Overview
ISO 592:1998 specifies a laboratory method for determining the optical rotation (also called specific rotation) of essential oils. Measurements are made using a polarimeter with sodium D-line illumination (589.3 nm ± 0.3 nm). For solid, partially solid, highly viscous or strongly coloured essential oils, the standard requires measurement on a prepared solution of the oil.
Key topics and requirements
- Measurement principle: Polarimetric determination of the angle by which plane‑polarized light is rotated when passing through the sample (reported in milliradians and/or degrees).
- Wavelength: Use light at 589.3 nm ± 0.3 nm (sodium D‑line), preferably from a sodium‑vapour lamp.
- Instrument precision and verification:
- Polarimeter precision: at least ±0.5 mrad (±0.03°).
- Verification against water (0°/180°) and a quartz plate or a saccharose solution (26.00 g/100 ml gives +604 mrad at 20 °C for 200 mm).
- Cells and tubes:
- Standard tubes: 100 mm ± 0.5 mm; alternatives 200 mm for low rotation, 50 mm or 10 mm for strongly coloured samples.
- Use double‑walled tubes with circulating water and a thermometer for controlled‑temperature measurements.
- Temperature control: Measurements normally at 20 °C ± 0.2 °C (thermostatted bath and thermometer required); thermometer graduations 0.1–0.2 °C.
- Solvent & sample preparation:
- Use analytical‑grade reagents; water must be distilled or equivalent.
- Preferred solvent for solutions: ethanol 95% (verify solvent has zero rotation).
- Follow ISO 356 for drying and sample preparation when required.
- Calculation and precision:
- Optical rotation α = (measured angle × 100) / tube length (mm).
- Apparent specific rotation = optical rotation of solution / concentration (g oil per ml solution).
- Measurement precision: ±3 mrad (±0.17°).
- Reporting: Include sample ID, sampling method (if known), whether analysis was on the oil or a solution (with solvent and concentration), all operational details and test results.
Applications and users
ISO 592:1998 is used for quality control, specification and authentication of essential oils in:
- Manufacturing and QA laboratories in flavor, fragrance and aromatherapy industries
- Essential oil producers and packers
- Analytical testing and certification bodies
- Research labs studying chiral composition or stability of oils
Optical rotation is a rapid, non‑destructive indicator of purity, identity and batch consistency and supports compliance with product specifications.
Related standards
- ISO 356 - Essential oils: Preparation of samples for analysis (referenced for sample drying/preparation).
- ISO 212 - Essential oils: Sampling (recommended sampling practices).
Keywords: ISO 592:1998, essential oils, optical rotation, polarimeter, specific rotation, sodium D-line, ethanol 95%, temperature control, sample preparation.
