Determination of Dichlorodifluoromethane in Fumigation Mixtures with Ethylene Oxide By Measurement of Thermal Conductivity WILBUR A. AFFENS,' EDWARD 0.HAENNI,* and
R. A. FULTON U. S. Department
Entomology Research Division, Agricultural Research Service,
b A practical procedure for field use has been developed for the sampling and determination of dichlorodifluoromethane and ethylene oxide in fumigation and sterilization otrnospheres. The dichlorodifluoromethane is determined by. a therma I-conductivity method, and the ethylene oxide is determined as described b y Lubatti. A commercially available instrument is used for the thermal conductivity measurements. The method is rapid and simple. It can b e used for laboratory applications of greater sensitivity by modification of the instrument.
D
investigations on the use of pressurized propellants for dispensing sterilizing agents, fumigation mixtures of ethylene oxide and propellants of the chlorofluorohydrocarbon series, including dichlorodifluoromethane, have been of particular interest. The use of ethylene oxide as a fumigant for insect control dates back to the work of Cotton and Roark ( I ) , and its use as a sterilizing agent to the work of Phillips and Kaye (10). Dichlorofluoromethane (propellant 12) is used either alone or in mixtures with other chlorofluorohydrocarbons as a propellant in aerosol-type insecticides and numerous other kinds of pressurized sprays and in refrigerants. Fumigation with ethylene oxide introduced the possibility of fire and explosion, and a suitable flame-suppressing agent was needed. Dichlorodifluoromethane may be used for this purpose (6). This problem has been investigated, and limiting flammability ratios have been determined (4). If the concentration ratio of dichlorodifluoromethane to ethylene oxide exceeds a certain limit, the mixture mill be nonURING
Present address, Naval Research Laboratory, U. s. Department of Defense, Washington 25, D. C. Present address, Food and Drug Administration, U. s. Department of Health, Education and Welfare, Washington 25, D. C.
o f Agriculture, Beltsville, Md.
flammable, and will not propagate a flame when mixed in any proportion with air. If a fumigation mixture is to be safe, the limiting concentration ratio must be exceeded a t all times during the fumigation period. Because the time necessary for effective fumigation is several hours, the mixture in the fumigation chamber might become flammable because of different loss of dichlorodifluoromethane relative to that of the ethylene oxide, and it was decided to keep a constant check on the concentrations of dichlorodifluoromethane and ethylene oxide during the fumigation. Such information would also be useful because the time necessary for effective fumigation is a function of the ethylene oxide concentration. The concentration levels of interest were 2 to 15% by volume of ethylene oxide, and two to three times these levels of dichlorodifluoromethane. The U. S. Army Chemical Corps had been using an adaptation of the method of Lubatti (6) for the determination of ethylene oxide. This laboratory attempted to develop a simple, rapid, and portable field method for the determination of dichlorodifluoromethane. It was decided t o investigate the applicability of the nork of Phillips and Bulger (11) in the determination of methyl bromide in grain-fumigation mixtures by the thermal conductivity method to dichlorodifluoromethane and similar propellants. The theoretical and historical background of the thermal conductivity method has been covered by Daynes (S), Palmer and Weaver ( 8 ) ,and more recently by Weaver (12). The method has gained popularity as a sensing component in gas chromatography. A method for the determination of fluorinated hydrocarbons by gas chromatography a t rather low concentration levels has recently been reported (9).
