Guidance for Using the Volatilization Algorithm in the
Pesticide in Water Calculator and Water Exposure Models
Prepared by: Gabe Rothman, Meridith Fry, Chuck Peck, Jim Lin, Dirk Young, Faruque Khan, and Jim
Hetrick
Environmental Fate and Effects Division
December 8, 2015
1
-------
Guidance for Using the Volatilization Algorithm in the
Pesticide in Water Calculator and Water Exposure Models
Overview
This document provides guidance for using the volatilization algorithm in the Pesticide in Water
Calculator (PWC). The release of the PWC (Version 1.5) includes additional inputs needed to
execute the volatilization algorithm. These include the Diffusion in Air Coefficient, Heat of
Henry, and Henry's Law Constant. Additional volatilization-related outputs are also now
available, including pesticide mass distribution within the soil profile and the amount of pesticide
lost due to volatilization.
This document describes these new additions and provides guidance for assessing pre-emergent
and bare soil applications in the PWC with the volatilization algorithm.
Applicable Use of the Volatilization Algorithm
The volatilization algorithm calculates the daily pesticide mass flux from soil over the simulation
period. It should only be used to evaluate aquatic exposure associated with bare soil and pre-
emergent applications of fumigant and conventional pesticides. In addition, the impact of
volatilization is not large for aquatic exposure estimates for semi-volatile chemicals with Henry's
Law Constants less than 10"7 atnrm Vmol. As such, the volatilization algorithm should not be
used to evaluate the following at this time:
1. Aquatic exposure associated with foliar applications. The portion of the volatilization
algorithm associated with the crop canopy has not been verified at this time. As such,
one critical input for crops, the foliar volatilization dissipation rate constant, is not
available in the current PWC. This variable parameterizes the contribution of off-gassing
of residues from crop surfaces.
2. Aquatic exposure associated with compounds possessing Henry's Law Constants less
than 10"7 atm*m3/mol. Figure A-l in Appendix A shows that aquatic exposure does not
change for chemicals with the same loading possessing Henry's Law Constants less than
10"7 atm*m3/mol.
3. Inhalation exposure or other terrestrial exposure resulting from vapor-phase
concentrations resulting from volatilization. Daily volatilization fluxes estimated from
the PWC do not provide the precision required for addressing shorter-term inhalation
exposure with external air exposure modeling tools. The daily average volatilization flux
values potentially underestimate peak flux values, which can spike over short time scales,
on the order of hours. 1
The volatilization algorithm can now be executed entirely within the latest version of the PWC
interface. Additional inputs required to execute the volatilization algorithm include: Diffusion in
1 While daily flux rates can under represent peak flux rates which spike over shorter time scales, underestimation of
the equivalent daily total mass loss (percent of applied) by the volatilization algorithm is not expected.
2
-------
Air Coefficient, Heat of Henry, and Henry's Law Constant. The graphical user interface
prompts the user for these parameters regardless of the chemical or application method. For
situations which require the volatilization algorithm to be shut off (as described above), the
user should specify an input parameter value of 0 for the Diffusion in Air Coefficient and
Heat of Henry to effectively eliminate the volatilization dissipation pathway.
Guidance on Execution of the Volatilization Algorithm
Parameterization guidance is provided below, organized by menu (or tab) in the PWC interface.
The Chemical Menu includes the physical-chemical properties needed to execute the
volatilization algorithm. The Applications Menu specifies bare soil and pre-emergent
application input parameters. The Crop/Land Menu defines the soil, land surface, and soil-air
boundary layer parameters, and the More Output Parameters Menu includes options for
generating various output distributions and fluxes throughout the soil profile.
• Chemical Menu
In the Chemical Menu, the Henry's Law Constant, Air Diffusion Coefficient, and Heat of Henry
are required to execute the volatilization algorithm. These parameters have been added to this
menu as shown in Figure 1 below.
