Overview
ISO 21493:2019 - "Petroleum products - Determination of turbidity point and aniline point equivalent" defines an automated/automatic laboratory method to measure the turbidity point of petroleum products based on distillates from crude oil and to convert that value to an aniline point equivalent. The procedure is intended for transparent samples with an initial boiling point above ambient temperature and uses a p‑anisaldehyde/sample mixture to detect phase separation (turbidity).
Key topics and technical requirements
- Principle: Mix equal volumes of p‑anisaldehyde and the petroleum sample, heat until miscible, then cool while stirring and detect the temperature at which cloudiness first appears - the turbidity point (τ).
- Reagents & reference materials:
- p‑Anisaldehyde, ≥97.5% purity (gives τ = 73.06 ± 1.576 °C when tested with heptane).
- Drying agents (anhydrous calcium sulfate or sodium sulfate) and spectroscopic/HPLC grade heptane.
- Apparatus:
- Automated turbidity‑detection apparatus able to heat/cool and detect changes in turbidity; compatible with ISO 2977 method 5.
- Thermometers/temperature sensors with calibration traceable to national standards (sensitivity/accuracy requirements given).
- Pipettes (ISO 648 Class A, 10.0 ml ±0.02 ml), balances (0.01 g), and standard PPE (goggles, gloves).
- Procedure parameters:
- 10 ml p‑anisaldehyde + 10 ml sample (or equivalent mass if viscous).
- Heating rate: 1 °C/min to 3 °C/min (until miscible).
- Cooling rate: 0.5 °C/min to 1 °C/min.
- Record turbidity point to 0.1 °C; perform three determinations and report the average if the range ≤ 0.5 °C.
- Conversion: For turbidity points in the range 66 °C to 124 °C, convert to aniline point equivalent using:
- Âeq = 0.926 × τ + 5.63 (example in standard: τ = 115.5 °C → Âeq = 112.6 °C).
- Quality & precision: The standard includes precision data from an interlaboratory study (samples in the 59 °C to 124 °C range) and defines repeatability/reproducibility considerations and reporting requirements.
- Safety: The document warns of hazardous materials and operations; users must apply appropriate laboratory safety measures.
Applications and who uses this standard
- Petroleum laboratories performing physical property characterization of distillate fuels and refinery streams.
- Product quality, R&D and blending teams that need an estimate of aromatic content (aromaticity) for process control or specification compliance.
- Regulatory and testing organizations comparing turbidity‑based results with aniline point results (ISO 2977).
- Useful for correlative hydrocarbon analysis where turbidity point and aniline point equivalent serve as indicators of aromatic vs. paraffinic content.
Related standards
- ISO 2977 (Aniline point determination - complementary method and method 5 compatibility)
- ISO 3170 / ISO 3171 (Sampling of petroleum liquids)
- ISO 648 (Laboratory pipettes)
Keywords: ISO 21493:2019, turbidity point, aniline point equivalent, petroleum products, p‑anisaldehyde, automated turbidity apparatus, aromaticity, distillates from crude oil, ISO 2977.
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