A chemical study of the preparation and stability of aqueous methyl bromide solutions

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Title:
A chemical study of the preparation and stability of aqueous methyl bromide solutions
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Creator:
Chisholm, R. D
Kobilitsky, Louis
United States -- Bureau of Entomology and Plant Quarantine
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U.S. Department of Agriculture, Bureau of Entomology and Plant Quarantine ( Washington, D.C )
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STATE PLANT BOARD
September 1942 E-575










A CHEMICAL STUDY OF THE PREPARATION AND STABILITY

OF AQUEOUS METHYL BROMIDE SOLUTIONS

By R. D. Chisholm and L. Koblitsky,
Division of Insecticide Investigationsl/



INTRODUCTION



An aqueous solution of methyl bromide (0.3 percent by volume)2/ has
been used to kill the white-fringed beetle and is now authorized/ for the
treatment of various plants shipped out of the quarantined area. Another
aqueous solution (0.15 percent by volume) has shown considerable prcmise4/
for control of larvae of the Japanese beetle. Either solution is made
by dissolving methyl bromide in twice its volume of ethyl alcohol, adding
this mixture to water, and mixing thoroughly. It was observed that during
the preparation of the solution there was a considerable loss of gas. It
was also suggested that the methyl bromide might be hydrolyzed in the mixture
rapidly enough to modify its insecticidal value. A study was made to deter-
mine the influence of different methods of preparation and the temperature

1/ The authors deeply appreciate the cooperation of the Divisions
of Fruit Insect Investigations, Japanese Beetle Control, and Control Inves-
tigations in furnishing much of the equipment and materials used in this
investigation.
The authors especially appreciate the cooperation of H. C. Donohoe,
Division of Control Investigations, during this investigation, particularly
in the work involving the preparation of the solution in large quantities.
2/ Livingstone, E. M., Easter, S. S., and Swank, G. R. Methyl
bromide in aqueous solutions to control Pantomorus leucoloma and P ErfEgrinus.
Jour. Eqon. Ent. 33: 531-533. 1940.
3/ Administrative instructions--Modifying the restrictions of the
white-fringed beetle quarantine by authorizing treatment by Mfethybrmie
Solution of balled nursery stock of specified thickness. U.S. Dept. Agr.,
B. E. P. Q. 503 Revised. March 15, 1940.
4/ Donohoe, H. C., Unpublished report. May 1940.





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of its components at the time of mixing on the composition of the solu-
tion. Consideration was also given to the rate at which methyl bromide
is hydrolyzed in the solution.

MATERIALS USED

Ethyl alcoho1.-The ethyl alcohol used in these experiments was
either denatured or 95 to 98 percent pure.

MeI'i bromide.--The methyl bromide was of the commercial grade and
was supplied in cans, each holding substantially 1 pound. The average net
weight in 10 cans was 456.5 grams, with a range from 451.5 to 462.0 grams.
Since the average net weight was within 1 percent of 1 pound, these cans
were considered to contain a uniform weight of 1 pound of methyl bromide,
and no compensation for the differences in net weight was made in these
studies.

GENERAL PLAN AND PROCEDURES

General Plan

The general plan followed in these studies consisted of the prepara-
tion of the solution in open containers and in closed containers. The
ccnponents of the solution were mixed at different temperatures, and the
solutions were held for extended periods of time in closed containers and
also exposed to air in various ways. The total methyl bromide content, as
calculated from a bromide determination, and the free bromide ion were
determined at intervals.

Procedures

Method of preparation

In -one series of experiments the solutions were prepared in open
containers on the basis of 1 pound (262 ml.) of methyl bromide, 524 ml. of
ethyl alcohol, and 46 gallons (174 liters) of water, which is equivalent
to 2.6 mg. of methyl bromide per ml., or 0.15 percent of methyl brcmiCe
by volume. In the second series of experiments the solutions were pre-
pared in closed containers, using the same amount of methyl bromide, both
with and without alcohol, and in quantities of water ranging from 46 to 70
gallons.

Opn containers.--Preliminary experiments were conducted in which the
solutions we.. prepared in open 5-gallon bottles, the necks of which were
1.25 inches in diameter. The required amount (49 grams) of methyl bromide
was weighed in a Dreschsel high form gas-washing bottle of 125 ml. capacity,
the alcohol (57 ml.) was added, and the solution was mixed gently with the
stopper iii place. The inlet tube of the gas-washing bottle was closed, the
outlet tube was connected to a tube extending to the bottom of the bottle
coi~tainiig a little less than 5 gallons of water, and the methyl bromide-
alcohol solution was introduced at the bottom of the bottle. The gas-washing







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bottle was rinsed with water twice, the volume in the large bottle was made
up to 5 gallons, and the mixture was agitated to insure uniformity. The
temperatures of the components and of the final mixture were recorded.

