pH determines the acid and base characteristics of water. A pH of 7.0 is neutral; values below 7 are acidic and values above 7 are alkaline. Excessively high or low pH levels are often associated with nutrient deficiencies, metal toxicities, or other problems for aquatic life. High pH makes ammonia more toxic. During algal blooms, photosynthesis increases the water pH, especially in stagnant or slow-moving water.

pH is measured by a sensing electrode for Hydrogen and a reference electrode along with a meter to measure the electrode potential. The YSI pH sensor is a glass bulb filled with a solution of stable pH (usually 7), so the inside of the glass surface experiences constant binding of H+ ions. The outside of the bulb is exposed to a water sample where H+ varies. The resulting differential of H+ creates a potential which is read by the meter versus the stable potential of the reference electrode.