Estimation of Methyl Bromide in Air R. C. BLIXN AND F. A. GUNTHER Uniuersity of California, Citrus Experiment Station, Riverside, Ca1;f. IIE widespread use of methyl bromide as a fumigant for the Tcontrol of many insects has resulted in several analytical methods for its quantitative estimation. Most of these methods are based upon the hydrolysis of the methyl bromide Tvith subsequent determination of the bromide ion. Typical procedures have been described by Glaser ( 3 ) ; Busby and Drake ( 1 ) ; Stenger, Shrader, and Beshgetoor (6); Chisholm and Koblitsky (b); and Lewis (4). However, none of these methods has satisfactorily filled the requirements for a rapid and convenient procedure for field N-ork. Actually, the delimiting field sampling requirements are that a sample be taken rapidly and accurately without the use of complicated apparatus. A gross method of this sort was suggested by Quayle ( 5 )in work with hydrogen cyanide. This report deals mainly with an adaptation of this method for methyl bromide, wherein alcoholic potassium hydroxide is used as the absorbing and hydrolyzing medium.
gas-absorbing scrubbers each containing 100 ml. of 5% alcoholic potassium hydroxide. After the solutions from the absorbers were combined, they were allowed to stand for 2 hours a t room temperature to complete hydrolysis. The resulting potassium bromide was completely dissolved by the addition of 300 ml. ot water and 150 ml. of 10% acetic acid solution, then titrated with standard 0.1 IV silver nitrate solution with sodium eosin as the indicator.
Table I. Absorption of .Methyl Bromide at Various Rates Rate of Aspiration, Minutes 1.50 1.75 2.50 6.00 7.00 29.30
A 2-liter sample of air containing methyl bromide was drawn by aspiration at controlled rates through two series-connected Fisher
30
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0
10 15 TIME, M I N U T E S
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18
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3 0
1
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50
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Figure 2
Calcd. M g . of Methyl Bromide per Liter of Air 27.2 31.4 31.6 32.1 31.9 31.9
DISCUSSION
EXPERIMENTAL PROCEDURE
=
Total M1. of 0.1 N AgNOa to Titrate 5.80 6.70 6.75 6.85 6.80 6.80
PO0
Effect of Rate of Aspiration upon Absorption. The essentiality of a rapid sampling period required study of the effect of the rate of aspiration upon the completeness of absomtion. This WM done by varying the rate of aspiration, by means of capillary tubes inserted in the aspirating system, so that varying rates from 1.5 to 30 minutes were obtained. Two seriesconnected scrubbers were used for these runs. (The fumigator was charged with 48.9 mg. of methyl bromide per liter of air about 3 hours before sampling.) The results, graphically seen in Figure 1, show very clearly that a minimum of 2 minutes' aspiration will absorb all the methyl bromide under the conditions of this study. This was further demonstrated by using four scrubbers in the system, and taking each sample a t the 2-minute rate. Pooled results so obtained were no different from those given by two scrubbers. I t was shown, however, that one scrubber would absorb only 79% of the methyl bromide in parallel experiments. The third and fourth scrubbers were tested separately and gave no indication of the presence of even a trace of bromide. Rate of Hydrolysis of Methyl Bromide in Alcoholic Potassium Hydroxide. The rate of hydrolysis of methyl bromide in alcoholic potassium hydroxide was determined by pipetting 10 ml. of 107, alcoholic potassium hydroxide solution and 10 ml. of alcoholic methyl bromide solution (approximately 1.6 grams per liter) into test tubes which were placed in a water bath a t 25' C. and stoppered tightly with a waxed cork. The reactions were stopped at suitable intervals by pouring the contents of the tubes into 50 ml. of 2% acetic acid solution. The results, illustrated by Figure 2, indicate that a minimum of 2 hours was necessary for complete hydrolysis of the methyl bromide under the conditions utilized. Rate of Dissipation of Methyl Bromide in 100-Cubic Foot Fumigator. ll'hen a known concentration of methyl bromide was introduced into a fumigator (100-cubic foot California vacuum fumigator for the Vaeufume process, manufactured by the Union Tank and Pipe Go., Ltd.), the value determined was always lower than the theoretical. Therefore the variation of concentration of methyl bromide with time in the fumigator was evaluated. It was I believed that the methyl bromide ww either adsorbed on 250 the walls of the fumigator, was reacting with the copper heaters therein, or both. A charge of 40 ml. of methyl
1289
ANALYTICAL CHEMISTRY
*
1290
24
Table 11. Rate of Hydrolysis of Methyl Bromide i n 10% Alcoholic Potassium Hydroxide Solution Reaction Time, Minutes 0
2 4 8 16 32 64 96 128 160 192 224
Total MI. 0.1 N AgKOa t o Titrate 0.04 0.24 0.30 0.57 0.96 1.16 1.68 1.70 1.80 1.75 1.76 1.75
Methyl Bromide, Mg. 0.9 2.2 2.8 5.3
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9.0
10.9 15.7 15.9 16.9 16.4 16.5 16.4
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Table 111. Rate of Dissipation of Methyl Bromide i n 100Cubic Foot Fumigator Sampling Time, Hours 0 0. os
0.5 1 2
4 6
Total M1. of 0.1 N AgNOr to Titrate
Mg. of Methyl Bromide per Liter of Air
...
