Translations:Wasserparameter bei der Wasseraufbereitung/18/en

And divides the pH values of dilute aqueous solutions into:

• pH < 7 as acidic aqueous solution, here it is cH3O+ > cOH−
• pH = 7 as neutral aqueous solution, here it is cH3O+ = cOH−; along with a property of pure water
• pH > 7 as basic (alkaline) aqueous solution, here it is cH3O+ < cOH−

Theoretically, chemically pure water at 22 °C has a pH value of 7 (the equilibrium constant for dissociation of water is about 10-14). This value is specified as chemically neutral. However, chemically pure water has no buffer and hence reacts to the slightest contamination with a significant change in pH value. Therefore due to CO2 solution, a pH value of 4.5 and 5 is immediately set in previously chemically pure water with admission of air. <ref name="Wiki">Wikipedia</ref>