ISO 20336:2017

Overview

ISO 20336:2017 specifies a standardized laboratory method for determining the total sulfur content in solid mineral fuels (coal and coke) using high‑temperature combustion followed by Coulomb titration. The method uses automated instruments to combust a weighed test portion at controlled temperatures and to quantify sulfur oxides electrochemically. ISO 20336:2017 provides requirements for sample preparation, reagents, apparatus, calibration and routine operation to produce traceable sulfur results.

Key topics and technical requirements

• Principle: Combustion of a weighed sample in purified air at high temperature converts sulfur to sulfur oxides which are absorbed in an electrolytic solution and titrated by iodine/bromine produced by electrolysis; the integrated electrical charge (Coulomb integrator) yields sulfur dioxide amount.
• Temperature control: Combustion zone maintained at (1150 ± 10) °C with a preliminary decomposition zone around 500 °C.
• Sample preparation: General analysis test sample ground to pass 212 µm (or 100 µm for samples with >5 % sulfur). Typical test portion: 0.05 ± 0.005 g (0.03 ± 0.005 g for >5 % sulfur).
• Reagents: Electrolytic solution prepared from potassium iodide, potassium bromide and glacial acetic acid (5 g KI + 5 g KBr in 250–300 ml water + 10 ml acetic acid). Tungsten trioxide is used as catalyst.
• Apparatus: Includes tube resistance furnace with silicon or corundum combustion tube, electrolytic cell (>400 ml) with platinum electrodes, magnetic stirrer, program controller, purified air supply and Coulomb integrator (integration linearity error < 0.1 % over 0–350 mA).
• Calibration and quality control: Instrument calibration using certified reference materials (CRMs) by multi‑point or single‑point methods; regular calibration checks after 10–15 runs. Precision (repeatability and reproducibility) and test reporting requirements are addressed in the standard.
• Safety and purity: Use analytical‑grade reagents and distilled or equivalent water; ensure gas‑tight connections and air purification (NaOH and silica gel) where required.

Applications and users

• Laboratories performing routine sulfur determination in coal and coke for quality control, fuel specification and compliance testing.
• Coal and coke producers, utilities and power plants monitoring fuel sulfur for combustion performance and emissions control.
• Environmental and regulatory testing labs assessing sulfur content for reporting and permitting.
• Instrument manufacturers and metrology labs implementing or validating automated Coulomb titration analyzers.

Related standards

• ISO 687 - moisture determination for coke
• ISO 5068-2 - moisture in brown coals and lignites
• ISO 11722 - moisture in hard coal (drying in nitrogen)

Keywords: ISO 20336:2017, total sulfur, Coulomb titration, solid mineral fuels, coal and coke, high‑temperature combustion, sulfur determination, instrument calibration.