Setting up a reflux

Reflux is a commonly used method of heating a reaction. When setting up apparatus for reflux, you need to clamp glassware securely, and use an appropriate heat source such as a stirrer hotplate.

The principle of the reflux operation is to safely provide a controlled amount of heat to the reaction, while ensuring that the solvent does not boil off. This is achieved by fitting a reflux condenser to the reaction vessel, so that evaporating solvent is condensed and returned to the reaction flask.

1. Plan where you will assemble the apparatus. You will need two clamps, one above the other, so they can both be mounted on a single clamp stand. The apparatus needs to be close to a water supply and to a sink, to run the condenser.

2. Select the correct glassware. Refluxes should be conducted in round-bottomed flasks, which are available in different sizes and configurations.

(a) Size. RBFs are available in many sizes (10cm3, 25cm3, 50cm3, 100cm3, 250cm3 etc.). The flask should be large enough to contain the reagents, and the solvent, without being much more than half-full.

(b) Joint size. RBFs commonly have a single neck, with a ground glass joint. Your condenser will need to fit into this joint, so ensure that they are the correct size...

(c) Configuration. RBFs can be designed to have a single neck, or two necks, or even three necks. RBFs in multineck configurations can be useful when a reaction requires the addition of reagents after it has begun.

3. Clamp the round-bottomed flask. The RBF should be clamped securely, upright, with the clamp on the ground glass joint. This is the main support point for the apparatus, so it needs to be secure. Arrange the heating apparatus under the flask, then adjust the height of the clamped flask to bring it to the correct height. For a stirrer-hotplate/isomantle set-up, a raised lab-jack should be placed under the heating apparatus, as it allows the heat source to be removed quickly by lowering away from the reaction vessel. This also allows fine adjustments to be made to the position of the heating apparatus, to ensure that the reaction vessel is set up to be heated effectively, without being placed under stress.

4. Add reagents/solvent/stirrer bar to the flask. Some chemicals may have specific requirements for how they are handled; in general, you can use conical funnels to add liquids, and weighing boats to add powder solids. Most refluxes are stirred as they are heated; you can add a magnetic stirrer bar at this point.

5. Add the condenser. Fit the ground glass joints together, so that the condenser sits upright in the neck of the flask. It should be vertical.

(a) Clamping the condenser. The weight of the apparatus is already supported by the securely-clamped RBF, so the addition of a loosely-fitted (i.e. barely touching the glass) second clamp on the condenser serves to protect the apparatus from accidents. Tightly clamping the condenser should be avoided, unless necessary, as this can stress (and even break) the ground glass joint.

(b) Condenser hoses. These are thin-walled rubber tubes, which bring the water from the tap to the condenser, then take the water to the sink. The water should go in at the lower point on the condenser, and come out of the upper point.

The hoses need to be attached carefully, so that they fit securely and do not leak. If left alone, water leaks cause a mess in your workspace, with the associated hazards – but they can also potentially ruin the chemistry you are attempting, and can also be seriously hazardous if they drip into equipment (e.g. hotplates, oil-baths).

The hoses should also be arranged so that they are not in the way of the experimental work; for example, they should not be hanging near the heat source. Hoses can be held out of the way by threading behind clamps if necessary, and the water outlet hose can be secured in the sink by fixing on a clamp boss.

Some condensers have smooth glass inlets, where the hoses are fitted directly to the glass. This is most easily (and safely) done by twisting the hose onto the glass, rather than trying to push it straight on. Wetting the end of the hose with water can also help.

Other condensers have screw threads, which require hoses with plastic joints. The rubber hose is fitted to the plastic joint, which is then screwed onto the condenser. Fitting the plastic joint to the condenser, then attaching the hose, should be avoided, as this puts undue stress on the glass screw thread.

(c) greasing the ground glass joint. The use of vacuum grease on the joints is generally to be avoided, if possible, as it can easily enter the reaction as an impurity. However, there are some circumstances in which the use of grease is required. The most common reason for using grease is that exposure to strongly basic chemicals (such as NaOH, whether solid or in solution) can fuse ground glass joints, so that they become irretrievably stuck. So, when using these chemicals, a thin protective coating of vacuum grease should be applied to the ground glass joint. Greased joints are also used to make apparatus properly airtight, if it is necessary to control the atmosphere of the reaction e.g. preventing the entry of air or water into the reaction vessel.

6. Switch on the water flow to the condenser. The optimal water flow rate will vary between reactions, depending on the solvent, conditions, glassware dimensions etc.; however, in general, a slow flow of water (a few drops per second) is adequate. Fast flows of water are wasteful, and are much more likely to cause problems, as the pressure could cause the hoses to detach during the reaction. The flow rate can be checked as the water exits the hose into the sink. If there are any problems such as joints which are leaking water, they need to be fixed at this stage, before the reaction begins.

7. Switch on the stirring and heating.

(a) The appropriate amount of stirring may vary between reactions; in general, the stirring should agitate and mix the reagents, keeping them suspended/dissolved in the solvent, without causing the liquid to rise too high in the flask. The stirred reaction mixture should not rise to near the neck of the flask. Careful adjustment of the stirring rate is important, as small changes in rate can cause the magnetic stirrer bar to change from even, moderate stirring (desirable) to chaotic motion, which then fails to agitate the reagents as required.

(b) The appropriate amount of heating may vary between reactions, depending mainly on the solvent. For example, very volatile solvents (such as diethylether, b.p. 35°C) require very little heating to achieve reflux, while higher-boiling solvents (such as toluene, b.p. 110°C) would require significant heat. Some heating sources (e.g. hotplates, isomantles) have a control to set the heat input, which may be indicated as a target temperature (°C) or as an arbitrary power rating. With a target temperature, the corresponding sensor generally detects the temperature of the heating element, which may be significantly higher than the temperature of the reaction vessel - for this reason, it is prudent to set the control to exceed (but not by too much) the boiling point of the solvent, in order to achieve reflux in a manageable time. As the reaction heats up, the solvent will approach its boiling point, and begin to bubble. Solvent vapour will begin to visibly condense inside the condenser, and drip back into the flask. This condensation should only be in the lower half of the condenser; if it rises higher, this indicates that too much heat is being supplied. Continuing to do this might lead to some of the solvent escaping the condenser, which is undesirable and potentially dangerous. Heat supply can be reduced slightly by moderating the heating controls, or reduced significantly by removing the heat source (i.e. lowering the lab-jack in the case of a stirrer-hotplate set-up).

8. Carry out the reflux. Once the reaction is refluxing as described, it can be safely left for the duration of the reaction.

9. Finishing the reflux. At the end of the reflux period, switch off the heat source. Removing the flask from contact with the heat source helps the reaction to cool more quickly; slightly lowering the labjack with the hotplate should achieve this.