Organic Chemistry at CU Boulder
Drying Organic Solutions
The process of synthesizing and isolating an organic compound often results in an organic
compound or solution contaminated with traces of water. For instance, in aqueous extractions
some water will be transferred into the organic phase because of the partial miscibility of the
organic phase and water. Also, many reactions themselves are performed in an aqueous solution.
This water must be removed before the required compound can be properly characterized.
In the Organic Chemistry Teaching labs at CU, two methods of drying solutions are commonly
used: saturated aqueous sodium chloride and solid drying agents. These two methods are
described below.
Saturated Aqueous Sodium Chloride
The bulk of the water can often be removed by shaking or "washing" the organic layer with
saturated aqueous sodium chloride (otherwise known as brine). The salt water works to pull the
water from the organic layer to the water layer. This is because the concentrated salt solution
wants to become more dilute and because salts have a stronger attraction to water than to organic
solvents.
To dry your organic product by this method, place the organic solution in a separatory funnel.
The organic solvent can be any solvent that is immiscible with water. Add an amount of
saturated aqueous sodium chloride, less than or equal to the amount of organic solution you
have.
Stopper the funnel and shake as in an extraction. Allow the layers to separate. The rules as to
which layer is on top are the same as for extraction. Since there is a lot of salt dissolved in the
water, the density of the saturated aqueous sodium chloride solution is 1.2 g/mL. Drain off the
lower layer. In this case, this is the organic layer and the layer you want to save. Dispose of the
aqueous layer in the aqueous waste carboy. (In some cases, for instance if you are using diethyl
ether, the upper layer will be the desired organic layer.)
Solid Drying Agents
Final traces of water are removed by treating the organic solution with a drying agent. A drying
agent is an inorganic salt which readily takes up water to become hydrated. Several such salts are
used routinely in the organic chemistry teaching labs:
Drying Agent
Capacity Speed Applications
Calcium chloride, CaCl2
High
Medium Used for hydrocarbons
Calcium sulfate, CaSO4 (Drierite) Low
Fast
Generally useful
Magnesium sulfate, MgSO4
High
Fast
Not used for very acid-sensitive compounds
Potassium carbonate, K2CO3
Medium Medium Not for acidic compounds
Sodium sulfate, Na2SO4
High
Slow
Generally useful
Of the five drying agents in the above table, magnesium sulfate is available as a fine powder and
the rest are of a larger particle size. Calcium chloride, magnesium sulfate, and sodium sulfate are
the three most commonly used agents. These are shown below.
Because these compounds will deliquesce (absorb moisture from the air), you MUST cap bottles
of drying agents between uses! Otherwise, they will end up looking like the photo below.
When the Lab Manual tells you to "Dry over..." it refers to the process below. Usually, you will
perform a wash with saturated sodium chloride solution to remove the bulk of the water before
treating with an inorganic salt.
Add a small amount of the solid drying agent directly to the organic solution. Swirl the solution.
Observe the drying agent; if it is all clumped together, add more. The pictures below show how a
solution with drying agent looks when it is clumped and when it is free-flowing.
Each drying agent will have a slightly different appearance when clumped, and practice will
make you better at judging whether or not the inorganic salt is wet or dry. There is no set rule as
to how much drying agent needs to be added. The amount required depends on the amount of
water in the solvent solution which you are drying, and this amount varies from experiment to
experiment. Use as much as it takes to dry the solution. In most cases, drying is as complete as it
will get in 20 minutes. When drying is complete, you need to remove the dried organic solution
from the drying agent. There are several methods by which you can do this.
If the powder is quite fine (as when using magnesium sulfate) or if the volume is large, gravity
filtration is the method of choice. If the drying agent is of larger particle size (as when using
sodium sulfate or calcium chloride), decanting is the method of choice. An alternative to
decanting is removing the liquid from the drying agent simply by drawing it off with a Pasteur
pipet. Squeeze the bulb of the pipet, carefully place it flush with the bottom of the flask, and
slowly draw liquid into the pipet, leaving the desiccant behind. Squirt the liquid into a fresh,
clean flask. These two methods are shown below.
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Biochemistry. The information on these pages is available for academic use without restriction.
This page was last updated on May 20, 2015.