the dissolved gas or liquid. This comparison would yield the solubility of the gas without the uncertainty of small volumes. The GC can be calibrated to within f 3 % for a particular compound, and the standard can be diluted in the solvent to within f2%. A specific standard is not presently known, but the necessary properties of such a standard can be examined. I t must be soluble in the solvent, but unreactive with solvent and solute. The standard must be separable from the solute and solvent by chromatography and sensitive a t low levels to electron capture detection. Finally, a t very low concentrations, it must not affect the solubility of the gas in the solvent. In many ways, low molecular weight fluorinated alcohols fit this description. However, thorough testing for the above properties would be necessary before a specific selection could be made. Such a standard, when proved suitable, should limit solubility measurement uncertainties to f5%. The extension of this technique to include other compounds under saturated and nonsaturated conditions may involve three distinct sample handling techniques. For solubility studies, the saturated sample must generally be diluted in a suitable manner (Le., dilution chamber) prior to injection in order to avoid saturation of the electron capture detector. For tracer studies or for a few environmental trace analyses, the dissolved compound concentrations may be such that samples can be injected directly into the gas chromatograph. For most environmental trace analyses, the concentration of the substances is low enough to require stripping and concentrating of the compound prior to injection into the chromatograph ( I , 13). The electron capture gas chromatography method of measuring electron capturing gases dissolved in liquids is capable of accurate concentration determinations under
I I 1
1
I
1
1
1.5 3.0 4.5 6.0 7.5 9.0 CHAiN LENGTH OF SOLVENT(A)
Figure 3. Solubility of SF6 in various solvents: (OA)this work: (0) Wilhelm and Battino (1973)
conditions of well-specified equilibria with analysis times three to four times faster than usual volumetric-manometric methods. Since dissolved gases represent the most difficult analytical challenge, one may expect that the technique will be easily applied to the measurement of detectable liquids dissolved in other liquids. With this procedure, gas or liquid solute concentrations may be determined over an orders-of-magnitude range under a variety of conditions. The combined properties of this technique result in an analytical procedure with exciting possibilities for application in a wide spectrum of theoretical and real systems. Future work will be concentrated on the development of several applications.
LITERATURE CITED (1)R. Battino and H. L.Clever, Chem. Rev., 66,395 (1966). (2)P. J. Drivas and F. H. Shair, Atmos. Environ., 8,475 (1974). (3)P. J. Drivas. P. G. Simmonds, and F. H. Shair, Environ. Sci. Techno/., 6, 609 (1972). (4)J. T. Ashton. R. A. Dawes, K. W. Miller, E. B. Smith, and B. J. Stickings, J. Chem. SOC.A, 1793 (1968). (5)T. J. Morrison and N. B. B. Johnstone, J. Chem. Soc., 3655 (1955). (6) H. L Friedman, J. Am. Chem. Soc., 76,3294 (1954). (7) E. Wilhelm and R. Battino, Chem. Rev., 73, 1 (1973). (8)R. C. Weast. Ed., "Handbook of Chemistry and Physics," 50th ed., The Chemical Rubber Co., Cleveland, Ohio, 1970,p F-155. (9)P. R. Bevington, "Data Reduction and Error Analysis for the Physical Sciences," McGraw-Hill, New York, 1969,pp 56-65. (10)B. K. Lamb, J. D. Bruchie, and F. H. Shair. "Tracer Studies for Characterizing the Transport and Dispersion of Plumes Emitted at Ground and at Elevated Levels," (Preliminary Report); prepared for the U.S. Environmenlal Protection Agency, EPA Grant No. R802160-03-2,1975, pp
45-53. (1 1) K. G. Donohoe, personal communication, 1975. (12)N. E. Hester, E. R. Stephens, and 0. C. Taylor, A m o s . Environ., 9, 603 (1975). (13)P. Hanst. L. Spiller, D. Watts, J. Spence, and M. Miller, J. Air follut. Conk Assoc.. in press.
RECEIVEDfor review September 19, 1975. Accepted December 4, 1975. This work was supported in part by the
U S . E.P.A. under contract No. 68-03-0434 and Grant No. R802160-03-2. The contents do not necessarily reflect the views and policy of the Environmental Protection Agency.
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