Henry constant values

The Henry s law constant value of 2.Ox 10 atm-m /mol at 20°C suggests that trichloroethylene partitions rapidly to the atmosphere from surface water. The major route of removal of trichloroethylene from water is volatilization (EPA 1985c). Laboratory studies have demonstrated that trichloroethylene volatilizes rapidly from water (Chodola et al. 1989 Dilling 1977 Okouchi 1986 Roberts and Dandliker 1983). Dilling et al. (1975) reported the experimental half-life with respect to volatilization of 1 mg/L trichloroethylene from water to be an average of 21 minutes at approximately 25 °C in an open container. Although volatilization is rapid, actual volatilization rates are dependent upon temperature, water movement and depth, associated air movement, and other factors. A mathematical model based on Pick s diffusion law has been developed to describe trichloroethylene volatilization from quiescent water, and the rate constant was found to be inversely proportional to the square of the water depth (Peng et al. 1994). 

The net retention volume and the specific retention volume, defined in Table 1.1, are important parameters for determining physicochemical constants from gas chromatographic data [9,10,32]. The free energy, enthalpy, and. entropy of nixing or solution, and the infinite dilution solute activity coefficients can be determined from retention measurements. Measurements are usually made at infinite dilution (Henry s law region) in which the value of the activity coefficient (also the gas-liquid partition coefficient) can be assumed to have a constant value. At infinite dilution the solute molecules are not sufficiently close to exert any mutual attractions, and the environment of each may be considered to consist entirely of solvent molecules. The activity... 

The transport processes that may move disulfoton from soil to other media are volatilization, leaching, runoff, and absorption by plants. Volatilization of disulfoton from wet soil may be greater than from relatively dry soil (Gohre and Miller 1986). Like other pesticides, disulfoton in soil partitions between soil-sorbed and soil-water phases (Racke 1992). This latter phase may be responsible for the volatilization of disulfoton from soil however, due to the low Henry s law constant value, the rate of disulfoton volatilization from the soil-water phase to the atmosphere would be low. 

Since 1,4-dichlorobenzene is slightly soluble (79 ppm at 25 °C) in water (Verschueren 1983), partitioning to clouds, rain, or surface water may occur. The Henry s law constant value (H),... 

In principle, the Henry constant may be predicted theoretically by evaluation of the configuration integral for an occluded molecule. Such calculations are subject to the considerable uncertainties implicit in theoretical potential calculations (17), and the utility of this approach is now limited to simple spherical molecules such as the inert gases (18, 19). A fair estimate of the standard entropy of sorption or of the value of K0 may, however, be obtained from a simple idealized model. 

Table I. Values of Kq and q0 Giving Temperature Dependence of Henry Constants for Sorption in 5A Zeolite and Chabazite according to Equation 2 ...

As the two sorbates methane and krypton on 5A appeared to have different mechanistic behaviour, further theoretical study appeared warranted. Two hypothetical gases P and Q whose properties are tabulated in Table 1 were used for comparison with the behaviour of methane and krypton. Hypothetical gas P was designed to have a Henry constant equal to methane, but to be a localized sorbate having entropy of sorption values decreasing incrementally as for krypton. Conversely, hypothetical gas Q had a Henry constant equal to that of krypton, but entropy of sorption values non-localized and decreasing incrementally as for methane. 

Now by Henry s law, 2 approaches a constant value, at infinite dilution which may be... 

Table 1 lists several compounds and their Henry s constants taken from Ashworth et al. [5]. For compounds of similar structure, heavier compounds tend to have smaller Henry s constant values. For compounds of similar size, those with polar functional groups (e.g., oxygen, nitrogen, sulfur) tend to have smaller Henry s constant values. This explains why methyl ethyl ketone (MW=72) has a Henry s constant that is orders of magnitude less than chloroethane (MW=64). 

Table 2 Temperature dependence of Henry s constant values...

The pte value of 4.4 (see Table 3-2) for DNOC suggests that in natural waters with a pH 5-9, >50% of the compound exists in the ionic state at pH 5 and the percent of ionic forms increases as the pH increases. In addition to this dissociation effect, DNOC may form H-bonds in water (EPA 1979), reducing its vapor pressure and chances of volatility from water. Using a Henry s law constant value of 1.4x10" atm-m /mole (Shen et al. 1982a, 1982b) and an estimation method (Thomas 1990), the estimated volatilization half-life of DNOC from a typical river 1 meter deep, with a current speed of 1 m/second, and an overhead wind speed of 3 m/second, is 36 days. Therefore, direct volatilization from water will not be significant for DNOC. 

