Paper Chromatography

Transcript

0:00Introduction

Hello everyone! Let us start with a little experiment. Put a small spot of ink near one end of a strip of special paper, and suspend the paper in a solvent. Then just wait, and nobody touches anything. Slowly, the solvent climbs up the paper all by itself, and the single spot of ink splits into separate spots, at different heights. So here are two puzzles for you. What makes the solvent climb? And what is really holding the spots back? Is it the paper itself? Hold on to your answers; by the end of this video, both will be clear. Questions on paper chromatography and R f values do come up in JEE Main, so this is worth your time. So, let's begin.

0:43The set-up: paper, water and a rising solvent

So, first, the paper. Paper chromatography uses a special paper, called chromatography paper. And here is the key point: this paper holds water in its pores. Okay? Now, that water stays on the paper; it does not move. So this water is the stationary phase. Remember this line, because it is the most important line of today's lesson. Right? Next, we put a small spot of the mixture near one end of the paper. With me? Then the paper is suspended in a solvent. This solvent is the mobile phase, the phase that moves. Clear? Now, this is our first puzzle from the start. Nobody pumps the solvent up. So what makes it climb the paper, all by itself? It is capillary action, very good! Liquid rises on its own through narrow pores, and the paper is full of them. So, one phase stays, the water, and one phase moves, the solvent. Got it?

1:44Who travels further? Partition between two liquids

Now, as the solvent moves up past the spot, what happens to each component of the mixture? It is partitioned, which means shared, between two liquids. Okay? Which two liquids? The water held on the paper, and the solvent moving up. Each component distributes itself between the two. Right? So let me ask you this. One component is more soluble in the water on the paper, and another is more soluble in the moving solvent. Which one travels further up the paper? Yes, exactly, the one more soluble in the mobile phase, the moving solvent! The solvent carries it along, so it travels further. And the one that prefers the water on the paper is held back. With me? So, because of this sharing between two liquids, paper chromatography is called partition chromatography. Remember that name. Clear? Now, careful here. Some students write adsorption, because thin-layer chromatography, TLC, from the last lesson, is adsorption, on a solid adsorbent. But on paper, the stationary phase is a liquid, the water. So it is partition, not adsorption.

2:55The Rf value: defined just as in TLC

Got it? Next, how do we say how far a spot has moved? We use the R f value, written as a capital R, with a small f below it, as a subscript. It is defined just as in TLC: the distance moved by the substance, over the distance moved by the solvent front, the top edge of the rising solvent. The line in the middle is a fraction bar, so read it as over. Okay? And both distances are measured from the base line, the line where the mixture was spotted, not from the edge of the paper. Right? Now tell me. Both distances are in centimetres. So what are the units of the R f value? It has no units at all, correct! It is a ratio of two lengths, so the centimetres cancel, and we get a pure number. With me? And for a given solvent, the R f value is characteristic of the compound, like its fingerprint. But change the solvent, and the R f value changes too. Keep this in mind, because we need it in a minute. Clear?

3:58Uses, and what to do when spots overlap

Alright, where is paper chromatography used? For amino acids, sugars, inks and plant pigments. So yes, our spot of ink from the start fits right in. Okay? Now, a real problem. Two amino acids give overlapping spots with one solvent, so we cannot see them apart. How can we separate them on paper? Here is the trick. Turn the paper through ninety degrees; the small circle on screen means degrees. Then run it in a second, different solvent. Right? And why does this work? Remember, the R f value changes with the solvent. So the second solvent separates the spots that overlapped in the first. This is called two-dimensional paper chromatography. Got it?

4:46Example 1: is the stationary phase cellulose or water?

Now, Example one, and this is our second puzzle from the start. What forms the stationary phase in paper chromatography: the cellulose, or the water in its pores? Cellulose is what the paper itself is made of. Pause the video and try it first. Done? Okay, let me test you first. The paper is the solid that stays fixed, so the stationary phase must be the cellulose, right? No, no, careful, that is exactly the trap here! The cellulose only holds the water. It is just the support. Okay? So the stationary phase is the water in the pores. And that answers our puzzle: what holds the spots back is the water on the paper, not the paper itself. Two liquids, so partition, exactly as we said.

5:36Example 2: ratio of Rf values of two compounds

Clear? Now Example two, a little harder, and in the JEE Main style. In a paper chromatogram, the solvent travels three point two five centimetres, compound A travels two point zero eight centimetres, and compound B travels one point zero five centimetres, all from the base line. Find R f of A over R f of B; the brackets on screen just name the compound. Pause the video and try it first. Done? Okay, let's check. R f of A is its distance over the solvent distance, so two point zero eight over three point two five. And R f of B is one point zero five over three point two five. Right? Now, do we really need to divide both of these out? No, we don't, and this is the topper's trick! It is the same run, so both have the same solvent distance below the line. In the ratio, the three point two five cancels. So R f of A over R f of B is just two point zero eight over one point zero five. With me? Now divide. Two point zero eight over one point zero five gives about one point nine eight. The wavy equals sign means approximately. So the ratio is about two. That's it! And notice one more thing. Compound A moved further up the paper, so it is the one more soluble in the mobile phase. Clear?

7:05Quick revision: phases, partition, Rf and 2D runs

So, quick revision. In paper chromatography, the stationary phase is the water in the pores, and the mobile phase is the solvent. It is partition, so the component more soluble in the mobile phase travels further. The R f value is defined just as in TLC, and if two spots overlap, turn the paper through ninety degrees and use a second solvent. Now go and try the two practice problems on this page. You will do well. All the best!