PAPER CHROMATOGRAPHY

There are two phases in paper chromatography:

The stationary phase – the paperThe movable phase – the solvent

The molecules we wish to separate have attractions to both phases, but if they have different amounts of attractions to different phases they can be separated.

Let’s look at a specific example:

This is cellulose (filter paper)

Let’s abbreviate this…

Cellulose

OH

OHHOH2C

O

O

OH

OHHOH2C

O

O

This is allura red, a.k.a. F.D.&C. Red 40, a.k.a. red food coloring:

Let’s abbreviate this…

Red 40

Because of the similar O – H and O pieces on each molecules, these molecules attract each other with an intermolecular force called Hydrogen bonding.

Cellulose

OH

OHHOH2C

O

O

OH

OHHOH2C

O

O

Red 40

Water also can do hydrogen bonding, so it will attract to both the cellulose and Red 40.

Cellulose

OH

OHHOH2C

O

O

OH

OHHOH2C

O

O

Red 40

O HH

O HH

Let’s review:

Is Red 40 attracted to cellulose?

Let’s review:

Is Red 40 attracted to cellulose? Yes

Let’s review:

Is Red 40 attracted to cellulose? YesIs Red 40 attracted to water?

Let’s review:

Is Red 40 attracted to cellulose? YesIs Red 40 attracted to water? Yes

Let’s review:

Is Red 40 attracted to cellulose? YesIs Red 40 attracted to water? YesIs water attracted to cellulose?

Let’s review:

Is Red 40 attracted to cellulose? YesIs Red 40 attracted to water? YesIs water attracted to cellulose? Yes

Let’s review:

Is Red 40 attracted to cellulose? YesIs Red 40 attracted to water? YesIs water attracted to cellulose? Yes

Is Red 40 more attracted to cellulose or water?

Let’s review:

Is Red 40 attracted to cellulose? YesIs Red 40 attracted to water? YesIs water attracted to cellulose? Yes

Is Red 40 more attracted to cellulose or water? We don’t know, yet…

To separate mixtures (mixtures must be separable by physical means) of dyes by paper chromatography, we start with a piece of filter paper.

Then we draw a start line, and place our mixture (for example, purple Kool Aid). The Kool Aid dyes sticks to the cellulose with Hydrogen bonds.

Now place the paper into a container with a solvent, in this case water.(How did we know Kool Aid dyes would be soluble in water?)

What will happen between the water and the filter paper?

What will happen between the water and the filter paper?

The dyes also Hydrogen bond to the water, so as the water moves, what might happen to the dyes?

The dyes also Hydrogen bond to the water, so as the water moves, what might happen to the dyes?

This is not what would actually happen. The purple dye is made up of (at least) blue and red.

Here are three common food dyes. Notice how their structures have similar parts for Hydrogen bonding, but very different structures overall.

Will these dyes Hydrogen bond with cellulose and water in identical ways?

Here are three common food dyes. Notice how their structures have similar parts for Hydrogen bonding, but very different structures overall.

Will these dyes Hydrogen bond with cellulose and water in identical ways?Obviously not!

This is more like what would happened:(let’s pretend red moves more than blue)

When this happens, how can we compare the amount of Hydrogen bonding the red did with the water, and not the cellulose, compared to the blue?

When this happens, how can we compare the amount of Hydrogen bonding the red did with the water, and not the cellulose, compared to the blue?

Red Hydrogen bonds more to the water than the cellulose, whereas blue Hydrogen bonds more to the cellulose than the water.

So our mixture of red and blue dyes are separated, physically, based on molecular bonding. Furthermore, the one that travels the highest bonds more to water.

To compare our results to the results of others, we calculate a resolution factor, or Rf, for each dot.

Rf

=

Distance dot moved

Total distance solvent moved

A separate Rf calculation is needed for each color dot!

Round to the hundredths place!

Not all dyes would stick to cellulose, and not all dyes would dissolve in water. You must appropriately pick your surface and solvent!

As a general guide, the saying is “like dissolves like”, so the more alike two molecules are, the more they will bond, with either Hydrogen bonding, dipole bonding, or London Dispersion forces.

Let’s try this…

Oils have a general shape like this:

If we want to separate oils, which is better?

Water

Hexane

What parts can bond with water?

Water

Hexane

What parts can bond with Hexane?

Water

Hexane

Which would be better for oils; which is most like the oil?

Water

Hexane

With a good choice of stationary phase and mobile phase, many mixtures can be separated by chromatography.

Here is the separation of a black Sharpie.

Here are some more examples:

Paper chromatography is not the only type of chromatography. What conditions would make this method of paper chromatography not a good choice for separating a mixture?

SETTING UP FOR PAPER CHROMATOGRAPHY

Get some chromatography (filter) paper.

Cut it to size for the container.

Measure 1 cm from bottom for starting line and write your name on the top.

1 cm

Fit it into the container and the suspending devices (in this case paper clip and fuzzy wire).

Barely touching the bottom

Double dot your sample (in this case grape).

Place water in beaker.

Carefully lower the paper into the beaker with water.

Wait for it to develop. (and take quiz)

When finished,be sure to mark…

Solvent finish line

Center of each color

Then dump the beaker andplace the wire and clip nearthe beaker. Save your paperfor tomorrow!