Buffering of pH in natural waters

(1) Carbonic acid formation:

then carbon dioxide (CO2) enters water, a small amount hydrates to form carbonic acid (H2CO3):

CO2 + H2O = H2CO3

(2) Carbonic acid dissociation:

Some of the carbonic acid then dissociates into bicarbonate (HCO3), carbonate (CO32-) and hydrogen ions (H+):

H2CO3 = HCO3 + H+

HCO3 = CO32- + H+

(3) Hydrolysis of bicarbonate:

Bicarbonate and carbonate ions also dissociate to yield hydroxyl ions (OH):

HCO3 + H2O = H2CO3 + OH

CO32- + H2O = HCO3 + OH

Hydrolysis of bicarbonate (and carbonate) gives rise to a pH ‘buffering’ mechanism in waters that have appreciable bicarbonate concentrations. Hydrogen ions liberated during carbonic acid dissociation (2) neutralise the hydroxyl ions formed during the hydrolysis of bicarbonate (3). However, more hydroxyl ions are quickly produced and pH change is resisted unless the supply of bicarbonate and carbonate ions is exhausted. Bicarbonate and carbonate ions are derived principally from chemical weathering of silicate and carbonate rocks. Sulfate reduction also produces bicarbonate in an equivalent amount to the sulfate reduced.

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