The thermal conductivity method of gas analysis is best applicable to a twocomponent system. As a rule, the reference gas is of constant composition, and differs from the unknown
gas only in that it does not contain the ingredient being determined. Should the unknown gas contain measurable amounts of extraneous substances which are not components of the reference gas, they must be removed from the sample or compensated for in some other manner before electrical readings are taken. Ethylene oxide and water vapor were the chief interfering substances in the determination of dichlorodifluoromethane when a dry air reference was used. After these substances are removed, the thermal conductivity method can be applied readily, using a commercially available instrument with minor instrument modifications. The moisture is removed from the gas sample by anhydrous calcium sulfate. To remove ethylene oxide, the technique of the Lubatti (5) determination is used. A gas sample of known volume is bubbled through traps containing normal sulfuric acid solution saturated with magnesium bromide. The ethylene oxide absorbed in these traps reacts with the acid, and the residual acid is back-titrated with standard alkali; the quantity of ethylene oxide in the sample may then be calculated. It was decided to use the standard acid from the Lubatti method for removal of ethylene oxide from the gas sample intended for the dichlorodifluoromethane determination, and to combine the methods by determining the ethylene oxide in the same absorber. EXPERIMENTAL
Gas-Sampling System (Figure 1).
A light-walled, steel gas-pressure cylinder, H , 9 inches long, 5 inches in diameter, and of 136-cubic inch (2.23liter) capacity, was used as a sampling vessel. The cylinder had two l/*-inch I.P.T. threaded outlets a t one end, one, G, a t the center and the other I , ll/z inches away a t an angle of 30" from the center axis. A 31/2 X 1 inch strap handle was raised 1 inch from the cylinder side. The outlets were fitted with needle valves a t G and I. The V O L . 31, NO. 9, SEPTEMBER 1959
1565
center valve, G, was connected internally to a '/&ch copper eductor tube which extended to about '/* inch from the bottom of the cylinder. The valves of the cylinder and some of the other fittings were equipped with the handtightened type of coupling commonly used in the refrigeration industry, in order to facilitate connecting and disconnecting the components of the system when necessary. This sample cylinder, evacuated to less than 1 mm. and .its valves then closed, was connected in series to the following components: A 1-mm. capillary tube about 1 inch long, F , connected valve G with a drying tube, E, containing anhydrous calcium sulfate. The purpose of the capillary was to limit the speed of flow of incoming gas into the sample cylinder, and avoid possible carry-over of acid from the absorption bulbs, D. Conventional glass drying tubes were too fragile, and were replaced by transparent polymethacrylate plastic tubes, 1 inch in diameter and about 6 inches long. They were filled with 8-mesh anhydrous calcium sulfate (indicating Drierite), and were plugged with a small piece of cotton and a 1-hole rubber stopper with '/&eh copper tubing insert at each end. A similar drying tube waa used in the analysis system. The absorption bulbs, D,D,consisted of two Vigreux bubblers in series containing 20 and 10 ml. (left and right) of 1N sulfuric acid saturated with magnesium bromide. (This solution was prepared by dissolving 3 pounds of M g B r 2 . 6 B 0 in a minimum of distilled water, adding 27 ml. of sulfuric acid, and diluting to 1 liter. It was standardized against 0.2N sodium hydroxide solution.) The absorption bulbs were connected by Tygon tubing to E and to a valve, C, to which was attached a T-tube connected with an evacuated steel gas cylinder, B, and a '/rinch copper-tubing sample line, A , leading to the fumigation chamber. The function of B was to flush the lines to the chamber before sampling. A cylinder of about 8 liters was found to be suitable, such ax an oxygen-breathing cylinder, D-2 type or larger. The fumigation chamber, which was a t atmospheric pressure, was very large (about 2000 cubic feet) compared with the sample taken (about 2.5 liters). Method of Samulinrr. After tht? "Y"'y""LL1YY
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valve C was closed, and the lines were flushed by opening the valve of cylinder B for 5 to 10 seconds. This valve was then closed and C and G were opened. Gas from the fnmigation chamber began to enter the system, as was noted by bubbling through the absorption bulbs. After the sample cylinder, H , was at atmospheric pressure, as indicated by cessation of bubbling through the Vigreux tubes, valve G was closed. The sampling took from 1% to 2 minutes. The sample cylinder was then ready for the determination of dichlorodifluoromethane. The temperatures of the fumiga1566
ANALYTICAL CHEMISTRY
Figure 1 sampling system
.
