Water quality concept: At what pH value would the alkalinity of water be at its maximum (considering the standard pH scale used in water treatment)?

Introduction / Context:
Alkalinity in water treatment refers to the acid-neutralizing capacity of water, primarily due to hydroxide, carbonate, and bicarbonate species. The pH scale governs the distribution of these species. Understanding where alkalinity peaks helps operators manage coagulation, corrosion control, and stabilization.

Given Data / Assumptions:

• Standard pH scale from 0 to 14 in aqueous systems.
• Alkalinity is the capacity to neutralize strong acid to a specific endpoint (often pH about 4.5).
• Speciation of carbonate system depends on pH.

Concept / Approach:

Alkalinity increases as pH rises because more bases (HCO3−, CO3^2−, and OH−) are present. At very high pH, hydroxide and carbonate dominate, maximizing neutralizing capacity for an equivalent total inorganic carbon. Therefore, the upper end of the pH scale corresponds to maximum alkalinity potential.

Step-by-Step Solution:

Verification / Alternative check:

Titration curves show greater acid required to reach the endpoint from high-pH waters than from neutral or low-pH waters, confirming increased alkalinity with rising pH.

Why Other Options Are Wrong:

• Zero / less than 7: Acidic to neutral conditions have low alkalinity.
• More than 7: True that alkalinity increases above 7, but the maximum within the scale is specifically at 14.
• 21: Not a standard pH value in aqueous systems.

Common Pitfalls:

• Confusing alkalinity with hardness; they are related but distinct.
• Assuming “more than 7” is the maximum, rather than identifying the upper bound 14.

Final Answer:

14.