3
-------
wc. Pesticide Water Calculator (PWC)
File Scenario Help
r^ii 0 i-^i
Chemical j| Applications j Crop/Land J Runoff j Watershed ] Batch Runs [ More Options | Out Pond f Out Reservoir j Out Custom | OutGW | Advanced |
Chemical ID (optional)
~
Parent Daughter
{% Koc Kd Sorption Coeff (mL/g)
Water Column Metabolism Halflife (day)
Water Reference Temperature (°C)
Benthic Metabolism Halflife (day)
Benthic Reference Temperature (°C)
Aqueous Photolysis Halflife (day)
Photolysis Ref Latitude (°)
Hydrolysis Halflife (day)
Soil Halflife (day)
soil ref (°C)
Foliar Halflife (day)
MWT
Figure 1. Chemical menu and parameters of interest relevant for volatilization algorithm.
Henry's Constant (dimensionless): The dimensionless Henry's Law Constant (Kh) is the
partitioning coefficient of a chemical between air and moist soil. The graphical user interface
enables the user to calculate Kh automatically from input vapor pressure and solubility values by
clicking the "Estimate It!" button. In order to obtain the proper value, the chemical's vapor
pressure value at 25°C should be used. The PWC calculates Kh according to the set of equations
below:
Q10 2
Ready-
Working Directory: Working Directory: Not yet specified. Use save or retrieve.
Run
IO Family Name: File name not yet determined. Use Save or retrieve.
Universal Gas Constant
HLC
where:
Universal Gas Constant R = 8.21 x 10
Temperature = 298 K
_5 atm«m3
mol»K
4
-------
hlc fatm • m3^ _ Vapor Pressure at 25°C (torr)x 7^orr
\ mol / c, , , fmg\ 1 /moP\ g 1,000 L
Solubllty V L J x MWT \ g J x 1,000 mgx m3
It is important to note that the Henry's Constant input is referenced to 25°C. However, the
Henry's Constant for the chemical will vary throughout the simulation based on soil temperature
variation, and the Heat of Henry value for the chemical (see description of input parameter above
this section). Soil temperature is simulated along with volatilization and varies based on albedo
and soil properties. The input of related parameters is discussed within the Crop/Land menu.
Air Diffusion Coefficient (cm2/day): The air diffusion coefficient is related to the kinetic
energy associated with molecular motion and is dependent on the molecular weight of the
compound. The air diffusion coefficient for a specific compound is calculated as (ACS, 1982):
/cm2\ 0.001T175Mr/2 3,600 seconds 24 hours
^airydayJ p(v1/3 + V1/3)2 X hour X day
where:
T is the temperature in K (default of 298 K)
(Ma + Mb)
mamb
Ma is the molecular weight of air (approximately 29 g/mol)
Mb is the molecular weight of the chemical (g/mol)
P is the pressure in atm (default of 1 atm)
Va is the molar volume of air (approximately 20.1 cmVmol)
Vb is the molar volume of compound of interest calculated from the following expression
(cmVmol):
The air diffusion coefficient also may be retrieved from sources such as chemical information
reports, MSDS documents, or physical chemical property databases, or it can be calculated
interactively from EPA's Tools for Site Assessment website, available at:
http://www3.epa.gov/ceampubl/learn2model/part-two/onsite/estdiffusion.html. It should be
noted that the air diffusion coefficient is adjusted for soil porosity and tortuosity based on
Millington and Quirk (1960). The air diffusion coefficient is also more directly used as part of
the Jury et al. (1983) equation to determine the volatile flux across the soil-atmosphere boundary.
An input of zero for the air diffusion coefficient effectively shuts off dissipation of the chemical
due to volatilization. This value is recommended at this time for simulating foliar applications.
