Thin-Layer Chromatography and R_f

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle. Two students run the same compound on the same kind of plate, with the same solvent. One stops the run early, and the other lets the solvent climb much higher. So the same compound ends up at two different heights. Then how can a spot's height ever tell us which compound it is? Strange, right? Today we find one number that stays the same however far the solvent runs: the R f value. It comes up often in JEE Main, so let's begin.

0:36Setting up a TLC plate

So, how is a TLC plate made? Take a glass plate, and coat it with a thin layer of silica gel, or alumina. The drawing on the right shows a plate after a run, and we will read it part by part. Okay? How thin is this layer? About zero point two millimetres. The wavy equals sign on screen means about, and m m stands for millimetres. With me? Next, a tiny spot of the mixture goes about one centimetre above the lower edge, and c m stands for centimetres. This is the base line: in the drawing, it is the line near the bottom, with two small circles where the mixture was spotted. Right? Now the plate is stood in a little solvent, in a closed jar: that is the pale blue band at the bottom of the drawing. But the spots sit above it. So let me ask you this: what makes the solvent climb up the plate? Yes, capillary action, exactly! As it rises, the solvent carries the components to different heights. See spots A and B, and the dashed line near the top, the solvent front, which shows how far the solvent reached. Clear?

1:46The retardation factor Rf

Now, back to our puzzle. The height of a spot depends on how far the solvent ran. So what should we compare the spot's distance with? Yes, exactly, with the distance the solvent itself moved! This ratio is the retardation factor, written as capital R with a small f below it, and read as R f. R f equals the spot's distance, over the solvent front's distance. The drawing on the right writes the same formula in words. Okay? And both distances are measured from the base line. In the drawing, the green arrow, d with a small A below it, runs up to spot A, the shorter orange arrow, d with a small B, runs up to spot B, and the long black arrow, d with the word solvent below it, runs up to the solvent front. So the R f value of spot A is d A over d solvent. Got it? Now let me ask you this: can an R f value ever be more than one? No, it can never be more than one. No substance moves ahead of the solvent that carries it, so the top of the fraction is never bigger than the bottom. And a ratio of two lengths has no units. So every R f value lies between zero and one. Right? Now, why is this number so useful? Keep three things the same: the adsorbent, the solvent, and the temperature. With me? Then the R f value is characteristic of the compound, like its fingerprint. Run the solvent further, and the spot runs further in the same proportion, so the ratio stays the same. That solves our puzzle! Clear?

3:25Trap: measure from the base line, not the edge

Now, a very common trap. Take a plate six centimetres long, with the base line zero point five centimetres from the bottom, and the solvent front zero point five centimetres from the top. So how far did the solvent move? Six centimetres, right? No, no, careful, that is the trap! The solvent starts at the base line, not at the edge of the plate. So six, minus zero point five, minus zero point five, equals five centimetres, not six. Always measure from the base line.

3:56Seeing the spots

Okay? Now, how do we actually see the spots after the run? Coloured compounds are easy, we see them directly. But how would you find a colourless spot, one that you cannot see? Good, there are three ways. The first way is ultraviolet light, on a fluorescent plate. Many compounds quench, or dim, the glow of the plate, so the spots show up. Right? Second, keep the plate in a jar of iodine crystals. The organic spots take up the iodine and show as brown spots. With me? Third, spray a reagent. Ninhydrin gives purple spots with amino acids, and this one comes up often. Got it? And here is a rule to remember. The visualising agent, which makes the spots visible, is never put in the mobile phase, the moving solvent.

4:48Polarity and Rf

Clear? Now, polarity. Silica is polar, so it holds a polar compound strongly. So tell me, will a strongly held compound have a high R f value, or a low one? It will be low, yes, exactly! It moves less, and stays near the base line. The double arrow on screen means so, or therefore. The notes on the drawing agree: spot A is less polar and weakly adsorbed, with a high R f value, while spot B is more polar and strongly adsorbed, with a low one. Okay? And now the solvent. Switch to a more polar solvent, and all the R f values rise, because the solvent competes with the compounds for the adsorbent, and moves them further. So the R f value changes when the solvent changes. Right?

5:35Purity, identity and preparative TLC

Now, what is TLC used for? First, purity: a pure compound gives just one spot. With me? Second, identity. Run your compound beside an authentic sample of the expected compound. Spots at the same height, under the same conditions, mean it is very probably the expected compound. Got it? And third, preparative TLC, where a thicker layer separates small amounts, tens to hundreds of milligrams, written m g. The band is scraped off, and the compound is extracted from it. Clear?

6:10Example 1: Rf when spot moves 4.5 cm, front 7.5 cm

Now, Example one, an easy one to warm up. The solvent front moves seven point five centimetres, and a spot moves four point five centimetres from the base line. Find the R f value. Pause the video and try it first. Done? Okay, let's check. Both distances are from the base line, so it is spot over solvent: four point five over seven point five gives zero point six zero. And it lies between zero and one, just as it must. Right?

6:42Example 2: Rf on a 10 cm plate, measured from base line

Now a slightly harder one, Example two. Take a plate ten centimetres long, with the base line one centimetre from the bottom, and the solvent front stopping one centimetre from the top. Now, a spot is five centimetres from the bottom edge. Find its R f value. Pause the video and try it, and remember our trap. Done? Okay, let's check. First, the solvent. It starts at the base line, and stops one centimetre below the top. So ten, minus one, minus one, equals eight centimetres. Okay? Now the spot. It is five centimetres from the bottom edge. So the spot moved five centimetres, right? No, no, wait, that five is measured from the edge of the plate, not from the base line! So five minus one, equals four centimetres. With me? So the R f value is four over eight, which is zero point five. Same trap as our six centimetre plate: always measure from the base line, for the spot and for the solvent. Got it?

7:50Quick revision: Rf formula, polarity, seeing the spots

So, quick revision. R f is the spot's distance over the solvent front's distance, both from the base line. It has no units, it lies between zero and one, and polar compounds on silica have low R f values. Colourless spots are seen with ultraviolet light, iodine, which gives brown spots, or ninhydrin, which gives purple ones. Now go and try the four practice problems on this page. All the best!