Overview
ISO 3733:1999 - "Petroleum products and bituminous materials - Determination of water - Distillation method" sets a standardized laboratory procedure to measure water content (up to 25% v/v) in petroleum products, bitumens, tars and derived materials (emulsions excluded). The method is a co‑distillation (Dean and Stark style) technique in which the sample is heated under reflux with a water‑immiscible solvent, the condensate is collected in a graduated trap and the separated water is read directly. The standard notes the result can be used for refining control, custody transfer volume corrections and commercial transactions.
Key topics and technical requirements
- Scope and limits: Validated for water up to 25% (may be used above 25% but without precision data). Volatile water‑soluble substances are measured as water.
- Principle: Co‑distillation with a water‑immiscible solvent; continuous separation in a graduated trap.
- Solvents: Three solvent classes specified - aromatic (e.g., mixed xylene, toluene/xylene blends), petroleum distillate (specified boiling fractions per ISO 3405), and paraffinic (light petroleum or iso‑octane) chosen according to material type.
- Apparatus: Glass or metal still, reflux condenser and graduated trap (Dean and Stark assembly). Trap sizes, scale divisions and maximum scale error are specified; leak‑tight joints (ground glass or O‑rings) are required.
- Verification & recovery: Mandatory trap calibration and apparatus recovery tests before use. Verification example: add 0.05 ml increments of distilled water using a microburette or micropipette (readable to 0.01 ml); reject trap if deviation > 0.05 ml. Recovery tolerances depend on trap size (typical permissible limits range from ±0.025 ml to ±0.25 ml).
- Sampling & preparation: Requires representative sampling per ISO 3170 / ISO 3171. Solid samples crushed/mixed; liquid samples homogenized and rinsed with solvent. Annex A covers sample handling details.
- Safety: Standard contains a warning - users must observe appropriate laboratory safety for hazardous materials and operations.
Applications
- Quality control and laboratory testing of bitumen, fuel oils, lubricants, road oils, tars and related products.
- Determination of free water for custody transfer adjustments, refining process control, and commercial acceptance.
- Regulatory and inspection testing where a recognized distillation method is required.
Who uses this standard
- Petroleum and bitumen testing laboratories
- Refinery and terminal QA/QC personnel
- Third‑party inspectors and custody transfer auditors
- Manufacturers of bituminous products, lubricants and fuel oils
Related standards
- ISO 3170 - Petroleum liquids: Manual sampling
- ISO 3171 - Petroleum liquids: Automatic pipeline sampling
- ISO 3405 - Distillation characteristics (referenced for solvent specifications)
- Annex A (ISO 3733) - Sample handling procedures
Keywords: ISO 3733:1999, water determination, distillation method, petroleum products, bituminous materials, Dean and Stark, custody transfer, trap calibration, solvent‑carrier.