Purification

Purification Purpose and Importance

When preparing an organic solid or liquid, what kind of steps do we need to take to purify and extract it? These are the types of questions this area of chemistry asks.

We can therefore justify how this branch of science is so crucial to modern industries of food and medicine. For example, any type of drug after it is synthesised has to be extracted and purified, which is accomplished by the techniques set out int his article. If you can chemically make a compound but cannot eliminate your other reagents, by-products, and other impurities, what does it matter if you created the product?

Purification Methods

In this section, we will go over some common methods of purification that you will come across with the most. You will need to know these techniques and how they work, as you can apply them in many different settings. Some of the ones we will be discussing will be filtration, extraction, and separation.

Filtration

So what is filtration?

There are two main reasons to use filtration for, and they include to recover the insoluble solid particles which are in a solvent, or to purify the solution you are working with from impurities.

The main principle of filtration depends on the solubility of the particles which are suspended in the solution. If a particle is soluble in a solution it will be dissolved into the solvent, and produce a clear solution. On the other hand, if a particle or compound is not soluble, it will produce a discoloured and cloudy solution.

Filtration can help separate the insoluble particles from solutions. This is achieved by passing the solution by different types of filters. These filters are able to separate the liquid and insoluble particles based on the size of pores in the filter. Small pores will allow for the liquid to pass but leave the insoluble particles to remain. This is especially useful for the purification of liquids from impurities or for recovering insoluble compounds.

There are two main types of filtration you will come across: gravity filtration and vacuum filtration.

Gravity Filtration

Gravity filtration relies on gravity to pull the filtrate through a filter, usually flutter filter paper. This causes the gradual dropwise purification of the solution, with the insoluble particles being left on the filter. A variation of this

is filtration through celite.

Vacuum Filtration

As gravity filtration is often a slow and tedious process, vacuum filtration allows this process to be sped up. By connecting a vacuum pump to the filtration machinery, you can create an environment with a reduced pressure. Reduced pressure will cause the solution to be sucked through the filter rapidly.

Solid Extraction and Purification

In this section we will mention how you can retrieve solid particles from you liquid that don't involve filtration. These include drying and recrystallisation. These techniques are crucial for organic solid synthesis. You might wonder why we need these techniques if we already have filtration, but in the next couple of sections you will understand how these are crucial for purifying your substance.

Drying

So after you got your substance in question after filtration what do you do? The substance you will obtain will often be not 100% pure due to some small amount of solvent being left on your solid. This means that your solid might be still bit "wet".

Drying involves exposing your collected solid to different conditions for the purity of the compound to increase. This will happen due to the evaporation of the solvent that will occur in the process. This can be accomplished by leaving your extracted solid in a well-ventilated environment. Another method would be to leave your compound in a vacuum or desiccator.

Recrystallisation

Recrystallisation relies on growing crystals of your desired compound from a solution. This is a purification technique as the produced crystals are very pure and usually contain only the compound you are tiring to extract.

Crystals are formed only when the solution is a saturated one. This is done by heating a solvent with your impure compound, as the solubility of the compound increases at higher temperatures. As the solution cools, pure crystals will form in the beaker. Removal of the crystals can occur through filtration and further drying.

Liquid Extraction

Liquid extraction requires on the separation of two compounds in the liquid state from each other. This is a purification technique that exploits different properties of liquids, such as their physical properties. There are two ways in which two liquids can be separated, which are through distillation or a separation funnel.

Separation Funnel

A separation funnel can separate two liquids which don't mix. This can be due to their polarity being different, which causes the molecules of the two liquids not to exhibit intermolecular forces.

A separation funnel is a large vessel with an adjustable stopper at the bottom, which allows for the decanting of the 'bottom' layer of the solution.

Distillation

Distillation is a purification method that is used for two different liquids which are mixed together. Distillation exploits different boiling points of the solutions you are working with to separate them.

The distillation apparatus is comprised of a flask which is heated, which is connected to a condenser. Vapour condenses in the condenser as cold running water is pumped through it. This means that particles which are in the gas phase travel from the flask to the condenser, where they are turned back into liquid, and then collected in another flask.

Distillation machinery is connected to a thermometer that tracks the progress of the purification. This is because different compounds boil at different temperatures, which means you can extract different compounds at different times. The compounds with the lowest boiling point will be collected first, and later the compounds with the higher boiling points will be collected.

Fig. 2: Distillation Apparatus. Source: Wikimedia Commons.

Purification Method Differences

In this section, we will describe some of the most important differences that arise when using different types of purification methods.

The most common difference that arises when using different methodologies for extracting substances is if the solution needs to undergo some kind of physical or chemical property change or not. This means if the solution needs to be for example heated up, or mixed with another reagent. Examples of this include distillation, where the solution needs to be heated, or liquid separation. On the other hand, many processes exploit the physical properties already present in the solution, such as solubility of compounds.

The other important distinction to be made is regarding the compound you are trying to extract. This means you need to be careful about different parts of the experimental setup when extracting a liquid as opposed to a solid using the same apparatus. For example when using filtration, as the main concept will rely on if you are trying to reduce the impurities in the solution or get the insoluble particles.

Preparation and Purification Process (of Aspirin)

Here we will cover how the process of preparation and purification of an organic substance is performed. The preparation of aspirin (acetylsalicylic acid) is a versatile process that explored the preparation of an organic solid compound using different techniques.

Aspirin Synthesis and Reaction

For the preparation of aspirin, we will be exploiting the reaction between its precursor, salicylic acid, and excess acetic anhydride (ethanoic anhydride). The reaction mechanisms are not required to be explored. This reaction is sped up with an acid catalyst, in this example it will be sulfuric acid.

Below you can see the reagents and amounts required for the reactions as well as the steps needed for the synthesis:

1. Add 6.0g of salicylic acid and 10ml of acetic anhydride into a flask. Additionally, 0.5ml of sulfuric acid are added (around 5 drops).
2. Put the flask into a water bath set at 60°C for around 20 minutes.
3. The mixture is set aside and cooled, after which 75ml of water is added.
4. The aspirin should crystallise overtime, and later filtered out.
5. The crystals are left to dry.

Aspirin Purification and Analysis of Purity

The dry aspirin crystals can be recrystallised as a part of purification. This involves the addition of ethanol to the dry crystals. Only enough ethanol to create a saturated solution. Once the crystals are dissolved, under heat, the solution is poured over 40ml of water for new crystals to form. Additional filtration, preferably vacuum filtration, and drying are required to recover the crystals.

Testing the purity of an organic solid can be performed by a melting point test.

The melting point of aspirin is 135°C. You can perform a melting point test using three methods: using a melt station, a Thiele tube, or an oil bath. A melting point test relies on noticing the phase change of the compound in question, and recording the temperature at which it occurs.

In this article, you should have understood some common preparation and purification methods you will see not only in your school lab, but any lab! These techniques are used in both simple organic analysis as well as drug design and various other industries.