.
..
..
nirlotion of dichloroditluoromethone
tlon chamber and of the sample cylinder
at the time of sampling were recorded. Blank samples (before discharge of fumigant into the chamber) were also taken. The volume of air space in the sampling system between C and G was determined by evacuating this section and measuring by differential pressure measurements the amount of air required to fill it. Determination of Ethylene Oxide. After standing for about 15 minutes following sampling, the contents of the absorption bulbs were transferred to a n Erlenmeyer flask, and the ethylene oxide was determined by back-titration with 0.2N sodium hydroxide. The number of milligrams of ethylene oxide in the sample was calculated. The total volume of sample represented by the ethylene oxide is a function of the quantity of ethylene oxide and moisture absorbed and the amount of dichlorodifluoromethane, and can be calculated from the combined data. Determination of Dichlorodifluoromethane. The apparatus for the determination of dichlorodifluoromethane is shown in Figure 2. The chief component of the system was the thermal conductivity gas analyzer, D (Gas Master, Model 30-S, with M / T filaments, and sealed air reference, Cow-Mac Instrument Co., Madison, N. J.). The cell, which was of the Minter diffusion type, and the bridge circuit, of the Naval Research Laboratory plan, have been described (8, 6, 7, 11-18). Bridge balance was effected by means of a %ohm zero-con-
trol potentiometer prior to admitting the sample gas for analysis. The gas analyzer was energized by a 6-volt direct current power supply, S (Gas-Master Helper, Gow-Mac Instniment Co.). The same cylinder, A , was connected into the system as shown. Connections were made by hand-tightened refrigeration couplings and Tygon tubing. After the cylinder valves were opened, gas from the sample cylinder flowed through a drying tube, B, containing anhydrous calcium sulfate, to a circulating pump, C , and then to the cell, F , inside the gas analyzer, from which it was returned to the sample cylinder through the side valve and then recirculated. A flowmeter, E (Flowrator 0438A, Fischer and Porter Co., Hatboro, Pa., or from the Gow-Mac Instrument Co.), attached to the side of the analyzer was inserted in the gasflow system in advance of the cell. Pump C was fitted with a control knob for adjusting flow rate. A flow rate of 1 cubic foot per hour was used. The flowmeter and pump (originally supplied as component parts of the GasMaster Helper) were mounted as shown for convenience and versatility, as i t was sometimes desirable to use other sources of electric power. Certain other modifications were also made in the gas-analyzer and Helper units. Current to the bridge, and hence filament temperature, were controlled by means of a 20-ohm potentiometer (one terminal unused), which was in series with the current from the power source. It was found helpful to mount a second potentiometer of 200-ohm resistance in' series with the other, so as to have
greater control of current flow when necessary. The original 20-ohm control was used as a fine adjustment. For similar reasons a 200-ohm potentiometer was also connected into the Helper unit, since the original power supply had no current control. The bridge voltage was measured by means of a millivoltmeter connected across a span adjuster. The millivoltmeter scale was in arbitrary units from 0 to 100; full scale was equivalent to 5 mv. This dual-purpose meter was used either for current or for bridge potential readings by means of a switch and shunt system on the instrument panel. The gas analyzer was also equipped with a double pole-double throw switch for reversing the polarity of the leads to the meter, if necessary, when it was being used to measure bridge potential. This polarity depended on the relative thermal conductivities of the reference and sample gases, and also on which of the two pairs of cell chambers was used