Application principles and foundations
Ash determination aims to accurately quantify the content of inorganic minerals that remain after complete combustion of a sample under specific high-temperature conditions. The core function of the high-temperature muffle furnace is to ensure sufficient oxidation of the sample's organic components while avoiding volatilization or agglomeration of inorganic matter due to temperature fluctuations or local overheating, thereby guaranteeing accuracy and reproducibility of results. The determination typically consists of the following steps: sample weighing, loading into a crucible, predetermined heating and thermostatic burning in a muffle furnace, weighing the residue after cooling, and calculating the ash mass fraction based on a formula.
Key parameter setting
The accuracy of ash determination is highly dependent on the temperature control accuracy of the muffle furnace and the programmed temperature rise and fall capability. Standard methods for different materials have clear requirements for temperature and time, such as:
Typical application scenarios
1. Food and feed industry: According to international standards, samples are usually burned at 525°C ± 15°C for 4-6 hours to constant weight. This condition completely breaks down proteins, fats, and carbohydrates, retaining mineral salts and trace elements.
2. Coal and coke field: The standard method mostly uses slow heating to 500°C and then rising to 815°C ± 10°C for 2 hours to ensure that sulfur, silicon and other elements are not lost.
3. Soil and fertilizer testing: It is often set to 550°C ± 20°C for 4 hours to determine the difference in organic matter content. Too low a temperature may prevent the organic carbon from being completely removed, and too high a temperature may cause carbonate decomposition.
4. Polymers and composites: To avoid violent burning splashes, it is recommended to use segmented heating: first maintain at 300°C for 30 minutes, then increase to 600°C and maintain a constant temperature for 1 hour. This method prevents packing loss.
Temperature calibration tips
The temperature distribution of the muffle furnace is not absolutely uniform, so it is recommended to calibrate the commonly used working areas in the furnace (e.g., front, middle, and rear points) with a meter-certified platinum-rhodium thermocouple every month. If the temperature difference exceeds ±10°C, adjust the thermostat parameters or optimize the placement of the crucible (avoid being close to the furnace door and heating elements). The calibration results should be recorded in the technical file.
Constant weight operation requirements
Constant weight is the basis for determining whether burning is complete in ash determination. Move the crucible and ash to the dryer to cool to room temperature, weigh accurately, and then return to the muffle furnace to continue burning for 30-60 minutes, cool and weigh until the two mass differences do not exceed 0.5 mg. Failure to achieve constant weight may indicate the presence of incompletely oxidized carbon particles (longer burning time) or constant loss of volatile inorganic matter in the sample (temperature reduction).
Common interference and exclusion
1. Sample splashing: Caused by excessive furnace temperature during the initial heating phase. The countermeasure is to use the program to heat up or carbonize at low temperature first and then ash at high temperature, and cover the crucible with a lid with a seam.
2. The furnace door is not tightly sealed: the entry of outside air affects the oxidation efficiency or increases the weight of ash. Check the seal and make sure there are no visible gaps when the furnace door is closed.
3. Crucible material influence: Porcelain crucibles may react with alkaline samples at high temperatures. A platinum or quartz crucible is recommended for the determination of samples with high salt content.
Safe operation reminders
Although the temperature of the muffle furnace shell is insulated, it may still reach more than 100°C, and high-temperature resistant gloves must be worn during operation. Corrosive gases (e.g., chlorine, sulfur dioxide) may be produced by combustion of different samples, so ensure that the muffle furnace is equipped with smoke exhaust ducts or placed in a fume hood. When not used for a long time, the furnace door should be slightly opened and kept dry to prevent the refractory brick from absorbing moisture and cracking.
Data calculation example
The constant weight mass of the crucible is m0, the total mass after adding the sample is m1, the total mass of the crucible and ash after burning is m2, then the ash mass fraction A is calculated according to the following formula (in g/g):
A = (m2 - m0) / (m1 - m0) × 100%
Example: If m0 = 20.000 g，m1 = 22.000 g，m2 = 20.245 g, then: (20.245 - 20.000) / (22.000 - 20.000) × 100% = 12.25%.
Routine maintenance guide
- Clean up the residue that falls from the bottom of the furnace chamber after each use, use a vacuum cleaner or soft brush, and avoid using hard objects to scratch the refractory layer.
- Check the connection between the thermocouple and the thermostat every week for oxidation and looseness, and replace the contacts if necessary.
- Test whether the furnace door switch and smoke exhaust fan are operating properly every month to ensure that the ventilation volume meets the maximum usage needs.
Quote the instructions
1. The ISO 2171:2020 Ash content determination of cereals and legumes by the International Organization for Standardization (ISO) on temperature and constant weight procedures.
2. The heating rate and end point determination techniques introduced in ASTM D3174-12 Standard Method for Determination of Coal Ash Content of the American Society for Testing and Materials.
3. The crucible treatment and constant weight operation process in the national standard "GB/T 5009.4-2016 Determination of ash content in food".
4. Technical requirements for muffle furnace temperature calibration in the Laboratory Instrument Maintenance and Calibration Manual (3rd Edition).