In fact, it is a general expression for any composition, i.e., for any value of N. Only Y5 varies over the entire composition range. At very small values of the constituent normally obeys Henry s Law and 1b acquires a constant value. 

Here, G is the adsorption per gram of zeolite (at low vapor pressure, p, the value of G is close to the adsorption level, a) Ki and Ki are Henry constants characterizing the adsorbate—adsorbent interaction K2 and K2 are constants characterizing the pair-wise adsorbate-adsorbate interaction which is influenced by the adsorbent field. 

Figure 11. Dependence of the values of the Henry constant, InKi, on 1/T for the adsorption of Ne and A on zeolite NaA calculated curves and experimental points are shown...

Figure 1 shows the representation of the experimental isotherm (B. G. Aristov, V. Bosacek, A. V. Kiselev, Trans. Faraday Soc. 1967 63, 2057) of xenon adsorption on partly decationized zeolite LiX-1 (the composition of this zeolite is given on p. 185) with the aid of the virial equation in the exponential form with a different number of coefficients in the series i = 1 (Henry constant), i = 2 (second virial coefficient of adsorbate in the adsorbent molecular field), i = 3, and i = 4 (coefficients determined at fixed values of the first and the second coefficients which are found by the method indicated for the adsorption of ethane, see Figure 4 on p. 41). In this case, the isotherm has an inflection point. The figure shows the role of each of these four constants in the description of this isotherm (as was also shown on Figure 3a, p. 41, for the adsorption of ethane on the same zeolite sample). The first two of these constants—Henry constant (the first virial constant) and second virial coefficient of adsorbate-adsorbate interaction in the field of the adsorbent —have definite physical meanings. 

The coefficient of the first term of the virial series in Equation I is the inverse Henry constant for the temperature considered. From the graph of the isotherm in Figure 3 of the survey paper, it follows that the linearity of the initial section of the adsorption isotherm of ethane on zeolite LiX in accordance with the calculated Henry constant is observed for adsorption values not exceeding 0.7 mmole/g. According to K. N. 

Li per unit cell, isosteric heats were determined by R. M. Barrer and R. M. Gibbons for 68 Li" 6 Na", calorimetric measurements were made by N. N. Avgul, E. S. Dobrova, and A. V. Kiselev for 40 LL + 25 Na", calorimetric (points) and isosteric (filled curve) heats were obtained by N. N. Avgul, B. G. Aristov, A. V. Kiselev, L. Ya. Kurdyukova, and N. V. Frolova. In the case of GO2 adsorption, the calorimetric heat values coincide with the isosteric. These examples clearly show that the physicochemical constants calculated from experiments (Henry constant, second virial coeflBcient, corresponding heat of adsorption, etc.) are influenced by the zeolite structure and chemical composition. Therefore, it is quite necessary to indicate this composition in the representation and discussion of the thermodynamic results. Uncertain results were often obtained for zeolites having a binding material. 

The primary variable that determines whether the controlling resistance is in the liquid or gas film is the H or Henry constant. As shown in Figure 5.15, and as is apparent from equation 39, for small values of H the water phase film controls the transfer, and for high values of H the transfer is controlled by the air phase film. Gas transfer conditions that are liquid film controlled sometimes are expressed in terms of thickness, Zw, of the water film. As indicated by equation 38, this can be done from a measured value of (or K,o,) and the diffusion coefficient of the substance Zw decreases with the extent of turbulence (current velocity, wind speed, etc.). Typical values for are in the range of micrometers for seawater, a few hundred micrometers in lakes and up to 1 nun in small wind-sheltered water bodies (Brezonik, 1994). 

The Henry constant is defined as the limiting value of the ratio of the gas partial pressure to its mole fraction in solution as the latter tends to zero [8]. 

Usually, the differential enthalpy is determined from Equation (2) using two or more adsorption isotherms. Alternatively, the differential enthalpy can be measured directly using a calorimeter. In either case, a reference isotherm should be measured for the lowest temperature at which an accurate value of the Henry constant can be extracted. In the example shown in Figure 1, the reference isotherm is at 25 °C. For a particular gas and solid, the combination of a reference isotherm with the differentia] enthalpy provides complete thermodynamic information about the system. 

