Reflux

Reflux is another simple technique in which the sample is placed in a flask containing the solvent with a condenser fitted on top of the flask.  The flask is heated for a set time, allowed to cool and then the solution can be analysed.  The fundamentals of the extraction are around the boiling point of the solvent which makes the technique universal.

With any setup using a condenser, the efficiency of the condenser can be important. Each condenser has a potential cooling capacity and efficiency. Different types of condenser exist, from the basic ‘Liebig condenser’. To the more complex ‘Graham condenser’ which has a spiral coil running the length of the condenser.

Graham Condenser

Graham condenser, with three possible configurations: a) cooling with jacket, b) cooling with spiral and c) combined jacket and spiral cooling. [1]

There are two basic configurations for a Graham condenser. In the first, the spiral contains the coolant, and the condensation takes place on the outside of the spiral. This configuration maximises flow capacity because vapors can flow over and around the spiral.

In the second configuration, the jacket tube contains the coolant, and condensation takes place inside the spiral. This configuration maximises collected condensate because all the vapors must flow through the entire length of the spiral, thereby having prolonged contact with the coolant. Other condensers such as the Allihn condenser and ‘Friedrich condenser’ are available.

For reflux extraction and also for any other type of extraction, the correct sample-to-solvent ratio should be used.  This is to make sure that a solution is obtained that spans a suitable concentration range e.g. too concentrated or dilute, as well as consideration of the amount of solution required for analyses and the required limit of detection based on the Analytical Evaluation Threshold (AET).

Heating is commenced and the material extracted at the boiling point of the solvent. The boiling point of the solvent is the key controlling factor with this technique, along with the duration of extraction. Extraction should continue until asymptotic levels or exhaustive conditions are reached, depending on the guidelines that are being followed.

Reflux may not be suitable if the material to be extracted is particularly thermally labile because it is exposed to complete heating of the solvent. This potential thermal impact can be exacerbated by the material sinking and being in contacted with the bottom of the flask and potentially be exposed to higher temperatures.  Reflux extraction does have the advantage that any solvents can be used including mixtures.

Other related techniques that also heat the sample in a solvent such as microwave-assisted extraction (MAE) or pressurised solvent extraction (PSE) will be discussed in more detail in following weeks.

  1. Update on Undertaking Extractable and Leachable testing  A Feilden ISBN 978-1-84735-455-6