pH is a measure of?

• A. Hydrogen ion activity
• B. Hydrogen ion concentration
• C. Acidity
• D. Alkalinity

Answer: (A)

pH is a measure of hydrogen ion activity—the effective concentration of H+ in the water. Activity reflects how those ions actually behave in the solution, taking into account interactions with other dissolved ions and the water’s overall ionic strength. Because of these interactions, pH isn’t just a simple count of hydrogen ions; it’s the logarithmic scale of their activity.

Acidity and alkalinity describe the water’s buffering capacity—the ability to neutralize added bases or acids—based on dissolved species like carbonates, bicarbonates, and minerals. Those properties are not directly what pH quantifies, which is why pH specifically reflects hydrogen ion activity rather than the total acidity or buffering capacity.