Estimation, from Henry’s law constant

Estimation from Henry s Law Constant In certain cases the water solubility at temperature T can be calculated as the ratio of the liquid vapor pressure at saturation, PSL, and Hc, or as the ratio of the solid vapor pressure at saturation, P, and Hc, if these data are known at T ... 

Volatilization t,/2 = 3000 d estimated from Henry s law constant for a body of water 1 m deep, flowing at 1 m/s and with a wind speed of 3 m/s (Howard 1991). 

We can calculate the amount of each substance in each phase, in both the initial state and final state of the isothermal bomb process, from the following information the internal volume of the bomb vessel the mass of solid or liquid reactant initially placed in the vessel the iiutial amount of H2O the initial O2 pressure the water vapor pressure the solubilities (estimated from Henry s law constants) of O2 and CO2 in the water and the stoichiometry of the combustion reaction. Problem 11.7 on page 361 guides you through these calculations. 

The vapor pressure, Pvp, of a liquid or solid is the pressure of the compound s vapor (gas) in equilibrium with the pure, condensed liquid or solid phase of the compound at a given temperature [5-9]. Vapor pressure, which is temperature dependent, increases with temperature. The vapor pressure of chemicals varies widely according to the degree of intermolecular attractions between like molecules The stronger the intermolecular attraction, the lower the magnitude of the vapor pressure. Vapor pressure and the Henry s law constant should not be confused. Vapor pressure refers to the volatility from the pure substance into the atmosphere the Henry s law constant refers to the volatility of the compound from liquid solution into the air. Vapor pressure is used to estimate the Henry s law constant [equation (2.4)]. 

Solubility is also used to estimate the Henry s law constant [equation (2.4)]. Solubility is the maximum amount of a chemical that can be dissolved into another at a given temperature. Solubility can be determined experimentally or estimated from molecular structure [6,10-12],... 

Volatilization volatilization t,/, = 296 h from a model river was estimated using Henry s law constant for a model river of 1 m deep with 1 m/s current and a 3 m/s wind (Howard 1989)... 

Even for reactions in which the Sij2 contribution to ionization is negligible, one does not have a means of estimating from solvolysis rates, the solvent activity coefficients for the transition state corresponding to ionization of RX. Although Vrx easily found from Henry s Law constants, and equation (16) does produce an activity coefficient for some transition state, this may not be a simple transition state corresponding to ionization of RX. Solvolysis rates may be smaller than ionization rates of many compounds, in certain solvents, because of ion-pair return, a phenomenon which has been firmly established by the investigations of Winstein et al. (1965). No matter whether Ag is a titrimetric rate constant 7... 

Major uncertainties have to be considered especially when an estimate from the Meissner method (Rechsteiner, 1990) is used instead of an experimental boiling point. Furthermore, these errors are propagated when, for example, calculated p values are used together with calculated data to estimate the Henry s law constant. 

We might try to estimate the Henry s law constant by extrapolating the vapor-pressure curve from the critical point, but the extrapolation is so large and thus uncertain that instead we normally use the experimental concentration values for dissolved gases, plotted as in Figure 3.9 this topic is discussed again in Chapter 9. 

The tendency of acrylonitrile to partition between air and water is described by Henry s law constant (H). The value of H for acrylonitrile has not been determined experimentally, but has been calculated to be 8.8 x 10 5 atm-m3/mole (Mabey et al. 1982). This value indicates that acrylonitrile will occur in both air and water, tending to transfer between air and water phases only slowly. Cupitt (1980) estimated the half-time of acrylonitrile clearance from air in wet precipitation to be greater than 10 months. 

