pH Scale

In aqueous solutions of acids and bases the concentration of H₃O⁺ and OH^- ions may vary from about 10 M to 10^-14 M. It is inconvenient to express these concentrations by using powers of 10. In 1909, S.P.L. Sorensen proposed a logarithmic scale (called pH scale) for expressing the concentrations of H⁺ ions. He defined pH as the negative logarithm of the molar concentration of hydrogen ions.

pH = -log₁₀ [H⁺]

pH = -log₁₀ [H₃O⁺]

For neutral solutions (or pure water)

[H₃O⁺] = [OH^-] = 1 × 10^–7

pH = 7.0

For acidic solutions

[H₃O⁺] > [OH^-]

pH < 7.0

For basic solutions

[H₃O⁺] < [OH^-]

pH > 7.0