Estimate the calcium content of the given food sample by the permanganate titration method.

1. 1 · Aim. To estimate the calcium content of the given food (ragi) by precipitation as calcium oxalate and titration with potassium permanganate.
2. 2 · Principle. Calcium is precipitated as calcium oxalate at pH about 5. The washed precipitate is dissolved in hot dilute sulphuric acid, freeing oxalic acid equal to the calcium, which is titrated with 0.01 N KMnO₄ (its own indicator, pale pink end point). Write the three reactions. 1 mL 0.01 N KMnO₄ ≡ 0.2 mg calcium.
3. 3 · Requirements.
   • Ash solution of the food
   • Methyl red, dilute ammonia, dilute acetic acid
   • Ammonium oxalate, 2% ammonia (wash)
   • 1 N sulphuric acid, 0.01 N potassium permanganate
   • Centrifuge, water bath, burette, pipettes, conical flask
4. 4 · Procedure.
   • 10 mL ash solution + methyl red; adjust to faint pink (about pH 5).
   • Add 5 mL ammonium oxalate; stand overnight.
   • Centrifuge; wash the precipitate with 2% ammonia.
   • Dissolve in 5 mL 1 N H₂SO₄; heat to 60-70 °C.
   • Titrate hot with 0.01 N KMnO₄ to a pale pink lasting 30 seconds; run a blank.
5. 5 · Observation. [Table: burette readings (initial, final, titre) for the sample trials and the blank; concordant value]
6. 6 · Calculation. Ca (mg/100 g) = (T − B) × 0.2 × (100 ÷ 10) × (100 ÷ sample weight). Example: (17.4 − 0.2) × 0.2 × 10 × 10 = 344 mg per 100 g.
7. 7 · Result. The calcium content of the given food (ragi) is 344 mg per 100 g.
8. 8 · Precautions.
   • Gloves and goggles; never pipette by mouth (oxalate is poisonous).
   • Keep the pH near 5.
   • Wash out extra oxalate.
   • Titrate hot; read the top of the KMnO₄ meniscus.

Scoring tip: Examiners look for the three reactions, the 0.2 mg factor with its basis, and a neat burette table.