Experiment 63

Objective

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

Introduction

The Lassaigne test is used to detect the presence of nitrogen, sulfur, and halogens in organic compounds. By heating the sample with sodium in a fusion tube, sulfur and nitrogen are converted to sodium thiocyanate (NaSCN). When hydrochloric acid and ferric chloride (FeCl₃) are added to the filtrate, the formation of a blood-red color due to ferric thiocyanate indicates the presence of both sulfur and nitrogen.

Reaction Equation:

FeCl3 ​+ NaSCN → Fe(SCN)Cl2 ​+ 2H2​O

The blood-red color of ferric thiocyanate confirms the presence of sulfur and nitrogen.

Materials

Chemicals

·       Sodium metal

·       Ferric chloride (FeCl₃)

·       Hydrochloric acid (HCl)

·       Supplied sample containing sulfur and nitrogen

Apparatus

·       Fusion tube

·       Test tube and holder

·       Mortar

·       Bunsen burner

·       Filter paper

Preparation of Reagents

·       Weigh approximately 0.1 mg of the sample in a fusion tube.

·       Prepare sodium thiocyanate solution if necessary.

Procedure

1.    Sample Fusion:

·       Place the sample and a small piece of sodium metal in a fusion tube.

·       Heat the tube over a Bunsen burner until the sodium reacts with the sample, forming sodium thiocyanate (NaSCN).

2.    Solution Preparation:

·       Carefully break the fusion tube in a mortar and add approximately 250 mL of distilled water to dissolve the fusion products.

·       Filter the mixture using filter paper to remove any solid particles.

3.    Detection of Nitrogen and Sulfur:

·       Take a small amount of the filtrate in a test tube.

·       Add a few drops of sodium thiocyanate solution, followed by 1-2 drops of hydrochloric acid.

·       Add a few drops of ferric chloride (FeCl₃) to the test tube.

·       Observe any color change in the solution

Observation

A blood-red color in the solution indicates the presence of both sulfur and nitrogen in the compound.

Questions

1.    What is the Lassaigne test?

2.    How is the presence of both sulfur and nitrogen identified?

3.    Why is the Lassaigne test necessary?

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

5.    What is sodium thiocyanate?