Qualitative tests for minerals in food ash
Burn the food to ash, dissolve it in dilute acid, then use one reagent per mineral: oxalate for calcium, molybdate for phosphate, thiocyanate for iron.
Aim and principle
Aim: To prepare the ash solution of a food (for example ragi flour) and test it for calcium, magnesium, iron, phosphate, chloride and sulphate. Principle: Heating the food at 550-600 °C burns off the organic matter (carbohydrate, protein and fat) as carbon dioxide and water. The minerals stay behind as a white or grey ash, mostly as oxides, carbonates, phosphates and sulphates. Dilute acid dissolves the ash, so each mineral is present as an ion. Each ion then reacts with its own reagent to give an insoluble precipitate or a coloured complex: calcium forms white calcium oxalate, phosphate forms yellow ammonium phosphomolybdate, ferric iron forms red ferric thiocyanate or Prussian blue, chloride forms white silver chloride and sulphate forms white barium sulphate.
Requirements
| Requirement | What it does |
|---|---|
| Dilute hydrochloric acid (1:1) | Dissolves the ash, so the minerals go into solution as ions |
| Dilute nitric acid | Dissolves a separate portion of ash for the chloride test (HCl would add chloride) and gives the acid medium for silver nitrate |
| Ammonium hydroxide and dilute acetic acid | Adjust the pH: calcium oxalate comes down in a neutral to faintly acetic medium but dissolves in strong mineral acid |
| Ammonium oxalate | Precipitates calcium as white calcium oxalate |
| Ammonium chloride, ammonium hydroxide, disodium hydrogen phosphate | Precipitate magnesium as white magnesium ammonium phosphate; ammonium chloride stops magnesium hydroxide forming |
| Potassium thiocyanate, with a few drops of conc. nitric acid | Nitric acid makes sure all the iron is ferric; thiocyanate then gives blood-red ferric thiocyanate |
| Potassium ferrocyanide | Gives Prussian blue (ferric ferrocyanide) with ferric iron |
| Conc. nitric acid + ammonium molybdate | Give canary-yellow ammonium phosphomolybdate with phosphate on warming |
| Silver nitrate | Gives white curdy silver chloride with chloride |
| Barium chloride | Gives white barium sulphate with sulphate |
| Apparatus | Silica crucible, hot plate, muffle furnace, long tongs, desiccator, water bath, funnel and filter paper, 100 mL volumetric flask, test tubes, droppers |
Procedure
Steps 1-5 make the ash solution; step 6 tests 2 mL portions of it. Typical amounts are given. Follow the exact volumes in your lab manual.
- 1Weigh 5-10 g of the food into a clean, dry silica crucible.
- 2Char it slowly on a hot plate or low flame until it stops smoking.in a fume hood; do not let it catch fire
- 3Ash in a muffle furnace at 550-600 °C for 4-6 hours, until the ash is white or grey.cool in a desiccator
- 4Add about 5 mL dilute HCl (1:1) to the ash, warm on a water bath, and filter into a 100 mL volumetric flask.rinse the crucible into the flask with hot distilled water
- 5Make up to 100 mL with distilled water and mix. This is the ash solution.for chloride, take up a second ash portion in dilute nitric acid instead
- 6Take about 2 mL of ash solution for each test and add the reagents as in the table below.look at colours against a white background
Scheme: from food to ash solution
The same ash solution is used for the calcium, phosphorus and iron estimations in this unit.
Test, observation and inference
| Test | Observation | Inference |
|---|---|---|
| Calcium: 2 mL ash solution + ammonium hydroxide till alkaline, then dilute acetic acid till just acid; add 1 mL ammonium oxalate and stand 5 minutes | White crystalline precipitate, insoluble in acetic acid but soluble in dilute HCl | Calcium present (calcium oxalate) |
| Magnesium: to the filtrate of the calcium test add ammonium chloride, ammonium hydroxide and disodium hydrogen phosphate; scratch the inside of the tube with a glass rod | White crystalline precipitate forms slowly | Magnesium present (magnesium ammonium phosphate) |
| Iron (thiocyanate): 2 mL ash solution + 2 drops conc. nitric acid + 1 mL potassium thiocyanate | Blood-red colour | Iron present (ferric thiocyanate) |
| Iron (ferrocyanide): 2 mL ash solution + a few drops dilute HCl + 1 mL potassium ferrocyanide | Deep Prussian-blue colour or precipitate | Iron present (ferric ferrocyanide) |
| Phosphate: 2 mL ash solution + 1 mL conc. nitric acid + 2 mL ammonium molybdate; warm in a water bath at about 60 °C | Canary-yellow crystalline precipitate | Phosphate present (ammonium phosphomolybdate) |
| Chloride: 2 mL ash extract in dilute nitric acid + a few drops silver nitrate | White curdy precipitate that dissolves in ammonium hydroxide | Chloride present (silver chloride) |
| Sulphate: 2 mL ash solution + dilute HCl + 1 mL barium chloride | White precipitate, insoluble in conc. HCl | Sulphate present (barium sulphate) |
Precautions and result
- Wear gloves and goggles. Char the food, and handle concentrated nitric and hydrochloric acids, in a fume hood.
- Use long tongs and heat-proof gloves at the muffle furnace, and cool hot crucibles in a desiccator.
- Never pipette acids, ammonium oxalate or silver nitrate by mouth; use a pipette filler. Silver nitrate stains skin black.
- Use distilled water and clean glassware: tap water contains calcium, chloride and sulphate, and rusty stands add iron, giving false positives.
- Warm, do not boil, in the phosphate test (about 60 °C), and remove calcium before the magnesium test.
- Result: The ash solution of the given food (ragi) contains calcium, magnesium, iron, phosphate, chloride and sulphate. (Write only the minerals that tested positive.)
The ash value in food tables is the total mineral matter of a food. These quick tests show which minerals are present before you measure them. The same ash solution is then used to estimate calcium (bones and teeth), phosphorus and iron (anaemia), the minerals most often checked in Indian diet surveys. Ragi, for example, gives a heavy white calcium oxalate precipitate, a first sign of why it is promoted as a calcium-rich millet.
Short answer: “Why is food ashed before it is tested for minerals?” Ashing at 550-600 °C burns off the organic matter, which would hide colours and bind the minerals. The minerals stay in the ash and dissolve in dilute acid as ions.
Short answer: “Why is the ash taken up in nitric acid, not hydrochloric acid, for the chloride test?” Hydrochloric acid itself contains chloride, so silver nitrate would give white silver chloride even if the food had none. Nitric acid adds no chloride.
Structured question: “Describe the qualitative tests for calcium, iron and phosphate in a food ash.” Make the ash solution (ash at 550-600 °C, dissolve in dilute HCl, make up to 100 mL). Calcium: ammonium oxalate in a faintly acetic medium gives white calcium oxalate. Iron: thiocyanate gives blood red; ferrocyanide gives Prussian blue. Phosphate: conc. nitric acid + ammonium molybdate, warmed, gives canary-yellow ammonium phosphomolybdate. Present it as a Test | Observation | Inference table.
"Calcium is white like milk, iron is red like blood (or blue like ink), phosphate is yellow like a canary."