PURIFICATION TECHNIQUE - I

 

Dear students,

It is known that organic compounds can be derived from natural sources and these are mixed with other substances and are impure. Similarly compounds prepared in the laboratory  are also mixed with other by products formed during the course of the reaction. To purify such compounds, various methods are adopted. Some of the common methods are as follows

Crystallization

Sublimation

Distillation

Extraction

Chromatography

In this post we are about to discuss two methods of purification. They are as follows.

Solvent Extraction: 

Extraction means drawing (pulling out) a compound out of a mixture using a solvent. It means organic compounds have a "choice" of two solvents that they can dissolve in. It requires two solvents that are not miscible in each other. Usually one of the solvents is water. The other solvent is a liquid that does not dissolve very well in water, such as diethyl ether.

The process of removing organic substance from its aqueous solution by mixing with suitable organic solvent is termed solvent extraction. It is also called as liquid-liquid extraction and partitioning. It is an equilibrium process and is a convenient technique can be carried out using separating funnel.

Principle of Solvent Extraction

It is based on the fact that different substances have different solubility in different solvents.

Procedure:

The aqueous solution of the given solute (organic substance) is taken in a separating funnel. It is mixed with the organic solvent. The funnel is closed, and its contents are shaken well. It is then allowed to remain undisturbed for some time. Water and organic solvent will form separate layers, and the  solute will be transferred from aqueous layer to the organic layer. In the funnel, the solvent forms the upper layer while the water forms the lower layer. The two layers can be recovered by opening the stop cock and collecting them in separate bakers. On evaporating the organic solvent, the solute can be recovered.

It is important to note that extraction is more efficient (i.e., higher purification) when the process is repeated. For example, benzoic acid can be extracted from its water solution using benzene. 

It is based on the distribution law, also called the Nernst’s distribution law. It states that if a solute X distributes itself between two immiscible solvents A and B, at constant temperature and X is in the same molecular condition in both solvents, then

The constant 𝐾D (or simply K) is called the distribution coefficient or Partition coefficient or Distribution ratio. 

Applications:

It is used in perfumes, vegetable oil, biodiesel processing, etc.

It is used to segregate hazardous contaminants from sludge and sediments.

In the petrochemical industry, it is used to separate and purify different components of crude oil.

It is used for the separation and purification of organic compounds.

It removes oil and grease that  spilled into water. This helps clean up environmental pollution.

Crystallization:

It is a purification technique used to separate solids from a solution. In this method, the impurities are dissolved in a suitable solvent and then filtered to remove impurities.

Principle: 

An impure organic solid is completely dissolved in a small amount of solvent by boiling. The hot solution is allowed to cool slowly. The impurities remain in the solution (called the "mother liquor") while the organic substance comes out as pure crystals . It is then filtered and dried.

Procedure:

Step 1: Take some water in a beaker

Step 2: Add organic substance (impure) and stir it

Step 3: Now heat the solution

Step 4: Add more impure substance.

Step 5: After some time there will be a point at which no more substance can be dissolved in water. This stage is the saturation point, and the solution is referred to as a saturated solution

Step 6: Now filter the substance with the help of a filter paper

Step 7: Collect the filtrate in a glass bowl and cool it slowly

Step 8: You will observe that some fine crystals are formed in the bowl

Step 9: The process of filtration can separate these crystals.

Selection of solvent:

(a) It dissolves more of the substance at higher temperature than at room temperature 

(b) The impurities are either insoluble or dissolve in solution (in the mother liquor) upon crystallization, 

(c) which is not highly inflammable and 

(d) which does not react chemically with the compound to be crystallized. 

The most commonly used solvents for crystallization are: water, alcohol, ether, chloroform, carbon- tetrachloride, acetone, benzene, petroleum ether etc. 

Fractional crystallization: 

The process of separation of different components of a mixture by repeated crystallizations is called fractional crystallization.

Examples:

(a) Sugar having an impurity of common salt can be crystallized from hot ethanol since sugar

dissolves in hot ethanol but common salt does not.

(b) A mixture of benzoic acid and naphthalene can be separated from hot water in which

benzoic acid dissolves but naphthalene does not.