May, 1945
295
ANALYTICAL EDITION
apparatus has been used very successfully in the continuous deterrnination of carbon tetrachloride. I n this application, no hylrogen is required, providing a relatively high humidity is maintained. This can readily be accomplished by bubbling the bamplr through water prior t o combustion. The determination of other volatile chlorinated hydrocarbons could presumably be dct cirmined similarly. ACKNOWLEDGMENT
The authors wi-iqh to acknowledge the valuable asii-tance of L. .A. Bedwell in fabricating quartz and glassn-arc. They also
wish to expreys thanks for the permission of Westvaco Chlorine Products Corporation to publish this paper. LITERATURE CITED
Kolthoff, I. XI.,and Yutzy, H. C., ISD. ESG.CHEM.,ANAL.ED., 9, 75 (1937). ( 2 ) Olsen, J. C.. Smyth, H. F., Jr., Ferguson, G. E., and Scheflan, L., Ibid., 8, 260-3 (1936). (3) Reilly, J., and Rae, W ,K., "Physico-Chemical Methods", 2nd ed.,New York. D.Van Nostrand Co., 1933. I+ J. O., .-\bersold, J . N., and Hendrirks, R.H., (4) Thomas. AI. D., IXD.ESG. CHEN..A S L L . E D . ,15,287-90 (1918).
(1)
Ijn-ight. I!,id., 17, 295 (1045j. ( 5 ) IViIliai~i.~,
Determination of' Low Concentrations of Methyl Bromide in the Atmosphere DWIGHT WILLIAMS Research Department, Westvaco Chlorine Products Corporation, South Charleston, M e t h y l bromide in concentrations as low as I part per million is determined b y combustion in a quartz tube at 950' to 1000' C. and absorption of the combustion products in dilute sodium hydroxide. The bromine content of the sodium hydroxide solution is determined b y the Kolthoff-Yutzy procedure. The method i s readily adjusted to any concentration level b y adjusting the sample size. Recovery of larger amounts is 98% complete.
A
I I E T H O D for the determination of methyl bromide in the atmosphere in low concentrations was desired to detect the presence of hazardous concentrations when manufacturing and handling this compound. Stenger, Shrader, and Beshgetoor ( 2 ) recommended hydrolysis of methyl bromide with monoethanolamine followed by titration of the bromides formed by hydrolysis. They applied this method to the determination of methyl bromide a t relatively high concentrations. Preliminary tests indicated that the method was useful a t high concentrations where the bromides could be titrated by the Volhard method. However, a t low concentrations, it was necessary to titrate the bromide by the procedure of Kolthoff and Yutzy ( f ) and to remove the monoethanolamine before applying this latter method. The removal of the monoethanolamine proved troublesome and other means for analysis were sought. C'ombustion (or py a t high temperatures in a quartz tube has been ividely d to the determination of the chlorine content of chlorinated hydrocarbons in the atmosphere. Apparently no attempt has been made t o apply this principle to brominated hydrocarbon vapors. Khile somewhat more elaborate apparatus would be required than in a hydrolysis procedurcx, no foreign substance need be introduced and the sample size could hc xaried more readily. I n addition, experience showed that a t lon- concentrations ~ h c r eit ua.: nccesxuy to apply the Kolthoff-Tutzy titration procedurcx, coinbustion vias more rapid than hydrol. APPARATUS AND REAGENTS
The sample bottle, combustion tube and furnace, diffuser and absorber, flowmeter, and needle valve illustrated in Figure 1 are required. COMBUSTION FURXACE. A 20-cm. (8-inch) tube furnace was made by winding resistance \Tire around a 20-cn1. (8-inch) length of 2.5-cm. (1-inch) quartz tubing and insulating with hlundum cement and magnesia pipe lagging. The voltage to the furnace was controlled by means of a Variac, type 200-CM. The temperature was measured by means of a chromel-alumel thermocouple and a milliammeter. The milliammeter was calibrated against a n L. & N. portable potentiometer. CoMsrsTIoN TUBE. fi 50-cm. (20-inch) length of 4-mm.