5
-------
Heat of Henry (J/mol): This parameter is defined as the energy required for the phase change
of a chemical from aqueous solution to air solution. The Heat of Henry can be calculated from
the Clausius-Clapeyron equation if Henry's Constants at two different temperatures are available
(Staudinger and Roberts, 2001). For further instructions, see Appendix B. Otherwise, the Heat
of Henry can be approximated by the enthalpy of vaporization, which is more commonly
available from product chemistry literature, chemical information reports, MSDS documents, or
physical chemical property databases. The enthalpy of vaporization is used along with the
temperature of the soil to determine the Henry's Law Constant at a specific time. Measured or
estimated values (with the cited method description) of the Heat of Henry should be used as
much as possible in the PWC input parameterization. Generic default values previously
recommended in water model documentation should be avoided.
• Applications Menu
In the Applications Menu, the A initial chemical soil distribution under the options for
application methods to address shank injection applications has been added. This chemical
distribution initializes the distribution of the chemical mass in the soil in a linearly increasing
fashion from the surface to the specified depth of incorporation. This initial chemical
distribution associated with the A option is illustrated in Figure 2 below. Figure 3 provided
below shows the unique application methods associated with bare soil applications (e.g., surface
applications and incorporated applications). Input parameter guidance related to the selection of
the appropriate application method is provided in Table 1 below.
Q.
Cl)
¦a
Atmosphere
Soil
Chemical
Distribution
<4
Injection Depth
Figure 2. Illustration of the new A initial chemical distribution developed in the PWC.
6
-------
wc- Pesticide Water Calculator (PWC)
Scenario Help
0
File
Chemical ] Applications | Crop/Land | Runoff | Watershed | Batch Runs | More Options | Out Pond | Out Reservoir | Out Custom") OutGW [ Advanced j
Number of Applications
50
Update
Applications
Absolute Dates
© Relative Dates
Days Since
Emergence
Application Method
Below^Above Uniform @ T
Crop Crop Below Depth Bam
Depth T-Band
(cm) Split
Hide
Reservoir
~
Hide
Pond
n
Hide
Custom
~
Eff Drift Eff. Drift Eff. Drift
Figure 3. Application menu and parameters of interest relevant for volatilization
algorithm.
Table 1. PWC crop management input parameter guidance for consideration using
the volatilization algorithm with bare soil pesticide applications.
PWC Input
Parameter
Input Value and Unit
Comment
Crop Scenarios
and
Application Date
While bare soil and pre-emergent
applications are being simulated,
existing crop scenarios in the PWC
should be used, relevant to the
registered or proposed crop use, to
account for representative variations in
soil and meteorological conditions.
Select the relative date button and
enter the days prior to emergence
(using a negative sign (e.g., -7) days or
any number of days since emergence)
considering the plant back interval, if
specified on the label.)
Exposure from runoff and
leaching will be determined
from the occurrence of
rainfall, relative to the
application date on a field
with bare soil.
Application Rate
(kg/ha)
Maximum broadcast or area-treated
application rate for each crop use.
The maximum broadcast
application rate usually
applies. However, the
application rate may be
adjusted to account for
banded applications (such as
T-Band, injected1, or bedded
applications) provided an
area-treated normalized
application rate is specified
on the label.
7
-------
Tsihle 1. PWC crop niiuiiigement input piiriimeler giiirisince lor consideration using
(lie volsilili/silion ;il«ori(hin with hare soil pesticide :ipplic;itions.
PW'C Inpul
I'ii rmiieler
1 iiput Value siihI I nil
Comment
Application
Method and
Incorporation
Depth (cm)3
Surface Applications:
(Including Overhead Sprinkler,
Overhead Chemigation, Drip
Chemigation, Flooded Applications,
and Drench Applications):
Application Method: select Below
Crop button (soil applied, uniform
chemical mass distribution to 4 cm
default depth)
In this case, no depth is
specified because the model
will use a uniform chemical
mass distribution to a
default depth of 4 cm.
