SOP: Determination of Total, Fixed and Volatile Acidity in Vinegar

GIFSQ — Global Institute for Food Safety & Quality

Principle

The acids in vinegar are determined by titration with standard sodium hydroxide. The total acidity is titrated directly; the fixed acidity is titrated after the volatile acids have been removed by repeated evaporation with water; and the volatile acidity is the difference between the two, or is determined directly by steam distillation of the volatile acids.

Apparatus required

• Porcelain dish, about 5 in. in diameter
• Distillation flask with condenser
• 25-ml graduated cylinder
• Litmus paper

Reagents

a. 0.5 N NaOH.

b. Phenolphthalein indicator.

c. Water.

Procedure

Total acidity: dilute 10 ml of the sample with water in the porcelain dish, add phenolphthalein, and titrate carefully with 0.5 N NaOH

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Fixed acidity: evaporate 10 ml of the vinegar at least 5 times with water and titrate as for total acidity

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Volatile acidity: express the volatile acidity by difference between total and fixed acidity

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Alternatively, steam distil the volatile acids and determine the acidity in the distillate: transfer 30 ml of the vinegar to a distillation flask and connect with the condenser; heat to incipient boiling and pass the steam; receive the distillate in the 25-ml graduated cylinder; after 15 ml have collected, test the drops of distillate from time to time with litmus paper as they fall into the receiving vessel, and when they are free from acid discontinue the distillation; note the volume of the distillate; mix by shaking and determine the acidity in an aliquot; express the volatile acids as acetic acid

Calculation

Total acidity as acetic acid % (w/v) = (Titre × Normality of alkali × 60 × 100) ÷ (Volume of sample taken × 1000)

Where,

• Titre = volume of standard NaOH used in the titration (ml)
• Normality of alkali = normality of the NaOH solution
• Volume of sample taken = volume of vinegar taken for the titration (ml)
• 60 = equivalent weight of acetic acid

Precautions

• Evaporate at least five times with water when determining fixed acidity, so that all volatile acids are driven off.
• During steam distillation, keep testing the falling drops with litmus until they are free from acid before stopping.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.