From pH 0 to pH 7,solutions are acid.The activity of hydrogen ions can be most conveniently expressed in logarithmic units and the abbreviation “pH” is designated to represent the negative base 10 log of the hydrogen ion activity in moles per liter.
From pH 7 to pH 14, solutions are alkaline.
The pH may be expressed mathematically as follows pH = Log 1/[H+] where the H+ is the amount of hydrogen ions in solution in moles per liter.
Pure water is itself disassociated to a slight extent into the species H+ and OH-. This disassociation may be written as [H20] = [H+] + [OH-].
The equilibrium for this equation can be expressed as [H+][OH-]/[H20] = Kw. By convention, the activity of the liquid water is taken to be unity in this very dilute solution and the constant KW is then equal to the product of the activities of [H+] and [OH-]. This ion activity product for water at 25° C. is in exponential terms 10-14.000 .
The two place log of Kw is -14.00. By definition, the [H+] = [OH-] at neutrality and therefore pH = 7.
At higher temperatures, Kw increases, and the neutral value for pH becomes smaller. For instance, the neutral value pH at 30° C is 6.92. when there is an increase in the concentration of [H+] ions, a 7 resultant lowering of the pH value is observed. Conversely, a reduction in the [H+] concentration brings about a higher pH value.
From this relationship a pH scale has been devised. This scale ranges from 0 corresponding to an acidic solution with the hydrogen ion concentration equal to 1 through pH 7. The pH 7 is considered neutral. From neutrality to pH 14, the scale corresponds to a basic solution.