In the multimedia models used in this series of volumes, an air-water partition coefficient KAW or Henry s law constant (H) is required and is calculated from the ratio of the pure substance vapor pressure and aqueous solubility. This method is widely used for hydrophobic chemicals but is inappropriate for water-miscible chemicals for which no solubility can be measured. Examples are the lower alcohols, acids, amines and ketones. There are reported calculated or pseudo-solubilities that have been derived from QSPR correlations with molecular descriptors for alcohols, aldehydes and amines (by Leahy 1986 Kamlet et al. 1987, 1988 and Nirmalakhandan and Speece 1988a,b). The obvious option is to input the H or KAW directly. If the chemical s activity coefficient y in water is known, then H can be estimated as vwyP[>where vw is the molar volume of water and Pf is the liquid vapor pressure. Since H can be regarded as P[IC[, where Cjs is the solubility, it is apparent that (l/vwy) is a pseudo-solubility. Correlations and measurements of y are available in the physical-chemical literature. For example, if y is 5.0, the pseudo-solubility is 11100 mol/m3 since the molar volume of water vw is 18 x 10-6 m3/mol or 18 cm3/mol. Chemicals with y less than about 20 are usually miscible in water. If the liquid vapor pressure in this case is 1000 Pa, H will be 1000/11100 or 0.090 Pa m3/mol and KAW will be H/RT or 3.6 x 10 5 at 25°C. Alternatively, if H or KAW is known, C[ can be calculated. It is possible to apply existing models to hydrophilic chemicals if this pseudo-solubility is calculated from the activity coefficient or from a known H (i.e., Cjs, P[/H or P[ or KAW RT). This approach is used here. In the fugacity model illustrations all pseudo-solubilities are so designated and should not be regarded as real, experimentally accessible quantities. 

As described earlier, Henry s law constants can be calculated from the ratio of vapor pressure and aqueous solubility. Henry s law constants do not show a simple linear pattern as solubility, Kqw or vapor pressure when plotted against simple molecular descriptors, such as numbers of chlorine or Le Bas molar volume, e.g., PCBs (Burkhard et al. 1985b), pesticides (Suntio et al. 1988), and chlorinated dioxins (Shiu et al. 1988). Henry s law constants can be estimated from ... 

Most agricultural pyrethroids have a very low vapor pressure (Vp) - around 10 8 mmHg at an ambient temperature - which is usually measured by the gas saturation method [8] and, therefore, its distribution to an air compartment is considered less important, as listed in Table 1. Tsuzuki [27] has improved the modified Watson method to estimate the vapor pressure of pyrethroids with reasonable precision just from their chemical structures. The volatilization from water can be conveniently evaluated by the Henry s law constant defined as vapor pressure divided by water solubility [28] and the small values of synthetic pyrethroids... 

Pollutants with high VP tend to concentrate more in the vapor phase as compared to soil or water. Therefore, VP is a key physicochemical property essential for the assessment of chemical distribution in the environment. This property is also used in the design of various chemical engineering processes [49]. Additionally, VP can be used for the estimation of other important physicochemical properties. For example, one can calculate Henry s law constant, soil sorption coefficient, and partition coefficient from VP and aqueous solubility. We were therefore interested to model this important physicochemical property using quantitative structure-property relationships (QSPRs) based on calculated molecular descriptors [27]. 

Based on its very small calculated Henry s law constant of 4.0xl07-5.4xl0"7 atm-m3/mol (see Table 3-2) and its strong adsorption to sediment particles, endrin would be expected to partition very little from water into air (Thomas 1990). The half-life for volatilization of endrin from a model river 1 meter deep, flowing 1 meter per second, with a wind speed of 3 meters per second, was estimated to be 9.6 days whereas, a half-life of greater than 4 years has been estimated for volatilization of endrin from a model pond (Howard 1991). Adsorption of endrin to sediment may reduce the rate of volatilization from water. 

The transport of disulfoton from water to air can occur due to volatilization. Compounds with a Henry s law constant (H) of <10 atm-m /mol volatilize slowly from water (Thomas 1990). Therefore, disulfoton, with an H value of 2.17x10" atm-m /mol (Domine et al. 1992), will volatilize slowly from water. The rate of volatilization increases as the water temperature and ambient air flow rate increases and decreases as the rate of adsorption on sediment and suspended solids increases (Dragan and Carpov 1987). The estimated gas- exchange half-life for disulfoton volatilization from the Rhine River at an average depth of 5 meters at 11 °C was 900 days (Wanner et al. ] 989). The estimated volatilization half-life of an aqueous suspension of microcapsules containing disulfoton at 20 °C with still air was >90 days (Dragan and Carpov 1987). 

