Overview
ISO 20336:2025 specifies an alternative analytical method for determining the total sulfur mass fraction in coal and coke. The method uses high‑temperature combustion (1 150 ± 10 °C) followed by Coulomb titration of the evolved sulfur oxides in an electrolytic cell. It is intended for laboratories and instrument users who require an automated, calibrated procedure to report total sulfur on an air‑dried or other specified basis.
Key topics and technical requirements
- Principle: Combustion of a weighed test portion in a catalyst‑assisted furnace converts sulfur to sulfur oxides that are absorbed and titrated coulometrically; the integrated electrolysis charge is used to calculate sulfur mass fraction.
- Operating conditions: Combustion at 1 150 °C in a combustion tube capable of >1 300 °C, with a high‑temperature zone ≥70 mm and a preliminary decomposition zone (~500 °C).
- Electrolytic system: Electrolytic cell with platinum electrodes, magnetic stirring, and an electrolytic solution (recommended: 5 g potassium iodide + 5 g potassium bromide in 250–300 ml water + 10 ml glacial acetic acid).
- Air supply and purification: Purified air stream supply (typical flow arrangement in the standard: supply ~1 500 ml/min, aspirate ~1 000 ml/min) with sodium hydroxide and self‑indicating silica gel drying/cleaning stages.
- Sample preparation: General analysis test sample ground to pass 212 µm (or 100 µm for sulfur >5 %), air‑dried and mixed; moisture determination required if results are expressed on a different basis.
- Calibration: Multi‑point or single‑point calibration using certified reference materials (CRMs) of coal/coke; calibration verification with additional CRMs and triplicate determinations per sample.
- Quality and precision: Instrument performance checks, calibration checks, and precision requirements (repeatability and reproducibility limits) are specified in the standard.
- Equipment and documentation: Requirements for tube furnace, program controller, Coulomb integrator (linearity ≤0.1 % recommended), sample boats, and reporting requirements.
Applications and users
ISO 20336:2025 is used for:
- Regulatory and compliance testing where accurate total sulfur determination in coal and coke is required.
- Power plants, coke producers, coal traders, environmental testing labs, and quality control laboratories needing coal and coke analysis.
- Instrument manufacturers and service providers who supply or calibrate automated Coulomb titration analyzers.
Benefits include automated operation, traceable calibration with CRMs, and suitability for routine analysis of a wide sulfur concentration range.
Related standards
- ISO 687 - moisture determination for coke (general analysis sample)
- ISO 5068‑2 - moisture in brown coals and lignites (analysis sample)
- ISO 11722 - moisture in hard coal (analysis sample)
Keywords: ISO 20336:2025, total sulfur determination, Coulomb titration method, high‑temperature combustion, coal and coke analysis, sulfur mass fraction, certified reference materials.