SOP: Determination of Alkalinity of Ash

GIFSQ — Global Institute for Food Safety & Quality

Principle

The ash is treated with a measured excess of standard acid, and the excess acid is back-titrated with standard alkali using methyl orange indicator. The alkalinity, expressed as potassium carbonate, is calculated from the acid consumed. The same titration is applied to the ash as a whole and, separately, to its water-soluble and water-insoluble fractions.

Apparatus required

• One of the silica dishes containing the total ash (from the total ash determination)
• Water bath
• Watch glass
• Burette

Reagents

a. 0.1 N hydrochloric acid (HCl).

b. 0.1 N sulphuric acid (H₂SO₄).

c. 0.1 N sodium hydroxide (NaOH).

d. Methyl orange indicator.

Procedure

Alkalinity of the ash

Add 10 ml of 0.1 N HCl or H₂SO₄ to the ash in the dish

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Dissolve the ash by warming on a water bath

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Cool and titrate the excess acid with 0.1 N NaOH using methyl orange indicator

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Record the volume of alkali consumed as B ml

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Carry out a blank titration on 10 ml of 0.1 N HCl and record the blank value as A ml

Alkalinity of water-soluble ash

Take the water-soluble fraction — the combined filtrate and washings from the water-soluble and water-insoluble ash determination

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Titrate it with 0.1 N H₂SO₄ or HCl using methyl orange indicator

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Calculate the alkalinity as below

Alkalinity of insoluble ash

Add 10 ml of 0.1 N HCl to the insoluble ash in the dish

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Cover with a watch glass and heat gently to boiling over a low flame

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Cool and titrate with 0.1 N NaOH using methyl orange indicator

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Carry out a blank titration and calculate the alkalinity as below

Calculation

1 ml of 0.1 N acid = 0.00691 g of potassium carbonate (K₂CO₃)

g of K₂CO₃ per g of ash = [(A − B) × 0.00691] ÷ Weight of ash taken

The alkalinity may also be expressed as the number of ml of 0.1 N acid required to neutralize the ash from 100 g of the sample.

Where,

• A = blank titration value (ml of 0.1 N acid)
• B = volume of 0.1 N alkali consumed in the sample titration (ml), equivalent to the excess acid
• Weight of ash taken = weight of the ash in the dish (g)

Precautions

• Warm the dish gently on the water bath until the ash dissolves completely.
• Cool the solution before titrating the excess acid.
• Run a blank titration alongside every sample titration.

References

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