Separation Techniques
There are a variety of ways to separate a mixture into its components by taking advantage of differences
in their properties. For example, we could separate a mixture of gold pieces and iron pieces by either
picking them out into two piles based on their color or by using a magnet (the iron will be attracted to
the magnet, the gold will not). This is a simple example but what if we had a precipitate mixed in a
solution or a solid dissolved in a solvent? We must employ different techniques based on what type of
mixture we have.
The four main types of separations techniques that we will study are:
1. Filtration
2. Distillation
3. Paper Chromotography
4. Column Chromotography
Filtration
Filtration is a separation technique used to separate a solid and liquid from each other.
The apparatus consists of:
1. Funnel with filter paper
2. Beaker (to catch the filtrate)
3. Glass rod (to prevent splattering)
The mixture of the solid and liquid is poured through a piece of filter paper in a funnel. The liquid will
pass through the paper and the solid will remain on the paper.
Filtrate – the liquid that is able to pass through the filter paper
Residue – the solid left in the filter paper following fitration
Link from video in video lesson: http://www.youtube.com/watch?v=-yhLepvhM4s
Distillation
Distillation is a process that can separate two miscible liquids from each other based on their differing
boiling points.
The apparatus consists of three main parts.
1. Distillation Flask - it is used to heat the mixture and volatize the components.
2. Condenser – used to cool the vapors back to the liquid state
3. Collection Vessel - used to collect the vaporized gas that has been condensed back into a
liquid
A common distillation is the separation of
ethanol from water. The ethanol-water
mixture is placed in the distilling flask and
heat is applied. The ethanol has a lower
boiling point than the water (due to the
weaker intermolecular forces in ethanol) so it
will evaporate first. Once the boiling
temperature of the ethanol is reached
(78oC), you will see the bubbles throughout
the liquid.
The evaporated/boiled gas
travels up the neck of the flask and into the condenser. The condenser is surrounded by a constantly
refreshing cool water source. This cools/condenses the gas back into a liquid and it will drip into the
collection vessel. You know you have collected all the ethanol when you start to see the boiling stop as
it will take more energy to get to water’s boiling pont (this is an indication that the phase change for the
ethanol is complete).
Link for video shown in video lesson: http://www.youtube.com/watch?v=mP4Hgui-g6U
Paper Chromotography
Paper chromatography is a separation technique that allows you to separate out the components of a
mixture into the components.
Paper choromotography consists of a stationary phase (the chromatography paper) and a mobile phase
(the solvent in the container). The mobile phase flows through the stationary phase and carries the
components of the mixture with it. Different components travel at different rates.
The process for setting up a paper chromatography:
1. Obtain a piece of chromatography paper. Make a line IN PENCIL about 2 centimeters from the
bottom of the paper. You use pencil as the pencil will not run or dissolve.
2. Make a spot on the line using the mixture you are testing. The spot should be small and
concentrated. Make sure to allow the spot to dry before continuing.
3. Pour the solvent you are using into the bottom of a flask or beaker but no deeper than 1
centimeter.
4. Place the chromatography paper into the solution so just the end of the paper is submerged (the
pencil line and spot should NOT be submerged into the liquid and should be above the liquid
level.
5. Secure the paper, cover the flask/beaker and allow time for the solvent to move up the
chromotography paper. Covering the container allows the solvent vapor to saturate the inside of
the flask. This stops the solvent from evaporating as it travels up the paper.
6. Remove the chromatography paper. Immediately mark with your pencil the “solvent front” ( that
is where the solvent ended up) as well as circle the spots where the separation occurred.
What is happening?
As the solvent slowly travels up the paper, the different components of the ink mixtures travel at different
rates and the mixtures are separated into different colored spots.
The stationary phase is usually a piece of high quality filter paper. The mobile phase is a developing
solution that travels up the stationary phase, carrying the samples with it. Components of the sample will
separate readily according to how strongly they absorb onto the stationary phase versus how readily they
dissolve in the mobile phase.
