SOP: Determination of Ash Content and Alkalinity of Ash

GIFSQ — Global Institute for Food Safety & Quality

Principle

The organic matter of the juice is burned off, leaving the mineral ash, which is weighed. The ash of fruit juices is alkaline (mainly carbonates of the alkali metals); its alkalinity is measured by warming with a known excess of standard hydrochloric acid and back-titrating with standard sodium hydroxide using methyl orange.

Apparatus required

• Evaporating dish
• Water bath
• Muffle furnace (capable of 550 °C)
• Burette

Reagents

a. 0.1 N hydrochloric acid.

b. 0.1 N sodium hydroxide solution.

c. Methyl orange indicator.

Procedure

Evaporate a weighed quantity of the sample on a water bath

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Ignite the dish on a burner and ash the residue at 550 °C

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Cool and weigh the ash

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Warm the ash with a measured excess of 0.1 N HCl, cool, and titrate the excess acid with 0.1 N NaOH using methyl orange as indicator

Calculation

Ash content, % (w/w) = (Weight of ash / Weight of sample taken) × 100

Alkalinity of ash is calculated from the acid consumed: 1 ml of 0.1 N NaOH = 0.0069 g of K₂CO₃

Alkalinity of ash (as K₂CO₃) = (ml of 0.1 N HCl added − ml of 0.1 N NaOH required for back-titration) × 0.0069 g

Precautions

• Evaporate on the water bath first so the sample does not froth or spatter when ignited.
• Ash at 550 °C as specified.

References

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