W. V a .
inside diameter quartz tubinq was bent a t a 90" angle 2.5 cm. (1 inch) from the exit end. This tube should extend 15 cm., (6 inches) beyond the furnace on the exit side to permit dissipation of the heat of the exit gases. DIFFUSERAKD . ~ B S O R B E R . The diffuser is a 6 X 37 mm. Alundum extraction thimble S o . R.198 sealed to the end of B 32-mm. length of 7-mm. glass tubing. The absorber is made from a 14-cm. length of 14-mm. glass tubing. Flowmeter, Scientific Glass iipparatus Co., Catalog KO. J1912. Seedle valve, Hoke, straight, 0.3 cm. (0.125 inch), V-point, 1-0. 341. Sodium hydroxide, approximately 0.1 S. Sodium hypochlorite about 5y0. -4commercial bleaching agent (Clorox) was used. Sodium dihydrogen phosphate, SaH,P04.H?0,200 grams per liter
Sodium formate, 500 grams per liter. This solution is usually turbid and is filtered before using. Ammonium molvbdate. 50 prams oer liter of MOO,. Potassium iodide, 300 ,rami per l&er. Sulfuric acid, 6 S. Sodium thiosulfate, 0.01 S containing 0.4 gram per liter of sodium carbonate. ' Starch, 10 grams per liter. The sodium phosphate, sodium formate, potassium iodide, and sulfuric acid are added by means of dispensers consisting of pipets cut off a t the bottom of the bulb. The pipet is fitted in a one-hole rubber stopper, so that the bulb is a t the bottom of the reagent bottle. notch is cut in the stopper to permit equalization. TT-lien dispensing the reagent the analyst closes the t,op of the filled dispenser with his finger, raises the dispenser, and discharges it into the reaction flask. PROCEDURE
Set u p the combu~tionfurnace, ahorber, flowmeter, and needle valve shon-n in Figure 1. Charge the absorber with 10 ml. of 0.1 S sodium hydroxide. If the sample is to be taken directly, connect the inlet side of the furnace by rubber or Saran tubing directly to the point st xhich the sample is to be taken. Turn on the needle valve and adjust the air flow to 500 nil. per minute. Start the stop vatch simultaneously. Continue the. air f l o for ~ 4 minutes or longer, depen(1ing on the sensitivity which is required. If the sample is taken in a bottle, connect it t o the furnace throuEh a n L-tube at F and attach a bot,tle containing acidified saturated sodium chloride a t E . Open stopcock D. Turn on the needle valve until an air rate of about 500 ml. per minute i:, obtained. K h e n the sample bottle has been filled with brine, stop the brine flow by closing stopcock D , then turn off the needle valve. Disconnect the sample bottle from the furnace to allow the air pressure to equalize. Disconnect the absorber, first from the quartz tube, then from the flowmeter. Pour the liquid from the absorber into a 230ml. conical flask. Pipet 5 ml. of distilled witer into the diffuser and force it through the diffuser into the absorber by means of a rubber bulb. Combine this washing with the absorber liquid and wash three mdre times in the same manner, transferring each washing separately to the flask.
296
Vol. 17, No. 5
INDUSTRIAL AND ENGINEERING CHEMISTRY
T o the combined absorber liquid a i d mabhings add 5 ml. of sodium hypochlorite solution by means of a pipet. Add 8 to 10 ml. of sodium dihydrogen phosphate and heat the solution just to boiling. Bdd 3 to 4 ml. of sodium formate and again bring to a boil. Cool to room temperature in a stream of water. T o the cooled solution add 100 ml. of cold water (about 10" C.). Add 2 drops of ammonium molybdate solution, 8 to 10 ml. of potassium iodide solution, and 20 to 25 ml. of 6 S sulfuric acid. Titrate promptly with 0.01 S sodium thiosulfate solution. Add 3 to 4 ml. of starch solution just before the end point is reached and titrate until the last visible trace of pink or blue color juqt tlisappears.
-4 reagent blank containing all the reagents including the 0.1 A' sodium hydroxide is prepared and carried through the titration procedure and is subtracted from the sample titration. Ml. of0.010 N SazS203 X 41.5 = p.p.m. of methyl liters of sample
The volume, L, of G in liters is determiued approximately by filling with water from a graduate. The concentration in H is calculated as follows: c =
(Pz
- PdCl P,
where P1is pressure in G before adjustment I is pressure in H before adjustment is pressure in both G and H after adjustment To determine the volume of air which must be passed through the system to recover all the methylbromide, approximately 500 p.p.m. of methyl bromide were introduced into H and air was swept through the system a t the rate of 1000 ml. per minute for 2.5-minute intervals. The air flow was stop ed after each interval and fresh caustic introduced into the agsorber. Each portion of caustic was titrated separately for bromides. The data obtained (Table I) indicate that a minimum of 17.5 liters of air must be passed through the system for maximum recovery. X 20-liter air sweep was used in subsequent tests.
f'z
The Kolthoff-Yutzy procedure is best applied to small volumes.
