SOP: Determination of Total Ash Content
GIFSQ — Global Institute for Food Safety & Quality
1. Principle
Ash is the mineral residue that remains after complete incineration of a sample. A known quantity of sample is incinerated without flaming at a specified temperature until the residue is free from carbon. The residue is cooled in a desiccator, and the total ash is determined gravimetrically.
2. Apparatus required
- Silica crucible
- Muffle furnace (600°C)
- Oven
- Desiccator with effective desiccant (silica)
- Tongs
- Balance (±0.0001 g)
3. Procedure
Take a clean ashing crucible and dry it at 100°C for 30 minutes
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Cool in a desiccator to room temperature and weigh the crucible
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Weigh 3–5 g of well-mixed sample in the crucible
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Ignite the sample on a hot plate until charred
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Ignite the charred sample in the muffle furnace at 550°C for 3–4 hours, until light grey ash is obtained
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Allow the crucible temperature to fall to 100°C, then transfer it to a desiccator for cooling
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Repeat heating, cooling and weighing at half-hour intervals until the difference between two consecutive weighings is less than 1 mg
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Weigh the crucible with ash as soon as it reaches room temperature
4. Calculation
Total ash (%) = (Weight of ash after incineration ÷ Weight of sample taken for ashing) × 100
= [(Z − X) ÷ (Y − X)] × 100 (weight basis)
= [(Z − X) ÷ (Y − X)] × 100 × [100 ÷ (100 − M)] (dry basis)
Where,
– X = weight of empty crucible
– Y = weight of crucible with sample
– Z = weight of crucible with ash
– M = % moisture
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5. Precautions
- Ash containing black particles should be moistened with 2–3 drops of pre-heated water (60°C), dried at 100–110°C and re-ashed at 550°C until the ash is white or slightly grey.
- Clean crucibles carefully with a lab brush to remove adhering substances. Remove any stain with cold concentrated HCl. Never use abrasive substances such as sand, hot concentrated nitric acid, free alkalis or aqua regia.
- Do not transfer a crucible directly from high temperature into a desiccator — the thermal shock may crack the crucible and the desiccator.
- Keep the muffle furnace operating temperature within 550 ± 25°C.
References
AOAC, 21st edition, 2019; Official Method 923.03.