A separatory funnel, also known as separation funnel, separating funnel, or
colloquially sep funnel, is a piece of laboratory glassware used in
liquid-liquid extractions to separate the components of a mixture into two
immiscible solvent phases of different densities. Typically, one of the phases
will be aqueous, and the other a non-polar lipophilic organic solvent
such as ether, MTBE, dichloromethane, chloroform, or ethyl acetate. All of
these solvents form a clear delineation between the two liquids. The two layers
formed are usually known as the organic and aqueous phases. Most organic
solvents float on top of an aqueous phase, though important exceptions are
most halogenated solvents. The organic solvent used for the extraction must not
react with the substance to be extracted or with water. It should also have a low
boiling point so it can be easily removed from the product.
Description A separating funnel takes the shape of a
cone with a hemispherical end. It has a stopper at the top and stopcock, at the
bottom. Separating funnels used in laboratories are typically made from
borosilicate glass and their stopcocks are made from glass or PTFE. Typical
sizes are between 30 mL and 3 L. In industrial chemistry they can be much
bigger and for much larger volumes centrifuges are used. The sloping sides
are designed to facilitate the identification of the layers. The
stopcock-controlled outlet is designed to drain the liquid out of the funnel.
On top of the funnel there is a standard taper joint which fits with a ground
glass or Teflon stopper. To use a separatory funnel, the two
phases and the mixture to be separated in solution are added through the top
with the stopcock at the bottom closed. The funnel is then closed and shaken
gently by inverting the funnel multiple times; if the two solutions are mixed
together too vigorously emulsions will form. The funnel is then inverted and
the tap carefully opened to release excess vapor pressure. The separating
funnel is set aside to allow for the complete separation of the phases. The
top and the bottom tap are then opened and the lower phase is released by
gravitation. The top must be opened while releasing the lower phase to allow
pressure equalization between the inside of the funnel and the atmosphere. When
the bottom layer has been removed, the stopcock is closed and the upper layer
is poured out through the top into another container.
Before using the separatory funnel, make sure it is placed safely in a ring
stand. Also place an Erlenmeyer flask below the separatory funnel to ensure
that any drops which may leak out of the funnel are caught in the flask. Finally,
it is of vital importance to make sure that the stopcock is tightly closed.
Theory The separatory funnel runs on the
concept of "like dissolves like". This means that if a compound is polar, then
it can only be dissolved in a polar solvent. Following this logic, a
non-polar compound can only be dissolved in a non-polar solvent. In a separatory
funnel, there are two phases, the aqueous layer is polar, and the organic
layer is non-polar. Because these two layers share a surface, any polar
substances that were initially in the solution will be pulled into the
polar/aqueous layer, and any non-polar substances will be pulled into the
non-polar/organic layer. The denser of the two layers is then removed via the
stopcock. The vast majority of the time water is used for the polar layer, so
the position of the organic layer depends on its relative density to that
of water. There should be a distinct phase boundary between the two layers,
however if an emulsion forms, it may take time for the layers to completely
separate. Once the bottom layer is removed and the desired layer is
isolated, a recrystallization is used to isolate the target compound. Because a
recrystallization commonly follows a separation and it is often the organic
layer that is isolated, an organic layer with a low boiling point is preferable
to reduce the evaporation time. Emulsions
Emulsions can be formed while liquids are being mixed in the separatory
funnel. This can occur when small droplets are suspended in an aqueous
solution. If an emulsion is formed, try to slowly swirl the solution in the
separatory funnel. If the emulsion is not eliminated, then try adding a small
amount of saturated aqueous sodium chloride solution. The main drawback to
the use of a separatory funnel is that oftentimes, it takes significant time
for the emulsion to dissipate. Research is being done on alternative, more
efficient techniques, mostly utilizing stir bars to decrease or even eliminate
the chance of emulsification, thus decreasing the amount of waiting time.
Safety concerns The largest risk when using a separatory
funnel is that of pressure build-up. Pressure accumulates during mixing if
gas evolving reactions occur. This problem can be easily handled by simply
opening the stopper at the top of the funnel routinely while mixing. This
should be done with the top of the funnel pointed away from the body. When
shaking, hold the stopper in place or it can become dislodged causing the liquids
to spill. To account for this, simply hold the stopper in place with one hand.
Gallery See also
Dropping funnels are similar in shape and design, and may be used as
separatory funnels. They have standard taper ground glass joints at the stem.
Partition coefficient is a measure of the distribution of an analyte between
the two phases in a separation. References