Quantitative estimation of calcium (permanganate titration)
Calcium is precipitated as calcium oxalate, dissolved in sulphuric acid, and the freed oxalic acid is titrated with KMnO₄: 1 mL 0.01 N ≡ 0.2 mg calcium.
Aim and principle
Aim: To estimate the calcium content of a food (ragi) using its ash solution. Principle: At a pH of about 5, ammonium oxalate precipitates calcium as white, insoluble calcium oxalate; magnesium stays in solution at this pH. The precipitate is washed free of extra oxalate and dissolved in hot dilute sulphuric acid, which releases oxalic acid in an amount equal to the calcium. This oxalic acid is titrated with standard 0.01 N potassium permanganate, which is reduced from purple MnO₄⁻ to colourless Mn²⁺. Once all the oxalic acid is used up, the next drop of KMnO₄ stays pink, so KMnO₄ is its own indicator. Each calcium ion gives one oxalate, which gives up two electrons, so the equivalent weight of calcium is 40 ÷ 2 = 20 and 1 mL 0.01 N KMnO₄ ≡ 0.2 mg calcium. Alternative: titration with EDTA at pH 12-13 using murexide or Patton-Reeder indicator.
Requirements
| Requirement | What it does |
|---|---|
| Ash solution of the food (10 g in 100 mL) | Source of calcium ions |
| Methyl red indicator, dilute ammonia, dilute acetic acid | Bring the pH to about 5, where calcium oxalate precipitates fully but magnesium does not |
| Ammonium oxalate solution (4% or saturated) | Precipitates calcium as calcium oxalate |
| Dilute ammonia (about 2%) | Washes away extra oxalate, which would otherwise use up KMnO₄ |
| 1 N sulphuric acid | Dissolves calcium oxalate to free oxalic acid, and gives the acid medium KMnO₄ needs |
| 0.01 N potassium permanganate | Titrant and its own indicator; standardise it against 0.01 N oxalic acid or sodium oxalate |
| Apparatus | Centrifuge with conical tubes (or ashless filter paper), water bath at 60-70 °C, burette, pipettes, conical flask, glass rod |
Procedure
Typical volumes for a food ash solution. Follow the exact volumes in your lab manual.
- 1Pipette 10 mL of ash solution into a centrifuge tube or small beaker.use a pipette filler
- 2Add 2 drops methyl red. Add dilute ammonia till yellow, then dilute acetic acid drop by drop till faint pink.about pH 5
- 3Add 5 mL ammonium oxalate solution, mix and stand at least 4 hours, preferably overnight.for complete precipitation
- 4Centrifuge (about 3000 rpm, 10 minutes) and pour off the liquid carefully.or filter through ashless paper
- 5Wash the precipitate 2-3 times with 2% ammonia, centrifuging each time.removes extra oxalate
- 6Dissolve the precipitate in 5 mL 1 N H₂SO₄, rinse into a conical flask with about 20 mL distilled water and heat to 60-70 °C.
- 7Titrate hot with 0.01 N KMnO₄ until a pale pink lasts 30 seconds. Record the titre (T); repeat for concordant values.
- 8Blank: titrate 5 mL 1 N H₂SO₄ + 20 mL water, heated the same way (B).
Scheme: calcium to titration
Reactions and the 0.2 mg factor
Worked example: calcium in ragi
10 g ragi flour was ashed, and the ash was dissolved and made up to 100 mL. Calcium in a 10 mL portion was precipitated as oxalate and titrated with 0.01 N KMnO₄. Burette readings: sample 17.4 mL, blank 0.2 mL. Find calcium in mg per 100 g.
- Net titre = sample − blank = 17.4 − 0.2 = 17.2 mL of 0.01 N KMnO₄.
- Factor: 1 mL 0.01 N KMnO₄ ≡ 0.2 mg calcium.
- Calcium in the 10 mL portion = 17.2 × 0.2 = 3.44 mg.
- Calcium in 100 mL ash solution (from 10 g ragi) = 3.44 × 100 ÷ 10 = 34.4 mg.
- Calcium per 100 g = 34.4 × 100 ÷ 10 = 344 mg.
- In one line: Ca (mg/100 g) = (T − B) × 0.2 × (total volume ÷ portion taken) × (100 ÷ sample weight) = 17.2 × 0.2 × 10 × 10 = 344.
Ragi contains about 344 mg calcium per 100 g, close to Indian food-table values (roughly 340-365 mg).
Precautions and result
- Wear gloves and goggles. Ammonium oxalate is poisonous and acids burn: never pipette by mouth, and handle concentrated acid in a fume hood.
- Keep the pH near 5: in strong acid calcium oxalate dissolves; in alkali magnesium and phosphate come down too.
- Wash the precipitate well with dilute ammonia; leftover oxalate uses up KMnO₄ and gives a falsely high value.
- Titrate hot (about 60-70 °C; some manuals say up to 80 °C, never boiling) because the reaction is slow in the cold; take the first pale pink that lasts 30 seconds.
- Read the dark KMnO₄ at the top of the meniscus. If it is not exactly 0.01 N, multiply the result by (actual normality ÷ 0.01).
- Result: The calcium content of the given food (ragi) is ___ mg per 100 g.
Calcium builds bones and teeth and is needed for muscle contraction, nerve signals and blood clotting. The ICMR-NIN 2020 RDA for adults is 1000 mg/day. Food calcium values from this estimation fill food composition tables and labels, and show why ragi (about 340-365 mg/100 g) is promoted over polished rice (only about 7-10 mg/100 g) for children, pregnant women and older adults at risk of osteoporosis.
Short answer: “Why is 1 mL of 0.01 N KMnO₄ taken as equal to 0.2 mg calcium?” Each calcium ion gives one oxalate, which loses 2 electrons to permanganate, so the equivalent weight of calcium is 40 ÷ 2 = 20. 1 mL of 0.01 N is 0.01 meq, and 0.01 × 20 = 0.2 mg.
Short answer: “Why is the oxalic acid titrated hot?” Permanganate oxidises oxalic acid slowly at room temperature. At 60-70 °C the reaction is fast, so the pale pink end point is sharp.
Structured question: “Explain the principle and calculation for estimating calcium in a food by the permanganate method.” Calcium in the ash solution is precipitated as calcium oxalate at pH about 5, washed with dilute ammonia, and dissolved in hot 1 N H₂SO₄ to free oxalic acid. This is titrated with 0.01 N KMnO₄ to a pale pink. Write the three reactions. Ca (mg/100 g) = (T − B) × 0.2 × (total volume ÷ portion) × (100 ÷ weight), e.g. 17.2 × 0.2 × 10 × 10 = 344.
"Two charges, two electrons": Ca²⁺ gives one oxalate that gives up 2 electrons, so 40 ÷ 2 = 20, and 0.01 N × 20 = 0.2 mg per mL.