Question

Explain why for most natural waters alkalinity can be assumed to be: [HCO3-] + 2[CO32-].

Answer 1

Alkalinity of water refers to ability of water to neutralize strong acids.

In natural waters, alkalinity is mainly due to bicarbonate and carbonate ions present in water. Other basic ions like PO₄³⁻, SiO(OH)₃⁻, HSO₄⁻ etc., are present in insignificant quantity.

Neutralization reactions:

HCO₃^- + H+ --> H₂CO₃

CO₃^2- + 2 H⁺ --> H₂CO₃

1 mole of HCO₃^- neutralizes 1 mole of acid

1 moles of CO₃^2- neutralizes 2 moles of acid

Total alkalinity = [HCO₃^-] + 2 [CO₃^2-] + [OH^-] - [H⁺]

For natural water, [OH^-] = [H⁺] at pH = 7

Total alkalinity ~ [HCO₃^-] + 2 [CO₃^2-]