JEE Chapter - Principles of Qualitative Analysis

Quantitative analysis is an analysis method that is used to determine the number of elements or molecules produced during a chemical reaction. Organic compounds are mainly composed of carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur and halogens. 

It is the analysis of the species present in a given compound. For example, if a compound has been taken, the qualitative analysis would be more focused on finding the elements and the ions present in the compound rather than study as to how much they are present.

Importance of Qualitative Analysis of Organic Compounds

The identification and analysis of the unknown organic compounds is a very important aspect of experimental organic chemistry. A systematic approach based on the scheme helps in fetching good results as there is no definite set of procedures which can be applied for all.

Important Topics of Qualitative Analysis

• General Qualitative Analysis Scheme

• Procedure for the detection of Carbon and Hydrogen

• Detection of Phosphorus

• Detection of Nitrogen

• Detection of Halogens

• Detection of Oxygen

• Detection of sulphur

General Qualitative Analysis Scheme

Procedure for the Detection of Carbon and Hydrogen

A small quantity of the pure and dry compound is taken and then it is mixed with nearly five to six times its weight of dry and pure cupric oxide powder. This mixture is strongly heated in a hard glass test tube fitted with a delivery tube having a bulb in the centre. Hence, the other end of the delivery tube is dipped in lime water taken in a test tube. After that, the bulb in the delivery tube is packed with anhydrous copper sulphate supported over glass wool. Upon heating, the carbon is oxidised to carbon dioxide which turns lime water milky and hydrogen is oxidised to water which turns anhydrous copper sulphate blue. 

Detection of Phosphorus

• For determining the phosphorus, the organic compound is heated with an oxidising agent that helps in oxidising the phosphorus present in it to phosphate. After that, the solution is boiled with concentrated HNO3 and is then treated with ammonium molybdate. We can confirm the presence of phosphorus with a yellow precipitate.

• The reactions involved in the process of estimation of phosphorus methods are given as:

Na3PO4 + 3HNO3 → H3PO4 + 3NaNO3

H3PO4 + 12(NH4)2MoO4 + 21HNO3 → (NH4)3PO4.12MoO3 + 21NH4NO3 + 12H2O

Detection of Nitrogen

Dumas Method

• The detection of Nitrogen is done with the help of the Dumas method where a known mass or weight of the compound is heated with copper oxide (CuO) in the carbon dioxide atmosphere. This yields free Nitrogen along with carbon dioxide and water.

• The balanced chemical reaction is as follows:

CxHyNz + (2x+ 0.5y) CuO → xCO2 + 0.5y H2O + 0.5z (N2) + (2x+ 0.5y)Cu

• When the gases are passed over a hot copper gauge, it converts the trace amounts of nitrogen oxides to dinitrogen. The gaseous mixture is collected over KOH solution which absorbs carbon dioxide. After that, the nitrogen gas is collected in the upper part of the graduated tube.

Detection of Halogens 

The presence of halogens in an organic compound is detected by the following tests: 

(1) Beilstein Test

In this test, a clean copper wire is heated in the non-luminous flame of the Bunsen burner until it ceases to impart any green or bluish-green colour to the flame. The heated end is then dipped in the organic compound and again introduced into the Bunsen flame. The appearance of a green or bluish flame due to the formation of volatile cupric halides indicates the presence of halogens in the organic compounds. 

(2) Lassaigne's Test 

It is a very useful test for the detection of halogens in an organic compound. One part of Lassaigne's extract(Sodium fusion extract) is boiled with dilute nitric acid and cooled. After that, a few drops of silver nitrate solution are then added. 

(a) A white precipitate soluble in ammonia and insoluble in dilute nitric acid indicates the presence of chlorine in an organic compound. 

(b) A pale yellow precipitate partially soluble in ammonia indicates the presence of bromine in an organic compound.

(c) A yellow precipitate insoluble in ammonia indicates the presence of iodine in an organic compound.

Detection of Oxygen 

The given organic compound is tested for the presence of oxygen containing functional groups such as -OH, -CHO, -COOH, etc. The presence of any one of these groups in the compound, in turn, confirms the presence of oxygen in it. 