Soil Incorporated (e.g., shank iniection
The most shallow injection
depth is the most protective
assumption for surface
water exposure estimates.
or knifed-in):
> Application Method: select
A button (soil applied, chemical
mass linearly increasing from the
surface to the incorporation depth).
> Depth: Use the most shallow
incorporation depth specified on the
label.
Application
Efficiency and
Spray Drift
Fraction
Surface (Overhead Chemigation) and
Incorporated Applications:
Eff = 1.0
Drift = 0.0
No spray drift is assumed to
accompany overhead
chemigation, drip
chemigation, or knifed-
in/ shank injection
applications.
8
-------
Table 1. PWC crop management input parameter guidance lor consideration using
the volatilization algorithm with hare soil pesticide applications.
PWC Input
Parameter
Input Value and I nil
Comment
Surface (Overhead Sprinkler)
Applications2;
Follow standard guidance from Off-
Site Transport Guidance (USEPA,
2013)
Overhead sprinkler
applications with semi-
volatile compounds are
generally subject to spray
drift deposition to water
bodies.
Notes:
1 For flat fume knifed or shank injection applications, the broadcast application rate will generally apply as opposed
to bedded shank injection applications.
2 For fumigant applications through an overhead sprinkler, a drift of zero and application efficiency of one may be
specified, since the spray material remaining in air is expected to volatilize sufficiently in air to prevent spray drift
deposition to water bodies.
3 Generic guidance is presented here for surface and incorporated applications. Based on other specific applications
on the label, the user may decide to use Uniform below, @Depth, T-Band, or the V options. Please refer to the
PWC Manual for further explanation.
• Crop/Land Menu
The Crop/Land Menu includes various inputs that are critical to the execution of the
volatilization algorithm and dynamic soil temperature routine. The dynamic soil temperature
routine should be executed to account for variations in the phase-change resulting from changes
in temperature in the soil. The dynamic soil temperature routine can be activated by clicking the
checkbox beside "Simulate Temperature". While the standard PWC crop scenarios are used
with the volatilization algorithm, several additional parameters are needed to execute the
volatilization algorithm and dynamic soil temperature routine. These parameters, their physical
significance, and the related input parameter guidance are provided below.
9
-------
wc- Pesticide Water Calculator (PWC)
| File | Scenario Help
Chemical [ Applications | Crop/Land [ Runoff | Watershed [ Batch Runs [ More Options ] Out Pond j Out Reservoir J Out Custom | OutGW | Advanced j
Weather File
Growth Descriptors
Day Month
Emerge
Mature
Harvest
Post-Harvest Foliage
O Surface Applied
Removed
1 Left as Foliage
Hydro Factors
Root Depth (cm)
Canopy Cover (%)
Canopy Height (cm)
Canopy Holdup (cm)
Pan Factor
Snowmelt Factor (cm/°C/day)
Evaporation Depth (cm)
Irrigation
Extra Water Allowed Max Rate
None Fraction Depletion (cm/hr)
Over Canopy
Under Canopy
Soil Irrigation Depth
9 Root Zone
User Specied (cm)
Soil Layers
Number of Horizons: 8 | Update Horizons |
Thick p Max. Min.
(cm) (g/cm3) Cap. Cap. OC (%) N
Ready...
Working Directory: Working Directory: Not yet specified. Use save or retrieve.
IO Family Name: File name not yet determined Use Save or retrieve.
Figure 4. Crop/Land menu and parameters of interest relevant for volatilization
algorithm.
Boundary Layer Thickness (cm): Thi s is the depth of the stagnant boundary layer. The Jury et
al. (1983) equation for the determination of volatile flux across the soil-atmosphere boundary
constrains the atmospheric compartment to this depth. Since the Jury model for flux is based on
diffusion factors only, it does not consider dispersion resulting from wind speed and turbulence
above this stagnant boundary layer. By definition, the stagnant boundary layer height is
analogous to the aerodynamic roughness length, given that there is essentially no wind below
this height (due to frictional drag being completely dependent on the underlying bare soil
surface2). For bare soil, the surface roughness length is very low considering its relatively
smooth surface compared to more complex landscapes containing more significant obstacles to
wind flow, such as trees or terrain. A default boundary layer thickness depth of 5.0 cm is
recommended corresponding to the appropriate surface roughness length for bare soil specified
in several sources (Stull, 1988; USEPA, 2004). However, the user may decide to use a different
value to evaluate situations involving hillier terrain.
