The acidity of water is its quantitative capacity to react with a strong base to designated pH or it can be defined as the base neutralizing capacity (BNC).In this article, we are going to read about the determination of acidity of water.
Dissolved CO2 is usually a major acidic component of unpolluted surface water. Industrial wastes usually contain mineral acidity.
Determination of acidity is important because acids contribute to corrosiveness and influence certain chemical and biological processes.These are the main sources of acidity in water.
Theory based on acidity of water sample calculation
Hydrogen ions present in a sample as a result of dissociation hydrolysis of solutes is neutralized by titration with standard alkali. The acidity thus depends on endpoint pH or indicator used. CO2 is usually a major acidic component of unpolluted surface water.
In a sample containing only CO2 – bicarbonate – carbonates, titration to pH 8.3 at 25oC corresponds to stoichiometric neutralization of carbonic acid to bicarbonate.
As the colour change of phenolphthalein indicator is close to pH 8.3, this value is generally accepted as a standard endpoint for titration of total acidity including CO2 and mineral acids.
Mineral acids are measured by titration to a pH of about 4.5 using methyl orange as indicator. Mineral acidity is also called methyl orange acidity.
Apparatus used for the acidity test of water
- Erlenmeyer flask
Reagents used in the acidity test of water
- CO2 free distilled water
- 0.02 N standard NaOH
- Methyl orange indicator
- Phenolphthalein indicator
Procedure for determination of acidity of water
- Pipettes V mL (say 50 ml) of the sample to the flask.
- Added 1 or 2 drops of methyl orange indicator.
- The sample is then titrated against 0.02N standard NaOH. The endpoint is noted as colour changed from orange-red to yellow. The titrate value is recorded as V1.
- Added one or two drops of phenolphthalein indicator.
- Titration is continued until the colour changed to faint pink. The volume of titrant used is noted as V2.
Acidity of water
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The below video shows the determination of acidity of water by titration