pH is the scientific term used to describe how acidic or alkaline a solution is. Lemon juice and vinegar are acids, bicarbonate of soda is an alkali.

Acid conditions are usually caused when there is an excess acid causing hydrogen ions (H⁺) present. Alkaline conditions occur when there is an excess of alkaline ions (OH^-) present.

At pH7 there is an exact balance between OH^- and H⁺ ions. This pH is known as Neutral and is neither acidic nor alkaline.

Generally, fish from jungle streams prefer acidic conditions, while rift valley cichlids and marine species prefer alkaline conditions.

Fish have been acclimatised to pH as low as 3 and as high as 11, though this is not to be recommended.

The measurement of pH

pH can be measured using test kits, narrow range pH paper or electronic meters.

The pH scale

The pH scale is logarithmic, which means that there is a ×10 change in the number of H+ ions (acid causing particles) or OH- ions (alkaline causing particles) present for each change of pH of 1 unit.

Thus pH5 is ten times more acidic than pH6 and 100 times more acidic than pH7.

pH6 is ten times more acidic than pH7 (neutral).

pH9 is ten times more alkaline than pH8 and 100 times more alkaline then pH7.

pH8 is ten times more alkaline than pH7 (neutral).

A change of 0.5 of a pH unit alters the number of H⁺ ions or OH^- ions by a factor of approximately 5. Thus even a small change in a pH reading represents a large change in the concentration of either H⁺ or OH^- ions.

Buffering

Hard water and seawater contain dissolved materials that prevent rapid changes in pH - they are buffered.

The buffering system in seawater is overcome at about pH 8.1. Once the buffering system runs out a very rapid fall in the pH may occur jeopardising livestock.

Rising pH

At low pH's the toxicity of ammonia is low. Low pH's may be brought about by carbon dioxide, produced by animals all the time and by plants at night, dissolving in water and forming carbonic acid.

If water of a higher pH is added, particularly if buffered, then there may be a sudden increased rise in pH. Associated with the rise will be a rapid increase in the toxicity of any ammonia present. This situation may arise in the transport of livestock in either hard, or seawater.