ANALYTICAL EDITION
May, 1945
iiririg electrodes is then adjusted so that the recorder indicates zero. The gas flow from the sample line is started, and grooved stopcock 8 1 is adjusted so that the top of the mercury column of the temperature and pressure compensator is a t the calibration mark. The sampling stopcock is started and the analyzer is in operation. The analyzer is calibrated with gas mixtures of known composition. The accuracy of the calibration was checked during regular plant operation by comparison with analyses obtained by means of a Fisher precision gas analysis unit over a 3-day period (Table I). There is good reason for helieving that the accuracy of the electroconductometric method is better than that of the Orsat method. MAINTENANCE
The sariiple metering and preparation train is so mounted that the pressure stabilizers, flowmeter, and compensator are readily visible. The gas sample and air flows seldom require adjustment, but the pressure in the gas sample line must be adjusted at Sn (Figure 2 ) when significant changes in temperature or atmobpheric pressure occur. The rate of flow of electrolyte and the 7 ~ 1 ' 0qetting of the recorder are checked daily. The wro hetting
289
of the recorder is obtaiiietl by stoppiug the rotation of the . m i pling stopcock for sufficient time for all the carbon dioxide to be flushed from the train leading to the absorption-conductivity cell. The solution in thc potassium hydroxide scrubber is changed once each week, and the sampling stopcock is lubricated as required. The catalytic umt in n-hich the carbon monoxide and hydrogen are oxidized requires no attention other than the maintenance of its temperature. The original charge of copper oxide catalyst has shown no decrease in activity in more than a year of continuous use. ACKNOWLEDGMENT
The authors express their appreciation to the staff of the m i monia plant control laboratory for their cooperation in the inqtallation and initial operation of the analyzer. LITERATURE CITED
(1) Brown, E.H.,Isu. ENG.CHEM.,ANAL.ED.,14,551 (1949). (2) Brown, E.H., Cline, J. E., Felger, M.M., and Howard, R. E., Jr., Ibid., 17,280 (1945). (3) Brown, E. H., and Felger, M. M., Ibid., 17,277 (1945). (4) Miller, A.M.,and Junkins. J. N..C'hnn. & -Wet. Eng., 50, No. 1 1 . 119-25,152-5 (1943).
Continuous Determination of Methyl Bromide in the Atmosphere DWIGHT WILLIAMS, GEORGE S. HAINES, A N D F. D. HEINDEL Research Department, Westvaco Chlorine Products Corporation, South Charleston, W. V a .
A method and apparatus for the continuous determination of the concentration of methyl bromide at several points in the atmosphere are described. The methyl bromide is converted to hydrogen bromide b y adding hydrogen to the sample and passing the mixture through a quartz tube which is maintained at a temperature above 800" C. The hydrogen bromide is continuously absorbed in a stream of water and the electrical conductivity of the water solution measured b y means of a recording conductivity meter. Concentrations as
T
Hkl determinatioii of lo\\. concentrittions of methyl bromide in the atmosphere by combustion on passage through a quartz tube followed by titration of the bromine by the KolthoffYutzy ( 1 ) procedure has been described (6). This method has numerous disadvantages when considering the safety of personnel exposed to the vapors. The most serious of these is the time lag between the sampling operation and the completion of the analysis. Since the methyl bromide wa5 converted on combustion either to hydrogen bromide or to free bromine, which could presumably be readily converted to hydrogen bromide, it appeared that electrical conductivity offered a ready means for the continuous determination of methyl bromide. The possibility of determining carbon tetrachloride in air continuously and automatically by measuring the electrical conductivity has been suggested by Olsen, Smyth, Ferguson, and Scheflan ( 2 ) . Thomas, Ivie, Abersold, and Hendricks ( 4 ) have described an automatic apparatus suitable for determining volatile chlorine compounds by combustion and conductivity measurements. However, they abborbed the combustion product- hatch\$-ise and obtained their quantitative results from the rate of change of conductivity. It appeared that this procedure rould he improved if the combus-
low as 1 part per million may b e detected and concentrations up to
600 parts per million have been determined with a precision of *5% of the amount present. The method is not specific for methyl bromide but gives the total chlorine and bromine compounds in the air. A similar apparatus in which the hydrogen is omitted and the air stream is saturated with water vapor is used for the determination of chlorinated hydrocarbons when brominated hydrocarbons are absent.