2 Roughness length is constant within discrete land surface types. It does not vary with wind speed or temperature.
10
-------
Lower Boundary Condition (BC) Temperature (°C): This parameter is specified as the
constant temperature occurring at a specific depth below the surface. At this depth, heating and
cooling cycles (often observed in top soil resulting from air mass temperature changes, diurnal
and annual insolation cycles, and changes to land cover over the course of the year) no longer
occur. The ground water temperature is roughly equivalent to this temperature. The national
distribution of the lower boundary condition can be derived referencing the contour map in
Figure 5.
Figure 5. Average shallow groundwater temperatures (°C) in the United States (from
http://www3.epa.gov/ceampubl/learn2model/part-two/onsite/teinpmap.html derived from
Collins ri925R
Albedo: Albedo is the fraction of total incident solar radiation reflected by the surface back to
space without absorption. Surface color highly influences albedo values since lighter colors tend
to reflect more effectively than darker colors. Albedo is a critical value for the dynamic soil
temperature routine, as it dictates the amount of heating through the top soil given air
temperature and downward solar radiation from the weather file. Consistent with USEPA
(2004), a daytime albedo value of 0.2 is recommended for bare soil. This value is
representative of conditions associated with the majority of bare soil pesticide applications
nationwide.
Sand (%), Clay(%): Soil textural components also need to be specified for the dynamic soil
temperature routine. The user will need to retrieve this data from the equivalent soil series of the
PWC crop scenario. Consistent with the other soil properties, the percent sand and clay contents
need to be included for each soil horizon specified. Soil property data throughout the soil profile
11
-------
is available from the USGS Soil Survey Geographic Database (SSURGO), and may be retrieved
following the steps below:
1. Proceed to the USGS SSURGO Web Soil Survey website at the link provided below:
http ://web soil survey. sc. eeov.usda. gov/ App/WebSoil Survey. aspx
2. Click on the download soils data tab.
3. Click on the Soil Survey Area (SSURGO) section, and fill in the information with the
area of interest.
4. With the results shown below the query area, click on the desired link containing the zip
file with the information for the county area of interest.
5. Download the zip file.
6. Extract the directory named (tabular) containing the soil profile data attributes, and save
it on a known location on your local computer.
7. Open the Microsoft access database file (.mdb) containing the soil database report
template, and enable all macros.
8. In the SSURGO Import Dialogue box which appears, copy the link to the location of the
tabular directory, and paste in the field.
9. After waiting several minutes for the soil profile data attributes to load, click on the soil
series of interest which appears in the dialogue box. After the soil series is selected, then
select the, "Physical Soil Properties", report name in the drop down menu below the soil
series in the dialogue box. After the preceding two entries are selected, click on the,
"Generate Report" button at the bottom of the dialogue box.
10. The tabular report of Physical Soil Properties, including the %Sand and %Clay contents
specified for each soil horizon, appears on the screen. Parameterize the PWC with the
data preserving the horizon-specific data to the existing scenario in the best manner
possible.
• More Output Menu
The PWC includes an additional menu with options to examine outputs related to the soil profile.
Specific functions include (see Figure 6):
1. Top left-hand section: Users may select outputs related to the residual pesticide mass
distribution retained in the soil as well as specific upward and downward fluxes within
the soil profile, including volatilization. These are outputted as daily values to the time
series file (.zts) upon running the PWC.
