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 at 25oC is 7, but this combination results in a pH of about 5.2 when exposed to carbon dioxide in the atmosphere.