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.