The scientific definition of pH is that it is a hydrogen ion activity indicator (H+) and is stated as the inverse of the hydrogen ion activity logarithm.
Therefore there are 10-7 moles per litre of hydrogen ions in water with a pH of 7; while a pH of 6 is 10-6 moles per litre. The scale of the pH varies between 0 and 14.
Water with a pH of < 7 is considered acidic, and water with a pH of > 7 is considered basic.
The standard range of pH is 6.5 to 8.5 for surface water systems and 6 to 8.5 for groundwater systems.
Alkalinity is a measure of the water's capacity to tolerate a pH change that appears to make the water more acidic.
To calculate the corrosivity of the water, the calculation of alkalinity and pH is required. The pH of pure water (H2​0) at 25oC is 7, but this combination results in a pH of about 5.2 when exposed to carbon dioxide in the atmosphere.