Gas Dissolution and Acidification of Natural Waters

The amount of a given gas in water depends on its ability to react chemically with water molecules or to dissolve as a free molecule, on its partial pressure in the atmosphere, and on its vapor pressure it also depends on both temperature and the amount of dissolved salts. 

Mass transfer phenomena govern the rate of dissolution of a gas due to the exposed water surface, local turbulence, and the degree of air and water mixing. Consequently, large water surfaces under turbulence (as the rapids in a river) favor gas dissolution in this way, a turbulent cold river is richer 

When the amount of a dissolved gas is small—as in the case of covalent, non-reactive gases in water under normal conditions—Henry s law governs this amount. This law states that the solubility of a non-reactive gas in water (or in any other liquid) at constant temperature is directly proportional to the partial pressure p, of that gas above the liquid. Because a higher dissolution means a higher concentration of the gas in the aqueous phase, one can write a simple mathematical expression for this law as  

Non-polar compounds such as CH4 or N2, do not easily dissolve in a polar medium like water, and they have small Henry s law constants. On the other hand, compounds such as S02 and NH3 have large constants due to their polar characteristics and their chemical reactivity in aqueous media. 

Henry s law works best with gases that are not too soluble, do not associate or dissociate in solution, and do not react chemically with the solvent. The components in a gas mixture act separately, and the mass of each gas that dissolves at a constant temperature is directly proportional to its Henry s law constant and to its partial pressure in the gas mixture. In addition, Henry s law is sometimes used in conjunction with the inverse process, 

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