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Design - Module Specifications

Constituent : con - Physical Parameters

Constituent ID:

This unique identifier provides absolute identification for a pollutant in the database. This identifier is the Chemical Abstract Service Registry (CAS) number for chemical pollutants and the radionuclide name for radionuclides. The radionuclide name is composed of the element symbol and atomic weight of a particular radionuclide (e.g., PU239 for the plutonium isotope with atomic weight of 239).

Full Name and Common Synonyms:

This field contains the full name of the pollutant and well known synonyms, as space allows.

Constituent Name:

A 20-character short name is provided for use in constituent lists (e.g., constituent search list).

Type:

The type parameter indicates whether pollutant is a chemical or radionuclide, and if a chemical, mode of toxicity (carcinogenic or non-carcinogenic) by inhalation and ingestion. usage of parameter is as follows:

Type 1 = radionuclide (EPA class A carcinogen)
Type 2 = carcinogen by inhalation and ingestion
Type 3 = carcinogen by inhalation, non-carcinogen by ingestion
Type 4 = carcinogen by ingestion, non-carcinogen by inhalation
Type 5 = non-carcinogen by inhalation and ingestion

Molecular Weight (g/g-mole):

The molecular weight of chemicals is determined by a chemical formula. For radionuclides, molecular weight is set to atomic weight, except for special cases, such as tritium, which is given molecular weight of water (tritiated, HTO). The molecular weight is used in several models, including soil and pond volatilization models, applicable to organic chemicals. For mixtures, an approximate molecular weight is entered based on either types of chemicals in mixture or, as in case of gasoline, constituent that poses greatest potential for risk (benzene).

Vapor Pressure (mm Hg):

The vapor pressure is pressure exerted by a material's vapor, at a given temperature, in equilibrium with pure compound (liquid or solid). The vapor pressure is used to estimate rate of emission of volatile chemicals from soil and water sources into atmosphere. Vapor pressures in database are generally taken at 25 °C. The Editor does not provide methods to estimate vapor pressure. Some of the values for vapor pressure have been estimated by methods based on structural information and boiling point of constituent. methods are indicated by references as follows:

Method VP-1 (Antoine Method) - used for liquids and gases (10^-3 to 760 mm Hg).
Method VP-2 (Modified Watson Method) - used for liquids and solids (10^-7 to 760 mm Hg).

Henry's Law Constant (atm m^3/g-mole):

Henry's Law Constant is ratio of concentration of a volatile chemical in air to concentration in an aqueous solution (at equilibrium). It can be used as a general indicator of volatility of a chemical, and to estimate amount of a volatile chemical available for inhalation during domestic showering. Values for Henry's Law Constant are reported in two forms: dimensional and non-dimensional. values contained in database are given in dimensional form. relationship between two forms is as follows:

H' = H / (RT)
where H' = non-dimensional Henry's Law Constant
H = Henry's Law Constant, atm m^3/g mole
R = Gas law constant, atm m^3/g mole K
T = absolute temperature, K.

At 20 °C, value for RT is 2.4x10^-2 atm m^3/g-mole, and conversion between forms of Henry's Law Constants is given by equations:

H' = 41.67 H
and
H = 0.024 H'

Values for Henry's Law Constants are based on measured values where possible. When an estimate must be relied upon, the value may be estimated as ratio of vapor pressure to water solubility, both values being taken at temperature of interest (25 °C) for gases; vapor pressure is assumed to be 760 mm Hg.

Water Solubility (mg/L):

The solubility of a pollutant in water is expressed in units of mg/L at a temperature in range of 20 to 30 °C. Measured values are used where information is available. Alternatively, values for some chemicals are based on estimation methods, referenced as follows:

Method S-1: non-acidic compounds when 0.5 < log Kow < 8
Method S-2: non-acidic compounds when -0.5 < log Kow < 0.5
Method S-3: acids with -0.5 < log Kow < 3.2

The measured solubility values provided in database were generally determined under laboratory conditions without considering effects of complex mixtures or varying aquatic chemical or geochemical conditions. These values should therefore be used with caution, as should values determined by estimation methods.

Carbon Matter Partition Coefficient, Koc (ml/g):

The carbon matter partition coefficient (Koc) describes partitioning of a chemical between aqueous phase and soil in contact with water. The carbon matter partition coefficient is used to estimate absorption coefficient, Kd.

When measured values of carbon matter partition coefficient are not available, an estimate can be made using correlation methods. The methods are referenced as follows:

Method K-1: correlation based on water solubility
Method K-2: correlation based on octanol-water partition coefficient

Octanol-Water Partition Coefficient, Kow (ml water/ml octanol):

The octanol-water partition coefficient describes equilibrium partitioning of a chemical between octanol and water phases. This parameter is used to estimate, as necessary, bioaccumulation in aquatic foods, concentration ratios for transfer between soil and plants, transfer factors between animal feed and animal products (meat and milk), and other physical parameters as described in these sections. Values for many chemicals are available from literature. When literature values are not available, estimation methods are available (as commercial software) based on chemical structure. Estimation methods for Kow are not available in the Editor.

Skin Permeability Constant, Kp (cm/hr):

The skin permeability constant defines rate of absorption of a pollutant through skin for exposure pathways involving dermal contact with aqueous solutions. Showering and swimming are typical exposure pathways using skin permeability constant to estimate dermal absorption. Values for permeability constants have been summarized by EPA (1992a) and many of these values have been entered into MMEDE database. There is also a method of estimation available in the complete Database document.

Gastrointestinal Absorption Factor:

The fraction of a pollutant absorbed on the passage through the human gastrointestinal system is provided in the MMEDE database. This parameter may be of use in comparing dermal absorption and ingestion exposure routes for chemicals.

Skin Absorption Fraction from Soil:

This parameter provides fractions of a pollutant in the soil on skin that transfers to blood during a typical soil contact exposure scenario. Values provided in MMEDE database are taken primarily from EPA compilation experimental measurements (EPA 1992a). The default value for organic chemicals is 1.0, and default value for radionuclides and inorganic chemicals is 0.001.

Constituent Database Links