for each. The span adjuster was a 100-ohm precision linear potentiometer with a 100-turn microdial locking indicator. I n addition to the electrical modifications, the gas-flow system of the gas analyzer to and from the cell was fitted with copper tubing and brass connecters (refrigeration - type) throughout, and carefully sealed and tested for leakage. Four identical brass nipples were screwed into the ports of the cell, and were connected by short lengths of Tygon to the copper tubing in order to preserve the thermal-conductance symmetry of the cell. The reference side of the cell was filled with dry air and sealed. The pump and drying tube were mounted in a separate box, and an on-off switch was placed in the circuit. All experiments were done where 115-volt alternating current was available. The pump and power supply required this, but in field experiments where this current n-as not available the storage battery of a n automobile could be used. The gas-analyzer could operate directly 08 the battery, and the power supply could be put aside. Where 115-volt alternating current was not available, the pump was operated with a small direct currentalternating current converter commonly used for automobiles to operate small alternating current appliances. Calibration of Gas Analyzer. Dichlorodifluoromethane of refrigeration grade, Federal Specification BB-C310, was used throughout this work. The instrument was calibrated against mixtures of dichlorodifluoromethane and air prepared in the sample cylinders. The cylinder was connected to a manifold system (4) equipped with a manometer, cylinders containing liquid dichlorodifluoromethane and ethylene oxide, a pump for evacuating the system, and a dry-air reservoir (air stored in a 5-gallon bottle over a layer of anhydrous calcium sulfate, and fitted with an inlet tower also containing the desiccant). After the system was evacuated, dichlorodifluoromethane was
40,
I
Figure 3. Calibration curves for gas analyzer
- I
1
8
0
20 40 60 BRIDGE W T E N T I L (METER SCALE OIVISIONS>
admitted in the desired proportion as determined by its partial pressure, and then dry air was admitted until atmospheric pressure was achieved. The cylinder was disconnected from the manifold system after its valve was closed, and then connected to the gas analyzer, its valves were opened, and the equilibrium reading of the instrument was recorded with the circulating pump on. The instrument was warmed up for 15 to 20 minutes and the zero adjustment made before it was ready for use. The volume of the internal free-air space of the system (cell, drying tube, pump, and connectors less the volume of the cotton and drying agent) was determined to be approximately 71 ml., which represents a dilution of about 3% for the Pliter sample cylinder. This dilution factor was canceled out by the fact that the calibration and determination of the unknown were made with the same equipment. About a dozen cylinders were prepared and checked for volume uniformity; they were found to average 2.23 liters. The rest of the system was prepared with this volume in mind. When the instrument was used for the determination of dichlorodifluoromethane in other sample vessels, or in large spacps, appropriate corrections were made for the dilution factor.
1
00
80
of 100, 62, and 52 (measured a t 146, 90.6, and 75.9 ma. for the cell filament) for concentration ranges of 0 to 7, 0 to 25, and 0 to 35%, respectively. Results are shown in Figure 3. These curves were used to obtain direct readings in the determination of dichlorodifluoromethane. In later calibrations span-adjuster settings of 10 and 0.5 ohms were made with current constant a t 100 scale divisions. At these settings the curves were almost linear, and it was possible to calculate the concentrations of dichlorodifluoromethane from the meter readings by applying a factor. However, the original curves were found to be more accurate, and these data are given.
