LSS_Methods of Separation Lesson

Methods of Separation

Often solutions or mixtures need to be separated in order to be studied or to be purified. Because all mixtures are physical combinations, they can be separated by physical means. Sometimes separating a mixture is a easy as picking out the pieces that are larger than the others, like picking rocks out of a bucket of soil. However, many mixtures contain particles that are too small or are too difficult to be separated by hand. There are three main methods for separating such mixtures.

• Filtration
• Chromatography
• Distillation

Filtration

Filtration is the simplest of these methods. It is a separation technique that is used to separate a solid that has not dissolved in a liquid. This solid is often the precipitate of a chemical reaction.  

In lab, the common technique is to place a piece of filter paper inside a funnel. At home, coffee filters or paper towels can work as well. The funnel is placed over some sort of collection container.  

 

The mixture of the solid and liquid is stirred and poured into the filter paper. The liquid flows through the filter paper and into the container.

The liquid in the container is called the filtrate. The solid that remains in the filter paper is called the residue.

The residue can be dried by spreading it out on the filter paper and allowing the liquid to evaporate.

If a solid is dissolved in a liquid, it can be separated by crystallization simply by leaving the solution open to the air, allowing the solvent to evaporate.

As the saturated solution is allowed to cool, the solid will come out of the solution and crystals will start to grow. The crystals can then be collected and allowed to dry.

Chromatography

Chromatography is a method that separates chemical species by taking advantage of the different strengths of intermolecular interactions between and among the components. It is an extremely useful analytical technique available to chemists. It can separate complex mixtures with great precision. Even very similar components, such as proteins that may only vary by a single amino acid, can be separated with chromatography. Chromatography is used to separate and identify all sorts of substances in police work. Drugs from narcotics to aspirin can be identified in urine and blood samples, often with the aid of chromatography.

If you have ever written on a piece of paper with a marker and accidentally spilled water on the paper, you may have noticed that the ink splits up into colored patterns that spread across the page. You may have even done something like this as part of a craft project! This is an examle of chromatography, specifically paper chromatography. Chromatography literally means "color writing" (from the Greek words chroma and graphe). However, it does not have to involve colors or paper at all. There are many different types of chromatography.

We will discuss paper and column chromatography. And because paper chromatography is easy to understand, the process will be explained in that context.

Paper Chromatography

All chromatography has a mixture in one state of matter that moves over the surface of something in another state of matter that stays in place. The moving substance is called the mobile phase and the substance that stays put is the stationary phase.

In paper chromatography, the dissolved ink is the mobile phase and the paper is the stationary phase. The dissolved ink (the mobile phase) slowly travels up the paper (the stationary phase) and separates into different components that can then be identified.

A great analogy for how chromatraphy works is to think of it as a race. Visualize a group of kids about to race.

The kids all start together, but once they hear Go! they take off running. Kids don't all run at the same speed, so even though they start together, as the race progresses, the kids spread out according to how fast they can run. The chemicals in the mobile phase spread out in the same way.

The movement of the components in the mobile phase is controlled by the significance of their interactions with the mobile and/or stationary phases. Because of the differences in factors such as the solubility of certain components in the mobile phase and the strength of their affinities for the stationary phase, some components will move faster than others, thus facilitating the separation of the components within that mixture.

A molecule in the mobile phase can have a greater affinity for either the paper (the stationary phase) or for the solvent (also in the mobile phase). The phases are chosen such that components of the sample have differing solubilities or intermolecular attractions in each phase. A component which is quite soluble (meaning similar IMAs) in the stationary phase will take longer to travel through it than a component which is not very soluble in the stationary phase but very soluble in the mobile phase. As a result of these differences in mobilities, sample components will become separated from each other as they travel through the stationary phase. When a solute is made up of larger molecules, it will not react with the solvent as much to move up the paper.

This results in it appearing lower on the paper than other solutes with smaller molecules. 

Column Chromatography

In column chromatography, instead of paper, the stationary phase is a vertical column packed with a highly adsorbent solid, such as crystals of silica or silica gel, or a solid coated with a liquid.

The mobile phase is pumped at high pressure through the column and splits into its components, which are then removed and analyzed. In liquid-column chromatography, the mixture being studied is placed at one end of the column and an extra added substance called an eulant is poured in to help it travel through.

Thin-film chromatography is a variation of this technique in which the "column" is actually a film of glass, plastic, or metal coated with a very thin layer of adsorbent material. The smaller the affinity a molecule has for the stationary phase, the shorter the time spent in a column. 

Distillation

Distillation is a technique used to separate chemical species by taking advantage of the different strengths of intermolecular interactions between and among the components as they relate to vapor pressures and boiling points of the components in the mixture.

It is commonly used for the identification and the purification of organic compounds by separating them from other substances that have a lesser tendency to vaporize (less volatile). Scientists have been using the principles of distillation for thousands of years.

Remember that boiling point is the temperature where the vapor pressure of a liquid equals the atmospheric pressure. You learned earlier that as temperature increases, vapor pressure increases. Also, the stronger the intermolecular attractions, the lower the VP. So, compounds with higher vapor pressures will boil at lower temperatures.

The solution is heated until it boils. The liquid with the lowest boiling point boils first and becomes a vapor (gas). The vapor moves into the condenser which has cold water running through a jacket around the outside to keep the temperature below the boiling point of the vapor. The vapor is cooled in the condenser until the temperature falls back below the boiling point. As it condenses back into a liquid, it is collected in another container. The collected liquid is called the distillate.

After the liquid with the lowest boiling point has been collected, the temperature of the remaining mixture will rise to a new temperature when the liquid with the next lowest boiling point will boil and be collected. The process can be continued to separate all the liquids in the mixture.

Mixtures of liquids whose boiling points are similar (separated by less than 70°C) cannot be separated by a single simple distillation. In these situations, fractional distillation is used. The process is similar to simple distillation but adds in an additional step. A fractionating column is placed between the boiling flask and the condenser. In the fractionating column the gases condense and boil a second time before being collected. Fractional distillation is used to separate the components of crude oil.

Remember to work on the module practice problems as you complete each section of content.  

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