How many mg/L of Alkalinity are needed to produce 1 mg/L NO₃?

To understand why the correct answer states that 7.1 mg/L of alkalinity is needed to produce 1 mg/L of nitrate (NO₃), it's important to comprehend the underlying chemistry of nitrification and the role of alkalinity in this process.

During nitrification, ammonia (NH₃) is oxidized to nitrite (NO₂) and then to nitrate. This biochemical process consumes alkalinity, which is typically measured in terms of calcium carbonate (CaCO₃) equivalents. The nitrification reaction is described by the following stoichiometric equation:

NH₃ + 2 O₂ → NO₃⁻ + H₂O + 2 H⁺

For every mole of nitrate produced, two moles of hydrogen ions are generated, which will decrease the pH of the water. To counter this acidity and maintain a stable pH level, a certain amount of alkalinity is required.

The relationship established through various studies indicates that approximately 7.1 mg/L of alkalinity (as CaCO₃) is necessary to neutralize the acidity produced for each mg/L of nitrate formed. This conversion factor arises from the stoichiometry of the chemical reactions involved and the buffering capacity of the water to