2. Bottom left-hand section: Users may also select daily outputs related to inputs,
movement, and removal of soil moisture (e.g., evapotranspiration, irrigation, infiltration,
and average soil profile moisture). These are also outputted to the .zts file.
3. Top right-hand section: Users have the option of examining the localities of the soil
compartments discretized in the Crop/Land menu, relative to user-specified depths of
12
-------
i nterest or to the depth of incorporation specified. After the user enters the depth (in
centimeters) of interest in the top box and clicks the "Node Examiner" button, the
closest node and the actual simulated depth appear in the text boxes below. While the
user should follow the guidance provided in this document on input incorporation depth,
the Node Examiner provides the actual incorporation depth simulated, based on the
closest soil compartment node.
|wc. Pesticide Water Calculator (PWC)
Help
| File | Scenario
Chemical j Applications j Crop/Land J Runoff [Watershed j Batch Runs ]7 More Options jj Out Pond j Out Reservoir j Out Custom j OutGW \ Advanced
F^ll a ^
Optional Waterbody Output Files
[T~] Create HED Files
Additional Frequency of Return (years): 15
Output water input, water removal, or
water movement parameters
Ready...
Working Directory: Working Directory: Not yet specified. Use save or retrieve.
IO Family Name: File name not yet determined. Use Save or retrieve.
Figure 6. More Output menu and parameters of interest relevant for volatilization
algorithm, and movement of chemical and water in the soil.
References
1. ACS, 1982. Handbook of Chemical Property Estimation Methods, Warren J. Lyman,
William F. Reehl and David H. Rosenblatt, editors, American Chemical Society, 1982.
2. Collins, W. D., 1925. Temperature of water available for industrial use in the
United States: U. S. Geol. Survey Water-Supply Paper 520-F, p. 97-104.
13
-------
3. Gilreath, J. P., Jones, J. P., Santos, B. M., and Overman, A. J.,2004. Soil Fumigant
Evaluations for Soilborne Pest and Cyperus rotundus Control in Fresh Market Tomato.
Crop Prot. 23: 889-893.
4. Jury, W. A., R. Grover, W. F. Spencer, and W. J. Farmer, 1983. Behavior assessment
model for trace organics in soil: I. Model description. Journal of Environmental Quality
12:558 - 564.
5. Macleod, M., Scheringer, M., and Hungerbiihler, K. 2007. Estimating Enthalpy of
Vaporization from Vapor Pressure using Trouton's Rule. Envion. Sci. Technol. 2007, 41:
2827 - 2832.
6. Millington, R.J. and Quirk, J.P., 1960. Permeability of Porous Solids. Trans. Faraday
Soc., 1961, 57, 1200-1207. DOI: 10.1039/TF9615701200.
7. Overman, A. J., Csizinszky, A. A., Jones, J. P., and Stanley, C. D., 1987. Efficacy of
Metam Sodium Applied via Drip Irrigation on Tomato. In: 46th Annu.Meet.of the Soil
and Crop Sci.Soc.of Fla., Oct. 14-16, 1986, Longboat Key, FL, Soil Crop
Sci.Soc.Fla.Proc. 4-7.
8. Staudinger, J. and Roberts, P.V., 2001. A Critical Compilation of Henry's Law Constant
Temperature Dependence for Organic Compounds in Dilute Aqueous Solutions.
Chemosphere, 44(4), 561-576.
9. Stull, R.B., 1988. An Introduction to Boundary Layer Meteorology, p. 11. Kluwer
Academic Publishers. Boston, MA.
10. USEPA, 2004. User's guide for the AERMOD Meteorological Preprocessor (AERMET).
USEPA, Office of Air Quality Planning and Standards, Emissions, Monitoring, and
Analysis Division, Research Triangle Park, North Carolina 27711. EPA-454/B-03-002,
November 2004 (http://www.epa.eov/ttn/scram/metobsdata procaccprogs.htm#aermef)
11. USEPA, 2013. Guidance on Modeling Offsite Deposition of Pesticides via Spray Drift for
Ecological and Drinking Water Assessments. USEPA Office of Chemical Safety and
Pollution Prevention, Office of Pesticide Programs, Environmental Fate and Effects
Division. December 20, 2013.
