SOP: Destruction of Organic Matter
GIFSQ — Global Institute for Food Safety & Quality
Principle
Fruits, vegetables and their products contain organic matter that must be destroyed before minerals can be estimated. Destruction is carried out by dry ashing or by wet digestion. The choice depends on the nature of the organic material, the inorganic constituents present, the metal to be determined and the sensitivity of the determination method. Dry ashing suits most common minerals except mercury and arsenic; calcium, phosphorus and iron in fruits and vegetables can be ashed safely by it, while potassium is retained only below about 480 °C and zinc only at or below 450 °C. Wet digestion is used for arsenic, copper, lead, tin and zinc. The ash or digest solution prepared here is the starting material for all the mineral determinations.
Apparatus required
- Long-necked 300-ml Kjeldahl digestion flask with B24 ground-glass joint
- Extension with side-tap funnel for introducing reagents (fits into an acid fume condenser)
- Acid fume condenser with water inlet, connection to pump and outlet to waste
- Mild steel rack with asbestos top, with circular holes sized so the flask receives no direct heat above the level of the acid
- Water (suction) pump
- Nickel tongs with jaws covered in asbestos string
- Muffle furnace, temperature controlled
- Silica dish
- Watch glass
- Water bath
- Volumetric flask, 100 ml
- Whatman No. 44 filter paper
Reagents
a. Concentrated nitric acid (HNO₃), analytical grade
b. Concentrated sulphuric acid (H₂SO₄), analytical grade
c. Perchloric acid, analytical grade
d. 30% (100-volume) hydrogen peroxide (H₂O₂), analytical grade
e. Dilute hydrochloric acid (HCl) 1+1
f. Glass beads
Use analytical grade reagents and distilled (preferably glass-distilled) water of low metal content throughout. Run a reagent blank using the same quantities of reagents as in the tests.
Sample preparation
Wet digestion using nitric and sulphuric acids
Weigh an amount of sample sufficient to contain 5 to 10 g of solids into the digestion flask; a sample containing not more than 5 g of solids (20 to 50 g of fresh sample) may be taken initially
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Add a few glass beads, 10 ml of H₂SO₄ and 10 ml or more of HNO₃ to keep the sample fluid
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Heat very gently until the liquid appreciably darkens in colour; avoid excessive frothing
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Add HNO₃ in small portions (1 to 2 ml) and continue heating until darkening takes place again; continue adding acid and heating to fuming for 5 to 10 min until the solution fails to darken
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When all organic matter is oxidized, allow to cool, add 10 ml of distilled water (the solution is colourless or pale yellow when iron is present) and boil gently to fuming
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Allow to cool again, add a further 5 ml of water and boil gently to fuming
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Cool and make up the digest to a known volume.
Notes: avoid charring, either initially or later, since arsenic, if present, may volatilize. If a wet sample is taken, boil it down to a small bulk with HNO₃ before adding H₂SO₄, then continue as for solid samples.
Wet digestion using sulphuric, nitric and perchloric acids
Weigh the sample into the Kjeldahl digestion flask
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Add a few glass beads, 4 ml of perchloric acid and sufficient HNO₃ to ensure complete oxidation (about 7 ml for every gram of material), then add 5 ml of H₂SO₄ and mix gently
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Apply very low heat slowly for 5 to 10 min until dense fumes appear; remove the burner for 5 min and allow to cool
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Replace the burner and continue digestion slowly at low heat for 5 to 10 min after the appearance of dense white fumes of H₂SO₄; then increase the heat and continue for 1 or 2 min — the liquid should be colourless or pale yellow if iron is present
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If any carbon remains, add 1 to 2 ml of HNO₃, digest again, cool and make up to a known volume.
Wet digestion using sulphuric acid, nitric acid and hydrogen peroxide
Digest the sample as under wet digestion using HNO₃ and H₂SO₄ until the digest on cooling is pale yellow or light brown
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Add 2 to 3 ml of 30% H₂O₂ and a few drops of HNO₃ at a time
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Heat to fuming after each addition until the residue is colourless or no further reduction of the pale yellow colour can be obtained
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Cool, dilute with 10 ml of distilled water and evaporate to fuming; again dilute with 5 ml of distilled water and evaporate to fuming
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Finally dilute the solution to volume with water.
Procedure — dry ashing
Weigh accurately a suitable quantity of the well-mixed sample into a tared silica dish (the sample used for moisture determination may be taken for ashing)
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Heat first over a low Bunsen flame to volatilize as much organic matter as possible, until no more smoke is given out
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Transfer the dish to the muffle furnace at about 300 °C until all carbon has ceased to glow, then raise the temperature to 420 °C (generally 450 °C; most fruits, vegetables and their products ash in 5 to 7 h; do not exceed 480 °C if potassium is to be retained or 450 °C for zinc)
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If all carbon is not oxidized, remove the dish, cool, add 1 to 2 ml of concentrated HNO₃, evaporate to dryness and heat in the muffle for another hour or so
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Remove, cool and, if required, note the weight of the ash
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Cover the dish with a watch glass and add gently 40–50 ml of dilute HCl (1+1) with a pipette (the cover prevents spattering)
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Heat over a water bath for 30 min, remove the cover and rinse it; continue heating for another 30 min to dehydrate silica
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Add another 10 ml of HCl (1+1) and water to dissolve the soluble salts
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Filter into a 100-ml volumetric flask using Whatman No. 44 filter paper, washing the residue in the dish once or twice with dilute HCl and the residue on the filter paper with HCl
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Make up to volume with water; the solution is the ash solution for mineral determination.
(To estimate silica: return the filter paper to the dish, ignite, place in the muffle for 1 h at 450 °C, cool and weigh.)
Precautions
- Perchloric acid digestions have caused explosions: carry out digestion in an isolated digestion room under a fume cupboard; wear a face mask; never raise the temperature or the burner until the vigorous oxidation by nitric and sulphuric acids has subsided; never let the digest dry — at least 2 to 3 ml of H₂SO₄ must always be present
- Keep a current of water flowing through the fume condenser and connect the upper outlet to the suction pump to remove acid fumes
- Use nickel tongs with asbestos-covered jaws to handle hot flasks
- Excessive heating may make some metallic compounds insoluble, as in the case of tin
- Always run a reagent blank with the same quantities of reagents
References
- Analytical Methods Committee, Analyst, 85 (1960). (The printed page number is OCR-illegible; carried without it.)