The Hashimoto process (b) was formd to be infeasible for 99.9% purity. In fact, the process is thermodynamically infeasible for the production of pure A. The scheme achieves only low performance for 90% purity. This is in agreement with literature [2]. The hypothetical option of a fully integrated process with distributed reactivity (c) allows for a significant improvement of process performance. This holds in particular for component A, where SR and EC are strongly reduced. The main reason is that here the required 3 / -value is 16% lower than in case (a) m [ is even lower than the Henry constant of B. The explanation is that any B in the reactive zone I also reacts to produce A, which is desorbed more easily and transported towards the non-reactive zones. A similar benefical effect (which is somewhat less pronounced) is found for m [v, which is higher for the fully integrated schemes than for the flowsheet-integrated processes. 

The H in Tables 2-123 to 2-134 is the proportionality constant in Henry s law, p = Hx, where x is the mole fraction of the solute in the aqueous liquid phase p is the partial pressure in atm of the solute in the gas phase and H is a proportionality constant, generally referred to as Henry s constant. Values of H often have considerable uncertainty and are strong functions of temperature. To convert values of H at 25 C from atm to atm/(mol/m ), divide by the molar density of water at 25 C, which is 55,342 mol/m. Henry s law is valid only for dilute solutions. 

In the simplest case the adsorption isotherm is of the Langmuir type with a steep slope at the initial part and reaching a saturation value at higher amounts of adsorbed solute. The slope of the isotherm corresponds to the distribution coefficient K, discriminating two parts of the isotherm the linear and nonlinear parts. In the linear range K is equal to the Henry constant. For the nonlinear part K becomes... 

Regardless of which method is used, the initial slope of the isotherm must be determined with great care. Either the lower bound for integration must be approximated with the lowest recorded concentration or the slope for zero concentration must be determined separately. As the latter task is identical to the determination of the Henry constant, this value can be obtained from pulse experiments with very low injection concentrations using momentum analysis (Section 6.5.7.2). 

This equation is widely used in gas-solid adsorption studies [4], especially to derive an accurate value of the Henry constant from experimental data, which can rarely be acquired at low enough partial pressures [18]. Equation 3.21 shows that a plot of log bP/tiads versus ngds is linear at low values of riads- The Henry constant is derived from its intersection with the ordinate axis. 

Note that K-cjg t is the Henry constant b as given in Eq. (9.16a) (mol kg Pa" or mol m" Pa" ) which can be directly determined from experiments without separate knowledge of the value of qgaf Equation (9.47) shows that the flux / is activated with an apparent activation energy (E, - Qa ) which is determined directly from permeation experiments. Since both parameters are positive quantities, positive as well as negative values can be expected and the flux can be increase as well as decrease with temperature depending on the relative values of Ej,- and Equation (9.47) has been used by several authors to describe, analyse and/or simulate permeation and diffusion in silica [59,63,92] and in zeolite membranes [69,72,75]. ... 

It is well known that the energy of interaction of an atom with the continuous solid is 2-3 times less than with the discrete (atomic) model (cf., e.g., Ref. [38], Figs. 2.2-2.4). Thus, to obtain the same Henry s Law constants with the two models, one has to increase e for the continuous model. This, however, does not discredit the continuous model which is frequently used in adsorption calculations. In particular, we can use the above mentioned results of Ref. [37] to predict the value of e for Ar which would have been obtained if one had carried out Henry s Law constant calculations for Ar in the AO model of Ref. [17] and compared them with experiment. One can multiply the value of e for CH4 obtained from AO model by the ratio of e values for Ar and CH4 in the CM model [36] to obtain tjk = 165A for Ar in the AO model. This is very close to the value of 160 K obtained in Ref. [21, 28] by an independent method in which the value of the LJ parameter e for the Ar - oxide ion interaction was chosen to match the results of computer simulation of the adsorption isotherm on the nonporous heterogeneons surface of Ti02. Considering the independence of the calculations and the different character of the adsorbents (porous and nonporous), the closeness of the values of is remarkable (if it is not accidental). The result seems even more remarkable in the light of discussion presented in Ref. [28]. Another line of research has dealt with the influence of porous structure of the silica gel upon the temperature dependence of the Henry constants [36]. 

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