Purification in Chemistry: When compounds are isolated from natural sources, they are contaminated and mixed with other impurities in the form of other chemicals, dirt and sand, and so on. In order to obtain pure substances, they need to be purified using specific methods of purification used in chemistry. Not only are the compounds impure when extracted from natural sources, those prepared in the laboratory sometimes become impure due to side reactions and other contaminations. Hence, purification is an essential step in the extraction or manufacturing of any chemicals, and therefore, it needs a careful selection of the process for it to become effective. 

After purification, the purity of an organic compound is checked and determined by measuring any of the physical constants of the substance, such as melting and boiling point, refractive index, and so on. Apart from this, spectroscopic methods and others can also be used to determine the purity of an organic compound after purification.

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Methods of Purification 

There are specific purification methods for every extracted compound or manufactured substance, and some of the commonly used and significant purification processes include:

1. Crystallization or recrystallization
2. Filtration
3. Fractional distillation
4. Simple distillation
5. Steam distillation
6. Fractional crystallization
7. Sublimation
8. Differential Extraction
9. Chromatography
10. Distillation under reduced pressure

Some of the common methods of purification used to purify organic compounds include:

a. Filtration Process

Filtration is the simplest of purification methods. An insoluble solid component can be separated from a soluble component from a solvent in the filtration method. This method is used in the separation of urea and naphthalene, with water as a solvent. Since urea is

soluble in water, and naphthalene is insoluble, when filtered, naphthalene separates out while urea is extracted from the filtrate through evaporation of water.

The filtration process can be selective with respect to the solvent used. Sometimes, the temperature of the solvent also plays a major role. For example, some solutes are only sparingly soluble in cold water while completely soluble in hot water. In such cases, a hot water funnel is used for filtering the hot solution. 

b. Sublimation

‘Sublimation is a process where a solid gets converted directly to the gaseous state on heating, without passing over the intermediate liquid state, and back to solid when it is cooled.’

Sublimation is possible in cases of compounds whose vapour pressure reaches the atmospheric pressure much before they reach their melting points. The substances which can undergo the sublimation process are called sublimable compounds. While sublimation is used for sublimable substances, it is not a common process since the number of such compounds which can undergo the process is very limited. 

Process: Impure substances are placed in a china dish and covered with an inverted funnel with a perforated filter paper attached to it. The open end of the funnel is covered with cotton. When the substance is heated on a sand bath, the vapours of volatile substances rise up and pass through the perforated filter paper, and get condensed on the walls of the funnel. The non-volatile substance is left behind in the china dish.

Examples of sublimable substances include naphthalene, benzoic acid, camphor, etc.

c. Crystallization method

‘Crystallization is a process where impure substances are converted into pure crystals.’ 

Crystals are considered the purest form of compounds with definite geometrical shapes. The crystallization method, therefore, finds use in purifying several organic compounds which exist as solids. The crystallization process is based on the difference in the solubility of the original compound and the impurities present in it. 

Process: The compound with impurities is dissolved in a suitable solvent selected on the basis of the fact that the impurities will dissolve in the solvent, sparingly at a lower temperature and completely at a higher temperature. When this solution cools down, the pure crystals of the organic compound separates out and are removed by filtration. Sometimes due to comparable solubilities between impurities and the original compound, repeated crystallizations are done to get pure crystals. The steps involved in the crystallization or recrystallization process include:

1. Selecting a relevant solvent 
2. Preparing the solution for crystallization
3. Filtering the solution
4. Crystallization of organic compound and separation of pure crystals formed

The crystals are then dried and stored. If necessary, and if the crystals are coloured due to impurity, the crystals are again dissolved in the same solvent and the crystallization process is repeated.

Purification of Liquids 

a. Distillation is used to separate:

1. Non-volatile impurities from volatile liquid
2. Mixtures with liquids having a difference in boiling point 

Distillation is defined as:

‘A process where liquids are converted into vapours by heating and followed by condensation of vapours through cooling.’

There are different types of distillations depending upon the process used and is given.

1. Simple Distillation

Simple distillation is used to purify substances like benzene, acetone, chloroform, ethanol, \(\mathrm{CCl}_{4}\), etc., which are stable at boiling points and the impurities present in them are non-volatile. 

Process: The liquid mixture to be distilled is placed in a flask and heated up. With the rise in temperature, the liquid mixture starts boiling, and the vapour of the most volatile liquid escapes through the condenser and are condensed and collected in the process. The liquid which distils at a constant temperature is collected to ensure the purity of the collected substance. The apparatus is as shown:

2. Fractional Distillation

When two or more compounds have very similar boiling points, simple distillation cannot work. This is because, when the boiling points of two liquids are closer to each other, before the vapours of one liquid start condensing, the other vapours of the other one might rise and mix up with the first liquid. Hence, fractional distillation is done to ensure the complete separation of liquid mixtures. 

Fractional distillation is repeated distillation of the mixture followed by condensations. Such distillation is carried out with the use of fractionating columns instead of simple condensation tubes. The fractionating columns provide enough surface area for the liquids to cool and also to provide hindrances to the ascending vapour and the descending liquid. It, therefore, prevents two vapours from getting mixed.

