The apparatus should be set up as outlined in Part 2 of this series, with a known weight of product for testing placed in the flask. If a two litre capacity apparatus is being used, then around 100g of accurately weighed sample would typically be used. About one litre of tap water should then be added to the flask: the easiest way for this is by to remove the stopper in the flange lid, pouring in the water via a funnel placed in the flange-top side socket. Then remove the funnel and replace the stopper; the heating can be now be switched on to maximum and the water to the condenser turned on to allow it to flow as a steady trickle. Ensure that the tap at the base of the calibrated arm on the Dean & Stark is closed, whilst the tap situated mid-way up the return arm is open.
Carefully observe the contents of the flask; when the temperature is getting close to boiling point, turn down the heating via the temperature control on the mantle and adjust it so that a gentle 'rolling boil' is achieved. The steam, as a colourless vapour, will rise up the flask, then up the main arm of the Dean & Stark apparatus and will finally reach the condenser. Here the steam will condense back to water and then drip vertically down into the calibrated 'collection section' of the Dean & Stark. The steam should not reach more than a third of the way up the condenser or essential oil may be lost, giving an incorrect lower QC value. If the vapour appears to be climbing up the condenser above the half-way point, then slightly increase the water flow through the condenser to increase its cooling capability. Alternatively lower the heating to achieve the same result.
For the vast majority of essential oils (say 98% or so), the oils will simply float on the surface and collect in the calibrated arm of the Dean & Stark without a problem. For some (e.g. Lavender), the distillation will be completed within 30 minutes or so, whereas other plant materials can take many hours to complete.
Completion is determined when the lower oil level stops moving down the calibrated arm and the level remains unchanged after a set period e.g. 20 minutes or so.
There are a very small number of essential oils that are heavier than water (e.g. clove) and in this case the both taps at the base of the calibration arm and the one mid-way up the return arm on the Dean & Stark should remain closed. The essential oil will then collect at the base of the collection arm. When the oil level stops rising up the calibrated arm, it should be transferred to an appropriately sized measuring cylinder: any water floating on the top can then be ignored and the oil yield can then be calculated.