SOP: Specific Gravity of Essential Oils
GIFSQ — Global Institute for Food Safety & Quality
Principle
The specific gravity is the weight of oil contained in a pycnometer divided by the water equivalent of the same pycnometer at the same temperature, measured in air; a temperature correction is applied when the test temperature differs from the specified temperature, within ±3 °C of it.
Apparatus required
- Pycnometer (a specific gravity bottle may be used in place of the pycnometer)
- Water bath
Procedure
Immediately after opening the sample containers, note the colour, clarity and separated water, if present — if the container is opaque and the specific gravity of the oil is more than 1.0, draw the sample with the help of a glass tube (open at both ends) from the bottom of the container; if the specific gravity is less than 1.0, insert the tube slowly and draw from the middle region
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Test the sample for any bynote; if moisture is present, remove it by adding freshly fused anhydrous sodium sulphate or freshly calcined anhydrous magnesium sulphate, and store overnight
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Fill the clean, dry pycnometer with the oil which had been previously cooled to a temperature 3 °C below the test temperature
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Place the pycnometer in a water bath and permit it to warm slowly to the test temperature; adjust the oil to the proper level, put the cap in place and wipe the pycnometer dry
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Weigh accurately after 30 min
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Empty the pycnometer, thoroughly clean and dry; fill with distilled water in the same way as in the case of oil and note the weight of water equivalent
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The weight of the oil contained in the pycnometer divided by the water equivalent gives the specific gravity of the oil at the test temperature (in air)
Calculation
Specific gravity = (Wt of oil in pycnometer) / (Wt of water equivalent)
Temperature correction: a temperature of 30 °C is recommended unless otherwise agreed to. From the specified temperature, if the temperature of testing is higher, add 0.00064 for each °C increase in the temperature above the specified temperature; if low, subtract 0.00064 for each °C below the specified limits. This correction holds good within ±3 °C of the specified temperature.
Temperature corrections per °C specified for Florida oils in USA:
- Orange oil: 0.00078
- Lemon oil: 0.00077
Temperature corrections per °C suggested by EOA:
- Bitter orange oil: 0.00057
- Bergamot oil: 0.00061
- Lemon oil, Arizona: 0.00057
- Lime oil, cold pressed: 0.00054
- Lime oil, distilled: 0.00059
Precautions
- Terpenes sometimes used to adulterate essential oils have a lower specific gravity than the oil — a sample having a lower reading should be viewed with suspicion.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.
- Methods of Sampling and Test for Natural and Synthetic Perfumery Materials (First Revision), IS: 326—1969, ISI, New Delhi.
- Kesterson, J.W., R. Hendrickson & R.J. Braddock, Florida Citrus Oils Bulletin, 747 (technical).