Solutions were also made in a 50-gallon drum, the bung of which was
2.25 inches in diameter. A similar procedure was used, except that a quart
jar equipped with inlet and outlet tubes was substituted for the gas-washing
bottle. The amounts of materials used were 1 pound of methyl bromide,
524 ml. of ethyl alcohol, and 46 gallons of water.

Closed containers.--Solutions were prepared in a drum of apprcxi-
mately 82 gallons' capacity, with the bung in place and sealed by means of
a rubber gasket. A 1/8-inch copper tube was soldered vertically into a hole
drilled in the bung, and the tube extended to the bottom of the drum. An
applicator designed to discharge the methyl bromide from a 1-pound can was
attached to the upper end of this tube. Another tube in the bung was
provided for pressure gauge. The volume of water in the drum ranged frcm
46 to 70 gallons. The method of preparation was as follows: The required
amount of water was weighed in the drum, the alcohol added, and the drum
closed. A 1-pound can of methyl bromide was inserted in the applicator,
and its contents were discharged into the drum. A record was kept of the
gauge pressure at intervals.

Method of analysis

The method used to determine the total amount of bromide in the
solution was adapted from the method described by Stenger, Shrader, and
Beshgetoor.5/ This method involves the use of ethanolamine to hydrolyze
the methyl bromide, with the subsequent determination of the bromide ion by
the Volhard method.6/ In preliminary tests, using a lC-ml. sample and 2 ml.
cf ethanolamine, it was found that about 6 hours was required for complete
hydrolysis to take place. When the amount of ethanolamine was increased to
10 ml., hydrolysis was complete in 1 hour. The exact method used follows.

Ten ml. of the solution were pipetted into a'glass-stoppered 250-ml.
Erlenmeyer flask containing 10 ml. of ethanolamine, and the flask was closed
immediately with a stopper previously lubricated with ethanolamine. The
flask was shaken at intervals during 1 hour, after which its contents were
acidified with concentrated nitric acid. The bromide ion was determined
by the Volhard method. One ml. of 0.1 normal silver nitrate is equiva-
lent to 9.50 mg. of methyl bromide.

The amount of bromide ion originally present was determined by acidi-
fying the solution at once with concentrated nitric acid and titrating it.

5/ Stenger, V. A., Shrader, S. A., and Beshgetoor, A. W. Analytical
methods for methyl bromide. Indus. and Engin. Chem., Anal. Ed., 11: 121-
124. 1939.
6/ Scott, W. W., and Furman, N. H. Standard methods of chemical
analysis. Ed. 5, v. 1, p. 271. D. Van Nostrand Co., Inc., New York. 1939.






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EXPERIMENTAL RESULTS

Composition of Solutions Prepared in Open Containers

In a 5-gallon bottle

Methyl bromide solutions were prepared in a 5-gallon bottle with the
components of the solution at temperatures ranging from 10 to 250 C. The
uniformity of these solutions was established by analysis of samples drawn
from both the top and bottom of the bottle. The calculated methyl bromide
content was 2.6 mg. per ml. or 0.15 percent by volume. When the components
were mixed at 10 C., solid methyl bromide hydrate was formed. This solution
was then warmed slowly until the hydrate disappeared, which took place at
about l0 C. The average percent of charge recovered immediately after
preparation is shown in table 1.


Table l.--The composition of methyl bromide-ethyl alcohol-water
solutions prepared at different temperatures in open 5-gallon
bottles, immediately after preparation

Temperature in C. Percent of charge
Water Alcohol recovered

25 25 57,7
20 20 61.5
10 25 69.2
1 1 96.1


In a 50-gallon drum

Two solutions were also prepared in quantities of 46 gallons in an
open 50-gallon drum. In one case the quantity of the water was roughly
measured in small units by volume, while in the second case the water was
weighed in the drum, the temperature of the water being 260 C, and '5 C.,
respectively. The amount of methyl bromide recovered was 61.5 percent and
53.8 percent, respectively, of the amount introduced, which indicates that
measuring the water accurately is important.

Another solution was prepared under the above conditions except that
the amount of methyl bromide in the charge was increased to 2 pounds in 46
gallons of water at a temperature of 220 C. The amount of methyl bromide
recovered was 59.6 percent of the amount introduced. Considering the dif-
ference in temperature, this percentage recovered was only slightly larger
than the percentage recovered when the solution was prepared at the rate of
1 pound of methyl bromide in 46 gallons of water at a temperature of 250 C.;
i. e., the weight of methyl bromide per unit of volume recovered from the
solution prepared with a 2-pound charge was about double that recovered when
a 1-pound charge was used.