tion products could be absorbed ill a coiitinuous f l u x of water. This would involve the continuous passage of a constant stream of the air sample through t>hefurnace and the continuous pumping of a constant stream of water through the absorber. Obviously this method is not specific for bromide, as is the KolthoffYutzy procedure, but it should prove useful where methyl bromide is the most probable atmospheric contaminant and will in any case give results on the high side, which is in tho interest of safety. A consideration of tlie coiiceutration of methyl bromide which it was desired to detect and of the probable optimum air and liquid flows indicated that a conduct.ivity cell having a cell (*onstant of about 0.1 reciprocal centimeter was required. The conductivity cell was designed to permit the complete absorption of the combustion products, reduce holdup to a minimum, aiicl permit the compact assembly of the apparatus. Low and erratic recoveries of methyl bromide were obtairicd in early tests based on the assumption that the methyl bromidc was quantitatively converted to hydrogen bromide. Thi. was traced to the formation of free bromine. The addition of hydrogen to the air stream eliminated this sourw of error.
INDUSTRIAL AND ENGINEERING CHEMISTRY
290
Vol. 17, No. 5
figure I. Diagram of Analyzzr Hydrogen inlet B . A i r inlet C. Rotameter
A.
D . Burner E . Furnace F . Absorber conductivity cell
APPARATUS’
A scale diagram of an individual system of the analyzer is shown in Figure 1. Details of construction of the parts and the flow of water and gases through the system are indicated.
WATERPUMP. The piston and cylinder are made from a 5-ml. Pyrex hypodermic syringe. The stops are adjusted to give a stroke of about 1 ml. The hollow core of the piston is filled with iron \Tire (KO.22 B. and S. lacquered iron wire was used) and the glass sealed together again. The check valves are made by grinding glass marbles into the preformed valve seats before assembly. The authors found single-colored marbles superior to the varicolored variety. Hand grinding on a stationary rubber plate using 240-, 400-, and 600-mesh Carborundum powder, in that order, and then rouge gave good results. Each valve seat is tested individually and, if found defective, is reground before assembly. The test is made under a 150-rm. (5-foot) head of water using a 20-mm. head tube. A drop of 30 em. (12 inches) in the water level in the head during 24 hours is cause for regrinding the valve. This all-glass pump assembly provides a small, precise flow of water without danger of contamination which would affect the conductivity of the nater. All pump-valve assemblies were tested for constancy of delivery with variation in head. I n some cases variations in delivery were observed which could not be traced to leaky valves or cylinders. Leaks resulted in deliveries less than the displacement Uf the piston but deliveries greater than the displacement, sometimes as much as 2.5 times, were observed in these anomalous cases, It was observed that all four valves moved a t each stroke of the piston, two during the stroke and the other two just a t the end of the stroke. I t v a s concluded that this was caused by the inertia imparted to the water by the rapid motion of the piston, which tended to force water through the inlet valves at the end of the stroke. This source of error was eliminated by insert,ing short lengths of capillary a t either end of the cylinder. This slowed the motion of the piston, reduced the inertia, and gave the rated delivery. 1
