Interchange of Solutes between Solid, Liquid and Gas Phases - The Biogeochemistry of Submerged Soils - page 45

3.1.1 ACIDS AND BASES

The concentrations of dissolved species in natural waters depend ultimately on the
dissolution of basic rocks-carbonates, silicates and aluminosilicates-induced by
the action of weak acids in the water derived from dissolved gases-e.g. H 2 CO 3
derived from CO 2 . Anions produced in acid-base reactions balance cations pro-
duced in dissolution reactions. The charge balance is:

m [cation m + ] = n [anion n − ] ( 3 . 1 )

Table 3.1 shows the main weak acids present in natural waters and typical
concentration ranges. Table 3.2 shows the corresponding equilibrium constants.
Table 3.1 shows that carbonic acid is by far the dominant acid with concentrations
typically of the order of several mM. It arises from the dissolution of carbonate
rocks and atmospheric CO 2 , and from the respiration of aquatic and soil organ-
isms. The concentrations of dissolved silica are 5-10 times smaller, and those
of ammonium and orthophosphate smaller again, although NH 4 + concentrations
in the mM range may arise in the water in ricefields following fertilizer appli-
cations. The hydrolysis products of certain metals, such as Fe(III) and Al(III),
also behave as weak acids and may be important under particular circumstances.
Dissolved amino, organic and humic acids are rarely a large part of the charge
balance in solution but may be important as metal ligands.
The distributions of different acid-base pairs with pH are shown in Figure 3.1.
Bicarbonate ( HCO 3 − ) is the dominant carbonate species at near neutral pH;
silicic acid ( H 4 SiO 4 ) is essentially undissociated at all pHs of interest; and the
ammonium ion ( NH 4 + ) is the dominant form of ammoniacal-N at pHs below
about 8. Orthophosphate and sulfide have acidity constants near neutral pH.
For a given concentration of a particular dissolved acid, the proportions of the
component species in the equilibrium solution will depend on the alkalinity of
the solution; that is, the balance of cations and non-dissociating anions present.
This can be calculated as shown in Table 3.3 for the aqueous carbonate equilibria

Table 3.1 Concentrations of weak acids and bases in natural waters
Global mean
for freshwater a

Range in water Range in

submerged soil
solutions

Carbonate 0.97mM 0.01-10mM 5-100mM

Silicate 0.22mM 0.1-0.5mM 0.1-1.5mM

Ammonium 0-10 µ M 0.001-1mM 0.001-1mM

Phosphate 0 . 7 µ M 0 . 5-25 µ M 0 . 5-100 µ M

Sulfide — Trace 0 . 01-10 µ M

Amino acids — Trace 0 . 1-10 µ M

Organic acids — 0.001-1mM 0.1-10mM

Source :
a Morel and Herring (1993).