Detection of Sulphur 

The sulphur in the organic compound is detected by the following tests:

Lassaigne's Test: If sulphur is present in the organic compound then on fusion with sodium metal, sodium sulphide is formed. The balanced chemical reactions are given as:

2 Na + S → Na2S 

Then we will perform following two sub-tests.

(a) Sodium Nitroprusside Test

A small portion of the Lassaigne's filtrate is treated with a few drops of sodium nitroprusside solution when a violet colouration is obtained. This colour slowly fades on standing. It signifies the presence of sulpur. The chemical reaction is given as

Na2S + Na2[Fe(CN)5(NO)] → Na4[Fe(CN)5 (NO)] 

(b) Lead Acetate Test

Another portion of Lassaigne's filtrate is then acidified with dilute acetic acid and a few drops of lead acetate solution are added to it. After that, the formation of black lead sulphide confirms the presence of sulphur in the given compound. The chemical reaction is given as

Na2S + (CH3COOH)2Pb → PbS + 2CH3COONa 

Solved Examples from the Chapter

Example 1: Carbon and hydrogen are detected by heating the compound. Name the compound.
Solution: Carbon and hydrogen are detected by heating the compound with copper (II) oxide. Therefore, the carbon that is present in the compound will be oxidised to carbon dioxide and hydrogen to water. Carbon dioxide is tested by lime water which develops turbidity, and hydrogen is tested with anhydrous copper sulphate, which turns blue.

Key Point to Remember: The carbon that is present in an organic compound is oxidised to carbon dioxide which turns lime water milky and hydrogen is oxidised to water which turns anhydrous copper sulphate blue.

Example 2: What is the colour of the precipitate obtained in the lead acetate test for sulphur?

Solution: In the lead acetate test for sulphur, the sodium fusion extract is acidified with acetic acid and lead acetate is added to it. After some time, a black precipitate is formed. This black precipitate is lead sulphide, indicating the presence of sulphur.

Key Point to Remember: The formation of lead sulphide in the lead acetate test for sulphur. 

Solved Questions From The Previous Years’ Question Papers

Question 1: Copper wire test for halogens is known as:

(1) Duma’s Test

(2) Beilstein’s Test

(3) Lasssigne’s Test

(4) Liebig’s Test

Solution: Copper wire test for halogens is called Beilstein's Test.

Therefore, option (2) is the answer.

Trick: In Beilstein's test, a clean copper wire is heated in the non-luminous flame of the Bunsen burner until it ceases to impart any green or bluish-green colour to the flame. 

Question 2: Beilstein test is used for estimation of which one of the following elements?

(1) S

(2) Cl

(3) C and H

(4) N

Solution: The Beilstein test is a chemical test for organic halides. From the given options, Cl is in the halogen group. Therefore, option (2) is the answer.

Trick: The presence of halogens in an organic compound is detected by the Beilstein test.

Question 3: A ‘X’ colour precipitate, which is Y in ammonium hydroxide indicates presence of chlorine. Identify X and Y.

a) X = yellowish, Y = soluble

b) X = yellow, Y = insoluble

c) X = white, Y = insoluble

d) X = white, Y = soluble

Solution: During the detection of chlorine, the organic compound reacts with sodium, it forms sodium chloride. Therefore, sodium chloride gives the white precipitate of silver nitrate with silver nitrate solution. The white precipitate obtained is also soluble in ammonium hydroxide. Therefore, option (d) is the correct option.

Trick: A white precipitate soluble in ammonia and insoluble in dilute nitric acid indicates the presence of chlorine in an organic compound. 

Practice Questions

Question 1: What is Lassaigne’s test extract called as?

Answer: Sodium fusion extract.

Question 2: In the test for nitrogen, the sodium fusion extract is acidified with which of the following?

a) Dilute sulphuric acid

b) Dilute hydrochloric acid

c) Concentrated hydrochloric acid

d) Concentrated sulphuric acid

Answer: Concentrated sulphuric acid

Conclusion

Analytical chemistry is necessary as it is required to identify, quantify, and separate the matter. The three-thing mentioned, which are, identifying, quantifying, and separating the matter, can either be an entire analysis, in practice or combined with another method. The science of obtaining information about the composition and structure of matter, the processing of the same, and the communication of this information, is called Analytical Chemistry. 

Hence, qualitative analysis of the chemical compound does not help in measuring the quantity, which helps in determining the number of elements that are being produced during the chemical reaction.