Which property does pH specifically describe in water quality?

• A. Total dissolved solids
• B. Turbidity
• C. Temperature
• D. Hydrogen ion activity

Answer: (D)

pH describes the activity of hydrogen ions in water, indicating how acidic or basic the water is. It is defined as the negative logarithm of the hydrogen ion activity, not the amount of dissolved solids, the amount of suspended particles, or the temperature. The pH scale (0–14 at 25°C) reflects how available H+ ions are in solution, with lower values meaning more acidic and higher values more basic. This is what drives chemical behavior in water systems, such as corrosion potential and disinfection effectiveness. The other properties—total dissolved solids, turbidity, and temperature—are different measurements and do not define pH.