MadSci Network: Chemistry
Query:

Re: separation of mixtures

Date: Wed Sep 30 20:52:06 1998
Posted By: Jason Henrie, Graduate Student, Enology, UCDavis
Area of science: Chemistry
ID: 901329993.Ch
Message:

Valerie,

Valerie--

You have asked a question that would take a textbook to answer sufficiently! However, the short answer is that if you have a mixture of chemicals, and there is any sort of difference between them, then they can be separated (to a certain extent) by exploiting that difference! (However, it may be easier said than done!) Most chemists do this type of stuff at work every day so I'll use examples from my day at work.

Selective Dissolution

Different compounds may be more or less soluble in a particular solvent. For example, table salt dissolves very easily in water but not in olive oil. Gasoline, however, would dissolve in olive oil but not water! This is due to the interactions of the solute and the solvent. It takes a huge amount of energy to separate charged particles (like sodium and chloride in salt) and so salt is very stable. Water, however, is somewhat charged itself so that one end of water surrounds the sodium and the other end surrounds the chloride and so they move apart without separating any charges! The electrons in water are free to move over the whole molecule, but they tend to stay around the oxygen the most. The result is that the area around the oxygen is slightly negatively charged and the areas around the hydrogens are slightly positively charged. Water is shaped like a stubby "V" so there is an overall negative charge near the point. When charges are distributed unevenly like this the molecule is called polar. If there isn't an overall charge then it is nonpolar. Olive oil and gasoline are nonpolar and can dissolve eachother. Nonpolar compounds interact through a different mechanism; but the result is that "Like dissolves like"--that is, polar dissolves polar and nonpolar dissolves nonpolar.

In organic chemistry, you may have a mixture of 10 compounds after a reaction and you only want one. If you know that the one you want is very nonpolar and most of the rest are polar, you can mix your mixture with water (polar) and toluene (nonpolar) in a separatory funnel. There will now be two layers (like oil on water) and the one you want is in the water layer and everything else is in the toluene! Today at work I wanted to extract some tannins (somewhat nonpolar) from some grape skins. When I put the grape skins in water the tannins didn't dissolve but all the sugars did.. So I pulled the skins out and next put them in acetone (somewhat nonpolar) and the tannins dissolved! So I separated the tannins from the sugars based on polarity.

Decantation and Filtration (and Centrifugation)

The more massive a particle is the closer to the bottom it is when placed in a liquid. For example, soil chemists may want to know how much sand (massive) and clay (tiny) a soil has. They take a hand full of dirt and put it into a column of water and wait for a day or a week. Then they will have layers in the column with the most massive particles at the bottom and the smallest ones at the top. They could then decant or pour off the first layers (clay) and leave the bottom layers (sand) and it is now separated!

Filters act like nets that only let things through that are smaller than the pore sizes. So clay and sand could be separated that way also. Soil scientists often put a series of these together with the net with largest holes on top and, then a medium sized one under that, and then a small one on the bottom--so that they can add soil to the top and divide it into fractions based on size.

I used a filter today to separate the grapeskins from the water. The water (and pectin, sugars, and other polar things) passed through a filter and the grapeskins stayed on top (and so did the tannins and other nonpolar things that were inside the grapeskins that were not yet extrated by acetone).

Filters pores can be extremely small. You can separate bacteria from viruses with a filter.

A centrifuge spins tubes around in a circle at something like 10,000 times a minute. This putes a huge force on the tubes much like gravity but stronger. The result is like gravity for decanting, but it works much more quickly and it can differentiate between things more selectively. You can separate DNA from cells with a centrifuge.

Evaporation and Distillation (and Freezing)

Any compound can be either a liquid, gas, or solid. It depends on the pressure and the temperature. The compound is exactly the same, it is just how the compound interacts with its surroundings that changes. Water, for example, is liquid at room temperature. If you heat it, the temperature will raise until you reach the boiling point and the temperature will not raise after that! All of the liquid will eventually just boil away. (This is why you add salt to water for cooking! It raises the boiling point so your water is hotter and food cooks faster.) So where does all of the energy from the stove burner go if the temperature is not raising? The energy is used to overcome a phase transition--that is, it is used to convert water to steam. The stronger the interactions beetween the liquid molecules, the more energy it takes to overcome them, and the higher the boiling temperature. Adding salt to water makes the interactions stronger (salt-water solutions are stable, that's why salt dissolves in water AND why salt-water boils hotter) and raises the boiling point.

Tommorow I will have to separate tannins from acetone. Fortunately this is very easy and I can sleep in! The boiling point of acetone is somewhere around 50 degrees Celcius (water, for reference, is 100) and that of tannins is probably around 200-300. So I can simply boil away the acetone and the stuff left over is what I want! However it isn't quite this easy. What if the tannins are damaged by so much heat? Instead I will put them under a vacuum (which reduces the boiling point) and use a very low heat to evaporate the acetone.

Distillation is just evaporation, except you collect the stuff that boils and leave the rest behind. For example, to make brandy you boil wine and catch the steam with a funnel above the wine. Then you cool the seam and it condenses into brandy. Actually, it makes a really nasty brandy at this point because some of the stuff that you didn't want still managed to volatilize (say 1/3 of it). So you distill the nasty brandy again! This time again say 1/3 of the nasty stuff came over also. But now only 1/9 of the original nasty stuff is there. So do it again and now only 1/27 of the nasty stuff is there and the brandy tastes pretty good! This technique of multiple distillations (or freezings or dilutions or whatever) is important.

To make alcohol-free beer you can put beer in the freezer and all of the water freezes and the alcohol is still a liquid which you can simply pour off (decant).

Chromatography

Chromatorgaphy started with a botonist separating plant compounds with a column full of clay. He washed the column with a nonpolar solvent and the nonpolar things came through the column faster than the polar things (which stick to clay because it is polar). So the more polar a compound is, the longer it takes to come through the column.

The trick is to pack the column with anything that separates your compounds--and a lot of genius has gone into this and a huge variety of things can be separated by chromatography!

 

As you can see, usually it takes several of these separation techniques to actually make a separation! It would be difficult to just use one of them and not any other to purify a mixture. This stuff can get quite detailed and whole text books are availabe one just Gas Chromatography or Distillation. Here are some web sites:

http://kerouac .pharm.uky.edu/ASRG/HPLC/hplcmytry.html All about High Performance Liquid Chromatography

http://ull.chemistry.uakron. edu/chemsep About separations in general, taught at the University of Akron

 

I hope this information is useful!

Jason

 


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