An Overview of Purification of Crude Mixtures

An Overview of Purification of Crude Mixtures

We’ll go through a few fundamental techniques of separation and purification of crude mixtures. In this article, we’ll cover five key techniques. Let’s go through without further ado.

 

Acid-base chemical properties

Modification of pH is one of the oldest and most widely used methods used for natural product isolation. Certain acidic and basic molecules can be easily converted to salts. This conversion substantially increases their water solubility. The reaction is usually reversible and hence, after the separation of salt, the recovery of the initial acid and the base can be done easily.

The general overview of the process is shown in the image below.

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The crude mixture has one molecule that acts as a base- say, for example, an amine. Most amines are soluble in organic solvents like dichloromethane when they are in their neutral form. If pH is adjusted to about 1, the amine will get protonated resulting in the formation of an ammonium salt. The salt, having a charge will have considerable water solubility. Therefore, the extraction of this mixture with water will result in separation of salt from the crude mixture. The collection of the aqueous phase can then be done. The pH is adjusted back to neutral by adding base. That will result in neutralization of the acid, resulting in precipitation of amine. The neutral amine can be easily extracted through the use of an organic solvent.

The pictorial representation of the process is shown in the image below:

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Image courtesy: https://11452-presscdn-0-51.pagely.netdna-cdn.com/wp-content/uploads/2016/08/Acid-Base-Extraction.pdf

Distillation (Separation by boiling point differences)

This process is one of the most familiar methods of purification. A flask having the mixture is heated to the point of boiling, and then the condensation and collection of vapor are done. The temperature of the vapor is monitored with a thermometer.

Gasoline is a mixture of 200+ hydrocarbons. Through the use of large distillation setup, the components can be distinguished from several individual components like pentane, hexane, heptanes, etc., etc.

Crystallization

Crystallization can be used to acquire pure compounds from a crude mixture. A simple example can be the wine-like diamonds found on corks of wine bottles. If you examine corks of specific wine bottles, you’ll come across tiny clear shards resembling that of broken glass. These “wine-like diamonds” are actual crystals of potassium bitartrate that slowly crystallizes out of wine with the increase in alcohol content. (Potassium bitartrate is completely soluble in grape juice, but is poorly soluble in ethanol).

Crystallization is as close to a flawless purification method as one can get. The products obtained through this process are usually highly pure. The process is pretty simple as well as relatively cheap.

Chromatography

This is another effective process of purification.

We’ll try to explain the process of chromatography figuratively as much as possible. Let’s take the example of a Velcro. Imagine a floor having a carpet covered in Velcro “hooks.” If you start walking on it with your normal running shoes, there won’t be much of a problem. Now if you line the bottom of those shoes with Velcro “Fuzz,” you’ll see that you’ll walk much slower. So why did we provide you with that analogy?

The Velcro in the analogy represents the interactions between silica gel packed in a column containing free OH groups and polar groups dissolved in the solvent as they pass through the column. The more polar groups the compound has, the slower it will move down the column (similar to our Velcro example). Greasy compounds have few polar groups. These compounds can move quickly down the column because they have little interaction with silica gel (similar to walking on the carpet with normal running shoes).

Gas Chromatography and High-Performance Liquid Chromatography

GC (Gas Chromatography) and HPLC (High-Performance Liquid Chromatography) require expensive instruments for their implementation.

GC and HPLC are the forms of chromatography that are based on the same principle as that of a column chromatography with an exception. The column, in this case, has a much smaller diameter, and it runs are a significant high pressure.

So that should be all for now then. Hope you had a good read.

Sudipto Das

Sudipto writes technical and educational content periodically for wizert.com and backs it up with extensive research and relevant examples. He's an avid reader and a tech enthusiast at the same time with a little bit of “Arsenal Football Club” thrown in as well. He's got a B.Tech in Electronics and Instrumentation.
Follow him on twitter @SudiptoDas1993

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