Effective Henry coefficient

Fig. 4.7 Scheme of the processes while phase transferring. Indices oo far from interface, 0 beginning of diffusion layer, ads outer surface, aq inner surface and aqueous phase, diss -dissolved, prot - protolyzed form, k reactions rate coefficients in gas phase (g), at surface ads) and in aqueous phase aq). Dg diffusion coefficient gas phase, sorption equilibrium coefficient, Dag diffusion coefficient aqueous phase, Kag equilibrium coefficient of protolysis, H Henry coefficent, Hgff effective Henry coefficient, a accommodation coefficient, /uptake coefficient. 

The equilibrium (4.207) only describes the physical dissolved gas species (Fig. 4.7) without considering the subsequent protolysis equilibrium as discussed for the example of CO2 absorption in Chapter 2.S.3.2. In Eq. (2.117), we introduced an apparent Henry coefficient, where the dissolved matter comprises the anhydride (for example CO2 or SO2) and the acid (H2CO3, H2SO3). The acid can dissociate according to Eq. (4.174) and thereby increases the total solubility of gas A, as described by the effective Henry coefficient Hgf/. 

Dissociation of the neutral acid in water necessitates modifications for air-sea exchange in the model, which is based on Henry s law. Other possible pathways, e.g. sea spray, are neglected. Henry s law is restricted to concentrations of physically solved, non dissociated substances. Since only the non-dissociated acid is volatile, it is important to correct the air-water partition coefficient as to reflect the relative proportions of volatile and non-volatile components. The corrected parameter is the effective Henry s law coefficient, which is related to the Henry s law coefficient as a function of pH (modified Henderson-Hasselbalch equation) ... 

Cdg Concentration of D in gas phase, moles/volume Cdp Concentration of D in crystallite phase, moles/zeolite pore volume Deff Effective diffusion coefficient, (length) 2/time F Volumetric feed rate to reactor, volume/time H Henry s law-type constant relating gas phase mole fraction to crystallite phase mole fraction... 

The equilibrium concentration of the total amount of S(IV) dissolved in the liquid can be expressed according to Schwartz ( ) in terms of an effective Henry s Law coefficient H as... 

In the case of gases such as S02> which are anhydrides of weak acids (i.e., for SO2, sulfurous acid), assumption also of pertinent hydration and acid dissociation equilibria, which, together with the cloud droplet pH, determine the total solubility. This pH-dependent total solubility is conveniently expressed as an effective Henry s law coefficient, H. ... 

Henry s law coefficients and effective Henry s law coefficients for gases of concern in cloud chemistry are summarized in Figure 3. 

Figure 3. pH-Dependence of the effective Henry s law coefficient for gases which undergo rapid acid-base dissociation reactions in aqueous solution, as a function of solution pH, Buffer capacity of solution is assumed to greatly exceed incremental concentration from uptake of indicated gas. Also indicated at the right of the figure are Henry s law coefficients for non-dissociative gases. For references see (28). 

Solution of equations 5-1 and 5-2 requires site-specific values as well as chemical-specific values. Critical chemical-specific properties in the above equations are the Henry s law constant (H), the organic carbon/water partition coefficient (Koc), and the effective diffusion coefficient through soil (Deff). Henry s law constants are estimated using solubility and vapor pressure values obtained from data compilations. The Koc is estimated from the octanol-water coefficient (Kow) and is commonly estimated from the Hansch and Leo (1979) fragment constant approach (TPHCWG 1997b) ... 

To examine the effect of reaction kinetics on the reaction region the derived design criteria are applied for the reversible solid-phase reaction A B + C. A linear adsorption isotherm of the components is assumed, with Henry coefficients of 0.4 (reactant), 0.2 and 0.6 (products) respectively. A process with an equal number of columns in sections II and III is considered. The conversion that has to be reached is set to 99.99%. 

Equilibrium of adsorption on a solid is characterized by an adsorption isotherm, which shows the concentration on the solid as a function of the concentration in the contacting fluid. A quantitative measure of uptake of a gaseous species by a liquid is the distribution coefficient, defined as the ratio of the concentration on the solid to that in the contacting fluid. If concentration-independent, the coefficient is also called Henry coefficient. Diffusion of a species in a porous solid is expressed in terms of an effective diffusion coefficient, whose value accounts for the retardation by the solid matrix. Mass transfer to or from a solid is expressed in terms of a mass-transfer coefficient, the flux being the product of that coefficient and a concentration difference as "driving force."... 

Figure 7. The effective diffusion coefficient for small molecules in glassy polymers as a function of penetrant concentration. Dp is the mutual diffusion coefficient for the penetrant population dissolved by the Henry s-law mode. Other symbols are defined in the text.

The modified Henry s law coefficient relates the total dissolved S(IV) (and not only SOz H20) with the S02 vapor pressure over the solution. The effective Henry s law coefficient always exceeds the Henry s law coefficient, indicating that the dissociation of a species enhances its solubility in the aqueous phase. 

Another problem that is encountered in the calculation of atmospheric equilibrium concentrations is the dependence of the Henry coefficient on the concentration of dissolved ions, the so-called salting out effect. Difficulties are that (1) sometimes fresh-... 

The effective Henry s law constant for S02 increases by almost seven orders of magnitude as the pH increases from 1 to 8 (Figure 7.6). The effect of the acid-base equilibria is to pull more material into solution than predicted on the basis of Henry s law alone. The Henry s law coefficient for S02 alone, HSOl, is 1.23 M atm"1 at 298 K, while for the same... 

So under atmospheric conditions practically all dissolved ammonia in clouds is in the form of ammonium ion. The aqueous-phase concentrations of [NH ] in equilibrium with 1 ppb of NH3(g) are shown in Figure 7.9. The partitioning of ammonia between the gas and aqueous phases inside a cloud can be calculated using (7.9) and the effective Henry s law coefficient for ammonia, // = Wnh, Ka [H+]/ATm. If the cloud pH is less than 5 practically all the available ammonia will be dissolved in cloudwater (Figure 7.10). 

Formaldehyde has a Henry s law constant Hhcho = 2.5 M atm-1 at 298 K (Betterton and Hoffmann 1988), but its water solubility is enhanced by several orders of magnitude as a result of the diol formation. Combining the Henry s law equilibrium and the hydrolysis relationships, we can calculate the effective Henry s law coefficient for the total dissolved formaldehyde //((CHO as... 

Both Cg and Ceq are gas-phase concentrations and therefore have units of moles of A per unit volume of air, whereas W, has units of moles of A transferred per unit surface area per time. Using Henry s law, Ceq = Caq/ H RT), where H is the effective Henry s law coefficient of the species, and Caq is its aqueous-phase concentration (mol A per unit volume of droplet). Therefore (20.10) can be written as... 

Let us estimate the above timescales for a 10 pm radius droplet at 298 K. For O3, a gas with low water solubility, the calculated timescale using (11.62) is 5 x 10" s. For SO2 at pH 5, and an accommodation coefficient of 0.1, the timescale using (11.61) is approximately 0.05 s. Since the timescale depends linearly on the effective Henry s law constant, it will be even smaller for lower pH values, while it will increase to approximately 1 s at pH 6. For H2O2 with an accommodation coefficient on water over 0.2, the timescale is less than 0.1 s. For relatively soluble species like NH3 the characteristic time is roughly 1 s for a = 1 and 18 s for a = 0.1. For extremely soluble species like nitric acid the timescale... 

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