pH scale

The concentration of H₃O⁺ in a solution ([H₃O⁺]) can be used as a measure of its acidity or basicity.

pH = – log [H₃O⁺]

– log 10^–6 = 6
– log 10^–2 = 2
– log 10^–0 = 0
– log 10⁴ = –4

[H₃O⁺]

0.000001 = 10^–6
0.01 = 10^–2
1 = 10⁰
10,000 = 10⁴

Because [H₃O⁺] in most aqueous solutions is very small, [H₃O⁺] is described more conveniently using the logarithmic pH scale defined as shown at the right. This scale was first used by the Danish chemist Soren Sorenson when working in his father's brewery.

pH meters are electrochemical cells that measure [H₃O⁺] in aqueous solutions and display the result as pH.

Solutions having higher [H₃O⁺] have lower pH because of the negative sign in the definition of pH.

The pH of pure water is 7 because [H₃O⁺] = 10^–7.

An aqueous solution of an acid has a higher [H₃O⁺] than pure water and a pH < 7.

Because aqueous solutions with higher [H₃O⁺] have lower [OH^–], pH indirectly measures [OH^–]

In pure water [H₃O⁺] equals [OH^–] which is equal to 10^–7

.

An aqueous solution of a base has a higher [OH^–] and a pH > 7 (because [H₃O⁺] is lower).