Overview
ISO 625:1996 - Solid mineral fuels - Determination of carbon and hydrogen - Liebig method specifies a classical wet-combustion (Liebig) procedure for determining the total carbon and total hydrogen in hard coal, brown coal, lignite and coke. The method burns a known mass of sample in oxygen, converts hydrogen to water and carbon to carbon dioxide, then absorbs and gravimetrically determines the products. Results include carbon bound in carbonates and hydrogen in moisture and structural water; concurrent moisture determination and corrections are required.
Key topics and technical requirements
- Scope and principle: Total C and H by the Liebig combustion technique; hydrogen corrected for moisture and water of constitution; optional correction for carbonate carbon via CO2 determination.
- Reagents & safety: Use analytical-grade reagents (e.g., magnesium perchlorate, sodium hydroxide on inert base, granular manganese dioxide, lead chromate). WARNING: many reagents are toxic and magnesium perchlorate must not be regenerated due to explosion risk.
- Apparatus:
- Combustion tube (fused silica or hard glass), 1.25 m typical length; combustion boat (platinum/porcelain/fused silica).
- Three-furnace arrangement with typical temperatures: ~925 °C, ~800 °C and ~500 °C for sequential oxidation and absorption zones.
- Purification train and absorption train assembled using Midvale tubes, glass wool, copper/silver gauze, and specific packing order (magnesium perchlorate, NaOH, manganese dioxide, lead chromate).
- Flow-rate control for oxygen (typical flows up to ~100 ml/min; procedure uses ~12 ml/min during conditioning).
- Preparation & conditioning: Detailed packing, heating and oxygen-purging procedures for both combustion tube and absorption train; conditioning until absorber mass stability (e.g., within 0.0005 g) is achieved.
- Sample handling and procedure: Typical test portion ~0.2–0.3 g spread in an ignited boat; blank tests to check system background (blank mass increase tolerance noted).
- Quality control: Blank tests and absorber mass-change limits (e.g., blank >0.001 g indicates repacking; tube E >0.05 g may require repacking).
Applications and who uses it
ISO 625:1996 is intended for:
- Coal and coke testing laboratories performing fuel quality analysis, calorific value calculations and compositional reporting.
- Industrial QA/QC teams in mining, coke production and power generation verifying feedstock composition.
- Research groups and standardization bodies comparing analytical methods (Liebig vs. high-temperature combustion).
- Environmental and certification labs that need reproducible, internationally recognized procedures for carbon and hydrogen determination in solid mineral fuels.
Practical benefits include established gravimetric accuracy, compatibility with moisture and CO2 corrections, and well-defined apparatus and conditioning steps for reproducible results.
Related standards
Keywords: ISO 625:1996, Liebig method, determination of carbon and hydrogen, solid mineral fuels, coal analysis, coke testing, absorption train, Midvale tubes, magnesium perchlorate.