12. USEPA, 2015. Pesticide in Water Calculator User Manual. USEPA Office of Chemical
Safety and Pollution Prevention, Office of Pesticide Programs, Environmental Fate and
Effects Division. July 1, 2014.
14
-------
Appendix A - Limits of Volatilization Impact on Aquatic Exposure Estimates
16
14
12
10 i
s a
6
4
2
0
l.OOEll 1 JO (IE ID ljOOEuy lJOOEOS lJOGEir? IjOuEK 1J00EO5 lflOEW lJOOEtB 1.ULG-CQ lJOOEOl 1MEHX)
MaiVf LbiChw tort' MMtT' (L(f Scie)
Credit: Mohammed Ruhman
Figure A-l. Distribution of estimated exposure concentrations (EECs) in surface water for
surface applied compounds of same loading possessing range of Henry's Law Constants.
• >Podt
EECs without
Volatilization
—06 hr
21 Day
• ' \\
X
-
60 Day
—*-90 Day
V
IP—
—Yearfy
—•- All yeats
\
EECs with
Vnlatili7atinn
, , , 1
15
-------
Appendix B - Calculation of Henry's Law Constants and Heats of Henry
• Calculation of Unknown Henry's Law Constant at a Given Temperature: An
unknown Henry's Law constant can be calculated for one temperature given a known
Henry's Law Constant at another temperature according to a Van't Hoff relationship
derived from the Clausius Clapeyron equation as follows (Staudinger and Roberts, 2001):
Ahvoi = Heat of Henry or enthalpy of phase change for volatilization of a solute
from solution (J/mol)
R = universal gas constant =8.314 J/K/mol
Href = known Henry's Law constant at Tref (atm-m3/mol)
Tk = temperature for H(T) value of interest (K)
Tref = temperature at which Href was measured (K)
• Calculation of Heat of Henry (Ahvoi): Ideally, the Heat of Henry would be calculated
from two different Henry's Law Constants measured at two different temperatures. In
these cases, the Heat of Henry would be calculated with the following form of the Van't
Hoff relationship derived from the equation above:
Since Henry's Law Constants at two different temperatures may not be known many times,
there are two other different approaches that can be used to calculate or approximate the
Heat of Henry.
1. Heat of Henry can be estimated by the US EPA EPI Suite software. Open the software,
then select the HENRYWIN subprogram on the left of the EPI Suite screen. On the top
menu of the HENRYWIN window item, select the Show Options, then select Show
Temperature Variation with Results. Enter the chemical name of interest and then push
the Calculate button. EPI Suite will give the temperature variation results in the form
of an equation: HLC (atm-m3/mole) = exp(A-(B/T)) {T in K}. The enthalpy of
solvation in Joules/mol is equal to 8.314*B.
2. An alternative approach approximates the Heat of Henry using the chemical-specific
enthalpy of vaporization (Ahvap), which may also be calculated from the Van't Hoff
relationship using measured physical chemical properties, including vapor pressure and
boiling point according to the following equation:
H(T) = H
V K 1 Ref J
where: H(T) is the Henry's Law constant at temperature of interest
16
-------
R
(r~~t~)
^Ref lrtP<>
X In
VP
= A/i.
vap
BP'
where variables are the same as above except specified below:
Tref = temperature at which vapor pressure was measured (K)
Tbp = chemical-specific boiling point, obtained from product chemistry (K)
VP = chemical-specific vapor pressure, obtained from product chemistry (torr)
P = standard atmospheric pressure (= 760 torr).
Note:
1 If TK>TRef in the above equations, the negative sign from the equations should be
removed.
17
------- |