;OARD September 1942 STATE PLANT BOARD E-575 A CHEMICAL STUDY OF THE PREPARATION AND STABILITY OF AQUEOUS METHYL BROMIDE SOLUTIONS By R. D. Chisholra and L. Koblitsky, Division of Insecticide Investigations!/ 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 authorized3/ 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 leucolcma and P geregrinus. Jour. Econ. Ent. 33: 531-533. 1940. 3/ Administrative instructions — Modifying the restrictions of the %hite-fringed beetle quarantine by authorizing treatment by Methyl , brom ide 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 March 15, 1940. 4/ Donohoe, H. C. Unpublished report.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. 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 alcohol . — The ethyl alcohol used in these experiments was either denatured or 95 to 98 percent pure. Me thy 1 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 brcmide, and no compensation for the differences in net weight was made in these studies. GENERAL PLAN AND FROCEDURES 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 components 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 bromide by volume. In the second series of experiments the solutions were pre- pared in closed containers, using the same amount of methyl brcmide, both with and without alcohol, and in quantities of water ranging frcm 46 to 70 gallons. Open, containers. — Preliminary experiments were conducted in which the solutions we; a prepared in open 5-gallon bottles, the necks of which were 1.25 inches in diameter. The required amount (49 grams) of methyl brcmide 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 in 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 containing a little less than 5 gallons of water, and the methyl brcmicc- alcohol solution was introduced at the bottom of the bottle. The gas-washing 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 approxi- 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 bottcm 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 a 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 frcm the method described by Stenger, Shrader, and Beshgetcor.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 10-ml . sample and 2 ml. of 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. 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 1° to 25° 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 brcmide content was 2.6 mg. per ml. or 0.15 percent by volume. When the components were mixed at 1° C, solid methyl bromide hydrate was formed. This solution was then warmed slowly until the hydrate disappeared, which took place at about 10° C. The average percent of charge recovered immediately after preparation is shown in table 1. Table 1. — The composition of methyl bromide-ethyl alcohol-water solutions prepared at different temperatures in open 5-gallon bottles, immediately after preparation Percent of charge recovered 57.7 61.5 69.2 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 26° C. and 25° C, respectively. The amount of methyl brcmide 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 22° 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 25° 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. Tempei ratu re in °C. Water Alcohol 25 25 20 20 10 25 1 1 These results indicate that in open containers between 40 and 50 percent of the methyl bromide was lost when the solution was prepared at 25° C, which is a temperature that might be encountered under field condi- 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 1° 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 v/hich 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 11.5° to 26° 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. Percent of charge recovered 80.8 90.0 S4.1 £4.1 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 net 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. Water Pressure in ,.per_ square pounds Volume Temperature _inch in gallons in.°l£0 upon the bns under which it is'held after pre, tioii. The extenl eh loss of methyl bromide" occurs appears to be dependent upon time, temperature, degree of 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 open 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 cf 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 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 bromide 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 25° 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 bromide 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.