The acidity of water is a measure of the quantitative capacity to react with a mineral base. The measured value usually varies with the pH indicator used in the experiment. The solution of phenolphthalein is colourless in acidic or mild basic medium (pH<8.0), while in basic medium (pH<9.6) solution is pink. The solution of methyl orange is generally red at pH lower than 3.1 and above pH 4.4 the solutions are yellow. The dissolved strong mineral acids, weak organic acids, dissolved carbon dioxide and hydrolyzed mineral salts may contribute to the measured acidity of natural water. The acidity of water is a significant parameter of water as the presence of acids in natural water contributes to corrosiveness and influences chemical reaction rates. The biological processes are dependent on the acidity of water. The quantitative acidity measurement also reflects the quality of the source of water. The acidity of water is usually expressed in terms of CaCO3.