NEET Chemistry Notes Some Basic Principles and Techniques -Chromatography

NEET Chemistry Notes Some Basic Principles and Techniques -Chromatography

Chromatography

Chromatography

• It is an important technique extensively used to separate mixtures into their components.
•  In this technique, the mixture of substances is applied on to a stationary phase (solid or liquid), a pure solvent. A mixture of solvents or a gas is allowed to move slowly over the stationary phase.
•  Based on the principle involved, chromatography is classified into different categories.
  Two of these are

 

• Adsorption chromatography
• Partition chromatography.

Following are two main types of chromatographic techniques based on the principle of differential adsorption

• Column chromatography
• Thin layer chromatography

Qualitative Analysis of Organic Compounds

This method involves the conversion of covalently bonded nitrogen, sulphur or halogens present in the organic compounds to corresponding water-soluble ions, in the form of sodium salts.

Detection of Carbon and Hydrogen

Organic compound when heated with CuO, carbon changes into C0₂ (tested with lime water which develops turbidity) and hydrogen to H₂O (tested with anhydrous copper sulphate which turns blue).

Detection of Other Elements

Nitrogen sulphur, halogens and phosphorus present in an organic compound are detected by Lassaigne’s test.

Lassaigne’s test is given by compounds carrying nitrogen and carbon. So, hydrazine (NH₂ -NH₂ ) and hydroxyl amine (NH₂ OH) do not give this test.

Test for Halogens

The sodium fusion extract is acidified with nitric acid and then treated with silver nitrate.

X represents a halogen — Cl, Br or I. .
AgCl-white ppt, AgBr- Dull yellow ppt, Agl-bright yellow ppt.

Test for Phosphorus

The compound is heated with an oxidising agent (like sodium peroxide), the phosphorus present in the compound is oxidised to phosphate. The solution is boiled with nitric acid* and then treated with ammonium molybdate.
A yellow colouration or precipitate indicates the presence of phosphorus.

Quantitative Analysis of Organic Compounds

The qualitative analysis tells about the elements present in an organic compound. The quantitative analysis is finally carried out to determine the proportions in which different elements are present in an organic compound.

Estimation of Carbon and Hydrogen

The carbon and hydrogen are estimated by Leibig method

Estimation of Nitrogen .

There are two methods for the estimation of nitrogen

•  Duma’s method
• Kjeldahl’s method

Duma’s Method

 Kjeldahl’s Method

Let the mass of organic,, compound taken = w g, Volume of H₂S0₄ of molarity of taken = V mL and
V₁ mL of NaOH of molarity M

Kjeldahl method is not applicable to compounds containing nitrogen as nitro and azo groups.

• Estimation of Halogens (Carius Method)

Let the mass of organic compound = w g Mass of AgX formed = g Percentage of halogen

Percentage of halogen

• Estimation of Sulphur (Carius Method)

Percentage of halogen

Estimation of Phosphorus

Determination of Empirical and Molecular Formulae

Empirical formula expresses the relative number of . atoms present in the molecule. It is calculated from percentage composition of the compound.
Molecular formula = (empirical formula) x n

Example

An organic compound having molecular mass 60 is found to contain C= 20%, H= 6.67% and N= 46.67% while rest is oxygen. On heating it gives NH₃ along with a solid residue. The solid residue gives violet colour with alkaline copper sulphate solution. Identify the compound

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