pH is a direct measure of the ____ of the ____ ion, and an indirect measure of the ___ion activity in water or sewage.

• A. Hydrogen ion activity and sulfate ion activity
• B. Oxygen ion activity and nitrogen ion activity
• C. Hydrogen ion activity and hydroxyl ion activity
• D. Calcium ion activity and magnesium ion activity

Answer: (C)

pH reflects how acidic or basic a solution is by measuring the activity of hydrogen ions. The pH value is defined as the negative logarithm of the hydrogen ion activity, so it directly tells you about H+ in the water. Because water autoionizes (H2O ⇌ H+ + OH−) and the activities are linked by Kw, higher hydrogen ion activity means lower hydroxyl ion activity, and vice versa. This makes pH a direct measure of hydrogen ion activity and an indirect measure of hydroxyl ion activity. At standard temperature, pH and pOH add up to about 14, reinforcing their connected relationship. In water or sewage, pH information helps predict chemical reactions, corrosion potential, and ion speciation, which are central to treatment processes. Non-hydrogen ions like sulfate, oxygen, nitrogen, calcium, or magnesium aren’t what pH directly measures, so they don’t fit this description.