Water Quality Parameters: Alkalinity

Alkalinity can be defined as the ability of water to neutralize acids. It is a measure of buffering capacity against a pH drop.

Sources:

Alkalinity results from the presence of the following ions in water:

1. Hydroxides ion: OH ⁻

2. Carbonates ion:  CO₃^-

3. Bicarbonates ion: HCO^-

These ions can be present with elements such as calcium, magnesium, Sodium, potassium and ammonia. Calcium and magnesium carbonates and bicarbonates are the most common. It should be mentioned here that wastewater is normally alkaline.

Impacts:

 →) Imparts a bitter taste to water.

→) Precipitate due to reactions between alkalinity and certain cations in the water can foul pipes and other waste water appurtenances.

Measurement:

→) Alkalinity measurements made by titrating water with an acid and determining the hydrogen equivalent.

→) Alkalinity is then express as mg/liter of CaCO3. If 0.02N H₂SO₄ is used in the titration, then 1mL of the acid will neutralize 1 mg of alkalinity as CaCO_3. Hydrogen ions from the acid react with the alkalinity according to the following equations:

          H⁺ +OH^- ↔ H₂O

          CO₃^-+ H⁺ ↔ HCO₃^-

          HCO₃^- + H⁺ ↔ H₂CO₃

→) If acid is added slowly to the water and the pH is recorded for each addition, a titration curve similar to that shown in Figure: 1. 

Use:

→)  Alkalinity is used to determine the buffering capacity of water.

→) Also used frequently as a process control variable in water and waste water treatment.

→) Maximum levels of alkalinity level have not been set by EPA for drinking water of for wastewater discharges.