When a colored chemical sample is placed on a filter paper, the colors separate from the sample by
placing one end of the paper in a solvent. The solvent diffuses up the paper, dissolving the various
molecules in the sample according to the polarities of the molecules and the solvent. If the sample
contains more than one color, that means it must have more than one kind of molecule. Because of the
different chemical structures of each kind of molecule, the chances are very high that each molecule will
have at least a slightly different polarity, giving each molecule a different solubility in the solvent. The
unequal solubilities cause the various color molecules to leave solution at different places as the solvent
continues to move up the paper. Most times we use a polar solvent. In this case, the more soluble a
molecule is (remember “like dissolves like”), the higher it will migrate up the paper. If a chemical is very
nonpolar it will not dissolve at all in a very polar solvent (remember “like dissolves like”). This is the
same for a very polar chemical and a very nonpolar solvent. It is important to note that when using
water (a very polar substance) as a solvent, the less polar the color, the lower it will rise on the paper.
Calculations of Rf values
Rf values are the “rentention factors”. These values will be consistent from one trial to another as long
as the same sample is used and same solvent is used.
Rf =
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑓𝑟𝑜𝑚 𝑠𝑡𝑎𝑟𝑡𝑖𝑛𝑔 𝑙𝑖𝑛𝑒 𝑡𝑜 𝑐𝑒𝑛𝑡𝑒𝑟 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡 𝑠𝑝𝑜𝑡
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑓𝑟𝑜𝑚 𝑡ℎ𝑒 𝑠𝑡𝑎𝑟𝑡𝑖𝑛𝑔 𝑙𝑖𝑛𝑒 𝑡𝑜 𝑡ℎ𝑒 𝑠𝑜𝑙𝑣𝑒𝑛𝑡 𝑓𝑟𝑜𝑛𝑡
 Solvent front line
Link for video shown in video lesson: http://www.youtube.com/watch?v=XPKjHkm3A_0
Column Chromatography
Column chromatography is a technique that allows the components of a mixture to be separated. It is
particularly useful if you are trying to separate a larger mixture into two or more smaller samples of each
part. Paper chromatography only allows to you to separate a small “spot” out to measure the Rf values.
Before we start, it would be beneficial to color some areas on the diagram below. In the first column,
color the area by A green. For columns 2 through 6, color the B yellow and the C blue.
In column chromatography, the stationary phase is the silica gel and the mobile phase is the solvent.
To carry out a column chromatography, the
experimenter would use column apparatus (we
usually use a Pasteur pipet for this process…it looks
like a small buret or a large pipet). The tip of the
column is blocked with a small piece of wool to
prevent larger particles from passing through. Silica
A
B
C
B
(a dry granular substance is then added to the
C
B
column to fill about three-fourths full and packed
down (this is called “dry packing” because you are
B
making sure the dry material in the column is
C
C
B
packed down). Make sure the silica is flat at the top. A small amount of sand can be placed on top of
the silica to help keep it flattened.
Add the first solvent you will be using (the least polar solvent) to the column. Allow time for the solvent
to travel down the gel (this is called “wet packing” because you are wetting down the silica after it was
packed).
Place the mixture to be separated on top of the silica gel in the column (see green A area in the first
column). Add a small amount of solvent on top of the mixture. The mixture will start to travel down the
column and will start to separate (see the blue (C) and yellow segments (B) to the right). More solvent
may need to be added as the process continues to ensure the silica stays wet. As the first band of
color reaches the bottom end of the column, put a fresh beaker or flask under to catch the component.
As soon as it all passes through, switch to a new beaker. You have one of the components separated.
To get the second one out you may need to add a NEW solvent to the top of the column. This solvent
will need to more polar as the first one we used was less polar. This will allow the more polar second
band of color to be able to travel farther down the column.
Link from video in video lesson:
http://chem-
courses.ucsd.edu/CoursePages/Uglabs/143A_Weizman/Recrystflash/Chromatography.html