SEPARATING FUNNEL: CHARACTERISTICS, WHAT IT IS FOR AND EXAMPLES - CHEMISTRY - 2025

The funnel, pear or separating ampoule is a glassware used in liquid extractions and separations. It is recognized by its peculiar inverted cone shape. In laboratories it rests on iron rings attached to the universal support.

As it is a separating funnel, by definition it must be able to allow separation due to the effect of gravity, and without the rest of the mixture being washed away. To achieve this, necessarily (usually) there must be a liquid two-phase system; that is to say, that formed by two immiscible liquids and of different densities.

Separating funnel with a yellow biphasic system. Source: Markdegeus via Wikipedia.

For example, in the image above there is a separatory funnel supported by an iron ring whose edges are covered by a rubber. Inside it contains a biphasic mixture of two yellow liquids; although the color is more intense in the lower phase than that of the upper phase.

Thanks to this funnel, it is possible to extract some solutes or analytes from water samples, or from the solutions of samples of interest (soils, ashes, plant masses, fuels, etc.). Likewise, it serves to explain the concept of solute distribution between the interface formed between two immiscible solvents.

Separating funnel features

The image above alone shows the most important features of the separatory funnel. Note that the volume of the liquid mixture is much less than the capacity of the funnel. In fact, it is always recommended that extractions do not cover volumes greater than half the total volume of the funnel.

Going over the funnel in more detail, at the bottom we have the stopcock (bluish thread), which allows the liquid to fall through the glass neck. Above, we have the mouth of the funnel sealed by a plastic plug; however, the stopper can also be glass with a ground seal.

Both the stopcock and the cap can be greased to ensure proper operation. The key must turn without much difficulty and also ensure that when closed it does not allow the lower liquid to pass through. Meanwhile, the cap has to seal hermetically, well enough so that when the funnel is shaken, the liquid does not escape from above.

The advantage of the iron ring being covered by a rubber is that it prevents scratching the glass of the funnel when holding it.

What is it for?

The separatory funnel is used in principle for only two purposes: to extract solutes from samples without carrying other unwanted solutes, or directly to separate immiscible liquids.

The methodology of both processes is in itself the same: a liquid mixture, previously obtained from the sample, is placed inside the separatory funnel and one or two solvents are added. This in order to create an easy to separate two-phase system.

Then, separating the two liquids, and knowing that our solute or analyte is more soluble in one of the two phases (lower or higher), one is collected while the other is discarded. The lower phase has a higher density, so it is extracted by opening the stopcock; while the upper phase is obtained through the mouth of the funnel by removing the cap.

Then, with the phase of interest, aliquots are taken to perform chromatographic or spectroscopic analysis, acidifications, precipitation reactions, or it is simply subjected to heating to evaporate the solvent and recover the remaining solid: the solute (impure).

How to use

Shaking the funnel

While the exact methodology varies depending on the analysis, the sample, the type of analyte, and the solvents used, the way the separatory funnel is used is almost always the same.

Keeping the funnel carefully wedged in the iron ring, and making sure that the stopcock is closed, proceed to fill it with the mixture to which the extracting solvents have been added. It is covered and shaken to guarantee that the solute in the mixture interacts with the solvent that we want to decant later.

During stirring, which is usually done inside a hood and by repeatedly tilting the funnel away from the body, it is important to open the stopcock to allow the internal vapor pressure to release.

This vapor pressure is due in part to the fact that the extractions are usually carried out with very volatile solvents, so the high pressures of their vapors can burst the glass of the funnel and hurt those who handle it.

Phase extraction

The funnel is allowed to rest until two well-defined phases appear. If we are interested in the lower phase, we open the stopcock and collect it in a container. We add more solvent to the funnel and repeat the agitations to then extract again. In this way we ensure that the largest amount of solute is extracted.

Meanwhile, if we are interested in the upper phase, we discard the lower phase by opening the stopcock, and the upper phase is decanted through the mouth of the funnel. The lower phase is returned to the funnel and more solvent is added to it again to repeat several extractions.

In short: it is stirred, we release the vapor pressure, it is left to rest, and we decant. In the phase of interest we will have the solute that has been extracted from the sample. It is recommended to carry out several extractions with a low volume of solvent.

The following video shows an experiment using a separatory funnel:

Usage examples

Some general examples of the use of the separating funnel will be mentioned below:

-Extractions of organic compounds dissolved in water by adding organic and apolar solvents, which is of great relevance to assess water quality or detect a source of contamination

-Extractions of analytes from finely ground plant materials, so that some are more soluble in the upper phase, and others in the lower phase.

-Separate two immiscible liquids in sufficient quantities so that two phases are defined, and also adding a species to destabilize the emulsions formed

-Aqueous extractions of iodine with dichloromethane, this being one of the most common practices in teaching laboratories

-Extractions of essences or oils to remove at the same time their content of salts and other soluble compounds in the aqueous phase (which is usually the lower phase due to its higher density)

References

1. Separatory Funnel: Preparing to use the separatory funnel. Recovered from: dept.harpercollege.edu
2. Elsevier BV (2020). Separatory Funnel. ScienceDirect. Recovered from: sciencedirect.com
3. K. Gable. (2020). Use of a separatory funnel. Oregon State University. Recovered from: sites.science.oregonstate.edu
4. Wikipedia. (2020). Separatory funnel. Recovered from: en.wikipedia.org
5. Yuraima Register. (2018). Separating funnel. Recovered from: iquimicas.com