fact section
Ref: L11

A moorland stream

Acidity of the water – pH values

pH is the unit used to describe the acidity of the water. A neutral figure is pH7. Higher figures show increasing alkalinity, lower figures increasing acidity.

The pH scale is logarithmic so an increase in acidity by one point (e.g. from pH6 to pH5) represents a ten times increase in acidity. Few animals can tolerate low pH (high acidity), fish are particularly sensitive (the EEC fish directive recommends a pH of 6 to 9). Less sensitive species include beetle and midge larvae.

To test the pH of water in a stream or river special paper which changes colour according to acidity can be used. This paper is called litmus paper.

Remember acidic water is not necessarily polluted water. The rocks and vegetation close to and in streams and rivers can cause acidity, this is especially the case in moorland areas.