Henry s Law constant (i.e., H, see Sect. 2.1.3) expresses the equilibrium relationship between solution concentration of a PCB isomer and air concentration. This H constant is a major factor used in estimating the loss of PCBs from solid and water phases. Several workers measured H constants for various PCB isomers [411,412]. Burkhard et al. [52] estimated H by calculating the ratio of the vapor pressure of the pure compound to its aqueous solubility (Eq. 13, Sect. 2.1.3). Henry s Law constant is temperature dependent and must be corrected for environmental conditions. The data and estimates presented in Table 7 are for 25 °C. Nicholson et al. [413] outlined procedures for adjusting the constants for temperature effects. 

The partitioning exhibited through the Henry s Law constant can be used to estimate the vaporization of various PCB contaminants from solid surfaces. In the presence of water, organic compounds volatilize more rapidly than would be expected based upon vaporization of the pure compound. This tendency accounts for the presence of low vapor pressure contaminants, such as the PCBs, in the atmosphere at higher concentrations than one would estimate from the chemistry of the pure compounds [403,408,409]... 

Physical and Chemical Properties. The physical and chemical properties of bromomethane are sufficiently well known to allow estimation of environmental fate. Although there is some disparity in reported values for the solubility in water and Henry s law constant for bromomethane (see Table 3-1), further studies to define these parameters more precisely do not appear essential, since volatilization from water is so rapid. 

The dominant fate process for chloroform in surface waters is volatilization. Chloroform present in surface water is expected to volatilize rapidly to the atmosphere. An experimental half-disappearance range of 18-25 minutes has been measured for volatilization of chloroform from a 1 ppm solution with a depth of 6.5 cm that was stirred with a shallow pitch propeller at 200 rpm at 25 °C under still air ( 0.2 mph air currents) (Dilling 1977 Dilling et al. 1975). Using the Henry s law constant, a half-life of 3.5 hours was calculated for volatilization from a model river 1 meter deep flowing at 1 meter/second, with a wind velocity of 3 m/second, and neglecting adsorption to sediment (Lyman et al. 1982). A half-life of 44 hours was estimated for volatilization from a model pond using EXAMS (1988). 

Vapor pressure The vapor pressure of a substance is defined as the pressure exerted by the vapor (gas) of a substance when it is under equihbrium conditions. It provides a semi-quantitative rate at which it will volatilize from soil and/or water. The vapor pressure of a substance is a required input parameter for calculating the air-water partition coefficient (see Henry s law constant), which in turn is used to estimate the volatilization rate of compounds from groundwater to the unsaturated zone and from surface waterbodies to the atmosphere. 

Surface Water. In a 5-m deep surface water body, the calculated half-lives for direct photochemical transformation at 40 °N latitude, in the midsummer during midday were 3.2 and 13 d with and without sediment-water partitioning, respectively (Zepp and Schlotzhauer, 1979). The volatilization half-life of benzo [a] pyrene from surface water (1 m deep, water velocity 0.5 m/sec, wind velocity 1 m/sec) using experimentally determined Henry s law constants is estimated to be 1,500 h (Southworth, 1979). 

A simple environmental chamber is quite useful for obtaining volatilization data for model soil and water disposal systems. It was found that volatilization of low solubility pesticides occurred to a greater extent from water than from soil, and could be a major route of loss of some pesticides from evaporation ponds. Henry s law constants in the range studied gave good estimations of relative volatilization rates from water. Absolute volatilization rates from water could be predicted from measured water loss rates or from simple wind speed measurements. The EXAMS computer code was able to estimate volatilization from water, water-soil, and wet soil systems. Because of its ability to calculate volatilization from wind speed measurements, it has the potential of being applied to full-scale evaporation ponds and soil pits. 

Henry s law constant for 1,3-DNB was estimated to be 2.33x10 atm-m /mol (HSDB 1994). Based on this value, volatilization from deep quiescent water bodies is expected to be a slow fate process for... 

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