Overview
ISO 3012:1999 specifies a potentiometric method for the determination of thiol (mercaptan) sulfur in light and middle distillate fuels (e.g., gasolines, naphthas, kerosines, gas oils). The method covers mercaptan sulfur in the range 0.0003% to 0.0100% (m/m) (3 mg/kg to 100 mg/kg). The test uses silver nitrate titration in an alcoholic sodium acetate solvent and detects the end‑point by the potential between a glass reference electrode and a silver/silver sulfide indicating electrode.
Key topics and technical requirements
- Scope and sensitivity: Measures thiol (mercaptan) sulfur from 3 mg/kg to 100 mg/kg in light and middle distillates.
- Principle: Mercaptans are precipitated as silver mercaptide; endpoint identified by an inflection in the plot of titrant volume versus cell potential.
- Interferences:
- Sulfides, disulfides and thiophene: do not interfere.
- Elemental sulfur: no interference below 0.0005% (m/m).
- Hydrogen sulfide: interferes unless removed as described.
- Sample handling:
- Sampling per ISO 3170 / ISO 3171.
- Light distillates should be kept cold (<4 °C) to avoid loss of volatile mercaptans.
- Reagents:
- Propan‑2‑ol (isopropanol) free of peroxides.
- Silver nitrate standard solutions (0.1 mol/L and daily 0.01 mol/L dilution).
- Potassium iodide for standardization.
- Sodium sulfide, sodium acetate titration solvents (alkaline or acidic versions).
- Apparatus:
- Burette conforming to ISO 385‑1, glass reference electrode, silver/silver sulfide indicating electrode, sensitive voltmeter (±2 mV), and titration stand/stirrer.
- Electrode conditioning and silver sulfide coating procedures are specified.
- Safety: The standard includes a warning about hazardous materials and operations; users must implement appropriate safety and waste disposal measures.
Applications and users
- Fuel quality control laboratories and refinery analytical labs performing mercaptan sulfur analysis.
- Regulatory and environmental testing labs verifying fuel sulfur species for compliance and emissions control.
- Manufacturers or purchasers of light and middle distillate fuels requiring routine QC of mercaptan content.
- Instrument suppliers and labs implementing potentiometric titration systems for petroleum analysis.
Related standards
Normative references incorporated in ISO 3012 include:
Keywords: ISO 3012:1999, mercaptan sulfur, thiol sulfur, potentiometric method, petroleum products, silver nitrate titration, light distillates, middle distillates, fuel analysis.