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.

1. SHREE MALLIKARJUN COLLEGE CLASS: SYBSC
SAFETY: Refer to MSDS of sodium hydroxide, phenolphthalein and methyl orange.
-Dr. Mithil S. Fal Desai
Aim: Determination of acidity of a water sample.
Chemicals: Sodium hydroxide, phenolphthalein and methyl orange.
Apparatus: Standard volumetric flask, pipette, burette, conical flask, beakers, funnel and glass rod.
Theory: 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.
H+
+ (OH)-
→ H2O
Procedure:
1) Methyl orange acidity
Pipette out 100 mL water sample into a conical flask and add 1-2 drops of methyl orange (if the color of the
solution is yellow methyl orange acidity is absent). Rinse and fill the burette with standardized NaOH. Titrate
the red color of the water sample from the conical flask against standardized NaOH and note down the
constant reading.
2) Phenolphthalein acidity
Pipette out 100 mL water sample into a conical flask and add 1-2 drops of phenolphthalein. (if the color of
the solution is pink solution is highly basic). Rinse and fill the burette with standardized NaOH. Titrate the
colourless solution from the conical flask against standardized NaOH and note down the constant reading.
Results: i) Methyl orange acidity is = ___g/L of CaCO3
ii) Phenolphthalein acidity is =___g/L of CaCO3
Further reading (available in the library)
1) Experiments in Applied Chemistry, S. Rattan, S. K Kataria and Sons.
2) Vogel’s textbook of quantitative chemical analysis, J. Mendham, R. C. Denney, J. D. Barns, M. Thomas,
B. Sivasankar, Pearson India Education, 6th
Edition.
Questions.
1) Name any three primary standards.
2) What are primary standards?
3) What is the pH of distilled water?
4) What is the biological pH range?
5) What precautions you will take while handling strong acids/base?
6) What action a laboratory student must take in case of accidental spillage of strong acids/base?