Table I. Determination of Ethylene Oxide and Dichlorodifluoromethane in laboratory-Prepared Fumigation
(Simultaneous samples taken from 34.42liter chamber) DichlorodifluoroEthylene Oxide, V 0 1 . s methane, Vol. % 0 0 2 16 9 16 4 0 0 2 18 2 17.9 5 0
5 1
6 9 12 1 12 2
6 9
12 2 12 0
101
12 0 19 0 19 2
9 7
ii
9 19 8 20 8
Filament Temperature and Span Adjustment. For gas with a low
response, the current control of the gas analyzer is adjusted a t 100 meterscale divisions (approximately 138 ma.). However, the instrument was found t o be extremely sensitive t o dichlorodifluoromethane - in - air mixtures, and it was necessary t o diminish its sensitivity both by using the span adjuster and by operating a t lower filament temperatures (lower current) a t the higher levels of concentration. It was found convenient to operate a t a span-adjuster setting of 20 ohms, and to standardize the instrument for three ranges a t currents (in seale divisions)
After the cylinder containing the gas sample was connected to the gas analyzer, a reading was made, and the per cent by volume of dichlorodifluoromethane was read off the appropriate calibrat.ion curve. Calculations. After the sampling and the determinations of ethylene oxide and dichlorodifluoromethane were completed, calculations were necessary t o correct the volume of the sample on which t o base the analysis. The sample volume, V,, was equal t o the volume of the sample cylinder, V,, plus the original gas-phase volume VOL. 31, NO. 9, SEPTEMBER 1’959
1567
of the absorbed ethylene oxide, V., and the water vapor. Vc had been measured previously, and V. a t sample temperature was calculated from the number of milligrams determined by the Lubatti method, by assuming that ethylene oxide followed the ideal gas law. This was well within the accuracy of the method. By similar considerations, based on relative humidity, \vater-vapor saturation tables, and temperature, a relationship was derived for determining the volume of the water vapor in the original sample. This equation was extremely complicated and led to very tedious calculations. Because the volume of moisture was relatively small (1 to 2%) in this work, it was decided to ignore this correction as being within experimental error. However, if fumigation is done under very moist conditions, this factor would have to be taken into account. With moisture ignored, V, was equal to the sum of V, and V,, from K hich the per cent by volume of ethylene oxide in the original sample was calculated. The volume of dichlorodifluoromethane, V,, was calculated from the per cent by volume obtained by the thermal conductivity method, F’, Vc,and the volume of the free air space in the sample system, V,, previously referred to.
v/ = loo(VC2F‘ - Val v c
The per cent of volume, F , of dichlorodifluoromethane in the sample was calculated from Vi and V,. Analysis of Laboratory-Prepared Fumigation Mixtures. To determine the accuracy of the analytical method, mixtures of ethylene oxide, dichlorodifluoromethane, and dry air were prepared by the partial-pressure procedure described under calibration. The ethylene oxide to be used for calibration was freshly distilled to remove polymer whenever necessary. For the fumigation chamber a G-1 oxygentype cylinder of 34.42-liter capacity, V,, was used. Several coiled springs were inserted into the cylinder, to mix the gases by shaking to ensure homogeneity. After the mixture was thoroughly mixed, the sampling system was connected, and a sample drawn into the evacuated sample cylinder, as described under method of sampling. Because the G-1 cylinder was smaller than the usual fumigation chamber for which this method was devised, the relative volume of the sample cylinder was much more important. The entire system, including both cylinders, was under reduced pressure after sampling. The valves were then closed, and the sample cylinder was disconnected. Dry air was admitted to bring the cylinder contents to atmospheric pressure, and the cylinder was then connected to the 1568
ANALYTICAL CHEMISTRY
gas analyzer for a thermal conductivity reading, from which F’ was obtained from the calibration curves. The number of milligrams of ethylene oxide in the absorbers was determined as usual. Because of the reduced pressure a t the termination of sampling, a correction had to be applied to the equations for calculating V I and V,. This correction was a function of the pressure, p z , in the system after sampling (before air &-asadmitted). The equations were:
Several mixtures were prepared.
Sensitivity of Method for Dichlorodifluoromethane. Applicability of the gas analyzer as a laboratory and research tool for the determination of dichlorodifluoromethane in air (ethylene oxide absent) a t lower concentration levels was also investigated. The analyzer was set to its maximum sensitivity by adjusting the span adjuster at 100 ohms and the current setting at the rated maximum current of 138 ma. (100 scale divisions). By actual measurement with an ammeter, the current was 146 ma. At these settings the range of the instrument was between 0.04 and 2% by volume of dichlorodifluoromethane. The meter on the gas analyzer was of limited sensitivity, however; furthermore, it introduced a voltage drop in the bridge potential due to its own relatively low resistance (16 ohms). To increase the sensitivity of the analyzer the meter was replaced by a potentiometer (Catalog No. 2730, Rubicon Co., Philadelphia 32, Pa.) This no-load-type potentiometer had a low range of 0 to 161 mv. in 0.05-mv. subdivisions, and did not cause a voltage drop in the circuit whose potential it was measuring. The gas analyzer was equipped with connector terminals for an external meter, or a recorder, and a switch for changing over from one to the other. With the potentiometer as a substitute for the meter, the instrument range was from 0.006 to 9% by volume of dichlorodifluoromethane.