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These results indicate that in open containers between 40 and 50
percent of the methyl bromide was lost when the solution was prepared at
250 C., which is a temperature that might be encountered under field conCi-
tions. The amount of loss was decreased as the temperature of the solution
was decreased, until substantially all the methyl bromide was in the solu-
tion prepared at 10 C. Since the temperature of solutions prepared in the
field cannot be controlled, the composition of the solutions prepared in
open containers would vary widely in methyl bromide content.

It was also indicated that the loss from the 1-pound charge was not
due to the solution being saturated with methyl bromide but rather to the
method of preparation which did not allow sufficient time for the methyl
bromide to be dissolved.

Composition of Solutions Prepared in Closed Containers

Solutions were prepared in a drum of 82 gallons' capacity by the
method previously described. The temperature of the water ranged from
11.50 to 260 C., and the quantity of water ranged from 46 to 70 gallons.
The data gathered in these experiments are shown in table 2.


Table 2.--The composition of methyl bromide-ethyl alcohol-water
solutions prepared at different temperatures in a closed drum.

Water Pressure in pounds
Volume Temperature _p s guare inch Percent of charge
in gallons in C. Maximum Minimum recovered

46 26 6.5 3.0 80.8
60 25 8.5 1.5 90.0
70 25 10.5 1.5 94.1
70 11.5 5.5 1.0 94.1


These data indicate that when the solution is prepared in a closed
drum its composition is not dependent on the temperature of the water at the
time of preparation. The greatest percentage of recovery was obtained when
the drum was nearly full and sufficient time had been allowed for the gas to
go into solution, as indicated by the minimum gauge pressure. Considering
the amount of methyl bromide in the vapor phase, it is indicated that sub-
stantially all the methyl bromide charge was dissolved when the drum con-
tained 70 gallons of solution.

A few experiments in which the solution was similarly prepared, except
that it did not contain any alcohol, indicated that under these conditions
alcohol at the concentrations used is not an important factor in the solu-
bility of methyl bromide.






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Stability of Solution

In order to determine the rate at which methyl bromide is lost from
the solution, experiments were conducted in which the solution was held
for periods of time under different conditions with respect to temperature
and exposure to air.

Two solutions prepared in 5-gallon bottles by the method previously
described were kept tightly stoppered -and maintained at a temperature
ranging from 100 to 120 C. for a period of 48 hours. An average of 2.2
percent of the original bromide content was lost during this period. The
average loss of methyl bromide from two solutions similarly prepared and
treated, except that the temperature was approximately 251 C., was equal to
5.3 percent of the average original methyl bromide content.

A solution was prepared in a 5-gallon bottle by the same method and
kept in the open bottle maintained at a temperature ranging from 100 to
120 C. for a period of 48 hours. During this period 4.3 percent of its
methyl bromide content was lost. Two solutions were similarly prepared and
treated, except that the temperature was maintained at approximately 250 C.
The average loss of methyl bromide during 48 hours was equal to 14.9 percent
of the average original methyl bromide content of the solutions,

Forty-six gallons of the solution were prepared in a 50-gallon drum
at a temperature of 250 and allowed to remain in the open with the bung
removed, During 48 hours 35.7 percent of the original methyl bromide content
of the solution was lost and in 96 hours the loss was 57.1 percent.

The above experiments were on the basis of limited exposure to air,
Other experiments were performed in order to determine the loss of methyl
bromide from the solution when a relatively large surface was exposed. Six
liter beakers (low form) were filled with the solution and held at room
temperature (approximately 270 C.). During a period of 48 hours the average
amount of methyl bromide lost was equal to 71.0 percent of the average
initial content of the solutions and during 72 hours the average loss was
87.5 percent. All the methyl bromide was lost during 2 hours from a thin
layer of the solution in a shallow glass dish placed on the ground in the
sunlight.

Experiments were conducted to determine the influence of agitation
combined with exposure to air on the loss of methyl bromide from the solu-
tion, Forty-six gallons of the solution were prepared in an open 50-gallon
drum in the usual manner. Six gallons of this stock solution were drawn
from the drum by means of a spigot, analyzed, and diluted With an equal
volume of water in a small drum. One gallon of this solution was drawn from
this drum as above, analyzed, and sprinkled on 1/3 of a square yard of soil,
on the center of which was located a shallow glass dish. It was found, on
the basis of 1 pound of methyl bromide in 46 gallons of water, that there
was a loss o. methyl bromide of 46.2 percent during preparation. There was
an additiona! loss of about 7.5 percent during each of the other operations





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enumerated above. The solution retained in the glass dish contained only
23.1 percent of its calculated methyl bromide "content.