EXPERIMENTAL
The sampling apparatus shown in Figure 1 was employed to obtain the small concentrations required for this work.
It appeared that some care might have to be exercised in the de-
sign of the absorber and in the manipulations to avoid a subsequent evaporation prior to the oxidation and titration. Several portions of sodium bromide, equivalent to 460 micrograms of quantity of methyl bromide is sealed in a glass capmethyl bromide, were diluted to various volumes and analyzed sule following the procedure employed by Stenger, Shrader, and Beshgetoor ( 2 ) . The sealed capsule is placed in the 9-liter bottle, in the usual manner. Negative errors were obtained a t all diluG, the bottle is closed by means of the standard-taper joint asincreased s h a d y above tions and the magnitude Of the sembly, and the capsule broken by shaking the bottle. The 280 ml. (Figure 2). These data indicate that the volume should be resliter bottle, H,is partially evacuated to a kept as small as possible for the best results. sure, depending on the concentration desired in H, by app&ing suction a t E. The pressure in G is measured by means of the The absorber (Figure 1) requires about 10 ml. of fluid to fill Zimmler-type mJ-Iometer with stopcock A open and B closed; to the bottom of the spray bulb. To determine the minimum A is now closed and B opened to obtain the pressure in H. Both volume of water required for washing the diffuser a %liter sample A and B are now opened (c is kept closed throughout these manipcontaining 500 p.p.m. of methyl bromide was analyzed in the ulations) and the pressure again measured. The concentration Of methyl bromide in G in parts per million is given by the followusual manner except for the washing. The diffuser was washed ing formula, calculated a t 27' C. anti 750 mm. pressure: with six 5-ml. portions of water. The caustic in the diffuser, the first and second, the third and fourth, and the fifth and sixth grams of CH8Br X 263,000 c1 = washings were oxidized and titrated separately. The data obI, tained (Table 11) indicate that even six washings may not remove all of the bromides. HowLegend ever, since in the final procedure each washing is transferred W,C.2mm. stopcocks separately, fewer washings D. 2-mm. capillary stopcock would be required and, since G. 9-liter bottle excessive dilution introduces an H. 2-liter bottle error, the use of four separate I . manometer, Zimmler 5 - d . washes was ado pted. A study of the effect of furJ. furnace nace temperatureindicat thd that K . absorber satisfactory results were obL. flowmeter tained above approximately M. needle valve 900' C. R e c o v e r y b r o k e sharply below this temperature and no recovery was obtained below a temperature of about 700' C. Tebbens (3) reported that platinum reduces the minimum temperature required for the decomposition 01 carbon tetrachloride in thc air. S o change was observed in the minimum temperature required for complete combustion of methyl bromide when 2 grams of scrap platinum wile were incm troduced into the combustion I 5 10 15 tube. Subsequent work was done a t a temperature of 950" to loooo c. The data in Table I11 show that the recovery decreaseswith Figure 1 . Sampling, Combustion, and Absorption Apparatus ~
-
Table I.
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ANALYTICAL EDITION
May, 1945
a
Volume of Air Required to Sweep M e t h y l Bromide from 2-Liter Bottles Recover>IncreCumumental lative
(:umulative Volume
% 54.3 28.0 7.0 3.0
.lf I . 2,500 5,000 7,500 10,000
Table
11.
Cumulative Volume
c/c
M1.
