Principle of Chromatography

Transcript

0:00Introduction

Hello everyone! Let us begin with a simple green leaf. Crush it in a little solvent, and you get a green liquid, right? It looks like one single colour. But the green of a leaf is really a mixture of several pigments. So here is a puzzle for you: how would you pull those pigments apart, without any chemical reaction at all? Not so easy, is it? The method that does exactly this is called chromatography, and today we will see the simple principle behind it. Chromatography does come up in JEE Main questions, so this is worth your time. So, let's begin.

0:37The idea: one phase holds, one phase moves

So, first the name. Chromatography comes from the Greek word chroma, which means colour. Why colour? Because the method was first used to separate plant pigments, just like the pigments in our leaf. Okay? Now, every chromatography has two parts. The mixture is applied to the first part, called the stationary phase. Stationary means it stays fixed. It is a solid, or a liquid held on a solid. With me? The second part is the mobile phase, which simply means the moving phase. It is a liquid or a gas, and it moves over the stationary phase. Right? So now picture two components of the mixture. The stationary phase holds one of them strongly, and the other only weakly. And the mobile phase keeps moving over them. Let me ask you this: which component falls behind? Yes, exactly, the one held more strongly moves more slowly! The double arrow on screen simply means so, or therefore: held more strongly by the stationary phase, so it moves more slowly. Clear? And that is the whole principle. Different components are held with different strengths, so they move at different speeds, and they separate. That is how the pigments of our leaf get pulled apart, with no reaction at all. Got it?

1:55Two main kinds

Now, chromatography comes in two main kinds. The first is adsorption chromatography. Here the stationary phase is a solid adsorbent, such as silica gel or alumina. An adsorbent is a solid that holds other molecules on its surface. Okay? The components are adsorbed to different extents, so we say separation is by differential adsorption. Column chromatography and thin-layer chromatography, written TLC on screen, are both of this kind. With me? The second kind is partition chromatography. Here the stationary phase is a liquid, held on a support. Right? Each component is shared, or partitioned, between the stationary liquid and the mobile liquid, depending on how well it dissolves in each. So here, separation is by differential solubility. Clear? Now, the best-known partition method is paper chromatography. So tell me, what is the stationary phase there? The paper itself, right? No, no, careful, that is a trap! The stationary phase is the water held in the pores of the paper. The paper only holds that water in place. And that is exactly why paper chromatography is a partition method, and not an adsorption method. Got it?

3:12Polarity decides who moves

Now, the most useful rule of this topic. Silica gel and alumina are polar adsorbents. So let me ask you this: on silica, will a polar compound move more, or less? It moves less, very good! On a polar adsorbent, the more polar compound is held more strongly, so it moves less. Read the two double arrows on screen: more polar, so held more strongly, so it moves less. Okay? Now, a non-polar compound is the opposite case. The polar adsorbent hardly holds it, so it simply moves along with the solvent. Keep this rule ready, because both our examples today use it. Right?

3:55What does the separation depend on?

Now, let us list what decides the result of a chromatography. First, the adsorbent and the solvent you choose. With me? Next, the polarity and the solubility of the compounds themselves, which is what we just used. Clear? And for a column, its length and the flow rate of the solvent matter as well. Okay? Now, here is a question that comes up in statement form. Suppose one pure compound is a solid, and another pure compound is a liquid. Does that change the separation? The answer is no, it does not. Every compound is applied in solution, so its state as a pure compound does not matter. The result does not depend on whether the pure compound is a solid or a liquid. Remember this one. Got it?

4:44Example 1: hydrocarbon vs alcohol, which moves further

Now, Example one, an easy one to warm up. On silica gel, which moves further: a hydrocarbon, or an alcohol of similar size? Pause the video and try it first. Done? Okay, let's check. Look at the alcohol first. It has the O H group, oxygen joined to hydrogen, and this group is polar. Right? And silica is polar too, so it adsorbs the alcohol strongly. Held strongly means it moves slowly. Clear? The hydrocarbon is non-polar, so silica hardly holds it, and it moves along with the solvent. With me? So the answer is the hydrocarbon: it moves further. Polarity decided everything, just as our rule said. Okay?

5:32Example 2: adsorption order of three compounds on silica

Now a slightly harder one, Example two. Arrange naphthalene, benzophenone and benzoic acid in order of increasing strength of adsorption on silica gel. Pause the video and try it. Here is a hint: first decide what kind of compound each one is. Done? Okay, let's check. Naphthalene is a hydrocarbon, made of only carbon and hydrogen. Right? Benzophenone, as the ending of its name tells you, is a ketone. Clear? And benzoic acid is a carboxylic acid. Okay? Now the polarity. The hydrocarbon is the least polar, then comes the ketone, and the carboxylic acid is the most polar. The less-than sign on screen means less polar than. With me? Now careful here. Benzoic acid is the most polar, so it moves the least on silica. So it should come first in our answer, right? No, no, wait, read the question again: it asks for increasing strength of adsorption. The most polar compound is held the most strongly, so benzoic acid comes last. So the answer is naphthalene, less than benzophenone, less than benzoic acid. Notice that the order of how far they move is exactly the reverse. Always check which order the question wants. Got it?

6:55Quick revision: phases, adsorption vs partition, polarity

So, quick revision. The component held more strongly by the stationary phase moves more slowly, and that is how a mixture separates. Adsorption chromatography covers column and TLC, while paper chromatography is partition, with the water in its pores as the stationary phase. And on silica or alumina, the more polar compound is held more, and moves less. Now go and try the three practice problems on this page. You will do well. All the best!