Experiment 62

Objective

To identify the presence of nitrogen and sulfur in a carbonyl compound using the Lassaigne’s test.

Introduction

The Lassaigne’s test is a qualitative method used to detect elements like nitrogen, sulfur, and halogens in organic compounds. In this test, nitrogen and sulfur present in the sample react with sodium to produce sodium thiocyanate. When this is treated with hydrochloric acid and ferric chloride, a blood-red solution of ferric thiocyanate is formed, confirming the presence of both nitrogen and sulfur.

Reaction:

FeCl3 ​+ NaCNS → Fe(CNS)Cl2 ​+2H2​O

Materials

Chemicals

·       Sodium metal

·       Ferric chloride (FeCl₃)

·       Sample containing sulfur and nitrogen

·       Hydrochloric acid (HCl)

·       Sodium thiocyanate (NaSCN)

Apparatus

·       Fusion tube

·       Test tube

·       Mortar

·       Bunsen burner

·       Test tube holder

·       Filter paper

Reagents

·       mg of sample with a piece of sodium metal in a fusion tube

·       250 ml of distilled water in a mortar

Procedure

1.    Add the sample and a piece of sodium metal to a fusion tube.

2.    Heat the fusion tube in the Bunsen burner until the reaction completes.

3.    Break the fusion tube in the mortar and add distilled water to dissolve the reaction product.

4.    Filter the solution using filter paper.

5.    Take the filtrate in a test tube.

6.    Add sodium thiocyanate solution to the test tube.

7.    Add 1–2 drops of HCl, followed by ferric chloride.

8.    Observe any color change.

Observation

A blood-red color forms in the solution, indicating the presence of both nitrogen and sulfur.

Questions

1.    What is the Lassaigne’s test?

2.    How are nitrogen and sulfur identified together in this test?

3.    Why is the Lassaigne’s test necessary?

4.    What color appears when both nitrogen and sulfur are present?

5.    What is sodium thiocyanate?