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United States Department of Agriculture

Agricultural Research Service

Methyl Bromide
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Chemical and Physical Properties
 
Some of the basic physical and chemical properties of MeBr are listed in Table 1. Due to its high vapor pressure, MeBr can readily penetrate many matrices and is extremely difficult to contain even in the laboratory. Since MeBr is colorless and virtually odorless at room temperature, even at potentially toxic concentrations, severe exposure can occur unknowingly (Yang et al., 1995). In commercial formulations of MeBr, various percentages (0.5 - 33%) of chloropicrin are added as a warning agent to protect workers and residents during and immediately after MeBr applications (and to assist in protecting plants from disease). However, it should be noted that since the vapor pressure of MeBr is many times that of chloropicrin, the safety of using low ratios of chloropicrin in the mixture as a warning agent is questionable (Van Assche, 1971). Methyl bromide is considered to be acutely toxic, with an 8-hr time-weight averaged limit for human exposure in air to be only 5 ppm (ACGIH, 1988). Acute toxicity to workers upon exposure to MeBr vapor has been a major concern in the many years of MeBr use, and is one of the reasons for some early modifications of its application method (e.g., use of surface tarp, mixing with chloropicrin, and use of buffer zones). Fatalities and injuries resulting from exposure to MeBr have been reported, but most incidents are related to structural fumigations rather than soil fumigations (Yang et al., 1995).
 
Table 1.  Selected Physical Properties of Methyl Bromide
 
Property Value
Synonym Bromomethane
Trade Name Brom-O-Gas,
Meth-O-Gas,
Terr-O-Gas
Molecular Weight, g mole-1 94.94
Vapor Density, g L-1 3.974
Dipole Moment, debye 1.8
Liquid Density, g cm-3  
  20 °C 1.676
  25 °C 1.737
Solubility, mg L-1  
  20 °C 16000
  20 °C 17500
  25 °C 13400
CAS Registry Number 74-83-9
Freezing Point, °C -93.7
Boiling Point (at 1.0 atm), °C 3.56
Koc, cm3/(g %Foc) 22
Log(Kow) 1.19
Vapor Pressure, mmHg  
  20 °C 1395 ±19
Henry's Law Constant  
  20 °C 0.23
  21 °C 0.30 ±0.02
   

Last Modified: 10/20/2005
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