To determine the acidity of the given sample of water.
Principle Acidity of water is its quantitative capacity to neutralise a strong base to a designated pH. Strong minerals acids,
weak acids such as carbonic and acetic and hydrolysing salts such as ferric and aluminium sulphates may contribute
to the measured acidity. According to the method of determination, acidity is important because acid contributes to
corrosiveness and influences certain chemical and biological processes. It is the measure of the amount of base
required to neutralise a given sample to the specific pH.
Hydrogen ions present in a sample as a result of dissociation or hydrolysis of solutes are neutralised by titration
with standard alkali. The acidity thus depends upon the end point pH or indicator used. Dissolved CO2 is usually
the major acidity component of unpolluted surface water. In the sample, containing only carbon dioxide-bicarbonatecarbonate,
titration to pH 8.3 at 25°C corresponds to stoichiometric neutralisation of carbonic acid to carbonate.
Since the colour change of phenolphthalein indicator is close to pH 8.3, this value is accepted as a standard end
point for the titration of total acidity. For more complex mixture or buffered solution fixed end point of pH 3.7 and
pH 8.3 are used. Thus, for standard determination of acidity of wastewater and natural water, methyl orange
acidity (pH 3.7) and phenolphthalein acidity (pH 8.3) are used.