SOP: Determination of Acidity in Water

GIFSQ — Global Institute for Food Safety & Quality

Principle

The acidity of a water may be caused by the presence of uncombined CO₂, mineral acids, and salts of strong acids and weak bases. The determination is based on the titration of a sample with a standard solution of a strong base in the presence of an indicator. Titration to the methyl orange end point, pH 4.5, is arbitrarily defined as “free acidity”; it measures the relatively strong acids such as mineral acids. Titration to the phenolphthalein end point, pH 8.3, is defined as “total acidity”, and it includes the weak acids, acid salts, and some acidity due to hydrolysis.

Reagents

a. 0.02 N NaOH.

b. Phenolphthalein indicator.

Procedure

Determine total acidity on the spot, on a fresh sample collected in a bottle and stoppered immediately to prevent the escape of CO₂

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Take 50 or 100 ml of the sample in a conical flask and add 3 drops of phenolphthalein indicator

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Titrate over a white surface with 0.02 N NaOH until a faint pink colour appears

Calculation

Total acidity as CaCO₃, mg/litre = (ml of NaOH × Normality of NaOH × 50 × 1000) / (ml of sample)

Where,

• ml of NaOH = volume of standard NaOH solution used
• Normality of NaOH = normality of the standard NaOH solution
• ml of sample = volume of the water sample taken