The pH scale

In pure water, for every dissociated H⁺ there is a OH^- ion. In other words, the ratio of H⁺ to OH^- is always 1:1.

In 1909, a chemist called Sørensen devised a system of measuring the amount of free H⁺ ions in a solution. He called it the pH scale because, in German, potenz Hydrogen means “Hydrogen concentration”.

Sørensen’s scale ran from 0 to 14. He based it on pure water, which he placed in the middle at pH 7.

Solutions with a pH of 7 are neutral. This means that for every H⁺ ion in solution there is a matching OH^- ion.

In the diagram on the right, you can see:

• an empty beaker
• a line representing the pH scale and
• a bar graph, showing the relative concentrations of H⁺ and OH^- ions

When pure water is added to the beaker there are equal number of H⁺ and OH^-. The pH of pure water is 7 or neutral.