From these- experiments it'is idi cted that the rate of icss of methyl
bromide from- thed solu on's 'ependeht not only upon the method of propara-
under which t is held aftdr prcp.ra-
unot T ee w inss of sneth CM ireies appqars. to ke
dependent upon time, tempeabure, degreee or exposure, and agitation after
preparation.

Rate of Hydrolysis of Methyl Bromide

To determine the rate of hydrolysis of methyl bromide in the solution,
samples were taken from four solutions at intervals during periods as long
as 144 hours after preparation. These samples were titrated immediately on
being taken, the same method being used as for the determination of total
methyl bromide except that ethanolamine was not used. The average amount of
methyl bromide determined was in the order of 0.1 mg. per ml. at each
interval. The determination of this quantity of methyl bromide is at
about the limit of accuracy of the method of analysis. Since there was no
increase in the amount of methyl bromide determined during a period of 144
hours after preparation of the solution, it is indicated that the rate of
hydrolysis of this compound under these conditions is not important in
relation to its use as an insecticide. Under field conditions, when the
solution is applied to soil, the rate of hydrolysis may be modified in the
presence of the soil and soil solution. Further studies will be required
to determine the behavior of methyl bromide under these variable conditions.

DISCUSSION

It was demonstrated that the methyl bromide content of an aqueous so-
lution of this compound is dependent upon the conditions under which it is
prepared and stored. Temperature, exposure to air, and agitation are all
variables which modify the methyl bromide content. It was indicated that,
since it is difficult to standardize these variables when using an cpen
container, the solution should be prepared in a closed container. Since
substantially all the methyl bromide charge is retained in solution when
the latter method is used, the use of the closed system would result in a
saving of about half of the methyl bromide cost and produce a solution of
predetermined methyl bromide content. The closed system referred to is
described in detail under "Method of Preparation," which appears in the fore
part of this paper.

It is suggested that under field conditions the solution should be
prepared by the following method: The container could be a heavy reinforced
drum of 100 to 110 gallons capacity. The bung of this drum would be equipped
with a pressure gauge and a tube extending to the bottom of the drum and
with a strap applicator attached to its upper end. The drum would contain
92 gallons of water. An amount of alcohol equal to twice the volume of
methyl bromide may be used if desired. The methyl bromide content of the




UNIVERSITY OF FLORIDA

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8 3 1262 09230 3691

solution would be dependent upon the amount of this compound introduced.
A charge of 1 pound would produce a solution roughly equivalent to the
solution prepared in an open drum containing one-half this amount of water.
Precautions should be taken to eliminate leakage of methyl brcmide from
the applicator and at the points where the tube, pressure gauge, and bung
are sealed. Sufficient time should be allowed for the methyl bromide to go
into solution, as indicated by the pressure gauge. About 1 hour should be
sufficient for the pressure to become less than 1 pound.

Since the methyl bromide content of the solution is dependent on the
variables listed above, it is indicated that it should be stored in closed
containers to avoid contact with air. In drawing the solution from the drum
in which it was prepared, an effort should be made to avoid agitation
and exposure as much as possible. It is suggested that a tube be attached
to the end of the spigot and extended to the bottom of the container into
which the solution is drawn in order to reduce the amount of methyl bromide
lost in this operation. After having been drawn from the drum, the solution
should be applied to the soil as soon as possible and with a minimum of
agitation. It would appear better to pour the solution on the ground rather
than to sprinkle it.

SUMMARY

A chemical study was made of the preparation and stability of an
aqueous methyl bromide solution used to kill various insects. The composi-
tion of the solution prepared in the usual way was found to be dependent
upon the temperature of the solution at the time of preparation, which is
an uncontrollable variable under field conditions. A loss of between 40 and
50 percent of the methyl bromide charge might be expected when water at a
temperature of 250 C. is used. Apparatus was constructed and a method
established for the preparation of this solution in a closed container.
Solutions prepared in this way retained substantially all the methyl brcmide
charge. The rate of loss of methyl bromide from the solution was dependent
upon time, temperature, degree of exposure to air, and agitation after
preparation. The hydrolysis of methyl bromide in the solution during 144
hours was not an important factor. Apparatus and a method for the prepara-
tion of the solution in a closed system are described. Suggestions are made
for the storage and use of the solution.