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.