Effective Henry’s law constants

There is evidence from laboratory studies that heterogeneous reactions on sulfate particles may be important in the upper troposphere as well. For example, HCHO uptake into sulfuric acid solutions or ternary mixtures of sulfuric and nitric acids and water has been observed in laboratory studies (e.g., Tolbert et al., 1993 Jayne et al., 1996 Iraci and Tolbert, 1997). In sulfuric acid, the effective Henry s law constant at the low... 

Table 8.3 shows the solubilities of some potentially important aldehydes in the form of the Henry s law constant (H) and the effective Henry s law constant (// ) (Betterton and Hoffmann, 1988 Olson and Hoffmann, 1989). These aldehydes not only dissolve in aqueous solutions but also hydrate to form gera-diols (Buschmann et al., 1980, 1982) ... 

Because of this hydration, the total solubility, i.e., effective Henry s law constant, is larger than expected based on physical solubility alone. The data in Table 8.3 show that most aldehydes have quite large effective Henry s law constants (// ), the exceptions being acetaldehyde and benzaldehyde. As a result of these high solubilities, significant concentrations can occur in fogs and clouds and hence be available to complex with S(IV). 

In this equation the brackets indicate molar concentrations for liquid phase species. The concentration of (S02)g must be given as partial pressure and H S02 represents the effective Henry s law constant of S02 considering the dissolution equilibria reactions of S(IV)aq As to be seen from Equation 8 the transformation of (S02)g to S(VI)aq in a multiphase system must be measured as a function of (S02)g/ L, [H202]aq and [H+]aq in order to determine the constant k4 and the exponents, p, X and . 

Noting the similarity of this expression to Henry s law, we can define the effective Henry s law constant for CO2, // 02, as... 

The effective Henry s law constant always exceeds the Henry s law constant... 

For pH <5, the dissolved carbon dioxide does not dissociate appreciably and its effective Henry s law constant is, for all practical purposes, equal to its Henry s law constant. For a gas-phase C02 mixing ratio equal to 330 ppm, the equilibrium aqueous-phase concentration is 11.2 pM (Figure 7.4). As the pH increases to values higher than 5,C02 H20 starts dissociating and the dissolved total carbon dioxide increases exponentially. However, even at pH 8, Hq0i is only 1.5 M atm-1, and practically all the available carbon dioxide is still in the gas phase. The aqueous-phase concentration of total carbon dioxide increases to hundreds of pM for alkaline water. 

FIGURE 7.4 Effective Henry s law constant for CO2 as a function of the solution pH. Also shown is the corresponding equilibrium total dissolved C02 concentration [COf] for a C02 mixing ratio of 330 ppm. 

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... 

This simplification frees us from the need to estimate the aqueous-phase concentration Caq (z, t). This will be a rather good approximation for a very soluble species, that is, a species with sufficiently high effective Henry s law constant H. Nitric acid, with a Henry s law constant of 2.1 x 105 M atm-1, is a good example of such a species. Recalling that dissolved nitric acid dissociates to produce nitrate, we obtain... 

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... 

One of the factors limiting the rate of heterogeneous reactions such as (9), (10) and (11) is the availability of the second reactant at the surface. For a trace species like HCl, this may be determined by the solubility of the species in sulfuric acid. Measurements of the effective Henry s law constant for HCl in sulfuric acid solutions between 50 wt.% and 60 wt.% [24] have been made. Because the solubility is low, the range of temperatures and compositions over which the Knudsen cell experiment could be performed is limited. However, extrapolation to room temperature measurements [28] looks reasonable, and the agreement with the results of other groups [29,30] is good. 

The solubility of HNO3 in sulfuric acid will determine whether the products of reactions (7) to (10) remain in solution or enter the gas phase. The effective Henry s law constant, H, for HNO3 in sulfuric acid as a function of temperature for solutions between 58 wt.% and 87 wt.% has been determined [35]. The data in Ref. [35] have been reanalyzed to correct an error of a factor of the square root of k in one of the equations, and with a steeper dependence of the viscosity (from which the diffusion... 

Table 1 Temperature dependence of effective Henry s Law constants for several species in H2SO4. standard state correction was used to convert AS = -30 cal/mol-K to an intercept. Assuming a standard state of 1 atm, the intercept is equ to (AS/4.58 + log [solvent]), where [solvent] is the molarity of the sulfuric acid. ( )A standard state correction was used to convert AS = -21 cal/mol-K to an intercept. Same as in (a). )A standard state correction was used to convert AS = -27 cal/mol-K to an intercept. Same as in (a).

For many gases, reversible ionization reactions occur and the ordinary Henry s law constant, H, is replaced by an effective Henry s law constant, /fy. In the case of SO2 this can be written as ... 

Less dissociation takes place if the pH is decreased, which also results in a decrease of the effective Henry s law constant and a further slowdown of the dissolution of SO2 in the aqueous layer [41]. 

Fig. 7.29 Effective Henry s law constant Kh. s (IV) of SO2 to water as a function of solution pH at 298 K(Seinfeld and Pandis 2006)...

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