pH is a measure of how acidic or basic water is. The pH scale ranges from 0 to 14, with 7 being neutral. A pH of less than 7 indicates acidity, while a pH greater than 7 indicates a base. The pH of pure water is 7 at 25°C, but when exposed to the carbon dioxide in the atmosphere, this equilibrium results in a pH of approximately 5.2 because CO2 in the air dissolves in the water and forms carbonic acid. The normal range for pH in surface water systems is 6.5 to 8.5, and for groundwater systems, it is 6 to 8.5.

The pH of water is an essential measurement concerning water quality. It can affect the solubility and toxicity of chemicals and heavy metals in the water, and if the pH of water is too high or too low, the aquatic organisms living within it will die. The majority of aquatic creatures prefer a pH range of 6.5-9.0, though some can live in water with pH levels outside of this range.

In general, a water with a low pH (< 6.5) could be acidic, soft, and corrosive. Therefore, the water could leach metal ions such as iron, manganese, copper, lead, and zinc from the aquifer, plumbing fixtures, and piping. On the other hand, basic water typically has a high concentration of healthy minerals.

It is essential to test the pH of water as soon as possible after the water sample is taken because the pH of water is associated with atmospheric gases and temperature. The pH of the water is not a measure of the strength of the acidic or basic solution and alone does not provide a full picture of the characteristics or limitations of the water supply.