Overview
ISO 1041:1973 defines a laboratory method for the determination of the freezing point of essential oils by slow, progressive cooling and observation of temperature changes as the oil passes from liquid to solid. The method is intended for routine physical testing and quality control of essential oils, with a specified apparatus and repeatable procedure. Note: the standard does not apply to rose essential oil.
Key Topics
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Principle: Controlled cooling of a small oil sample while recording temperature; identification of a maximum or a plateau in the temperature–time curve during latent heat release marks the freezing point.
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Apparatus: A wide-mouthed container (~500 ml) with a bored cork or rubber stopper holds a stout-walled test tube. A smaller test tube is fitted inside the stout-walled tube; the thermometer is placed in the inner tube. A schematic device is provided in the standard.
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Thermometer requirements: The thermometer must be calibrated and suitable to measure in the range from about –20 °C to +50 °C. Recommended reservoir dimensions are:
- reservoir length: 10–20 mm
- reservoir diameter: 5–6 mm
- graduation: 0.1 °C
(Thermometers should conform to the related ISO thermometer guidance.)
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Test tube dimensions:
- inner test tube (sample): ~20 mm diameter × 100 mm length
- stout-walled outer tube: ~30 mm diameter × 125 mm length
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Procedure summary:
- Prepare a preliminary test: liquefy the oil if necessary, cool a few millilitres in a small tube and agitate until solidification to create a seed crystal and to record an initial temperature.
- Fill the apparatus bath with ice and water or a suitable refrigerant to about 5 °C below the preliminary temperature.
- Introduce 10 ml of the oil into the inner test tube with the thermometer centered in the liquid. Cool to the preliminary temperature, then insert the inner tube into the outer tube and wait for an additional 2 °C drop.
- Initiate solidification with the seed crystal and agitate with the thermometer to avoid wall-adhering particles. Observe temperature closely until a maximum or a plateau lasting more than 1 minute is seen.
- Repeat until two successive determinations differ by no more than 0.2 °C. The mean of the last two determinations is reported to 0.1 °C.
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Expression of results: Final freezing point reported in °C to one decimal place.
Applications
- Quality control and batch verification of essential oils
- Specification compliance and physical property profiling
- Laboratory testing where a reproducible freezing point measurement is required
Practical benefits include a standardized, repeatable approach to detect freezing/solidification behaviour and to support product specification limits.
Related Standards
- ISO 212 - Essential oils - Sampling (reference for sampling practice)
- ISO/R 356 - Essential oils - Test methods - Sample preparation
- ISO/R 653 - Thermometer selection and requirements (referenced for thermometer performance)
For test reporting, ISO 1041:1973 requires inclusion of method reference, numeric results and their expression, details of any deviations or special operations, and any particular observations recorded during the test.
