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Diffusion coefficients, along with other parameters, are required inputs to some environmental transport or risk assessment models. Literature values are sometimes available for these parameters, although, often the literture values are determined at 25 oC. Regulatory programs may have developed preferred sets of parameter values, which should be used if avaialble. Further information on parameter sets is available on the chemical properties page. When parameter values are otherwise unavailable, either for an unusual chemical, or for a temperature not reported in the literature, a calculated value may be useful. Two versions of a calculator for estimating water and air phase diffusion coefficients are available on these pages. The first (this page) contains preset inputs for benzene, toluene, ethylbenzene, and xylenes (BTEX) and oxygenated additives of gasoline. The second version requires generation of input parameters for the chemicals of interest.

  Estimated Diffusion Coefficients in Air and Water
Dair = 0.001 T1.75 Mr1/2 / (P (VA1/3 + VB1/3 )2)

D is the diffusion coefficient in cm2/sec
T is the temperature in K
Mr is the following function of molecular weight:
Mr = (MA + MB)/ (MA MB)
where MA is the molecular weight of air, approximately 28.97 g/mol
and MB is the molecular weight of the compound of interest
VA is the molar volume of air (approximately 20.1 cm3/mol)
VB is the molar volume of the compound of interest
P is the pressure in atm.
Dwater = 13.26 x 10-5 / h1.14 (VB')0.589
h is the viscosity of water
VB' is the LaBas estimate of molar volume

These estimation methods were developed by Diffusion Coefficients in Air and Water by William A. Tucker and Leslie H. Nelken, Chapter 17 in Handbook of Chemical Property Estimation Methods, Warren J. Lyman, William F. Reehl and David H. Rosenblatt, editors, American Chemical Society, 1982. 		 
Inputs
Date
Pressure ATM
Temperature oC
Default Inputs
Chemical
Boiling Point (oC)
Atom Special Conditions Number of Atoms (or rings)
Diffusion Coefficient Results
Estimated Diffusion Coefficient in Air cm2/sec FSG Method
Estimated Diffusion Coefficient in Air cm2/sec FSG/LaBas Method
Estimated Diffusion Coefficient in Air cm2/sec WL Method
Estimated Diffusion Coefficient in Air cm2/sec Average of WL, FSG and FSG/LaBas Methods
Estimated Diffusion Coefficient in Water cm2/sec Hayduk and Laudie method
Quality Checks against values given in Tucker and Nelken: Recalculated each time an estimate is made.
FSG Method Diffusion Coefficient in Air for m-chlorotoluene cm2/sec Tucker and Nelken result: 0.0731 cm2/sec Expected Percent Error = -0.184 Calculated:
FSG-LaBas Method Diffusion Coefficient in Air for isopropyl iodide cm2/sec Tucker and Nelken result: 0.0815 cm2/sec Expected Percent Error = -0.0987 Calculated:
WL Diffusion Coefficient in Air for m-chlorotoluene cm2/sec Tucker and Nelken result: 0.0758 cm2/sec Expected Percent Error = -0.0959 Calculated:
WL Diffusion Coefficient in Air for isopropyl iodide cm2/sec Tucker and Nelken result: 0.0857 cm2/sec Expected Percent Error = -0.444 Calculated:
Hayduk and Laudie Diffusion Coefficient in Water for aniline cm2/sec Tucker and Nelken result: 9.5 x 10-6 cm2/sec Expected Percent Error = -0.127 Calculated:
Hayduk and Laudie Diffusion Coefficient in Water for ethyl acetate cm2/sec Tucker and Nelken result: 9.6 x 10-6 cm2/sec Expected Percent Error = -0.312 Calculated:
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Page author: Jim Weaver, of U.S. EPA, Office of Research and Development, Athens Georgia who last modified this content on: September 15, 2006

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