DISCUSSION
This method has been used in the laboratory and field for more than 3 years, with satisfactory results at all times. The gas analyzer was checked and recalibrated on several occasions, and only minor changes were noted. The instrument appears to be rather rugged and if not mistreated, as by the analysis of corrosive gases, will give good service for a long time. For re-
search or laboratory use onlv, and not as a dual-purpose tool, certain modifications might increase its sensitivity. A more sensitive no-load-type potentiometer might be used in place of the meter supplied with the instrument. It might also be advisable to remove the span adjuster (whose sole function is to reduce sensitivity) from the system entirely, as it places a low resistance across the bridge and creates a significant voltage drop, even a t its highest setting. The manufacturer also makes an 8-filament cell which is capable of ta ice the bridge voltage a t no-load potential. Increased instrument sensitivity could lead to problems, wpecially those due to electrical noise. The method was found to be feasible for other chlorofluorohydrocarbons, such as trichloromonofluoromethane, but, although some data were obtained, no attempt was made to investigate this application further, nor to improve on the ethylene oxide determination. ACKNOWLEDGMENT
The authors are grateful to G. L. Phillips, Agricultural Marketing Service, U. S. Department of Agriculture, for his advice on the thermal conductivity method a t the time of the inception of this work, and to David Spinner, Army Chemical Corps, for help and information on the apalyticsl method for ethylene oxide. LITERATURE CITED
(1) Cotton, R. T., Roark, R. C., Ind. Eng. Chem. 20, 805 (1928). (2) Coull. J.. Enael, H. C., Miller, J., I N D . ENG.‘ CHEh;., .4.1’.4L. ED. 14, 459 (1942). (3) Daynes, H. A., “Gas>.Analysis by
Thermal Conductivity, Cambridge University Press, London, 1933. (4) Haenni, E. O., Affens, IT. -I., Lento, H. G., Jr., Yeomans, -4.H., Fulton, R. .4., Ind. Eng. Chem. 51,685 (1959). ( 5 ) Lubatti, 0. F., J . SOC.Chem. Ind. 54, 4248 (1935).
(6) Midgley, T., Jr., U. S. Patent 1,926,395 (Sept. 12, 1933). (7) Minter, C. C., Burdy, L. 11. J., ANAL.CHEM.23, 148 (195lj. (8) Palmer, P. E., Weaver, E. R., Natl. Bur. Standards, Tech. Paper 249 (1024). (9) Percival, W. C., ANAL. CHEU. 29, 20 (1957). (10) Phillips, C. R , &ye, S.,Ant. J. Hyg. 50, 270 (1949). (11) Phillips, G. L., Bulger, J. W.,U. S. Dept. rlgr. Bur. Entomol. Plant Quarantine, E-851 (1953);( (12) Weaver, E. R., Physical Methods in Chemical Analysis,” ed. by W. G. B d . Vol. 2. 387. Academic Press, Yew York, 1951. (13) Webb, G. A., Black, G. S.,IND. ENG. CHEM., A N A L . ED. 16, 719 (1944). I
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RECEIVED for review November 6, 1958. Bccepted March 27, 1959. Division of Agricultural and Food Chemistry, Pesticides Subdivision, 134th Meeting, ACS, Chicago, Ill., September 1958. Work supported by contract with the U.,S. Army Chemical Corps, Fort Detrick, Frederick, Md.