24.5 19.9 18.7 17.1 16.1 15.5
4.25 4.40 3.65 3.47 3.31 3.19
$18
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14.9
14 0
bromide (24.5 mg. per liter) was placed in the fumigator and circulated with an electric fan. Samples were withdrawn by the described method a t 5 minutes, 30 minutes, and 1, 2, 4, and 6 hours. The results, shown in Figure 3, indicate that both adsorption and chemical reaction reduce the concentration of methyl bromide under these conditions; hence the seeming contradiction of Table I. ACKNOWLEDGMENT
The authors wish to thank D. L. Lindgren for suggesting the problem and for certain materials and assistance.
1
2 3 4 TIME, HOURS
5
6
7
Figure 3 LITERATURE CITED
(1) (2) (3) (4) (5) (6)
Busby and Drake, IND.ENG.CHEY.,ANAL.ED.,10, 390 (1938). Chisholm and Koblitsky, I b i d . , 16, 638 (1944). Glaser, Deut. 2.ges. gerichtl. M e d . , 12, 470 (1928). Lewis, J. SOC.Chem., Ind., 64, 57 (1945). Quayle, Hilgardia, 3, 220 (1928).
Stenger, Shrader, and Beshgetoor, IND. ENG.CHEW,ANAL.ED., 11, 121 (1939).
RECEIVED September 13, 1948.
Improvements in Microaeration Technique for Determination of Kjeldahl Nitrogen HARRY G . DAY, EILEEN BERNSTORF, AND R. T. HILL Indiana University, Bloomington, Ind.
THIS laboratory difficulty was experienced in obtaining I Nsatisfactory nitrogen recoveries with the microaeration method
of Sobel et al. (a). A study of the method has shown that it is re-
from known amounts of ammonium sulfate or from protein-free blood plasma filtrates, even with prolonged aeration a t room temperature or by increasing the aeration rate to the point where
liable and practical if larger amounts of alkali are used and if the aeration is carried out a t approximately 70" C. instead of room temperature as recommended by Sobel et al. In principle the latter is to be expected, because Dillingham ( 1 ) and others have stressed the importance of aeration temperature in analytical Kjeldahl nitrogen macro- and semimicromethods. In order to aerate the Kjeldahl digest at an elevated temperature and trap the displaced ammonia in boric acid a t a lower temperature, it is desirable to use a modified Sobel et al microaeration tube. The authors have constructed tubes similar to those used by Sobel et al., except that the side arm is at an angle of 45' with the inlet of the sealed-in bubbIer of the microaeration tube (Figure 1). The tubes are assembled in zigzag formation in a rack with the digestion tubes inside the bath and the receiving tubes outside the bath. The rack can be easily constructed by making four 90" folds in a rectangular piece of 2-mesh hardcloth cloth, so that the ends of the original rectangle join a t the base to clamp over the side of the bath. Short pieces of rubber tubing are used to complete the glass-to-glass connections between tubes, as in the Sobel method. It was found that nitrogen could not be completely recovered
Figure 1. Setup of Aeration Tubes
