The definition of pH is: The negative logarithm of the hydrogen ion activity (concentration) in equivalents per liter [eq./l] or written as an equation:
pH = -log[H+] = log 1/[H+] (3)
The mathematical expression, pH of 7, is much easier to say than “a hydrogen ion concentration of 0.0000001 gram equivalents per liter” as shown in Equation 4.
pH = 7.0 = -log(.0000001 eq.H+/1) (4)
pH = 5.0 = -log(.00001 eq.H+/1) (5)
pH = 9.0 = -log(.000000001 eq.H+/1) (6)
As shown in Equation 5, there is 100 times more hydrogen present for pH 5 than for pH 7 and similarly for pH 9, there is 100 times less hydrogen than for pH 7. The pH scale, by definition, ranges from 1 to 14. In perspective, the pH of lime juice is about 2, while aqua ammonia is about 12.