More about the pH of natural waters

The pH of natural waters depends on the relative concentrations of carbonate ions (CO32-), hydrogen carbonate ions (HCO3-) and dissolved carbon dioxide (CO2).

Rain water is usually slightly acidic (pH = 5.7) due to the reaction of water and dissolved carbon dioxide from the atmosphere that the rain has fallen through

Ground water is usually slightly alkaline (pH = 8.0) due to the reaction of water with dissolved carbonate ions from rocks that the water has passed over

Water in rivers and lakes can be acidic or alkaline depending on whether it is precipitation dominated, rock dominated or evaporation dominated.

The way that the relative concentrations of CO2(aq), HCO3-(aq) and CO32-(aq) vary with pH is shown below.

Note that the concentrations of CO2(aq) and HCO3-(aq) are equal at about pH = 6.3 and the concentrations of HCO3-(aq) and CO32-(aq) are equal at about pH = 10.3.

Also note that under acidic conditions (pH < 6.3) the dominant species is carbon dioxide, whereas under strongly alkaline conditions (pH > 10.3) the carbonate ion dominates. Under intermediate conditions (6.3 < pH < 10.3) the hydrogencarbonate ion dominates.

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