What does pH measure in a solution?

• A. Oxidation-reduction potential of the solution
• B. Hydrogen ion concentration in solution
• C. Electrical conductivity
• D. Dissolved oxygen concentration

Answer: (B)

pH measures how acidic or basic a solution is by describing the amount of hydrogen ions present. It’s defined as the negative logarithm of the hydrogen ion activity, so pH ≈ -log10[a_H+]. In dilute aqueous solutions this activity closely mirrors hydrogen ion concentration, which is why pH is often described as a hydrogen ion concentration gauge. The scale ranges roughly from 0 to 14, with about 7 being neutral, values below 7 indicating acidity, and values above 7 indicating basicity. In wastewater treatment, pH is crucial because it affects chemical reactions (like metal hydroxide precipitation), microbial activity, and disinfection efficiency, as well as protecting equipment from corrosion. It’s different from oxidation-reduction potential, electrical conductivity, or dissolved oxygen, which measure other properties of the water.