---

More about the pH of natural waters

---

The pH of natural waters depends on the relative concentrations of carbonate ions (CO₃^2-), 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

H₂O(l) + CO₂(aq) HCO₃^-(aq) + H⁺(aq)

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

H₂O(l) + CO₃^2-(aq)HCO₃^-(aq) + OH^-(aq)

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 CO₃^2-(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 CO₃^2-(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.