Overview
ISO 16591:2010 specifies an oxidative microcoulometry method for the determination of sulfur content in petroleum light and middle distillates (final boiling point ≤ 400 °C). The method is applicable to samples with sulfur contents from 1 mg/kg to 100 mg/kg; lower-level analysis (< 1 mg/kg) is addressed by a standby technique in Annex A. ISO 16591:2010 is widely used for sulfur analysis in fuels, naphtha and related petroleum streams where accurate sulfur content determination is required.
Key topics and technical requirements
- Analytical principle: sample pyrolysis and combustion; sulfur as SO2 reacts with tri‑iodide in an electrolyte titration cell. Coulometrically generated current required to replace consumed tri‑iodide is proportional to sulfur content.
- Scope and range: quantitative range 1–100 mg/kg (with dilution for higher levels; Annex A for <1 mg/kg).
- Apparatus: pyrolysis furnace (multi‑zone), quartz pyrolysis tube or boat inlet, titration cell with sensor/generator electrodes, microcoulometer and recorder/integrator, heating tape to prevent condensation.
- Reagents and gases: potassium iodide electrolyte with sodium azide and acetic acid; high‑purity oxygen (≥ 99.995 %) and inert carrier gas (Ar, He or N at ≥ 99.995 %). Sodium azide addition mitigates interferences from nitrogen and chlorine.
- Calibration and standards: use of sulfur stock and standard solutions (prepared from certified reference materials or known sulfur compounds such as thiophene, dibutyl sulfide, dibenzothiophene).
- Interferences and limitations: nitrogen (>0.1 % m/m) and chlorine (>1.0 % m/m) can interfere (treated with NaN3), bromine and organometallics interfere at high concentrations (~500 mg/kg). Combustion of high‑boiling fractions can form carbonaceous deposits requiring maintenance.
- Quality and precision: the precision statements in the standard apply to the 1–100 mg/kg range; normative sampling references include ISO 3170 and ISO 3171.
Practical applications and users
ISO 16591:2010 is intended for:
- Petroleum and petrochemical laboratories performing sulfur analysis for product specification and quality control
- Refineries and fuel producers monitoring sulphur levels in naphtha, gasoline blendstocks and middle distillates
- Regulatory and environmental testing where sulfur limits must be verified
- Analytical chemists implementing microcoulometric sulfur methods and instrument manufacturers validating systems
Benefits include rapid, sensitive sulfur measurement with direct electrochemical detection and suitability for routine laboratory workflows.
Related standards (if applicable)
Safety note: the method involves hazardous reagents (sodium azide) and high‑temperature combustion-users must follow appropriate safety and health practices.