Patent applied for
C. Wa!er bath H. Water pump I.
Water inlet
The piston is driven by two electromagnets which are actuated alternately by means of an automatic electric timer having a speed of about 1 r.p.m. (Sutomatic Electric Manufacturing Co., Mankato, hfinn., Catalog S o . 1005). The timer is set a t 10 cycles per minute. The solenoids are made by winding 2500 turns of No. 22 enameled copper wire on an arbor 6.25 cm. (2.25 inches) long by 2.5 cm. (1 inch) in diameter. A perforated steel shell 7.8 em. (3.125 inches) inside diameter by 14 cm. (5.625 inches) long encloses the two solenoids which are required to drive a single pump assembly. There is a 2.19-cm. (0.875-inch) gap between the two solenoids. The length of this gap is fixed by means of two washers 7.8 cm. (3.125 inches) in diameter which are machined to fit the inside of the shell, to which they are xelded. The syringe passes through a 2.5-cm. (1-inch) hole in the center of the washers. Water is pumped alternately to each of two conductivity cells by means of a single pump. Water passes from the conductivity cell to a water separator from which it is discharged through a 90-em. (%foot) standleg to a small reservoir having an overflow to the sevier. HYDROGEN SUPPLY SYSTEM. Hydrogen is supplied by means of a cylinder or other source at about 2.3 kg. ( 5 pounds) pressure. The flow to the analyzer is controlled by means of a 0.3-cm. (0.125-inch) S’-point needle valve (Hoke, Inc., S e w Tork, S.Y., Catalog S o . 341). The hydrogen flow rate is measured by means of a rotameter, having a range 0 to 1000 ml. per minute. h R FLOX SYsmii. The air sample is conducted continuously through Saran tubing to the analyzer. -1rotameter having a range of 0 to 1500 ml. per minute is used tor measuring the air flow rate. The hydrogen and air flow are combined by means of the Pyrex burner. The pas streams should not combine until inside the iurnace. Othern-ise, burning may be erratic. Clear quartz ground joints (Thermal Syndicate, Ltd., Sen- Tork) are used on the combustion tubes. One complete 10,’30 T quartz joint is required for each combustion tube, the outer member being at the inlet end of the furnace and the inner member at the outlet end. The burner and the conductivity cell, to which the combustion tube is attached by means of these standard taper quartz joints, are made of Pyrex. The air stream passes through the furnace and conductivity cell to the wateraeparator and from there to a needle valve of the same type as that which is used to control the hydrogen flow. The needle valve is connected to a manifold which is evacuated continuously by means of a Cenco-Pressovac pump. COKDUCTIVITY CELL. The electrodes are made of two 1 X 2 cm. sheets of 0.013-cm. (0.005-inch) platinum foil held approxi-
May, 1945
mittely 2 mm. auart by means of dass suacers. It i s convenient
showed resistances of from 35,000 to 38,000 ah& corresponding to about 1 u.u.m. of methyl bromide. The exact cell constant i s
uraeedure outlined 'by Reilly and Rae (Zj, anb. ulaeed inside a
having the highest resistkce. The platinum black is then removed by eleotrolysis in aqua regis. for a few minutes and the electrodes are permanently mounted in 14mm. outside diameter tubes by sealing the platinum lead wires through the glitss. Sections of appropriate tubine are sealed to each end of the eleotrode envelope to permit
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291
ANALYTICAL EDITION
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The ele&odk issgmblies are then reulstinized rand calibrated with several Concentrations of h y d r o b r k i e acid before final assembly. This calibration should be carried out a t 25" C., since Mieromax oonductivity recorders are calibrated to show the corre$ resistanoe a t this temperature.
rate of 1000 ml. per minute and a watcr rate of 10 ml. per minute. The cell has 8 resistaneo of 1000 to 1500 ohms a t this concentration and 25" C. Tho range of the Microm3.n recorder which was uscd in conjunction with the analyzcr was 250 t o 50,000 ohms. I t was possible by means of this combination to detect 1 p.p.m. and thc top of the range corrcspondod to 103 t o 200 p.p.m. The maximum safe working concentration, which wm considered to be 30 p.p.m., wm easily read from the recorder chart. A scale reitding in parts per million of methyl bromide was provided for the recorder as shown in Figure 7, in order to permit the instantaneous roading of the methyl bromide concentration. The stopcock in the conductivity cell is open during the operation of the analyzer to permit escape of the air through this leg of the cell. When the analyzer i s shut down, the stopcock i s closed and most of the water is drawn out of the cell to prevent backpressure from forcing water into the furnace. The conductivity cells are immersed in a water bath equipped with a thermoregulator, flexible-type immersion heater, and electric stirrer. Water is added to the bath continuously to main,tain its level and prevent the temperature from rising above 40" C. Without the cooling effect of the added water, the bath temperature rises, even when the heater i s off, owing to !he heatine effect of the uvrolvsis furnace and the hot eases emerzing from if to the cond&ivi