54.3 82.3 89.3 92.3
12,500 15,000 17,500 20,000
Recovery IncreCumumental lative
% 1.0 1.0 0.3 0.0
% 93.3 94.3 94.6 94.6
Effect of Washing Diffuser Recovery, ";I
Absorber First arid second washing Third and fourth washing Fifth and sixth washing
53.1 28.8 5.2 2.,
_________~~~ ~~
,
*
incwawing air flow rate. White vapors were observed above the absorber liquid at the highest air flow rate (2000 ml. per minute), suggesting incomplete absorption of hydrogen bromide. However, no significant amount of hydrogen bromide was recovered in a second absorber and white vapors were observed there also. ctory explanation can be advanced for this phenomenoli. M-hile recovery a t the lowest rate (500 ml. per minute) was not quantitative, it appears that further reduction in flow rate is riot likely to improve recovery further. The arcuracy of the met>hodwas determined using the sampling apparatus shown in Figure 1. In each case 20 liters of air were drawn through the 2-liter bottle, H , at a rate of 500 ml. per ininute. ,4n average recovery of about 98% of the amount taken was obtained at, the higher concentrations (Table IV). A possible explanation for these slightly low recoveries at high caoncentrations is suggested below. Errors a t lower concentrations tend to be positive and do not exceed 2 p.p.m. The positive t'rrors may be due t'o the presence of a small amount of methyl bromide iri the laboratory atmosphere. Since the volume of air which was passed through the furnace was 10 times the volunie of the sample bottle, a concentration of 0.2 p.p.m. of methyl bromide in the atmosphere would account for this error. These data show that this method will detect methyl bromide in concentrations as low as 1 or 2 p.p.m. when using a 2-liter sample. The sensitivity of the method may readily be increased several-fold by increasing the sample size. The technique of riweeping air through a sample bottle until free of methyl bromide is not practical for routine work, owing to the long time required to complete the air sweep. Two methods of sampling may be used. The easiest method, where feasible, is to pass air directly from the point which it is desired to sample to the combustion furnace. In case this method is not feasible the air can be swept from the sample bottle into the furnace by filling the bottle with acidified sat'urated sodium chloride solution. The solubility of methyl bromide in water is substantial, between 1 and 2% by weight a t 760-mrn. pressure, and that in brine is probably of the same order of magnitude. However, the eolubility would be slight when only a few parts per million were present in the air and experimentation which was carried out in connection with a continuous method (4)indicates that therr is no significant error from this source. An analysis of a 2liter sample could be completed in 15 to 20 minutes if either of these sampling techniques were used. Dilute sodium hydroxide TTas used as the absorbent for the combustion products in this work. In an investigation of a continuous method for methyl bromide (4)it was found that part and perhaps all of the methyl bromide is converted to free bromine on passing through the combustion furnace and that the bromine is only partially absorbed in water. The recoveries obtained by absorption in 0.1 N sodium hydroxide indicate that a higher concentration of sodium hydroxide is not necessary. 111several applications of the Iiolthoff-Yutzy procedure in this laboratory departurt:i froni the conoentionxl procedure have been
volume, ml. Figure 2.
Volume
VI.
Error in Titration of Bromide
made. These departure, have been incorporated into the present procedure. For convenience, some commercxial bleaching agent, usually Clorox, has been used as a source of hypochlorite. The small bromine content of the bleaching agent 14 corrected for by means of the blank. Commercial bleaching agents almost always contain some chlorate and the chlorate may increase the magnitude of the blank slightly. However, the chlorate content is usually many times the magnitude of the blank. The solution is diluted with 100 ml. of cold water prior to the titration. This gives a dilution having a temperature a little below 20" C., even in the summer, which is ideal for the subsequent iodometric titration. I n this application, no sodium chloride was added prior to oxidizing the bromide. This last factor may have contributed to the low results a t higher conceiitrations noted in Table I V but, since the method wai intended primarily for low concentrations, this point was not investigated.
Table
Table
Ill. Effect of Air Flow Rate
Air Flow Ml./min.
Recovery
2000 2000 2000 1000 500
81 81 81 92 94
IV. Range and Accuracy of M e t h o d
Methyl Bromide Added Found Error P.p.m. P.p.m. P.p.m. 0.7 1.0 +0.9
1.9 2.2 3.6 5.4 5.9 9.2 11.2 11.8 17.0
70
3.4 3.5 5.3 6.3 6.3 8.3 11.4 11.3 17.0
+1.5 +1.3 t1.r +0.9 +0.4 -0.9 +0.2 -0.5 0.0
Added P.p.m. 18.0
Methyl Bromide Found Error P.p.m. P.p.m. 18.0 0.0
24.5 40.3 90.0 132.0 213.0 286,9 356.4 439.0 486.0
23.5 40.3 87.0 129.4 213.0 283.5 348.4 416.0 476.0
-1.0 0.0
-3.0 -2.6 0.0 -3.4 -8.0 -13.0 -10.0
ACKNOWLEDGMENl
The writer wishes to acknowledge the assistaxe of Mary P. Brevoort, George S. Haines, and Frank D. Heindel i r i performing the esperimental work. LITERATURE CITED
(1) Kolthoff,I. AI., and Yutzy, C. H., IND. ENG.CHEX.,ANAL.ED., 9, 75-6 (1937). (2) Stenger, V. A., Shrader, S. A,, and Beshgetoor, A.
W., Ibid., 11, 121-4 (1939). ( 3 ) Tebbens, B. D., J . I n d . H y g . Tozicol., 19, 204-11 (1937). (4) Williams, Dwight, Haines, G. S., and Heindel, F. D., IND. ENO. CHEM.,AS.,;. ED., 17, 289 (1945)
