Acidity:

The acidity of water is a measure of its capacity to neutralize bases. Acidity of water is caused by the presence of uncombined carbon dioxide, mineral acids, salts of strong acids and weak bases. It is expressed as mg/1 in terms of calcium carbonate.

Acidity in unpolluted water is usually due to dissolved carbon dioxide, which produces weak carbonic acid. Humic, fulvic and other organic acid produced by decomposition of vegetation also give rise to acidity in water but the pH value is usually more than 3.7. Due to presence of mineral, acids, water polluted by industrial effluents has pH less than 3.7. Acidic water having pH less than 4.5 is considered to be undesirable as water becomes unpalatable and it affects aquatic life besides causing corrosion in water carrying pipes, etc.