Monday, July 3, 2017

Paper Chromatagraphy

I need to prepare a paper chromatagraphy lab for Advanced Placement Chemistry. The subject is a lab that lots of high school chemistry students have performed, but it sounds like AP students could use a refresher. Besides, it gives me a lab that these advanced students can do their first day back to school without over racing them. It doesn't hurt that the AP test asked a question on this topic. So here is the results of the first paper chromatagraphy lab I've done privately for myself at home.

First, find a colorful leaf to extract the pigments from. I choose a plant that had strong red pigmentation this spring.

A leaf from my front yard. This use to be more redder this spring.

Before performing chromotagraphy, the sample's pigments must be extracted. This procedure was the first experiment I performed for this lab. So two popular solvents are water (the universal solvent) and propanol and I tested both for this experiment.

The solvents can't do their job unless the pigments inside the leaf are not exposed to the solvents. Therefore I cut two leaves per solvent into thin slices. Then I added them to small glass bowls filled with their designated solvent.

Sliced leaves in water on the left and in 91% isopropanol on the right. Barely cover the leaves in solvent (unlike what I initially did) or else the concentration of pigment will be too small.

The solvents were hot, I microwaved the water and soaked the glass bowl of alcohol in a hot water bath (I have a bad feeling about what would happen if I were to microwave alchol). After soaking the leaves for over 30 minutes, place chromatagraphy strips into each glass container. For my strips, I used strips of a coffee filter my wife was unhappy with.

You'll notice that the alcohol was more effective at pulling pigment out of the leaves. Does this mean the green pigment (chlorophyll) is not polar?

There are intermolecular forces that pull solvent up the chromatagraphy strip. It's strongest for water since it rose to the height of the strip much faster than the alcohol. In this case, the intermolecular force was probably hydrogen bonding between water and cellose.

After the water was finishing climbing up and soaking the coffee filter chromatagraphy strips, I removed them and made some measurements.

The water-solvent strip is on the left and the alcohol-solvent strip is on the right. It's easy to see the pigment when extracted by alcohol. The pigment was not extracted by hot water.

Just extracting the pigment is not enough. The pigment's R-factor, or retention factor must be measured. It's calculated by dividing the length the pigment travelled by the length the solvent traveled, or the solvent front. My measurement of the alcohol strip was as follows.

Solvent Front: 1.50 inches
Pigment Line: 1.15 inches
R-factor: 0.77

What's Next?
So first off, I didn't extract enough pigment. I need a more concentrated solution of pigment and a larger quantity. Using more leaves, less solvent, and crushing the leaves should be effective in this regard. Greater pigment concentration means the pigment line will be darker and Amy pigments of lower concentration will become visible.

Second, I want to see how far water will draw up the year's pigment. Therefore, I will need to extract the pigment with alcohol and place a drop on the chromotagraphy strip. Then place the strip in a water solvent. A comparison of R-factor in water and alcohol should indicate the degree of polarization in the pigment molecule.

I'll let you know what happens next time.

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