ISO 592:1998 pdf download – Essential oils — Determination of optical rotation.
4 Reagents Reagents shall be of analytical grade. Use distilled water or water of at least equivalent purity. 4.1 Solvent (only for essential oils that need to be tested in solution). Use preferably 95 % ethanol by volume. It is advisable to check that the optical rotation of the solvent used is nil. 5 Apparatus 5.1 Polarimeter, having a precision of at least ± 0,5 mrad (± 0,03°) and adjusted to give 0° and 1 80° with water. The polarimeter shall be checked with a quartz plate of known optical rotation or, if that is unavailable, with an aqueous solution containing 26,00 g of anhydrous pure saccharose per 1 00 ml of solution. The optical rotation of this solution shall be +604 mrad (+34,62°) in a 200 mm layer, at a temperature of 20 °C. The instrument shall be under conditions of stability when in use, and non-electronic instruments shall be used in the dark. 5.2 Light source, comprising any device giving a light of wavelength 589,3 nm + 0,3 nm, preferably a sodium vapour lamp. 5.3 Polarimeter tubes, usually 1 00 mm ± 0,5 mm long. When testing slightly coloured samples of low optical rotation, tubes of length 200 mm + 0,5 mm may be used. Tubes of length 50 mm ± 0,05 mm or 1 0 mm ± 0,05 mm or even less may be used, if necessary, for strongly coloured samples. For determination at 20 °C or at another specified temperature, use double-walled tubes, equipped with a thermometer (5.4) to ensure water circulation at the required temperature. For determination at ambient temperature, any type of tube may be used, although it is advisable to use the type described above in this case too. 5.4 Thermometer, graduated in 0,2 °C or 0,1 °C, allowing determination of temperatures between 1 0 °C and 30 °C. 5.5 Thermostatically controlled device, for maintaining the temperature of the sample at 20 °C ± 0,2 °C or any other specified temperature.
7 Procedure 7.1 Preparation of test sample See ISO 356 if it is necessary to dry the sample. When determining specific rotation (see 3.2), prepare the oil solution in the appropriate solvent (4.1 ), at the concentration specified in the corresponding appropriate International Standard for the essential oil being analysed. 7.2 Determination Switch on the light source (5.2) and wait until full luminosity is obtained. If necessary, bring the temperature of the test sample (7.1 ) to 20 °C ± 1 °C or to another specified temperature (see the note), then pour the sample into the appropriate polarimeter tube (5.3), which should be at approximately the same temperature. Start water circulation under thermostatic control so as to keep the whole at the specified temperature (± 0,2 °C) during the determination. Fill the tube with the test sample, and ensure the absence of air bubbles. Place the tube in the polarimeter (5.1 ) and read the dextrorotatory (+) or laevorotatory (–) optical rotation of the oil on the scale of the instrument. NOTE As a rule, the determination will be carried out at 20 °C, exceptions to that being set out in the specifications concerning a number of particular oils.