Acid Base Behavior

The species H3O+ represents the hydrated proton. Because of its small size and consequent high charge density, a proton will not exist alone in solution but will interact with the electron clouds of solvent molecules. In water, it will share one of the lone pairs from an oxygen atom. The actual species present probably is more complex than H3O+ (perhaps H9O+), but this is not particularly important for our purposes. This proton transfer process, however, is a very important type of reaction in aqueous chemistry, and acid-base behavior is widespread. Typical waters have a hydrogen ion concentration between 10—6 and 10—9 M (pH 6-9; pH = — log [H+]). This is a comparatively narrow range, which is determined by the balance of acidic and basic substances dissolved in the waters. Absolutely pure water at 25°C would have a hydrogen ion concentration of 10—7 M, from the equilibrium expression

Both biological processes and purely inorganic processes, such as dissolution or precipitation of minerals, are dependent upon pH. We are interested in what determines the pH of natural waters, and equally important, how resistant the pH value is to natural or human influences.

The oxygen atom of a water molecule possesses two pairs of nonbonding electrons (unshared pairs). These may be donated to empty, low-energy orbitals in another atom to form a coordinate bond, and such activity is the basis of Lewis acid-base behavior. This has particular importance with metal ions, as discussed in more detail later.

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