Experiment 72

Objective

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

Introduction

The Lassaigne’s test is a qualitative method used to detect elements such as nitrogen, sulfur, and halogens in organic compounds. In this test, a carbonyl compound containing sulfur is fused with sodium to produce sodium sulfide, which reacts with sodium nitroprusside to produce a violet-colored complex.

Reaction:

Na2​S + Na[Fe(CN)5​NO] → Na4​[Fe(CN)5​(NOS)] (violet color)

Materials

Chemicals

●      Sodium metal

●      Sodium nitroprusside

●      Sample containing sulfur

Apparatus

●      Fusion tube

●      Test tube

●      Mortar

●      Bunsen burner

●      Test tube holder

●      Filter paper

Reagents

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

●      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 using the Bunsen burner.

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

4.    Filter the solution using filter paper.

5.    Take the filtrate in a test tube.

6.    Add sodium nitroprusside solution to the test tube.

7.    Cool the sample in water.

8.    Observe the color change.

Observation

A violet color appears in the solution, indicating the presence of sulfur in the sample.

Questions

1.    What is the Lassaigne’s test?

2.    How is sulfur identified in this test?

3.    Why is the Lassaigne’s test necessary?

4.    What color is observed in the sulfur test?

5.    What is sodium nitroprusside?