ISO 625:2025

Overview

ISO 625:2025 - "Coal and coke - Determination of carbon and hydrogen - Liebig method" specifies a laboratory procedure for measuring total carbon and total hydrogen in coal and coke by combustion (Liebig method). The method combusts a known mass of sample in oxygen, oxidizes incomplete combustion products over copper oxide, and collects the resulting water and carbon dioxide for gravimetric determination. The standard replaces ISO 625:1996 and updates apparatus, reagents and procedural details.

Key technical topics and requirements

• Principle: Complete combustion in oxygen, secondary oxidation over copper oxide, absorption of products and gravimetric measurement. Corrections are applied for moisture and carbonate carbon when required.
• Apparatus: Detailed requirements for combustion and absorption systems, including:
  • Combustion tube (fused silica or hard glass), typical length 1.25 m and diameter 12–15 mm.
  • Three furnaces (approx. heated zones described) to achieve required temperatures (e.g., boat region ~925 °C, copper oxide region ~800 °C, lead chromate/Ag gauze area ~500 °C).
  • Combustion boat (platinum/porcelain/fused silica) and analytical balance (0.1 mg resolution).
  • Purification train for oxygen (U-tubes with magnesium perchlorate and sodium hydroxide).
  • Absorption train using Midvale tubes and a guard tube with reagents in sequence: magnesium perchlorate, manganese dioxide, sodium hydroxide, etc.
• Reagents and packing: Specifications for magnesium perchlorate, sodium hydroxide on inert base (self-indicating preferred), granular manganese dioxide, copper oxide wire, lead chromate, and pure silver gauze.
• Preparation and procedure: Clauses cover conditioning of tubes, packing of absorbents, sample preparation, blank tests, and expression of results (total carbon, organic carbon, total hydrogen, hydrogen corrected for moisture).
• Safety & handling: Warnings on toxic reagents and disposal (e.g., do not attempt to regenerate magnesium perchlorate due to explosion risk).
• Quality aspects: Clauses on precision (repeatability and reproducibility limits) and required test report content.

Practical applications and who uses it

• Laboratories performing proximate/ultimate analysis of coal and coke for quality control, fuel specification and research.
• Coal producers, coke plants, power generation and metallurgical industries monitoring fuel composition.
• Environmental and regulatory testing labs requiring accurate carbon and hydrogen determinations for emissions accounting or material characterization.
• Standards bodies and test method developers comparing combustion-based methods (ISO 625 vs alternative methods such as ISO 609).

Related standards

• ISO 609 - alternative method for C and H determination (referenced in the introduction).
• Normative references that support sampling and moisture corrections: ISO 687, ISO 925, ISO 5068-2, ISO 11722, ISO 13909-4/6, ISO 18283.

Keywords: ISO 625:2025, Liebig method, coal and coke analysis, determination of carbon and hydrogen, combustion analysis, absorption train, Midvale tube, magnesium perchlorate.