SOP: Determination of Calcium

GIFSQ — Global Institute for Food Safety & Quality

Principle

Calcium is precipitated as calcium oxalate from the ash solution. The precipitate is filtered, dissolved in hot dilute sulphuric acid and titrated with standard potassium permanganate; the volume of permanganate consumed is proportional to the calcium present (1 ml of 0.01 N KMnO₄ = 0.2 mg of Ca).

Apparatus required

• Beaker, 250 ml
• Burette
• Whatman No. 42 filter paper
• Platinum wire or pointed glass rod

Reagents

a. Saturated ammonium oxalate solution

b. Methyl red indicator: dissolve 0.5 g methyl red in 100 ml of 95% alcohol

c. Dilute acetic acid 1+4

d. Dilute ammonium hydroxide 1+4

e. Dilute sulphuric acid 1+4: add the acid to water slowly with constant stirring; cool and make up to volume

f. 0.1 N potassium permanganate (KMnO₄)

g. 0.01 N potassium permanganate — working standard: dilute 10 ml of the 0.1 N solution to 100 ml with water (1 ml = 0.2 mg of Ca); prepare fresh solution before use

Sample preparation

Destroy the organic matter of the sample by dry ashing and prepare the ash solution as described in SOP: Destruction of Organic Matter.

Procedure

Pipette an aliquot (20 to 100 ml) of the ash solution into a 250-ml beaker; add 25 to 50 ml of water if necessary

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Add 10 ml of saturated ammonium oxalate solution and 2 drops of methyl red indicator

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Make the solution slightly alkaline with dilute ammonia, then slightly acid with a few drops of acetic acid until the colour is faint pink (pH 5.0)

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Heat the solution to the boiling point; allow to stand at room temperature for at least 4 h, preferably overnight

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Filter through Whatman No. 42 paper and wash with water until the filtrate is oxalate-free (absence of oxalate is tested with silver nitrate)

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Break the point of the filter paper with a platinum wire or pointed glass rod; wash the precipitate first with hot dilute H₂SO₄ (1+4) from a wash bottle into the beaker in which the calcium was precipitated, then wash with hot water

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Titrate while still hot (70 to 80 °C) with 0.01 N KMnO₄ to the first permanent pink colour; finally add the filter paper to the solution and complete the titration.

Calculation

Calcium, mg/100 g = (Titre × 0.2 × Total volume of ash solution × 100) ÷ (Volume of ash solution taken for estimation × Wt of sample taken for ashing)

Where,

• Titre = volume of 0.01 N KMnO₄ used in the titration (ml)
• 0.2 = mg of Ca equivalent to 1 ml of 0.01 N KMnO₄
• Total volume of ash solution = volume to which the ash solution was made up (ml)
• Volume of ash solution taken for estimation = aliquot taken for the determination (ml)
• Wt of sample taken for ashing = weight of sample ashed (g)

If the KMnO₄ standard solution is not exactly 0.01 N, correct the titre proportionally: Calcium, mg/100 g = (Titre × Normality of KMnO₄ / 0.01 × 0.2 × Total volume of ash solution × 100) ÷ (ml of ash solution taken for estimation × Wt of sample taken for ashing).