The liquid with a higher boiling point will condense well before that of the liquid with a lower boiling point. So, the vapours that ascend to the top are more volatile and rise up to the top of the fractionating column. At one point, the vapours are pure of the high volatility, low boiling point liquid, and thus, both liquids can be separated easily.

Fractional distillation is used in the petroleum industry for separating crude oil into different highly useful fractions.

3. Vacuum Distillation

Vacuum distillation or distillation under reduced pressure is used to separate liquids that have very high boiling points or those compounds which decompose just before or at their boiling point. 

The principle behind distillation under reduced pressure is that a liquid reaches its boiling point when its vapour pressure is equal to the atmospheric pressure. However, the same liquid will boil at a lower temperature when the pressure is controlled and reduced. 

Examples include:

The sugar manufacturing industry employs vacuum distillation for the concentration of sugar, and glycerol decomposes at \(563 \mathrm{~K}\), and hence, distillation under reduced pressure is used to separate it from spent lye in the soap industry.

4. Steam Distillation

Steam distillation is used for the separation as well as purification of both solid and liquid organic compounds from other non-volatile inorganic or organic impurities. This method is used for those compounds which are:

1. Volatile in steam
2. Have a vapor pressure of \(10 – 15\;\,{\rm{mm}}\,{\rm{Hg}}\)
3. Insoluble in water
4. Have non-volatile impurities

Also, steam distillation is even easier to perform when the original substance has a boiling point of greater than \(373 \mathrm{~K}\) at \(760\;\,{\rm{mm}}\,{\rm{Hg}}\) pressure and decomposes at or below the boiling point. It is because if the to-be purified substance is a high-boiling one, then it will distill at a lower temperature and, therefore, avoid problems of decomposition. Examples of substances that are purified by steam distillation include \(0\)-nitrophenol, nitrobenzene, essential oils, bromobenzene, etc.

b. Differential Extraction

Differential extraction is used to recover organic compounds from their aqueous solutions. Both solids and liquids can be extracted using this method. When the substance to be purified is shaken with a suitable solvent that is not miscible in water, but in which the organic compound shows very high solubility. Substances such as chloroform, carbon tetrachloride, ether, and many more can be purified using this method.

c. Chromatography

In \(1906\), a Russian botanist, Tswett, discovered a new and versatile purification technique called chromatography. It was first used to separate coloured substances, and hence, the name ‘chromatography (kroma – colour, graphy – writing, in Greek) was given to this technique. 

Chromatography finds several uses, both in the purification of compounds and also in testing their purity. Chromatography is a physical method of desperation and can be defined as:

‘The technique in which differential movement of individual components on a stationary phase is used to separate the individual components from a mixture, with the support of an additional mobile phase’

There are different types of chromatography, and each has its applications in specific cases of separation and purification of organic compounds. The different chromatographic techniques are as follows:

Chromatographic Technique	Base	Adsorption medium	Principle
Column Chromatography	Long glass column with a bottom stop cock	Alumina, Silica gel, starch, charcoal	Differential adsorption
Thin Layer chromatography	Plastic or a glass plate	Silica gel or alumina	Migration and differential adsorption
Paper Chromatography	Chromatographic paper	Suitable liquid phase	Migration and Selective adsorption

Summary

Organic compounds need to be purified due to the impurities present in them. There are many purification methods used in chemistry, and they are selected depending upon the properties of the substance to be purified and also on the type of impurities present in them. Filtration, sublimation, distillation, and crystallization are techniques that are often used in the purification process. Chromatography is a more versatile technique and employs different types of mediums to separate substances and also to identify their purity.

Learn About Chromatographic Techniques

Frequently Asked Questions

Q.1. Why do we have to purify chemical substances? What is the purification process?
Ans: Chemical compounds obtained from nature, and even those prepared in the laboratories, have impurities in them in the form of other compounds. These need to be removed to get pure compounds, and therefore, purification processes are employed to get the relevant purity of compounds.

Q.2. What are the four major methods of purification?
Ans: The four major purification methods include:
1. Filtration
2. Crystallization
3. Sublimation
4. Distillation

Q.3. What are the basic laboratory techniques or operations used in purifying organic compounds?
Ans: There are several laboratory techniques used to purify organic compounds, such as fractional distillation, crystallization, sublimation, and differential extraction. Chromatography is also used as a purification technique.

Q.4. How can we use sublimation to purify organic compounds? 
Ans: The sublimation process involves converting a solid into a gas without passing over the liquid state. Compounds that are sublimable or those with vapour pressures become equal to the atmospheric pressure well before it reaches the melting points can be purified using this technique.

Q.5. How is the purity of organic compounds determined?
Ans: After purification processes, the purity of organic compounds are determined by measuring their physical quantities such as density, melting, and boiling point measurements, and also using chromatographic techniques such as thin-layer chromatography.

We hope this detailed article on the methods of purification of organic compounds helped you in your studies. If you have any doubts, queries or suggestions regarding this article, feel to ask us in the comment section and we will be more than happy to assist you. Happy learning!