Solubility Of Gases In Rain Fogs And Clouds Henrys Law And Aqueousphase Equilibria

The distribution of a species between the gas and the aquated forms due simply to physical solubility of the gas is described by the Henry's law constant, hx (Fig. 8.1). The Henry's law constant (hx) for a particular species X is, in effect, the equilibrium constant for the reaction

Table 8.1 shows the values of these constants for some species of tropospheric interest. The most soluble gases have Henry's law constants of approximately 105 M atm~', whereas the least soluble have values about eight orders of magnitude smaller.

However, for species such as C02 and S02 whose aquated forms can react further, much larger amounts of the gases can be taken up into solution. For C02, for example, the bicarbonate ion (HCO^) and the carbonate ion (CO2-) are formed as the aquated form of C02 reacts:

h[ = 3.4 X fCT2 M atm~(1,-1) C02 • H20(aq) ^ HCO3- + H +

FIGURE 8.1 Henry's law applied to atmospheric systems.

(The Henry's law and equilibrium constants are from Sillen and Martell, 1964.) In such cases, the Henry's law constant reflects only the physical solubility (i.e., reaction (1, — 1). A "pseudo-Henry's law constant," h*, is often defined to take into account the increased uptake compared to that expected based on simple dissolution of the gas without further reaction. With C02 as an example, this pseudo-Henry's law constant is defined by Eq. (A):

That is, h* is the sum of all aqueous-phase forms of the species divided by its equilibrium gas-phase pressure. The usual expressions for the equilibrium constants k2 = [HCO, ][H+]/[C02 • HzO] and k3 = [C02-][H+]/[HC0^] can be used to replace [HCO^] and [CO2-] in Eq. (A), resulting in the following expression (see Problem 1) for the pseudo-Henry's law constant as a function of pH:

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